SPRAY COOLING OF A VERTICAL GRID OF HORIZONTAL TUBES

Abstract

Transient heat dissipation experiments were performed on a single heated horizontal tube and a vertical grid of horizontal tubes with water droplets formed by an air-atomized spray nozzle. A hot, dry surface is quenched by bringing it into abrupt contact with a coolant, causing a fast fall in surface temperature. Various factors were analyzed in this experiment, such as tube diameter, tube thickness, initial tube temperature, nozzle-to-tube distance, and air-water pressure at the nozzle inlet. To assess how each spray parameter affects heat transfer during spray cooling, the tests systematically adjusted each spray parameter separately. During the cooling process, constant heat flux was provided on each tube with the help of a step-down transformer with Variac. Temperature variations in the circumferential direction were measured over time during cooling. A high-speed video camera was employed to investigate the flow behavior on the top tube surface as well as the spread of the re-wetting front around it. Experiments and visual analysis show that wetting front velocity rises with the water flow rate. The rewetting velocity on the tube surface was determined using the outcome of thermocouples mounted on the tube wall and an imaging system used to record the video picture during the runs. The two techniques of calculating rewetting velocity are compared. An excellent agreement regarding rewetting velocity has been reported utilizing thermocouples and a high-speed camera. To estimate the local spray impingement density on the top tube surface, an in-house mechanical patternator was designed and developed. The heat flux dissipated across the tube surface during spray cooling was calculated using the response of thermocouples. The main aim of the study is to provide a fundamental understanding of the extremely complicated heat transfer mechanism during the airatomized spray cooling process. The heat transfer and the patterns in the experimental data are explained using quantitative analysis. For a fixed water pressure at the nozzle inlet, many mechanisms are found that contribute to the maximum heat flow. Air-atomized spray cooling on the surface of a vertical grid of horizontal tubes is the first comprehensive work attempting to fully explain the heat transfer process and to relate detailed measurements of the cooling and rewetting rates to the observed physical phenomena. For the safety of nuclear reactors, quench cooling research is crucial to reduce the magnitude of core damage in the early stages of severe accidents after coolant loss accidents (LOCA).