Dissertationen zum Thema „Convective mixers“

Comprendre le comportement d'écoulement des poudres lors de l'agitation mécanique est crucial pour optimiser les processus de mélange dans les applications industrielles. Cette étude introduit la rhéologie μ(I), une loi rhéologique développée pour analyser la rhéologie des poudres dans les écoulements denses au sein d'un dispositif de mélange en laboratoire. Le cadre se concentre sur les interactions entre les pales et le lit de poudre, en abordant les défis de mesure de paramètres complexes des poudres tels que le coefficient de friction effectif µeff. La rhéologie μ(I), développée grâce à l'analyse dimensionnelle et à la visualisation des bandes de cisaillement, démontre de fortes capacités prédictives pour différentes configurations de poudres ayant des formes de particules similaires mais des tailles variées. Le cadre de la rhéologie μ(I) s'est révélé applicable à diverses configurations de systèmes agités et a posé les bases des premières études de mise à l'échelle incluant les caractéristiques des poudres. Les comparaisons avec l'équation de Hatano confirment la robustesse de la rhéologie μ(I), notamment pour les lits de poudre profonds. Les améliorations futures se concentreront sur le raffinement de l'évaluation de la largeur des bandes de cisaillement et la réévaluation des hypothèses de contrainte normale pour améliorer la précision du modèle. Cette recherche contribue à une compréhension approfondie de la dynamique des poudres dans les systèmes de mélange et soutient l'efficacité de la mise à l'échelle des processus industriels<br>Understanding powder flow behaviour during mechanical agitation is crucial for optimising mixing processes in industrial applications. This study introduces μ(I)-rheology, a novel rheological law developed to analyse powder rheology in dense flows within a laboratory mixing setup. The framework focuses on the interactions between paddles and the powder bed, addressing the challenges of measuring complex powder parameters such as the effective friction coefficient µeff. μ(I)-rheology, developed through dimensional analysis and shear band visualisation, demonstrates strong predictive capabilities across different powder configurations with similar particle shapes but varying sizes. The μ(I)-rheology framework proved applicable across various agitated system configurations and has laid the groundwork for initial scale-up studies that includes powder characteristics. Comparisons with Hatano's equation confirm the robustness of μ(I)-rheology, particularly for deep powder beds. Future improvements will focus on refining shear band width evaluation and reassessing normal stress assumptions to enhance model accuracy. This research contributes to a deeper understanding of powder dynamics in mixing systems and supports efficient scaling-up of industrial processes

Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale. In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.

Turbulent mixed convection is a complicated flow where the buoyancy and shear forces compete with each other in affecting the flow dynamics. This thesis deals with the near wall dynamics in a turbulent mixed convection flow over an isothermal horizontal heated plate. We distinguish between two types of mixed convection ; low-speed mixed convection (LSM) and high-speed mixed convection (HSM). In LSM the entire boundary layer, including the near-wall region, is dominated by buoyancy; in HSM the near-wall region, is dominated by shear and the outer region by buoyancy. We show that the value of the parameter (* = ^ determines whether the flow is LSM or HSM. Here yr is the friction length scale and L is the Monin-Obukhov length scale. In the present thesis we proposed a model for the near-wall dynamics in LSM. We assume the coherent structure near-wall for low-speed mixed convection to be streamwise aligned periodic array of laminar plumes and give a 2d model for the near wall dynamics, Here the equation to solve for the streamwise velocity is linear with the vertical and spanwise velocities given by the free convection model of Theerthan and Arakeri [1]. We determine the profiles of streamwise velocity, Reynolds shear stress and RMS of the fluctuations of the three components of velocity. From the model we obtain the scaling for wall shear stress rw as rw oc (UooAT*), where Uoo is the free-stream velocity and AT is the temperature difference between the free-stream and the horizontal surface.A similar scaling for rw was obtained in the experiments of Ingersoll [5] and by Narasimha et al [11] in the atmospheric boundary layer under low wind speed conditions. We also derive a formula for boundary layer thickness 5(x) which predicts the boundary layer growth for the combination free-stream velocity Uoo and AT in the low-speed mixed convection regime.

An experimental apparatus was designed and constructed for the study of laminar mixed convection heat transfer in vertical, horizontal and inclined tubes. The working fluid was distilled water, with bulk temperatures in the range of 8$ sp circ$C to 31$ sp circ$C.<br>An innovative design allows, for the first time, flow visualization over the entire heated portion of the test section. The key element of this design is a thin, electrically conductive gold-film heater suitably attached to the outside surface of a plexiglas pipe: the gold film is approximately 80% transparent to electromagnetic radiation in the visible wavelength band. This test section was mounted inside a transparent vacuum chamber to insulate it from the environment. A dye injection technique was used to visualize the mixed-convection flow patterns. The apparatus was also designed and instrumented to allow the measurement of both circumferential and axial temperature variations over the heated tube.<br>The flow-visualization results revealed the following: (i) a steady recirculating flow pattern, followed by laminar flow instability in vertical tubes; (ii) steady spiralling flow patterns in inclined and horizontal tubes, that confirmed earlier numerical predictions. The temperature results agreed qualitatively with earlier published experimental and numerical data. Local and overall Nusselt numbers can be calculated using the data presented, but this is not within the scope of this thesis.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.<br>Includes bibliographical references (leaves 28-29).<br>The Mars Gravity Biosatellite will house fifteen mice in a low Earth orbit satellite spinning about its longitudinal axis. The satellite's payload thermal control system will reject heat through the base of the payload module and provide air circulation vital to maintaining a habitable environment for the mice. The centripetal acceleration due to rotation creates the tendency for heated air to move by free convection toward the axis of rotation. Dominance of forced convection throughout the payload module will ensure nearly isothermal air and effective heat rejection from the payload to the bus module via fan/heatsink/thermoelectric cooler units. Circulation effectiveness is measured by the Richardson number, which expresses the ratio of the influence of free convection to the influence of forced convection in a mixed-convection flow. Experiments were executed with the current circulation system to determine the forced convection flow velocity. The free convection flow parameter was determined theoretically. Cross-flow fan/heatsink units mounted on the baseplate rim created low Reynolds number (88-985) flow throughout the enclosure. The calculated Richardson number for the worst-case 19°C difference between heated components and cooled air is between 0.78 and 2.34.<br>(cont.) For a realistic steady 3°C-80C temperature difference, the calculated Richardson number for the overall flow field is between 0.22 and 0.37. It was found that the flow capacity of the fan/heatsink assemblies must be increased from 1CFM to 5CFM to achieve the desired dominance of forced convection (a Richardson number of 0.1) in the worst-case on-orbit scenario. Increasing the capacity of the circulation system would allow for recovery from worst-case thermal scenarios under spaceflight conditions and allow a fraction of the cooler units to be powered during normal spaceflight conditions. The methods used here are scalable to analysis for the design of rotating human habitation vehicles.<br>by Jesse B. Marsh.<br>S.B.

This work focuses on magnetohydrodynamic (MHD) mixed convection flow of electrically conducting fluids enclosed in simple 1D and 2D geometries in steady periodic regime. In particular, in Chapter one a short overview is given about the history of MHD, with reference to papers available in literature, and a listing of some of its most common technological applications, whereas Chapter two deals with the analytical formulation of the MHD problem, starting from the fluid dynamic and energy equations and adding the effects of an external imposed magnetic field using the Ohm's law and the definition of the Lorentz force. Moreover a description of the various kinds of boundary conditions is given, with particular emphasis given to their practical realization. Chapter three, four and five describe the solution procedure of mixed convective flows with MHD effects. In all cases a uniform parallel magnetic field is supposed to be present in the whole fluid domain transverse with respect to the velocity field. The steady-periodic regime will be analyzed, where the periodicity is induced by wall temperature boundary conditions, which vary in time with a sinusoidal law. Local balance equations of momentum, energy and charge will be solved analytically and numerically using as parameters either geometrical ratios or material properties. In particular, in Chapter three the solution method for the mixed convective flow in a 1D vertical parallel channel with MHD effects is illustrated. The influence of a transverse magnetic field will be studied in the steady periodic regime induced by an oscillating wall temperature. Analytical and numerical solutions will be provided in terms of velocity and temperature profiles, wall friction factors and average heat fluxes for several values of the governing parameters. In Chapter four the 2D problem of the mixed convective flow in a vertical round pipe with MHD effects is analyzed. Again, a transverse magnetic field influences the steady periodic regime induced by the oscillating wall temperature of the wall. A numerical solution is presented, obtained using a finite element approach, and as a result velocity and temperature profiles, wall friction factors and average heat fluxes are derived for several values of the Hartmann and Prandtl numbers. In Chapter five the 2D problem of the mixed convective flow in a vertical rectangular duct with MHD effects is discussed. As seen in the previous chapters, a transverse magnetic field influences the steady periodic regime induced by the oscillating wall temperature of the four walls. The numerical solution obtained using a finite element approach is presented, and a collection of results, including velocity and temperature profiles, wall friction factors and average heat fluxes, is provided for several values of, among other parameters, the duct aspect ratio. A comparison with analytical solutions is also provided, as a proof of the validity of the numerical method. Chapter six is the concluding chapter, where some reflections on the MHD effects on mixed convection flow will be made, in agreement with the experience and the results gathered in the analyses presented in the previous chapters. In the appendices special auxiliary functions and FORTRAN program listings are reported, to support the formulations used in the solution chapters.

This work focuses on magnetohydrodynamic (MHD) mixed convection flow of electrically conducting fluids enclosed in simple 1D and 2D geometries in steady periodic regime. In particular, in Chapter one a short overview is given about the history of MHD, with reference to papers available in literature, and a listing of some of its most common technological applications, whereas Chapter two deals with the analytical formulation of the MHD problem, starting from the fluid dynamic and energy equations and adding the effects of an external imposed magnetic field using the Ohm's law and the definition of the Lorentz force. Moreover a description of the various kinds of boundary conditions is given, with particular emphasis given to their practical realization. Chapter three, four and five describe the solution procedure of mixed convective flows with MHD effects. In all cases a uniform parallel magnetic field is supposed to be present in the whole fluid domain transverse with respect to the velocity field. The steady-periodic regime will be analyzed, where the periodicity is induced by wall temperature boundary conditions, which vary in time with a sinusoidal law. Local balance equations of momentum, energy and charge will be solved analytically and numerically using as parameters either geometrical ratios or material properties. In particular, in Chapter three the solution method for the mixed convective flow in a 1D vertical parallel channel with MHD effects is illustrated. The influence of a transverse magnetic field will be studied in the steady periodic regime induced by an oscillating wall temperature. Analytical and numerical solutions will be provided in terms of velocity and temperature profiles, wall friction factors and average heat fluxes for several values of the governing parameters. In Chapter four the 2D problem of the mixed convective flow in a vertical round pipe with MHD effects is analyzed. Again, a transverse magnetic field influences the steady periodic regime induced by the oscillating wall temperature of the wall. A numerical solution is presented, obtained using a finite element approach, and as a result velocity and temperature profiles, wall friction factors and average heat fluxes are derived for several values of the Hartmann and Prandtl numbers. In Chapter five the 2D problem of the mixed convective flow in a vertical rectangular duct with MHD effects is discussed. As seen in the previous chapters, a transverse magnetic field influences the steady periodic regime induced by the oscillating wall temperature of the four walls. The numerical solution obtained using a finite element approach is presented, and a collection of results, including velocity and temperature profiles, wall friction factors and average heat fluxes, is provided for several values of, among other parameters, the duct aspect ratio. A comparison with analytical solutions is also provided, as a proof of the validity of the numerical method. Chapter six is the concluding chapter, where some reflections on the MHD effects on mixed convection flow will be made, in agreement with the experience and the results gathered in the analyses presented in the previous chapters. In the appendices special auxiliary functions and FORTRAN program listings are reported, to support the formulations used in the solution chapters.

On etudie les conditions dans lesquelles un ecoulement de convection peut penetrer un milieu poreux et les consequences que cette circulation engendre sur les transferts thermiques. Etude theorique a partir d'une approche de type couche limite. Mesure de la vitesse, des temperatures et des flux thermiques dans une cavite rectangulaire partiellement occupee par un isolant fibreux

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1986.<br>MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE.<br>Bibliography: leaves 322-326.<br>by Tae Sun Ro.<br>Ph.D.

Mixed convection heat transfer in enclosures has been studied in order to enhance the associated heat transfer performance through the use of either different convective fluid types, domain configurations, boundary conditions, or combinations thereof. Analysing the enhancement in heat transfer has been accomplished through the isotherm and streamline contours, temperature isosurfaces, flow vectors, mean and root mean square velocity profiles, turbulence kinetic energy profiles and Nusselt number profiles. Firstly, laminar mixed convection in a lid-driven trapezoidal cavity using different nanoparticle types and various parameters other than configuration parameters has been investigated. It was found that any nanofluid types can provide greater heat transfer than water, especially, at high nanoparticle volume fraction and low diameter. Heat convection can be affected by changing either rotational and inclination angles, aspect ratio, or flow direction. Secondly, turbulent mixed convection due to the moving sidewalls of a lid-driven cuboid has been analysed. Remarkable enhancement in heat transfer has been achieved by either increasing the turbulent flow circulation or using nanofluids. Thirdly, turbulent mixed convection in a top wall lid-driven cuboid containing a clockwise- or anticlockwise-rotating cylinder has been studied. Significant enhancement in heat convection was noticed with the use of the rotating cylinder, especially when the direction of rotation can enhance the top wall movement. In addition, the Reynolds number and nanofluids have a positive impact on the heat transfer in the presence of the rotating cylinder. Finally, the study has been extended by artificially roughening the heated wall in order to increase the heat transfer rate. A noteworthy enhancement has been found due to the use of two rib shapes, particularly in combination with the rotating cylinder. Overall, in terms of the comparison between the URANS and LES predictions, even though both methods have performed well, the LES approach is more successful in capturing more detail of the secondary eddies.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1986.<br>MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE<br>Bibliography: v.1, leaves 287-292.<br>by Victor Iannello.<br>Sc.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005.<br>Includes bibliographical references (p. 79-84).<br>For decay heat removal systems in the conceptual Gas-cooled Fast Reactor (GFR) currently under development, passive emergency cooling using natural circulation of a gas at an elevated pressure is being considered. Since GFR cores have high power density and low thermal inertia, relative to the high temperature gas-cooled thermal reactor (HTGR), the decay heat removal (DHR) in depressurization accidents is a major challenge to be overcome. This is due to (1) a gas has inherently inferior heat transport capabilities compared to a liquid and (2) the high surface heat flux of the GFR strongly affects the gas flow under natural circulation. The high heat flux places the flow into a mixed convection regime, which is not yet fully understood. One of the issues of mixed convection is that the transition from laminar to turbulent flow is not clearly defined in the existing literature. Review of previous work on heat transfer mechanisms and flow characteristics of the mixed convection transitional regime shows that two transitional zones exist between laminar or laminar-like flow and fully turbulent flow for the upward heated case. Previous work has focused on liquids and thus is not applicable to gas mixed convection.<br>(cont.) An experimental facility is designed to obtain the data in the regions not covered in previous work, using nitrogen, helium and carbon dioxide. The facility is expected to operate with heat fluxes up to 10kW/m2 and gas velocities up to 2.5m/s by natural circulation only. A velocity calibration method is designed in addition to the hotwire probe for velocity and temperature profiles measurement. Finally, computational simulations, using the commercial code FLUENT, are performed to select an appropriate turbulence model for investigating mixed convection transitional flow regimes. It was concluded that the basic models in FLUENT were not capable of predicting the transitional flow as the Launder-Sharma turbulence model does. Nevertheless, the advanced numerical algorithm and convenient post processor of FLUENT can still be utilized by using UDF to incorporate other turbulence models into the code.<br>by Jeongik Lee.<br>S.M.

A FORTRAN code was developed to numerically simulate the mixed convective flow over a three-dimensional horizontal backward-facing step. The momentum and energy equations under the assumption of the Boussinesq approximation were discretized by means of a finite volume technique. The SIMPLE algorithm scheme was applied to link the pressure and velocity fields inside the domain while an OpenMP parallel implementation was proposed to improve the numerical performance and to accelerate the numerical solution. The heating process corresponds to a channel heated from below at constant temperature keeping insulated all the other channel walls. In addition, the back-step was considered as a thermally conducting block and its influence in the heating process was explored by holding different solid to fluid thermal conductivity ratios. The effects over the velocity and temperature distribution of buoyancy forces, acting perpendicular to the mainstream flow, are studied for three different Richardson numbers Ri=3, 2, and 1 and the results are compared against those of pure forced convection Ri=0. In these simulations the Reynolds number is fixed at 200 while the bottom wall temperature is adjusted to fulfill the conditions for the different Ri. Under this assumption, as Ri increases the buoyancy effects are the dominant effects in the mixed convective process. The numerical results indicate that the velocity field and the temperature distribution for pure forced convection are highly distorted if compared with the mixed convective flow. If the Ri parameter is increased, then the primary re-circulation zone is reduced. Similarly, as the buoyancy forces become predominant in the flow, the convective rolls, in the form of spiral-flow structures, become curlier and then higher velocity components are found inside the domain. The temperature field distribution showed that as the Ri is increased a thicker layer of high temperature flow is located at the channel??s top wall as a result of the higher rates of low-density flow moving to the top wall. The flow is ascending by the channel sidewalls, while descending by the channel span-wise central plane. The parallel numerical strategy is presented and some results for the performance of the OpenMP implementation are included. In this sense, linear speedup was obtained when using 16 possessors in parallel.

Cooling of electronic components is one of the most important issues concerned in the electronic industry for design of equipment. Maintaining the temperature of an electronic device within its safe operating temperature limits is essential to operate the equipment safely with proper functionality. According to the Arrhenious law of failure rate, for a device with activation energy 0.65eV, every 10°C increase in temperature doubles the failure rate. Recent miniaturisation of components and high device heat dissipation rates lead to high heat fluxes, which cause temperature rise. Hence, there is an increasing need for research to achieve high heat removal rates and optimal design. Several cooling techniques are used for cooling of electronics based on the application and cooling rate requirements. Air-cooling of electronics has a wide range of applications due to its greater reliability, simplicity, easy maintenance, low cost, easy availability of coolant (air), and light weight. Air-cooling is also free from boiling and dripping problems. Air-cooling is used in applications such as avionics, cooling of personal computers, cooling of data centers, and in automobile electronics. In a typical electronic cooling application, cooling fluid is driven by the combination of external pressure forces and buoyancy forces. Based on the relative contribution of these forces towards the total driving force, the cooling techniques can be categorized as forced, natural or mixed convection cooling. However, in many of the electronic cooling situations, such as in the applications with very high heat fluxes, tall Printed Circuits Boards (PCBs) with low forced convection velocity, and in large scale applications such as data centers, the contributions of the buoyancy forces and external pressure forces for the total driving force are comparable, which results in a mixed convection situation. In the present study, mixed convection in vertical channels heated with five heating configurations, which represent typical electronic cooling applications, is studied numerically. The five different heating configurations are channels with flush-mounted continuous heater, flush-mounted strip heaters, flush-mounted square block heaters, protruding rib heaters and protruding square heaters. The first three configurations are categorised as flush-mounted heating configurations and the latter two configurations are categorised as protruded heating configurations. One of the channel walls represents the substrate on which the heaters are mounted and the heat sources represent the heat generating electronic components. Heat transfer under steady state conditions is considered in the study. The study includes laminar as well as turbulent heat transfer. For a systematic study of mixed convection, an analytical or semi-analytical formulation is desirable for a simplified model, as it can highlight the effect of relevant non-dimensional parameters on the heat transfer characteristics of a system. The results of a simplified model can be used for benchmarking the results of practical situations. Hence, before numerically solving the governing equations for mixed convection in channels, mixed convection boundary layer flows over a heated vertical plate is considered for study. Perturbation technique is used to solve the boundary layer equations with non-isothermal boundary conditions. The perturbation analysis is carried out for an arbitrarily variation of wall temperature or heat flux. Subsequently, the results are extended to find heat transfer rates in the cases of power-law variation of temperature and heat flux, as special cases. It is always required to design a cooling system to remove maximum possible amount of heat, keeping the device temperature within its safe operating limits. Hence, optimization of heat transfer in boundary layers is attempted, whose results can be used as guidelines to achieve optimal heat transfer in practical situations of channels with continuous as well as discrete heating. Similarity analysis is used for the optimization of heat distribution in boundary layer flows. In the similarity analysis, in the search of optimal heat transfer from the plate, the boundary layer equations are solved for various power-law heat flux variations and the appropriate power-law variation of optimal heat transfer is found. Similarly, the heat flux variation for optimal heat transfer is found for the cases of natural and forced convection, as they are the limiting cases of mixed convection. In the numerical part of the study, the generalised three-dimensional governing equations for the five heating configurations considered for the study are solved numerically with appropriate boundary conditions. Separation of natural, forced and mixed convection regimes is carried out in all the heating configurations using a criterion based on individual contributions of pressure force and buoyancy force towards the total driving force for the fluid movement. Heat transfer characteristics are studied in laminar as well as turbulent regimes in terms of parameters such as Grashof number, Reynolds number, Nusselt number, maximum temperature of heaters, pressure drop across the channel, and so on. The influence of conjugate effects on the heat transfer characteristics is studied by varying the substrate thermal conductivity. A systematic comparison of various effects such as the effect of discrete heating in plain channels, effect of discrete heating in channels with heated ribs, and the effect of three-dimensional protrusions on heat transfer, is achieved. The parameters in the individual configurations, which affect heat transfer, are explored for better cooling solutions. Optimal heat distribution among the heaters to minimise the temperature of the hottest heater for a given total amount of heat generation in the channel is found for all the channel configurations, which are heated either continuously or discretely. In the process of finding the optimal heat distribution among heaters, guidelines are taken from the optimal heat distribution in boundary layer flows. Compared to usual optimization approaches such as genetic algorithm, the present physics based optimisation procedure requires fewer runs to arrive at the optimal distribution. The fluid flow characteristics in all the three configurations with flush-mounted heaters are found to be similar. However, heat transfer characteristics in channels with flush-mounted square heaters differ from those in the other two flush-mounted channel configurations. Hot spots with higher temperatures are found at heater locations in channels with flush-mounted square heaters. The effect of substrate follows the same trend in all the flush-mounted configurations. At lower thermal conductivities, the maximum temperature decreases sharply with increasing thermal conductivity. However, at higher conductivities, the influence reduces. In all the flush-mounted configurations, heat transfer will not be influenced by substrate thermal conductivity increment at conductivities more than 150 times the fluid thermal conductivity. The fluid flow and heat transfer characteristics in channels with protruded heaters differ significantly from those in channels with flush-mounted heaters. The protrusions in the channels interact with the fluid flow and make it different from that of smooth channels. In turn, the protrusions affect heat transfer characteristics in the channels. The influence of the protrusions on the heat transfer and locations of hot spots in the domain is examined. Effect of thermal conductivity in channels with protruded square heaters is similar to that in channels with flush-mounted heaters. However, conductivity in channels with protruded rib heaters affects the heat transfer in a wider range of conductivities than in the other heating configurations. Unlike in the other configurations, at low thermal conductivities, maximum temperature does not drop sharply with increase of conductivity. In channels with protruded square heaters, staggering arrangement of heaters results in higher heat transfer rates than those with in-line heater arrangement. In all the configurations, pressure drop is found to be independent of Grashof number in the range of heat dissipation rates considered in the study. Heat transfer rates in turbulent region are much higher than the heat transfer rates in laminar regime. However, the pressure drops encountered are also high in the turbulent regime. Turbulent heat transfer results in a more uniform temperature distribution in channels. The cooling performances of the individual configurations are compared. For a given pressure drop the cooling performances decreases in the order of flush-mounted strip heating, protruded square heating, flush-mounted square heating, protruded rib heating. For a given inlet fluid flow rate, the cooling performances decreases in the order of protruded rib heating, protruded square heating, flush-mounted square heating, flush-mounted strip heating. However, for a given inlet fluid flow rate, the pressure drop increases in the order of increasing cooling performance.

Cooling of electronic components is one of the most important issues concerned in the electronic industry for design of equipment. Maintaining the temperature of an electronic device within its safe operating temperature limits is essential to operate the equipment safely with proper functionality. According to the Arrhenious law of failure rate, for a device with activation energy 0.65eV, every 10°C increase in temperature doubles the failure rate. Recent miniaturisation of components and high device heat dissipation rates lead to high heat fluxes, which cause temperature rise. Hence, there is an increasing need for research to achieve high heat removal rates and optimal design. Several cooling techniques are used for cooling of electronics based on the application and cooling rate requirements. Air-cooling of electronics has a wide range of applications due to its greater reliability, simplicity, easy maintenance, low cost, easy availability of coolant (air), and light weight. Air-cooling is also free from boiling and dripping problems. Air-cooling is used in applications such as avionics, cooling of personal computers, cooling of data centers, and in automobile electronics. In a typical electronic cooling application, cooling fluid is driven by the combination of external pressure forces and buoyancy forces. Based on the relative contribution of these forces towards the total driving force, the cooling techniques can be categorized as forced, natural or mixed convection cooling. However, in many of the electronic cooling situations, such as in the applications with very high heat fluxes, tall Printed Circuits Boards (PCBs) with low forced convection velocity, and in large scale applications such as data centers, the contributions of the buoyancy forces and external pressure forces for the total driving force are comparable, which results in a mixed convection situation. In the present study, mixed convection in vertical channels heated with five heating configurations, which represent typical electronic cooling applications, is studied numerically. The five different heating configurations are channels with flush-mounted continuous heater, flush-mounted strip heaters, flush-mounted square block heaters, protruding rib heaters and protruding square heaters. The first three configurations are categorised as flush-mounted heating configurations and the latter two configurations are categorised as protruded heating configurations. One of the channel walls represents the substrate on which the heaters are mounted and the heat sources represent the heat generating electronic components. Heat transfer under steady state conditions is considered in the study. The study includes laminar as well as turbulent heat transfer. For a systematic study of mixed convection, an analytical or semi-analytical formulation is desirable for a simplified model, as it can highlight the effect of relevant non-dimensional parameters on the heat transfer characteristics of a system. The results of a simplified model can be used for benchmarking the results of practical situations. Hence, before numerically solving the governing equations for mixed convection in channels, mixed convection boundary layer flows over a heated vertical plate is considered for study. Perturbation technique is used to solve the boundary layer equations with non-isothermal boundary conditions. The perturbation analysis is carried out for an arbitrarily variation of wall temperature or heat flux. Subsequently, the results are extended to find heat transfer rates in the cases of power-law variation of temperature and heat flux, as special cases. It is always required to design a cooling system to remove maximum possible amount of heat, keeping the device temperature within its safe operating limits. Hence, optimization of heat transfer in boundary layers is attempted, whose results can be used as guidelines to achieve optimal heat transfer in practical situations of channels with continuous as well as discrete heating. Similarity analysis is used for the optimization of heat distribution in boundary layer flows. In the similarity analysis, in the search of optimal heat transfer from the plate, the boundary layer equations are solved for various power-law heat flux variations and the appropriate power-law variation of optimal heat transfer is found. Similarly, the heat flux variation for optimal heat transfer is found for the cases of natural and forced convection, as they are the limiting cases of mixed convection. In the numerical part of the study, the generalised three-dimensional governing equations for the five heating configurations considered for the study are solved numerically with appropriate boundary conditions. Separation of natural, forced and mixed convection regimes is carried out in all the heating configurations using a criterion based on individual contributions of pressure force and buoyancy force towards the total driving force for the fluid movement. Heat transfer characteristics are studied in laminar as well as turbulent regimes in terms of parameters such as Grashof number, Reynolds number, Nusselt number, maximum temperature of heaters, pressure drop across the channel, and so on. The influence of conjugate effects on the heat transfer characteristics is studied by varying the substrate thermal conductivity. A systematic comparison of various effects such as the effect of discrete heating in plain channels, effect of discrete heating in channels with heated ribs, and the effect of three-dimensional protrusions on heat transfer, is achieved. The parameters in the individual configurations, which affect heat transfer, are explored for better cooling solutions. Optimal heat distribution among the heaters to minimise the temperature of the hottest heater for a given total amount of heat generation in the channel is found for all the channel configurations, which are heated either continuously or discretely. In the process of finding the optimal heat distribution among heaters, guidelines are taken from the optimal heat distribution in boundary layer flows. Compared to usual optimization approaches such as genetic algorithm, the present physics based optimisation procedure requires fewer runs to arrive at the optimal distribution. The fluid flow characteristics in all the three configurations with flush-mounted heaters are found to be similar. However, heat transfer characteristics in channels with flush-mounted square heaters differ from those in the other two flush-mounted channel configurations. Hot spots with higher temperatures are found at heater locations in channels with flush-mounted square heaters. The effect of substrate follows the same trend in all the flush-mounted configurations. At lower thermal conductivities, the maximum temperature decreases sharply with increasing thermal conductivity. However, at higher conductivities, the influence reduces. In all the flush-mounted configurations, heat transfer will not be influenced by substrate thermal conductivity increment at conductivities more than 150 times the fluid thermal conductivity. The fluid flow and heat transfer characteristics in channels with protruded heaters differ significantly from those in channels with flush-mounted heaters. The protrusions in the channels interact with the fluid flow and make it different from that of smooth channels. In turn, the protrusions affect heat transfer characteristics in the channels. The influence of the protrusions on the heat transfer and locations of hot spots in the domain is examined. Effect of thermal conductivity in channels with protruded square heaters is similar to that in channels with flush-mounted heaters. However, conductivity in channels with protruded rib heaters affects the heat transfer in a wider range of conductivities than in the other heating configurations. Unlike in the other configurations, at low thermal conductivities, maximum temperature does not drop sharply with increase of conductivity. In channels with protruded square heaters, staggering arrangement of heaters results in higher heat transfer rates than those with in-line heater arrangement. In all the configurations, pressure drop is found to be independent of Grashof number in the range of heat dissipation rates considered in the study. Heat transfer rates in turbulent region are much higher than the heat transfer rates in laminar regime. However, the pressure drops encountered are also high in the turbulent regime. Turbulent heat transfer results in a more uniform temperature distribution in channels. The cooling performances of the individual configurations are compared. For a given pressure drop the cooling performances decreases in the order of flush-mounted strip heating, protruded square heating, flush-mounted square heating, protruded rib heating. For a given inlet fluid flow rate, the cooling performances decreases in the order of protruded rib heating, protruded square heating, flush-mounted square heating, flush-mounted strip heating. However, for a given inlet fluid flow rate, the pressure drop increases in the order of increasing cooling performance.

碩士<br>國立虎尾科技大學<br>機械與機電工程研究所<br>98<br>This study is aimed to treat the problem of convective heat transfer introduced by combined electrokinetic and pressure forces in microchannels. Analytical solution are presented for a slit microchannel with constant surface temperature, taking into account the effect of joule heating and viscous dissipation for different Peclet number. First , the Poisson-Boltzmann equation is solved to obtain the electrokinetic potential. Next, the fluid velocity distribution across the channel is determined form the momentum equation. Finally, we solve the energy equation by using the integral transformation method to obtain the temperature distribution for the thermally developing flow within the microchannel. Governing parameters include the velocity scale ratio Γ ( the ratio of the pressure-driven velocity scale for Poiseuille flow to Helmhlotz - Smoluchowski velocity for electroosmotic flow i.e. ) , Joule heating parameter , Peclet number and Brinkman number . Representative results for the results for the mean fluid temperature and the local Nusselt number are presented at selected governing parameters. The mean fluid temperature attains the fully - developed value at a smaller distance as Peclet number increases . The similar behavior is observed for the local Nusselt number. For pure electroosmotic flow (Γ=0) , the effect viscous dissipation on the thermal transport is not important. However, for mixed electro-osmotic and pressure-driven flow , viscous dissipation plays a role in the heat transfer. When viscous dissipation is neglected (Br=0) , the local Nusselt number decreases monotonously with streamwise location to the fully developed value . On the other hand, for Br≠0 the local Nusselt number occurs a rise in the fully developed value . As compared to the effect of joule heating , the effect viscous dissipation on the thermal transport is of less importance.

碩士<br>國立中央大學<br>化學工程學系<br>84<br>This paper studied laminar mixed convection from combined heat and mass transfer. The investigated system are vertical and horizontal plates which are maintained with uniform temperature and concentration. The case of plates maintained with uniform fluxes of heat and spesies are also included. By introducing some proper dimensionless variables and parameters from scale analysis, the governing equations can be transformed and solved efficiently by a finite-difference method. Very accurate numerical results have been obtained for dilute gaseous solutions (Pr=0.7,0.3<Le<3) and aqueous solutions (Pr=7,20<Le<200) over the whole regions of mixed convection parameter and buoyancy ratio. The effects of mixed convection parameter, buoyancy ratio, Lewis number on the variation of primary physical quantities are presented. As can be seen from the figures, an increase in mixed convection intensity accerates fluid flow and thus enhances the rates of heat and mass transfer. It is also shown that, the effects of Lewis number is significant for gaseous solutions but not for aqueous solutions. For the convenience of application, comprehensive correlations of heat transfer and mass transfer have been proposed for mixed convection and also for natural convection. These correlations are very simple and precise.

碩士<br>國立暨南國際大學<br>土木工程學系<br>94<br>Abstract The study of combined free and forced magnetohydrodynamic convection flow between two parallel vertical plates is performed by taking into account the effects of internal heat source, viscous dissipation and ohmic heating. The channel boundaries are kept isothermal and maintained at either equal or at different temperature. The velocity and temperature fields are solved analytically by perturbation series method and numerically by the foruth-order Runge-Kutta shooting technique together with the Newton-Raphson scheme to satisfy the boundary conditions. The results are given for various Brinkman number, GR (the ratio of Grashof number to Reynolds number), small parameter (BrGr) as well as internal source parameter for the symmetric and asymmetric heating cases. It is found that the effect of internal heat source parameter can affect the reversal flow significantly.

碩士<br>國立交通大學<br>機械工程學系<br>99<br>An investigation of heat transfer in a three-dimensional tapered chimney with consideration of the flow compressibility is studied numerically.The finite difference method is adopted and the computational approaches are divided into two parts. One is the Roe scheme applied for the flux of inviscid terms and the preconditioning matrix is added for the efficiency in all speed fields. The other one is the central difference method of second order utilized to solve viscous terms. The temporal term is solved by LUSGS. Non-reflection conditions at the outlet is derived in order to resolve reflections induced by acoustic waves. In many important natural convection problems, the temperature differences are often higher than 30K. Boussinesq assumption is unreasonable. Besides, the OpenMP method is also used to promote the computing efficiency. By numerical results, there is the greatest flow speed near the outlet in the three-dimensional vertical natural convection pushed upward by buoyancy effect. The enhancement of heat transfer of Reynolds number 400 is worse than the enhancement of Reynolds number 100, 200. It is mainly due to the more flow rate sucked from exterior near the outlet in the case of Reynolds number 100, 200. And the flow sucked from the exterior impacted the flow exiting the outlet in the case of Reynolds number 100. The heat transfer is worse than the case of Reynolds number 200. Besides, the flow field is unstable due to the backflow near the outlet in the case of Reynolds number 100, 200.

碩士<br>建國科技大學<br>機電光系統研究所<br>92<br>The objective of the present work is to study the convective heat transfer in sintered porous channel. The sintered test section is made of three diameters of copper beads of 0.704, 0.830 and 1.163 mm. The characteristics of the wall temperature distribution with constant heat flux were measured. This study also reveals that heat transfer coefficients and Nusselt numbers were presented for various Reynolds numbers and heat flux. Use compress air to proceed cooling work fluid, Also facing sintered porous channel weigh rib. Using the Brinkman-extended Darcy model for fluid flow and the two-equation model for heat transfer, the analytical solutions for both velocity and temperature distributions are obtained and compared with the exact heat transfer in order to validate the porous medium approach. The sintered test section weigh ribbed and the effective conductivity ratio is parameters of engineering importance are identified, Main purpose weigh ribbed exist understand increase heat coefficient, Reynolds numbers, heat flux and not axial seat facing heat transfer effect in influence, by under consideration great effect. Finally, the results of the heat transfer coefficients and Nusselt numbers in sintered porous channels are compared with those of the previous study with a high degree of accuracy.

碩士<br>國立高雄應用科技大學<br>機械與精密工程研究所<br>96<br>Mixed convective heat transfer in a channel with a periodic oscillating heated block is numerically investigated in this study. The fluid flows into the channel from the left opening and exits from the opposite end. Both the temperature and velocity of the inflow fluid are kept constant. Periodic oscillating heated block in the center of the channel. The governing equations for micropolur fluid were first presented by A.C. Eringen, wherein we furthermore expend the applications to non-Newtonian fluids. The numerical computations were obtained using the cubic spline collocation method in a personal computer. The governing equations, including stream function, vorticity, microrotatin and energy, were first put in dimensionless form. The governing parameters appearing in present study are Re、R、Gr、F、Pr、λ. The numerical results of the flow fields are discuss with plot of isotherms, streamlines and concentration. The results indicate that the effect of mixed convective heat transfer depends on the moving block to a large extent.

碩士<br>逢甲大學<br>機械工程學所<br>94<br>This research is to investigate mixed convection heat transfer phenomena of nanofluids in vertical channels by using numerical technique. The CFDRC software in conjunction with the self-developed adjustment subroutines for boundary conditions and for specified thermal properties is applied in this research. The mixed convection heat transfer phenomena of six different nanofluids, Water-Cu, Water-Al2O3, Water-SiO2, Water-TiO2, Engine oil-Cu, and Ethylene glycol-Cu, with various volume fractions, entrance velocities, and wall calefaction conditions are studied in this research. Moreover, the effects of the dispersion term on the heat transfer phenomena are also studied independently. It is observed that the heat transfer is enhanced the most by adding nanoparticle Cu. As Cu is the adding nanoparticle, the heat transfer of the Engine oil is enhanced the most for different basefluids. Increasing the nanoparticle volume fraction will enhance the heat transfer. Besides, it would also lengthen the hydrodynamic entry length, thin the hydrodynamic boundary layer thickness, shorten the thermal entry length, and thicken the thermal boundary layer thickness except for adding nanoparticle SiO2. The thermal entry length would be shorter and the thermal boundary would be thicker by simulating with the dispersion term than without it. The heat transfer is also more enhanced. Similar to common fluid, nanofluid’s heat transfer of mixed convection is better than that of forced convection. The effect of increasing particle volume fraction on the entrance length and Nusselt number would not be affected by temperature difference or convection type. Based on the research data and by curve fitting, the equations between Nu and Re and Pr based on various volume fractions are also obtained.

碩士<br>國立成功大學<br>航空太空工程研究所<br>80<br>MCVD製程是目前製造光纖的方法中最廣為採用的方法，其製造成本與反應氣體反應後 所產生之粒子的堆積效率有關，如何提高粒子堆積效率是本文探討的課題所在。 本文以雷射光束加熱－旋轉圓管，改變雷射強度及電射光束寬度，以及電射的中心位 置，並考慮流場高溫時所不能忽略的熱輻射效應。在沒有電射加熱時，探討圓管轉速 、入口速度、入口溫度對流場、溫度場和粒子濃度場之影響，尋求一較佳的粒子堆積 效的狀況。 對圓管旋轉時，在θ方向引入強制對流，使得浮力效應對二次流的影響較小，當轉速 為60rpm 和120rpm時溫度場變化不大。當轉速提高為300rpm時，溫度場趨向對稱，使 得管壁附近的溫度梯度減小，故粒子堆積效率較低。 以雷射光束加熱流場時，雷射強度越大，流場溫度越高，有助於提高粒子堆積效率， 並且存在一最佳的雷射光束分佈，而獲得最高的粒子堆積效率。當雷射光束分佈不是 十分集中時，雷射中心在管中央時有較高的粒子堆積效率；當雷射光束分佈十分集中 時，雷射中心位於圓管底端時粒子堆積效率較高。考慮熱輻射時，流場的溫度分佈較 為平緩，粒子堆積效率也會降低。

碩士<br>國立臺灣海洋大學<br>輪機工程系<br>95<br>Abstract This study numerically investigates the laminar flow for mixed convection heat transfer problem between parallel plates. Constant plate wall temperature and uniform flow entry are assumed. Parameters of Reynolds number, inclined angle, and are varied to obtain the velocity and temperature distributions in the channel. Introducing the dimensionless quantities, the governing equations as well as boundary conditions are first non-dimensionalized. Then, all the equations are written in finite difference forms and simultaneously solved. Employing the trapezoidal integral method, Nusselt numbers in the channel are obtained with the calculated velocities and temperatures. The results show that backflow in the center of the flow is observed in the channel at a certain distance from the inlet for =10. This phenomenon also takes place for =1 but it occurs with the position shifting downstream. No backflow is found for =0.1. The effects of inclination on is insignificant for =0.1. As the inclined angle increases, Nusselt number decreases for =10. In addition, the in the channel increases with . Finally, it is expected that the results of this study may facilitate the prospective design of relevant heat exchangers. Keywords: Mixed convection, Channel flow, Finite difference method

碩士<br>中原大學<br>機械工程研究所<br>84<br>A transient mixed convection of a second grade viscoelastic fluid past an inclined backward facing step is studied to reveal combined effects of the Reynolds number, the elastic coefficient and the inclination angle of the channel on the flow and heat transfer. A numerical simulation method is used in the present study. A finite difference method is applied to the stream-vorticity function and energy equations. Related numerical schemes are point and line Gauss-Seidel methods, successive over-relaxation method and alternating-direction method. Results indicate that a primary recirculation zone is formed on the bottom plate, and is located at downstream of the step. Generally speaking, the reattachment length increases with an increase in the inclinati- on angle to a maximum length, and then decreases when the inclination increases further. However, a few cases show a diff- erent trend due to a complicated interaction of forces. The maximum value of the reattachment length is occurred when the inclination angle is 150°or 180°. The reattachment length increases with an increase in Reynolds number and a lowering in elastic coefficient. Under this condiction, the reattachment length is close to or over-shooting the position of local maximum Nux value. In other inclination angle, the position of local maximum Nux value is in the downstream of the reattachment point. With increasing inclination angle, the contact point of isotherm and upper plate moves upward originally, and then it moves downward. The inclination angle is arround 150°when the contact point is close to upstream flow. The moving phenomian is more obvious in the lower elastic coefficient range.

碩士<br>國立成功大學<br>航空太空工程學系<br>87<br>Placing the power cable underground is an engineering tendency in the present days, especially in city areas and industrial zones. The constant heat flux is generated from the electrical resistance of the power cable which lies on the bottom of the conduit, while the concrete wall is adiabatic. This configuration does not permit of steady-state solution of a two-dimensional case. Many theoretical and experimental studies on natural convection in horizontal, eccentric annuli have been carried out. In most of these studies, a two-dimensional model was used in which the annuli were assumed to be coupled with thermal boundary conditions on the cylinder surfaces specified as either with two constant wall temperatures or one with constant wall temperature while the other with constant wall heat flux (including adiabatic surface). A comprehensive literature survey revealed that published work is largely nonexistent on the three-dimensional eccentric annuli between two horizontal cylinders, where their geometric configurations possess an open end. The existed three-dimensional studies on natural convection were limited to the cavity flow problem. The boundary conditions for this problem are as follows. The adiabatic condition is given on the outer cylinder (concrete conduit) surface, while a constant heat flux which is specified with the heat dissipated from the power cable is given on the inner cylinder (power cable). Due to the symmetric natural convection of the flow field with respect to the two free ends and to a vertical plane crossing the center of the cylinders, zero-gradient conditions are given there. Thus, the free end consists of inflow (fresh air) and outflow (heated air) at the same plane. This flow configuration leads to that there is no thermal fully developed field at the open end. Instead, the computational domain has to be extended into the outside environment so that the proper boundary condition can be specified. The "zonal grid" method is used to tread numerically this two-zone approach. In addition to natural convection, mixed convection is studied in this work. In mixed convection, the fluid enters the annuli at one free open end and the other one becomes the outlet plane. For a long enough axial distance in the flow field, the "fully developed" boundary conditions can be reasonably specified at the outlet plane. It is economic to use the "fully developed" boundary conditions at outlet plane as compared to the two-zone approach. The present work investigates that under what conditions (in terms of Gr/Re2 which represents the ratio of buoyant force to inertia force), the fully-developed outlet boundary conditions cab be properly used in the modeling. Effects of the eccentricity on the surface temperature distribution of the inner cylinder are also investigated in this work.

碩士<br>國立高雄應用科技大學<br>機械與精密工程研究所<br>98<br>Mixed convection heat transfer in a square confined enclosure with partition plates is simulated in this study. The cavity is constructed with two adiabatic horizontal walls and two isothermal walls at different temperatures. The CFD scheme is applied to analyze the flow fields in the cavity. This study analyzes the effect configuration of the partition plates in the cavity on the thermal and flow fields. The results of this study reveal that raising of the Reynolds number will increase the strength of the circulation inside the cavity. In addition, changing the orientation and the distance between the plates also affect the streamlines and the thermal phenomenon significantly. In general, increasing the distance between the plates results in an increase on the values of the Nusselt number. The orientation of the plates has little effect on the values of the Nusselt number.

The problem of laminar and turbulent conjugate mixed convection flow and heat transfer in shallow enclosures with a series of block-like heat generating components is studied numerically for a Reynolds number range of zero (pure natural convection) to typically 106, Grashof number range of zero (pure forced convection) to 1015 and various block-to-fluid thermal conductivity ratios, with air as the working medium. The shallow enclosure has modules consisting of heat generating elements, air admission and exhaust slots. Two problems are considered. In the first problem, the enclosure has free boundaries between the modules and in the second problem, there are partitioning walls between the different modules. The flow and temperature distributions are taken to be two-dimensional. Regions with the same velocity and temperature distributions can be identified assuming repeated placement of the blocks and fluid entry and exit openings at regular distances, neglecting end wall effects. One half of such rectangular region is chosen as the computational domain taking into account the symmetry about the vertical centreline. On the basis of the assumption that mixed convection flow is a superposition of forced convection flow with finite pressure drop and a natural convection flow with negligible pressure drop, the individual flow components are delineated. The Reynolds number is based on forced convection velocity, which can be determined in practice from the fan characteristics. This is believed to be more meaningful unlike the frequently used total velocity based Reynolds number, which does not vanish even in pure natural convection and which makes the fan selection difficult. Present analysis uses three models of turbulence, namely, standard k-ε (referred to as Model-1), low Reynolds number k-ε (referred to as Model-2) and an SGS kinetic energy based one equation model (referred to as Model-3). Results are obtained for aiding and opposing mixed convection, considering also the pure natural and pure forced convection limiting cases. The results show that higher Reynolds numbers tend to create a recirculation region of increasing strength at the core region and that the ranges of Reynolds number beyond which the effect of buoyancy becomes insignificant are identified. For instance, in laminar aiding mixed convection, the buoyancy effects become insignificant beyond a Reynolds number of 500. Results are presented for a number of quantities of interest such as the flow and temperature distributions, local and average Nusselt numbers and the maximum dimensionless temperature in the block. Correlations are constructed from the computed results for the maximum dimensionless temperature, pressure drop across the enclosure and the Nusselt numbers.

The problem of laminar and turbulent conjugate mixed convection flow and heat transfer in shallow enclosures with a series of block-like heat generating components is studied numerically for a Reynolds number range of zero (pure natural convection) to typically 106, Grashof number range of zero (pure forced convection) to 1015 and various block-to-fluid thermal conductivity ratios, with air as the working medium. The shallow enclosure has modules consisting of heat generating elements, air admission and exhaust slots. Two problems are considered. In the first problem, the enclosure has free boundaries between the modules and in the second problem, there are partitioning walls between the different modules. The flow and temperature distributions are taken to be two-dimensional. Regions with the same velocity and temperature distributions can be identified assuming repeated placement of the blocks and fluid entry and exit openings at regular distances, neglecting end wall effects. One half of such rectangular region is chosen as the computational domain taking into account the symmetry about the vertical centreline. On the basis of the assumption that mixed convection flow is a superposition of forced convection flow with finite pressure drop and a natural convection flow with negligible pressure drop, the individual flow components are delineated. The Reynolds number is based on forced convection velocity, which can be determined in practice from the fan characteristics. This is believed to be more meaningful unlike the frequently used total velocity based Reynolds number, which does not vanish even in pure natural convection and which makes the fan selection difficult. Present analysis uses three models of turbulence, namely, standard k-ε (referred to as Model-1), low Reynolds number k-ε (referred to as Model-2) and an SGS kinetic energy based one equation model (referred to as Model-3). Results are obtained for aiding and opposing mixed convection, considering also the pure natural and pure forced convection limiting cases. The results show that higher Reynolds numbers tend to create a recirculation region of increasing strength at the core region and that the ranges of Reynolds number beyond which the effect of buoyancy becomes insignificant are identified. For instance, in laminar aiding mixed convection, the buoyancy effects become insignificant beyond a Reynolds number of 500. Results are presented for a number of quantities of interest such as the flow and temperature distributions, local and average Nusselt numbers and the maximum dimensionless temperature in the block. Correlations are constructed from the computed results for the maximum dimensionless temperature, pressure drop across the enclosure and the Nusselt numbers.