Experimental study on the effects of sodium and potassium proportions on the heat transfer performance of liquid metal high-temperature oscillating heat pipes

Abstract

Liquid metal high-temperature oscillating heat pipes (LMHOHPs) are highly efficient heat transfer devices which can work in high temperature environments above 500 °C. In this paper, LMHOHPs with different proportions of sodium-potassium alloy (hereinafter referred to as NaK) as working fluids were developed, and their start-up and heat transfer performance was investigated experimentally. 310S stainless steel with an inner diameter of 6mm was employed as the pipe material, and the filling ratio was kept as 50%. Under the conditions of input powers 1500∼3500 W and inclination angles 0°∼90°, LMHOHPs with five proportions of NaK (namely, 0, 10, 46, 78 and 100 wt.% of potassium) as working fluids were tested. The results showed that: (1) The wettability between NaK and 310S stainless steel was poor, but it was improved with the increase of temperature and potassium mass fraction. At 350 °C, The contact angles of NaK (K100%) and NaK (K78%) were 72.3° and 81.1°, respectively, which meant that the liquid metal changed from non-wetting state to wetting state. (2) Except that the LMHOHP with NaK (K0%) could not start up at the inclination angle of 0°, the five LMHOHPs could start up successfully and transfer heat through oscillating motion at all inclination angles and input powers. (3) When the heat flux exceeded 4.56 × 106W/m2, the flow patterns of LMHOHPs with NaK (K10%), NaK (K46%), NaK (K78%) and NaK (K100%) changed from oscillating motion to stable unidirectional circulating flow, which significantly improved their heat transfer performance. (4) Due to the influences of saturated vapor pressure gradient, viscosity and wettability, the heat transfer performance of LMHOHPs increased with the growth of potassium mass fraction. The minimum thermal resistance of the LMHOHP with NaK (K100%) was 0.071 °C/W at the input power of 3528 W and the inclination angle of 90°. This study could provide guidance for the working fluids selection of oscillating heat pipes in high temperature environments.