Multiphase flow visualization of alternative refrigerant mixtures
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BACKGROUND: The tropical temperature for most part of the year in the Middle East region is driving the demand for cooling devices in an exponential way. The refrigeration and air conditioning market in the Middle East is experiencing an accelerated growth due to high demand for cooling devices such as refrigerators and air conditioners. The cooling devices work by circulating a fluid (called refrigerant), that often undergoes phase change from liquid to vapor and back again in a cyclic manner. Most of the cooling devices in use today are built using synthetic (man-made) refrigerants. Although the synthetic refrigerants possess good thermophysical properties, their environmental damage potential (like ozone depletion and global warming) are extremely high when compared to naturally existing refrigerants like water, carbon dioxide, ammonia, hydrocarbons etc. Therefore, the synthetic refrigerants can cause severe damage to the environment, if released in to the environment. Consequently, the global community (governments, organizations, and scientific communities) have formulated policies like Montreal, Kyoto, and Kigali protocols to limit and phase-out such harmful refrigerants in a phased manned in the near future. For example, these protocols have completely banned the use of Chlorofluorocarbons (CFC), and planning to phase-out Hydrochlorofluorocarbons (HCFC) within the next few years. ABOUT NATURAL REFRIGERANTS: On the other hand, although natural refrigerants are inherently safe to the environment, they have their own limitations in terms of usage in cooling devices. For example, hydrocarbons have fire safety hazard, carbon dioxide may not be suitable for geographical locations with high ambient temperatures, and ammonia is toxic in nature. Nevertheless, there has been a considerable interest recently in using the mixture of natural refrigerants (like CO2 and hydrocarbons) as refrigerants in cooling devices. The inert carbon dioxide is mixed with hydrocarbons to reduce the fire hazard associated with the usage of hydrocarbons. However, the key challenge in using the mixture of natural refrigerants like carbon dioxide and hydrocarbons is related to the mixture phase change phenomena due to the difference in their boiling points. This phenomena has a profound impact on the overall performance of the cooling devices. Thus, it is essential to have a sound knowledge of their flow pattern to gain insights on their cooling (i.e., heat transfer) performance. MOTIVATION / OUTCOMES: These factors serve as the motivation for the proposed research, which will investigate the flow pattern exhibited by the mixture of carbon dioxide and hydrocarbons at different mixture concentrations at operating conditions that are relevant to practical cooling devices. It is worth mentioning here that until now, such flow visualization studies have not been reported in the literature for the mixture of natural refrigerants. Therefore, the outcomes of this proposal not only will help to address the knowledge gap in the scientific literature but also, will give an opportunity to the undergraduate students to gain research experience that can have a significant impact. The research mentors have performed a preliminary research to evaluate the performance and safety aspects of the natural refrigerant mixtures. The proposed research can be executed using the existing experimental facility at Texas A&M University at Qatar with minimal modifications. The major scientific instruments like high speed camera and high power background lights required for shadowgraph imaging are already available with the research group, and they will be used to perform the proposed research. The research mentors will provide necessary orientation and training to the undergraduate students to successfully achieve the proposed goals. The outcomes of the proposed research will be disseminated through international conference and journal publications.
