Phase Change Heat Transfer – a Perspective for the Future

During the last half of the twentieth century, significant advances have been made in developing an understanding of phase change heat transfer (e.g., boiling and condensation). Further advances in phase change heat transfer will continue to take place motivated by new technologies such as microelectronics, thermal management in space, advanced terrestrial and space power systems and processing of designed materials. In the past, because of the complexity of the processes, very often we have “oversimplified”, maybe out of necessity, the modeling of the processes. The resulting weaknesses in our models and correlations have continued to haunt us whenever we have encountered new applications. In order to address the phenomena from basic principles, in my opinion, we need to pay attention to processes occurring at nano to micro to macro scales, capitalizing on recent advances that have been made in experimental and numerical techniques. These phenomena include nucleation, evolution, merger and breakup of vaporliquid interfaces, contact line behavior; coupling of the bulk and surface features of the solid; and the role of nano and micro inhomogeneties and intermolecular forces between solid and liquid. Prediction of nucleate boiling transfer is taken as an example to demonstrate the value of coupling different scales in meeting the overall objective. INTRODUCTION Phase change heat transfer is a broad field that finds applications in almost all of the engineering disciplines. Boiling and condensation are two of the most important phase change processes as they are generally associated with high heat transfer rates. Boiling and condensation (drop wise) are very complex processes as well, and have been investigated extensively over the last half of the twentieth century. Professor Warren Rohsenow, in whose honor this paper is written, was a pioneer in this area of heat transfer. Past studies have lead to an increased understanding of the processes as well as to the development of correlations and semi-theoretical models at the global and subprocess level. These correlations and semi-theoretical models have served us well in their intended application. However, the simplifications we have made in developing correlations or models for the process have haunted us whenever new applications are encountered. Although further advances in phase change heat transfer will continue to occur in the future driven by new technological needs in micro-electronics, thermal management, power systems and material processing, in my view, future research trends in two-phase flow heat transfer will be in developing and solving conservation equations similar to those used for singlephase flows. In obtaining these solutions we must tie the underlying physics from nano to micro to macro scale. A significant effort will be required in relating results from different scales. Of course, advances in new instrumentation techniques [1] such as x-rays, liquid crystal thermography, high-speed infrared thermometry, nuclear magnetic resonance (NMR), neutron tomography, and laser induced fluorescence will play an important role in validating the physical models at various scales. In discussing the future research direction and needs, an example of nucleate boiling is considered. Nucleate boiling involves most of the basic elements of interest in generic two-phase heat transfer problems. FUTURE RESEARCH DIRECTION In nucleate boiling, vapor bubbles form on discretely located sites on heater surface. The formation and departure of vapor bubbles leads to enhancement in heat transfer. The interacting physical processes that lead to the enhancement in heat transfer beyond the single phase value are the evaporation at microlayer underneath the bubble, evaporation around the bubble, heat transfer due to bubble created flow field, and convective motion resulting from buoyancy. The contributions