MHD-ENHANCED MIXING AND COMBUSTION IN COCURRENT STREAMS

The process of mixing of gaseous fuel and oxidant is one of fundamental subjects of combustion and propulsion research. It is basically clear that the better (faster) mixing the higher combustion efficiency can be reached under similar conditions. The mixing can be critically important for short residence time combustion systems such as scramjet. The theory of combustion in so-called premixed systems is developed for a long time [13]. The key question in such a system is the flame propagation. Two different modes of the flame propagation are usually considered: laminar and turbulent. The laminar process of combustion is defined by balance of heat release in the flame front due to chemical reactions between fuel and oxidant and energy transfer by heat conductivity. In the case of turbulent combustion the effective flame propagation velocity increases significantly due to important role of convection in energy transport from the heat release region to the undisturbed flammable mixture. In fact the turbulence in the combustion zone can be created due to intensive heat release (heating resulting in non-steady state expansion flow of gaseous products and fresh mixture) or/and it can be transported by the flow to the combustion zone as well [3]. In contemporary combustion systems such as combustion chamber of (turbo)jets, ramjet, scramjet combustion starts in only partially mixed flow where the fuel/oxidant ratio varies significantly in working volume. In this particular but very important practical case the combustion and mixing can develop simultaneously affecting each other [4-6] (see also [7] with many example of particular recently performed studies in the field). The combustion as an elementary chemical reaction between fuel and oxidant molecules needs the direct contact (collision) of these molecules. Thus, the combustion can occur if in a particular location the collision between fuel and oxidant molecule can take place. This «molecular» level of mixing is provided by conventional molecular diffusion. However, the molecular diffusion is generally speaking very slow process in scale of available mixing time, which is limited, by residence time in combustion zone. To enhance the mixing process the other «large scale» mechanism of mixing is introduced. The main idea is to increase the contact surface between fuel and oxidant. The simplest well-known method is rotating mixing widely used in mankind practice for whole human history. The turbulence motion is basically a vortex motion and one can consider the turbulence enhancement of mixing as a combined effect of different scale rotating mixing cells. This, kinematics, stage of mixing increases the all inter-component contact surfaces. (The theory of kinematics mixing based on a Lagrangian’ non-turbulence approach is recently presented in a textbook by J.M.Ottino [8]. The kinematics treatment of the turbulence for the mixing study is independently proposed and developed in our recent papers [9-12].) Simultaneously the conventional molecular diffusion across the contact surface develops providing the mutual penetration of fuel into the oxidant fluid domain and, the oxidant, to the fuel fluid domain. As a result at the contact fuel and oxidant domain a intermediate diffusion region is formed where the fuel-to-oxidant ratio varies between its limits 0 and . The characteristics thickness of such a layer is estimated by