Browsing by Author "Roos, Thomas Helm"
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- ItemA prediction method for flow in axial compressors(Stellenbosch : Stellenbosch University, 1995) Roos, Thomas Helm; von Backstrom, T. W.; Stellenbosch University. Faculty of Engineering. Department of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: A procedure is presented for predicting the flow through axial compressors. The matrix throughflow equation is transformed to yield an expression of radius dependent on stream function and axial position. The solution of the resultant equation combines the advantages of following streamlines through the calculation domain (as in the streamline curvature method, SCM) with the stability of the matrix throughflow method (MTFM), and is correspondingly called the streamline through flow method (STFM). The predictions of the method were compared to analytical results for a number of inviscid test cases and gave good results. As with the SCM and MTFM, using STFM to predict turbomachinery flows requires empirical models for cascade loss and deflection as well as endwall loss. The off-design loss and deflection model of Howell was used as the basis for a new off-design correlation, H2, valid for axial velocity density ratios (AVDR) of unity. The H2 correlation was developed from the NACA 65-Series database as carpet-plotted by Felix. A simple subcritical Reynolds number correlation for loss and deflection was adapted from a method of Roberts, using inlet blade chord Reynolds number, camber angle, pitch-chord ratio, maximum blade thickness-chord ratio and turbulance factor as parameters. The H2 correlation together with the adapted Roberts correlation gave good predictions of loss and deflection for low-Reynolds number cascade flows at AVDR values of unity. Measurements were taken at the compressor inlet and behind each blade row of a low speed, three stage axial flow compressor at three flowrates: near-design, near-surge and near-choke. The predictions of STFM using Howell's endwall loss models, the modified low Reynolds number correlation and respectively Howell's original off design method and H2 were compared with the experimental results. Howell's method predicted pressure rise to within 3% at design and 10% at off-design, compared to 4% at design and 9% at off-design for the H2 method. The prediction of flow angles for H2 were considerably worse than that of Howell. This was deemed to be caused by AVDR effects. An interim AVDR correlation, dependent on stagger angle, was used together with H2. Choosing values of AVDR for the interim correlation which together with H2 would predict flow angles to match the experimental values, predictions of total pressure rise within 3% at design and 8% at off-design were achieved. As a measure of confidence can be placed in H2 and the modified low Reynolds number correlation, the endwall loss correlation of Howell was determined to be the cause of lack of further gains in accuracy.