Relation between triple phase contact line and vapor groove width for enhancing thermal performance of a loop heat pipe evaporator

Abstract

• The effects of changing vapor groove width of a porous wick and improving wettability of a LHP evaporator on heat transfer performances were evaluated experimentally and theoretically. • The micro-grooves processed on the heating plate improved heat transfer performances regardless of the vapor grooves width. • The micro-grooves increased heat transfer performances more greatly as the vapor grooves width increased. • Numerical model explained that the length of triple phase contact line has a great influence on the evaporative heat transfer coefficient. In this study, the effects of an increased evaporation area known as the triple-phase contact line (TPCL), of the porous wick and improved evaporator wettability on the loop heat pipe (LHP) heat transfer performance were evaluated experimentally and theoretically. An infrared camera and a microscope were used to observe the thermo-fluid behavior of porous wicks with different vapor grooves widths and evaporator heating plates with different inner wall structures in the experiment. The porous wick's vapor groove widths were 1.0 mm, 0.5 mm, and 0.2 mm. A normal heating plate and a micro-grooved heating plate were used as the evaporator case. As a result, it was clarified that processing the micro-grooves on the heating plate improved the evaporative heat transfer coefficient and maximum heat flux, regardless of the vapor grooves width of the porous wick. Furthermore, it was proposed that both of them increased significantly as the vapor grooves width increased. The thermal-hydraulic numerical model that predicted the heat transfer coefficients for each case was developed. The calculation results showed a significant increase in the heat transfer coefficient as the TPCL appearance rate, which expresses how much the TPCL appears in the micro-grooves, increased, and this increasing tendency was in good agreement with the experimental results. Furthermore, it was proposed that the TPCL appearance rate at a high heat flux was reduced as the vapor grooves width decreased. The results indicate a new promising approach for improving the heat transfer performance of LHP evaporators.