Realization And Characteristics Study On Flameless Combustion Of Liquid Hydrocarbon

Conventional energy reserves are dwindling accompanied by serious environmental pollution, so energy-saving and emission-reduction becomes the focus of conventional energy utilization. Flameless combustion technology becomes a research hotspot for its high efficiency and low pollutant emission. The present main focus of flameless combustion research is the mechanism and application of the gasous fuel under the conditions of high temperature air, therefore the flameless combustion research of liquid hydrocarbon reported is much less. The realization and combustion characteristics of flamelss combustion of liquid hydrocarbon under non-preheating air condition were investigated with an application background of micro gas turbine in this paper. The work is of great theoretical significance in exploring the flameless combustion mechanism and of practical value in studying the application in industrial applications.The major work of the paper is as follows:(1)A nozzle with surrounding vortex air was designed for flameless combustion, and the non-reactive spray flow in the vicinity of the nozzle exit and the atomization characteristics was tested using Adaptive Laser Phase/Doppler Velocimetry (APV/LDV) system. The test illustrates that the spray droplet sizes are between30-68μm with a skew distribution and an average diameter of about50μm. The initial fuel injection momentum is larger than that reported in the literature. It shows that the nozzle meets the basic research requirements of flameless combustion of liquid hydrocarbon.(2)A flameless combustor that can provide sufficient exhaust gas recirculation with multiple air jets was designed with the aid of CFD according to the atomization results. After the hot test devices had established, a series of combustion experiments was carried out by changing the fuel mass flow, fuel/air equivalence ratio (Φ), air shares and air inlet sizes. The results demonstrate that the dilution and mixing extent of liquid spray nearby the nozzle have an important influence on the transition of combustion regime. The fuel/air equivalence ratio range of the model combustor operating at flameless regime is0.25-0.50. With the same power input (Qp) and Φ, the total injection momentum (GT) and pollutant concentration of flameless combustion regime are higher than those of conventional combustion regime. Lowering Φ only, GT will increase, the average temperature of the combusor chamber (Tavg) will decrease, and emission concentration of CO will increase. In addition, the lower0is, the faster the increase of CO emission concentration will be with a little increase of NOx. Lowering Qp only, GT and Tavg decreases, the range of Φ(ΔΦ) at the flameless regime becomes narrower, the lower limit of0decreases, and CO concentration increases with a bit change of NOx emission. Note that the lower Op is. the faster the increase of CO will be. The process of flameless combustion was analyzed from the macroscopic and microcosmic perspective. In macrocopic, the high-temperature inertial species entranined by large reactant injection momentum is an important factor to realize and sustain stable flameless combustion. In microcosmic, the incorporated high-temperature inertial species lowers the probability of the effective collision of reactant molecules but the heat transfer at the uneffective collision moment enhances the energy of reactant molecules and the probability of proceeding collisions and continues the reaction.(3)A3D combustor was modeled to simulate the reactive flow and study the characteristics of the reaction zone. The computation reveals that flameless combustion can be achieved under the condition of high oxygen concentration when the reactant injection momentum is very important. It doesn’t quench when increasing the injection momentum at the flameless regime, which indicates that the heat and mass transfer between the combustion products and the reactants plays an important role in local reaction to release heat at low Φ. The heat transfer enhanced by the baffer in the chamber varies with Φ. The staged air jets improve the field synergy of the flow and the heat flow fields, promote the rapid heat transfer of species in small combustor chamber and provide energy for persistent ignition of local reaction.(4)The chamber was divided into13zones according to the computation results of CFD. The zones were connected to build a CRN model on the platform of CHEMKIN4.1. The pollutant emissions were computed using heptane reactional mechanism of low and medium-temperature and high-temperature respectively combining NOx mechanism. The main reaction paths of NO and CO were analyzed. Compared with the experimental measurements the computation results show that there are smaller differences in the computations of the flameless combustion regime using the low and medium-temperature reaction mechanism and the conventional combustion regime using high-temperature reaction mechanism. It is believed that the prompt NO accounts for the most of NO emission.