Two-phase flow instability in distributed jet array impingement boiling on pin-fin structured surface and its affecting factors

Abstract

The two-phase flow, heat transfer and pressure drop of distributed jet array impingement boiling of HFE-7000 on pin-fin surfaces are investigated with particular emphasis on the characteristics, triggering mechanism and affecting factors on two-phase flow instability. It is found that the two-phase flow in a jet chamber becomes unstable at certain high heat fluxes when large bubbles intermittently choke effusion holes. However, the two-phase flow returns to a stable condition as heat flux is further increased after stable vapor columns are formed between fin tops and effusion holes. Although this kind of instability only causes slight decrease in the heat transfer coefficient, it leads to significant fluctuations of inlet and outlet pressures as well as an abrupt increase in pressure drop. The transitional heat flux at which pressure oscillations occur corresponds to that of two-phase flow instabilities, but the frequency of pressure oscillation is much lower than that of two-phase flow patterns. Two-phase instabilities can be delayed by increasing flow rates, but the degree of instability cannot be reduced once it occurs. These instabilities can also be attenuated or even eliminated by decreasing jet-to-target distance or enlarging effusion ports.