Institution: University of Cincinnati
Department: Engineering : Mechanical Engineering
Degree: MS
Year: 2005
Record ID: 1771154
Full text PDF: http://rave.ohiolink.edu/etdc/view?acc_num=ucin1132344889


The Micro Loop Heat Pipe (LHP) is a self-circulating cooling device with extremely high thermal conductivity where heat is removed by phase change in micron pores of the evaporator and the working fluid circulates by means of the thermodynamic pressure difference developed between the evaporator and the condenser. There are several studies which have considered evaporation within very small diameter pores such as occurring within the evaporator of loop heat pipe. The significance of these results is that they can be used to determine whether the heat flux being supplied to each pore in the wick of an LHP exceeds any thermodynamic limit. Hamdan[4] developed a global model that predicts the temperatures and pressures in the loop heat pipe . It accounts for heat leak and loop performance. It predicts that if either nucleation or the capillary limit is exceeded, the maximum heat flux has been exceeded. The major disadvantage of this model is that it is 1-dimensional and assumes that the heat flux is uniformly distributed across the top of the wick. Heat distribution on the surface of the wick is non-uniform due to the lateral conduction of heat within the wick. This leaves many of the pores at the center unheated resulting in high temperature gradients from the side to the center of the wick. The power dissipation ability of the micro loop heat pipe is drastically affected by this non-uniform heat distribution. Without conduction pathways in the top cap, the energy must be transferred through the periphery of the primary wick. If the distance between conduction pads is too large, the pores in the center wick will not be heated, and will be non evaporating. The global model is conduction and convection controlled. It does not account for the evaporation that occur from the meniscus within the pores and hence no limit has been put on the amount of heat flux that can be supplied to the pores. If too few pores are used, there is some limit on the evaporation that occurs in the meniscus of the pores. The global LHP model does not predict the maximum amount of flux that can be supplied to each pore before it starts increasing the system temperature. The Oinuma [13] evaporation model accounted for thin film evaporation within the pores. Single pore analysis takes into account non uniform heat flux distribution in the pores. The present research work deals with application of the Oinuma[13] evaporation model to Hamdan[4] global LHP model. The goal was to include the interfacial phenomena in the pores. This helps in taking into account the evaporation from the meniscus and in predicting the evaporation limit for global LHP model. The evaporation rate was independent of the assumption of uniform heat flux distribution. The application also accounts for heat flux in each of the pores. In addition to the prediction of temperatures, pressures and maximum heat flux based on capillary limit and nucleation limit, it can also predict optimum number of pores per unit area based on evaporation limit. The evaporation limit was a function of…