Thermal effects in packaging high power light emitting diode arrays

A detailed overview of heat pipes is presented in this paper, including a historical perspective, principles of operations, types of heat pipes, heat pipe performance characteristics, heat pipe limitations, heat pipe frozen startup and shutdown, heat pipe analysis and simulations, and various applications of heat pipes. Over the last several decades, several factors have contributed to a major transformation in heat pipe science and technology . The first major contribution was the development and advances of new heat pipes, such as loop heat pipes, micro and miniature heat pipes, and pulsating heat pipes. In addition, there are now many new commercial applications that have helped contribute to the recent interest in heat pipes, especially related to the fields of electronic cooling and energy. For example, several million heat pipes are now manufactured each month since all modern laptops use heat pipes for CPU cooling. Numerical modeling, analysis, and experimental simulation of heat pipes have also significantly progressed due to a much greater understanding of various physical phenomena in heat pipes, as well as advances in computational and experimental methodologies.

http://m.thermalfluidscentral.org/journals/index.php/Heat_Pipes/article/view/360/362

Heat pipes: review, opportunities and challenges

A review of melting and freezing processes of PCM/nano-PCM and their application in energy storage

Boiling heat transfer characteristics of nanofluids in a flat heat pipe evaporator with micro-grooved heating surface

Publisher Summary This chapter discusses the mesoscale and microscale phase-change heat transfer. The chapter focuses on the boiling and condensation heat transfer processes occurring in various meso- or micro-devices such as heat exchangers, heat sinks, heat pipes, thermal inkjet printheads, and thermal microbubble actuators. The chapter also reviews the most recent literature on phase change heat transfer phenomena in these devices. The chapter discusses many new phase change heat transfer phenomena such as bubble growth and collapse under pulse heating, effects of dissolved gas, wall roughness, corners, and tube diameter of the channels and size of the heater on the nucleation temperature in microchannels. New flow patterns and instabilities in flow boiling and flow condensation in microchannels because of surface tension effects have also been illustrated in the chapter. The chapter concludes with a discussion on the process of flow condensation in meso- or micro-channels.

Mesoscale and Microscale Phase-Change Heat Transfer

Two flat copper-water axially grooved miniature heat pipes are fabricated employing the electric-discharge-machining (EDM) wire-cutting method. Because of the advantage of the EDM method, axial grooves are provided around the entire interior perimeters of the miniature heat pipes. The two miniature heat pipes are tested under different heat inputs, cooling temperatures, and orientations. The maximum heat transfer rate for the heat pipes tested is about 40 W, and the maximum heat flux achieved is about 20 W/cm 2 , based on the outer surface of the evaporator. The effective thermal conductance of the heat pipe is on the order of 40 times that of copper based on the external cross-sectional area of the miniature heat pipe. If the effective thermal conductance is evaluated based on the interior cross-sectional area of the heat pipe, it can be 100 times higher than the thermal conductivity of copper. Analyses for heat-pipe limitations are also performed based on certain analytical relations. It is found that the capillary limit is the dominant heat transfer limitation for the miniature heat pipes tested in this paper. To improve the accuracy of the analytical model, the hydraulic radius in the capillary limit is corrected using the exact two-dimensional solutions found in the literature, and the expression for the friction factor that takes into account the shear stress at the liquid/vapor interface is adopted. The analytical results based on these modifications are compared with corresponding experimental results with good agreement.

Experiments and Analyses of Flat Miniature Heat Pipes

Two flat copper-water axially-grooved miniature heat pipes are fabricated employing the electric-discharge-machining (EDM) wire-cutting method. Due to the advantage of the EDM method, axial grooves are provided around the entire interior perimeters of the miniature heat pipes. The two miniature heat pipes are tested under different heat inputs, cooling temperatures, and orientations. The maximum heat transfer rate for the heat pipes tested is about 40 W, and the maximum heat flux achieved is about 20 W/cm/sup 2/ The effective thermal conductance of the heat pipe is on the order of 40 times that of copper based on the external cross-sectional area of the miniature heat pipe. If the effective thermal conductance is evaluated based on the interior cross-sectional area of the heat pipe, it can be 100 times higher than the thermal conductivity of copper. Analyses for heat pipe limitations are also performed based on some analytical relations. It is found that the capillary limit is the dominant heat transfer limitation for the miniature heat pipes tested in this paper. To improve the accuracy of the analytical model, the hydraulic radius in the capillary limit is corrected using the exact two-dimensional solutions found in the literature, and the expression for the friction factor that takes into account the shear stress at the liquid/vapor interface is adopted. The analytical results based on these modifications are compared with corresponding experimental results with good agreement.

M. Gao

Papers

Experiments and Analyses of Flat Miniature Heat Pipes

Experiments and analyses of flat miniature heat pipes