TWO-PHASE FLOW INSTABILITIES: PRESSURE-DROP TYPE OSCILLATION THRESHOLDS (BOILING, SUBCOOLING, HEAT TRANSFER AUGMENTATION)

Experimental and theoretical work have been carried out to find out the effects of heat transfer augmentation on two-phase flow instabilities in a single channel system. An open loop vertical, forced convection up-flow boiling flow experimental set-up is built for this purpose and Freon-11 is used as the test fluid. The effect of mass flow rate, heat input and inlet subcooling on the system behavior is studied. Six different heater surfaces are tested at six different heat inputs with constant inlet temperature and six different inlet temperatures with constant heat input. Mass flow rate is varied over a wide range for both cases, heat input and the inlet temperature of the test fluid are varied from 0 to 3(.)10('4) W/m('2) and from -7.5(DEGREES)C to +38(DEGREES)C respectively. The steady-state pressure drop versus mass flow rate curves, stability boundary maps, along with the curves of additional inlet pressure drop required to stabilize the system during the oscillations, and tables have been generated using the experimental data.; A one dimensional, homogeneous equilibrium flow model is used in theoretical analysis, and conservation equations with the constitutive relations are solved by finite differences under the specified boundary conditions to obtain the steady-state characteristics. In the course of the model development, correlations for the two-phase flow pressure drop over an orifice, and the two-phase flow heat transfer coefficient for Freon-11 have also been developed. A satisfactory agreement is obtained between the experimental and theoretical analysis. Linearized analysis and steady-state data have been used to determine the pressure-drop type instability thresholds. Dynamic equations of the overall system, surge tank and the heater tube are derived by employing the basic conservation laws, and then they are linearized. Characteristic equation for the system is found and analyzed to determine the oscillation thresholds. The results are found to be conservative but in good agreement with the experimental findings.