NASA EPSCoR: Fundamental Studies on Boiling: Gas Entrapment in Microcavities

Boiling is a common phenomenon that is encountered in a broad range of applications, from industrial processes to everyday-life appliances. For example, in the electronics industry there is a strong need to increase device cooling rates in order to keep pace with increasing processing power, and nucleate boiling heat exchangers are among the most efficient solutions for this application. Another example comes from the nuclear energy field, where boiling phenomena play a critical role in nuclear plant safety as it determines the rate of loss of coolant in LOCA accidents. Each of these applications need fundamental understanding of boiling in order to develop reliable, predictive models of boiling rates. However, current designs are based on empirical models rather than on a first principles approach, in part due to the limited knowledge of some of the processes that control nucleate boiling. The research proposed here, motivated by our previous work on boiling waves that occur in LOCA accidents, will contribute to fill this knowledge gap by addressing the early dynamics of bubble formation, an up to now missing key ingredient in boiling modeling. Boiling occurs commonly in two steps: first an initial nucleus of vapor must originate in the liquid; and second, once a vapor initial nucleus is available, it will grow if the degree of supersaturation of the liquid phase is large enough. Thus, the nucleation of the vapor phase is a key process in the boiling dynamics. Among the several known routes leading to new vapor nuclei, the most accessible is the entrapment of gas/vapor pockets in cavities or defects on the walls of liquid containers as liquid flows over them. Once the cavity is filled partially or completely with gas/vapor, it will be able to sustain bubble growth/release events typically for a large number of these events. However, the entrapment dynamics is not still well understood. Most of the analyses, starting with the pioneering Bankoff's work, either deal with a purely static evolution, or are based on thermodynamic approaches, the dynamic nature of the process being lost. Nevertheless, these effects are essential in determining the initial size of the entrapped vapor nucleus. In turn, this size controls the subsequent boiling dynamics: the delay time for growth or the frequency of bubble formation and release for instance, that are treated in literature up to now on a purely phenomenological basis, depend critically on this size. The proposed research will address this important problem of the dynamics of gas entrapment in microcavities, which then serve as nucleation sites for boiling. Preliminary analyses show that as liquid flows over a microcavity (with typical sizes ranging from some microns to several tens), surface tension, along with the contact angle between liquid and surface, drive the liquid motion filling the cavity. This is a low Reynolds number flow with characteristic times that are typically of the order or shorter than the evolution at the macroscopic scale, that governing the motion of the bulk liquid flowing over the cavity. Different regimes are encountered according to the value of the capillary number, the ratio between the characteristic cavity filling time to that of the bulk liquid flow over the cavity. When this ratio is large, the flow over the cavity is much faster than that into the cavity, so only a small amount of liquid will enter. In this case the whole cavity gets filled with gas or vapor, leading to the so-called Cassie-Baster wetting model. The cavity is activated to bubble growth. In the opposite limit, for small values of the capillary number, the flow into the cavity is much faster than the outer one, and complete filling with liquid (called Wenzel wetting regime) could happen. The cavity cannot support bubble formation in this case. Clearly, the size of the entrapped vapor pocket depends on the capillary number, but also on the cavity geometry and on the contact angle between the liquid and the surface. We will address the influence of these effects by means of numerical simulations and also experimentally, with flow visualization on microfabricated controlled geometry cavities. Thus, a main outcome of this research will be the quantitative, first-principles understanding of the basic mechanism of cavity activation, providing access to, for instance, how the initial size of the entrapped vapor nucleus depends on the cavity geometry, the characteristics of the bulk flow, or on physicochemical fluid properties as the contact angle, surface tension, viscosity... This knowledge is essential in determining and controlling the vapor production rate and, consequently, the cooling rate needed in applications. In addition, these results will also provide a rational basis to the development of tailored micropatterned surfaces aimed at promoting or preventing boiling. It is our feeling that the slow progress experienced in boiling modeling is due in part to lack of fundamental knowledge of these phenomena, in particular, about the initial stages of bubble formation. Good quantitative knowledge of the basic physics that will result from this work will represent a breakthrough able to trigger rapid progress, both in applications and in basic research related to boiling physics.