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Degree

MS, University of Cincinnati, Engineering : Mechanical Engineering, 2006.

Abstract

The Micro Loop Heat Pipe (LHP) is a two-phase device that may be used to cool electronics, solar collectors and other devices in space applications. A LHP is a two-phase device with extremely high effective thermal conductivity that utilizes the thermodynamic pressure difference developed between the evaporator and condenser and capillary forces developed inside its wicked evaporator to circulate a working fluid through a closed loop. While previous experiments have shown reduction in chip temperature, maximum heat flux was less than theoretically predicted. This paper addresses the main problem with the past designs of top caps which has been the conduction of heat from the heat source to the primary wick. The new top cap design provides conduction pathways which enables the uniform distribution of heat to the wick. The provision of conduction pathways in the top cap increases the pressure losses and decreases the temperature drop. The feasible competitive designs of the top cap with conduction pathways from the fabrication point of view are discussed in detail. Calculation of pressure drop and temperature drop is essential for the determination of optimal solutions of the top cap. Approximate pressure drop was calculated for the top cap designs using simple 2-D microchannel principles. Finite element modeling was performed to determine the temperature drop in the conduction pathways. The conditions used for arriving at the optimal design solutions are discussed and presented. A trapezoidal slot top cap design and trapezoidal mesas top cap were chosen for fabrication as they were relatively easy to fabricate with available MEMS fabrication technologies. Geometry of the external vapor reservoir for the trapezoidal slot top cap was designed for optimum pressure drop. Variation of pressure drop in the top cap with respect to the porosity in the coherent porous silicon wick was discussed and analyzed in detail. The exact pressure drop calculations were performed numerically using a finite volume commercial flow solver FLUENT 6.1 with appropriate boundary conditions. The temperature drop calculations were performed using finite element modeling in ANSYS 6.1. It was assumed that all the pores have uniform mass flow rate and were at saturation conditions during the phase change. Obtained values of pressure drop and temperature drop for chosen geometries of trapezoidal slot and trapezoidal mesa top cap were found to be within the optimal limits and are ready to be fabricated.

Subject Headings

Engineering, Mechanical

Keywords

LHP; Loop Heat Pipe; CPL; Heat Pipes; Electronics Cooling; Pressure drop; Temperature Drop; Fluent; Ansys; CPS Wick; Microchannel; porosity modeling

Advisor

Dr. Frank M Gerner

Pages

88p.

Document number: ucin1152120112
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=ucin1152120112

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