Hi Rob, thanks for your quick reply. nAfter running a lot more simulations with different settings, i found out a few possible reason, but i am not really sure how to solve it.nAfter inspecting the pathline of water from the water inlet, it seems as though the water is travelling on a single path towards the outlet and not filling up the whole cavity. (The water is supposed to fill up the whole cavity while swirling around the inner pipe to reach the outlet.) From the temperature contour, it also shows the same result. Only where the pathline of water touches the inner pipe has a higher temperature which means heat is transfered from the hot gas to the cold water. For all other places, even though there's heat transfer happening, they have a lower temperature which I think it's just convection but not conduction. nAlso from the pathline, a few of my simulation turn out to have totally incomplete pathline which I assume it's due to reverse flow. I have move my outlet further downstrem but it doesnt really help much. when running calculation, the reverse flow message still keeps on appearing.nFrom the temperature contour, i realized there's somehow a lower temperature near the region of outlet which caused the outlet temperature higher than the inlet temperature, which is not possible. Does the specified backflow temperature plays a role in this? I have set mine to 20C. Should I set it nearer to the estimated outlet temperature?n(Here is a rough sketch of my design)nnDoes any of these reason even possible, especially the first one? I will be really grateful if you can advice me on this matter. Thanksn