|dc.description.abstract||This thesis investigates experimentally combustion characteristics of a small premixed excess enthalpy burner, a two dimension Swiss-roll burner whit the maximum diameter varying from 7.5~21 cm and with a height of 5.0 cm. We visualize variations of the flow fields the combustion zone of Swiss-roll burners with six different designs. Temperature distributions and pollutant emissions in these excess enthalpy burners are measured quantitatively for the first time to develop a compact, very low fuel consumed water heater. The excess enthalpy burners apply the principle of heat recirculating and the conversion of thermodynamics to minimize heat losses, increase burning efficiency, and make extra lean premixed burning possible. Propane/air mixtures are used, because propane is a liquid, with a density nearly one thousand times greater than that of gaseous fuels such that the space for the fuel storage can be significantly reduced.
It is found that the present Swiss-roll burner can be operated at extra lean conditions, where the equivalence ratio f is slightly smaller than 0.1. This φ»0.1 is much much less than the common lean flammability limit of C3H8/air mixtures in which φ»0.57. We also found a better design of the combustion zone of the Swiss-roll burner in which flames can be stabilized in the combustion zone. Using the heat recirculation rate (HR) as a criterion for the performance of the burners, the optimal design of the burners was examined in terms of φ, the fluid velocity Vf, the flow Reynolds number Ref , the number of rolls of the burner N, and the interval channel width of the Swiss-roll burner D. Results show that the mean temperature (Tm) inside the combustion zone is strongly influenced by the operating (heating) time period t, N andf. For examples, when φincreases from 0.30 to 0.50, Tm increases from 1054oC to 1282oC at fixed N=3.0, D=1.0 cm, and t =10 min conditions. Emission measurements on the outlet of the burner show that the concentrations of [NOx] are less than 10 ppm and [CO] »40~60 ppm for any value of φ between 0.4 and 0.5, when φ＜0.3, [CO] increases largely, indicating that burning is incomplete and unstable near the critical equivalence ratio (φc). HR increases with N but decreases with D, so that extra lean combustion can be performed at larger N and smaller D.
Concerning the preliminary application of the energy saving water heater, we use a long water tube wrapped along the upper product channel of the Swiss roll burner so that the water can be heated via the heat conduction from the high temperature product channel. It is found that this heating arrangement is inefficient. Alternative heating methods have to be considered, such as the direct injection of the exhausted gas (~300oC) into a water tank for developing a compact (20 cm×20 cm×5 cm), high efficiency bath water heater.||en_US|