Experimental and numerical study on bluff-body and swirl stabilized diffusion flames

摘要

Bluff-body and swirl flow are commonly utilized to stabilize diffusion flames in industrial applications, such as gas turbines, ramjets and furnaces. Flame stabilization mechanisms of these two kinds of burners are similar with each other: the interaction between the recirculation zone and the fuel jet. In the present paper, flow fields within flames stabilized by combinations of swirl flow and bluff-body were captured using high-speed PIV; while the flame structures were visualized by high-speed CH2O PLIF, CH* chemiluminescence and broadband chemiluminescence. The global CO emissions from the flames were captured as well. In addition, based on the CFD software OpenFOAM, simulations were adopted to better understand the interactions between flames and flow structures. Flames stabilized by bluff-bodies with different diameters (d(b) = 14mm and 20 mm), or only by swirl flow without a bluff-body, were studied. All reacting experiments were carried out with a constant mass flow rate of the central fuel jet (with thermal power 2.01 kW) and a constant mass flow rate of the total air flow (m = m(t) + m(a) = 200 ln/min). The swirl strength was controlled by the mass flow rate ratio of the tangential to the axial air flow. The geometrical swirl number was altered between Sg=0 and Sg= 4.08. Simulation results matched well with experimental data, especially in predicting the spatial distribution of CH2O. The position of the outer recirculation zone would be affected by the size of the bluff-body and the swirl strength. In addition, the recirculation zone determined the flame structures and the global CO emission levels. With a larger bluffbody, the air driven recirculation zone located more upstream near the burner exit. Flame prone to be more stable with a larger bluff-body and/or a stronger swirl flow. Flame was observed propagating into the upstream region in cases without a bluff-body or in cases with the small bluff-body (d(b)=14 mm), when the swirl strength was sufficiently strong. The mechanism for the diffusion flame 'flashback' was proposed. Flames in cases with a larger swirl number were shorter while its CO emission levels were higher.