Сравнительный анализ характеристик течения в горелках с эшелонированным расположением стабилизаторов пламени в изотермических условиях и при горении топлива

Анотація

COMPARATIVE ANALYSIS OF THE FLOW CHARACTERISTICS IN BURNER WITH ECHELONED POSITIONING OF THE FLAME STABILIZER UNDER ISOTHERMAL CONDITIONS AND FUEL BURNING

N. Fialko, Y. Sherenkovsky, V. Prokopov, O. Maletskaya,  S. Aleshko, N. Polozenko, E. Milko, S. Stryzheus

Fuel combustion in the burners with echeloned grids of flame stabilizer has a number of well-known advantages. The main of them is the possibility of the formation of the desired temperature profile in the burning zone, pressure loss in the burner, elimination of spontaneous symmetry breaking the flow and so on.

Design tasks of these burners require in-depth studies of the flow structure of the fuel and oxidant. This is of particular interest in comparing flow characteristics in combustion conditions and cold flow.

The objective of this work is the identification of the effect of non-isothermal flow on the flow characteristics in the staircase echeloned grids of the flame stabilizers of microjet burners.

The flow structure of natural gas, air and combustion products in the stabilizer burner with a staircase echeloned grid of flame stabilizer  is considered.

A mathematical model of the studying processes includes the equation of motion, continuity, energy for reacting turbulent flows, the equation of conservation of mass of the mixture component and the equation of state of a multi-component mixture.

The boundary conditions for the reduced system of equations was determined in such a way. Velocity values, concentrations and temperatures are wondered constant in the sections corresponding to the entrance of the burner channel and a gas supply openings. So-called soft boundary conditions - the vanishing of the derivatives in the longitudinal direction of the dependent variables - are set in the output section. Conditions of adhesion and impermeability are wondered on hard surfaces. The side surface of the channel considered adiabatic. The solution of the problem was based on the approach the DES, which is a combination of RANS and LES models and combines the best qualities.

Regarding the choice of turbulent transport model, according to the made research the k-ε realizable model of turbulence model is most appropriate to this situation.

As shown by the results of research, the flow pattern under isothermal and non-isothermal conditions is significantly different; it is obvious to a large extent due to the thermal expansion of gases in the combustion conditions.

Under burning character of redistribution of air flows in grid channels is opposite ones at isothermal flow. Under isothermal conditions in grid channels adjacent to the first, average speed significantly higher than them values in the channels adjacent to the last downstream stabilizer. The reverse situation is observed in the combustion conditions. This is due to a significant increase of static pressure in astern areas of stabilizers primarily the first upstream stabilizer.

The speed levels of the stabilizer grid under burning are significantly higher than these levels in the case of isothermal flow

As follows from the results of computer simulation in a burning velocity values for different of the flame stabilizer are essentially different. Namely, in general, the highest velocities take place for the first upstream stabilizer, less - for the lowest and the second - for the third. This is due to two factors. Thus, the character of the velocity field of the stabilizer grid in the case of reacting flows is in a certain way correlated with the corresponding temperature field.

According to computer modeling of the extent of reverse currents in combustion zones increase somewhat, due to the increase in static pressure in astern areas of track. The maximum velocities y increase significantly in burning conditions.

As studies have shown, the levels of velocity fluctuations in the astern  areas of stabilizers during combustion in general significantly higher than those for cold flow. The dimensions of the zones with elevated levels of velocity fluctuations after stabilizers increase markedly in the case of reacting flows.

Noteworthy is the fact that the size of these zones after stabilizers in conditions of cold flow are ranked as follows. The biggest of them are answered the first upstream stabilizer and the least - the last. In this case maximum levels of pulsations in these areas are also responsible for the first upstream stabilizer, and the minimum - last.

It is also shown that under the conditions of the combustion ratio of zone size with a higher level of velocity fluctuations after different stabilizers is feedback described above. This is obviously due to the reallocation of air flow characteristics in the stabilizer grid for compared situations.

With regard to pressure losses in the burner, in case reacting flows they increase significantly. Studies have shown that due to the echeloning stabilizers a reduction in the pressure loss in the burner is achieved.

Keywords: echeloning of flame stabilizers, isothermal flow, fuel burning, computer simulation