Andre Hildebrandt

Numerical Investigation of the Effect of Different Back Sweep Angle and Exducer Width on the Impeller Outlet Flow Pattern of a Centrifugal Compressor With Vaneless Diffuser

This paper presents a numerical investigation of the effect of different back sweep angles and exducer widths on the steady-state impeller outlet flow pattern of a centrifugal compressor with a vaneless diffuser. The investigations have been performed with commercial computational fluid dynamics (CFD) and in-house programmed one-dimensional (ID) codes. CFD calculations aim to investigate how flow pattern from the impeller is quantitatively influenced by compressor geometry parameters; thereby, the location of wake and its magnitude (flow angle and relative velocity magnitude) are analyzed. Results show that the increased back sweep impeller provides a more uniform flow pattern in terms of velocity and flow deviation angle distribution, and offers better potential for the diffusion process inside a vaneless (or vaned) diffuser Secondary flux fraction and flow deviation angle from CFD simulation are implemented into the ID two-zone program to improve ID prediction results. (Less)

Steady-State and Transient Compressor Surge Behavior Within a SOFC-GT-Hybrid System

Future pressurized Solid Oxide Fuel Cell- (SOFC) Gas Turbine Hybrid Systems (HS) promise high efficiency at both full-and part-load on account of the upper and lower SOFC temperature limit and the compressor surge line [1]. The compressor surge constraint is also evident in transient HS operation, caused by the slow transients of SOFC temperature imposed by large SOFC plenum, and fast turbo-machinery transients. This paper presents steady-state and unsteady-state HS modeling and calculation results with regard to surge. The transient compressor and SOFC models have been validated against literature. Calculation results of the coupled SOFC-GT-HS reveal a small operational window in case of unmatched turbine and the critical transient characteristics for HS shut-down. (Less)

Operation and Performance Limitations for Solid Oxide Fuel Cells and Gas Turbines in a Hybrid System

Temperature limitations of Solid Oxide Fuel Cells (SOFC) in transient single operation and steady-state Hybrid System (HS) operation with Gas Turbines (GT) are presented. For transient SOFC simulations, an unsteady-state SOFC model was developed by upgrading a detailed validated steady-state model. As critical SOFC single operation modes, concerning the risk of material cracking due to exceeding SOFC transient temperature gradients, heat-up and cool-down are investigated. For minimization of transient SOFC temperature gradients at start-up and shut-down, a stepwise heat-up and cool-down procedure is proposed. Concerning HS off-design and part-load operation, the impact of SOFC temperature limitations on the operational window is investigated. Results show a reduced operational window due to exceeding local SOFC temperature gradients, which can be reduced by optimal adaptation of GT to SOFC size.

Sensitivity analysis of transient compressor operation behaviour in SOFC-GT hybrid systems

This paper presents a sensitivity analysis of unsteady-state SOFC-GT-HS operation based on two different characteristic maps of centrifugal compressor taken from open literature and scaled by the law of similitude to match the design point of the Hybrid System. The system layout under investigation is a pressurised type comprising a low and high temperature recuperator. Computations are based on a one-dimensional finite element model of planar high temperature SOFC, which is validated against open literature. The reduced Moore and Greitzer model is used for compressor modelling. Calculation results of the coupled SOFC-GT-Hybrid System show that unsteady-state part-load operation is sensitive to the characteristics of compressor speed-lines but also to the load change operation procedure. Copyright (Less)

Numerical Investigation of Effects of Different Hub Tip Diameter Ratios on Aerodynamic Performance of Single Shaft Multistage Centrifugal Compression Systems

Regarding industrial centrifugal compressors in single shaft design, different configurations with e.g. varying numbers of stages or diverse circumferential speeds, necessitate different shaft diameters. Thus the application of impellers with different hub/tip ratios (dh/d2) is daily routine in industrial practice. Increasing hub/tip ratio leads to higher radii and therefore higher relative speeds, to a reduction in the impeller’s meridional length and hence more rapid diffusion, and to a sharper bending from axial to radial direction. In this paper the impact of hub/tip ratio on stage performance is investigated for three different centrifugal compressor stages, by steady state CFD-calculations. The hub/tip ratio is varied between 0.325 < dh/d2 < 0.45. The relation between design stage flow coefficient and hub/tip ratio is also analysed, both at design and off-design. Thermodynamic behaviour is assessed by 1D-data and also by the investigation of secondary flow features. The current analysis shows, that hub/tip ratio’s influence on characteristics is strongly dependent on the particular stage’s design flow coefficient and circumferential Mach-Number. Increasing a high flow stage’s hub/tip ratio is shown to decrease peak efficiency as well, as to limit the operating range. On the contrary, in case of a low flow stage, design point efficiency is hardly affected, but the characteristic curve is tilted around design point, by applying a different hub/tip ratio. However severity of hub/tip ratio’s impact on thermodynamic behaviour shows to decrease together with stage design flow coefficient.Copyright