Experimental analysis of the drop film boiling at ambient pressure

Abstract

The paper deals with the evaporation of large liquid drops having a mass of ∼1g under stable film boiling conditions at ambient pressure. Water drop evaporation was expressed by the heat balance, which provides basis for determining instantaneous values of the heat transfer coefficient. The measurement stand, comprising three independent measurement paths, namely mass registration, temperature measurement and thermal visualisation, was described in detail. The system maintaining a constant temperature of the heating surface was located on the scales, the recordings of which were taken at constant frequency of 2Hz. The measurement results come in the form of mass change versus time. On this basis, together with the measured area of the perpendicular drop projection onto the heating surface, it was possible to compute instantaneous values of the heat transfer coefficient. Those values decrease with a change in the area and the drop mass. At the beginning of measurements, at the constant temperature of the heating surface 337.5°C, the heat transfer coefficient equalled 0.32kW/m2K, and it was over twice higher for a drop with the mass of ∼0.2g. The thermal (infrared) mapping of the drop upper surface was performed using a thermovision camera (THV). The mapping indicates a complex interaction of heat and mass transfer processes, which result in intensive convection movements in the near-surface zone. That is manifested in the form of a highly diversified thermal field of the drop upper surface. The difference between the maximum and minimum temperatures can be as much as 10K. For the adopted method of heat transfer coefficient computation, an analysis of uncertainties was also performed. The scales accuracy and the camera resolution were found to have the greatest impact on uncertainty in the heat transfer coefficient measurements. Uncertainty in measurement is inversely proportional to the drop size. As a result, measurements and later analysis of results were limited to the bottom of the range determined by the drop mass of ∼0.2g.