Liquid crystal images of the transition from jet impingement convection to nucleate boiling .1. Monotonic distribution of the convection coefficient

Liquid crystal thermography was used to study nucleate boiling incipience on a thin, uniform-heat-generation surface that was cooled by a submerged impinging jet. The fluid was the refrigerant R-11. Thermographic images of the temperature distribution on the test surface were acquired synchronously with single-point temperature measurements from three locations on the surface. These measurements provide a framework for interpreting the images and quantifying the incipience temperature overshoot. The nozzle-to-plate spacing was such that the convective heat transfer coefficient in the single-phase flow decreased monotonically with radial distance from the stagnation point. The images clearly reveal a large temperature gradient, or boiling front, that existed at the interface between areas cooled by convection and areas cooled by boiling. In the wall jet region of the flow, the orientation and average radial location of the front were determined by the distribution of the convection coefficient in the single-phase flow field. A simple model was developed to demonstrate this dependence. The shape of individual segments of the front indicated that the precise position of the front on a specific portion of the surface depended on the local distribution of surface microcavities. Near the stagnation point, boiling was initiated when fluctuations in the direction of the jet flow allowed a momentary rise in the surface temperature to above the nucleation temperature.