Two stage atmospheric burners: Development and verification of a new mass-energy balance model

Abstract

Nitrogen oxides (NOx) from combustion devices are responsible for the formation of acid rain and photochemical smog. This led to a growing environmental concern resulting in the creation of rigorous legislation for harmful emissions. For domestic hot water production appliances, particularly water heaters, there are already some solutions in the market able to achieve low pollutant emissions. However, the complexity of the solutions makes the price tag significantly higher, hampering its mass adoption. It is therefore essential to design and develop simpler and low cost solutions that meet the limits imposed by legislation. This work reports the analysis and validation of the air-gas fuel mixture in a water heater low NOx gas burner element. This is based on a pre-mixed flame made of two separate fuel systems. One is a lean mixture that provides the bulk of the heat load. The other is a rich pilot flame, used to stabilize the flame. For that, a mathematical model wasdeveloped and applied, which, upon initial settings, allows for the prediction of the amount of primary air drafted. To validate the theoretical results, experimental tests were performed using a Laser Doppler Anemometer to evaluate the influence of the throat length in the amount of entrained air, as well as, the air induced in the burner element under study. Furthermore, a parametric analysis was performed which evaluated the influence of some variables (geometry, physical properties of fluids, etc.) in the amount of entrained air. The numerical results for the air-fuel equivalence ratio of a conventional burner element (between 0.645 and 0.794) proved to be reasonably close to the value announced by the manufacturer (0.7). Numerical analysis of the burner element with pilot stabilization resulted in values that verify the difference between the richness of the two mixtures. By adjusting the throat-diffuser geometry one may tune the air-fuel ratio of each mixture.