Rotor Tip Gap Research Articles

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

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.

Effect of Rotor Tip Gap Variation at the Rear Stages of an Axial Flow Compressor

In this paper, detailed analysis of the tip flow of an axial compressor rotor blade has been carried out using the commercial CFD package ANSYS CFX. The rotor blade was designed such that it is reminiscent of the rear stages of a multi-stage axial compressor. The effects of varying tip gaps are studied using CFD simulations for overall pressure rise and flow physics of the tip flow at the design point and near the peak pressure point. Rig tests of a low speed research compressor rotor with 3% tip clearance provided characteristics plots for validation of the CFD results. With increase in clearance from 1% to 4%, the rotor pressure rise at the design point was observed to decrease linearly. Increase in the clearance increases the cross flow across the tip; however, the magnitude of the average jet velocity crossing the tip decreases. The tip leakage vortex was observed to stay close to the suction surface with increase in clearance.

Numerical investigation of tip clearance effects in an axial transonic compressor

Numerical investigations of the Darmstadt transonic single stage compressor (DTC), in the Rotor1-Stator1 configuration, aimed at advancing the understanding of the effect of different rotor tip gaps and transition modelling on the blade surfaces are presented. Steady three dimensional Reynolds Averaged Navier Stokes (RANS) simulations were performed to obtain the flow fields for the different configurations at different operating conditions using the RANS-Solver TRACE. The stage geometry and the multi-block structured grid were generated by G3DMESH and a grid sensitivity analysis was conducted. For the clearance gap region, a fully gridded special H-grid was chosen. Comparisons were made between the flow characteristic at design speed, representative for a transonic flow regime, and at 65% speed, representative for a subsonic flow regime. The computations were used to analyse the flow phenomena through the tip clearance region for the different configurations and their impact on the performance of the compressor stage.

Active flow control at a 1.5-stage low-speed research compressor with varying rotor tip clearance

The stable operating range of axial compressors is limited by the onset of rotating stall and surge. Mass injection upstream of the tip of an axial compressor rotor is a stability enhancement approach which can be effective in suppressing stall in tip-critical rotors, and thus increasing the operating range of compressors. In this article, investigations on active flow control related to the rotor tip gap sensitivity are discussed. The experiments were performed in a 1.5-stage low-speed research compressor. Measurements at part speed (80 per cent) and full speed (100 per cent) with varying injection rates are discussed. These tests were performed for two rotor tip clearances of 1.3 per cent and 4.3 per cent of rotor blade tip chord. Results on the compressor map, the flow field as well as transient measurements to identify the stall inception are discussed. Supplementary, the numerical results are compared to the experiments based on the configuration with the greatest benefit in operating range enhancement.

Geometrical uncertainty in turbomachinery: Tip gap and fillet radius

In turbomachinery, the prediction of component life is crucial. It has been found that the variability of gaps, fillets and small geometries, due to manufacturing process and in service operation, can be higher than expected. This has direct consequences on both component life and efficiency. The question arising is how to take into account the aleatory distribution due to manufacturing uncertainty during the design phase. The use of state of the art Computational Fluid Dynamics allows considering the uncertainties in the estimation of performance parameters. Uncertainty is treated including the statistical distribution of the variability in the geometrical models. In this work the methodology for the analysis of the impact of geometrical variations on components life and machine reliability is presented. A representative high pressure stage is used as a reference and fillet sizes and gaps are modified in order to generate the response surface to geometrical uncertainty. For that machine, the nominal rotor tip gap G 0 is 1.5% of the span. A preliminary analysis has been performed considering a value for the fillet radius r/ G 0 = 50%, to underline the high impact of geometrical variations on the flow field. Then, five fillet radii at the rotor tip and three tip gaps have been studied ( r/ G 0 = 0.0, 0.03, 0.075, 0.15, 0.3 and G/ G 0 = 1, 1.5, 2). A two dimensional Gaussian distribution for the fillet radius and tip gap is arbitrarily assumed. Starting from this distribution the probabilistic functions of corresponding mass-flow through the rotor gap and the heat loading of rotor tip are obtained. The overall variation of mass-flow can be more than 60% and the heat loading is modified by 15% if compared to nominal conditions. Moreover this work shows that by including the mean value without the probability distribution the discrepancy is about 6% for mass-flow and 7% of heat loading.

Desensitization of the Flowfield from Rotor Tip-Gap Height by Casing-Air Injection

This paper presents an investigation that aims to desensitize the flowfield of an unshrouded high-pressure gas-turbine stage from the radial clearance between the rotor tip and the casing. A novel method of cooling-air injection from the rotor casing into the rotor tip region is applied. The injection opposes the tip-leakage flow and affects the development of the rotor secondary flow. In a previous investigation, an increase of stage efficiency of 0.55 percentage points was achieved with an injection of 0.7% of the core mass flow. The present study investigates the effect of the injection on the flowfield for two different rotor tip clearances. Measurements conducted with a two-sensor fast-response pressure probe provide data describing the time-resolved behavior of flow angles and pressures as well as turbulence intensity. The experimental investigation is done on the tip-gap heights of 0.65 and 1.00% span and injection mass flow rates representing 0, 0.7, and 1% of the turbine core mass flow. The results show that an increasing injection rate reduces the difference of rotor exit flow angle and flow unsteadiness between the two different tip-gap heights. The stage efficiency increases with injection for both tip-gap cases, whereas identical values are obtained at the injection rate of 1 % of the turbine mass flow.

The Effect of Work Processes on the Casing Heat Transfer of a Transonic Turbine

This paper considers the effect of the rotor tip on the casing heat load of a transonic axial flow turbine. The aim of the research is to understand the dominant causes of casing heat transfer. Experimental measurements were conducted at engine-representative Mach number, Reynolds number, and stage inlet to casing wall temperature ratio. Time-resolved heat-transfer coefficient and gas recovery temperature on the casing were measured using an array of heat-transfer gauges. Time-resolved static pressure on the casing wall was measured using Kulite pressure transducers. Time-resolved numerical simulations were undertaken to aid understanding of the mechanism responsible for casing heat load. The results show that between 35% and 60% axial chord the rotor tip-leakage flow is responsible for more than 50% of casing heat transfer. The effects of both gas recovery temperature and heat transfer coefficient were investigated separately and it is shown that an increased stagnation temperature in the rotor tip gap dominates casing heat transfer. In the tip gap the stagnation temperature is shown to rise above that found at stage inlet (combustor exit) by as much as 35% of stage total temperature drop. The rise in stagnation temperature is caused by an isentropic work input to the tip-leakage fluid by the rotor. The size of this mechanism is investigated by computationally tracking fluid path lines through the rotor tip gap to understand the unsteady work processes that occur.