Maximum Heat Transfer Capacity Research Articles

Flat Miniature Heat Pipes for Electronics Cooling: State of the Art, Experimental and Theoretical Analysis

An experimental study is realized in order to verify the Mini Heat Pipe (MHP) concept for cooling high power dissipation electronic components and determines the potential advantages of constructing mini channels as an integrated part of a flat heat pipe. A Flat Mini Heat Pipe (FMHP) prototype including a capillary structure composed of parallel rectangular microchannels is manufactured and a filling apparatus is developed in order to charge the FMHP. The heat transfer improvement obtained by comparing the heat pipe thermal resistance to the heat conduction thermal resistance of a copper plate having the same dimensions as the tested FMHP is demonstrated for different heat input flux rates. Moreover, the heat transfer in the evaporator and condenser sections are analyzed, and heat transfer laws are proposed. In the theoretical part of this work, a detailed mathematical model of a FMHP with axial microchannels is developed in which the fluid flow is considered along with the heat and mass transfer processes during evaporation and condensation. The model is based on the equations for the mass, momentum and energy conservation, which are written for the evaporator, adiabatic, and condenser zones. The model, which permits to simulate several shapes of microchannels, can predict the maximum heat transfer capacity of FMHP, the optimal fluid mass, and the flow and thermal parameters along the FMHP. The comparison between experimental and model results shows the good ability of the numerical model to predict the axial temperature distribution along the FMHP.

THERMAL CONTROL OF LOOP HEAT PIPE WITH PRESSURE REGULATING VALVE

Innovative approach to regulate operation temperature of LHP with two-way valve is discussed. Comparison with traditional three-way bypass valve is done. It was investigated that two-way valve design provides better ability of thermal control as traditional three-way design and has significant advantages. Some examples of two-way valve application is presented. Heat transfer limitation of LHP with PRV was verified. Both modes of PRV application demonstrated practically the same maximum heat transfer rate of LHP. It was also obtained that maximum heat transfer capacity of LHP with valve is higher than for LHP without valve. Start up ability of LHP with and without PRV was compared. It was found that PRV can facilitate start up.

Analysis of the parameters of the sintered loop heat pipe

AbstractThe purpose of this paper is to establish an experimental formula for sintered dendritic nickel powder. For this reason, wick structures with different porosity ranging from 65 to 80% were fabricated by cold pressing sintering process at fixed porosity and their parameters that included porosity, pore radius, and permeability were also measured. According to both the capillary limitation and the present experimental formula of the sintered dendritic nickel powder, the wick structure parameters that would affect the heat transfer capacity of the loop heat pipe (LHP) were analyzed theoretically and then investigated experimentally. The results showed that there exists an optimal combination of wick structure parameters by which the performance of the LHP would achieve optimization. The maximum heat transfer capacity was up to 500 W and the thermal resistance was 0.12°C/W at the allowable working temperature 80°C. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 515–526, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20034

Design of miniature loop heat pipe

AbstractIn the present study, the loop heat pipe (LHP) was miniaturized for application to electronic cooling. According to the capillary limitation, the wick structure parameters that would affect the heat transfer capacity were analyzed theoretically. Among the various wick parameters, this study especially investigated the effect of wick thickness, which has rarely been mentioned in the literature. Here, various thicknesses were analyzed theoretically and then tested experimentally. The results showed that the temperature on the evaporator wall dropped with decreasing wick thickness. This effect would lead to the raising of heat transfer capacity and the descending of thermal resistance. According to the analysis and the practical demand for electronic cooling, a miniature LHP was fabricated with the evaporator outer diameter of 13 mm and the evaporator length of 50 mm. The testing results showed that, at the allowable working temperature of 80 °C, the maximum heat transfer capacity was up to 200 W and the thermal resistance was 0.17 °C/W. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 42–52, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10133

Thermal Characteristics of Conventional and Flat Miniature Axially Grooved Heat Pipes

A detailed mathematical model of low-temperature axially grooved heat pipes (AGHP) is developed in which the fluid circulation is considered along with the heat and mass transfer processes during evaporation and condensation. The results obtained are compared to existing experimental data. Both capillary and boiling limitations are found to be important for the flat miniature copper-water heat pipe, which is capable of withstanding heat fluxes on the order of 40 W/cm2 applied to the evaporator wall in the vertical position. The influence of the geometry of the grooved surface on the maximum heat transfer capacity of the miniature AGHP is demonstrated.

Flooding and heat transfer limits in horizontal and inclined two-phase thermosiphons

Results are presented of experimental studies of the limiting heat fluxes transferred by horizontal and slightly inclined thermosiphons. An optimum filling ratio providing maximum values of the limiting heat fluxes is established. It is shown that if the filling ratio is below its optimum value, there occur heat transfer crises due to an insufficient amount of liquid being delivered into the evaporator and due to drying out of the heating surface. If the filling ratio exceeds its optimum, the heat transfer crisis is the consequence of the loss of the liquid-vapor counterflow stability in the transport zone of the thermosiphon. Generalizations are given with respect to the optimum filling ratio and maximum heat transfer capacity of thermosiphons.

Thermal Analysis of a Micro Heat Pipe

A detailed mathematical model is developed in which the heat and mass transfer processes in a micro heat pipe (MHP) are examined. The model describes the distribution of the liquid in a MHP and its thermal characteristics depending upon the liquid charge and the applied heat load. The liquid flow in the triangular-shaped corners of a MHP with polygonal cross section is considered by accounting for the variation of the curvature of the free liquid surface and the interfacial shear stresses due to a liquid-vapor frictional interaction. The predicted results obtained are compared to existing experimental data. The importance of the liquid fill, minimum wetting contact angle, and the shear stresses at the liquid-vapor interface in predicting the maximum heat transfer capacity and thermal resistance of the MHP is demonstrated.

Heat pipe cathode-deposited capillary structures and heat exchangers based on them

Lately, heat pipes (HP) have found increased application in the cooling systems of radio electronic equipment. The problem of designing HPs with minimum thermal resistance and maximum heat transfer capacity is a real one. One of the ways to solve this problem is by designing new HP powder capillary structures (CS). These possess a number of advantages compared with the known CS. HPs with CSs obtained by the method of cathode electrolyte deposition were studied as an example. The main CS parameters are determined, these being porosity, permeability, capillary rise height and their dependence on the deposition conditions. Experimental HP samples were made; heat transfer performances and their dependence on the deposition conditions are determined. It is shown that the thermal resistance can be reduced to 0.015 K W −1, and the maximum heat transfer rate obtained was of the order of 10 5 W m −2. An example of air-air-type heat exchanger for cooling a power supply cabinet for numerically programmed control devices is made using HPs with cathode-deposited CS. Test results showed that the heat exchanger was reasonable for the given purpose, power dissipation being up to 1.5 kW.

Maximum heat-transfer capacity of a vertical two-phase thermal siphon

A survey of the experimental data on the maximum heat-transfer capacity of a two-phase thermal siphon is presented; a physical model that describes many of the experimental data on the heat-transfer limits for thermal siphons is proposed.

A two-phase closed thermosyphon

An experimental study on the heat transfer performance of a two-phase closed thermosyphon together with a simple theoretical analysis for its maximum heat transfer capacity has been made. Water and Freon-11 were used as the working fluids. Out of many possible controlling variables, the effects of (a) the amount of working fluid in the tube, (b) the ratio of heated-length to cooled length, (c) the operating pressure, (d) the heat flux and (e) the working fluid, were investigated.