Flow and heat transfer characteristics of liquid nitrogen in mini/micro-channels own many particular aspects and are very important for applications. In the present study, the investigation of flow and heat transfer characteristics of liquid nitrogen in mini/micro-channels is presented. It is found that small viscosity enables the flow state in mini/micro-channels to be turbulent state, which proves that the classical theory for pressure drop is still valid if the surface roughness of the passage is properly taken into consideration. Experiments of flow boiling of liquid nitrogen are conducted under both adiabatic and diabatic conditions. It is shown that confinement number Co = 0.5 can be applicable in classifying the heat transfer characteristics of liquid nitrogen in macro- and micro-channels. Flow visualization in micro-channels at low temperatures poses big challenges in image magnification and illumination. These two problems have been subtly overcome in the investigation and clear images have been obtained. The flow patterns and flow regimes of two-phase flow of liquid nitrogen indicate different features from the room-temperature fluidss. Furthermore, a very simple but effective method for 3D flow visualization by one high-speed camera is proposed and implemented. Finally, numerical analysis of the flow boiling of liquid nitrogen in mini/micro-channel is carried out to deepen the understanding of mechanism.
Experimental study on heat transfer characteristics of R134a flow boiling in “Ω”-shaped grooved tube with different flow directions
