Vapor flow resistance characteristics in ultra-thin flat heat pipes

Abstract

Miniaturization of electronic devices drives the development of ultra-thin flat heat pipes (UTFHPs). As the thickness of the heat pipe decreases, the microchannel effect of the vapor chamber exacerbates the deterioration of the vapor flow. The vapor flow resistance, which is often neglected in the conventional theory due to its extremely small effect compared to the liquid flow resistance, becomes a critical factor limiting the heat transfer performance of UTFHPs. To provide a theoretical basis for the design and performance evaluation of UTFHPs, this paper investigated the effect of the wicking structure and working fluid on the vapor flow resistance of UTFHPs based on theoretical derivations, numerical calculations, and experiments. The results indicate that conventional analytical theory is no longer applicable when the total thickness of the heat pipe is less than 1 mm. The vapor flow resistance obtained using the conventional equation is significantly smaller than the experimental results, which is caused by several factors: raised momentum loss at the wall and gas-liquid interface due to the growth of the velocity gradient in the boundary layer; exacerbated flow instability due to the mixing between the vapor and the working fluid; enhanced fluctuating effects on the flow channel due to the presence of the wicking structure. A dimensionless flow resistance coefficient equation was proposed with the vapor Reynolds number and the channel height ratio. An experiment was conducted to verify the accuracy of the flow resistance correlation. The difference between the experimental results and the calculated values is less than 15 %.