Experimental Investigations for Heat Transfer Characteristics of Under-expanded Impinging Jet

Abstract

In this study, under-expanded impinging gas jets are investigated experimentally for understanding heat transfer characteristics of the jets. As working fluids, nitrogen (N2) and carbon dioxide (CO2) are selected in order to observe heat transfer effect changed by different working fluid. CO2 jet has a potential way to enhance the heat transfer effect which is sublimation. The novel concept of dry-ice assisted jet impingement cooling is proposed in this study. When carbon-dioxide (CO2) passes through a tiny orifice gap or jet nozzle, it experiences a rapid temperature drop as well as a pressure decrease via the Joule-Thomson effect. Joule-Thomson coefficient of CO2 is proven to be higher than the coefficient of other gases’ such as nitrogen, hydrogen, air. This temperature drop causes the formation of small CO2 dry-ice particles. In addition to the enhanced cooling performance caused by lowered bulk-jet temperature, heat transfer is improved by the additional sublimation effect between the dry-ice particles and the cooling target surface. A comparison of the cooling performance between the suggested CO2 solid-gas two-phase jet and a single-phase nitrogen (N2) jet was performed experimentally as well. In order to form dry-ice particles, high pressure and velocity of jet fluid are inevitably required, which is enough for compressible effect to appear. Both jets have differences not only in phase change but also in jet flow structures. As jet velocity increased, shock structures at jet downstream appeared and surface temperature is changed as well. The structures are detected more clearly in N2 jet than CO2 jet because of the difference in total pressure at jet boundary, therefore, relationship between shock structure of N2 jet flow and heat transfer is investigated in this study. In case of high Reynolds number impinging jet which is enough to reach supersonic flow regime, stagnation temperature of impinged surface is affected by jet structure as well as other factors such as nozzle-plate distance or radial distance does. When the jet flow velocity becomes supersonic, shock structures are constructed at downstream of the nozzle exit. Complicated shock structure such as Mach shock disk, plate shock is highly expected to affect to the heat transfer behavior of impingement surface. In this study, a cooling performance of supersonic N2 jet is investigated by measuring the impinged surface temperature and the flow of the jet is visualized by Schlieren image system. Visualized image and surface temperature are compared to clarify the flow structure-related heat transfer characteristics. In all the experiments of present study, jet fluids are expanded through a circular nozzle and impinged on an electrically heated flat heater surface, and their heat transfer coefficients are measured. The performances of the impinging jet for both fluids are also evaluated via the variance of flow parameters, for example, the Reynolds number, and the jet geometry configurations.