Effect of nonuniform tip clearance on the performance of transonic axial compressors

Tip leakage significantly affects the axial-compressor performance (i.e., its pressure ratio, peak efficiency, and surge margin). The size of the clearanceis one of the most important parameters in compressor design. Generally, the tip clearance is uniform from the leading edge to the trailing edge of the compressor-blade section at the tip. In this paper, the effect of chordwise nonuniform tip clearance on the axial-compressor performance is investigated. In addition, the optimum distribution of clearance size is obtained by employing a surrogate model in conjunction with a multi-objective genetic algorithm for optimization. It is shown that the nonuniform tip clearance can effectively reduce the leakage mass-flow rate and eliminate the flow separation, thus enhancing the overall performance of the compressor. Based on the results of the sensitivity analysis, it is found that the changes in clearance size near the trailing edge of the blade tip contribute more toward the improvement of the compressor efficiency than those obtained by changing the clearance near the leading edge. The Pareto optimal front obtained by employing the multi-objective genetic algorithm shows that the transonic axial compressor with well-designed distributions of tip-clearance size gives 0.64% improvement in peak efficiency and 48.18% enhancement in surge margin.