Yiding Cao

Wickless network heat pipes for high heat flux spreading applications

Abstract The concept of the network heat pipe employing the boiling heat-transfer mechanism in a narrow space is described. Two flat-plate wickless network heat-pipes (or thermal spreaders) are designed, fabricated, and tested based on this concept. The fabricated thermal spreaders, which are made of copper or aluminum, are wickless, cross-grooved heat transfer devices that spread a concentrated heat source to a much larger surface area. As a result, the high heat flux generated in the concentrated heat source may be dissipated through a finned surface by air cooling. The network heat pipes are tested under different working conditions and orientations relative to the gravity vector, with water and methanol as the working fluids. The maximum heat fluxes achieved are about 40 W / cm 2 for methanol and 110 W / cm 2 for water with a total heat input of 393 W.

Closed-form analytical solutions for radially rotating miniature high-temperature heat pipes including non-condensable gas effects

Abstract The radially rotating miniature high-temperature heat pipe is a wickless heat pipe, which has a simple structure and low manufacturing cost, and can withstand strong vibrations in a high-temperature environment. In this paper, the radially rotating miniature high-temperature heat pipe having a diameter in the range of 1.5–2 mm is analyzed by employing appropriate flow and heat transfer models as well as experimental investigation. The diffuse effects of non-condensable gases on temperature distribution along the heat pipe length are investigated. Closed-form analytical solutions for the temperature distribution along the heat pipe length are obtained and extensive experimental tests are undertaken. These closed-form analytical solutions are in good agreement with the experimental data. The theoretical and experimental studies prove that the radially rotating miniature high-temperature heat pipe with sodium as the working fluid has a very large heat transfer capability and a high effective thermal conductance that is 60–100 times higher than the thermal conductivity of copper. Although the diffuse effects of the non-condensable gases would increase temperature drop along the heat pipe length, the heat pipes can still work effectively and reliably. As a result, the combination of the traditional air-cooling technology with radially rotating miniature heat pipes is a feasible and effective cooling means for the rotor blades of a high-temperature gas turbine.

Reciprocating heat pipes and their applications

Based on the engine piston cooling application, reciprocating heat pipes and an engine piston incorporating reciprocating heat pipes have been proposed

Experimental Investigations and Correlations for the Performance of Reciprocating Heat Pipes

Reciprocating heat pipes are novel heat pipes that are being developed for engine piston applications. These heat pipes have a high effective thermal conductance due to the impinging effects of liquid particles in the heat pipe. In this investigation, semiempirical correlations for the dimensionless temperature distribution and effective thermal conductivity of heat pipes are derived. Extensive experiments are conducted to investigate the effects of thermal and geometrical conditions on the performance of reciprocating heat pipes, and a large number of experimental data are generated. Experimental results indicate that the effective thermal conductance of the reciprocating heat pipe can be more than 300 times that of a solid copper bar of equal size. Comparison of the experimental data and correlation shows that the experimental data can be correlated to within ±30% by the correlation derived, which provides a quantitative relation for heat pipe design purposes.

Flat and U-shaped heat spreaders for high-power electronics

To achieve a high heat-flux level and reduce manufacturing costs associated with conventional heat pipes, the concept of network heat spreaders employing a boiling heat-transfer mechanism in a narrow space had been proposed, and several flat-plate wickless heat spreaders had been designed and fabricated. The heat spreaders had been tested under different working conditions and orientations relative to gravity with very good results. The previously tested network heat spreaders, however, were based on plates with a relatively large size for general heat spreading purposes. In the present study, network heat spreaders with overall dimensions of 78 2 62 2 3.2 mm are designed and fabricated. Spreaders of this size are intended for use as heat sinks of high-power electronic components. External cooling fins are attached to enhance air-cooling heat transfer rate. The network heat spreaders are tested under various working conditions with water as the working fluid. The maximum heat input rate achieved is about 150 W with a corresponding heat flux of 60 W/cm 2 . Compared to the performance of a solid copper plate having the same overall size as the spreader, the maximum temperature difference over the surface is reduced from about 32°C to 3.3°C. The heat transfer performance of the spreader is also largely dependent on the filling ratio of the working fluid and the boiling heat transfer in the narrow space. For these reasons, boiling heat transfer mechanisms in a narrow space are analyzed, and a spreader design that would improve the performance in a horizontal position is described.

Piston Assembly Design for Improved Thermal-Tribological Performance

The tribological system in the piston assembly of an internal combustion engine includes contacts at interfaces of piston/piston ring/cylinder liner, piston skirt/cylinder wall, and piston/piston pin/connecting rod. The thermal and tribological properties of the piston, piston rings, and cylinder wall are critical to the life and quality of the engine. Severe wear and scuffing failure, especially at the ring/ring groove and ring/liner interfaces, may present a major problem if the piston temperature is too high. Temperature considerations for the piston often limit the effort to increase the engine power. A new engine piston incorporating the heat pipe cooling technology has been developed for reducing the piston temperature, especially in the ring land and along the piston wall. The current work aims at investigating the effect of reciprocating heat pipes on heat conduction in the piston, and thus the tribological behavior of the piston assembly. Due to the high thermal conductance of the reciprocating heat pipe, a considerably large amount of combustion heat, which is conventionally conducted through the piston wall, is transferred through heat pipes. This new design will result in a lower temperature on the piston wall and a reasonably low temperature distribution in the piston.

Development of an Isothermal Journal Bearing Employing Heat-Pipe Cooling Technology

This paper reports the development of an isothermal journal bearing by employing heat-pipe cooling technology for improved thermal-tribological performance. A stainless-steel bearing with a number of heat-pipe grooves which use methanol as the working fluid was designed and constructed to verify the heat-transfer mechanism in the isothermal journal bearing. The constructed journal bearing was tested to evaluate its thermal performance. The experimental results indicated that the heat pipe uniformly distributes the “frictional heat” along the entire circumference of the bearing. As a result, the journal bearing becomes a nearly isothermal element. Experimental data also indicate that with the heat pipe as a heat sink, the bearing can work at a much higher thermal load. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

Critical working frequency of reciprocating heat-transfer devices in axially reciprocating mechanisms

Abstract The reciprocating heat-transfer device or reciprocating heat pipe has a high effective thermal conductance, and applications can be found in various reciprocating mechanisms, including offset cam mechanisms, Scotch yoke mechanisms, and piston assemblies of internal combustion engines. The liquid return mechanism presents the most important operating limitation for this type of heat-transfer device. In this paper, the potential applications of the reciprocating heat-transfer device to various reciprocating mechanisms are described. Theoretical analyses are performed for the critical working frequency of the device in connection with the reciprocating mechanisms. An analytical solution and semi-empirical correlation, with and without taking into account the flow resistance, are obtained. Extensive experiments for the critical working frequency are then undertaken on an engine tester. Comparison of the analytical and experimental results indicates that the analytical solutions and semi-empirical correlation provide an accurate prediction for the working limits of reciprocating heat-transfer devices.

Heat Pipe Solar Receivers for Concentrating Solar Power (CSP) Plants

This paper introduces separate-type heat pipe (STHP) based solar receiver systems that enable more efficient operation of concentrated solar power plants without relying on a heat transfer fluid. The solar receiver system may consist of a number of STHP modules that receive concentrated solar flux from a solar collector system, spread the high concentrated solar flux to a low heat flux level, and effectively transfer the received heat to the working fluid of a heat engine to enable a higher working temperature and higher plant efficiency. In general, the introduced STHP solar receiver has characteristics of high heat transfer capacity, high heat transfer coefficient in the evaporator to handle a high concentrated solar flux, non-condensable gas release mechanism, and lower costs. The STHP receiver in a solar plant may also integrate the hot/cold tank based thermal energy storage system without using a heat transfer fluid.Copyright

Ceramic Miniature Heat Pipes and Liquid Charging Methods

Three working-liquid charging methods for miniature heat pipes are introduced, and their advantages and disadvantages are described. The methods are referred to as the micro-syringe method, thermodynamic equilibrium method, and capillary-tubing method. Using these methods, two types of ceramic heat pipes were charged and tested. The ceramic heat pipes were made of alumina and have overall dimensions of 89 mm × 12 mm × 2.9 mm and a designed vapor space of 82.5 mm × 4.1 mm × 1.27 mm. Axial micro-capillary grooves were provided on the top and bottom or sidewalls inside the heat pipes as wick structures. Water was used as the working liquid. More than 20 W of heat input was achieved on a 5 mm × 5 mm heating surface. The corresponding heat flux was 80 W/cm2.