Conjugate heat transfer (CHT) can be defined as the simulation of solid zone and fluid zone simultaneously . In solid zone we will only solve conduction equation (simplified form of energy equation) and in fluid zone continuity, momentum and energy equation. We can activate body forces due to gravity and buoyancy forces by defining operating density in operating pressure panel and thereby activating natural convection mode. During initial phase of solution due to different behaviour of solid and fluid zones, solution diverges immediately when solved in steady state mode due to buildup of initial transients (they are not real and will have no effect on final solution). Therefore in this blog we have discussed two approaches i.e. transient solution with only one iteration per time step and pseudo transient solver. It is FAQ that what solver settings I should use for any CFD problem including conjugate heat transfer modeling. Even advanced users get confused, let alone the “novice”. This question is even more demanding with inclusion of advanced models such as heat transfer, multiphase flow modeling etc. For current problem Fluent 15.0 is used with two different settings discussed later. In this blog I have tried to clarify different flow schemes for the conjugate heat transfer problem. In this simulation heat transfer from computer chip (producing 75 W) is to be dissipated into the still air via natural convection heat transfer mode. Fins are attached to CPU chip to enhance the heat transfer rate. External air modelled with zero velocity using pressure inlet (zero gauge pressure) and pressure outlet (zero gauge pressure). For the fins, chip and board solid zones are defined. Inter fluid-solid or solid-solid coupling is done via coupled boundary conditions. Chip is modelled as source term to energy equation using volumetric heat generation. First we have tried steady state solver but in vain. After that we have used two settings with similar results with different no of iterations to converge solution. These cases are: 1. PISO coupling scheme with transient solver: Time step was chosen as 10 s arbitrarily and one iteration per time step. It is found lower time steps induces unsteadiness and due to that solution does not converge. It is recommended to initially use 1000s as time step to make the solution stabilize. 2. Coupled pressure based solver with pseudo transient solver: Time step method is user defined with 10s time step for fluid and 1000s for solid. Conclusions: 1. Solution converges faster (10x sometimes) with Pseudo transient. 2. Overall results are same.
Recommendation: For steady state coupled problems always use pseudo transient Solver with coupled pressure based solver
CAD model Solver settings for transient + Piso photo hosting
Solver settings for transient solver Solver settings for Coupled pressure based + pseudo transient
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