Direct method for optimization of a centrifugal compressor vaneless diffuser

A Direct Method for Optimization (DMO) is developed for investigating press ure rise and energy loss in a vaneless diffuser of a generic compressor use d in shipboard air-conditioning systems. The scheme uses Reynolds-Averaged Navier-Stokes (RANS) results and evaluates gradients of a predetermined obj ective function. The current Direct Method for Optimization differs from th e popular Inverse Design Method in the process of obtaining final configura tions and in the final configurations obtained. The Direct Method for Optim ization achieves a final shape from maximizing/minimizing a nonlinear funct ion, i.e., the objective function. Both gradient and nongradient Direct Met hods for Optimization are compared with respect to accuracy and efficiency. The coupled DMO/RANS optimization code is benchmarked using a plane turbul ent diffuser also investigated by Zhang et al. using an adjoint method. The benchmark indicates that ifa global optimum exists, the result should be i ndependent of the methodologies or design parameters used. The DMO/RANS met hod is applied to redesign a three-dimensional centrifugal vaneless diffuse r used in a modem generic compressor. The objective function is a composite function of the diffuser's pressure rise and total energy loss. The new op timum diffuser has a minimum width at a location far beyond the conventiona l diffuser pinch point. The new diffuser also provides an efficient section for pressure recovery, which takes place after the minimum width location. Test data for the new diffuser validate the current approach at the design condition. Furthermore, improved performance is also recorded experimental ly at off-design conditions for the optimized diffuser.