Fields on Flames

In the wake of Chapters 2 and 3, which were devoted to electro-hydrodynamics [JAN 63] and magnetohydrodynamics [CAB 70, MOL 07], in this chapter we give some examples of the influence of electrical and magnetic fields on flames. 

In parallel, we shall also discuss the influence of other fields such as gravity and acceleration fields. What these fields have in common with electromagnetic fields is that they are exerted on every microscopic particle of the medium in question. 

Non-premixed flames can be generated in a variety of ways, as detailed in [PRU 13]. Here, we examine the influence of an acceleration fleld, followed by that of an electrical field, on a candle flame. Finally, we look at the effect of a magnetic field on a diffusion flame from a burner formed of two coaxial cylindrical tubes, with the central tube providing the fuel and the outer tube the oxidant. 

Candle flames are created by the combustion of mixtures of paraffin with other solid fuels. The heat from the flame liquefies the solid fuel, and the combustible liquid soaks into a wick and evaporates. The flame is non-premixed. 

In addition, the liquid phase, which accumulates in the hoUow space left in the top end of the candle s cylindrical body, is animated by Benard—Marangoni convection. The behavior of the candle flame thus results from multiple interactions, and merits a systematic study in order to characterize it [FAR 60, TAK 09, SUN 11]. 

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Chapter 5, which discusses the influence of fields on flames, is primarily an application of the equations established for homogeneous media. 

The influence of magnetic fields on flames has also been studied experimentally. A variety of authors have taken an interest in the subject [FUJ 98, GIL 07, DEL 06, CHE 97, CHA 12, KAS 12] — partictilarly in relation to the problems of efficiency of the combustion. 

Part Two of this volume is entitled Introduction. It too has four chapters. Chapter 5 presents a study of the influence of diverse fields on flames Chapter 6 discusses a classic application of the Peltier effect Chapter 7 is devoted to metal/plasma interaction, and more speciffcally to the Langmuir probe, and finally Chapter 8 discusses space propulsion by the Hall effect. 

Based on the flame-hole dynamics [59], dynamic evolutions of flame holes were simulated to yield the statistical chance to determine the reacting or quenched flame surface under the randomly fluctuating 2D strain-rate field. The flame-hole d5mamics have also been applied to turbulent flame stabilization by considering the realistic turbulence effects by introducing fluctuating 2D strain-rate field [22] and adopting the level-set method [60]. 

The first item is easily treated by considering the eigen-modes of the system and expanding the pressure field on a basis formed by these modes. The second item is less well documented but is clearly important. The presence of boundaries not only modifies the structure of the mean flow but also influences the flame dynamics. This is demonstrated in a set of recent experiments in which the lateral confinement was varied systematically [45]. 

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. 

The complexity of the turbulent reacting flow problem is such that it is best to deal first with the effect of a turbulent field on an exothermic reaction in a plug flow reactor. Then the different turbulent reacting flow regimes will be described more precisely in terms of appropriate characteristic lengths, which will be developed from a general discussion of turbulence. Finally, the turbulent premixed flame will be examined in detail. 

An important result bearing on the influence of applied fields on gaseous systems detonating at their threshold conditions is the observation that a flame in a uniform electrical field is always bent toward the negative electrode, as was first observed by Lewis (Ref 3). 

Phase contrast observations in flames) 10) R.R. John M. Summer-field, Jet Propulsion 27, 169—175 178 (1957) (Effect of turbulence on flame radiation) 11) H. Selle, Explosivstoffe 8, 9 195—204(I960) (Investigations on flame... 

To illustrate the behavior of a stagnation flame impinging into a wall, consider the following example based on an atmospheric-pressure, stoichiometric, premixed, methane-air flame [271]. Geometrically the situation is similar to that shown in Fig. 17.1. The manifold-to-surface separation distance is one centimeter, the inlet mixture is at 300 K, and the surface temperature is maintained at Ts = 800 K. Figure 17.4 shows the flow field and flame structure for two inlet velocities. The flow is from right to left, with the inlet manifold on the right-hand side and the surface on the left. 

In the absence of catalysis on the surface, similarity of the concentration and temperature fields is achieved precisely at the ignition limit if the coefficients of diffusion and thermal diffusivity are equal, since in this case both the diffusion gradient and the temperature gradient at the igniting surface are equal to zero, and the equations of diffusion and thermal conductivity with the chemical reaction may be reduced to the form of an identity (see our work on flame propagation [3]). 

Some of the preeminent fire-safety and flame-retardant research organizations worldwide are listed in the following text. The list is by no means comprehensive, and focuses on the organizations that are known to publish often in the fields of flame-retardant research or fire-safety engineering. This list includes links to the organizations to learn more about the range of services and research they offer. 

First a steady turbulent two-phase flame is calculated. The 15 pm droplet motion follows the carrier phase dynamics so that the Centered Recirculation Zones (CRZ) are similar for gas and liquid, as illustrated on Fig. 10.4, showing the instantaneous backflow lines of both phases, plotted in the vertical central cutting plane. Maintained by this CRZ, the droplets accumulate and the droplet number density, presented with the liquid volume fraction field on Fig. 10.4, rises above its initial value a zone where the droplet number density n is larger than 2n j j (where is its value... 

To summarize, the field of flame retardancy for polymers is in a state of flux, and multiple new technologies and approaches are expected to arise in the coming decade. In this entry, we focus on some new technology from our laboratory, including nonhalogenated flame retardants synthesized from bromi-nated starting materials, and inherently fire-safe and low-flammability polymers. 

Searby and Rochwerger [9] developed a model describing the effect of an acoustic field on the stability of a laminar, premixed flame, treated as a thin interface between two fluids of different densities and under the influence of a periodic gravitational field. Their model is an extension of the work by Markstein [8] and is consistent with the more recent flame theory of Clavin and Garcia-Ybarra [16]. Bychkov [17] later solved the problem analytically, presenting the following linear equation for the perturbation amplitude, /, of a flame under the influence of an acoustic field [17] ... 

Hou, S.-S., and Ko, Y.-C., "Effects of Heating Height on Flame Appearance, Temperature Field and Efficiency of an Impinging Laminar Jet Flame used in Domestic Gas Stoves." Energy Conversion and Management 45, nos. 9-10 (2004) 1583-95. 

This chapter presents the experimental modeling of flares as turbulent diffusion flames in crossflow. We have reviewed the parameters that affect the flare performance in the field. Experimenfal facilities and insfru-mentation employed for model sfudies are presented. A summary of existing data on flame appearance, geometry, radiation, and stability has been included. Data on inflame temperature, velocity, and species concentration fields have also been discussed. Field fesf dafa are to be used in conjunction with laboratory model data to validate the results and derive scaling relations. 

A.M. Lentati, H.K. Chefliah Dynamics of water droplets in a counterflow field and their effect on flame extinction. Combustion and Flame, 115, 158-179 (1998). 

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