AbstractThis thesis describes the work undertaken by the author in collaboration with Wyman-Gordon Forgings, USA, to assist in the development of a cooling system,based on air assisted atomised water sprays primarily for the quenching of aerospace components from high temperatures.
The mechanical properties of forgings used in aircraft engines depend on the rate of cooling from the heat treatment solution temperature. It is well known that water quenching produces high cooling rate. Although, the severity of the quench can sometimes produce unacceptable distortion and high residual stresses in the component. For this reason water quenching is only used when a high cooling rate is definitely needed and it is often replaced by a less severe oil quench. However, over the last 10 years the trend to reduce manufacturing costs has led to the forging of parts that are closer to the net shape. In these cases even oil quenching can lead to residual stresses being developed that result in difficulties during the final machining of the engine component. Forced air cooling has been adopted in problem cases where the part is thin enough to attain the desired cooling rate. In many instances, however, the component is of intermediate size or varying in cross section and fan cooling cannot provide the cooling rate which is needed to obtain the desired mechanical properties, whilst oil quenching produces an unacceptable level of residual stresses.
The use of air assisted atomised water sprays can provide heat transfer coefficients whose values lie between those for air cooling and oil quenching. Another advantage is that control of the air pressure enables the spray nozzle to operate with a much wider range of water flow rates so that the cooling rate can be readily controlled over the range.
This study describes the investigation of the heat transfer characteristics of air assisted atomised water sprays to quench aeroengine components from temperatures of approximately 850°C. New data were obtained at high temperatures for air assisted atomised water sprays operating over a wide range of water mass fluxes, (8.01>w 0 >0kg/m2 .s).
In practice the geometry of a component can be complex in shape. Therefore an investigation was also carried out into the application of spray cooling on recessed surfaces. It was found that the surface recess contributes significantly to the reduction in the rate of heat transfer at low and high water mass fluxes, but had little effect at intermediate flow rates.
Pulsed sprays were investigated and proposed as a means of controlling heat transfer coefficients for both plane and recessed surfaces. The use of a pulsed spray makes it possible to control the amount of water impacting on a surface per second. It was found that "water off periods of 5 and 10 seconds resulted in a reduction in heat transfer coefficients at low temperatures and also reduced considerably the differences in cooling previously observed between plane and recessed surfaces.
A finite element code was used to predict the residual stresses produced in a forged component for a range of spray parameters, and spray arrangements. The data were compared with cooling rates and stress patterns produced by both air and oil quenching. It was found that spray cooling resulted in cooling rates which met the mechanical property specification and provided residual stresses lower than those obtained during oil quenching. Furthermore, simulations of residual stress formation using two different spray arrangements in a typical forging indicated that spray non uniformities can substantially disturb the resultant residual stress patterns which could result in less predictable distortions during final machining.
The study of spray cooling presented here suggests that the use of air assisted atomised water sprays has considerable potential and could provide the required cooling rate for individual forgings.
|Date of Award
|John Ward (Supervisor) & David (Roy) Garwood (Supervisor)
- cooling system
- air assisted atomised water spray
- aerospace components
- aircraft engines
- heat transfer characteristics
- spray cooling