Forced-convection surface-boiling heat transfer and burnout in tubes of small diameters

A basic heat-transfer apparatus was designed and constructed for the study of forced-convection boiling in small channels. The various regions of forced-convection surface boiling were studied experimentally and analytically. In the region of low wall superheat, the heat flux can be predicted by available correlations for forced convection. Data indicate, however, that these correlations do not properly account for the radial variation of properties for water at high temperature difference. The conventional Dittus and Boelter-McAdams relationship is recommended for design purposes on the basis of its simplicity and conservative predictions. An analysis for the prediction of the inception of first significant boiling was developed. Experimental results are in good agreement with analytical predictions. The analysis provides information necessary for the prediction of the complete forced-convection surface-boiling curve. Data for a small-diameter tube indicate that the bubbles formed at incipient boiling can trip the laminar or transition boundary layer to a fully-developed turbulent boundary layer. The region of vigorous boiling coincides approximately with the extrapolation of the pool-boiling curve in one set of experiments. In other experiments, pool-boiling data were strongly influenced by fluid and surface conditions, as well as by bubble-induced convection in the pool. Due to the complexities in these pool-boiling data, it is impossible to make a conclusive comparison with forced-convection-boiling data. The heat flux obtained by a superposition of pool boiling and forced convection is close to the apparent asymptote for fully-developed boiling. For design purposes, it is concluded that fully-developed -forced-convection boiling can be related to pool boiling by either direct extrapolation or superposition of forced convection.(cont.) The burnout heat flux under conditions of forced convection and surface boiling is shown to be a complicated function of subcooling at low values of subcooling. This appears to be due to the velocity increase caused by the relatively large volume fraction of vapor. Burnout flux is shown to increase with decreasing tube diameter. This effect can be attributed to an increase in void fraction with decreasing tube diameter. Entrance effects are significant in forced-convection surface boiling as shown by the decrease of burnout flux with increasing length. Flow oscillations caused by system compressibility can greatly reduce the burnout heat flux in the subcooled region. This instability is particularly difficult to avoid with tubes of very small diameter.Air Force Office of Scientific Research D.S.R. Projec