Homogeneous Bubbles during Jet Impingement Quenching

Islam Islam

doi:10.3844/ajassp.2010.756.762

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.

Highlights

Jet impingement quenching has a very effective and large cooling potential and is a very effective means of cooling for many applications
As a follow up of the study by Islam et al (2007b), the present analysis finds the average surface heat flux during jet impingement quenching at the early stages using a new approach by the concept of critical time t* and compares it with the maximum thermodynamic limit of surface heat flux determined from molecular dynamics
As the average heat flux never exceeds the thermodynamic limit of maximum heat flux as well as the average internal energy of liquid well-exceeds the minimum energy required for bubble formation throughout the investigation, it is evident that, homogeneous bubble nucleation will initiate during jet impingement quenching after proper periods of contact

Summary

Introduction

Jet impingement quenching has a very effective and large cooling potential and is a very effective means of cooling for many applications. It is very important in Loss Of Coolant Accident (LOCA) analysis, steel manufacturing, metallurgy, microelectronic devices and thermal management processes. It may be useful in elucidating poorly understood phenomena such as Leidenfrost phenomenon and homogeneous nucleation. A comprehensive review of jet impingement boiling was made by Wolf et al (1993).

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