Experimental Investigation Of Pool Boiling Heat Transfer Enhancement On Micro-structured Surfaces

With the development of electronic devices and equipment to compact,miniaturization and high power,the requirements for heat dissipation are further enhanced.Boiling heat transfer is widely used in industrial equipment and technologies such as heat exchangers,nuclear reactors,rocket nozzles and aerospace electronic devices because it can take advantage of a large amount of latent heat released in the process of liquid vaporization.Therefore,boiling heat transfer is the most effective way to solve the heat dissipation problem of high heat flux equipment.Changing surface structure to enhance boiling heat transfer is one of the main research directions in the field of phase change heat transfer enhancement.In this paper,an experimental system of pool boiling heat transfer was established to study the boiling heat transfer laws of micro-structured surfaces,including micro-pit surfaces and hybrid-wettability surfaces.Firstly,the boiling heat transfer performance of the micro-pit surfaces was tested experimentally.It was found that the micro-pits could become a stable core of vaporization and reduce the initial boiling superheat.The effects of pit diameter,depth and spacing on the heat transfer coefficient(HTC)and critical heat flux(CHF)were analyzed quantitatively.The results showed that the diameter of the micro-pit had minor effect on HTC and CHF.With the increase of the micro-pit depth,the reflux resistance of the surrounding liquid increased and the heat transfer performance of the surface decreased.The optimal pit-to-pit spacing was 2.5 mm,which can reduce the hydrodynamic instability and enhance the boiling heat transfer.The predicted values were in good agreement with the experimental results,and the mean absolute error(MAE)is 8.3%.Secondly,the boiling heat transfer performance of wettability surfaces was experimentally tested.The wettability surfaces were divided into homogeneous-wettability surfaces and hybrid-wettability surfaces.According to the experimental results of homogeneous-wettability surfaces,hydrophilicity can improve the CHF,and hydrophobicity can improve the HTC in the low heat flux region.According to the geometry of hydrophobic region,the hybrid-wettability surfaces can be divided into two types: dot pattern hybrid-wettability surfaces and stripe pattern hybrid-wettability surfaces.The CHF and HTC of the dot pattern hybrid-wettability surfaces were higher than those of the plain surface,and an optimized 1-mm dot diameter was identified successfully for the maximum CHF enhancement.The CHF of stripe pattern hybrid-wettability surfaces decreased significantly with the increase of stripe width.The optimal pattern spacing of the two hybrid-wettability surfaces was 2.5 mm,but the change of stripe spacing had a weak effect on nuclear boiling heat transfer.Meanwhile,CHF for the hybrid surface increased remarkably with increasing the pattern-substrate contact angle difference,but the nucleate boiling heat transfer coefficient declined with increasing the contact angle difference.In addition,the pattern-to-surface area ratio significantly affected the boiling heat transfer performance of the hybrid-wettability surfaces.When the pattern-to-surface area ratio exceeded6.3%,the CHF of the hybrid-wettability surfaces decreased with the increase of the area ratio.Finally,based on the Kandlikar’s model,a theoretical model for predicting CHF on homogeneous-wettability surfaces was proposed considering the three-dimensional force balance of bubbles.Compared with the Kandlikar’s model,the theoretical model predicted that the MAE of CHF on hydrophilic surfaces could be reduced from 26.7% to 19.7%,while for hydrophobic surfaces,the predicted trends of the two models were almost identical.Based on the theoretical model of homogeneous-wettability surfaces,a predicting method for CHF on the hybrid-wettability surfaces was proposed by accounting for the pattern-to-surface area ratio with MAE of 2.8%.