Highly subcooled flow boiling of freon-113 and water over cylinders

The primary intent of this paper is to determine the influence of high subcooling and velocity on the ratio of minimum to maximum heat flux. The configuration was upflowing liquid in crossflow over 0.635-cm electric heaters. As a part of the study, azimuthal temperature variations around the heater during nucleate and film boiling were measured, the manner of onset of film boiling and vapor film collapse was observed and photographed, and the effects of subcooling and velocity on qmin and qmax were determined. Experiments were conducted at subcooling levels up to 58.6 C and a velocities up to 3.81 m/sec for Freon-113 and 95 C and 2.98 m/sec for water. While both qmin and qmax were sensitive to velocity and subcooling, the qmin to qmax ratio was seen to be a stronger function of subcooling than velocity. Previous investigators have reported that the qmin to qmax ratio for Freon-113 increases from 0.083 for pool boiling to 0.303 for 6.87 m/sec at low subcooling levels. In the Freon-113 experiments at 57 C subcooling and 3.53 m/sec, the ratio was found to be 0.77. Discontinuities in measured wall temperatures were observed when the vapor film collapsed over certain portions of the heater prior to the entire heater being enveloped in nucleate boiling. Our observations confirm such behavior previously observed in our laboratory. These discontinuities are caused by film and nucleate boiling coexisting in near proximity of the location of thermocouple used to measure local heater temperatures. It was also observed that the bottom of the heater underwent an unexpected temperature drop during nucleate boiling for high levels of subcooling. Such temperature drop were very repeatable. They resemble the temperature overshoot phenomena that have been observed in pool boiling experiments on a variety of heating surfaces and configurations