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November 2012
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Overcoming CFD Challenges for Pulsed Turbine Design
Pulsed turbines are of great interest to aerospace and automotive turbocharger designers in search of enhanced performance and durability.  In this month's issue of The Flow, we sit down with Bob Ni to discuss how CFD can be used to account for pulse flow upstream of turbines during design.  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: What are the characteristics of a pulsed turbine?
BOB: Generally speaking, a pulsed turbine refers to a turbine that experiences time-varying pulsation flow at its inlet.  In aerospace, it could be from a pulse detonation combustor used to dramatically increase performance, or in automotive turbochargers it could be from the exhaust of a six cylinder piston engine.  
FLOW: What are some of the CFD challenges associated with pulsed turbine design?
BOB:  We think there are three primary challenges.  The first is how to account for the impact of pulsation flow on turbine performance. The second is how to assess the corresponding impact on durability.  The third challenge relates to the efficiency calculation itself and how it is best formulated in light of the pluse flow condition.  
FLOW: How can these challenges be overcome?
BOB: We believe time accurate simulation is well-suited for understanding the impact of pulse flow on turbine performance.  By combining time accurate simulation with time varying boundary conditions at the turbine inlet, it is possible to predict pulsation effects during design.  For example, we've been working closely with the Air Force over the past year on characterizing the impact of pulse detonation combustors on turbine performance.  We carried out 2D time accurate simulations on the turbine by imposing time-varying boundary conditions on one or more circumferentially arranged sectors to represent the pulse detonation flow and sequence. Depending on your cluster size, passage counts and pulsation frequency, it's certainly possible to generate results overnight.  
FLOW: And how about durability analysis?
BOB:  We also believe that time accurate simulation is well-suited for identifying adverse blade row interation effects leading to high cycle fatigue.  For example, with ADS CFD, every unsteady simulation includes a standard results file that contains the time-varying pressure loads on the surface of the blade.  This information can be used as input to a structural analysis package to identify potential durability problems.
FLOW: Makes sense.  What are your thoughts on turbine efficiency determination?
BOB:   There are three approaches to consider for calculating pulsed turbine efficiency.  The first is to use a mass-weighted average of total temperature and pressure. This is the conventional approach.  Another method is to use the mass-weighted average of total temperature and entropy.  Finally, you can also use a moving average of specific work output and work lost due to entropy increase.  Our research to date suggests that the first method is far less reliable that the latter two.  We plan to highlight our findings in an upcoming paper.
FLOW: Thanks, Bob.
BOB: My pleasure.
TECHTIPS: Using Mesh Diagnostics to Assess Section Data Quality  
A new mesh diagnostics data file has been introduced in ADS Release 5.3 to simplify mesh debugging.  In this article, Will Humber describes the organization of the diagnostic file and how it can be used to assess the quality of airfoil section data.   <more>
TECHTIPS: Using IPLOT to Streamline Code Wand Mesh Inspection in ParaView  
As of ADS Release 5.3, Code Wand can write VTK files directly to examine a mesh in ParaView.  This article describes how to modify the IPLOT parameter to easily inspect meshes for one or more passages and airfoil rows in ParaView.  <more>
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Each month we'll spotlight a topic of interest, discuss a case study and/or provide useful pointers about how to get the most out of the ADS CFD system.

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