Study on Critical Heat Flux of High Velocity Liquid Flow

Critical heat flux experiments of subcooled, thin, and high-velocity water flow were performed. The test flow channel was rectangular. The width of the flow channel was 2 mm and the height was 0.5 mm or 0.2 mm. The heat transfer surface was 2 mm × 2 mm. At the low heat flux, tinny bubbles were formed at the downstream part of the test heating surface. As the heat flux was increased, the bubble diameter increased and the coalescence of bubbles occurred. Then, the coalesced bubbles grew larger to cover the whole area of the heat transfer surface. Finally, the dried area appeared at the downstream end of the heat transfer surface to cause the critical heat flux condition. The critical heat flux was considerably higher than that of the subcooled flow boiling for the usual-size pipe as well as those of the saturated and the subcooled pool nucleate boiling. As the flow rate was increased, the period between the onset of boiling and the critical heat flux occurrence became narrow. The critical heat flux in the present experiments where the heat transfer surface was located at the just downstream of the flat channel outlet was considerably larger than those in the previous experiments where the heat transfer surface was located at the outlet end of the flat channel or the upstream of the outlet. By producing a fast jet along the surface and providing enough space for generated bubbles to leave from the surface, the critical heat flux was considerably augmented. Critical heat fluxes obtained in the present experiments were in in-between of the correlations for the flowing-upward film flow and for the flowing-downward film flow. The increasing trend for the flow rate was similar to that of the correlations.