Features of flame stabilization in near wall flows

The results of the experimental and numerical study of the influence of obstacle geometry on the conditions of flame stabilisation in the turbulent boundary layer with fuel injection near a porous surface are presented. The features of the detachment zones formation in the turbulent reacting flow behind the obstacle are studied. The obtained results are compared with the data on flame-off conditions when flowing over a flat surface.


Introduction
An active or passive impact on flow in the flame front edge can effectively control the combustion in the boundary layer.In particular, as it was shown in the works of E.P. Volchkov [1] when burning near a flat surface, through which the gaseous fuel was injected, the features of the flow formation played an important role.In the same paper, the regularities of the flame-off conditions were experimentally established, and the model was proposed.It was based on the diffusion combustion approximation, the boundary layer theory and the assumption that extinction occurs when the flame front reaches the wall.The range of conditions in which the combustion is stable near a flat surface can be significantly expanded by using the stabilizing devices.Traditionally, ribs or back steps of various geometries were used to form a recirculation zone with an extended residence time of initial reactants and reaction products mixture.The heat release and transverse momentum flow caused by injection of the fuel mixture can substantially change the regularities of this class of flows [2].A very limited amount of works were devoted to the investigation of conditions of flame detachment in a separated flow near a flat surface with injection.In [3][4][5], based on visual observations, the diagrams of combustion and flame-off modes were obtained for some hydrocarbon fuels and the geometry of the barriers forming the detached flow zones.There are other ways to stabilize the flame.In particularly the detached rib may be one of the promising areas.The structure of the flow and heat transfer over a detached rib for non-reactive flows was numerically studied in [6].It was shown that an increase in the gap between the rib and the surface leads to a change in the structure of the recirculation region.The regimes in which there was an increase in heat exchange with the wall were observed in comparison with the case of a rib placed on the surface.
In this paper, we have determined the conditions of flame-off at the detached rib located above the wall, while the main attention was paid to the peculiarities of the flow in the vicinity of the obstacle.To measure the velocity distribution in the reacting flow the PIV method was used.The plane of the light sheet was oriented perpendicular to the permeable surface, which allowed obtaining data on a two-dimensional flow field (longitudinal and vertical velocity components).Because of the peculiarities of the method used to diagnose the dynamics of the wall flow, the measurements were carried out sequentially and statistically independently in overlapping regions of the space of ~ 40x40 mm in size.When using the optical method in the wall flow above the permeable wall, it is required to ensure a uniform dusting of the flow by light scattering particles.For this purpose, separate, independent dusting of TiO2 particles with both the main stream and the fuel mixture was used.

Theoretical model
Under the conditions of the experimental studies the numerical simulation was carried out.This allowed obtaining a more complete picture of the processes in the turbulent reactive separated flow in the boundary layer behind the barrier.In particular, the simulation allowed obtaining detailed information on the distribution of concentrations of all the mixture components, refining the data on the parameters in the immediate vicinity of the walls and identifying some other factors that were not directly measured.Numerical simulation in this paper was based on the URANS approach.We solved non-stationary averaged equations of Navier-Stokes, energy and concentrations.As a model of turbulence, the SST k-model was used, thermal and diffusion turbulent flows were determined according to the gradient model.The interaction of turbulent pulsations and combustion was modeled according to the concept of the EDC vortex dissipation.

Fig. 1 .
Fig. 1.The scheme of the working section of the unit for investigating the wall flow with injection and gas fuel combustion in the air stream (A).Photo (B) of the wall stream with propane injection and burning, top view.The experimental studies were carried out in a subsonic wind tunnel with a channel section at the entrance to the working part 108x108 mm.The speed of air flow over the plate varied in the range 1...20 m/s.The diagram of the working part is shown in Figure 1.The fuel was uniformly blown into the boundary layer through the lower horizontal porous plate (1) with a plan dimension S = 95x145 mm.The injected fuel was propane (or methane) diluted with CO2.To prepare the fuel mixture, digital gas flow regulators were used.Immediately before the beginning of the porous section, a rib (2), being a ceramic cylinder 5 mm in diameter, was installed.The rib was located directly on the surface, or was fixed at a height of up to 3 mm above the wall (detached rib).To measure the velocity distribution in the reacting flow the PIV method was used.The plane of the light sheet was oriented perpendicular to the permeable surface, which allowed obtaining data on a two-dimensional flow field (longitudinal and vertical velocity components).Because of the peculiarities of the method used to diagnose the dynamics of the wall flow, the measurements were carried out sequentially and statistically independently in overlapping regions of the space of ~ 40x40 mm in size.When using the optical method in the wall flow above the permeable wall, it is required to ensure a uniform dusting of the flow by light scattering particles.For this purpose, separate, independent dusting of TiO2 particles with both the main stream and the fuel mixture was used.

Fig. 2 . 2 .
Fig. 2. Range of parameters of stable diffusion combustion in the boundary layer with the injection of the hydrocarbon mixture with СО2 (above the line).Lines: 1-СН4, 2-С3Н8.Figures -the experimental data of the authors for the СН4/СО2 fuel mixture, points P1…P4simulation results for propane.

Figure 3 5 Conclusion
Figure 3 shows the results of measurements of the field of the longitudinal velocity component when the propane-air flame is stabilized behind the detached rib.It is evident that two regions of the recurrent flow are formed by isotachs ±0.1 m/s (light lines in the picture).The visible flame leading edge in the section of the PIV system laser sheet plane was at 6-8 mm downstream from the surface of the detached rib.The formation of the region of the recirculating near wall flow is apparently determined by two factors: the heat release in the flame front and the transverse flow of matter specified by injection of the fuel mixture.