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September 2010
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Fluid and Solid Heat Conduction--A Tale of Two Time Scales
Effective cooling schemes are key to the success of today's high efficiency turbines.  As designers grapple with fluid and solid heat conduction during design, vastly different time scales for fluid and metal response must be accounted for.  In this month's issue of The Flow, we sit down with Bob Ni to discuss this difference and how to properly account for it during design.  Prior to founding ADS, Bob spent nearly 30 years at Pratt & Whitney leading turbomachinery CFD in support of compressor and turbine design. 

FLOW: As part of your work in conjugate heat transfer, you've been investigating how quickly solids and fluids respond to step changes in temperature.  Why?
BOB: In speaking with clients, we've found many have had trouble getting their CHT results to cooroborate well with rig data.  There are many possible reasons for this, including numerical models and mesh quality, but we felt an even more basic problem potentially had to do with the time scale of the collected rig data.  So we investigated.
FLOW: How did you set up your test?
BOB: First, we set up a fluid problem using Codes Wand and Leo, imposed a step change in inlet boundary condition and determined the time it took to achieve steady state.  Then we looked at the same airfoil and used our heat conduction module to determine how long it would take for the transient temperature field to respond to an increase in external surface temperature.  With these two results we were able to compare the relative response time scales.

FLOW: And what were your findings?
BOB: We found the fluid response to be on the order of 0.006 seconds, and the metal response to be on the order of 60 seconds.  In other words, a five order of magnitude difference in time scale.

FLOW: That's a fairly big difference, but what is the implication?
BOB: The fact that there was a difference in time scale is of no surprise.  However, the magnitude of the response time scale for the metal suggests that designers must be keenly aware of the limitations of their test rig when conducting CHT analysis.  A warm rig that is capability of measuring results on a 60 second time scale is fine, however, using a blow down facility designed to operate in millisecond time scales may prove problematic.  For wall temperature measurement, the blow down facility would simply be generating transient results that would not be indicative of steady state.

FLOW: What's your takeaway?
BOB: As always, stay keenly aware of the limitations of both your numerical methods and rig setup.  In the case of conjugate heat transfer analysis, don't assume that a rig that is capability of producing reliable data on aerodynamic performance is equally as capable of producing reliable data on metal temperatures.  For wall temperature measurement, make sure the rig is set up to capture results on a time scale of tens of seconds, not milliseconds.
FLOW: Thanks, Bob.
BOB: My pleasure.
CASE STUDY: Improving Centrifugal Compressor Design at United Technologies Research Center (UTRC)
Learn more about how UTRC was able to design a rotorcraft-relevant, high speed centrifugal compressor for NASA using Code Wand and Code Leo.  <more>
TECHTIPS: Animating a Multistage Turbomachine Using ParaView
  Using ParaView and ADS-VTK, designers can easily plot and animate the solutions for full multi-stage machines.   <more>
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