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.

Experimental Investigation of an Evaporator Enhanced With a Micro-Porous Structure in a Two-Phase Thermosyphon Loop

2008 ◽  
Author(s):  
Richard Furberg ◽  
Rahmatollah Khodabandeh ◽  
Bjo¨rn Palm ◽  
Shanghua Li ◽  
Muhammet Toprak ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Rectangular Channel ◽  
Large Diameter ◽  
Nucleate Boiling ◽  
Nano Particles ◽  
Working Fluid ◽  
Two Phase ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Following is an experimental study of six different evaporators in a closed two-phase thermosyphon loop system, where the influence of various evaporator dimensions and surfaces was investigated. The evaporators featured a 30 mm long rectangular channel with hydraulic diameters ranging from 1.2–2.7 mm. The heat transfer surface of one of the tested evaporators was enhanced with copper nano-particles, dendritically connected into an ordered micro-porous three dimensional network structure. To facilitate high speed video visualization of the two-phase flow in the evaporator channel, a transparent polycarbonate window was attached to the front of the evaporators. Refrigerant 134A was used as a working fluid and the tests were conducted at 6.5 bar. The tests showed that increasing channel diameters generally performed better. The three largest evaporator channels exhibited comparable performance, with a maximum heat transfer coefficient of about 2.2 W/(cm2K) at a heat flux of 30–35 W/cm2 and a critical heat flux of around 50 W/cm2. Isolated bubbles characterized the flow regime at peak performance for the large diameter channels, while confined bubbles and chaotic churn flow typified the evaporators with small diameters. In line with previous pool boiling experiments, the nucleate boiling mechanism was significantly enhanced, up to 4 times, by the nano- and micro-porous enhancement structure.

scholarly journals The effect of operating parameters on the heat transfer in the heat pipe

2021 ◽  
Vol 2119 (1) ◽  
pp. 012088
Author(s):  
A. A. Litvintceva ◽  
N. I. Volkov ◽  
N. I. Vorogushina ◽  
V. A. Moskovskikh ◽  
V. V. Cheverda
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Temperature Difference ◽  
Heat Pipes ◽  
Working Fluid ◽  
Operating Parameters ◽  
Temperature Stabilization ◽  
Ir Camera ◽  
Fluid Properties

Abstract Heat pipes are a good solution for temperature stabilization, for example, of microelectronics, because these kinds of systems are without any moving parts. Experimental research of the effect of operating parameters on the heat transfer in a cylindrical heat pipe has been conducted. The effect of the working fluid properties and the porous layer thickness on the heat flux and temperature difference in the heat pipe has been investigated. The temperature field of the heat pipe has been investigated using the IR-camera and K-type thermocouples. The data obtained by IR-camera and K-type thermocouples have been compared. It is demonstrated the power transferred from the evaporator to the condenser is a linear function of the temperature difference between them.

Optimization of Wettability Gradient for Enhancement of Thermal Performance of Micro Heat Pipes

2018 ◽  
Author(s):  
Manjinder Singh ◽  
Naresh Varma Datla ◽  
Supreet Singh Bahga ◽  
Sasidhar Kondaraju
Keyword(s):  
Heat Transfer ◽  
Thermal Performance ◽  
Variable Mass ◽  
Working Fluid ◽  
Maximum Heat ◽  
Continuous Increase ◽  
Maximum Heat Transfer ◽  
Integration Density ◽  
Heat Transfer Capacity ◽  
Micro Heat Pipes

Continuous increase in the integration density of microelectronic units necessitates the use of MHPs with enhanced thermal performance. Recently, the use of wettability gradients have been shown to enhance the heat transfer capacity of MHPs. In this paper, we present an optimization of axial wettability gradient to maximize the heat transfer capacity of the MHP. We use an experimentally validated mathematical model and interior point method to optimize the wettability gradient. For our analysis, we consider two cases wherein (i) the mass of working fluid is constrained, (ii) mass of working fluid is a design variable. Compared to MHP with uniform high wettability and filled with a fixed mass of working fluid, optimization of the wettability gradient leads to 65% enhancement in heat transfer capacity. Similar comparisons for MHP filled with variable mass of working fluid shows more than 90% increase in the maximum heat transfer capacity due to optimization of wettability gradient.

Static configuration of liquid films on horizontal tubes with low radial fins: implications for condensation heat transfer

1987 ◽  
Vol 410 (1838) ◽  
pp. 125-139 ◽  
Keyword(s):  
Heat Transfer ◽  
Liquid Films ◽  
Finned Tube ◽  
Tube Surface ◽  
Maximum Heat ◽  
Finned Tubes ◽  
Horizontal Tubes ◽  
Maximum Heat Transfer ◽  
Fluid Properties ◽  
Static Conditions

The configuration of a liquid film retained by surface tension forces on horizontal low-finned tubes has been analysed. It has been shown that liquid is retained on the upper parts of the tube surface previously regarded as 'unflooded'. The meniscus in a radial plane has been shown to approximate to a circular arc, with radius dependent only on the distance from the bottom of the tube, fluid properties and gravity. Four ‘flooding’ conditions are identified for trapezoidal-section fins and the positions around a tube at which these occur have been determined. Experimental measurements, for condensation of three fluids on tubes with rectangular-section fins, suggest that maximum heat-transfer en­hancement occurs when the spacing between the fins (the only geometric variable in these tests) is such as to maximize the ‘unblanked’ (by retained liquid under static conditions) finned tube surface area. The ‘unblanked’ or ‘active’ area is increased by using a radiused fillet at the fin root rather than a sharp corner. This might also be expected to give a corresponding increase in heat-transfer enhancement during condensation.

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.

Numerical Analysis of Natural Convective Heat Transfer over a Vertical Cylinder Using External Fins

2015 ◽  
Vol 813-814 ◽  
pp. 707-712
Author(s):  
Anwesha Panigrahi ◽  
D.P. Mishra ◽  
Deepak Kumar
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Numerical Study ◽  
Convection Heat Transfer ◽  
Vertical Cylinder ◽  
Working Fluid ◽  
Natural Convection Heat Transfer ◽  
Maximum Heat ◽  
Optimum Parameters ◽  
Maximum Heat Transfer

The present numerical study deals with the natural convection heat transfer on the surface of a vertical cylinder with external longitudinal fins. The aim of the study was to determine the effects of geometric parameters like fin height, fin number and fin shape on the heat transfer and thus obtain the optimum parameters that will maximize the rate of heat transfer have been discussed. The numerical investigation consists of an aluminium cylinder of length 1m and diameter 0.07m with air as the working fluid. It has been seen from the numerical investigation that the heat transfer increases with fin height. It is also observed that there exists optimum fin number for maximum heat transfer. Keeping the fin number, fin height and volume fixed, it was found that the heat transfer is maximum for rectangular shaped fin.

Enhanced Pool Boiling Performance on Micro-, Nano-, and Hybrid-Structured Surfaces

2011 ◽  
Author(s):  
Qian Li ◽  
Wei Wang ◽  
Chris Oshman ◽  
Benoit Latour ◽  
Chen Li ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Thermal Management ◽  
Pool Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Maximum Heat ◽  
Structured Surfaces ◽  
Maximum Heat Transfer ◽  
Functional Components

Thermal management plays an important role in both high power electronics and energy conversion systems. A key issue in thermal management is the dissipation of the high heat flux generated by functional components. In this paper, various microstructures, nanostructures and hybrid micro/nano-structures were successfully fabricated on copper (Cu) surfaces, and the corresponding pool boiling heat transfer performance was systematically studied. It is found that the critical heat flux (CHF) of hybrid structured surfaces is about 15% higher than that of the surfaces with nanowires only and micro-pillars only. More importantly, the superheat at CHF for the hybrid structured surface is much smaller than that of the micro-pillared surface (about 35%), and a maximum heat transfer coefficient (HTC) of about 90,000W/m2K is obtained. Compared with the known best pool boiling performance on biporous media, a much larger HTC and much lower superheat at a heat flux of 250W/cm2 have been obtained on the novel hybrid-structured surfaces.

Modelling Heat Transfer Performance of a Thin Bi-Porous Evaporator for a Heat Pipe

2005 ◽  
Author(s):  
Aleksander Vadnjal ◽  
Ivan Catton
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
High Heat ◽  
Heat Transfer Process ◽  
Heat Fluxes ◽  
Capillary Structure ◽  
Porous Wick ◽  
Capillary Limit ◽  
Porous Capillary Structure ◽  
Selection Of

The evaporator of a heat exchanger is made with a porous, capillary, structure. In the past researchers [7] noticed that the heat flux limits of a bi-porous capillary structure is much greater than that of a mono-porous capillary structure and will be the focus of this work. There are three distinct stages in the heat transfer process in a bi-porous wick. Each of the stages is explored in turn. In the first stage, heat is transferred from the wall across the saturated wick by pure conduction to the evaporating front located on the top of the bi-porous wick. When the boiling limit is reached, bubbles begin to nucleate and the second stage begins. The boiling becomes more and more intensive as the heat flux is increased until all of the liquid from big pores is evaporated, and only small pores remain wetted with liquid. The point reached here is called the capillary limit, which is basically the limit at which the capillary forces are still sufficient to provide the liquid for evaporation into the big pores. The modelling of the different thermal physical processes determining heat transfer within each of the three stages for a bi-porous heat wick are modelled and significant improvement in achievable heat flux is observed. Comparison with experiment is found to be reasonable. Optimal selection of the bi-porous wick characteristics is shown to yield very high heat fluxes.

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