The onset of nucleate boiling in water spray cooling on hot steel plate was investigated by a simultaneous boiling visualization and heat transfer measurement. The boiling phenomena were visualized with 4K video camera and the surface temperature of the hot steel plate was determined by solving 2-D inverse heat conduction during water spray cooling. The temperature was measured by a sampling rate of 10 data/sec. The hot steel plate was initially heated up to 900°C and the coolant temperature was kept at a constant temperature of 20°C. The spray nozzle with fullcone pattern was mounted with the three different heights (100, 200 and 300 mm). The more spray height was increased, the more scattered the spray pattern became, which could affect the partial spray intensity and overall cooling uniformity. The lower spray nozzle height of 100 mm shows the steep temperature gradient in inner zone. As the spray particles are more intense at inner zone which wets faster than outer zone. But the higher spray nozzle height of 300 mm, the temperature profile keeps constant within the 400 sec. After this time, the outer zone is wetted faster than inner zone. At the middle height of 200 mm, although the temperature gradient in inner zone is slightly higher than that in outer zone, the overall surface wetting is relatively uniform in the inner and outer zone. These results exhibit that the spray cooling uniformity can be controlled with optimized spray nozzle height. Furthermore the boiling visualization agrees well with the onset of nucleate boiling in surface temperature profiles.

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