# Double pipe heat exchanger with SST k-w turbulence model

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 July 15, 2015, 12:33 Double pipe heat exchanger with SST k-w turbulence model #1 New Member   Join Date: Apr 2015 Posts: 11 Rep Power: 9 I've made quite a few threads lately, but it seems I just can't understand what I'm doing wrong... And I'm seriously running out of time Here is my problem: A counterflow double pipe heat exchanger was chosen for it's simplicity. Through the inner pipe flows a gas starting at 600°C (873K) going to about 50°C (323K), this is a laminar flow according to previous calculations.. Water flow through the annulus starting at a temperature of 6°C (279K), again according to previous calculations this is a turbulent flow with a Reynolds number of approx. 80 000. The diameter of the water inlet and outlet is 19.05 mm From LMTD, etc equations the pipe needs to be 3.5 m long I'm using the pressure based solver, and I've set the gravity to -9.81m/s² in the z direction... The model is set to SST k-w with all model constants still as standard. For the cell zone conditions I've only changed the materials and set the inner pipe "flow" to laminar zone For the boundary conditions: -the inlets are set to velocity inlets and I've calculated the velocity by means of 'flow through a certain area'. the turbulence factors I've decided to used (because they where easiest to find/know) are turbulent intensity (from the ansys guide I=0.16(Dh)^(-1/8))and hydraulic diameter (for both of which I've simply used the inlet diameter of the fluid 19.05 mm) -the outlets are set to pressure outlet and the thermal backflow is still set at the standard 300K -the wall I've left standard (non slip condition, etc) I haven't touched the dynamic mesh or the reference values velocity coupling is set to SIMPLE and for discretization I've set certain factors to second order upwind (see pic) What I get from this seems to be a very efficient heat exchanger when looking at the contours of static temperature...The does also seem to be an instability in the x-direction when looking at the scaled residuals... http://s12.photobucket.com/user/MStu...tml?sort=3&o=3 http://s12.photobucket.com/user/MStu...tml?sort=3&o=5 http://s12.photobucket.com/user/MStu...tml?sort=3&o=6 http://s12.photobucket.com/user/MStu...tml?sort=3&o=7 Please help me out with this problem, I can't seem to understand this problem or what I'm doing wrong, my degree hangs in the balance and I'm seriously running out of time... Also, I'm sorry to make this so pic heavy, but as they say 'a picture tells a thousand words'. Thanks for any help you can give me, MStudent

 May 21, 2016, 03:31 #2 Senior Member   Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 348 Rep Power: 16 The main issue may be in the flow direction which is specified in the cell zone condition. Here in your image. it is in z direction while the other figures suggest that they should be in x direction. Please put x=1 and z=0