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1
Jing, Tingting, Guoqiang He, Fei Qin, Wenqiang Li, Duo Zhang e Minghao Wang. "Flow Distribution Characteristics of Supercritical Hydrocarbon Fuel in Parallel Channels with Pyrolysis". Xibei Gongye Daxue Xuebao/Journal of Northwestern Polytechnical University 37, n. 1 (febbraio 2019): 1–6. http://dx.doi.org/10.1051/jnwpu/20193710001.
Abstract (sommario):
Due to the asymmetric geometry of flowpath and combustion organization, the heat flux distribution on RBCC's walls are extremely non-uniform. With a validated numerical model considering the changes of thermophysical properties and chemical components, the present study analyzed the effects of heat flux intensity, non-uniformity of heat flux distribution and inlet manifold on the mass flow distribution of parallel regenerative cooling channels. Results show that the intensity of heat flux enlarges the non-uniformity of parallel channels, however, the non-uniformity is reduced when the outlet temperature is above the completely pyrolysis value; and the non-uniformity heat flux distribution increases the mal-distribution of parallel channels dramatically, the difference of mass flow rate reaches to 33.2% when the heat flux difference is only 0.25 MW/m2; increasing the flow area of inlet manifold would improve the flow distribution of parallel channels with decrease of heat transfer efficiency and increases of pressure drop.
2
Zhang, Donghui, Haiyang Xu, Yi Chen, Leiqing Wang, Jian Qu, Mingfa Wu e Zhiping Zhou. "Boiling Heat Transfer Performance of Parallel Porous Microchannels". Energies 13, n. 11 (10 giugno 2020): 2970. http://dx.doi.org/10.3390/en13112970.
Abstract (sommario):
Flow boiling in microporous layers has attracted a great deal of attention in the enhanced heat transfer field due to its high heat dissipation potential. In this study, flow boiling experiments were performed on both porous microchannels and a copper-based microchannel, using water as the coolant. As the heat flux was less than 80 W/cm2, the porous microchannels presented significantly higher boiling heat transfer coefficients than the copper-based microchannel. This was closely associated with the promotion of the nucleation site density of the porous coating. With the further increase in heat flux, the heat transfer coefficients of the porous microchannels were close to those of the copper-based sample. The boiling process in the porous microchannel was found to be dominated by the nucleate boiling mechanism from low to moderate heat flux (<80 W/cm2).This switched to the convection boiling mode at high heat flux. The porous samples were able to mitigate flow instability greatly. A visual observation revealed that porous microchannels could suppress the flow fluctuation due to the establishment of a stable nucleate boiling process. Porous microchannels showed no advantage over the copper-based sample in the critical heat flux. The optimal thickness-to-particle-size ratio (δ/d) for the porous microchannel was confirmed to be between 2–5. In this range, the maximum enhanced effect on boiling heat transfer could be achieved.
3
Kuznetsov, Vladimir, Alisher Shamirzaev e Alexander Mordovskoy. "High heat flux flow boiling of refrigerant R236fa in parallel microchannels". EPJ Web of Conferences 196 (2019): 00062. http://dx.doi.org/10.1051/epjconf/201919600062.
Abstract (sommario):
This paper presents the results of an experimental study of the heat transfer during flow boiling of refrigerant R236fa in a horizontal microchannel heat sink. The experiments were performed using closed loop that re-circulates coolant. Microchannel heat exchanger that contains two microchannels with 2x0.4 mm cross-section was used as the test section. The dependence of average heat flux on wall superheat and critical heat flux were measured in the range of mass fluxes from 600 to 1600 kg/m2s and in the range of heat fluxes from 5 to 120 W/cm2. For heat flux greater than 60 W/cm2, nucleate boiling suppression has significant effect on the flow boiling heat transfer, and this leads to decrease of the heat transfer coefficient with heat flux grows.
4
Shakier, Raed, Hussam Muhammed, Hussain Khathem e Haider Abdul-Khathem. "Two-Phase Flow In Mini-Scale Complex Geometry". Al-Kitab Journal for Pure Sciences 1, n. 1 (17 giugno 2018): 20–26. http://dx.doi.org/10.32441/kjps.v1i1.88.
Abstract (sommario):
Heat-transfer coefficients are reported for one surface, a plain surface, with 50 mm square base area. Parallel channel test piece has one mm by one mm, 25 channelsThe data were produced while boiling R113 at atmospheric pressure. For this surface, the mass flux range was 200 – 600 kg/m2s and the heat flux range was 5 - 80 kW/m2. The results obtained have been compared with standard correlations for tube bundles. The measured heat-transfer coefficients for the parallel micro-channel surface are slightly bigger for any plate channel surface. It is dependent on heat flux and reasonably independent of mass flux and vapor quality. Thus, heat transfer is probably dominated by nucleate boiling. The parallel channel heat transfer coefficients were typically bigger than other plate -channel values.
5
Salman, Yasin K., e Hazim S. Hamad. "LAMINAR NATURAL CONVECTION HEAT TRANSFER BETWEEN DUCTED PARALLEL PLATES". Journal of Engineering 14, n. 03 (1 settembre 2008): 2786–803. http://dx.doi.org/10.31026/j.eng.2008.03.18.
Abstract (sommario):
Laminar natural convection heat transfer to air flow in a ducted two parallel plates subjected to same constant heat flux has been studied experimentally. In this study a test rig has been designed and constructed to allow studying the effect of plate spacing and plates angle of inclination on the heat transfer process. The study covers three plate spacing 15 mm, 35mm and 60 mm that makes plate aspect ratio (AR) 24, 10.285 and 6, The heat flux implemented in all test runs varies between 55 W/m2– 340 W/m2 that makes the modified Rayleigh number (Ra) varies from 105-108. Experimental results presented as plate temperature distribution, variation of local heat transfer coefficient and the relation between Nu and Ra, reveal for the range of parameters mentioned above, an improvement in heat transfer process as the AR values change from 6 to 24 and the improvement rates is higher for the lower heat flux test (lower Ra). A correlation of the results were made in the form of Nuvr (Gr/AR) in which the effect of duct aspect ratio (AR) has been introduced.
6
Barghouthi, I. A., H. Nilsson e S. H. Ghithan. "O<sup>+</sup> and H<sup>+</sup> ion heat fluxes at high altitudes and high latitudes". Annales Geophysicae 32, n. 8 (26 agosto 2014): 1043–57. http://dx.doi.org/10.5194/angeo-32-1043-2014.
Abstract (sommario):
Abstract. Higher order moments, e.g., perpendicular and parallel heat fluxes, are related to non-Maxwellian plasma distributions. Such distributions are common when the plasma environment is not collision dominated. In the polar wind and auroral regions, the ion outflow is collisionless at altitudes above about 1.2 RE geocentric. In these regions wave–particle interaction is the primary acceleration mechanism of outflowing ionospheric origin ions. We present the altitude profiles of actual and "thermalized" heat fluxes for major ion species in the collisionless region by using the Barghouthi model. By comparing the actual and "thermalized" heat fluxes, we can see whether the heat flux corresponds to a small perturbation of an approximately bi-Maxwellian distribution (actual heat flux is small compared to "thermalized" heat flux), or whether it represents a significant deviation (actual heat flux equal or larger than "thermalized" heat flux). The model takes into account ion heating due to wave–particle interactions as well as the effects of gravity, ambipolar electric field, and divergence of geomagnetic field lines. In the discussion of the ion heat fluxes, we find that (1) the role of the ions located in the energetic tail of the ion velocity distribution function is very significant and has to be taken into consideration when modeling the ion heat flux at high altitudes and high latitudes; (2) at times the parallel and perpendicular heat fluxes have different signs at the same altitude. This indicates that the parallel and perpendicular parts of the ion energy are being transported in opposite directions. This behavior is the result of many competing processes; (3) we identify altitude regions where the actual heat flux is small as compared to the "thermalized" heat flux. In such regions we expect transport equation solutions based on perturbations of bi-Maxwellian distributions to be applicable. This is true for large altitude intervals for protons, but only the lowest altitudes for oxygen.
7
Guo, Zeng-Yuan, e Xiao-Bo Wu. "Thermal Drag and Critical Heat Flux for Natural Convection of Air in Vertical Parallel Plates". Journal of Heat Transfer 115, n. 1 (1 febbraio 1993): 124–29. http://dx.doi.org/10.1115/1.2910637.
Abstract (sommario):
Variable property effects on vertical channel natural convection in air are studied systematically. Numerical solutions of the governing equations show that both the mass flow rate and heat transfer in the channel are not only lower than the constant property results, but also show a nonmonotonic variation with increasing wall temperature or wall heat flux. This phenomenon, which seemingly conflicts with the conventional knowledge, has also been identified by experiments. For a vertical channel with a uniform heat flux boundary condition, the wall may experience a sharp rise in temperature up to damage of the channel if the wall heat flux is greater than the critical heat flux. This implies that the crisis phenomenon (or burnout) may occur in channel natural convection in gas as well as in the boiling process.
8
Zhang, Xiao Jing, Bing Qi Liu, Xiao Jie Xu, Xi Wu e Rui Ming Yuan. "A Study of the Enhancement in Near-Field Radiative Heat Transfer by Surface Polaritons". Applied Mechanics and Materials 448-453 (ottobre 2013): 3211–16. http://dx.doi.org/10.4028/www.scientific.net/amm.448-453.3211.
Abstract (sommario):
The influence of surface polaritons on spectral energy flux at different temperatures or distances to the surface of a plate was analyzed. The relations of the net heat flux between two parallel plates with the material type were also discussed. The results demonstrate that the effect of surface polaritons is dominated in the spectral energy flux at 300 K when the distance is decreased to 100 nm. In addition, the intensity of surface polaritons increases with the temperature. The net heat flux between two parallel plates has a nearly linear relation with the temperature and is closely related to the material type. It reaches up to 2.792×107 W/m2 between two SiC plates, approximately 3~6 orders of magnitude larger than that between two different materials. However, the net heat flux between SiC and Al is merely 2329.7 W/m2, even smaller than the result calculated by the classical stefan-boltzman law between two blackbodies.
9
Bergles, A. E., e S. G. Kandlikar. "On the Nature of Critical Heat Flux in Microchannels". Journal of Heat Transfer 127, n. 1 (1 gennaio 2005): 101–7. http://dx.doi.org/10.1115/1.1839587.
Abstract (sommario):
The critical heat flux (CHF) limit is an important consideration in the design of most flow boiling systems. Before the use of microchannels under saturated flow boiling conditions becomes widely accepted in cooling of high-heat-flux devices, such as electronics and laser diodes, it is essential to have a clear understanding of the CHF mechanism. This must be coupled with an extensive database covering a wide range of fluids, channel configurations, and operating conditions. The experiments required to obtain this information pose unique challenges. Among other issues, flow distribution among parallel channels, conjugate effects, and instrumentation need to be considered. An examination of the limited CHF data indicates that CHF in parallel microchannels seems to be the result of either an upstream compressible volume instability or an excursive instability rather than the conventional dryout mechanism. It is expected that the CHF in parallel microchannels would be higher if the flow is stabilized by an orifice at the entrance of each channel. The nature of CHF in microchannels is thus different than anticipated, but recent advances in microelectronic fabrication may make it possible to realize the higher power levels.
10
Fundamenski, W. "Parallel heat flux limits in the tokamak scrape-off layer". Plasma Physics and Controlled Fusion 47, n. 11 (6 ottobre 2005): R163—R208. http://dx.doi.org/10.1088/0741-3335/47/11/r01.
11
Wu, H. Y., e Ping Cheng. "Boiling instability in parallel silicon microchannels at different heat flux". International Journal of Heat and Mass Transfer 47, n. 17-18 (agosto 2004): 3631–41. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2004.04.012.
12
Li, Fuqin, William P. Kustas, John H. Prueger, Christopher M. U. Neale e Thomas J. Jackson. "Utility of Remote Sensing–Based Two-Source Energy Balance Model under Low- and High-Vegetation Cover Conditions". Journal of Hydrometeorology 6, n. 6 (1 dicembre 2005): 878–91. http://dx.doi.org/10.1175/jhm464.1.
Abstract (sommario):
Abstract Two resistance network formulations that are used in a two-source model for parameterizing soil and canopy energy exchanges are evaluated for a wide range of soybean and corn crop cover and soil moisture conditions during the Soil Moisture–Atmosphere Coupling Experiment (SMACEX). The parallel resistance formulation does not consider interaction between the soil and canopy fluxes, whereas the series resistance algorithms provide interaction via the computation of a within-air canopy temperature. Land surface temperatures were derived from high-resolution Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM) scenes and aircraft imagery. These data, along with tower-based meteorological data, provided inputs for the two-source energy balance model. Comparison of the local model output with tower-based flux observations indicated that both the parallel and series resistance formulations produced basically similar estimates with root-mean-square difference (RMSD) values ranging from approximately 20 to 50 W m−2 for net radiation and latent heat fluxes, respectively. The largest relative difference in percentage [mean absolute percent difference (MAPD)] was for sensible heat flux, which was ≈35%, followed by a MAPD ≈ 25% for soil heat flux, ≈10% for latent heat flux, and a MAPD < 5% for net radiation. Although both series and parallel versions gave similar results, the parallel resistance formulation was found to be more sensitive to model parameter specification, particularly in accounting for the effects of vegetation clumping resulting from row crop planting on flux partitioning. A sensitivity and model stability analysis for a key model input variable, that is, fractional vegetation cover, also show that the parallel resistance network is more sensitive to the errors vegetation cover estimates. Furthermore, it is shown that for a much narrower range in vegetation cover fraction, compared to the series resistance network, the parallel resistance scheme is able to achieve a balance in both the radiative temperature and convective heat fluxes between the soil and canopy components. This result appears to be related to the moderating effects of the air temperature in the canopy air space computed in the series resistance scheme, which represents the effective source height for turbulent energy exchange across the soil–canopy–atmosphere system.
13
Zenteno-Quinteros, Bea, Adolfo F. Viñas e Pablo S. Moya. "Skew-kappa Distribution Functions and Whistler Heat Flux Instability in the Solar Wind: The Core-strahlo Model". Astrophysical Journal 923, n. 2 (1 dicembre 2021): 180. http://dx.doi.org/10.3847/1538-4357/ac2f9c.
Abstract (sommario):
Abstract Electron velocity distributions in the solar wind are known to have field-aligned skewness, which has been characterized by the presence of secondary populations such as the halo and strahl. Skewness may provide energy for the excitation of electromagnetic instabilities, such as the whistler heat flux instability (WHFI), which may play an important role in regulating the electron heat flux in the solar wind. Here we use kinetic theory to analyze the stability of the WHFI in a solar-wind-like plasma where solar wind core, halo, and strahl electrons are described as a superposition of two distributions: a Maxwellian core, and another population modeled by a Kappa distribution to which an asymmetry term has been added, representing the halo and also the strahl. Considering distributions with small skewness, we solve the dispersion relation for the parallel-propagating whistler mode and study its linear stability for different plasma parameters. Our results show that the WHFI can develop in this system and provide stability thresholds for this instability, as a function of the electron beta and the parallel electron heat flux, to be compared with observational data. However, since different plasma states, with different stability level to the WHFI, can have the same moment heat flux value, it is the skewness (i.e., the asymmetry of the distribution along the magnetic field), and not the heat flux, that is the best indicator of instabilities. Thus, systems with high heat flux can be stable enough to WHFI, so that it is not clear whether the instability can effectively regulate the heat flux values through wave–particle interactions.
14
Hattori, Masanari, Soichi Tanaka e Shigeru Takata. "Heat transfer in a dense gas between two parallel plates". AIP Advances 12, n. 5 (1 maggio 2022): 055220. http://dx.doi.org/10.1063/5.0091390.
Abstract (sommario):
Time-dependent heat transfer in a dense gas between two parallel plates, which is initiated by an abrupt change in temperature of one plate, is numerically investigated on the basis of the Enskog equation under the diffuse reflection boundary condition. Numerical computation is carried out by a finite-difference scheme combined with the Fourier spectral method for the efficient computation of the collision term of the Enskog equation. As a result, macroscopic quantities of the gas, such as heat flux and temperature, are obtained for various Knudsen numbers and ratios of the molecular diameter to the distance between plates. Compared to the case of an ideal gas, the heat flux in the stationary state is enhanced due to an effect of the finite size of molecules for not only small but also intermediate Knudsen numbers. The results imply that the finite-size effect also affects the propagation of disturbances in the initial stage, particularly for small Knudsen numbers.
15
Torii, Shuichi, e Wen-Jei Yang. "Thermal-Fluid Transport Phenomena of a Strongly-Heated Gas Flow in Parallel Tube Rotation". International Journal of Rotating Machinery 4, n. 4 (1998): 271–82. http://dx.doi.org/10.1155/s1023621x98000232.
Abstract (sommario):
A numerical analysis is performed to study thermal transport phenomena in gas flow through a strongly heated tube whose axis is in parallel with the rotational axis. The velocity and temperature fields prevail when fluid flows in a rotating tube with uniform heat flux on the tube wall. The two-equationk-ωturbulence andt2¯-εtheat transfer models are employed to determine turbulent viscosity and eddy diffusivity for heat, respectively. The governing boundary-layer equations are discritized by means of a control volume finitedifference techniques. It is found that the Coriolis and centrifugal (or centripetal) forces cause fluid flow and heat transfer performance in the parallel-rotation system to be drastically different from those in the stationary case. Consequently, even if a tube rotating around a parallel axis is heated with high heat flux whose level causes a laminarizing flow in the stationary tube case, both the turbulent kinetic energy and the temperature variance remain over the pipe cross section, resulting in the suppression of an attenuation in heat transfer performance. In other words, an increase in tube rotation suppresses laminarization of gas flow.
16
R, Nyabuto, Sigey J.K, Okelo J.A e Okwoyo J.M. "Magneto-Hydrodynamics Analysis of Free Convection Flow between Two Horizontal Parallel Infinite Plates Subjected to Constant Heat Flux". SIJ Transactions on Computer Networks & Communication Engineering 01, n. 04 (3 ottobre 2013): 08–12. http://dx.doi.org/10.9756/sijcnce/v1i4/0104520101.
17
Niazmand, Hamid, e Behnam Rahimi. "MIXED CONVECTIVE SLIP FLOWS IN A VERTICAL PARALLEL PLATE MICROCHANNEL WITH SYMMETRIC AND ASYMMETRIC WALL HEAT FLUXES". Transactions of the Canadian Society for Mechanical Engineering 36, n. 3 (settembre 2012): 207–18. http://dx.doi.org/10.1139/tcsme-2012-0015.
Abstract (sommario):
Mixed convective gaseous slip flows in an open-ended vertical parallel-plate channel with symmetric and asymmetric wall heat fluxes are numerically investigated. Buoyancy effects on developing and fully developed solutions are studied using the SIMPLE algorithm. The velocity and temperature fields are examined for different values of Knudsen number, mixed convection parameter and heat flux ratio. It is found that increasing Gr/Re leads to an increase in the heat transfer rate and friction coefficient. Also, rarefaction effects decrease the heat transfer rate and friction coefficient. The friction coefficient decreases with an increase in heat flux ratio.
18
Zeng, Lunwu, Zhongliang Tang, Hua Li, Yanyan Zhao, Cunli Dai e Runxia Song. "Experimental observation of heat wave cloak". Modern Physics Letters B 28, n. 12 (19 maggio 2014): 1450098. http://dx.doi.org/10.1142/s0217984914500985.
Abstract (sommario):
According to the heat conduction theory and effective medium theory, we fabricated a six-layer cylinder heat diffusion cloak by mixing heat conduction materials. We measured the time-dependant heat wave at the iso-temperature lines and the heat flux lines with a self-made heat wave device. The experiment results show that the iso-temperature lines are parallel outside the outer circle, the heat fluxes are parallel and equal outside the outer circle, and the heat fluxes in the inner circle are much smaller than those outside the outer circle, namely, the heat fluxes are guided outside the cloaking region.
19
Taler, Jan, Dawid Taler e Andrzej Kowal. "Measurements of absorbed heat flux and water-side heat transfer coefficient in water wall tubes". Archives of Thermodynamics 32, n. 1 (1 aprile 2011): 77–88. http://dx.doi.org/10.2478/v10173-011-0004-6.
Abstract (sommario):
Measurements of absorbed heat flux and water-side heat transfer coefficient in water wall tubes The tubular type instrument (flux tube) was developed to identify boundary conditions in water wall tubes of steam boilers. The meter is constructed from a short length of eccentric tube containing four thermocouples on the fire side below the inner and outer surfaces of the tube. The fifth thermocouple is located at the rear of the tube on the casing side of the water-wall tube. The boundary conditions on the outer and inner surfaces of the water flux-tube are determined based on temperature measurements at the interior locations. Four K-type sheathed thermocouples of 1 mm in diameter, are inserted into holes, which are parallel to the tube axis. The non-linear least squares problem is solved numerically using the Levenberg-Marquardt method. The heat transfer conditions in adjacent boiler tubes have no impact on the temperature distribution in the flux tubes.
20
Weng, Xia, e Dong Yao Liu. "Experimental Study on Heat Transfer Characteristics of Water and Ethanol Flow Boiling in Micro-Channel". Applied Mechanics and Materials 330 (giugno 2013): 788–91. http://dx.doi.org/10.4028/www.scientific.net/amm.330.788.
Abstract (sommario):
The Heat transfer characteristics of water and ethanol flow boiling in micro-channel are studied. Flow boiling of different mass and heat flux is carried out in 61 parallel microchannel with hydro diameter of 0.293mm, and the local heat transfer coefficient (HTC) of is calculated. The results indicate that the HTC decreases with the increasing of vapor quality, and depends on heat flux. Two latest correlations are used to predict the experiment, and the results indicate that the Bond number plays an important role in the correlation.
21
Kostanovskiy, А. V., M. E. Kostanovskaya, M. G. Zeodinov e A. A. Pronkin. "Heat conductivity of pyrolytic graphite of mark UPV-1 at temperatures 1900–2950 K". Izmeritel`naya Tekhnika, n. 9 (2020): 50–53. http://dx.doi.org/10.32446/0368-1025it.2020-9-50-53.
Abstract (sommario):
The urgency of the work is caused by that at research of heat conductivity in anisotropic materials is necessary to consider infringement of parallelism between vectors of heat flux and a gradient of temperature. In the previous works at studying heat conductivity of pyrolytic graphites techniques which were applied to isotropic graphites were used. Results of measurement of heat conductivity of pyrolytic graphite of mark UPV-1 in a direction in parallel to a plane of sedimentation are presented. The experimental samples which are heated up by an electric current, represented hollow cylinders at which the plane of sedimentation of pyrolytic graphite has been located along radius. It is offered also heat flux which defined on energy of radiation from an external surface of the sample, and a gradient of temperature to measure along radius, that is strictly in parallel each other. The comparative analysis of experimental data of heat conductivity (in parallel a plane of sedimentation), measured in the given work and resulted in literature sources is brought. The range of the investigated temperatures makes 1900–2950 K and is expanded in area of higher values on 450 K.
22
Han, J. C., Y. M. Zhang e C. P. Lee. "Influence of Surface Heat Flux Ratio on Heat Transfer Augmentation in Square Channels With Parallel, Crossed, and V-Shaped Angled Ribs". Journal of Turbomachinery 114, n. 4 (1 ottobre 1992): 872–80. http://dx.doi.org/10.1115/1.2928042.
Abstract (sommario):
The effect of wall heat flux ratio on the local heat transfer augmentation in a square channel with two opposite in-line ribbed walls was investigated for Reynolds numbers from 15,000 to 80,000. The square channel composed of ten isolated copper sections has a length-to-hydraulic diameter ratio (L/D) of 20. The rib height-to-hydraulic diameter ratio (e/D) is 0.0625 and the rib pitch-to-height ratio (P/e) equals 10. Six ribbed side to smooth side wall heat flux ratios (Case 1—q″r1/q″s = q″r2/q″s = 1; Case 2—q″r1/q″s = q″r2/q″s = 3; Case 3—q″r1/q″s = q″r2/q″s = 6; Case 4—q″r1/q″s = 6 and q″r2/q″s = 4; Case 5—q″r1/q″s = q″r2/q″s = ∞; Case 6—q″r1/q″s = ∞ and q″r2/q″s = 0) were studied for four rib orientations (90 deg rib, 60 deg parallel rib, 60 deg crossed rib, and 60 deg V-shaped rib). The results show that the ribbed side wall heat transfer augmentation increases with increasing ribbed side to smooth side wall heat flux ratios, but the reverse is true for the smooth side wall heat transfer augmentation. The average heat transfer augmentation of the ribbed side and smooth side wall decreases slightly with increasing wall heat flux ratios. Two ribbed side wall heating (Case 5—q″r1/q″s = q″r2/q″s = ∞) provides a higher ribbed side wall heat transfer augmentation than the four-wall uniform heating (Case 1—q″r1/q″s = q″r2/q″s = 1). The effect of wall heat flux ratio reduces with increasing Reynolds numbers. The results also indicate that the 60 deg V-shaped rib and 60 deg parallel rib perform better than the 60 deg crossed rib and 90 deg rib, regardless of wall heat flux ratio and Reynolds number.
23
Fu, Ben-Ran, Shan-Yu Chung, Wei-Jen Lin, Lei Wang e Chin Pan. "Critical heat flux enhancement of HFE-7100 flow boiling in a minichannel heat sink with saw-tooth structures". Advances in Mechanical Engineering 9, n. 2 (febbraio 2017): 168781401668902. http://dx.doi.org/10.1177/1687814016689022.
Abstract (sommario):
A heat sink with convective boiling in micro- or mini-channels is with great potential to meet the requirement of the high heat dissipation of the electronic devices. This study investigates the flow boiling of HFE-7100, having a suitable boiling temperature at atmospheric pressure and dielectric property, in the minichannel heat sink with the modified surface (namely, the saw-tooth structure). The effect of the system pressure on the boiling characteristics was also studied. The results reveal that the critical heat flux can be significantly improved by introducing the saw-tooth structures on the channel surface or boosting the system pressure as well as by increasing the mass flux. Compared to the non-modified channel, the enhancements of the critical heat flux for the parallel and counter saw-tooth channels are 44% and 36%, respectively, at the small mass flux. The boiling visualization further indicates that the minichannels with the saw-tooth structures interrupt the boundary layer and restrain the coalescence of the bubble, which may be the reason for the critical heat flux enhancement. Moreover, the degree of the critical heat flux enhancement, contributed by the saw-tooth modification of the channel, decreases with an increase in the mass flux.
24
Krishnan, A. S., C. Balaji e S. P. Venkateshan. "An Experimental Correlation for Combined Convection and Radiation Between Parallel Vertical Plates". Journal of Heat Transfer 126, n. 5 (1 ottobre 2004): 849–51. http://dx.doi.org/10.1115/1.1795245.
Abstract (sommario):
Results of an experimental study of natural convection and surface radiation between three parallel vertical plates, symmetrically spaced, with air as the intervening medium are presented. The analysis consists of heating the central plate at different levels and recording the temperatures of both the central and the side plates at steady state conditions. Based on the measurements, a correlation for the maximum temperature excess of the “hot” plate in terms of the emissivity of the central and the side plates, the aspect ratio, and the dimensionless total heat flux is given, valid for a range of emissivity 0.05⩽εc,εs⩽0.85, aspect ratio 2.38⩽A⩽17, and total heat flux 32⩽q⩽1590 W/m2. Through this, the heat transfer enhancement due to radiation has been succinctly brought out.
25
Yang, Zhuqiang, Ruipu Miao, Zhen Jin, Feng Liu, Qiao Kang e Bo Zhang. "HYDROCARBON FUEL IN HORIZONTAL PARALLEL CHANNELS WITH NONUNIFORM HEAT FLUX BOUNDARY". Heat Transfer Research 53, n. 8 (2022): 55–74. http://dx.doi.org/10.1615/heattransres.2022041445.
26
Ng, Jonathan, Ammar Hakim, A. Bhattacharjee, Adam Stanier e W. Daughton. "Simulations of anti-parallel reconnection using a nonlocal heat flux closure". Physics of Plasmas 24, n. 8 (agosto 2017): 082112. http://dx.doi.org/10.1063/1.4993195.
27
Siddiqui, Perwez. "Density Modelling in Mixed Convection Flow in a Vertical Parallel Plate Channel". International Journal of Heat and Technology 39, n. 4 (31 agosto 2021): 1294–304. http://dx.doi.org/10.18280/ijht.390428.
Abstract (sommario):
In this paper, a novel way of modelling the density in buoyancy term of mixed convection flow problem is presented using equation of state and Boussinesq approximation without first-order approximation of density with respect to temperature. The presented density model is used to investigate the laminar mixed convection flow in a vertical parallel plate channel under symmetric constant wall heat flux. The results obtained are compared with the results obtained using first-order approximation of density with Boussinesq approximation, and also compared with the results obtained using variable thermophysical properties with negligible viscous dissipation. Investigation is performed on the basis of flow and thermal fields for Re=150 and 300, Ri=0.1 to 25. It is found that the presented density model produces relatively better results, which is able to describe the case of developing flow under constant heat flux condition that is not evident if Boussinesq approximation with first-order approximation of density is used. An appearance of recirculatory cells when reverse flow takes place is also witnessed in vertical channel flow with constant heat flux boundary condition which was not reported earlier.
28
Fujino, T., Y. Yokoyama e Y. H. Mori. "Augmentation of Laminar Forced-Convective Heat Transfer by the Application of a Transverse Electric Field". Journal of Heat Transfer 111, n. 2 (1 maggio 1989): 345–51. http://dx.doi.org/10.1115/1.3250683.
Abstract (sommario):
The effect of a uniform d-c electric field on laminar forced-convective heat transfer has been studied experimentally with a weakly conducting fluorocarbon refrigerant in the liquid state that is flowing in a channel confined by parallel-plate electrodes, one of which serves as a heat transfer surface of uniform heat flux. The dependencies of the heat transfer coefficient and the pressure drop on the sign and the magnitude of an applied voltage, the heat flux at the heat transfer surface, the electrical conductivity of the test fluid, etc. are presented, and the structure and the mechanism of the electroconvection causing the heat transfer enhancement are considered.
29
Kumar Parwani, Ajit, Prabal Talukdar e P. M. V. Subbarao. "Estimation of transient boundary flux for a developing flow in a parallel plate channel". International Journal of Numerical Methods for Heat & Fluid Flow 24, n. 3 (1 aprile 2014): 522–44. http://dx.doi.org/10.1108/hff-01-2012-0020.
Abstract (sommario):
Purpose – The purpose of this paper is to develop a numerical model for estimating the unknown boundary heat flux in a parallel plate channel for the case of a hydrodynamically and thermally developing laminar flow. Design/methodology/approach – The conjugate gradient method (CGM) is used to solve the inverse problem. The momentum equations are solved using an in-house computational fluid dynamics (CFD) source code. The energy equations along with the adjoint and sensitivity equations are solved using the finite volume method. Findings – The effects of number of measurements, distribution of measurements and functional form of unknown flux on the accuracy of estimations are investigated in this work. The prediction of boundary flux by the present algorithm is found to be quite reasonable. Originality/value – It is noticed from the literature review that study of inverse problem with hydrodynamically developing flow has not received sufficient attention despite its practical importance. In the present work, a hydrodynamically and thermally developing flow between two parallel plates is considered and unknown transient boundary heat flux at the upper plate of a parallel plate channel is estimated using CGM.
30
Jiang, Zhengyong, Mengjie Song, Jun Shen, Long Zhang, Xuan Zhang e Shenglun Lin. "Experimental Investigation on the Flow Boiling of Two Microchannel Heat Sinks Connected in Parallel and Series for Cooling of Multiple Heat Sources". Micromachines 14, n. 8 (10 agosto 2023): 1580. http://dx.doi.org/10.3390/mi14081580.
Abstract (sommario):
Cooling methods for multiple heat sources with high heat flux have rarely been reported, but such situations threaten the stable operation of electronic devices. Therefore, in this paper, the use of two microchannel heat sinks is proposed, with and without grooves, labeled Type A and Type B, respectively. Experimental investigations on the flow boiling of two microchannel heat sinks connected in parallel and in series are carried out under different mass fluxes. In addition, a high-speed camera is used to observe flow patterns in the microchannels. The cold plate wall temperature (Tw), heat transfer coefficient (HTC), and pressure drop (PD) are obtained with the use of two microchannel heat sinks. The flow patterns of the bubbly flow and elongated bubbles in the microchannels are observed. The results of the analysis indicated that the Tw, HTC, and PD of the two microchannel heat sinks connected in parallel were degraded, especially when using the Type A-B parallel connection. Compared to the use of a single heat sink, the maximum decrease in HTC was 9.44 kW/(m2K) for Type A heat sinks connected in parallel, which represents a decrease of 45.95%. The influence of the series connection on the Tw, HTC, and PD of the two heat sinks is obvious. The Type A-A series connection exerted the greatest positive effect on the performance of the two heat sinks, especially in the case of the postposition heat sink. The maximum increase in HTC was 12.77 kW/(m2K) for the postposition Type A heat sink, representing an increase of 72.88%. These results could provide a reference for a two-phase flow-cooling complex for multiple heat sources with high heat flux.
31
Budaev, Bair V., e David B. Bogy. "The role of EM wave polarization on radiative heat transfer across a nanoscale gap". Journal of Applied Physics 132, n. 5 (7 agosto 2022): 054903. http://dx.doi.org/10.1063/5.0094382.
Abstract (sommario):
This work presents a novel study of radiative heat transfer between closely separated plates based on an extension of Planck’s spectrum of thermal radiations to systems with a steady heat flux. This extension together with electromagnetic wave theory is chosen specifically to avoid the commonly used so-called fluctuation dissipation theory, which is also limited to equilibrium systems. The spectrum of thermal radiation with a heat flux is described by the introduction of an analog of a chemical potential, which creates a bias toward the direction of heat transfer. This is the first comprehensive study of radiative heat transfer based on the generalization of Planck’s spectrum for systems with a heat flux, which eliminates contradictions arising when a heat flux is described in terms of the laws limited to equilibrium systems. The total heat flux is split into fluxes carried by waves with different frequencies, directions of propagation, and polarizations. This simplifies the analysis because due to the stochastic independence, the energy fluxes of such waves are additive, and this also reveals that the heat carrying capacity of radiation with the parallel polarization is significantly higher than that of the perpendicularly polarized radiation. This suggests that the rate of radiative heat transfer may be noticeably increased by the control of the polarization of thermal radiation.
32
Hao, Yun, Kaituo Chen, Yueshe Wang e Tian Hu. "Effect of One-Target Focus Type on Hydrodynamic Characteristics of Tower Solar Cavity Receiver". Advances in Mechanical Engineering 6 (1 gennaio 2014): 615942. http://dx.doi.org/10.1155/2014/615942.
Abstract (sommario):
On account of one-target focus type of the heliostats in the tower solar power technology, the heat transfer was analyzed for the vapor-liquid two-phase or single-phase superheated steam in the parallel heated panel bundles of the solar cavity receiver. A nonlinear mathematical model of the hydrodynamic characteristics in the evaporation panels was developed to obtain the flow rate distribution, thermal deviation, and two-phase flow circulation reliability of the working fluid under the severe nonuniform heat flux from the one-target focus of the heliostats. The simulation results show that for the evaporation panels the flow distribution can synchronize with that of the heat flux at the low heat flux, while for the superheater sections the flow distribution decreases with the increase of heat flux. This desynchrony may give rise to stagnation or backflow of the working fluid and lead to the panels burst or erosion due to the local overheating in some extreme situation.
33
Guedes, R. O. C., M. N. Ozisik e R. M. Cotta. "Conjugated Periodic Turbulent Forced Convection in a Parallel Plate Channel". Journal of Heat Transfer 116, n. 1 (1 febbraio 1994): 40–46. http://dx.doi.org/10.1115/1.2910881.
Abstract (sommario):
The transient conjugated turbulent heat transfer with axial conduction in the wall and convection boundary conditions is solved with the generalized integral transform technique for the flow of a Newtonian fluid in a parallel-plate duct subjected to periodically varying inlet temperature. A lumped model that neglects transverse temperature gradients in the solid, but takes into account the axial heat conduction along the wall, is adopted. Accurate numerical results are presented for the fluid bulk temperature, wall temperature, and wall heat flux. The effects of the conjugation parameter, fluid-to-solid heat capacitance ratio, and Biot number on the behavior of the periodic responses are investigated.
34
Fallahzadeh, Rasoul, Fabio Bozzoli, Luca Cattani e Muhammad Waheed Azam. "Effect of Cross Nanowall Surface on the Onset Time of Explosive Boiling: A Molecular Dynamics Study". Energies 17, n. 5 (26 febbraio 2024): 1107. http://dx.doi.org/10.3390/en17051107.
Abstract (sommario):
Explosive boiling is a fast-phase transition from an ultra-thin liquid film to vapor under an extremely high heat flux, which typically has been studied using the molecular dynamics simulation (MDS) method. The present MDS study investigated the explosive boiling of a liquid argon nanofilm over different solid copper surfaces with different nanowall patterns, including parallel and cross nanowalls. For each surface, atomic motion trajectories, the number of liquid and vapor argon atoms, heat flux, and, mainly, the onset time of explosive boiling were investigated. The simulation results indicated that explosive boiling occurs earlier on parallel and cross nanowall surfaces than on an ideally smooth surface, regardless of the topology and configuration of the nanowalls. Moreover, the results revealed that by using the cross nanowall surfaces, the onset time of explosive boiling decreased by 0.7–4% compared to the parallel nanowall surfaces. In addition, it was found that the onset time of explosive boiling strongly depends on the potential energy barrier and the movement space between nanowalls for both parallel and cross nanowall surfaces. Furthermore, the simulation findings showed that even though increasing the height of cross nanowalls increases the heat flux and temperature of the fluid argon domain, it does not necessarily result in a shorter onset time for explosive boiling. These findings demonstrate the capability of cross nanowall surfaces for explosive boiling, thereby being utilized in future surface design for thermal management applications.
35
Spearpoint, Michael, Charlie Hopkin e Danny Hopkin. "Modelling the thermal radiation from kitchen hob fires". Journal of Fire Sciences 38, n. 4 (19 giugno 2020): 377–94. http://dx.doi.org/10.1177/0734904120923566.
Abstract (sommario):
Kitchen hob fires present a potential threat to occupants escaping from dwellings and calculations may be needed to assess the hazard. Determination of the thermal heat flux from flames to a target can be achieved through the use of hand calculation methods or computational tools. This article compares point source, parallel plane and cylindrical view factor hand calculations and computational simulations using B-RISK and Fire Dynamics Simulator of thermal heat flux with kitchen hob fire experiments presented in the literature. Knowing the level of accuracy of each method provides useful information to designers. Although the point source model is influenced by whether the radial distance is measured perpendicular to the heat flux target or is offset relative to the centre of the flame, the article concludes that it provides an adequate approach for the calculation of thermal heat flux in the case of kitchen hob fires.
36
Maughan, J. R., e F. P. Incropera. "Mixed Convection Heat Transfer With Longitudinal Fins in a Horizontal Parallel Plate Channel: Part I—Numerical Results". Journal of Heat Transfer 112, n. 3 (1 agosto 1990): 612–18. http://dx.doi.org/10.1115/1.2910431.
Abstract (sommario):
Numerical calculations for laminar, fully developed mixed convection in a longitudinally finned horizontal channel have been performed for two sets of boundary conditions: (i) an isothermal, heated bottom plate with an isothermal, cooled top plate, and (ii) a uniform heat flux at the bottom surface with an adiabatic upper surface. Heat transfer and the strength of the buoyancy-driven secondary flow increase with increasing Rayleigh number and fin height. Fin spacing affects heat transfer through changes in the axial velocity distribution, the strength of the secondary flow, and the heated surface area, with decreased spacing acting to inhibit secondary flow. For the uniform heat flux condition and a small available pressure drop, close fin spacing can significantly reduce the channel flow rate and increase maximum plate temperatures.
37
Hidalgo-Salaverri, J., J. Gonzalez-Martin, J. Ayllon-Guerola, M. Garcia-Munoz, B. Sieglin, J. Galdon-Quiroga, D. Silvagni et al. "Thermo-mechanical limits of a magnetically driven fast-ion loss detector in the ASDEX Upgrade tokamak". Journal of Instrumentation 17, n. 02 (1 febbraio 2022): C02020. http://dx.doi.org/10.1088/1748-0221/17/02/c02020.
Abstract (sommario):
Abstract A real-time control system is being developed for a magnetically driven Fast-Ion Loss Detector (FILD) at the ASDEX Upgrade tokamak. The insertion of the diagnostic head will be adjusted in real-time to react to changes in the graphite head temperature, plasma position and appearance of MHD instabilities. The graphite probe head of the detector is exposed to an intense heat flux (located ∼3–5 cm from separatrix). The control algorithm performance is constrained by: the graphite head sublimation temperature, the ultimate stress limit, the reaction time of the controller and the retraction time. In this work, the temperature and thermal induced stress distribution on the probe head are assessed to determine what temperature-related magnitude is the limiting factor. The heat flux at the probe head has been estimated using the time-averaged parallel heat flux measured at the divertor target via infrared thermography. A field line tracing algorithm determines which regions of the probe head receives a weighted heat flux due to shadowing (self-induced or from other structures) and the incidence angle of the field lines. A finite element model is used to simulate the temporal evolution of the graphite head temperature and to obtain the induced thermal stress. The temperature spatial distribution at the probe head is validated against measurements of the probe head for different FILD systems showing a good agreement. These measurements have been obtained from visible cameras with an infrared filter. The model concludes that the maximum stress (∼100 MPa) does not overcome the graphite mechanical limit (170 MPa) and that the probe head is not affected by fatigue. Therefore, the graphite sublimation temperature (2000 °C) is set as the limiting factor of the new control system.
38
Guo, Zehua, e Xian-Zhu Tang. "Parallel transport of long mean-free-path plasma along open magnetic field lines: Parallel heat flux". Physics of Plasmas 19, n. 6 (giugno 2012): 062501. http://dx.doi.org/10.1063/1.4725494.
39
Duhau, S., e A. De La Torre. "Hydromagnetic waves for a collisionless plasma in strong magnetic fields". Journal of Plasma Physics 34, n. 1 (agosto 1985): 67–76. http://dx.doi.org/10.1017/s0022377800002683.
Abstract (sommario):
A hydrodynamic system of equations, valid in the limit in which the Larmor radius and the electron to ion mass ratio are both zero, and including the thermo-dynamic variables and the energy equation of the electrons, is used to investigate the propagation of small-amplitude waves in a collisionless heat-conducting plasma. The result is compared with that derived from the Chew, Goldberger & Low equations. It is found that for zero heat flux, the inclusion of the electron pressure does not change the number and characteristic of the modes but modifies the mirror stability criterion. In the general case, the phase speed is symmetric with respect to two axes: one parallel to the heat flux vector and the other normal to it. The heat flux generates a new mode and couples strongly the slow and fast magnetosonic modes whose wavenumber vectors have projections in the positive flux vector direction, giving rise to a new overstability whose existence does not depend on the ion anisotropy.
40
Gao, Huaibin, Xiaojiang Liu, Chuanwei Zhang, Yu Ma, Hongjun Li e Guanghong Huang. "Design and Experimental Investigation of a Self-Powered Fan Based on a Thermoelectric System". Energies 16, n. 2 (15 gennaio 2023): 975. http://dx.doi.org/10.3390/en16020975.
Abstract (sommario):
Providing electricity for isolated areas or emergencies (snowstorms, earthquakes, hurricanes, etc.) is an important challenge. In this study, a prototype of a self-powered fan based on a thermoelectric system was built to enhance the heat dissipation of the thermoelectric generator (TEG) systems using household stoves as heat sources. To improve output performance of the system, a heat collector consisting of a heat-conducting flat plate and a heat sink with fan cooling was designed to integrate several thermoelectric modules (TEM). The effects of the fan operating conditions (airflow velocity), number of thermoelectric modules, electrical connection mode under different heat flux among the performance of the TEG system are studied. The data obtained showed a higher heat flux and lower flow velocity are required to realize self-sustained cooling of the system. The maximum electric power is more sensitive to the heat flux than the fan operation conditions. It is also observed that more modules provide a higher power output but lower efficiency. The maximum power of four modules in series is larger than that in parallel, and the difference between them increases with increasing heat flux of the heat collector. In the case of self-sufficiency: the maximum output power and maximum net power with four thermoelectric modules are 10.92 W and 5.26 W, respectively, at a heat flux of 30,000 W/m2. Additionally, the maximum conversion efficiency of 1.8% is achieved for two modules at a heat flux of 14,000 W/m2, providing an effective strategy for the installation of TEMs and cooling fans in TEG.
41
Singh, Munendra Pal, Abdallah Sofiane Berrouk e Suneet Singh. "A Comparative Assessment on Different Aspects of the Non-Linear Instability Dynamics of Supercritical Fluid in Parallel Channel Systems". Energies 15, n. 10 (16 maggio 2022): 3652. http://dx.doi.org/10.3390/en15103652.
Abstract (sommario):
The thermal-hydraulic behavior of supercritical water reactors with a parallel channel configuration was examined through a non-linear instability analysis. This analysis was performed under wide-ranging conditions and aspects, including different working supercritical fluids, varied heat-flux and flow-rate conditions, and channel inclinations. The supercritical fluid (SCFs) dynamics were captured using the density, enthalpy, and velocity analytical approximation functions. The major findings show that both SCFs (water and carbon dioxide) experienced density wave oscillations at a low pseudo-subcooling number. Static instability characteristics were observed for supercritical water, at a relatively high subcooling number. Further, it was found that at different heat flux, the hotter channel makes the overall system more unstable, whereas, at equal heat flux, parallel channels perform similar to a single-channel system. However, the effect of the inclination angle was found to be negligible owing to supercritical pressure conditions. Moreover, stable and unstable limit cycles along with out-of-phase oscillation characteristics were observed in dynamic stability regions. The present model was also compared with experimental and numerical data. Moreover, co-dimension and numerical simulations were performed to confirm the observed non-linear characteristics. This study helps to enhance the heat transfer characteristics during safe operation of heated channel systems, such as nuclear reactors and solar thermal systems.
42
Kaniowski, Robert. "Pool Boiling of Novec-649 on Inclined Microchannel". Energies 16, n. 5 (5 marzo 2023): 2476. http://dx.doi.org/10.3390/en16052476.
Abstract (sommario):
Significant amounts of heat flow can be removed with small temperature differences by boiling. This process is used in a variety of industries, e.g., cooling electronics, digital circuits, power sources, etc. Heat dissipation from equipment that generates significant heat flows involves the movement of thermal energy through a wall into a cooling fluid. In the present study, this mechanism was analysed during the boiling of Novec-649 fluid at atmospheric pressure. The heat transfer surfaces were samples made of copper with milled, parallel grooves with a depth of 0.3 mm and a width ranging from 0.2 to 0.5 mm in 0.1 mm increments for straight channels and channels inclined with respect to the vertical by 30° and 60°, respectively. The study was carried out from the onset of nucleate boiling, approximately q = 7 kWm−2 with a heat flux increase until the critical heat flux was reached. The maximum heat flux was 262 kWm−2 and the heat transfer coefficient was 19.4 kWm−2K−1, achieved for surfaces with straight microchannels. A maximum heat flux increased by 80% and a heat transfer coefficient twice as high compared to a smooth surface was obtained. The performance of the experiment can be deemed adequate, considering that it compares well with the correlation results of different authors.
43
Sheela-Francisca, J., e C. P. Tso. "Viscous dissipation effects on parallel plates with constant heat flux boundary conditions". International Communications in Heat and Mass Transfer 36, n. 3 (marzo 2009): 249–54. http://dx.doi.org/10.1016/j.icheatmasstransfer.2008.11.003.
44
Franc¸a, Francis H. R., Ofodike A. Ezekoye e John R. Howell. "Inverse Boundary Design Combining Radiation and Convection Heat Transfer". Journal of Heat Transfer 123, n. 5 (20 febbraio 2001): 884–91. http://dx.doi.org/10.1115/1.1388298.
Abstract (sommario):
This work investigates inverse boundary design for radiation, convection and conduction combined-mode heat transfer. The problem consists of finding the heat flux distribution on a heater that satisfies both the temperature and the heat flux prescribed on a design surface of an enclosure formed by two finite parallel plates. A gray participating medium flows in laminar regime between the walls, which are gray, diffuse emitters and absorbers. All the thermal properties are uniform. This problem is described by an ill-conditioned system of non-linear equations. The solution is obtained by regularizing the system of equations by means of truncated singular value decomposition (TSVD).
45
Kaniowski, Robert, e Robert Pastuszko. "Pool Boiling of Water on Surfaces with Open Microchannels". Energies 14, n. 11 (25 maggio 2021): 3062. http://dx.doi.org/10.3390/en14113062.
Abstract (sommario):
Boiling, as the most efficient type of convective heat transfer, is an area of interest in many fields of industry and science. Many works have focused on improving the heat transfer efficiency of boiling by altering the physical and chemical properties of surfaces by using different technological processes in their fabrication. This paper presents experimental investigations into pool boiling on enhanced surfaces with open microchannels. The material of the fabricated surface was copper. Parallel microchannels made by machining were about 0.2, 0.3, and 0.4 mm wide, 0.2 to 0.5 mm deep, and spaced with a pitch equal to twice the width of the microchannel. The experiments were carried out in water at atmospheric pressure. The experimental results obtained showed an increase in the heat flux and the heat transfer coefficient for surfaces with microchannels. The maximum (critical) heat flux was 2188 kW/m2, and the heat transfer coefficient was 392 kW/m2K. An improvement in the maximum heat flux of more than 245% and 2.5–4.9 times higher heat transfer coefficient was obtained for the heat flux range of 992–2188 kW/m2 compared to the smooth surface. Bubble formation and growth cycle in the microchannel were presented. Two static computational models were proposed to determine the bubble departure diameter.
46
KIM, NAE-HYUN, SOO-HWAN KIM e JI-HOON PARK. "EFFECT OF INLET CONFIGURATION ON DISTRIBUTION OF AIR–WATER UPWARD FLOW IN A HEADER OF A PARALLEL FLOW HEAT EXCHANGER". International Journal of Air-Conditioning and Refrigeration 18, n. 04 (dicembre 2010): 265–77. http://dx.doi.org/10.1142/s2010132510000289.
Abstract (sommario):
The effect of inlet configuration (parallel, normal, vertical) on flow distribution in a parallel flow heat exchanger consisting of round headers and ten flat tubes is experimentally studied using air and water. The effects of tube protrusion depth as well as header mass flux, and quality are investigated for upward flow configuration. It is shown that best flow distribution is obtained for vertical inlet configuration, followed by normal inlet and parallel inlet configuration. For upward flow, significant portion of the water flows through the rear part of the header. As protrusion depth increases, more water is forced to the rear part of the header. The effect is most significant for parallel inlet, followed by normal and vertical inlet. The effect of mass flux or quality is opposite to that of the protrusion depth. Possible explanation is provided from flow visualization results.
47
Meng, Jianping, Yonghao Zhang e Jason M. Reese. "Numerical Simulation of Rarefied Gas Flows with Specified Heat Flux Boundary Conditions". Communications in Computational Physics 17, n. 5 (maggio 2015): 1185–200. http://dx.doi.org/10.4208/cicp.2014.m343.
Abstract (sommario):
AbstractWe investigate unidirectional rarefied flows confined between two infinite parallel plates with specified heat flux boundary conditions. Both Couette and force-driven Poiseuille flows are considered. The flow behaviors are analyzed numerically by solving the Shakhov model of the Boltzmann equation. We find that a zero-heat-flux wall can significantly influence the flow behavior, including the velocity slip and temperature jump at the wall, especially for high-speed flows. The predicted bimodal-like temperature profile for force-driven flows cannot even be qualitatively captured by the Navier-Stokes-Fourier equations.
48
Irani, Mazda, e Ian Gates. "Understanding the Convection Heat-Transfer Mechanism in the Steam-Assisted-Gravity-Drainage Process". SPE Journal 18, n. 06 (28 novembre 2013): 1202–16. http://dx.doi.org/10.2118/167258-pa.
Abstract (sommario):
Summary Steam-assisted gravity drainage (SAGD) is the preferred method to extract bitumen from Athabasca oil-sand reservoirs in western Canada. In SAGD, steam, injected outward from a horizontal injection well, loses its latent heat when it contacts the cold bitumen at the edge of a steam chamber. Consequently, the viscosity of the bitumen falls several orders of magnitude, enabling it to flow under gravity toward a horizontal production well directly below to the injection well. It is commonly believed that conduction is the dominant heat-transfer mechanism at the edge of the chamber. Heat transfer by convection is not considered in classic SAGD mathematical models such as the one derived by Butler. Researchers such as Butler and Stephens (1981), Reis (1992), Akin (2005), Liang (2005), Nukhaev et al. (2006), and Azad and Chalaturnyk (2010) considered the conduction from steam to cold reservoir to be the only heat-transfer component. Farouq-Ali (1997), Edmunds (1999a, b), Ito and Suzuki (1996, 1999), Ito et al. (1998), Sharma and Gates (2011), and Irani and Ghannadi (2013) questioned the assumption that thermal conduction dominates heat transfer at the edge of a SAGD chamber. Sharma and Gates (2011) and Irani and Ghannadi (2013) studied convective flux from condensate flow at the edge of an SAGD steam chamber. Irani and Ghannadi (2013) derived a new formulation that solves the energy balance and pressure-driven condensate flow normal to the steam-chamber interface into the cold bitumen reservoir and concluded that the assumption of conduction-dominated heat transfer is valid; however, all previous analyses do not include convective heat transfer arising from draining bitumen and condensate. Although a few researchers have studied convective flux from condensate flow at the edge of an SAGD steam chamber (e.g., Sharma and Gates 2011; Irani and Ghannadi 2013), there is a lack of understanding of bitumen and condensate drainage parallel to the edge of the chamber and of its effect on transverse heat transfer into the oil sand beyond the chamber. In this study, the relative roles of convective heat flux both parallel and normal to the edge of a steam chamber are examined. The results suggest that the convective heat flux associated with flow parallel to the chamber edge is minor compared with that normal to the edge.
49
Atmaca, Ş. Ulaş, İlker Göktepeli e Ali Ateş. "The Effects of Nanofluids on Forced Convection Heat Transfer Inside Parallel Plate Heated with Flush Mounted Discrete Heater Sources". International Journal of Civil, Mechanical and Energy Science 9, n. 1 (2023): 01–09. http://dx.doi.org/10.22161/ijcmes.9.1.1.
Abstract (sommario):
A numerical solution on forced convection of Al2O3-water nanofluid for different volume fractions is investigated for laminar flow through a parallel plate with flush mounted discrete heat sources. The model used for nanofluid mixture is a single-phase approach and fluid properties are considered constant with temperature. The finite difference method is used for solutions and four different volume fractions are considered varying from 0% to 4%. A fully developed laminar velocity profile is considered and the parallel plate is assumed as heated with three discrete heat sources flush mounted to the top and bottom plate with the same lengths. Uniform wall temperature boundary condition is taken for discrete heaters. Peclet numbers are in the range of 20-100. For comparison and validity of the solution the results for a classical problem, laminar flow through a parallel plate which is heated at the downstream region with constant temperature, are obtained. Results are presented in terms of bulk temperature, heat flux, and local Nusselt number. Heat transfer is enhanced with the particle volume concentration. For comparison, pure water results are also shown in the figures. At the locations where heat is applied the heat flux values decrease as the volume fraction increase and the bulk temperature values are higher for the higher volume fractions at the heated locations. As the volume fraction increases the local Nusselt number can increase up to 30% than to pure water.
50
Kaniowski, Robert, e Robert Pastuszko. "Pool boiling of ethanol on surfaces with parallel microchannels". EPJ Web of Conferences 269 (2022): 01024. http://dx.doi.org/10.1051/epjconf/202226901024.
Abstract (sommario):
The paper presents investigations into pool boiling heat transfer for open minichannel surfaces. The experiments were carried out with saturated ethanol at atmospheric pressure. Parallel microchannels fabricated by machining were about 0.2, 0.3 and 0.4 wide and 0.2 to 0.5 mm deep. The measurements were performed with increasing heat flux and variable geometric parameters of the minichannels. The image acquisition speed was 493 fps (at resolution 400 x 300 pixels with Photonfocus PHOT MV-D1024-160-CL camera) and an EX-FH20 (Casio) camera was used to record the images of the entire surface of the specimen. The analysis of boiling curves for the tested surfaces does not give an unambiguous response to the influence of geometrical parameters, i.e. the height of the microchannels on the heat transfer process.