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THE SHOULDER EFFECT IN TRANSITION BOILING DURING SUBMERGED JET IMPINGEMENT
Two-phase jet impingement combines the latent heat absorbed by boiling heat transfer with the strong forced convection of an impinging jet. It is a compact and highly effective heat transfer method that is capable of high heat transfer coefficients and high boiling critical heat flux limits. This makes it a suitable technology for electronics immersion cooling applications when configured as a submerged jet of a dielectric coolant. Previous studies have focused on the heat-flux-controlled nucleate boiling performance of an impingement jet up to the critical heat flux. Exploration of other boiling regimes that occur under temperature-controlled surfaces is of fundamental importance to fully understand the design space. It has been shown for free jets that a high and consistent heat flux can be dissipated over a wide range of surface superheats in the transition boiling regime when the surface is temperature-controlled. This effect is strongest in the stagnation zone, directly beneath the jet. Literature that studies this so-called “shoulder effect”, or heat flux shoulder, is scarce and almost completely focused on applications in metals processing using free jets of water as the coolant. It has been hypothesized previously that the impinging subcooled liquid delays and disrupts the start of film boiling, thereby dissipating heat flux levels comparable to that during nucleate boiling. To exploit operation in this unique transition boiling regime for potential applications in immersion cooling of electronics, the occurrence of this shoulder effect, as well as means for estimating the shoulder heat flux across different operating conditions, must be investigated for submerged jets and dielectric coolants.
In this work, temperature-controlled submerged jet impingement is experimentally characterized using HFE-7100. A copper heater sized to be completely covered by the jet stagnation zone is increased in surface temperature throughout the transition boiling regime via a PID controller, which allows for steady-state temperature-controlled data to be acquired in this regime. The boiling curves, including critical heat flux and shoulder heat flux, are measured for jet velocities from 0.5-3 m/s and inlet subcooling from 5-30 K. The shoulder effect is shown to exist in these conditions. High-speed imaging is used to relate the flow behavior to the boiling thermal measurements and shows that the shoulder heat flux effect is an enhanced film heat transfer in the film-like mode of transition boiling. Trends and dependencies on inlet subcooling and jet velocity are measured and used to assess available predictive tools. It is observed that there is a proportionality between the critical heat flux and the shoulder heat flux. This implies a mechanistic similarity between the two effects. With further data to correlate, this similarity can potentially be used to predict the shoulder heat flux leveraging existing correlations for the critical heat flux, widening the design space of two-phase jet impingement systems.
- Master of Science
- Mechanical Engineering
- West Lafayette