The shoulder effect in the stagnation zone for temperature-controlled jet impingement transition boiling of submerged dielectric fluid

Abstract

Two-phase jet impingement is capable of high heat transfer coefficients and high boiling critical heat flux (CHF) limits, making it a suitable technology for electronics immersion cooling applications. Previous studies have focused on the heat-flux-controlled nucleate boiling performance of an impingement jet up to the CHF. Exploration of other boiling regimes that occur under temperature-controlled surfaces is of equal fundamental importance. 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 so-called “shoulder effect”, or heat flux shoulder, is strongest in the stagnation zone, directly beneath the jet. It has only observed using free jets of water as the cooling for interest in metals processing applications. To exploit operation in this unique 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 via a PID controller, which allows for steady-state, temperature-controlled data to be acquired. The boiling curves, including CHF and shoulder heat flux, are measured for 0.5–3 m/s jet velocities and 5–30 K inlet subcooling. The shoulder effect is shown to exist in these conditions and high-speed imaging reveals that the shoulder heat flux effect is an enhanced film-like mode of transition boiling. It is observed that there is a proportionality between the CHF and the shoulder heat flux for the conditions investigated. The measured trends and dependencies of the shoulder heat flux on inlet subcooling and jet velocity are used to assess available predictive tools.