Study on a thermal network-based model for predicting the thermal resistance of pulsating heat pipes

Abstract

In this thesis, experimental and theoretical analyses are performed to develop a thermal network-based model for predicting the thermal resistance of pulsating heat pipes (PHPs). Based on the experimental investigations, a complex flow mode inside a PHP is simplified as a quantitative parameter. Moreover, a network of thermal resistances associated with the heat transfer processes involved in a PHP is established based on the theoretical investigations. This in turn leads to developing a simple but accurate model considering the flow mode and heat transfer processes inside a PHP. In the first technical chapter, a flow mode inside a PHP is quantified as a governing parameter affecting the thermal resistance of PHPs, prior to developing the thermal resistance model for a PHP. Since a PHP transfers the heat through fluid motion, simplification of the flow mode inside a PHP is essential to developing the thermal resistance model for a PHP. Experiments on silicon-based PHPs are conducted to identify the governing parameter on the various flow modes inside a PHP. Using MEMS techniques, a serpentine channel is engraved on the silicon wafer. Pyrex glass covers the etched silicon wafer to visualize the flow mode inside a PHP. Ethanol, FC-72, HFE-7000, and R-134a are used as the working fluids. High-speed photography and thermometry are performed to investigate the flow and the thermal characteristics of PHPs. To simplify the complex flow modes inside a PHP as a governing parameter, an average volumetric fraction in the condenser section is introduced. The average volumetric fraction in the condenser section is shown to be a major contributor to the thermal resistance of a PHP

an increase of the average volumetric fraction in the condenser section leads a reduction of the thermal resistance due to enhanced latent heat transfer in the condenser section. In the second technical chapter, a thermal network-based model for predicting the thermal resistance of PHPs is developed and proposed through theoretical investigations. For this, the thermal network analysis, which is generally used to predict the thermal resistance of a conventional heat pipe, is applied to simplify the heat transfer processes involved in a PHP. Each of heat transfer processes involved in a PHP is expressed as individual thermal resistances using the average volumetric fraction in the condenser section. The proposed model is found to be accurate in predicting not only total thermal resistance of a PHP but also individual thermal resistances associated with each of heat transfer processes. Using individual thermal resistances associated with each of heat transfer processes, the predicted axial temperature profiles along the substrate are shown to match well with the experimental data. Based on the proposed model, it is found that an increase of latent heat transfer in a PHP leads to a reduction of the thermal resistance of a PHP. Finally, the thermal resistance of a PHP is predicted within 13% for various working fluids and geometric parameters using the proposed model.