FLOW INSTABILITIES IN PARALLEL CHANNELS WITH NUCLEATE BOILING AND FILM BOILING

Experimental and analytical results are presented on thermally induced flow instabilities of cryogenic fluid in parallel channels, using liquid nitrogen as a working fluid with nucleate boiling in the inlet section. The fluid flow pattern transfers from bubbly flow to annular flow with film boiling, and from inverted annular flow to dispersed flow, respectively. Two types of channels, consisting of two spiral tubes and two vertical tubes, are tested. Stability criteria are obtained in each experiment and the criteria are approximately given by constant exit thermodynamic equilibrium qualities. The flows with film boiling in the inlet section are stable for higher exit thermodynamic equilibrium qualities than those with nucleate boiling. Linear stability analysis for density wave oscillation is adopted to predict the stability criteria using a drift flux model. The stability criteria for nucleate boiling are well predicted by using the previous drift flux models, and those for film boiling are also well predicted by using the drift flux model formulated by analytical results obtained with a turbulent boundary layer theory for the inverted annular flow. These results indicated that the criteria of the flow instabilities should be estimated by the slip between liquid and vapor for both boiling states.