Heat transfer and Fluid Mechanics, COMSOL

Windforce

Hello,
I am trying to couple heat transfer and fluid mechanic together, model is a simple concrete slab (3'x6')with a water pipe beneath it and along 3' direction. For heat transfer, the heat flux will apply on the surface of concrete slab, other faces are thermally isolated, the temperature of concrete and water at inlet are 273K, and water outlet is set to be convective flux. activate convective heat transfer in water pipe with u, v, w.
For fluid mechanics, DE-activate concrete slab properties, and only activate water pipe, and inlet of water is set to be laminar flow with V=0.058m/s, and outlet is set to be Pa=0.
I tried both stationary and time-dependent,
I used Direct Spooles for stationary, the velocity field at center of water pipe is fluctuate, and as well as temperature, both number will move up and down along the water pipe.
For time-dependent, I could not find a solver to solve it, always show errors on "zero in diagonal matrix".
What should I do to improve the analysis? I increased mesh side, swept mesh, to very fine mesh size, still not work, I also try to use artificial diffusion, Isotropic, but the Vmax will not be V=0.058*2.
Can anyone give me some advice or point out what I did wrong? please, this has been puzzled for couple months,
Thanks

greencomputer

try to find a singularity in the fluid or thermal solution and start with a good first guess at the problem solution by running no flow and feed the temperature solution for a first guess

turn smoothing of corners on and refine your mesh and raise your iteration at the beginning of the solution... take like five time steps and plot the solution you may find where the matrix is singular or trouble is occuring