Dry-out limit enhancement strategy in sodium heat pipes: Overfilling effect and modified capillary limit model

Despite the significant impact of the filling ratio on dry-out of heat pipe, current operating limit models do not adequately address this factor. This study proposes an advanced capillary limit model that incorporates the effect of the filling ratio. An experimental study was conducted to compare the operating limits of sodium heat pipes with filling ratios of 107 % and 246 %. The results showed that the existing capillary model is difficult to predict the operating limits based on filling ratios, leading to significant deviations between theoretical predictions and actual outcomes. Specifically, a heat pipe filled to 107 % experienced dry-out at a point 31.1 % lower than expected, while a heat pipe filled to 246 % reached dry-out at 20.2 % higher than expected. Notably, the overfilled heat pipe maintained a steady state even at 10.5 % above the expected dry out point. The key point of the proposed model, based on optical imaging and temperature data at dry-out, is that the excess liquid in vapor region travels and forms a liquid plug at the end of the condenser, which shortens the length required for capillary transport. Based on this insight, the Chi model was modified to predict the operating limit of overfilled heat pipes more accurately within a 5 % margin of error. The study highlights the critical role of filling ratio in heat pipe design, noting that overfilling can shorten the effective capillary length through liquid plug formation, thereby increasing the dry-out limit. However, these pools also limit the reach of sodium vapor and potentially reduce the effective area of the condenser.