Heat and mass transfer characteristics of ultra-thin flat heat pipe with different liquid filling rates

The visualization method is used to study the heat transfer performance of ultra-thin flat heat pipe (UTFHP) with a vapor chamber thickness of only 0.8 mm under different liquid filling rates, and observe the change of vapor-liquid interface in mesh wick and the flow characteristics of working fluid in vapor chamber, which has important reference value and guiding significance for determining the optimal liquid filling rate, improving heat transfer performance and analyzing the flow of working medium in ultra-thin flat heat pipe. The experimental results show that the optimum filling rate is 15%, the working medium just fills the wick, and the thermal resistance is the smallest, which is 1.2 celcius/W. At the excessive liquid filling rates, the boiling phenomenon is observed in the heating area at 20 W, but the effect of excessive liquid filling on improving the heat transfer limit is very limited, which only increases the heat transfer limit by 4 W. Capillary limit is the key factor restricting the heat transfer limit of ultra-thin flat heat pipe. The excessive liquid forms a liquid bridge in the vapor chamber, which will hinder the vapor flow. Meanwhile, it will destroy the capillary flow of the condensing working medium, resulting in liquid accumulation in the condensing area and increasing the thermal resistance of the plat heat pipe. Under the condition of slight inclination angle, the condensed working medium accumulated in the condensation area is obviously reduced, at 45% liquid filling rate, the heat transfer limit reaches 28 W.