Дисертації з теми "Radiative Heat Flux Rate"
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Liu, Xianglei. "Tailoring thermal radiative properties and enhancing near-field radiative heat flux with electromagnetic metamaterials." Diss., Georgia Institute of Technology, 2016. http://hdl.handle.net/1853/54960.
Повний текст джерелаPhillips, Bren Andrew. "Nano-engineering the boiling surface for optimal heat transfer rate and critical heat flux." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/76536.
Повний текст джерелаCataloged from PDF version of thesis.
Includes bibliographical references (p. 130-133).
The effects on pool boiling characteristics such as critical heat flux and the heat transfer coefficient of different surface characteristics such as surface wettability, roughness, morphology, and porosity are not well understood. Layer-by-layer nanoparticle coatings were used to modify the surface of a sapphire heater to control the surface roughness, the layer thickness, and the surface chemistry. The surface was then tested in a water boiling test at atmospheric pressure while imaging the surface with high speed infrared thermography yielding a 2D time dependent temperature profile. The critical heat flux and heat transfer coefficient were enhanced by over 100% by optimizing the surface parameters. It was found that particle size of the nanoparticles in coating, the coating thickness, and the wettability of the surface have a large impact on CHF and the heat transfer coefficient. Surfaces were also patterned with hydrophobic "islands" within a hydrophilic "sea" by coupling the Layer-by-layer nanoparticle coatings with an ultraviolet ozone technique that patterned the wettability of the surface. The patterning was an attempt to increase the nucleation site density with hydrophobic dots while still maintaining a large hydrophilic region to allow for rewetting of the surface during the ebullition cycle and thus maintaining a high critical heat flux. The patterned surfaces exhibited similar critical heat fluxes and heat transfer coefficients to the surfaces that were only modified with layer-by-layer nanoparticle coatings. However, the patterned surfaces also exhibited highly preferential nucleation from the hydrophobic regions demonstrating an ability to control the nucleation site layout of a surface and opening an avenue for further study.
by Bren Andrew Phillips.
S.M.
Sopkin, Kristin L. "Heat fluxes in Tampa Bay, Florida." [Tampa, Fla] : University of South Florida, 2008. http://purl.fcla.edu/usf/dc/et/SFE0002398.
Повний текст джерелаBeaulieu, Patricia. "Flammability Characteristics at Heat Fluxes up to 200 kW/m2 and The Effect of Oxygen on Flame Heat Flux." Digital WPI, 2005. https://digitalcommons.wpi.edu/etd-dissertations/427.
Повний текст джерелаBurchfield, Nicole Ashley. "Narrow Angle Radiometer for Oxy-Coal Combustion." BYU ScholarsArchive, 2020. https://scholarsarchive.byu.edu/etd/8423.
Повний текст джерелаBaud, Germain. "Conception de récepteurs solaires à lit fluidisé sous flux radiatif concentré." Thesis, Toulouse, INPT, 2011. http://www.theses.fr/2011INPT0106/document.
Повний текст джерелаThe aim of this work is to evaluate the position and the potential of solar fluidized bed receivers compared to other methods for the solar heating of gases at high temperature. To this end, a thorough knowledge of the heat transfer and hydrodynamic of the receiver is necessary. To acquire this knowledge, we modeled the heat transfer in the receiver with a focus on the radiative transfer by taking into account the multiple scattering of light in the particle medium, the effect of walls on radiative heat transfer and the directionality of the concentrated solar radiation. The accurate determination of the distribution of particles within the fluidized bed has been a critical parameter for the calculation of heat transfer. With these models, later refined by a confrontation with experimental references, we have studied the effect of geometry on heat transfer in the receiver. This study highlighted the necessity to use a switching section fluidization column and the importance to optimize the pair : solar concentrator / receiver to avoid any overheating at the walls of the receiver. Moreover, it appears that the homogenization of the temperature in the fluidized bed of the receiver increase its performance
Zhang, Zihao. "Investigating the far- and near-field thermal radiation in carbon-based nanomaterials." Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/54433.
Повний текст джерелаFernandes, Cássio Spohr. "Implementação de modelos atualizados de gás cinza no software FDS para predição do fluxo de calor radiativo em incêndios." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2018. http://hdl.handle.net/10183/184710.
Повний текст джерелаThis work aims to implement and test updated gray gas models in the thermal radiation routine of the Fire Dynamics Simulator (FDS) software, as well as the use of the gray gas model available in the software to the prediction of radiative heat flux. The gray gas models studied were the default model of the FDS software (determined GC1), and the most current gray gas models: the GC2, in which the absorption coefficient of the participant medium is given by a polynomial relations, and the GC3, which is a gray gas model that was based on the calculation of the absorption coefficient in the WSGG model. The most recently gray gas models were implemented in the source code, which is an open source, and the verification of the implementation was performed by the numerical solution of the equations from the reported values of the software. With the new gray gas models already implemented, the next step was the computational simulation of the previously selected cases. For all the gray gas models, pool fires were simulated different scenarios of fire for different fuels (ethanol, nheptane and methanol), with and without considering soot presence in the system. The fire scenarios were: (i) fully closed, (ii) fully open and (iii) with an intermediate condition, closed but with an opening to the external environment. A study of a mesh analysis and different parameters, such as the study of the required amount of discrete solid angles, were performed to correct the standard parameters. The computational simulations were verified for the default gray gas model of the FDS by comparing the simulations results with those reported in the specific literature of each case. With the models already verified, each fire scenario was simulated with the different gray gas models previously implemented. From the analysis of the results, good agreements were obtained for the fields of temperature, molar fraction of CO2 and H2O and soot volume fraction. The radiative heat fluxes were correctly predicted for all gray gas models early implemented. The GC2 model present results with average deviation in the range of 15%, the gray gas model based on WSGG (GC3) presented the best results, with average deviation lower than 10%, while the default software model (GC1) presented intermediate results.
Alanazi, Mohammed Awwad. "Non-invasive Method to Measure Energy Flow Rate in a Pipe." Thesis, Virginia Tech, 2018. http://hdl.handle.net/10919/103179.
Повний текст джерелаMS
Ozler, Emrah Talip. "Modelling Of Dropwise Condensation On A Cylindrical Surface Including The Sweeping Effect." Master's thesis, METU, 2007. http://etd.lib.metu.edu.tr/upload/3/12608440/index.pdf.
Повний текст джерелаNasri, Nacer. "Étude thermique du comportement en régime transitoire d'une matrice poreuse soumise à un flux radiatif : application a la conversion thermique de l'énergie solaire." Nancy 1, 1994. http://www.theses.fr/1994NAN10134.
Повний текст джерелаMaluk, Cristian. "Development and application of a novel test method for studying the fire behaviour of CFRP prestressed concrete structural elements." Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/15926.
Повний текст джерелаAlshawaf, Hussain M. J. A. A. M. A. "A Novel Thermal Method for Pipe Flow Measurements Using a Non-invasive BTU Meter." Thesis, Virginia Tech, 2018. http://hdl.handle.net/10919/101528.
Повний текст джерелаMS
Kasali, Suraju Olawale. "Thermal diodes based on phase-change materials." Thesis, Poitiers, 2021. http://www.theses.fr/2021POIT2254.
Повний текст джерелаThe thermal rectification of conductive and radiative thermal diodes based on phase-change materials, whose thermal conductivities and effective emissivities significant change within a narrow range of temperatures, is theoretically studied and optimized in different geometries. This thesis is divided into three parts. In the first part, we comparatively model the performance of a spherical and cylindrical conductive thermal diodes operating with vanadium dioxide (VO2) and non-phase-change materials, and derive analytical expressions for the heat flows, temperature profiles and optimal rectification factors for both diodes. Our results show that different diode geometries have a significant impact on the temperature profiles and heat flows, but less one on the rectification factors. We obtain maximum rectification factors of up to 20.8% and 20.7%, which are higher than the one predicted for a plane diode based on VO2. In addition, it is shown that higher rectification factors could be generated by using materials whose thermal conductivity contrast is higher than that of VO2. In the second part, on the other hand, we theoretically study the thermal rectification of a conductive thermal diode based on the combined effect of two phase-change materials. Herein, the idea is to generate rectification factors higher than that of a conductive thermal diode operating with a single phase-change material. This is achieved by deriving explicit expressions for the temperature profiles, heat fluxes and rectification factor. We obtain an optimal rectification factor of 60% with a temperature variation of 250 K spanning the metal-insulator transitions of VO2 and polyethylene. This enhancement of the rectification factor leads us to the third part of our work, where we model and optimize the thermal rectification of a plane, cylindrical and spherical radiative thermal diodes based on the utilization of two phase-change materials. We analyze the rectification factors of these three diodes and obtain the following optimal rectification factors of 82%, 86% and 90.5%, respectively. The spherical geometry is thus the best shape to optimize the rectification of radiative heat currents. In addition, potential rectification factors greater than the one predicted here can be realized by utilizing two phase-change materials with higher emissivities contrasts than the one proposed here. Our analytical and graphical results provide a useful guide for optimizing the rectification factors of conductive and radiative thermal diodes based on phase-change materials with different geometries
Kianzad, Siamak. "Measurement of Thermal Insulation properties of TBC inside the Combustion chamber." Thesis, Luleå tekniska universitet, Materialvetenskap, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-61917.
Повний текст джерелаMangeon, Gaëtan. "Modélisation au second ordre des transferts thermiques turbulents pour tous types de conditions aux limites thermiques à la paroi." Thesis, Pau, 2020. http://www.theses.fr/2020PAUU3018.
Повний текст джерелаAdvanced modeling of turbulent heat transfer for all thermal boundary conditions is proposed. This work was motivated by two facts: first, the thermal turbulent models used in most of the industrial computations are based on eddy-viscosity models which cannot deal with complex physics such as natural convection or heat transfer in the near-wall region. Then, the thermal boundary condition at the wall (imposed temperature, imposed heat flux, conjugate heat transfer) influences the near-wall behavior of the turbulent thermal variables. The formulation of the low-Reynolds number second moment closure EBDFM (Elliptic Blending Differential Flux Model), which was originally developed for an imposed temperature at the wall, has been extended to an imposed heat flux and a conjugate heat transfer condition. This new formulation is based on rigorous asymptotic analysis of the terms of the transport equation of the turbulent heat flux for all thermal boundary conditions. One of the key elements is the thermal-to-mechanical time-scale ratio R. Its asymptotic behavior highly depends on the thermal boundary condition: R goes to the Prandtl number at the wall for an imposed temperature and tends to infinity otherwise. Thus, solving a transport equation for the temperature variance ¯(θ^'2 ) and for its dissipation rate ε_θ is necessary to reproduce the asymptotic behavior of R. Indeed, these two variables drive the behavior of R in the near-wall region. Therefore, low-Reynolds number models for ¯(θ^'2 ) and ε_θ, valid for all thermal boundary conditions, are proposed. The new formulation of the EBDFM and the models for ¯(θ^'2 ) and ε_θ have been validated by performing Code_Saturne computations of channel flows in the forced convection regime
Wong, William Chiu-Kit. "CFD Flame Spread Model Validation: Multi-Component Data Set Framework." Digital WPI, 2012. https://digitalcommons.wpi.edu/etd-theses/918.
Повний текст джерелаSrinath, S. "Development of Novel Heat Transfer Gauges Based on Large Carbon Clusters to Measure Total as well as Radiative Heat Flux for Planetary Entry Configurations in Hypersonic Shock Tunnels." Thesis, 2017. http://etd.iisc.ac.in/handle/2005/4243.
Повний текст джерелаWang, HsienDe, and 王憲德. "Ray Tracing of Radiative Heat Flux in a Vertical Chamber During Rapid Thermal Processing." Thesis, 2004. http://ndltd.ncl.edu.tw/handle/33733200360025928917.
Повний текст джерелаShiung, Hung-Jia, and 熊宏嘉. "Time Periodic Evaporation Heat Transfer of Refrigerant R-410A due to Refrigerant Flow Rate Oscillation or Heat Flux Oscillation in a Narrow Annular Duct." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/24677965736367363416.
Повний текст джерела國立交通大學
機械工程學系
98
An experiment is carried out in the present study to investigate the characteristics of time periodic evaporation heat transfer for refrigerant R-410A flowing in a horizontal narrow annular duct subject to an imposed time periodic mass flux oscillation or heat flux oscillation. The mass flux oscillation and heat flux oscillation are both in the form of triangular waves. The gap of the duct is fixed at 2.0 mm. In the study, the effects of the refrigerant mass flux oscillation, heat flux oscillation, R-410A saturation temperature, imposed heat flux and vapor quality of the refrigerant on the temporal evaporating flow heat transfer and the photos of the evaporating flow will be examined in detail. The present experiment is conducted for the mean refrigerant mass flux varied from 300 to 500 kg/m2s, the amplitude of the mass flux oscillation is fixed at 10, 20 and 30% with the period of the mass flux oscillation tp fixed at 20, 60 and 120 seconds. Besides, the mean imposed heat flux is varied from 0 KW/ m2 to 15 KW/ m2, the amplitude of the heat flux oscillation is chosen to vary from 10% to 50% of the mean heat flux , and the period of the q oscillation tp is also fixed at 20, 60, 120 seconds. The mean refrigerant saturation temperature is set at 5, 10 and 15 ℃ for the mean refrigerant vapor quality varied from 0.05 to 0.95. The measured evaporation heat transfer data are expressed in terms of the variations of the heated wall temperature and evaporation heat transfer coefficient with time. In the first part of the study the results for the R-410A evaporation subject to the mass flux oscillation are presented. The measured heat transfer data for the R-410A evaporating flow for a constant coolant mass flux are first compared with the time-average data for a time periodic mass flux oscillation. This comparison shows that the mass flux oscillation exerts negligible influences on the time-average evaporation heat transfer. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over wide ranges of the experimental parameters. The results indicate that the Tw oscillation is stronger for higher amplitude and a longer period of the mass flux oscillation. However, a small time lag in the Tw oscillation is also noted. Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and liquid film take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets better as the mass flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the mass flux increases the opposite processes occur. These unusual changes of the heating surface temperature and heat transfer coefficient with the mass flux oscillation are attributed to the strong effects of the mass flux oscillation on the state of the refrigerant at the duct inlet and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. In the second part of the study results for the R-410A evaporation subject to the heat flux oscillation are also presented. Effects of the mean level and oscillation amplitude and period of the heat flux on the time periodic R-410A evaporation heat transfer have been investigated in detail. We first note that the time-average heat transfer coefficients for the time periodic evaporation of R-410A are not affected to a noticeable degree by the amplitude and period of the imposed heat flux oscillation. Then, the heated pipe wall temperature and evaporating flow pattern also oscillate periodically in time and at the same frequency as the heat flux oscillation. Experiment also shows that the resulting oscillation amplitudes of the wall temperature get longer for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. A significant time lag in the heated surface temperature oscillation is also noted, which apparently results from the thermal inertia of the copper inner pipe. Moreover, at the intermediate vapor quality changes in the evaporating flow pattern between that dominated by the nucleation bubbles and liquid film take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets worse as the heat flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the heat flux increases the opposite processes occur. These changes of the heating surface temperature and heat transfer coefficient with the heat flux oscillation are attributed to the strong effects of the heat flux oscillation and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. The effects of heat flux oscillation at extremely short and long periods have been explored. Due to the existence of the thermal inertia of the heated copper duct, the resulting heated surface temperature does not oscillate with time at an extremely short period of the imposed heat flux oscillation. But the oscillation amplitude of the heated surface temperature gets noticeably stronger for an extremely long period of the imposed heat flux oscillation.
Chen, Chien-An, and 陳建安. "Time Periodic Flow Boiling Heat Transfer and Associated Bubble Characteristics of Refrigerant R-134a in a Horizontal Annular Duct due to Flow Rate or Heat Flux Oscillation." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/80296358322824178267.
Повний текст джерела國立交通大學
機械工程學系
98
Experiments have been conducted here to investigate how the imposed time periodic refrigerant flow rate or heat flux oscillation affects the saturated and subcooled flow boiling heat transfer and associated bubble characteristics for refrigerant R-134a in a horizontal narrow annular duct. Besides, the evaporation heat transfer of R-134a flow in the same duct are examined. The test section for the horizontal annular duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to measure the boiling heat transfer coefficient and to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow annular duct. In the study the gap of the duct is varied from 1.0 to 5.0 mm with the mean refrigerant mass flux, saturated temperature, imposed heat flux and mean vapor quality respectively ranging from 100 to 600 kg/m2s, 5 to 15℃, 0 to 45 kW/m2 and 0.05 to 0.95. The inlet subcooling is varied from 3 to 6℃. In particular, attention is focused on the time periodic flow boiling characteristics affected by the mean levels, amplitudes and periods of the flow rate or heat flux oscillation. Some results have been obtained and are reported here. In the first part of the present study, experiments have been carried out to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on the saturated and subcooled flow boiling and evaporation heat transfer and associated bubble characteristics of R-134a in a horizontal narrow annular duct. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial time interval of each periodic cycle. But the intermittent boiling prevails in narrower ranges of the experimental parameters in the subcooled flow boiling. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation is found to negligibly affect the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, active nucleation site density, and evaporating flow pattern are noted to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting Tw oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in dp, f and nac are noted. Specifically, in the first half of the periodic cycle in which the mass flux decreases with time the departing bubbles are larger and the departure rate is lower but the active nucleation site density is higher. The opposite is the case in the second half of the cycle in which the mass flux increases. The effects of the mass flux oscillation on the departing bubble size and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing G in the flow boiling, opposing to that in the single-phase flow. But the bubble characteristics are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the Tw oscillation is also noted. Finally, flow regime maps are provided to delineate the boundaries separating different boiling regimes for the R-134a saturated and subcooled flow boiling in the annular duct. Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and liquid film resulting from the refrigerant flow rate oscillation take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets better as the mass flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the mass flux increases the opposite processes occur. These unusual changes of the heating surface temperature and heat transfer coefficient with the mass flux oscillation are attributed to the strong effects of the mass flux oscillation on the state of the refrigerant at the duct inlet and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. In the second part of the present study, experiments are conducted to investigate how the imposed time periodic heat flux oscillation also in the form of nearly a triangular wave on the refrigerant R-134a saturated and subcooled flow boiling and evaporation heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The results also show that when the mean imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial interval of each periodic cycle. But the intermittent boiling appears in narrower ranges of experimental parameters in the subcooled flow boiling. At somewhat higher heat flux persistent boiling prevails. Besides, the heat flux oscillation does not noticeably affect the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, active nucleation site density, and evaporating flow pattern are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the persistent boiling the resulting oscillation amplitudes of the heated surface temperature, heat transfer coefficient and bubble parameters, such as dp, f and nac, get larger for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. A significant time lag in the Tw oscillation is noted. In the first half of the periodic cycle in which the heat flux decreases with time, after the time lag the heated wall temperature decreases with time, so does the bubble parameters. The opposite processes occur in the second half of the cycle in which q increases with time. Finally, flow regime maps are provided to delineate the boundaries separating different boiling regimes for the R-134a saturated and subcooled flow boiling in the annular duct. Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and by the liquid film resulting from the heat flux oscillation take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets worse as the heat flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the heat flux increases the opposite processes occur. These changes of the heating surface temperature and heat transfer coefficient with the heat flux oscillation are attributed to the strong effects of the heat flux oscillation on the changes of the vapor quality and liquid film thickness in the evaporating flow.
Hsu, Shu-Hao, and 許書豪. "Time Periodic Flow Boiling of Refrigerant R-410A due to Refrigerant Flow Rate or Heat Flux Oscillations in a Horizontal Narrow Annular Duct." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/67889326225694857405.
Повний текст джерела國立交通大學
機械工程學系
98
An experiment is carried out here to investigate the heat transfer and associated bubble characteristics in time periodic flow boiling of refrigerant R-410A in a horizontal narrow annular duct subject to a time periodic mass flux or heat flux oscillation. Both the imposed mass flux and heat flux oscillations are in the form of triangular waves. Effects of the refrigerant mass flux oscillation, heat flux oscillation, and refrigerant saturated temperature on the temporal flow boiling heat transfer and bubble characteristics are examined. The bubble characteristics at the middle axial location of the duct are obtained from the flow visualization of the boiling flow, including the time variations of the bubble departure diameter and frequency and active nucleation site density. The present experiment is conducted for the mean refrigerant mass flux varied from 300 to 500 kg/m2s, the amplitude of the mass flux oscillation is fixed at 10, 20 and 30% and the amplitude of the heat flux oscillation is fixed at 10, 30 and 50% of their respective mean levels and , and the period of the G or q oscillation is fixed at 20, 60 and 120 seconds. The mean refrigerant saturation temperature is set at 5, 10 and 15 ℃. The gap of the duct is fixed at 2.0 mm. The measured boiling heat transfer data are expressed in terms of the boiling curves and boiling heat transfer coefficients along with the time variations of the heated wall temperature. In the first part of the present study the measured heat transfer data for the R-410A flow boiling for a constant coolant mass flux are first compared with the time-average data for the flow subject to a time periodic mass flux oscillation. This comparison shows that the mass flux oscillation exerts negligible influences on the time-average boiling heat transfer coefficients. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over a wide range of the imposed heat flux covering the single-phase, intermittent and persistent boiling flow regimes. The results indicate that the Tw oscillation is stronger for a higher amplitude and a longer period of the mass flux oscillation. However, the mean saturated temperature of the refrigerant exhibits much weaker effects on the Tw oscillation and the mean refrigerant mass flux exerts nonmonotonic effects on the amplitude of the Tw oscillation. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time and at the same frequency as the mass flux oscillation. Furthermore, the oscillations of dp, f and nac are somewhat like triangular waves. In the first half of the cycle in which the mass flux decreases linear increases in dp and nac and linear decrease in f are found. The effect of on nac oscillation is much stronger than on dp and f oscillation causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing G in the flow boiling opposed to that in the single-phase flow. But they are only slightly affected by the period of the mass flux oscillation. Besides, a small time lag in the Tw oscillation is also noted. In the second part of the present study the measured heat transfer data for the R-410A flow boiling for a constant heat flux are also first compared with the time-average data for a time periodic heat flux oscillation. This comparison shows that the time-average heat transfer coefficients are not affected by the time periodic heat flux oscillation to a significant degree. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over a wide range of the mean imposed heat flux covering the single-phase, intermittent and persistent boiling flow regimes. The results indicate that the Tw oscillation gets stronger for a higher amplitude and a longer period of the imposed heat flux oscillation and for a higher mean imposed heat flux. Moreover, a significant time lag in the heated surface temperature oscillation is also noted, which apparently results from the thermal inertia of the copper inner pipe. The effects of the heat flux oscillation at extremely short and long periods have been explored. Due to the existence of the thermal inertia of the heated copper duct, the resulting heated surface temperature does not oscillate with time at an extremely short period of the imposed heat flux oscillation. When the mean imposed heat flux is close to the heat flux corresponding to that for the onset of stable flow boiling, intermittent flow boiling appears. A flow regime map and an empirical correlation are given to delineate the boundaries separating different boiling flow regimes in the annular duct subject to imposed heat flux oscillation. Furthermore, the bubble departure diameter and frequency, and active nucleation site density also oscillate periodically in time and at the same frequency as the heat flux oscillation. The results also show that the oscillations in dp, f and nac get larger for a long period and a larger amplitude of the impose heat flux oscillation and for a higher mean imposed heat flux. Furthermore, the bubbles become smaller and more dispersed after the time lag when the imposed heat flux decreases with time. The opposite processes take place at increasing heat flux.
Link, Roman Mathias. "The role of tree height and wood density for the water use, productivity and hydraulic architecture of tropical trees." Thesis, 2020. http://hdl.handle.net/21.11130/00-1735-0000-0005-13EF-9.
Повний текст джерела