Enhancement of thermal performance of micro heat pipes using wettability gradients

We present a methodology for enhancing thermal performance of micro heat pipes (MHPs) by creating wettability gradients on the inner surface of MHPs. We analyse the effect of wettability gradients on MHP performance using a quasi one-dimensional mathematical model that accounts for axially-varying solid-liquid contact angle. For our analysis, we consider MHPs with various wettability schemes, such as uniform, step-variation, and linear variation in the contact angle. Our model predictions show that increasing the wettability of the evaporator surface and reducing the wettability of the condenser surface of MHP can lead to an increase in the heat transfer capacity of MHP by over 35%. We demonstrate the favourable effect of wettability gradients on MHP performance for different working fluids over a wide range of operating temperature and fluid charge. We also discuss the underlying physical mechanism that leads to enhanced thermal performance of MHP with mixed wetting surfaces. We show that the optimal choice of wettability gradient in MHP is governed by the competing effects of high liquid flow resistance in the lower wetting condenser and high liquid mass in the higher wetting evaporator.