Date of Award
Doctor of Philosophy (PhD)
Professor S. Banerjee
The refilling and rewetting of a directly heated horizontal channel has been studied experimentally. The quenching characteristics and rewetting rates were obtained under different well-defined initial and boundary conditions. The parametric effects of initial wall temperature, inlet water flow rate and inlet water subcooling as well as effects of residual power input, tube insulation and dissolved air and ions in the inlet water were investigated.
The results show that, in horizontal channels, the transverse gravity forces significantly affect the hydraulic, and consequently thermal, behaviour of the system. The heat transfer mechanisms were found to vary during transients, both axially and circumferentially. This results in large differences in the pre-quench characteristics with regard to the bottom and top of the channel.
A simple physical model which can account for most of the observed characterisitcs was developed. The model consists of an inclined rewetting front and an entrained "liquid tongue" extending downstream from the rewetting front. The model was also supported by photographic studies.
The propagation of the rewetting front appears to be largely controlled by hydrodynamic mechanisms. It was found that surface quenching can occur at very different wall temperatures along the tube. Thus, there is no well defined rewetting temperature. This is contrary to the predictions of the generally accepted conduction controlled rewetting model.
Since hydrodynamic mechanisms dominate the thermohydraulic processes in the present flow situation, a simplified two-fluid model was used to analyze the processes. The wall temperature was obtained after the hydraulic equations of the liquid phase were solved. The wall was assumed to be in stable film boiling before quenching.
A quench model based on a critical water level, (hʟ)crit' determined the transition of heat transfer mode from film boiling to transition boiling and subsequent quenching of the surface. (hʟ)crit was obtained using a model based on initiation of a Kelvin-Helmholtz type instability at the vapor film-liquid interface.
The initiation of the interfacial instability is believed to be the governing mechanism that leads to surface rewetting. When the instability starts, local regions of enhanced heat transfer are assumed to form on the heater surface. This provides the necessary conditions for surface rewet. However, if the surface is highly conductive and has a high thermal capacity, these rewet spots may not grow and surface rewet may not occur. Therefore, the sufficient conditions would require that the rewet spots could grow or spread on the heating surface. For thin-walled tubes, because of low thermal capacity, rewet spots may spread once they are formed. Hence, interfacial instability is postulated to be both necessary and sufficient to quench thin-walled systems of the type studied.
The simplified two-fluid model, together with the quench model were found to be quite successful in predicting the rewetting rates and details of the quenching characteristics for the refilling and rewetting experiments.
Chan, Albert M. C., "Transient Two Phase Flows: Refilling and Rewetting of a Hot Horizontal Tube" (1980). Open Access Dissertations and Theses. Paper 634.