The effects of circumferentially non-uniform tip clearance on axial compressor performance and stability have been investigated experimentally and analytically. A theoretical model for compressor behavior with non-axisymmetric tip clearance has been developed and used to design a series of first-of-a-kind experiments on a four-stage, low speed compressor. The experiments and computational results together show clearly the central physical features and controlling parameters of compressor response to non-axisymmetric tip clearance.It was found that the loss in stall margin was more severe than that estimated based on average clearance. The stall point was, in fact, closer to that obtained with uniform clearance at the maximum clearance level. The circumferential length scale of the tip clearance (and accompanying flow asymmetry) was an important factor in determining the stall margin reduction. For the same average clearance, the loss in peak pressure rise was 50% higher for an asymmetry with fundamental wavelength equal to the compressor circumference than with wavelength equal to one-half the circumference. The clearance asymmetry had much less of an effect on peak efficiency; the measured maximum efficiency decrease obtained was less than 0.4 percent compared to the 8% decrease in peak pressure rise due to the asymmetric clearance. The efficiency penalty due to non-axisymmetric tip clearance was thus close to that obtained with a uniform clearance at the circumferentially-averaged level.The theoretical model accurately captured the decreases in both steady-state pressure rise and stable operating range which are associated with clearance asymmetry. It also gave a good 1 Permanent address: California Institute of Technology, Pasadena, California. Idescription of the observed trends of (i) increasing velocity asymmetry with decreasing compressor flow, and (ii) decreasing effect of clearance asymmetry with decreasing dominant wavelength of the clearance distribution. The time resolved data showed that the spatial structure of the pre-stall propagating disturbances in the compressor annulus was well represented and that the stability limiting process could be linked to the unsteady structure of these disturbance modes.The model was also utilized for parametric studies to define how compressor performance and stability is affected by the circumferential distribution of clearance, steady-state compressor pressure-rise characteristic, and system dynamic parameters. Sensitivity to clearance asymmetry was found to fall off strongly with the (asymmetry-related) reduced frequency and to increase with peak pressure rise and increasing curvature of the characteristic near the peak.2

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In this contribution we present different results of our investigations regarding the use of aluminum foil as rear side metallization for solar cells with dielectric passivation and laser fired contacts (LFC). We investigate the impact of different laser processes on the resistance of the contacts, the adhesion properties of the foil and the efficiency potential. By fabricating highly efficient, 20×20 mm 2 sized solar cells with a conversion efficiency of 21.0 %, we demonstrate the high potential of this approach, which is equal to that of LFC-cells with common screen-printed or PVD metallization on the rear side. We investigated the optical properties of such metallized rear sides which benefit from an embedded air gap between foil and passivation layer. Finally, we present first solar cell results on industrially sized wafers (A=238 cm 2 ) demonstrating again the equal efficiency potential compared to PVD metallization.

Effects of stator pressure field on upstream rotor performance in a high pressure compressor stage have been assessed using three-dimensional steady and time-accurate Reynolds-averaged Navier-Stokes computations. Emphasis was placed on: (1) determining the dominant features of the flow arising from interaction of the rotor with the stator pressure field, and (2) quantifying the overall effects on time averaged loss, blockage, and pressure rise. The time averaged results showed a 20 to 40% increase in overall rotor loss and a 10 to 50% decrease in tip clearance loss compared to an isolated rotor. The differences were dependent on the operating point and increased as the stage pressure rise, and amplitude of the unsteady back pressure variations, was increased. Motions of the tip leakage vortex on the order of the blade pitch were observed at the rotor exit in all the unsteady flow simulations; these were associated with enhanced mixing in the region. The period of the motion scaled with rotor flow-through time rather than stator passing. Three steady flow approximations for the rotor-stator interaction were assessed with reference to the unsteady computations: an axisymmetric representation of the stator pressure field, an inter-blade row averaging plane method, and a technique incorporating deterministic stresses and bodyforces associated with stator flow field. Differences between steady and unsteady predictions of overall rotor loss, tip region loss, and endwall blockage ranged from 5 to 50% of the time average, but the steady flow models gave overall rotor pressure rise and flow capacity within 5% of the time averaged values.

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