Timofeev K.   Goldfeld M.  

The mechanism of self-ignition and flame holding in supersonic combustion chamber

Reporter: Goldfeld M.

Ignition and flame holding of gaseous or liquid fuel in an air flow belong among fundamental problems of physics of the combustion lying on a boundary junction of aero -, thermodynamics and chemical kinetics. Occurrence of the new direction connected with research of   ignition and combustion of fuel at supersonic speeds of flow is dictated by projects of creation of hypersonic vehicles with the scramjet.
The presented research has been concentrated to definition of conditions of self-ignition of hydrogen in the combustion chamber at the entrance Mach number 4. Other task consisted in definition of effective model of fuel injection for self- ignition and flame stabilization and prevention of choking of the channel. The experimental model was made in the form of rectangular channel with a flame holder in the form of backward facing step (BFS). Fuel injection was carried out in front of BFS on the top and bottom walls of model through   8 circular holes, which was located with regular intervals    under angles of 45° or 90°.
As a result of the performed researches it has been established that initial ignition occurs on distance approximately (6 − 10) h (h is step height) from BFS. Stabilisation process was realised in three stages. The first stage corresponds to local combustion with small   pressure increase into separation zone. The second stage corresponds to intensive combustion and pressure increase on all combustion chamber owing to flame propagation upstream and essential expansion of near-wall layer, and its  thickness exceeds a thickness of a boundary layer and BFS height. The increase in a heat release and, as consequence,  a flow turbulization leads to an intensification of mixing and the further flame propagation upstream. The third stage of process is characterised by steady combustion and high level of pressure in all combustion chamber but  the flow core remains supersonic. The increase of fuel-air equivalence ratio leads to reduction of height of a supersonic flow core and to chocking of the channel with transition to subsonic combustion. It was shown that combustion intensity and probability of the channel choking essentially depends on Mach number and a angle of fuel injection. The obtained results are confirmed by measurements of pressure, heat fluxes, shadow visualization of the flow structure and registration of flame propagation in the channel.

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