Boiling Heat Transfer during Impingement of Two or Three Pipe Laminar Jets onto Moving Steel Sheet

The impingement of pipe laminar jets is commonly used in run-out-table cooling in hot rolling mills. In this process, a moving hot steel sheet is cooled by pipe laminar array jets. When the spacing between two neighbor jets is small in the sheet width direction, flow interaction of cooling water on the sheet is inevitable, resulting in complex heat transfer phenomena. In the present study, the boiling heat transfer during the impingement of two or three pipe laminar jets onto a moving steel sheet was studied by laboratory-scale experiments. The test coolant was water at room temperature. Water jets were produced from 5-mm-diameter pipe nozzles at a mean velocity of 0.8 m/s. The nozzle spacing between two jet centers was 8, 12, or 16 mm. A 0.3-mm-thick stainless steel sheet with a moving velocity of 1.5 m/s was used as the test substrate. The temperature of steel ranged from 300 to 500°C. The flow was observed by flash photography, and the heat transfer characteristics were studied by an infrared thermography technique. It was found that high heat flux regions were formed near the jet impact points on the moving solid. Flow interaction occurred between two jets, where the heat removal rate was relatively small compared to that in the jet impact regions. The effects of the nozzle spacing, number of nozzles, and temperature of the solid on the boiling heat transfer characteristics were studied in detail from an industrial viewpoint.