Microscale Cavitating Flow Patterns and Spray Characteristics with Applications
Mechatronics Engineering, PhD Thesis, 2016
Prof. Dr. Ali Koşar (Thesis Advisor), Prof. Dr. Mustafa Ünel
, Assoc. Prof. Dr. Mehmet Yıldız, Prof. Dr. Asiye Işın Doğan Ekici, Asst. Prof. Dr. Hüseyin Üvet
Date & Time: 28th, December, 2016 – 14:00-16:00
Place: SUNUM, G111
Keywords : Cavitation; Spray; Micro/Mini-Channel; Turbulence; Energy; Collapse
Spray formation occurring at the outlet of short micro/mini-orifices due to the cavitation phenomenon is of great importance in biomedical and engineering applications. Recent studies show the destructive effect of the energy released from the collapse of cavitation bubbles, which are generated in micro domains, on the targeted surfaces. The cavitation phenomenon occurs at low local pressures within flow restrictive elements and strongly affects fluid flow regimes inside micro-channels which results in spray formation. Extended cavitation bubbles toward the outlet of the micro-channel, droplet evolution, and spray breakup are among crucial mechanisms to be considered in spray structure. In this study, spray formation and atomization, bubble and droplet evolution, break-up, and corresponding cavitating flow at the outlet of short micro/mini-channels are discussed at different physical and thermo-dynamical conditions. Cavitation phenomenon inside micro/mini-channel configurations are numerically investigated in detail. The results of this study show that the static pressure drops down to a very low magnitude (tensile stress) in micro-channels while the minimum static pressure in mini-channels is found to be equal to vapor saturation pressure, and higher velocity magnitudes particularly at the outlet are visible in the micro-channels. It is shown that for higher upstream pressures, the cavitating flow extends over the length of the micro/mini-channel thereby increasing the possibility of collapse at the outlet. A detailed study on the effect of energy associated with turbulence is investigated at high Reynolds numbers for both micro/mini-channels and its impact is analyzed using wall shear stress, turbulence kinetic energy and mean velocity at various locations of the channels. We find that there is a considerable difference on the flow regime between the emerging sprays at the outlet of the channels in micro and mini/macro scales. The spray at the outlet of nozzle has a conical shape with separated droplet/bubbles, however, interestingly the spray shape entirly differs in macro scale presenting spray jet flow regime at the same thermo-physical conditions. we showed that with the aid of the hydrodynamic cavitation in a low-cost and clean system, the spray jet has the capability of heat generation in contrast to the common use of spray jet in the cooling applications. The emerging spray is under the effect of the micro scale cavitating flow inside the micro/min-channels which is much more intense in comparison to its correspondence at macro scale. The temperature measurements on a black-covered aluminum plate subjected to the spray interestingly show a considerable increase for a specific micro-channel. This temperature rise would be potentially utilized as a power source in miniature electric appliances with a simple energy conversion device. Herein, we present a complete set of numerical and experimental results on the micro/mini scale cavitating flow, spray emergence and its interaction with a solid body which will increase our undersatnding about the physics of the cavitating flow inside the micro/mini-channel and its relation with the emerging spray structure.