Study On Transient Jet Impingement Heat Transfer During The Fast Cooling Process Of Glass Physical Tempering

In order to meet the requirements of the thinning and miniaturization of liquid crystal display and solar cells,the tempered glass needs to be ultrathin.The jet impingement heat transfer is the most promising heat transfer enhancement technology and is widely applied in various engineering application.The transient jet impingement heat transfer under the conditions of the small jet-to-plate distance and the large temperature difference is beneficial to temper the ultra-thin glass.And however,there is little study on the transient heat transfer under the condition of the small jet-to-plate distance.Therefore,the transient jet impingement heat transfer under the small jet-to-plate spacing is numerically studied in detail in our current paper.For the air-single-jet impingement,numerical studies on the effects of Reynolds number,nondimensional jet-to-plate distance on the transient heat transfer characteristics are presented.The Reynolds number(Re)varied from 20,000 to 60,000;The nondimensional jet-to-plate distance(H/D)varied from 0.2 to 2.Variations in stagnation point and boundary point Nusselt numbers with cooling times are discussed.It was found that with the decrease of H/D the heat transfer near stagnation point is enhanced,while it is decreased near boundary point.For the air-array-jet impingement,numerical studies on the effects of nondimensional jet-to-plate distance and nondimensional jet-to-jet spacing on the transient heat transfer characteristics are presented.The total mass flow rate of the jet(8)?)is constant at 30.34kg/h.The nondimensional jet-to-plate distance(H/D)varied from 0.2 to 1.The nondimensional jet-to-jet spacing is S/D = 5,7,and 10.Variations in the transient heat transfer characteristics and flow velocity at different values of H/D and S/D with cooling time are discussed.It was found that the effect of the number of array jet nozzles on the transient heat transfer is dependence of H/D.As H/D is decreased,a turning point exists at H/D = 0.4.For the air-mist-jet impingement,numerical studies on the effects of nondimensional jet-to-plate distance,the nozzle inlet temperature and the mist-flow droplet diameter on the transient heat transfer characteristics are presented.The nondimensional jet-to-plate distance(H/D)is varied from 0.2 to 1.The nozzle inlet temperature from 283 – 303 K,and the mist-flow droplet diameter from 5 – 15 ?m.It was found that the quenching time decreases monotonously with the increase in H/D for the mist-flow-jet impingement,whereas there exists a turning point for the air-jet impingement.With the analysis of the transient jet impingement heat transfer characteristics under the conditions of the small jet-to-plate distance and large temperature difference,we can conclude that: when the jet-to-plate distance is small,the quenching time has a turning point for the air-jet impingement heat transfer,and whereas the quenching time decreases monotonously for the airmist-jet impingement;as an addition of a certain amount of water mist into the air-jet causes the faster surface cooling.And through the optimization of the array-jet impingement structure parameters,a more uniform surface heat transfer can be obtained.The heat transfer enhancement under the small jet-toplate distance is mainly due to the effects of the jet deflection and the increase if the fluid velocity at the stagnation point area.In addition,the current provides a theoretical basis for the study on the transient heat transfer characteristics for small jet-to-plate distance.