How to get recirculation factor (by mass) used for underhood frontend simulation?
Recirculation factor (RF) is a useful parameter for frontend underhood application. Air enters the top and bottom grille. Some of this flows through the cooling package (it enters the condenserinlet and then the radiator). All of this air that has entered the cooling package through the condenserinlet exits on the other side from the radiatoroutlet. Not all the air that enters the condenserinlet comes from the grille. Some comes from the tire region, some through recirculation, and some from radiator back flow. Some of the air that has exited the cooling package through radiatoroutlet exits the unerhood and some returns back (recirculates) to the condenserinlet. Client would like to know the fraction (by mass) of the air recirculated. How to calculate RF? 1. Obtain flow solution as usual. 2. Activate User Defined Scalar Define>UserDefined>Scalars a) increase "Number of UserDefined Scalar" to 1 b) select "mass flow rate" for "Flux Function" c) click on OK d) clicjk on OK for the new message window "mass flow rate" will use UDS = density * velocity. 3. Set UDS Deffusivity Define>Materials a) select 'fluid' under 'Material Type' b) select the material name under 'Fluid Materials' c) click on 'Edit...' for 'UDS Diffusivity' Another panel will open. d) select UDS0 under 'User Defined Scalar Di' e) select Constant under Coefficient f) enter a value 0.0. This implies that UDS transports only by convection process and not by diffusion. In many cases, this assumption is true where diffusion is very less. But for more accurate results, UDS diffusion also needs to be accounted for and the UDS diffusivity is given by turbulent_diffusivity*Schmidt_number. This could be provided to Fluent through an UDF. 4. Assign value of 1.00 for "User defined scale0 (UDS0) Define>Boundary Conditions a) select radiator cell zone under Zone b) click on Set c) select "Fixed Values" d) enter 1.00 for "User defined scalar0" e) click on OK Fluent will add proper source/sink terms so that the fixed value is 1.0. 5. Assign zero UDS VALUE at all inlets Define>Boundary Conditions a) select the inlet zone under Zone b) click on Set c) select "Specified Value" for Scale0 under UDS Boundary Condition" d) enter0.00 for Scalar0 under "UDS Boundary Value e) click on OK Repeat step 4 for other inlets. NOTE: By default all walls and outlets will have UDS flux of zero. NOTE2: We need to provide zero VALUE entering the domain and zero FLUX at all the walls. If we provide zero flux at all the boundaries, then it may not converge and/or give wrong result. 6. Patch UDS of 0.00 in all the cells. 7. Use Underrelaxation Factor (URF) of 1.0 for UDS Solve > Controls > Solution a) enter 1.0 for "User defined Scalar0" under "UnderRelaxation Factors" b) select ONLY "User defined scalar0" under Equations 8. You need to converge the UDS equation down to 3e5. Run only the UDS equation. You first start with underrelaxation factor (URF) of 0.8, iteration about 10 times, and then increase URF to 1.0 and then iterate to reach residual value of about 3e5. Then you can change to secondorder UDS and continue iteration. POSTProcess: 9. Report > Surface Integrals a) select condenserin under Surfaces. Note that condenserin should be selected. Radiatorin should not be selected because it is attached to radiator cell that has fixed value of UDS of 1.0. If radiatorin is selected, it will average the values of upstream and downstream cells. The downstream cell has a fixed value of 1.0 which should not be included in the reportintegrals. b) select "Flow Rate" under "Report Type" c) select "User Defined Scalars" under "Field Variables" d) select "Scalar0" under "User Defined Scalars" e) click on Compute The UDS will vary from 0.00 to its fixed value of 1.00. Integrating UDS using "Flow Rate" will lead to mass flow rate of UDS, which is the same as mass flow rate of air recirculated. See <a target=_blank href="http://www.fluentusers.com/fluent6/doc/ori/html/ug/node948.htm">http://www.fluentusers.com/fluent6/doc/ori/html/ug/node948.htm</a>http://www.fluentusers.com/fluent6/doc/ori/html/ug/node948.htm for definition for "Flow Rate" integration. It will print out the mass flow rates of UDS in the cortex window. It will be the amount recirculated. You can get the total massflow rate using ReportFluxes through this same surface to get the ratio. 

