Subcooled film boiling in non-aqueous fluids.

Abstract

Subcooled film boiling has been investigated experimentally for vertical up-flow in a directly heated tube using the refrigerants R-12, R-22 and R-134a as test fluids. The data cover a mass flux range of 530 to 3000 kgm$\sp{-2}$s$\sp{-1},$ an inlet subcooling range of 6 to 28$\sp\circ$C and a pressure range of 0.83 to 1.4 MPa (corresponding to an equivalent water pressure range of 5 to 7 MPa). These are the first flow film boiling data ever obtained for R-134a and R-22. The results show strong effects of mass flux, inlet subcooling and pressure on the heat transfer coefficient. Also, the data exhibit complex trends of the heat transfer coefficient as a function of thermodynamic equilibrium quality. Because of the wide range of conditions covered in this study, a systematic examination of the effect of flow parameters and fluid properties on the heat transfer coefficient was performed, and this has provided a unique insight into the heat transfer mechanisms. During this study, a number of post-CHF heat-transfer prediction methods have been assessed over a wide range of flow conditions obtained from this study and AECL PDO data bank. None of the assessed prediction methods is able to adequately predict the heated surface temperature for all flow conditions. For dispersed flow film boiling (DFFB), the Groeneveld-Delorme correlation underpredicts the data and occasionally exhibits an incorrect parametric trend at low mass fluxes. The Shah correlation is not applicable for conditions of mass fluxes beyond 3442 kgm$\sp{-2}$s$\sp{-1}$ and thermodynamic qualities above 1. Phenomenological models provide worse prediction accuracy than the empirical equations. Among them, the Moose and Ganie, the Saha, and the Yoder and Rohsenow models collapse abruptly at very high mass flux or/and high-pressure conditions. The constitutive equations employed in these models are often valid only for a narrow range of flow conditions. For inverted annular film boiling (IAFB), none of the existing models or correlations is accurate over a wide range of flow conditions. Most models are valid only at low-pressure, where data on interfacial parameters are available. A two-fluid model has been developed to predict the wall temperature of a tube during IAFB. This model correctly accounts for the effects of flow variables such as mass flux, inlet subcooling, heat flux, and pressure. This method reduces the degrees of freedom of the system: relationships between relevant flow variables are established based on the physical mechanisms in IAFB that satisfy thermodynamic limits. Also, a hybrid post-CHF model is derived by combining the two-fluid model of IAFB with the slightly modified Moose and Ganie (1982) DFFB model, to predict heated surface temperatures over the whole film boiling region. Comparisons between the two-fluid model predictions and experimental data from four fluids (water, Freon-12, Freon-22, and Freon-134a) have shown very good agreement for a wide range of flow conditions. The model has shown better performance than all the IAFB prediction methods assessed during the course of this study. (Abstract shortened by UMI.)