UCLA

Boiling heat transfer research spans several decades with extensive data accumulation from several experiments. Several mechanistic models and empirical correlations have also been put forth but their applicability is limited to the narrow range of parameters over which they have been developed. For the past few decades several numerical methods have also been developed and gained considerable momentum to study boiling process. The most popular of these numerical methods are volume of fluid method, level set method and front tracking methods, however some other computational methods like Lattice Boltzmann, Moving particle semi-implicit gridless and cellular automata SIMPLER methods have also been developed.

In the present study level set method is used to simulate the entire boiling curve in a temperature controlled mode spanning all the three regimes viz. nucleate, transition and film boiling with a unified numerical model supplemented with correlations specifying nucleation site density and bubble waiting time. In order to improve the performance of the code parallel computing has also been implemented. Both two-dimensional and three-dimensional simulations have been done for saturated water with different contact angles for a horizontal surface with uniform wall superheat applied to it. Temporal and spatial averaged wall heat flux and wall void fraction computed for a fixed wall superheat case are plotted and analyzed. For a specified contact angle and by incrementing the wall superheat as two independent input parameters, the entire boiling curve along with vapor removal patterns capturing its vital points like the maximum heat flux and minimum heat flux are shown.

The two-dimensional assumption yields opposite trend for the nucleate boiling regime heat flux variation with contact angle but this anomaly was not observed in the three-dimensional simulations which infers that the two-dimensional assumption is an incorrect representation to study the essential physics of the boiling process. The trend of critical heat flux with contact angle was found to be decreasing with increase in contact angle for both two-dimensional and three-dimensional case, with the trend being steeper for the former.

Wall void fraction was found to increase with increase in wall superheat as different regimes of boiling were traversed, and also with increase in contact angle at a given wall superheat. Mushroom type vapor bubbles are seen in the nucleate boiling regime with liquid macrolayer trapped underneath it while long column of sustained vapor is seen at the critical heat flux condition continuously being fed by nucleating cavities at the surface. Upon increasing the wall superheat beyond critical heat flux the negative slope of the boiling curve is captured characterized by the transition boiling regime with intermittent liquid solid contacts seen. Finally, the transition to film boiling is seen with entire surface covered with superheated vapor and wall void fraction reaching unity. Energy partitioning from wall into liquid, interface and microlayer has also been examined for the 3D cases. For the 3D coarse grid case it was found that, as the wall void fraction increases the percent energy going into liquid decreases from about 85% in lower nucleate boiling to 6% in film boiling while the microlayer contribution peaks around CHF to a value of about 45%. The energy partition from the fine grid cases were however inconclusive as they couldn`t be run long enough to obtain any meaningful results.