Experimental investigations of the influence factors for the boiling heat transfer characteristics of R134a coolant in a loop thermosiphon system

Abstract

This study focuses on the heat transfer and flow characteristics of the evaporator of a loop thermosiphon system. The loop thermosiphon is a type of heat pipe that is driven by gravity rather than a pump. Thus, the flow and heat transfer characteristics may be different from those for pump driven forced convection boiling. It is important to investigate the heat transfer and flow conditions of the loop thermosiphon. This study is focused on the working conditions at different filling ratios, particularly the relationship between the mass flow and boiling heat transfer inside the evaporator. In the experiment, the heat transfer capacity varies in the range of approximately 200–1000 W and the filling ratio is approximately 60%-130%. Three major results are obtained: (1) when the filling ratio is greater than 100%, the real mass flow rate is much higher than the pure vapor mass flow rate, and the higher the filling ratio, the larger the mass flow rate will be; (2) the filling ratio will influence the mass flow rate of the coolant and the mass flow rate will then influence the vapor quality of the evaporator; and (3) the boiling heat transfer coefficient does not vary with the filling ratio, even though the mass flow rate at different filling ratios changes significantly