| As environmental protection and emission standards become increasingly strict, many research institutions at home and abroad have begun to explore different ways and means to reduce combustion emissions and pollutants. Studies have shown that dimethyl ether (DME) and other alternative fuels or fuel additives can effectively reduce pollutant emissions. DME is one of the most promising candidates because it has many advantages such as clean, efficient, low pollution and easily synthesized.In recent years, the study of combustion characteristics of DME has been drawing extensive attentions. Compared to DME premixed flame investigations, the liftoff properties of the DME diffusion flame has not yet been studied in detail except for the works by Ju Yi Guang et al. Flame liftoff is one of the most common phenomena. The experimental system of the lifted flame is simple, while the parameters included in the system are complex, such as heat release, stretch effect, the mass fraction of fuel/oxygen/dilution and nozzle diameter. Comprehensive studies have been done to investigate the flame liftoff characteristics and its stabilization mechanism. However, the liftoff criterion and flame lifting stabilization mechanisms are not well understood.Experimental, theoretical analysis and numerical simulations methods were combined in this paper to study the liftoff characteristics of propane and DME flames. Liftoff characteristics of propane flames diluted with nitrogen and oxygen were studied to analyse the effects of dilution and oxygen atoms in fuels on the flame liftoff characteristics at room temperature. The liftoff characteristics of DME flames diluted with nitrogen, at different nozzle diameter and the preheating temperature were studied. Numerical simulations were completed for the diluted propane flames, and the results were compared with the experimental results. The main contents and conclusions of this dissertation are as follows:1. The nitrogen dilution effect was studied for the single nozzle laminar jet diffusion propane flame. The dilution reduced the chemical activities of fuel and the adiabatic flame temperature, in order to achieve stoichiometric conditions, the stabilized point of lifted flame moved downstream, and the flame liftoff height increased. DME presented a different liftoff phenomenon from the non-oxygenated hydrocarbon fuels. In order to verify the effect of the fuel oxygen atoms on the flame liftoff characteristics, the flame lifting characteristics with oxygen dilution was studied. The results showed that when oxygen was added into fuel, the propane flame transited from stable liftoff area to directly blowout area. When the oxygen concentration reached a critical value, the liftoff characteristics of propane flame with oxygen dilution presented exactly the same as DME flame, which could not be lifted off directly by increasing the jet velocity except for far field ignition at relatively low mass flow rate.2. The experimental research of DME liftoff characteristics mainly contains three aspects as follows:1) Liftoff properties of DME laminar axisymmetric diffusion flames were investigated experimentally with emphasis on N2dilution. Converging nozzle was used with exit inner diameter of0.43mm. The effects of mole fraction of dilution (Schmit Number) and nozzle exit velocity on flame stabilization mechanisms, and the similarities and differences of the DME and propane liftoff characteristics were studied.2) Six nozzles with different texture and type were used to study the nozzle diameter effect on flame liftoff characterics. The liftoff properties including the liftoff position, the critical liftoff velocity and the critical blowout velocity were studied experimentally. The experimental results showed that the nozzle inner diameter has a significant impact on flame liftoff characterics. As the nozzle diameter increased, four distinct liftoff features were observed. The flame was blown out directly without liftoff flames with nozzle diameter of0.17mm. The DME flame could only be stabilized liftoff by an ignition at a proper position downstream with nozzle diameter of0.25mm,0.386mm and0.45mm, while it could be lifted off directly by increasing the mass flow rate of fuel/dilution with nozzle diameter of0.693mm. When the nozzle diameter was increased to1.152mm, the DME flames could be lifted off by three different methods: increasing the flow rate of fuel/dilution, decreasing the flow rate of the fuel and ignition the flame downstream. Oscillation lifted DME flames were found with1.152mm inner diameter nozzle when the fuel was highly diluted by nitrogen.3) When fuel and dilution were preheated, the DME flame could be lifted off directly by increasing the jet velocity. Stainless steel straight nozzle with0.693mm nozzle exit diameter was used in the experiment. And the temperature was controlled by PID regulator. Volumetric flow ratio of DME to N2was fixed to1/0.9, to ensure the Sc of mixture remained constant. The range of the mass flow rate of stabilized DME liftoff flames became much narrower and the liftoff height became much lower at fuel preheating than that at ambient temperature. With the increase of the jet temperature, the DME liftoff flames exhibited as one of the following three types: stationary lifted flames, stable oscillating lifted flames and unstable oscillating lifted flames. Stationary lifted flames existed when the initial temperature was relatively low (no more than400K). Stable oscillating lifted flames were observed at relatively high preheated temperature (about350K-750K), and the trajectory of the liftoff flame base was nearly sinusoidal. Both the oscillating frequency and amplitude increased with the preheating temperature. The oscillating lifted flames were caused by thermal buoyancy effect, inertia and the instability in the inner flow. When the jet temperature exceeded800K, the oscillating lifted flames became unstable and easily to be blown out. The flame base of stabilized DME liftoff flames had a tribrachial structure at both ambient temperature and elevated temperature.3. The Landau-Squire similarity solution for the axisymmetric, steady laminar round jet flow was used to analyse flame stretch effect on the triple flame propagation velocity. Three different models were used to calculate the theoretical liftoff height of propane and DME lifted flames. Compared to the experimental results, the theoretical results showed that the flame stretch effect has important effect on the flame liftoff height and critical Sc number.4. The flame liftoff characterics of propane diffusion flames were also computed by Fluent calculation software using the laminar finite-rate model and Propane-air one step reaction mechanism. A large number of calculated results were compared with the experimental results, and the liftoff height and adiabatic flame temperature agreed well. In addition, dilution effect and flame stretch rate for different temperatures were studied by numerical simulation. |
