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American Journal of Applied Sciences
Year: 2010  |  Volume: 7  |  Issue: 6  |  Page No.: 756 - 762

Homogeneous Bubbles during Jet Impingement Quenching

M. A. Islam and S. F. Ahmed    

Abstract: Problem statement: Jet impingement quenching has a very effective and large cooling potential and is a very effective means of cooling for many applications. It gives rise to heterogeneous and/or homogeneous nucleation of bubbles, which is yet to be explored. Approach: Analytically investigate the phenomenon that happened during a brief contact of a water jet impinging on a hot surface. In particular, explore the possibility of homogeneous bubble formation during jet impingement quenching. A simple semi-infinite conduction heat transfer model was considered in this case and the heat transfer analysis had been carried out for two heating cases of the impinging liquid, namely with (i) Prescribed Surface Temperature (PST-case) and (ii) Time-dependent Surface Temperature (TST-case). For each of above cases, explicit equations for temperature distribution within liquid, average liquid temperature, average internal energy and surface heat flux had been derived as a function of liquid depth and contact time. These equations were solved numerically and their outcomes were discussed. Furthermore, a critical contact time, t* at which the temperature on the opposite side of the liquid depth reached saturation temperature, Tsat was determined. The heat flux (qs) at time t* was also determined and compared with the thermodynamic limit of the maximum heat flux (qmax) which decided the validity of this analytical investigation. Results: During jet impingement quenching of hot surfaces near/above the thermodynamic limiting temperature, water stored enough energy for a contact period of 5-10 μs depending on the type of boundary conditions considered for triggering homogeneous bubble nucleation. A contact time of 4μs was required to trigger homogeneous bubble nucleation for the conditions where the impinging surface had a fixed temperature throughout the cooling process. A contact time of 6.5 μs was required to initiate homogeneous bubble nucleation for the conditions where the impinging surface temperature dropped at a rate depending on time. For both cases, the average internal energy of the liquid exceeded the minimum energy necessary for bubble formation. Moreover, the average heat flux, q never exceeded the thermodynamic limit of the maximum heat flux, qmax. Conclusion: When water was heated above the thermodynamic limit of superheat during jet impingement, there is always a chance of homogeneous bubble nucleation.

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