Sustained and Transient Boiling Flow Instabilities in a Cross-Connected Parallel Channel Upflow System. Final Report

摘要

Two kinds of instabilities, viz., sustained instabilities and transient instabilities, have been investigated in a cross-connected parallel channel forced convection boiling upflow system. In the investigation of the sustained instabilities, distilled water has been used as the test fluid, and the heat inputs, heat input distributions, mass flow rates, inlet temperatures and inlet valve settings have been taken as variables. Two major modes of oscillation, viz., the density-wave oscillations and the pressure-drop oscillations, similar to those encountered in single channel boiling water flows and in single and parallel channel (with and without cross-connections) boiling Freon-11 flows, have been identified. In the high heat input experiments, in most instances, some temperature oscillations have been observed at the downstream ends of the heaters. The system could not be completely stabilized by introducing pressure drop(s) at the two heater inlets or at the upstream side of the inlet plenum. However, the introduction of inlet pressure drops reduced the amplitudes of the oscillations. The results have been compared with those of the Freon-11 two phase flow experiments in a cross-connected parallel channel system and the water two-phase flow experiments in a single channel system. In investigating the transient instabilities, Freon-11 has been used as the test fluid since it has the advantages of low boiling point and low burn-out heat flux, permitting operations up into the film boiling region. In the experiments, step changes in heat inputs or flow rates have been applied, and the resulting transients in flow parameters have been recorded. The step increases in heat inputs and the step decreases in flow rates have both produced changes in flow rates, pressures and heater wall temperatures. The greater the step change, the greater has been the decrease in flow rates and the increase in heater wall temperatures. (ERA citation 10:008933)