Experimental study of non-condensable gas effects on sonic limit of sodium heat pipe

•Sonic limit of the 1 m-long sodium heat pipes with artery and axial groove wick are obtained.•Effects of non-condensable gas on sonic limit is experimentally and theoretically studied.•The heat transfer phenomena under sonic limit with the heating power ranging from 500 to 1400 W are discussed.•A modified sonic limit model with effects of non-condensable gas is proposed. Due to unexpected gas leakage and long-term operation, the non-condensable gas (NCG) inevitably exists in high-temperature alkali metal heat pipe. Previous studies have mainly focused on its influences on startup and steady state operation characteristics of heat pipes. However, few of them conducted experiments to study the effects of NCG on sonic limit, which usually occurs during heat pipe startup or during steady state operation when the heat transfer coefficient at condenser section is high. In present work, experimental and analytical studies are conducted to the explore the effects of NCG on sonic limit of sodium heat pipes. With the heating power ranging from 500 to 1400 W, the sonic limit of 1 m-long sodium heat pipes with artery wick and axial groove wick are obtained based on oil-cooling system, which can widely adjust the cooling capacity at condenser section. The amount of NCG in heat pipe is estimated by Heat Pipe Transient Analysis Code (HPTRAN) and verified by experimental data. It is found that the existence of NCG in heat pipe leads to the decrease of maximum heat transfer rate under sonic limit. The mechanism of NCG effects on sonic limit is discussed based on phenomenological analysis. By introducing the added back pressure, a modified model to predict sonic limit considering NCG effect is proposed and the comparison with experimental data shows good agreements.