Experimental study on premixed combustion of spherically propagating methanol-air-nitrogen flames

Abstract

The outward propagation and development of surface instability of the spark-ignited spherical premixed flames for methanol-air-nitrogen mixtures were experimentally studied by using a constant volume combustion chamber and a high-speed schlieren photography system. The laminar burning velocities, the mass burning fluxes, and the Markstein lengths were obtained at different equivalence ratios, dilution ratios, initial temperatures, and pressures. The laminar burning velocities and the mass burning fluxes give a similar curve versus the equivalence ratios. They increase with the increase of initial temperature and decrease with the increase of dilution ratio. The laminar burning velocity decreases with elevating the initial pressure, while the mass burning flux increases with the increase of the initial pressure. Markstein length decreases slightly with the increase of initial temperature for the rich mixtures. High initial pressure corresponds to low Markstein length. Markstein length increases with the increase of dilution ratio, which is more obvious when the mixture becomes leaner. Equivalence ratio has a slight impact on the development of the diffusive-thermal cellular structure at elevated initial pressures. The initial pressure has a significant influence on the occurrence of the flame front cellular structure. At the elevated pressures, the cracks on the flame surface branch and develop into the cell structure. These cells are bounded by cracks emitting a bright light, which may indicate soot formation. For very lean mixture combustion, the buoyancy effect and cooling effect from the spark electrodes have a significant impact on the flame propagation. The hydrodynamic instability, inhibited with the increase of initial temperature around the stoichiometric equivalence ratio, is enhanced with the increase of initial pressure and suppressed by mixture dilution.

The constant-volume propagating spherical flame method for laminar flame speed measurement

“…Extrapolation to zero stretch rate is conducted to obtain the unstretched flame speed with respect to burned gas, S b 0 . Finally, the laminar flame speed is determined through S u 0 = rS b 0 , where r = q b /q u is the density ratio between burned and unburned gases [55,56]. There are two main advantages of CPM [51]: (1) direct view from schlieren/ shadow photograph helps to identify the flame instability, which thereby can be prevented in data processing; (2) there exists a quasi-steady spherical flame propagation period during which the stretch effect can be eliminated through extrapolation to zero stretch rate.…”

Calculation method of spherically expanding flame propagation radius to consider ignition electrode effects

“…They find out that the shape of the spherical flame is affected by electrode factors in the propagation process, which leads to a certain protrusion near the ignition electrodes. Wang et al 20 applies the Schlieren method to scale the recorded flame images and measure the flame radius. It is determined that the cooling effects of the ignition electrodes caused by the flame to propagate slowly along the ignition electrode direction.…”

“…Although the nonstandard circle is considered in this method, the effects of ignition electrodes on accurate flame radius measurement are not separately considered. Liang 19 and Wang 20 only proves the effects of the ignition electrodes on the flame radius, but they do not address the effects on the accurate measurement of the flame radius. Varea et al 23 proposes a method of flame radius post-processing based on the effects of ignition electrodes on the flame radius.…”