[xlworn]

I’ve tried to do some calculation of natural convection heat transfer using CFX 12 and FLUENT 12, but it cannot reach convergence. My geometry is similar with the one you described in those papers. It starts from a single vertical tube in a large tank, tube length is 1m, diameters are 19mm and 6mm, tank is 1m×0.2m×0.2m.

diameter 19mm, in FLUENT

Solver type is pressure-based, transient, gravity is -9.8m/s2.

Models choose the energy equation and viscous model choose SST k-omega. And the low-Re number correction is included.

Materials is water-liquid from FLUENT database, boussinesq is chose for density, thermal expansion coefficient is 0.000257.

Boundary condition in FLUENT is the same with condition in CFX.

Solution methods like this

Under-relaxation factors is below

Time step size 0.6s, after calculating, I received the residual plot, it is not good, so I’ve tried to adjust the relaxation factor, I’ve tried for 6 times.

Relaxation factor

finally I received this residual plot.

I've checked Y+, it is about 1 near the tube surface.

Contours of temperature on clip plane is below

It looks like right, but the sum of residuals was JUST below 1×10e-2!

So would you please give me some advice about this calculation.