Improving the performance of a condensing heat exchanger for biomass combustion at household scale

Abstract

A computational CFD model representing the behavior of a condensing heat exchanger used in domestic heating and hot water applications has been presented and validated, obtaining a flow output gas temperature error of 7%. An analysis of the heat exchanger internal aerodynamic performance revealed the need for geometry enhancements which involved increasing the number of combustion gas passages, reducing the cannon diameter, and incorporating turbulence generators.The geometric modifications produced a significant increase in variables of interest for the performance of the equipment when compared to the baseline case. The improvement is quantified as the 68% increase in turbulent intensity, 57% increase in velocity, doubling of flue gas residence time and 96% increase in total heat transfer rate.The design that contains turbulence generators presents the best performance, obtaining the highest heat transfer rate equal to 3.5 kW, which produces a decrease in the flue gas temperature equal to 62% and an increase in the water outlet temperature equal to 53% compared to the base case. In contrast, the prototype without turbulence generators showed a relatively lower performance in terms of heat transfer, but with a 9 times lower pressure drop.