| A forced convection boiling facility has been designed, fabricated, and tested to experimentally study small-scale heat transfer processes in vertical upflow and downflow forced convection boiling. The facility incorporates a transparent boiling test section, through which the ebullition process can be observed and recorded using digital single-frame and high speed motion cameras. A capacitance-based sensor was developed and calibrated for measurement of liquid film thickness in annular two-phase flows.; Analysis of a current model for nucleation site density has led to the development of a new dimensionless parameter, the cavity size ratio, which is demonstrated to correlate nucleation site density. Measurements of flow boiling suppression in annular flow has revealed that the cavity size ratio is, to a leading order, a fundamental parameter on which the suppression of nucleation depends. This has led to the introduction of a flow boiling regime map which delineates the purely convective and nucleate boiling regimes.; Experimental measurements of the ebullition process in single-phase inlet upflow and downflow boiling has yielded a wealth of data for vapor bubble growth rate, departure and lift-off diameters, sliding trajectories, and waiting times, in the isolated bubble regime. One very significant result of these experiments is that vastly different vapor bubble dynamics are observed for upflow, downflow, and vertical pool boiling. It is demonstrated that the differences in vapor bubble dynamics are responsible for a marked increase in the heat transfer rate for upflow compared with that of downflow.; Based on these observations, a bubble dynamic model is developed which predicts the departure diameter, sliding trajectory, and lift-off diameter in upflow, downflow, and vertical pool boiling. The model compares well to experimental data recorded in this work, as well as data reported in the literature. Furthermore, the model is applied to the case of horizontal stratitied flow boiling, for which it shows a marked improvement over previous models for horizontal flow. Analysis of the model reveals, among other trends, that a negative shear lift force is responsible for maintaining vapor bubbles at the wall in upflow and ejecting bubbles from the wall in downflow.; Studies of sliding vapor bubbles have revealed that the sliding vapor bubble mechanism contributes significantly to the macroscale heat transfer in forced convection boiling. Experiments incorporating air bubble injection at the heating surface suggest that the presence of sliding bubbles enhances the bulk turbulent heat transport from the surface, and that turbulence enhancement is at least as important as the latent heat contribution to the sliding bubble heat transport mechanism. |
