Martin N. Goodhand

The Impact of Real Geometries on Three-Dimensional Separations in Compressors

This paper considers the effect of small variations in leading edge geometry, leading edge roughness, leading edge fillet, and blade fillet geometry on the three-dimensional separations found in compressor blade rows. The detrimental effects of these separations have historically been predicted by correlations based on global flow parameters, such as blade loading, inlet boundary layer skew, etc., and thus ignoring small deviations such as those highlighted above. In this paper it is shown that this may not be the case and that certain, engine representative geometry deviations can have an effect equivalent to an increase in blade loading of 10%. Experiments were performed at the stator hub of a low-speed, single-stage compressor. The results show that any deviation which causes suction surface transition to move to the leading edge over the first 30% of span will cause a large growth in the size of the hub separation, doubling its impact on loss. The geometry deviations that caused this, and are thus of greatest concern to a designer, are changes in leading edge quality and roughness around the leading edge, which are characteristic of an eroded blade.

The Sensitivity of 2D Compressor Incidence Range to In-Service Geometric Variation

The loss mechanisms which control 2D incidence range are discussed with an emphasis on determining which real in-service geometric variations will have the largest impact. For the majority of engine compressor blades (Minlet>0.55) both the negative and positive incidence limits are controlled by supersonic patches. It is shown that these patches are highly sensitive to the geometric variations close to, and around the leading edge. The variations used in this study were measured from newly manufactured as well as ex-service blades. Over most the high pressure compressor considered, it was shown that manufacture variations dominated.The first part of the paper shows that, despite large geometric variations (∼10% of leading edge thickness), the incidence range responded in a linear way. The result of this is that the geometric variations have little effect on the mean incidence range of a row of blades. In the second part of the paper a region of the design space is identified where non-linear behavior can result in a 10% reduction in positive incidence range. The mechanism for this is reported and design guidelines for its avoidance offered. In the final part of the paper, the linear behavior at negative incidence and the transonic nature of the flow is exploited to design a robust asymmetric leading edge with a 5% increase in incidence range.Copyright

High fidelity replication of surface texture and geometric form of a high aspect ratio aerodynamic test component

This paper details, assesses and validates a technique for the replication of a titanium wind tunnel test aerofoil in polyurethane resin. Existing resin replication techniques are adapted to overcome the technical difficulties associated with casting a high aspect ratio component. The technique is shown to have high replication fidelity over all important length-scales. The blade chord was accurate to 0.02%, and the maximum blade thickness was accurate to 2.5%. Important spatial and amplitude areal surface texture parameter were accurate to within 2%. Compared to an existing similar system using correlation areal parameters the current technique is shown to have lower fidelity and this difference is discussed. The current technique was developed for the measurement of boundary layer flow‘laminar to turbulent’ transition for gas turbine compressor blade profiles and this application is illustrated.

The Limitations of Using “Ra” to Describe Surface Roughness

Current criteria used to determine whether rough surfaces affect skin friction typically rely on a single amplitude parameter to characterize the roughness. The most commonly used criteria relate the centerline averaged roughness, Ra, to an equivalent sandgrain roughness size, ks. This paper shows that such criteria are oversimplified and that Ra/ks is dependent on the roughness topography, namely, the roughness slope defined as the roughness amplitude normalized by the distance between roughness peaks, Ra/λ. To demonstrate the relationship, wake traverses were undertaken downstream of an aerofoil with various polished surfaces. The admissible roughness Reynolds number (ρ1u1Ra/μ1) at which the drag rose above the smooth blade case was determined. The results were used to demonstrate a 400% variation in Ra/ks over the roughness topographies tested. The relationship found held for all cases tested, except those where the roughness first initiated premature transition at the leading edge. In these cases, where the roughness was more typical of eroded aerofoils, the drag was found to rise earlier.

Admissible Roughness in Gas Turbines

Roughness below a certain, admissible level will not increase an aerofoil’s skin friction and thus will not impact on engine performance. In this paper a simple model is developed that demonstrates how this admissible roughness height changes through the engine. The model is determined by combining existing analytical models and is backed up by computational validation where necessary.It is shown that, given a fixed inlet/exit stagnation temperature and pressure to a blade row, the admissible roughness height is only a weak function of chord Reynolds number and Mach number for typical gas turbine blades. The aerofoil geometry/duty is also shown to have little impact. This allows the model to give a general picture of where roughness matters, irrespective of the exact details of the flow conditions/blade geometries.The model shows that the admissible roughness height decreases as the stagnation pressure increases. The lowest admissible roughness level occurs at the high pressure end of the compressor at sea-level; the admissible roughness increases as the stagnation pressure drops towards the front of the compressor. Turbines are also most sensitive to roughness close to the combustor, but even there the admissible roughness will be around three times greater than at the rear of the compressor.Copyright

The Limitations of “Ra” to Describe Surface Roughness

Current criteria used to determine whether rough surfaces affect skin friction typically rely on a single amplitude parameter to characterize the roughness. The most commonly used criteria relate the centreline averaged roughness, Ra, to an equivalent sandgrain roughness size, ks. This paper shows that such criteria are oversimplified and that Ra/ks is dependent on the roughness topography, namely the roughness slope defined as the roughness amplitude normalized by the distance between roughness peaks, Ra/λ. To demonstrate the relationship, wake traverses were undertaken downstream of an aerofoil with various polished surfaces. The admissible roughness Reynolds number (ρ1u1Ra/μ1) at which the drag rose above the smooth blade case, was determined. The results were used to demonstrate a 400% variation in Ra/ks over the roughness topographies tested. The relationship found held for all cases tested, except those where the roughness first initiated premature transition at the leading edge. In these cases, where the roughness was more typical of eroded aerofoils, the drag was found to rise earlier.