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UnconventionalVolute-DiffuserAssemblyforExtendedCompressorRangeValeriuDragan,MihaiMihaescu
Centrifugal compressor range is highly dependent on installation effects, which for turbochargers , means that volute geometriessometimes sacrifice stability in favor of space/weight. Herein we investigate the concept of a non-axisymmetric vaneless diffuser(VLD) in a typical turbocharger test case. The concept is compared against classical VLD treatments, i.e. pinching and shelving. Interms of stability there are clear advantages to the technique, especially on the peak efficiency speedline, however it is alsoapparent that the scroll section shape and area distribution also need to be redesigned in order to avoid high pressure losses.Counter intuitively, the key of the method appears to be the uneven pressure distribution induced by the trimmed VLD whichdominates over any parasitic scroll induced diffusion at low flow conditions.
Introduction and Motivation:Automotive turbochargers are subject to tight packagingrestrictions, hence compressor volutes often have to be designedwith certain aerodynamic compromises. Since range and stabilityare important factors for automotive manufacturers, any range-efficiency trade-offs have to be carefully managed.Existing literature suggests that a trimmed vaneless diffuser (VLD)internal volute can provide stability extension for centrifugalblowers with little efficiency penalties. The purpose of this researchis to investigate the applicability of this concept to a compressorstage relevant to the automotive industry.Questions regarding the actual cause of the increased stability aswell as the universal character of the method will be addressed.Understanding the root causes of both the stability increase andpressure losses will pave the way for optimal implementation ofthis method and ultimately to better overall fuel efficiency ofautomotive powerplants.
Setup:Thus far, RANS methods were employed to assess the variousconfigurations. Both the k-ω SST (RC) and the SA (RC) models weretested against the baseline experimental data with comparableaccuracy. The SARC model was ultimately chosen owing itscomputational speed and proven accuracy. Convergence wasassessed using both the conventional monitors (i.e. residuals, massflow imbalance, efficiency, PR) but also tailored parameters such asindividual torque distribution on the blade passages, radial forcemagnitude and direction. The geometric variations consideredwere proportionately cutback diffusers, ranging from 100% -baseline to 0% - fully trimmed.
Summary and Conclusion:The fully cutback VLD resulted stable operation across thespeedline, but at ~7% efficiency penalties. Trimming led to a steadyincrease in the radial velocity component, mismatching the volutearia distribution and also limiting the choke margin. A redesign ofthe volute scroll is in order, for achieving the full potential of themethod. A strong circumferential pressure distortion was observedin the trimmed case, potentially linking stability to mass flowdistribution on the impeller passages. URANS simulations havebeen started and will be analyzed in the future to observe thebehaviour as a function of the stage’s Greizer-Moore constant B.
Acknowledgement:Mihai Mihaescu, [email protected]
1,55
1,65
1,75
1,85
1,95
2,05
2,15
2,25
2,35
2,45
0,105 0,155 0,205 0,255
totalpressureratio
massflow[kg/s]
25% 50% 75% baseline
00%(flush)
Results:One trimmed variation with significantlyincreased stability has been identified, althoughhaving – in the current, raw form- high efficiencypenalties. The variation is characterized by anoverall increase in throughflow velocity as well astrong pressure distortion across thecircumference. Losses are attributed to the nowinadequate aria distribution of the scroll and canbe addressed through a future optimizationstudy. Since the modified volute has a greatervolume than the original, it would be expectedthat the diffusion would be higher. Instead thiscannot be observed, pointing towards a clearless-than-optimal shape and aria distribution.It is worth mentioning that the variation inquestion outperforms the classical pinching ofthe VLD in terms of range extension whilematching its efficiency.
0485[m/s]
50160[J/kg-K]
The flush volute has been proven to stabilize thecompressor stage across the peak efficiency speed line(top). However, due to the lower diffusion (bottom),flow velocity inside the scroll leads to greater entropybeing generated across the volute (right), making thecompressor less efficient.
The baseline volute (right) ismodified by trimming theprotruding part of the VLD byvarious percentage. The flushvolute (left) has the VLD fullytrimmed.
Velocitymagnitude
Entropy
