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Heat transfer during multiple jet impingement on the top surface of hot rolled steel strip

University of British Columbia

The cooling which occurs on the runout table (ROT) is a key processing step for hot rolled steel strip. It determines the final microstructure and thus mechanical properties as well as flatness of the hot band. The use of multiple jets during ROT cooling results in interactions between neighboring water jets which can affect the overall heat transfer rate. The heat transfer which takes place during cooling with multiple jets is fairly complex and the available knowledge is limited. The research work described was done to obtain an understanding of the effect of varying nozzle-to-nozzle distance, plate speed and flow rate of the impinging water on the heat transfer taking place on the ROT. Experiments were performed on the pilot scale runout table available at UBC, using instrumented test samples of steel. Each sample was instrumented with twenty thermocouples which measured the internal thermal history. This data was then used in conjunction with an Inverse Heat Conduction (IHC) model to calculate surface heat fluxes and temperatures. Some of the variables examined included: speed of movement of the test plate (0.22 m/s and 1 m/s), nozzle spacing (114.3, 76.2 and 38.1 mm) and water flow rate (15 1/min and 30 1/min). These experimental results provide important information for the development of improved runout table cooling models. The results indicated that, during multiple jet cooling, high heat extraction takes place directly below the nozzles and adjacent to them due to direct impact of water. Lower heat extraction occurs at the locations between the nozzles, i.e. the interaction region. Visual observations of the tests-indicate that, when the water jets hit the strip, a small darkened zone can be observed at the impingement point below each nozzle. In the interaction region, the water flowing from the two adjacent jets interacts with each other and large splashing of the water is observed in this region. The dark zones below all three nozzles expand with cooling of the strip, indicating that the water front is progressing outwards from the stagnation line and more water solid contact is taking place. The boiling curves below each nozzle are similar to each other and clearly show the different boiling regimes while the boiling curve for the interaction region does not show these regimes as clearly. In the interaction region, heat transfer remains relatively low until the water completely wets the strip. The investigation of the effect of strip speed indicated that the heat fluxes are higher for lower strip speeds as the strip spends a longer time under the nozzle. This effect was seen both below the nozzle and in the interaction region. In general, increase in water flow rate increases heat fluxes at all measuring locations due to higher amount of water impinged on the strip surface. The nozzle configuration having two adjacent nozzles at 38.1 mm apart has more cooling capacity than the other two configurations indicating that, having two nozzles close to each other enhances heat transfer.

Text

2011-02-17

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http://hdl.handle.net/2429/31402

Materials Engineering

University of British Columbia

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