Design Calculations of the Limiting Characteristics of Heat Pipes for Cooling Active Phased Antenna Arrays

The article provides an algorithm for calculating the limiting characteristics of heat pipes for cooling active phased antenna arrays at a given saturation temperature. The maximum transmitted power is determined taking into account the limitations of the heat pipes operation by the capillary limit, by boiling (transition to film boiling, boiling limit), by the sonic limit at which the speed of steam reaches the speed of sound (sonic limit), by the entrainment of droplets liquid coolant from the surface of the wick with a counter flow of steam (entertainment limit) and viscous limit, which is realized at low temperatures (viscous limit). It is shown that an increase in the thickness of the wick and its porosity may be necessary to increase the capillary limit of heat pipes, while an increase in the thickness of the wick increases the thermal resistance of the tube and, accordingly, can lead to overheating of the cooled elements. Based on the above algorithm, design calculations for two types of heat pipes have been carried out. The dependences of various limits of the heat pipe on the operating temperature are plotted. Based on the above algorithm, calculations were performed for two types of heat pipes.

The Effects of Bending on Heat Pipes

A comprehensive three-dimensional numerical model is developed to evaluate the effect of bending on water-copper cylindrical heat pipes. This model distinguishes itself from other models by its ability to uniquely determine the operating pressure of the heat pipe based on the operating and physical conditions. The effects of one 90-degree bend and two 90-degree bends are evaluated on the performance of a heat pipe. Two types of wicks are considered: a screen mesh wick and a sintered powder wick. The obtained results show that bending does affect the vapor pressure drop; however, the changes are not significant when compared to the operating pressure of the heat pipe. If the bending is performed in a manner where the wick is not damaged and the liquid is not blocked from returning to the evaporator, the performance of the heat pipe will not be affected significantly. In addition, if the heat pipe is operating in the horizontal direction, where both evaporator and condenser legs are at the same level, bending does not affect the liquid pressure drop significantly; however, the screen mesh does provide a higher capillary limit. The results also showed that the effects of gravity can be important when bending heat pipes and consideration should be given for this factor. When the bent heat pipe works against gravity, the sintered powder wick heat pipes showed higher capillary limits.

Numerical Investigation on the Dielectric Working Fluid Effect on the Flow and Thermal Parameters of an Electrohydrodynamic Flat Heat Pipe

The present work highlights the impact of the working dielectric fluid on the flow and the thermal parameters of an axially grooved flat mini heat pipe (FMHP) submitted to Electrohydrodynamic (EHD) effects. Three dielectric working fluids are considered: pentane, R123, and R141b. A model is developed by considering the Laplace-Young, mass, momentum, and energy balance equations. The numerical results show that the electric field affects the liquid distribution along the heat pipe and helps the condensate to flow back to the evaporator section. Moreover, under the electric field conditions, the vapor pressure drop increases, however, the liquid pressure drop decreases. The effect of the electric field on the liquid velocity depends on the FMHP zone, and the vapor velocity is hardly affected by the EHD effects. Furthermore, lower capillary driving pressures are required to provide the necessary capillary pumping under EHD conditions. Besides, pentane allows for higher vapor pressure drops compared to those obtained with R123 and R141b, while the liquid pressure drops are highest for R123. It is found that with R123, the liquid velocity is higher than that reached with R141b and pentane. It is also demonstrated that the capillary limit increases under EHD conditions, and for R141b, the capillary limit is the highest either in zero-field and EHD conditions. Best heat pipe thermal performances are observed for wide and deep grooves with R141b. Finally, the optimum fill charge allowing the maximum heat transfer capacity is determined for each working fluid and different groove dimensions. It is shown that the optimum fill charge is hardly affected by the electric field whatever the working fluid. R123 requires the highest optimum fill charge, however, the heat transport capacity of the FMHP is the lowest when using this working fluid.

Impact of Micropillar Density Distribution on the Capillary Limit of Heat Pipes

This paper shows the performance enhancement of heat pipes by tailoring the density distribution of micropillar wicks to minimize viscous pressure loss while maintaining sufficient capillary pumping. In a heat pipe, capillarity and permeability are linked, since small pores create higher capillary pumping while unfortunately inducing more pressure drop along the heat pipe. This pressure loss accumulates along the heat pipe, leading to a non-uniform pressure difference between the liquid and vapor. Therefore, we do not need a uniform capillary pressure to withstand this difference. This provides the opportunity to spatially tailor the wick structure, aiming for a high capillarity to pump the liquid, but a low permeability to induce less pressure loss. Our study offers a compromise between capillarity and permeability by designing the density distribution of the pillar wick structure. This density distribution, which was not studied before, will be shown to enhance the heat pipe performance. The theoretical models show that a tailored density distribution can enhance the heat pipe performance by a factor of 1.5. To support this result, ‘rate of rise’ measurements along a pillar array demonstrate that the liquid pressure loss in a tailored density array are less compared to a constant pillar density.

Design, Fabrication, and Performance Evaluation of a Hybrid Wick Vapor Chamber

The growing electrification of transportation systems is dramatically increasing the waste heat that must be dissipated from high-density power electronics. Transformative embedded heat spreading technologies must be developed in next-generation systems to enable air cooling of power semiconductors with heat fluxes exceeding 500 W/cm2 over large hotspot areas up to 1 cm2. In this study, vapor chamber heat spreaders, or thermal ground planes (TGPs), with customized wick structures are investigated as one possibility. A 10 cm × 10 cm TGP with hybrid wick, which is a blend of a biporous wick with a standard monoporous wick, was designed. The TGP was tested in combination with a straight pin fin heat sink under air jet impingement and a 1 cm2 size heat source. The experimental performance of the hybrid wick TGP was compared under the same air-cooled conditions with an off-the-shelf TGP of the same size from a commercial vendor and a TGP with a biporous wick only. The customized hybrid wick TGP exhibits ∼28% lower thermal resistance compared with a traditional commercial TGP, and the capillary limit heat flux is measured as 450 W/cm2. Technical challenges in extending this capillary limit heat flux value and TGP integration into packaged electronics are described.