Compressor Efficiency Variation With Rotor Tip Gap From Vanishing to Large Clearance

Abstract

Compressor efficiency variation with rotor tip gap is assessed using numerical simulations on an embedded stage representative of that in a large industrial gas turbine with Reynolds number ∼ 2 × 106 to 7 × 106. The results reveal three distinct behaviors of efficiency variation with tip gap. For relatively small tip gap (less than 0.8% span), the change in efficiency with tip gap is nonmonotonic with an optimum tip gap for maximum efficiency. The optimum tip gap is set by two competing flow processes: decreasing tip leakage mixing loss and increasing viscous shear loss at the casing with decreasing tip gap. An optimum tip gap scaling is established and shown to satisfactorily quantify the optimal gap value. For medium tip gap (0.8%–3.4% span), the efficiency decreases approximately on a linear basis with increasing tip clearance. However, for tip gap beyond a threshold value (3.4% span for this rotor), the efficiency becomes less sensitive to tip gap as the blade tip becomes more aft-loaded thus reducing tip flow mixing loss in the rotor passage. The threshold value is set by the competing effects between increasing tip leakage flow and decreasing tip flow induced mixing loss with increasing tip gap. Thus, to desensitize compressor performance variation with blade gap, rotor should be tip aft-loaded and hub fore-loaded while stator should be tip fore-loaded and hub aft-loaded as much as feasible. This reduces the opportunity for clearance flow mixing loss and maximizes the benefits of reversible work from unsteady effects in attenuating the clearance flow through the downstream blade-row. The net effect can be an overall compressor performance enhancement in terms of efficiency, pressure rise capability, robustness to end gap variation, and potentially useful operable range broadening.

Understanding the aerodynamics of the fan blade tip

“…A linear variation in axial compressor blade efficiency with tip clearance was also shown by Beheshti (2004) and Seshadri et al (2014). Seshadri et al (2014) and Sakulkaew (2013) however found that close to zero clearance the relationship becomes non-linear, resulting in an optimum, non-zero clearance. This is because shear loss increases at the casing as the tip clearance becomes very small.…”

“…It was shown that entropy generation due to the leakage vortex is proportional to the difference between the streamwise velocities of the two flows. Sakulkaew (2013) carried out a study of compressor efficiency variation from large to vanishing clearances. The loss variation with tip gap was found to be linear between tip clearances of 1 and 3%.…”

Experimental and numerical characterization of a ceramic matrix composite shroud segment under impact loading

“…The second avenue considered by designers to increase an aircraft engine efficiency-the reduction of aerodynamic losses-implies a very stringent control of operating blade-tips/casings clearances in order to avoid any leak from higher to lower pressure stages. While a reduction of operating clearances provides a very significant improvement of the overall efficiency [19,20,21], it also unavoidably favors structural contacts between rotating and static components of the engine [22,23]. Following such contact events, a variety of complex phenomena including blade failures [24], abnormal increases of temperature [25], and even modal interactions [26,27] have been reported in the literature.…”

Experimental Investigation of the Flow Mechanisms and the Performance Change of a Highly Loaded Axial Compressor Stage with/without Stator Hub Clearance

“…The second one is the effect of the clearance size on the compressor or the cascade performance. For example, Lakshminarayana and Horlock [18] and Peacock [24] both found that there exists an optimum clearance size that makes the performance of the compressor cascade to be the best and Lakshminarayana and Horlock [18] pointed out that this only happens when the strength of the leakage flow and the secondary flow are comparable and they can just cancel out; Sakulkaew, et al [25] also found that there is an optimum clearance at the rotor tip in an industrial gas turbine, and proposed that this phenomenon is mainly caused by the decrease of clearance flow mixing loss and increase of viscous loss in the tip clearance with the decrease of tip clearance size; in addition, Dong, et al [23] found that the leakage flow can eliminate corner separation at the stator hub, and improve the compressor performance, which has been verified by Wennerstrom [26], and McDougall [27] at the rotor tip, while Gbadebo, et al [22] revealed that the clearance flow tends to strengthen the corner separation when the clearance size is very small.…”