HEAT TRANSFER ASSOCIATED WITH AIR - WATER SPRAY COOLING OF FORGINGS FROM HIGH TEMPERATURES

摘要

The desired mechanical properties of the nickel-based or titanium forgings used in gas turbines for aircraft and power generation applications can be controlled by varying the rate of cooling from the so-called solution temperature during an initial heat treatment process. The use of dilute air-water spray cooling of these forgings is a technique which can provide heat transfer rates lying between those associated with conventional oil quenching or convective air-cooling. Air assisted atomisation can result in fine sprays over a wide range of water flow rates and it has a further advantage in that the air "sweeps" the surface and hence helps to prevent the build up of deleterious vapour films at high surface temperatures. The paper presents experimental data for the heat transfer rates associated with the use of these sprays to cool surfaces from temperatures of approximately 800℃. Many forgings contain surface recesses, which can lead to build up or "pooling" of the water so that the effect of variations in surface geometry is also reported. Periodic interruption of the water flow is a technique which can be employed to provide additional control of the heat transfer rate, particularly at temperatures below 500℃ so that data is also presented for pulsed sprays.