Pressure drop instability analysis in mini-channel evaporators under different magnitudes of gravity

Abstract

Pressure drop instability in mini-scale channels has been observed in a wide temperature/pressure range, including both the room temperature fluids and cryogenic ones. In the last decade, this instability and corresponding oscillations of flow rate and temperature have attracted numbers of investigations. In this paper, the pressure drop instability in micro/mini-channels is theoretically analyzed. Frictional, accelerational and gravitational pressure drops are considered, and then experimental studies of ammonia in mini-channels are conducted. Based on the analysis, it is found that the stability criterion for pressure drop oscillation can be simplified as a maximum allowable inlet sub-cooling degree. If this maximum degree is not exceeded, the flow boiling will be absolutely stable without pressure drop instability at any heat fluxes. This maximum allowable inlet sub-cooling degree is mainly influenced by the latent heat, the kinematic viscosity ratio and the gravity. By using the phase change number and sub-cooling number, a dimensionless stability limit is proposed. It predicts the occurrence of pressure drop instability successfully, for both ammonia and water. Besides, the effects of gravitational and accelerational pressure drop on the flow instability are also studied. It is found that the accelerational pressure drop has little influence on the flow instability. But the gravitational effect can be significant. By introducing a newly proposed dimensionless number, a novel stability map containing gravity effects is established. The map shows that the gravitational pressure drop stabilizes the system in upward flow cases. But in downward flow cases, it destabilizes the system.