In order to improve overall performance of a turbomachinery, it is important to reduce losses of stationary flow passages, such as diffusers and return channels, as well as impellers. For multi-stage pumps, to achieve high uniformity of the inlet flow of the latter impeller can prevent degradation of subsequent performance.
A two stages high pressure pump which was developed by the authors applied vaned diffuser sections located downstream of the first stage impeller and second stage impeller. The vaned diffusers can be replaced to fit the required specification of operating condition. This design is able to help pumps share the casings so that customers can purchase our products at a low cost.
However, the loss of the first stage diffuser section and crossover downstream of the first stage diffuser to the second impeller is large, so that it has a negative impact on overall performance of the pump. It was difficult to reduce loss of the both sections by conventional way such as trial and error approach by modifying geometrical parameters. If we try to reduce the loss of the diffuser section, loss of the crossover passage increases.
We therefore applied a technology called Adjoint method to the design optimization of the diffuser and crossover sections of the pump. The adjoint method has recently been put to practical use. By using this method it is possible to obtain a complex three-dimensional shape for realizing an optimum flow field in a very short time while maintaining a high degree of freedom.
In this study two objectives were specified in the design optimization of the stationery sections; one is to increase the uniformity of the flow field at the inlet of the second stage impeller and the other is to minimize the loss of the first stage diffuser and crossover passage. In the optimization process, the change of sensitivity of the geometry deformation to the weights of objective functions was analyzed by sensitivity maps. The sensitivity maps show directions of the geometry modification depending on the objectives. The geometry in this design was optimized with 50% - 50% weighting ratio between flow uniformity and pressure loss. The loss of the vaned diffuser and crossover passage, velocity uniformity at the outlet of the crossover passage were verified with Computational Fluid Dynamics (CFD). The improvement of pump stage performance was expected by the optimization.
This optimization design procedure applying Adjoint method is effective to improve overall performance of a two stages high pressure pump which has complex three-dimensional flow passages.