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* * * * * * * DLIT AND D The data presentcd herein consist of conventional aerodynamic probe surveys upstream and downstream of the impeller, and flow p visualization results from fluorescing oil (Jurkovich et aL, 1984). Results of computational analyses which support the experimental data are also presented. The main emphasis of the data and com- putational analysis presented herein is to illustrate the impact of synergism between experiments and computational analysis on the development of the low speed centrifugal compressor facility. Renearch operating conditions selected for detailed investiga- tions of thc low speed centrifugal compressor flow field were cho- sen to reflect design flow conditions'(30 kg/scc) as well as high (36.36 kg/sec) and low (23,64 kg/sec) flow conditions. Because of aproblem with the facility temperature control, that has since been fixed, and limitations on the maximum physical speed of the impeller (1950 rpm), the corrected speed for all research data was set at 97% of design speed (1862.4 rpm). Preliminary measurements indicate that the impeller inlet boundary layer thickness measured at survey station I is approximately 20% of span (5.921 cm) on the shroud and 2% of span (0.592 cm) on the hub. Impact of Experiment/CFD Synergism The history of the development of the low speed centrifugal compressor illustrates how synergism between computational analy- sOs and experimentation has affected the final design of the test hardware. The first research entry into the facility occurred Octo- ber 1, 1988, and due to a scheduled rehabilitation of the NASA variable-frequency control facilities, testing was concluded Novem- ber 1, 1988. Since this was a new facility, the intent of the initial entry was to check out the research data acquisition system. Dur- ing the one month of operation preliminary surveys were acquired upstream and downstream of the impeller. The data were not ana- lyzed ingreat detail, since the probes were uncalibrated and because several problems were uncovered with the data acquisition system, *0 0A4------.----- 0 20 40 60 80 100 SHROUD % Immersion HUB Figure 8 Ciicumfcrcnida uniformily/repeatibility of nimpelelxeitr low field, SLatdon 2a (4% Up clearance), About the same period of time, results of a computational anal- ysis of the compressor flow field by Moore and Moore (1989, 1990) of the Virginia Polytechnic Institute became available under a NASA 'grant.This grant was initiated in the spirit of a cooperative effort to minimize the time typically required to develop a new facility. The Moore's calculated the LSCC flow field with their Elliptic Flow Program, a 3-D pressure correction solution for discretized forms of the Navier-Stokes equations. The 3-D flow through the impeller was modelled using a 53 streamwis by 22 spanwise by 21 pitchwie grid. The Moore's CFD solution indicated separation along the im- peller shroud wall near the impeller exit as shown in figure 9. This separation continued downstream all the way to the outflow bound- ary condition which prevented the CFD solution from converging since the backflow within the separation region was bringing mass flow into the solution domain across the out-flow boundary. In order to obtain aconverged solution, the Moore's altered the geometry in the vaneless diffuser region, as shown in figure 10, to force the sep- arated flow region to close before it reached the outflow boundary. About the same time the Moore's were generating their predic- tions of the LSCC flow field, NASA was also generating predic- tions of the flow field using the 3-D computational analyses of Hah (1988,1989) and Dawes (1988). These predictions also indicated that separation would occur along the Impeller shroud. As a result of these CFD predictions, the previously acquired preliminary ex- perimental results were investigated and were found to indicate a severely retarded flow region near the shroud wall as shown In fig- ure II. Although the preliminary measurements were Inconclusive in substantiating the predicted shroud wall separation, the severely retarded flow region measured near the shroud wall indicated that shroud wall separation was probably imminent. Certainly, areverse flow region within the tip clearance gap is expected, although the survey probe was not able to detect It. Since the rig was intended for fundamental flow physics research and CFD assessment, it was considered undesirable to have separation which would affect the impeller flow field and produce an undesirable downstream bound- ary condition for CFD analysis. Considering that the facility wam ,1.21 0.7- V. i:V1 I 0.80' 0.6 0.4 - -U 2 V07 R/Rr 1.067 Survey 4 Quadrant . 3 %: Survey Quadrnt 234 i0,210.-1 2 3O4 0 20 40 60 80 100 Shroud % Immersion Centerline F;igure I Circu:.i'mrrnual unitormity/rcpeatability of inleftlow field, Station 0. 7PDF Image | NASA Low-Speed Centrifugal Compressor for 3-D Viscous Code
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