Numerical study of a condensing boiler heat exchanger

Condensing boilers can have a much higher efficiency than conventional ones due to the recovery of talent and sensible heat from the exhaust gases. However, many condensing boilers currently operating in the UK have demonstrated that further improvements to their design are needed and detailed stimulation of the flow and heat transfer processes is necessary in order io optimise them. In the current study, Computational Fluid Dynamics (CFD) has been applied to evaluate the performance of a condensing boiler heat exchanger. Submodels for the estimation of the convection heat transfer coefficient. the water vapour diffusion in the flue gas, condensation on the tube walls and tube bundle friction resistance were developed and incorporated into the commercial CFD code PHOENICS. The tube bundle was treated according to the porous media concept. Detailed data on the velocity and temperature distributions for both shell and tube fluids, heat transfer and condensation rate were obtained. The predicted flue gas temperature distribution was compared with available experimental data and good agreement was found between them.