Hi @jopoy and @AnnaFless!
I managed to have a successful run. It seems like the problem was in temperature residuals being exploding. This was due to high heat change in the system as there was a source but no sink.
I reran the simulation having also convection applied on the outer surfaces and this did work. This convection represents as if air is touching those surfaces and taking out heat from the system. Higher the convection coefficient, more dominant will be the convection effect. Moreover, I also merged the stainless steel solids in to one to make the simulation setup more simple. Please note that I have used advanced solver in this case as the normal solver was giving some problem related to non-positive Jacobian which is a known problem of non-advanced solver.
Please find the project here: BSF Burner - SimScale
Hope this helps. In case of question/s, feel free to ask.