Key
1197
Conference Year
2014
Keywords
compressor design, loss model, performance prediction, test data, model stages, flow rate coefficient, work coefficient
Abstract
Decades ago at pre – computer era design process consisted of empirically based set of rules application to choose main flow path dimensions. Serious model tests were obligatory before compressor manufacturing to check delivery pressure and efficiency. Better flow physical models and computer progress made possible to develop quickly operating programs to predict gas dynamic performance curves of an arbitrary flow path. TU SPb set of computer programs was named “The Universal modeling method” and its application still in mid 1990th had lead to elimination of model tests in a design process of industrial centrifugal compressors. The Universal modeling state of the art including 4th generation loss and work input models were presented in conferences in Germany, Japan, Great Britain and Poland. Set of algebraic equations describe surface friction losses, flow separation and following mixing losses. Flow deceleration along surfaces and velocity gradient along a normal to surfaces are taken into account. By 4-th generation programs several dozens of compressor with delivery pressure up to 12,5 MPa, number of stages up to 8, power up to 25 mWt were designed for some Russian and foreign manufacturers. Amount of compressor installed exceeds 400 with total power close to 5 000 000 kWt. In all cases the design parameters were achieved without preceding model tests. The 4-th generation model was perfect enough to predict design point efficiency with accuracy about 2,5% if a single set of coefficients was applied. To raise accuracy of calculations to 1% or less different sets of empirical coefficients were necessary for stages with different flow rate and work coefficients. The proposed text is focused on scientific background and realization of model improvements. The main of them are more precise impeller size presentation, impeller velocity diagram schematization very close to non viscid diagram, surface roughness introduction in the loss model, shroud leakage influence on flow at an impeller inlet, etc. As a result 5-th generation model predicts efficiency curves of stages with different flow rate and work coefficients with mean deviation less than 0,5% at design point and 1,5% along all performance curves – with a single set of empirical coefficients. Then the compressors’ test performance curves were carefully correlated with the calculated ones by proper selection of empirical coefficients in models of pressure loss ant work coefficient. The stages of 16 tested compressors can be considered as 99 model stages with the range of gas dynamic and constructive parameters: flow rate coefficients 0,025 – 0,064, Euler work coefficients 0,40 – 0,85, relative hub diameter 0,258 – 0,483, outer relative diameter of a diffuser 1,316 -1,720. Stages polytrophic efficiency is 0,765 – 0,885 and surge limit ratio is 0,30 – 0,93 depends on a stage specific speed. There are samples of useful application of the stages. The new loss model is applied to 6-th and 7-th generation computer programs. Newly is described 3D impeller flow path shape. Q3D approach to impellers adds information for more profound optimization. New solution were checked and approved by CFD calculations.
NEW VERSION OF THE UNIVERSAL MODELING FOR CENTRIFUGAL COMPRESSOR GAS DYNAMIC DESIGN