TRANSIENT AND TIME-AVERAGED BOILING HEAT TRANSFER OF THE OSCILLATORY TWO-PHASE FLOW IN HELICAL COILS

The transient and time-averaged boiling heat transfer characteristics of water during density wave oscillations are studied experimentally. A horizontally positioned helical coiled tube is used as test section and is healed with alternating current. The measurement of the fluid temperatures at different positions within one cross section indicate that the test section departs from saturation, a significant superheat occurs, and postdryout heat transfer is reached during the transient. It is interesting that fluid temperatures at different positions of the same cross section are not oscillating in the same phase but are rather delayed from tube outer side to inner side, while the wall temperatures in one cross section are oscillating in the same phase. The time-averaged boiling heat transfer of the oscillatory two-phase flow is different from that of the steady-state flow by its dependence upon the frequency and amplitude of the oscillation. The boiling heat transfer coefficient of the oscillatory flow is less than that of the steady-state flow and this difference can be even enlarged in a high mass quality region. Based on the analysis of heat transfer phenomenon, a set of dimensionless parameters is defined to embody the oscillatory boiling characteristics and a correlation is proposed to predicate the boiling heat transfer coefficient during the oscillation.