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enr_venkat October 12, 2010 01:21

Conjugate Heat Transfer of Motorized EGR
Dear all,

I'm currently working on a Motorized EGR project. The device is used in automotive exhaust systems to reduce NOx emissions. When combustion temperatures are high (2200 C), the EGR will be operated for certain cycles by recirculating exhaust gases back to the inlet manifold of an engine which dilutes the incoming air to the engine which consequently reduces the combustion temperature in the engine thus controlling NOx emissions.

I would like to predict the conjugate heat transfer of the EGR valve. The plunger in the valve is operated by a stepper DC motor with some plastic gears that drives the cam and camshaft. Following are the steps considered for CFD Modeling of the EGR valve

1. Extracted fluid volume from the solid body using ANSYS DesignModeler in ANSYS Workbench 12.1

2. Different solid parts were treated individually in the ANSYS Workbench 12.1 i.e. meshing individually for moderate mesh quality (skewness - 0.7-0.85) eg: piston assembly, metallic bush, oil seal etc.,.

3. Linked all mesh cells in Workbench to CFX Pre- setup cell. When CFX is edited, i can find all the components get connected

For choosing the flow model in steady state CFD analysis, i've considered Total Energy model and Fluid Discrete Transfer Media Radiation model as the Mach number exceeds 0.2 in compressible flows. Moreover the inlet exhaust temperature of the gas is around 1023 K. I've created Fluid solid interface between the fluid domain and the first point of contact i.e. Piston assembly and then i did Solid-solid interface for every part that is in contact with each other including bush, oil seal, plastic gears, dc motor, etc.,. My main objective is to study the effect of exhaust gas temperature on solid parts and determine the heat transfer at desired locations. For instance, selection of material for oil seal was challenging i.e. engineers don't know how much temperature propagates if different sets of oil seal materials are tried. FKM Seal cannot resist temperatures exceeding 250 C.

I would like to know whether my modeling approach is correct or is there anything else has to be considered for the analysis. Any suggestions or help from CFD Expertise is highly appreciated

Many thanks

ghorrocks October 12, 2010 18:17

Are you sure this can be done as a steady state analysis? The flow is going to be very fast, pulses at 10Hz or 100Hz or something like that and I doubt it will have time to get to thermal equilibrium. Alternately you could look for some sort of pseudo-steady state by getting the average conditions but this is an inaccurate approach as the heat transfer coefficients will change with flow rate too so averaging it all back to a pseudo-steady flow could be a problem.

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