Effect of Gap Distance on Confined Boiling Heat Transfer Over a Saturated Porous Structure

2005 ◽  
Author(s):  
M. J. Schertzer ◽  
M. Khammar ◽  
D. Ewing ◽  
C. Y. Ching ◽  
J. S. Chang
Keyword(s):  
Heat Transfer ◽  
Porous Media ◽  
High Speed ◽  
Imaging System ◽  
Porous Plate ◽  
Heat Removal ◽  
Heat Fluxes ◽  
Maximum Heat ◽  
Porous Wick ◽  
Maximum Heat Transfer

An experimental investigation was performed to study the effect that the introduction of a gap between a heated fin and a porous media would have on the heat removal characteristics of a capillary evaporator. In these experiments, a thin stainless steel resistive foil stretched between two copper electrodes was used to heat a saturated porous plate with an effective pore size of 50 microns. The temperature distribution on a 10 mm wide simulated fin was measured by a high-speed infra-red thermal imaging system. The heat removal performance was investigated for gap distances of 0.00 to 1.00 mm and for heat fluxes of 17 to 180 kW/m2. These results showed that the maximum heat transfer rate that could be achieved before persistent hot spots were observed on the surface increased with gap distance. Local temperature measurements made using thermocouples embedded in the porous media indicate that vapour penetration into the porous wick is intermittent, and that there is no stable single phase blanket of vapour. For a gap distance of 0.00 mm, this penetration is more uniformly distributed across the width of the heated fin than at a gap distance of 0.50 mm. In the latter case, the vapour distribution is much higher near the edge of the heated fin.

Nucleate Boiling From Micro-Machined Surfaces

2003 ◽  
Author(s):  
Adrian M. Holland ◽  
Colin P. Garner
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Imaging System ◽  
Laser System ◽  
Ccd Camera ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Frame Rate ◽  
High Speed Imaging ◽  
Machined Surfaces

This paper discusses the production and use of laser-machined surfaces that provide enhanced nucleate boiling and heat transfer characteristics. The surface features of heated plates are known to have a significant effect on nucleate boiling heat transfer and bubble growth dynamics. Nucleate boiling starts from discrete bubbles that form on surface imperfections, such as cavities or scratches. The gas or vapours trapped in these imperfections serve as nuclei for the bubbles. After inception, the bubbles grow to a certain size and depart from the surface. In this work, special heated surfaces were manufactured by laser machining cavities into polished aluminium plates. This was accomplished with a Nd:YAG laser system, which allowed drilling of cavities of a known diameter. The size range of cavities was 20 to 250 micrometers. The resulting nucleate pool boiling was analysed using a novel high-speed imaging system comprising an infrared laser and high resolution CCD camera. This system was operated up to a 2 kHz frame rate and digital image processing allowed bubbles to be analysed statistically in terms of departure diameter, departure frequency, growth rate, shape and velocity. Data was obtained for heat fluxes up to 60 kW.m−2. Bubble measurements were obtained working with water at atmospheric pressure. The surface cavity diameters were selected to control the temperature at which vapour bubbles started to grow on the surface. The selected size and spacing of the cavities was also explored to provide optimal heat transfer.

Numerical Investigation on Pool Boiling Over a Vertical Tube Coupled With In-Tube Condensation

2019 ◽  
Author(s):  
Shuai Ren ◽  
Wenzhong Zhou
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Boiling Heat Transfer ◽  
Power Plants ◽  
Pool Boiling ◽  
Heat Removal ◽  
Vertical Tube ◽  
Two Phase ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Abstract Pool boiling and in-tube condensation phenomena have been investigated intensively during the past decades, due to the superior heat transfer capacity of the phase change process. In passive heat removal heat exchangers of nuclear power plants, the two phase-change phenomena usually occur simultaneously on both sides of the tube wall to achieve the maximum heat transfer efficiency. However, the studies on the effects of in-tube condensation on external pool boiling heat transfer are very limited, especially in numerical computation aspect. In the present study, the saturated pooling boiling over a vertical tube under the influences of in-tube steam condensation is investigated numerically. The Volume of Fluid (VOF) interface tracking method is employed based on the 2D axisymmetric Euler-Euler multiphase frame. The phase change model combining with a mathematical smoothing algorithm and a temporal relaxation procedure has been implemented into CFD platform by user defined functions (UDFs). The two-phase flow pattern and bubble behavior have been analyzed. The effects of inlet steam mass flow rate on boiling heat transfer are discussed.

Guidelines for Designing Micropillar Structures for Enhanced Evaporative Heat Transfer

2021 ◽  
Author(s):  
Kidus Guye ◽  
De Dong ◽  
Yunseo Kim ◽  
Hyoungsoon Lee ◽  
Baris Dogruoz ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Interfacial Area ◽  
High Heat ◽  
Heat Fluxes ◽  
Air Cooling ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Evaporation Heat

Abstract Over the last several decades, cooling technologies have been developed to address the growing thermal challenges associated with high-powered electronics. However, within the next several years, the heat generated by these devices is predicted to exceed 1 kW/cm2, and traditional methods, such as air cooling, are limited in their capacities to dissipate such high heat fluxes. In contrast, two-phase cooling methods, such as microdroplet evaporation, are very promising due to the large latent heat of vaporization associated with the phase change process. Previous studies have shown non-axisymmetric droplets exhibit different evaporation characteristics than spherical droplets. For a droplet pinned atop a micropillar, the solid-liquid and liquid-vapor interfacial area, the volume, and thickness of the droplet are the major factors that govern the evaporation heat transport process. In this work, we develop a shape optimization tool using the particle swarm optimization algorithm to maximize evaporation from a droplet confined atop a micropillar. The tool is used to optimize the shape of a nonaxisymmetric droplet. Compared to droplets atop circular and regular equilateral triangular micropillar structures, we find that droplets confined on pseudo-triangular micropillar structures have 23.7% and 5.7% higher heat transfer coefficients, respectively. The results of this work will advance the design of microstructures that support droplets with maximum heat transfer performance.

Heat Transfer to Water-Oxygen Mixtures at High Pressure

2002 ◽  
Author(s):  
S. N. Rogak ◽  
S. Boskovic ◽  
D. Faraji
Keyword(s):  
Heat Transfer ◽  
Heat Capacity ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Water Oxidation ◽  
Pure Water ◽  
Heat Fluxes ◽  
Maximum Heat ◽  
Water Oxygen ◽  
Maximum Heat Transfer

The constant pressure heat capacity and forced convection heat transfer coefficient was measured in a horizontal, smooth, electrically-heated tube. For the supercritical pressures considered, flow rates and temperatures (330–430 °C), the flow in the 6.2 mm ID tube was fully turbulent. The fluid was distilled water and up to 9 wt% oxygen. This mixture and the experimental conditions are found in supercritical water oxidation systems. At subcritical temperatures, the oxygen and water are almost immiscible, but just below the critical temperature, the fluid becomes single-phase. By measuring bulk and surface temperatures, knowing the mass and heat flux, both the heat capacity and heat transfer coefficient could be measured. The water-oxygen system is a highly non-ideal mixture, and small amounts of oxygen significantly reduce the temperature at which maximum heat transfer occurs. The changes in heat capacity appear to dominate the effect of oxygen on heat transfer, however, the mixtures do exhibit heat transfer deterioration at slightly subcritical temperatures, at flows and heat fluxes for which pure water shows nothing similar.

scholarly journals Computational Fluid Dynamics of Heat Transfer in Stirred Tank Reactors

2021 ◽  
Author(s):  
Chaitanya Moholkar ◽  
Punit Gharat ◽  
Vivek Vitankar ◽  
Channamallikarjun Mathpati ◽  
Jyeshtharaj Joshi
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Reynolds Number ◽  
Heat Removal ◽  
Design Parameters ◽  
Operational Parameters ◽  
Power Requirement ◽  
Maximum Heat ◽  
Maximum Heat Transfer

In the present work, computational fluid dynamics study of stirred tanks of three sizes (20L, 400L and 5000L) provided with helical coils has been carried out. Various design parameters (impeller diameter, type and clearance) and operational parameters (Reynolds Number and Power per unit volume) have been varied and their effect on process side heat transfer coefficient has been studied. CFD model is validated with experimental work of Cummings and West[9] and in house experimentation. Design settings of D/T=0.5, C/T=0.33 for PBTD450 resulted in maximum heat transfer (5440 W/m2K for P/V=1000 W/m3). For constant RPM and constant D/T (Constant Reynolds Number), Increasing the power number of impeller increased process side HTC at the cost of increased power requirement (decreasing efficiency). In such cases, proper selection of impeller system needs to be made based on the requirements of heat removal and optimizing parameters such as product yield, product quality etc.

Heat Transfer Enhancement of Viscoelastic Fluid in the Rectangle Microchannel with Constant Heat Fluxes

2011 ◽  
Vol 117-119 ◽  
pp. 574-581
Author(s):  
Guo Fa Zhou ◽  
Ting Peng
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Secondary Flow ◽  
Viscoelastic Fluid ◽  
Radial Flow ◽  
Heat Fluxes ◽  
Transfer Enhancement ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Macro Scale

It has been found that viscoelastic fluid has evident heat transfer enhancement function in macro scale. But in micro scale, viscoelastic fluid’s flow and heat transfer characteristics are still unknown. In this paper, the heat transfer process of viscoelastic fluid in the microchannel is studied by numerical simulation method. The simulation results show that the maximum heat transfer enhancement of viscoelastic fluid is up to 800%, compared with pure viscous fluid. The viscoelastic fluid has such obvious heat transfer enhancement function because of its strong secondary flow. Laminar sub-layer can be damaged by the strong secondary flow, and thus radial flow generates in laminar sub-layer. The radial flow can increase the interference and mixing effect, and enhances fluid’s turbulence and convection which can enhance heat transfer as a result. So the heat transfer enhancement depends on the intensity of secondary flow which is caused by the second normal stress difference, and it will increase with the raise of the flow rate.

Correlation for Prediction of Peak Heat Flux Achieved With a Bi-Porous Wick

2009 ◽  
Author(s):  
Ladan Amouzegar ◽  
Ivan Catton ◽  
Aleksander Vadnjal
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Material Properties ◽  
Working Fluid ◽  
Maximum Heat ◽  
Porous Wick ◽  
Maximum Heat Transfer ◽  
Capillary Limit ◽  
Small Pore ◽  
Fluid Properties

In the past researchers noted three distinct stages of evaporative heat transfer in a bi-porous wick. The maximum heat transfer rate is postulated to occur at the end of the second stage when the maximum number of small pores interfaces the vapor space. For optimization purposes a reliable model that relates the maximum heat flux of a bi-porous wick to the wick material properties, wick geometry given with average large and small pore diameter, wick thickness, and working fluid properties is demanded. In this work, a semi-empirical model that relates the heat flux at the capillary limit to the wick material properties, working fluid properties and wick dimensions is further developed. The model is based on the hydrodynamics of the capillary limit. The result is employed to qualitatively and quantitatively optimize the wick parameters for some specific cases and the optimization can be further performed using the proposed model.

scholarly journals The usage of infrared thermography to investigate spatial variations of heat transfer at spray cooling

2019 ◽  
Vol 196 ◽  
pp. 00055
Author(s):  
Anton Surtaev ◽  
Aleksandr Nazarov ◽  
Anatoliy Serov ◽  
Nikolay Miskiv ◽  
Vladimir Serdyukov
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Infrared Thermography ◽  
High Speed ◽  
Heating Surface ◽  
Spray Cooling ◽  
Spray Parameters ◽  
Maximum Heat ◽  
Maximum Heat Transfer

In present paper new approach to study heat transfer at spray cooling, based on the using of high-speed infrared thermography with high spatial resolution is proposed. Also in the paper new data on basic spray parameters, including sizes and velocities of droplets at different pressure at the nozzle inlet were obtained with the use of shadow technique and high-speed video camera. It is found, that heat transfer coefficient is unequally spatially distributed value and essentially depends on flow rate in the stationary irrigation mode. The dependence of heat transfer coefficient on a distance between spray source and heat exchange surface is obtained and an optimal distance corresponding to the maximum heat transfer intensity at present configuration of irrigation points relatively to the heating surface is determined.

Enhanced Pool Boiling With HFE7000 Over Selectively Sintered Microchannels

2017 ◽  
Author(s):  
Travis S. Emery ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Heat Dissipation ◽  
Pool Boiling ◽  
Electronics Cooling ◽  
Fold Increase ◽  
High Heat ◽  
Heat Fluxes ◽  
Dielectric Fluid ◽  
Maximum Heat ◽  
Maximum Heat Transfer

As the need for efficient thermal management grows, pool boiling’s ability to dissipate high heat fluxes has gained significant interest. The objective of this work was to study the performance of pool boiling at atmospheric pressure using a dielectric fluid, HFE7000. Both plain and enhanced copper surfaces were tested, and these results were then compared to similar testing performed with water and FC-87. The enhanced surfaces utilized microchannels with porous coatings selectively located on different regions of the heat transfer surface. A maximum critical heat flux (CHF) of 41.7 W/cm2 was achieved here, which translated to a 29% CHF increase in comparison to a plain chip. A maximum heat transfer coefficient (HTC) of 104.0 kW/m2°C was also achieved, which translated to a 6-fold increase in HTC when compared to a plain copper chip. More notably, this HTC was achieved at a wall temperature of 38.4 °C. This HTC enhancement was greater than that of water and FC-87 when using the same enhanced surface. The effect of sintering location was found to have a similar effect on CHF with HFE7000 in comparison with water. The effect of microchannel size was shown to have similar effects on CHF when compared with FC-87 and water. From the results found here, it is concluded that the employment of selectively sintered open microchannels with HFE7000 has significant potential for enhanced heat dissipation in electronics cooling applications.

Performance of a Flexible Evaporator for Loop Heat Pipe Applications

2008 ◽  
Author(s):  
Mukta S. Limaye ◽  
James F. Klausner
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Heat Fluxes ◽  
Loop Heat Pipe ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Maximum Heat Flux

A flat and flexible evaporator, which conforms to contoured surfaces, has been developed for loop heat pipe applications. A loop heat pipe (LHP) is a passive, two phase heat transfer device that uses a porous membrane in the evaporator to circulate fluid. A number of flexible membranes have been tested as evaporator wicks that have a length of 12.7 cm and heated area of 50.6 cm2. For cellulose, polyethylene, and blotting paper membranes, maximum heat fluxes of 0.43, 1.5 and 2.9 W/cm2 have been observed, respectively. The maximum heat transfer coefficients measured for these membranes are 551, 876, and 2100 W/m2-K, respectively. The best performance was observed by a membrane made of a fibrous cotton matrix, typically used as gauze. This material has a large pore size and high wettability with water. When tested in a rigid, brass evaporator, the maximum heat flux observed is 5.95 W/cm2, and the maximum heat transfer coefficient is 2865 W/m2-K. A flexible evaporator is fabricated using a heat sealable, flexible barrier pouch, and the cotton matrix membrane is sealed inside. The maximum measured heat flux for the flexible evaporator is 3.2 W/cm2 and maximum measured heat transfer coefficient is 1165 W/m2-K. The observed reduction in heat transfer as compared to the rigid evaporator is due to the poor contact between the evaporator and membrane. It is concluded that for the flexible evaporator membranes considered, the heat transfer mechanism is boiling and the maximum heat flux is limited by the wicking rate of the membrane. For a given membrane, the wicking rate increases with a reduction in the wicking length and decreases with an increasing rate of evaporation. To further improve the performance of the flexible evaporator, it is necessary to ensure efficient vapor removal from the evaporator as well as maintaining good contact between the membrane and the evaporator surface.

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