Effects of Dilution and Nozzle Diameter on Laminar Dimethyl Ether Lifted Flames

Abstract

This work aimed at studying the effects of nitrogen dilution and nozzle exit inner diameter on the liftoff properties of the dimethyl ether (DME) jet diffusion flames. The liftoff properties including the liftoff position (HL), the critical liftoff velocity (Ulo) and the critical blowout velocity (Ubo) were studied experimentally. In nitrogen dilution experiments, a slowly converging nozzle was used with inner exit diameter of 0.43 mm. When mole fraction of N2 (Z) increased, a) HL increased because the dilution reduced the chemical activity of fuel, in order to achieve stoichiometric conditions, the stabilization point of the lifted flame moved downstream. b) at the critical liftoff condition, the flow rate of DME decreased with the increase of N2, while the total flow rate was almost unchanged, so the jet velocity was almost the same. c) as Z increased, the stabilization zone of the DME liftoff flames became narrow and small. In the experimental study of the effects of the nozzle diameter on the flame liftoff characteristics, six nozzles with i.d. of 0.17mm, 0.25mm, 0.386mm, 0.43mm, 0.693mm and 1.152mm were used. These nozzles had different materials and nozzle exit types. The experimental results showed that the nozzle inner diameter has a significant impact on the flame liftoff characteristics. As the nozzle diameter increased, four types of different liftoff features were observed. The flame was blown out directly with i.d. of 0.17 mm. The DME flame could only be observed liftoff by ignition at a proper position downstream with i.d. of 0.25 mm, 0.386 mm and 0.43 mm. The observations are agreed with that reported in the literatures. While it could be lifted off directly by increasing the mass flow rate of fuel/dilution with i.d. of 0.693 mm. This is the new observation in the present work. It is different from the report in the literatures that the DME flame could not be lifted off directly by increasing the jet velocity except for far field ignition at relatively low mass flow rate. When the nozzle i.d. was increased to 1.152 mm, 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 with i.d. of 1.152 mm when the fuel was highly diluted by nitrogen. The experimental results also showed that the critical liftoff velocity Ulo and the critical blowout velocity Ubo were strongly dependent on the inner diameter, which decreased with the increase of the nozzle diameter. When the jet velocity was kept constant, the flame liftoff height HL increased with the increase of the nitrogen mole fraction Z for all lifted flames.