Fuel mixtures flow structure

To examine the effect of turbulence on flames, and hence the mass consumption rate of the fuel mixture, it is best to first recall the tacit assumption that in laminar flames the flow conditions alter neither the chemical mechanism nor the associated chemical energy release rate. Now one must acknowledge that, in many flow configurations, there can be an interaction between the character of the flow and the reaction chemistry. When a flow becomes turbulent, there are fluctuating components of velocity, temperature, density, pressure, and concentration. The degree to which such components affect the chemical reactions, heat release rate, and flame structure in a combustion system depends upon the relative characteristic times associated with each of these individual parameters. In a general sense, if the characteristic time (r0) of the chemical reaction is much shorter than a characteristic time (rm) associated with the fluid-mechanical fluctuations, the chemistry is essentially unaffected by the flow field. But if the contra condition (rc > rm) is true, the fluid mechanics could influence the chemical reaction rate, energy release rates, and flame structure. 

In relatively low-reactive fuel-air mixtures, a detonation may only arise as a consequence of the presence of appropriate boundary conditions to the combustion process. These boundary conditions induce a turbulent structure in the flow ahead of the flame front. This turbulent structure is a basic element in the feedback coupling in the process by which combustion rate can grow more or less exponentially with time. This fundamental mechanism of a gas explosion has been described in Section 3.2. 

Much can be learned by analyzing the structure of a flame in more detail. Consider, for example, a flame anchored on top of a single Bunsen burner as shown in Fig. 4.3. Recall that the fuel gas entering the burner induces air into the tube from its surroundings. As the fuel and air flow up the tube, they mix and, before the top of the tube is reached, the mixture is completely homogeneous. The flow velocity in the tube is considered to be laminar and the velocity across the tube is parabolic in nature. Thus the flow velocity near the tube wall is very low. This low flow velocity is a major factor, together with heat losses to the burner rim, in stabilizing the flame at the top. 

Beginning with the innovative work of Tsuji and Yamaoka [409,411], various counter-flow diffusion flames have been used experimentally both to determine extinction limits and flame structure [409]. In the Tsuji burner (see Fig. 17.5) fuel issues from a porous cylinder into an oncoming air stream. Along the stagnation streamline the flow may be modeled as a one-dimensional boundary-value problem with the strain rate specified as a parameter [104], In this formulation complex chemistry and transport is easily incorporated into the model. The chemistry largely takes place within a thin flame zone around the location of the stoichiometric mixture, within the boundary layer that forms around the cylinder. 

In volume limited applications, high density propellant combinations are favored and some appropriate trade-off between performance and density is established. In a truly volume limited system as shown in section IV. A. 1., the appropriate performance parameter is the product of the specific impulse and the propellant bulk density, a quantity usually labeled the density impulse. Conceivably, mixture ratio may be determined by yet other vehicle system considerations. If a new propellant combination is to be utilized in an existing vehicle, the optimum mixture ratio may be influenced by such considerations as existing pump flow rate capacities, tank volumes, and structure load carrying capacities. Even other system considerations, such as the desirability of operating at equal fuel and oxidizer volume flow rates to allow interchange of fuel and oxidizer flow hardware, may determine the propellant mixture ratio. 

A honeycomb shape has been considered as the most desirable structure for the combustion catalyst due to a small pressure drop across the channel and a large surface-to-volume ratio. Stable combustion can be attained with laminar flow of gas mixture along the channel of the honeycomb, whereas turbulent flow and back-mixing are operative for the conventional flame combustion. The temperature at the honeycomb wall rises rapidly with fuel contact. The rate of homogeneous reaction depends on the fuel concentration and temperature therefore, the non-catalytic gas-phase reaction initiates from this hot wall where the temperature is raised by catalytic combustion. Once this catalytically initiated gas-phase reaction started, the reaction propagates rapidly toward the center of the channel. Then the high combustion efficiency can be attained. ... 

In the following description of the reactions occurring during this stage of a severe core damage accident, three different topics will be discussed the release of fission products from the fuel, the release of constituents of the core structural and control rod materials (although these two sources develop almost simultaneously in the reactor pressure vessel so that the volatilized substances can be assumed to enter the gas flow as a mixture) and, finally, volatilization of substances during the molten core - concrete interaction phase. The current state of the art will be discussed with special emphasis on the important chemical phenomena no attempts will be made to establish numerical values of source terms from the results of these experimental and theoretical efforts. 

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