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October 2012
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Using CFD for Improved Ported Shroud Design
Ported shrouds have become an indispensable feature in modern turbocharger design.  In this month's issue of The Flow, we sit down with Bob Ni to discuss ways in which CFD can be leveraged to optimize ported shroud performance and extend compressor operating range.  Bob is the Chairman and CTO of ADS.  Prior to founding the company, he was a senior fellow at Pratt & Whitney leading turbomachinery CFD development and application efforts.

FLOW: Why have ported shrouds become so important in turbocharger compressor design?
BOB: There are many options for improving low end performance using variable geometry or twin stage designs, but they are costly and more susceptible to durability issues. Ported shrouds are appealing because they are relatively simple, passive features that can be used to delay the onset of stall and thus improve low end compressor performance, often with minimal impact to compressor efficiency.  
FLOW: How do ported shrouds achieve this?
BOB:  As we all know, stall is usually driven by tip leakage/endwall secondary flow build up in the impeller.  A ported shroud delays the onset of stall by bleeding the bult up secondary flow and recirculating it upstream of the impeller to re-energize the low speed flow coming into the impeller.  
FLOW: So how can CFD assist with ported shroud design?
BOB: Good CFD can help in several ways.  At a fundamental level, it can provide important time-resolved insights to understand the factors driving the onset of stall. For design, it provides an effective means for locating and sizing the bleed and reinjection ports.  And once the casing treatment has been fully designed, the entire configuration can be meshed and analyzed to predict overall performance and stability limits.  
FLOW: Let's focus on design.  How can CFD be used to design the ported shroud?
BOB:  CFD can help at two levels.  First, you can use it during preliminary design to situate and size your bleed and reinjection ports.  This can be carried out in ADS CFD, for example, through the use of our 2-D ported shroud engine model.  The model lets you specify the location and size of the bleed and reinjection ports and will direct the flow based on the pressure gradient between the two locations.  For preliminary design, it's an effective way to hone in on the impact of port locations without having to grid them up and engage in heavy duty analysis.
Once CFD has been used to opimize the size and location of the ports, the actual design of the casing treatment can be carried out.  And when this is complete, the casing treatment can be fully meshed and analyzed using 3D steady or unsteady CFD depending on time/budget/resource constraints.  This second level of analysis can be used to fine tune your design--for example, to refine the shape of your ported shroud or to introduce features like vanes to deswirl the flow.
FLOW: Seems like design optimization requires a lot of CFD computing resource.  Doesn't this put it out of reach for some of your clients?
BOB:   The approach will certainly vary depending on industry application and level of sophistication.  Let's assume for now that we are focused on automotive turbochargers and in particular, low end performance.
  1. Generate a compressor map for your impeller without casing treatments. This establishes the baseline for assessing the impact of ported shroud designs.
  2. Select an operating point near stall for three part-speed operating conditions (say 20%, 40% and 60% speed) and interrogate the corresponding static pressure vs. axial location plots to determine where the buildup of secondary flow is located.  Use this information to establish an initial location for the bleed port.  Set a width equal to 4-5x the thickness of the leading edge as your initial size for the bleed port.
  3. Now look for a corresponding area upstream of the impeller where the static pressure is similar in value to the static pressure at the bleed port.  Use this to establish the initial location for the reinjection port. Use the size of the bleed port as the size of the reinjection port.
  4. With these baseline values, carry out parametric studies against a range of port locations and sizes. Use the resulting compressor maps generated for each run to hone in on the optimum set of parameters to address your design objectives.
FLOW: This seems like a lot of operating points to analyze and administer.  
BOB: Yes, it's a lot of manual labor in the absence of some level of scripting or automation.  For example, the ADS Workbench recently introduced a feature to automate speedline generation and post-processing from a single converged solution.  Multiple operating points can be queued for execution to generate a compressor map without manual intervention.  This removes much of the tedium while ensuring quality.   
FLOW: Thanks, Bob.
BOB: You're welcome.
CASE STUDY: A Comparison of ADS CFD Predictions to Experimental Data for the Radiver Centrifugal Compressor Impeller  
In this case study, the impeller row of the Radiver centrifugal compressor is analyzed using Code Leo and Code Wand and compared to experimental results obtained at the Institue of Jet Propulsion and Turbomachinery at RWTH Aachen, Germany.   <more>
TECHTIPS: Tips for Speedline Generation Using the ADS Workbench  
The ADS Workbench includes several useful features for constructing compressor speedlines.  In this article, we describe each of these capabilities and how they can best be used during design.  <more>
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