Fundamentals of Gravity and Tube Size Effects on Flow Boiling Heat Transfer

The research completed as a part of this fellowship maintained a general goal of advancing our understanding of flow boiling heat transfer at various gravity levels. This was done in an effort to provide more accurate heat transfer coefficient correlations to thermal management engineers so that they can design more efficient space-based heat exchangers, thereby reducing the cost of materials and launches. The experiment utilized for these studies is summarized in papers by the fellow [1–3]. Measurement methods include an infrared thermography technique in which simultaneous heat transfer measurements and flow visualization can be obtained, high-speed flow visualization, capacitance void fraction sensors, and a film thickness sensor. Early work included parabolic flights in Bordeaux, France, where heat transfer and void fraction measurements were made of boiling HFE 7100 at various gravity, mass flux, vapor quality, and mass flux conditions. The results indicated the heat transfer coefficient measured in the tubular test section was dependent on the flow regime. When transitions in the flow regime occur due to variation in the gravity level, in many cases the measured heat transfer coefficient correspondingly changed. This highlighted two main needs in the design tools for space thermal management systems: 1) comprehensive flow regime maps for flow boiling in microgravity, and 2) a mechanistic understanding of the heat transfer for each flow regime. The UMD experiment is best equipped to investigate the second of these needs. A new round of experiments was completed in the laboratory in which annular flow and slug flow were of particular focus. Annular flow is characterized by a thin liquid film flowing along the inner tube wall, while a vapor core flows at much higher speed in the center of the channel. The difference in liquid film and vapor core velocities causes instabilities to occur in the liquid film, which appear as waves. Simultaneous heat transfer measurements and flow visualization showed that while moderate heat transfer values occur due evaporation of the liquid film, higher values are seen when the disturbance waves pass. It is believed that recirculation patterns within each wave provide an increase in convective cooling of the tube surface as each wave passes. Slug flow is generally characterized by bubbles with diameter very close to that of the inner wall of the tube separated by liquid slugs. This flow regime is most commonly seen in microchannel flow boiling where both ends of the bubble are spherical. In the case of this research, however, the bubbles, known as Taylor bubbles, are bullet shaped and occur over a small range of vapor qualities at low flow rates. Several models have been developed to predict the heat transfer coefficient for slug flow [4,5], but local experimental heat transfer measurements have been lacking. Laboratory experiments were completed in which a single Taylor bubble was grown and released. As the bubble rose in co-current flow, infrared heat transfer measurements, high-speed flow visualization, and liquid film thickness measurements were obtained. It was observed that unlike microchannel slug flow, the greatest heat transfer enhancement over single-phase flow was caused by turbulent vortices in the bubble wake. The level of enhancement was found to decrease with increasing liquid mass flux due to a reduced residence time of the bubble over a particular location in the tube, decreasing the mixing potential of each vortex. Furthermore, liquid film thickness measurements were found to agree well with classical correlations when the measured bubbles were long enough such that their films were fully developed. Very recently, isolated rising Taylor bubble experiments were conducted on parabolic flights in Houston, TX. Early results show that with increasing gravity level the drift velocity of the bubble with respect to the liquid also increases, as was expected. At very low drift velocities, the bubbles possess spherical ends similar to microchannel slug flows. These cases had no vortices in the bubble wake and the heat transfer enhancement as the bubble passes occurred only in the thin liquid film. Above a critical drift velocity, vortices began to appear and the bubble shape resembled a typical Taylor bubble. The frequency at which vortices were generated was found to increase with increasing drift velocity. Further analysis is ongoing to extract more mechanistic understanding of slug flow heat transfer from these results. In conclusion, the research completed over the length of this fellowship has taken significant steps to improve the knowledge base from which fellow researchers can develop more accurate mechanistic heat transfer models for the flow regimes experienced in vertical upward flow boiling, particularly in reduced gravity environments.