Sergey Minaev

Nonlinear dynamics of flame in a narrow channel with a temperature gradient

The non-stationary behaviour of near-limit premixed flame propagating in a microchannel with temperature gradient was theoretically investigated. A one-dimensional (1D) nonlinear evolutionary equation of the flame front was obtained. The nonlinear model outlined the flame stabilization, nonlinear flame oscillations and flames with repetitive extinction and ignition processes that were observed in experiments.

Experimental study on flame pattern formation and combustion completeness in a radial microchannel

Combustion behavior in a radial microchannel with a gap of 2.0 mm and a diameter of 50 mm was experimentally investigated. In order to simulate the heat recirculation, which is an essential strategy in microscale combustion devices, positive temperature gradients along the radial flow direction were given to the microchannel by an external heat source. A methane–air mixture was supplied from the center of the top plate through a 4.0 mm diameter delivery tube. A variety of flame patterns, including a stable circular flame and several unstable flame patterns termed unstable circular flame, single and double pelton-like flames, traveling flame and triple flame, were observed in the experiments. The regime diagram of all these flame patterns is presented in this paper. Some characteristics of the various flame patterns, such as the radii of stable and unstable circular flames, major combustion products and combustion efficiencies of all these flame patterns, were also investigated. Furthermore, the effect of the heat recirculation on combustion stability was studied by changing the wall temperature levels.

Limits of Flame Propagation in a Narrow Channel with Gas Filtration

A model of gas–flame propagation in a narrow gap between two plates is proposed. Apart from the conventional regime, this model describes the regime of low velocities. A typical feature of the low–velocity regime is the fact that the flame propagates together with the heat wave in the plates generated by the flame. It is shown that the limits of flame propagation in the low–velocity regime are wider than the limits in the classical theory. Dependences of the flame–propagationvelocity and the critical Peclet number on the fresh–gas velocity are obtained. For Peclet numbers lower than the critical value defined by the classical theory, the flame may exist only within a certain range of velocities of the fresh mixture counterflow. A possible reason for the existence of the upper and lower limits of flame propagation relative to the flow velocity of the fresh mixture is discussed.

Experimental investigations on the combustion behavior of methane?air mixtures in a micro-scale radial combustor configuration

Experimental investigations on combustion stability limits for methane–air mixtures in a radial micro-scale combustor configuration are herein reported. To study the flame stability characteristics in this radial combustor configuration, two circular quartz plates were arranged parallel to each other and a fuel–air mixture was supplied at the center of the plates. The plates were externally heated to create a positive temperature gradient condition in the flow direction to simplify the heat recirculation process through the solid walls. The fuel–air mixture emanating from the center was subjected to a positive wall temperature gradient and a negative velocity gradient in the radial combustor configuration. Various stable and unstable flame propagation modes were observed during the experimental investigations. The appearance of these modes was a strong function of mixture flow rate, channel width, temperature distribution and mixture equivalence ratio. The effects of wall temperature and mixture equivalence ratios were investigated for a range of mixture flow rates. These investigations showed that the combustion zone moved linearly outward with the increase in the mixture flow rate. At intermediate channel widths (~2.5 mm), an unstable combustion mode appeared. In this combustion mode, large-scale random fluctuations in flame radius were observed along with the leakage of large amounts of unburnt fuel. Exhaust gas analysis was carried out to compare the combustion performance in stable and unstable combustion modes. Based on the experimental investigations, recommendations are made to facilitate the efficient design of a combustor for micro gas turbine applications.

Appearance of target pattern and spiral flames in radial microchannels with CH4-air mixtures

This paper presents the experimental evidence of the formation of rotating spiral flames with premixed methane-air mixtures introduced at the center of the two parallel circular quartz plates which are separated by a millimeter scale distance (≤5mm). Both plates are externally heated to create a positive wall temperature gradient in the flow direction to resemble heat recirculation through solid walls, which is a requisite to obtain stabilized combustion in microburners. Contrary to the general perception of a stable premixed flame front at a radial location, a variety of nonstationary flame propagation modes are observed. For lower mixture flow rates and a range of mixture equivalence ratios, a radial flame propagation mode is observed with simultaneous presence of two circular flames at different radial locations. For higher flow rates, a rotating spiral flame propagation mode is observed. In addition to radial and spiral flame propagation modes, random and unsymmetrical flame oscillations are also obse...

Characteristic regimes of premixed gas combustion in high-porosity micro-fibrous porous media

Dynamical behaviour of the premixed flame propagating in the inert high-porosity micro-fibrous porous media has been studied numerically. Effects of mixture filtration velocity, equivalence ratio and burner transverse size on the flame structure have been investigated and the regions of existence of different combustion regimes have been determined. It was found that the influence of the hydrodynamic instability on the flame dynamics is significant in the case of the moderate and high filtration velocities and this effect is negligible at the low velocities. At the moderate filtration velocities the effect of hydrodynamic instability manifests in the flame front deformation and in particular in the flame inclination. It was found that the flame can be stabilized within the whole interval of the filtration gas velocity, whereas in the ordinary porous media the standing wave is settled only at fixed value of gas filtration velocity. This finding is in line with recent experimental results on combustion in micro-fibrous porous media (Yang et al., Combust. Sci. Tech. 181 (2009), 1–16). Possible physical interpretation of the flame anchoring effect may be given on the base of present numerical analysis. At the high filtration velocities the hydrodynamic instability manifests itself in periodical appearance of the moving wrinkles on the flame front surface which forms non stationary high temperature trailing spots behind the leading part of the flame front. Such dynamics may be associated with splitting wave structures which were revealed in previous experiments (Yang et al., Combust. Sci. Tech. 181 (2009), 1–16).

Filtration Combustion of Methane in High-Porosity Micro-Fibrous Media

Filtration combustion of lean methane-air mixtures in inert, high-porosity micro-fibrous media was studied experimentally in quartz tubes with different diameters. First, an overall stability diagram was obtained. Between the upstream and downstream propagating regimes, a specific rigid standing wave regime was observed, whereas only one standing wave point existed in other ordinary porous media. A specific instability phenomenon of a combustion wave splitting into two or more parts during wave propagation downstream was also observed in smaller diameter tubes at high filtration velocities. In principle, the results of a conventional two-temperature 1D analytical model captured the main feature of the process obtained in the experiment, but failed to explain the process of flame anchoring to the porous carcass that was observed under variation of the mass flow rate. The possible causes of the failure of the conventional modeling approach were presented.

On self-drifting flame balls

Self-drifting flame balls whose existence has recently been suggested by an ad hoc one-dimensional sandwich model are studied within the framework of a more rational multi-dimensional formulation as bifurcations of the associated stationary spherical flame balls.

INVESTIGATIONS ON THE FORMATION OF PLANAR FLAMES IN MESOSCALE DIVERGENT CHANNELS AND PREDICTION OF BURNING VELOCITY AT HIGH TEMPERATURES

Flame stabilization studies of preheated mesoscale channels of various divergent angles and aspect ratios are reported in this article. Flame propagation modes such as planar flame and negatively and positively stretched flames were observed for a range of mixture flow rates and equivalence ratios. The present investigation is focused on the formation of the planar flames in these channels. The effect, of various parameters such as channel aspect ratio, divergence angle, and heating rate on the formation of planar flames and thereby on the prediction of laminar burning velocity of various fuel–air mixtures at high temperatures are discussed. Detailed investigations show that stretch-free planar flames can be stabilized in high-aspect-ratio channels with linear velocity and temperature gradient in the axial direction. Detailed numerical simulations confirm a negligible effect of heat loss on the burning velocity due to external preheating of the channel walls. Apparatus independence of the burning velocity is confirmed through experiments in channels of various aspect ratios and divergence angles for various fuel–air mixtures. The proposed method of burning velocity measurement at high temperature is quite promising.

Thermal Interaction of Two Flame Fronts Propagating in Channels with Opposing Gas Flows

A one‐dimensional nonstationary model is presented for propagation of two premixed flame fronts in narrow plane channels with regard for thermal interaction of flames through a dividing wall. The gas flows in channels considered are oppositely directed and equal in magnitude. It is shown that heat transfer through the heat‐conducting wall separating the plane channels leads to the appearance of a number of special features. The scheme of filtrational combustion of gases considered may be called the scheme of combustion with counter filtration; it is a new modification of systems with enthalpy excess. It is shown that the temperature at the combustion‐wave front may be greater than the adiabatic temperature of free plane flame with the same composition of the combustible mixture even if there are heat losses through external walls of the system. By solving the problem, dependences of the velocity of combustion waves on the distance between them are obtained, and the dynamic behavior of combustion waves is studied. The region of problem parameters, where self‐stabilization of combustion waves is possible, is found.