Vapor bubble departure in forced convection boiling

A forced convection boiling facility has been fabricated in which vapor bubble departure can be investigated. It has been observed that once a vapor bubble departs from its nucleation site, it typically slides along the heating surface and lifts off at some finite distance downstream. The probability density functions (pdfs) for bubble departure diameter, d , have been obtained for mass flux, C , ranging from 112 to 287 kg m −2 s −1 and heat flux, q w , ranging from 11.0 to 26.0 kW m −2 . The data indicate a systematic dependence of d on G and q w . A detailed analysis of various forces acting on the bubble is presented and is used to predict the mean departure diameter. The onset of imbalance between the quasi-steady drag, the unsteady component of the drag due to asymmetrical bubble growth, and the surface tension force in the flow direction is used as a criterion for departure and yields satisfactory agreement between the measured and predicted values of the mean departure diameter. The analytical prediction shows a strong influence of mean liquid velocity and wall superheat on the bubble departure diameter. At the point of departure the surface tension force in the flow direction is generally small.