Thank you again for your input!
1) I have following figures: pipe length: 13.1 m. Max water flow rate: 0.15 kg/s =1.5 10^-4 m3/s. Pipe inside diameter: 0.05 m, pipe section 2.0 10^-3 m2. Hence, Max water speed: 0.075 m/s. So I comply with the requirement â??fluid cannot manage to pass through the pipe section within a timestepâ?? for a timestep of 1/20 hour, and the particle would stay in the pipe for three timesteps for a timestep of one minute.
With these timesteps, I do get rid of HVAC convergence warnings. However, the curves of pipe ambient/fluid heat transfer rates still look similar (attached screenshots).
2) I am not sure what you mean here? Given the material, the thermal diffusivity of the pipe should not be so high:
Material,
distribution-SHSpace Heating pipe construction plastic,
Smooth,
0.004,
0.4,
900,
1200,
.9,
.7,
.7;
Material,
distribution-SHSpace Heating pipe construction insulation,
MediumRough,
0.016667,
0.035,
15,
1500,
.9,
.7,
.7;
Construction,
distribution-SHSpace Heating pipe construction,
distribution-SHSpace Heating pipe construction insulation,
distribution-SHSpace Heating pipe construction plastic;