Experimental studies of sub-atmospheric pressure boiling of water on various tubes and plates
Salem, Salem Moh. M.
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Different modes of experimental data are reported for water boiling at sub-atmospheric pressure. These experimental modes were pool boiling on an isolated tube and flow boiling over a tube bundle and on a vertical wall. As part of this study, a low-pressure evaporation rig was designed, constructed, commissioned and operated in the school of Engineering and Physical Science at Heriot-Watt University to investigate the effect of pressure on the thermal performance of isolated tubes, a tube bundle and a heated wall. The experimental study on an isolated tube was undertaken at sub-atmospheric pressures in the range 50-850 mbar for heat fluxes of 5-80 kW/m² using two different tube material; brass and stainless steel. The experimental study of water flow over a heated tube bundle was undertaken at the same sub-atmospheric pressures of 50-850 mbar but using flow rates of 20-70 l/min with heat fluxes of 10-65 kW/m². Boiling tests for flow on a heated vertical wall were undertaken at sub-atmospheric pressures of 50- 850 mbar for heat fluxes of 20-70 kW/m² and flow rates of 20-40 l/min. The experimental results show that the boiling characteristic changes significantly as the pressure is reduced. At lower pressure, a higher wall superheat was required to cause nucleation. The shape and size of bubbles varied, with multiple small bubbles observed at 850 mbar and single large bubbles observed at pressure of 50 mbar. The wall superheat changed considerably at sub-atmospheric pressure as the material of the heater changed. Different correlations were used to predict nucleate boiling at reduced pressures. The comparisons of the correlations with the experimental data showed that the predicted wall superheats for the brass tube by the Gorenflo correlation was most accurately predicted for pressures of 120 mbar and higher. The Churchill and Chu correlation predicted wall temperatures reasonably for the stainless steel tube boiling data indicating that natural convection dominated the regime, even at high heat fluxes where it was in the fully developed boiling region. The tube bundle operated in the natural convection regime, even at high flow rates. The Gorenflo - Churchill and Chu combination model showed a reasonable prediction of the measured data from the sub-atmospheric boiling tests. The data obtained for flow boiling on a heated vertical stainless steel wall indicated that the heat transfer regime was natural convection. The Churchill and Chu correlation gave well predictions to the measured data at single-phase and sub-cooled boiling conditions for pressures of 50 and 850 mbar.