Experimental study of water pool boiling on a single nucleation site, from atmospheric pressure down to low pressure
Résumé
Heat transfer during boiling is generally of great interest for many engineering applications. Among some of them, boiling occurs at low pressure levels. In these conditions, pressure and subcooling degree are no more homogeneous around the bubbles. These nonhomogeneities lead to changes in the boiling phenomenon: bubbles size, shape and frequency are modified compared to what is usually observed at atmospheric pressure. Experiments of water pool boiling were conducted at saturation state from atmospheric pressure down to 4.2 kPa with an imposed heat flux of 2.7 W/cm2 at the heater. Videos of each experiment were recorded owing to a high speed camera in order to study the bubble dynamics for the different pressure levels. As the pressure decreases, the size of bubbles increases and the detachment frequency of bubbles decreases. Close to atmospheric pressure, bubbles are near-spherical whereas at the lowest pressure range bubbles have an oblate spheroidal shape. In the range of intermediate pressures, bubbles get a mushroom shape and/or form vapour columns due to the rapid growth of the successive bubbles which are sucked into the wake of the first one. After detachment bubbles collapse quite rapidly because of the subcooling degree. Bubble growth curves were also plotted from the calculation of the instantaneous volume of each bubble through image processing.