[eddie4160]

Hi all, I am currently working on a simulatoin case of "crossflow over a tube", the shematic geometry and boundary conditions are shown in following figure

https://www.dropbox.com/scl/fi/c0082...lcznbc3jp&dl=0

- Introduction -
I've develpoed a mathematical model ( in-house code ) to simulate the heat transfer of the tube solid region including external flow ( see above figure ) and aim to couple it with Ansys Fluent. The in-house code require the outer wall heat flux from Ansys Fluent as the boundary conditions.

- How I calculate the outer wall heat flux using UDF -
The whole process is control by DEFINE_ON_DEMAND ( a macro in UDF ) which is a general-purpose macro in Ansys Fluent. I calculate the outer wall heat flux as below:

https://www.dropbox.com/scl/fi/mhli8...71jx2s6zw&dl=0

Using right hand side of Eq. (1) in above figure, the snippet of code see

        // Phase domain
        Domain *domain = Get_Domain(1) ;

        // The ID of the BC created in ANSYS Fluent
        const int wall_ID = 21 ;

        // Get the thread from the domain, the thread is partitioned
        Thread *f_thread = Lookup_Thread( domain, wall_ID ) ;
        
        // Create face ID
        face_t f_ID ;
        
        /* Adjacent cell information */

        // adjacent cell index of the boundary face
        cell_t c0 ;  

        // thread index of adjacent cell
        Thread *t0 ; 
        
        // xf represents facets location array and xc is cell center locatoin array
        real xf[ND_ND] ;
        real xc[ND_ND] ;

        begin_f_loop( f_ID, f_thread )
        {
            /* Get the cell center 
            F_CENTROID( xf, f_ID, f_thread ) ;
   
            /* Get the cell id of the cell adjacent to the face*/
            c0 = F_C0( f_ID, f_thread ) ;         
            
            /* Get the Thread id of the cells adjacent to the face*/
            t0 = F_C0_THREAD( f_ID, f_thread ) ; 
            
            C_CENTROID( xc, c0, t0 ) ;

            /* dx is ignored because the cell center and face center are align in radial direction
            double dy = abs(xc[1] - xf[1]) ;
            double dz = abs(xc[2] - xf[2]) ;
            double dr = sqrt(dy*dy+dz*dz)  ;

            // Calculate the surface heat flux, k_f * (T_cell - T_face ) / dr
            q_array[ f_ID ] = C_K_L(c0, t0) * (C_T(c0, t0) - F_T(f_ID, f_thread)) / dr ; // C_K_L(c0, t0) is the UDF macro return fluid thermal conductivity
        }
        end_f_loop( f_ID, f_thread )

- Main Issue -
The difference between the outer wall heat flux calcuted by UDF and from Ansys ( using built-in function on GUI, File->export->solution data->Total Surface Heat Flux ) are shown below

https://www.dropbox.com/scl/fi/yva9r...xpomlujum&dl=0

The outer wall heat flux is represented in 2D and θ=180° is bottom region of the tube. Refer to the figure, most of the error occurs at θ=120°~240° and the maximum error of 5.33%.

Does anyone have any experience with this? I would appreciate your advice and suggestions.