A Mathematical Model Predicting Heat Transfer Performance in a Oscillating Heat Pipe

A mathematical model predicting the oscillating motion in an oscillating heat pipe is developed. The model considers the vapor bubble as the gas spring for the oscillating motions including effects of operating temperature, non-linear vapor bulk modulus, and temperature difference between the evaporator and the condenser. Combining the oscillating motion predicted by the model, a mathematical model predicting the temperature drop between the evaporator and the condenser is developed including the effects of the forced convection heat transfer due to the oscillating motion, the confined evaporating heat transfer in the evaporating section, and the thin film condensation in the condensing section. In order to verify the mathematical model, an experimental investigation was conducted. Experimental results indicate that there exists an onset power input for the excitation of oscillating motions in an oscillating heat pipe, i.e., when the input power or the temperature difference from the evaporating section to the condensing section was higher than this onset value the oscillating motion started, resulting in an enhancement of the heat transfer in the pulsating heat pipe. Results of the investigation will assist in optimizing the heat transfer performance and provide a better understanding of heat transfer mechanisms occurring in the oscillating heat pipe. INTRODUCTION Because of the rapid development of the electronic industry, with chips packed closer together and the continuing decrease in the size of electronic packages, heat flux levels continue to increase. For example, the new design of highdensity computer chips for the next generation of desktop computers may reach a heat flux level of over 80 W/cm. Metal oxide semiconductor controlled thyristors generate heat fluxes from 100 to 200 W/cm. And some laser diode applications have reached a heat flux level of 500 W/cm. Conventional heat sinks or spreaders become severely inadequate at these high levels of heat fluxes. While the conventional heat pipe can significantly push the border of cooling power, the heat transfer limitations in the heat pipe restrict the applications for these high levels of heat fluxes. Compared with the conventional heat pipe, the successful oscillating heat pipe (OHP) has the following features: 1) because most or all of working fluid does not flow through the wick structure, there is a low pressure drop of working fluid; 2) because the vapor flow direction is the same as liquid flow, there is no vapor flow influence on the liquid flow; 3) the thermally driven, oscillating flow inside the capillary tube will effectively produce some “blank” surfaces to significantly enhance evaporating and condensing heat transfer; and 4) due to the oscillating motion in the capillary tube, the heat added on the evaporating area can be distributed by the forced convection in addition to the phase-change heat transfer. Clearly, the OHP exists a potential to remove an extra high level of heat flux. Extensive experimental investigations [1-6] and theoretical analysis [7-15] have been conducted, and show that there exist oscillating or/and oscillating motions in an oscillating heat pipe, which depend on working fluids [2-4], tilt angles [5-7], dimensions [2,6,9,10,13], filled liquid ratio [5,9,14], turns [2,4,13] and heat flux levels [2,4,7,14]. While these investigations have provided an insight into the mechanisms of oscillating motions occurring in the OHP, the primary factors affecting the heat transport capability have not been fully understood. As mentioned above, the OHP transfers heat through forced convection in addition to phase-change heat transfer. It is expected that the heat transport capability occurring in the OHP should be much higher than the convectional heat pipe. But, the available experimental data show that the effective thermal conductivity occurring in the OHP is lower. In this investigation, a mathematical model predicting the temperature drop from the evaporating section to the condensing section is developed to determine the primary