Effect of strain fields on burning rate

Abstract

Flame propagation can be highly influenced by the presence of strain fields. Initial stagesof flame growth in a turbulent medium, initiated by a point ignition, source such as a spark, may be dominated by straining effects if the flame thickness is less than the Kolmogorov scale. Straining may influence both the flame surface area and the local burning velocity. Area variation (usually referred to as flame stretching for an aear increase) is characterized by the so-called Karlovitz number. The effect of area variations has been investigated by a number of workers. However, little attention has been given to the effect of velocity gradients on the local burning velocity. An analytical study is presented for the influence of strain rate on the laminar burning velocity with heat loss from the reaction zone. The analytical results show that, in general, the laminar burning velocity decreases when the strain rate is increased, and that the decrease is more pronounced with increasing the heat loss and/or increasing the Lewis number. Experimental work is carried out for an axisymmetric stagnation point flow, in which a laminar flat flame is established. Different values of the strain rate and heat loss from the reaction zone are imposed on the flame, and their effect on the laminar burning velocity is found. The strain rate and the laminar burning velocity are determined by measuring the velocity field using laser doppler aneomometry. The work is of importance in operating spark ignition engines on lean mixtures and athigh compression ratios. Cycle-by-cycle variations in combustion increase as the mixture is made leaner and it is now established that this is primarily due to mixture motion around the spark plug during the initial stage of flame development. It is show that, in internal combustion engines, straining can cause variations in the initial stage of flame growth, and thus cause cycle-by-cycle varitions in the combustion process.