Guo-Xiang Wang

Thermal Characteristics of Flashing Spray of Volatile R134a Cryogens

This paper presents an experimental investigation of thermal behavior of flashing spray of refrigerant R134a. A high-speed camera up to 1000 frames per second is used to visualize the shape and size of the spray and an infrared thermograph is employed to visualize the temperature variation of the spray. The temperature field within the spray is measured by a small thermocouple whose position can be systematically adjusted to cover the entire spray. The high-speed photo images of sprays revealed an explosive spray formation near the exit of nozzle and the formation of a hot core region near the nozzle. The measurements found that the hot core region was quickly dissipated, followed by a cold region of almost uniform temperature that is far below the saturation point of R134a at the ambient pressure. The measurements provide detailed quantitative information of both radial and axial temperature distributions of droplets within the spray. These results provide insight into the formation of flashing spray of volatile liquids.Copyright

Theoretical Evaporation Model of a Single Droplet in Laser Treatment of PWS in Conjunction With Cryogen Spray Cooling

Cryogen spray is an effective cooling technique used for laser treatment of Port Wine Stain. The cooling process involves complex droplet evaporation and strong convective heat and mass transfer, therefore a deep understanding of spray characteristics is essential in order to optimize the nozzle design and improve the cooling efficiency of the spray. This paper improves a theoretical model to describe the equilibrium evaporation process of a single cryogen droplet in cryogen spray. The results of comparative analysis of gas phase models for single droplet heating and evaporation are presented. Six different semi-theoretical models based on various assumptions are compared and their effects on a single droplet heating and evaporation characteristics in the process of cryogen R-134a spray cooling are compared. It is pointed out that the gas phase model, in which the effect of superheat is taken into account, predicts the evaporation process closest to the experimental data. Finally, a parametric study of the influences of initial diameter and velocity on the droplet evaporation is then carried out. The results can be used to guide the Cryogen Spray Cooling of laser therapy.Copyright

An Experimental Study of Heat Transfer With an Impinging Circular Water Jet

Impinging jet is widely used in both traditional industrial and new high-tech fields. High efficiency heat transfer in impinging jet cooling makes it an important method for heat transfer enhancement, in particular in cooling of electronic devices with high heat density. This paper presents an experimental study of heat transfer by an impinging circular water jet. A Constantan foil with the size of 5 mm × 5 mm was used to simulate a microelectronic chip with heat generated by passing an electrical current through the foil. A high heat flux over 106 W/m2 was achieved. The surface temperature was measured by a thermocouple glued onto the back surface of the foil. Both a free surface jet and a submerged jet were investigated. Effect of the nozzle-to-surface spacing as well as the jet speed at the exit of the nozzle on cooling was examined. By positioning the jet away from the center of the heating foil surface, the radial variation of the heat transfer coefficients over the foil was also investigated. Quantitative heat transfer data have been obtained and analyzed.Copyright