Numerical Simulation And Experimental Study On Premixed Combustion Of Domestic Gas Instantaneous Water Heater

With the development of town gas, people increasely pay attention to the effect ofNOxand CO produced in combustion process on indoor air quality. Domestic gasinstantaneous water heater is widely used as a kind of domestic appliances. Partlypremixed combustion is employed generally by domestic gas instantaneous waterheater. However, partly premixed combustion would have high NOxemission, about90~95ppm, which caused serious indoor environmental pollution. Aiming at thisproblem, a premixed burner of gas instantaneous water heater, with stable combustionand low pollutant emission, was developed in this paper. Through numericalsimulation and experimental validation method, the effect of mixing gas system andthe combustion characteristics of the premixed burner were discussed, the keytechnology problem during the process of premixed burner development was sovled,and the potential application of this technology in the field of gas appliance was alsopointed out.In this paper, a concept that multi-ejectors were employed to realize premixing ofgas and air was put forward, and a mathematic model was set up based on this concept,then a numerically simulated study on it was carried out. It was found that the smallerthe nozzle diameter, the more gas pressure, and the influence of the NXP on the gaspressure required could be ignored; nozzle diameter should be greater than1.3mm,otherwise, the mass flow rate could not meet with the heat load requirement; when theNXP and air coefficient were constant, the greater the nozzle diameter, the greater theair pressure; when NXP>4mm, the mixing effect of gas and air was better, however,the uniformity of the velocity distribution at the mixed system outlet was poor. Basedon this problem, a technology that a distribution orifice plate was installed at theoutlet of the mixed gas system was proposed, which could effectively solve theproblem of the uniformity of the velocity distribution at outlet of the mixing system.The experiment proved that the mixed gas system developed in this paper couldrealize the premixing of gas and air.The numerical model simulating pollutant emission of burner was established,and the effects of air coefficient, fire hole diameter and thermal intensity of fire holeon the temperature field and the concentration fields of NO and CO in the combustion chamber were explored. Simulation results showed that the CO emission decreasedwith the increase of the thermal intensity of fire hole when air coefficient α wasconstant; and the CO emission decreased quickly with the increase of air coefficientwhen the thermal intensity of the fire hole was constant; the flow vortex produced inthe areas which include two parts, one area is between the square fire hole group andthe round fire hole group, another area is between the round fire hole groups, couldcause the rapid increase of the temperature and drop of the oxygen concentration,therefore, the concentration of NO and CO increased. Based on the simulated results,the fire orifice plate was optimised, the spaces between the small fire holes in the firehole cluster were increased, and the spaces between fire hole groups were decreased.The simulated results showed that in the new burner, the area of high NOconcentration reduced, therefore, the NO concentration also reduced.The whole machine experiment of the premixed burner of gas instantaneouswater heater was carried out, and the combustion condition was studied. Thelaboratory furniture used for premixed burner was set up, and the variation ofpollution emission and combustion stability of the premixed burner with NXP, nozzlediameter, air coefficient and heat load (thermal intensity of fire hole)were researched.Experiments showed that the effect of air coefficientα on the CO emission wassignificant, when1.1<α <1.2, if α increased slightly, the CO emission reducedsharply; whenα>1.2, the CO emission reduced slowly. When α>1, the variation ofNO emission with air coefficient was linear; the nozzle position away from ejecorentrance could help reducing pollutants emission; when the heat load (thermalintensity of fire hore) reduced, the pollutants emission was on a declining curve.Through the verification of the whole machine experiment, the completelypremixed burner with stable combustion and low pollutant emission, could supply animportant technology references for the research of the completely premixed gascombustion appliance.