Yu-Tai Lee

Direct Method for Optimization of a Centrifugal Compressor Vaneless Diffuser

A Direct Method for Optimization (DMO) is developed for investigating pressure rise and energy loss in a vaneless diffuser of a generic compressor used in shipboard air-conditioning systems. The scheme uses Reynolds-Averaged Navier-Stokes (RANS) results and evaluates gradients of a predetermined objective function. The current Direct Method for Optimization differs from the popular Inverse Design Method in the process of obtaining final configurations and in the final configurations obtained. The Direct Method for Optimization achieves a final shape from maximizing/minimizing a nonlinear function, i.e., the objective function. Both gradient and nongradient Direct Methods for Optimization are compared with respect to accuracy and efficiency. The coupled DMO/RANS optimization code is benchmarked using a plane turbulent diffuser also investigated by Zhang et al. using an adjoint method. The benchmark indicates that if a global optimum exists, the result should be independent of the methodologies or design parameters used. The DMO/RANS method is applied to redesign a three-dimensional centrifugal vaneless diffuser used in a modern generic compressor. The objective function is a composite function of the diffusers pressure rise and total energy loss. The new optimum diffuser has a minimum width at a location far beyond the conventional 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 experimentally at off-design conditions for the optimized diffuser.

Performance Evaluation of an Air-Conditioning Compressor Part I: Measurement and Design Modeling*

In order to comply with legislation to eliminate the use of refrigerants that damage the ozone layer, it is necessary to redesign centrifugal compressors, used by the US Navy for shipboard air-conditioning systems, to use an environmentally acceptable refrigerant. This paper describes an evaluation of a 125-ton compressor designed to use HCFC-124 as the refrigerant. The objectives are not only conducting the performance evaluation, but also pinpointing the design problems for achieving a high-performance compressor. The design method used to design the 125-ton compressor is first reviewed and some related performance curves are predicted based on a quasi-3D method. In addition to an overall performance measurement, a series of instruments were installed on the compressor to identify where the measured performance differs from the predicted performance. The measurement techniques for providing the diagnostic flow parameters are also described briefly. Part II of this paper provides predictions of flow details in the areas of the compressor where there were differences between the measured and predicted performance.

Performance Evaluation of an Air-Conditioning Compressor Part II: Volute Flow Predictions*

A numerical method that solves the Reynolds-averaged Navier-Stokes equations is used to study an inefficient component of a shipboard air-conditioning HCFC-124 compressor system. This high-loss component of the centrifugal compressor was identified as the volute through a series of measurements given in Part I of the paper. The predictions were made using three grid topologies. The first grid closes the connection between the cutwater and the discharge diffuser. The other two grids connect the cutwater area with the discharge diffuser. Experiments were performed to simulate both the cutwater conditions used in the predictions. Surface pressures along the outer wall and near the inlet of the volute were surveyed for comparisons with the predictions. Good agreements between the predicted results and the measurements validate the calculations. Total pressure distributions and flow stream traces from the prediction results support the loss distribution through the volute. A modified volute configuration is examined numerically for further loss comparison.

Unsteady Rotor Dynamics in Cascade

A time-accurate potential-flow calculation method has been developed for unsteady incompressible flows through two-dimensional multi-blade-row linear cascades. The method represents the boundary surfaces by distributing piecewise linear-vortex and constant-source singularities on discrete panels. A local coordinate is assigned to each independently moving object. Blade-shed vorticity is traced at each time step. The unsteady Kutta condition applied is nonlinear and requires zero blade trailing-edge loading at each time. Its influence on the solutions depends on the blade trailingedge shapes. Steady biplane and cascade solutions are presented and compared to exact soluions and experimental data. Unsteady solutions are validated with the Wagner function for an airfoil moving impulsively from rest and the Theodorsen function for an oscillating airfoil

Prediction of Vortex and Linear Cascade Interaction Noise

This paper is concerned with increasing the understanding of the noise generation mechanism assoicated with the interaction between foils and shed vorticity. An inviscid computational method is developed that predicts the flow interaction unsteadiness and the resultant acoustic pressures due to the acceleration and deceleration of vortices in the proximity to foils. This time-accurate prediction method is used to study the generation of shed vorticity and the interaction between a vortex and a foil or linear cascade of blades in terms of flow interaction phenomena, forces acting on foils, instability of free traveling vortices, and radiated acoustic pressure. The results show that the predicted acoustic pressure is proportional to the closeness of the free vortices to the foil or the cascade blades. The number of acoustic pulses, generated when a free vortex encounters a foil, depends on the inflow angle of attack. When compared to a single foil, a cascade produces effects that suppress the vortex/blade flow interaction. However, the acoustic response depends heavily on the free vortex location in relation to the cascade blades.Copyright