Flame temperature, fluctuations

Precision For samples and standards in which the concentration of analyte exceeds the detection limit by at least a factor of 50, the relative standard deviation for both flame and plasma emission is about 1-5%. Perhaps the most important factor affecting precision is the stability of the flame s or plasma s temperature. For example, in a 2500 K flame a temperature fluctuation of +2.5 K gives a relative standard deviation of 1% in emission intensity. Significant improvements in precision may be realized when using internal standards. 

Correa, S. M., A. Gulati, and S. B. Pope (1994). Raman measurements and joint pdf modeling of a nonpremixed bluff-body-stabilized methane flame. In Twenty-fifth International Symposium on Combustion, pp. 1167-1173. Pittsburgh, PA The Combustion Institute. Corrsin, S. (1951a). The decay of isotropic temperature fluctuations in an isotropic turbulence. Journal of Aeronautical Science 18,417 -23. 

It has been shown recently [25] that concentrations of NOj, tend to reduce with increase in the amplitude of discrete-frequency oscillations. The mechanisms remain uncertain, but may be associated with the imposition of a near-sine wave on a skewed Gaussian distribution with consequent reduction in the residence time at the adiabatic flame temperature. Profiles of NO, concentrations in the exit plane of the burner are shown in Fig. 19.6 as a function of the amplitude of oscillations with active control used to regulate the amplitude of pressure oscillations. At an overall equivalence ratio of 0.7, the reduction in the antinodal RMS pressure fluctuation by 12 dB, from around 4 kPa to 1 kPa by the oscillation of fuel in the pilot stream, led to an increase of around 5% in the spatial mean value of NO, compared with a difference of the order of 20% with control by the oscillation of the pressure field in the experiments of [25]. The smaller net increase in NO, emissions in the present flow may be attributed to an increase in NOj due to the reduction in pressure fluctuations that is partly offset by a decrease in NOj, due to the oscillation of fuel on either side of stoichiometry at the centre of the duct. 

The emphasis in this work has been on the acquisition of simultaneously-obtained instantaneous values of temperature and concentration, with as high a spatial resolution as practical for such experiments. The temporal and spatial resolution requirements result from the necessity to probe within (if at all possible) characteristic turbulence time and length scales. The accuracy of our experiments (which, in any case, utimately depends upon a trade-off with resolution (1)), is considered to be adequate to achieve the diagnostic goal of providing data of value to flame modelers this can be seen by comparison of the fluctuation temperature measurement uncertainty (characterized by a 5-7% standard deviation) with the broad temperature spread of the measured pdf s (extending, in Fig. 4, from values near ambient temperature to values in the vicinity of the adiabatic flame temperature). ... 

Robben and co-workers have exploited these facts to measure mean and rms temperature fluctuations in a turbulent flat flame (2) and above a catalytic surface (8). By measuring the postflame temperature on a flat flame burner, as a function of reactant flow rate, a precise measurement of laminar flame speed was reported by Muller-Dethlefs and Weinberg (9). 

For temperatures above the useful range of the oil and wax baths (approximately 160°), a sand bath or a Wood s metal bath is commonly used. A sand bath is simply a metal dish (usually in the form of a hemisphere) containing sand. The flask is immersed in the sand, and the bath is heated with a bunsen burner. The sand serves to make the heating more uniform than would be possible with a flame and to minimize temperature fluctuations because of its fairly larg e heat capacity. 

The mathematical models for simulating turbulent flame acceleration and the onset of detonation in chemically reacting flows were described in detail in [1-3]. The system of equations for the gaseous mixture was obtained by Favre averaging. The standard k e model was modified an equation was added that determined the mean squared deviate of temperature in order to model the temperature fluctuations. 

Mantzaras, J., and Van Der Meer, T. H. "Coherent Anti-Stokes Raman Spectroscopy Measurements of Temperature Fluctuations in Turbulent Natural Gas-Fueled Piloted Jet Diffusion Flames." Combustion and Flame 110 (1997) 39-53. 

Sodium and potassium in serum are determined in the clinical laboratory by atomic-emission spectroscopy, using an instrument designed specifically for this purpose [5]. Two filter monochromators isolate the sodium and potassium emission lines. A lithium internal standard is used, and the ratios of the Na/Li and K/Li signals are read out on two separate meters. The internal standard compensates for minor fluctuations in flame temperature, aspiration rate, and so forth. A cool flame, such as air-propane, is used to minimize ionization. Typically, the serum sample and standards are diluted 1 200 with a 100 ppm Li solution and aspirated directly. The instrument can be adjusted to read directly in meq/1 for sodium and potassium by adjusting the gain while aspirating appropriate standards. 

Measurement of pressure is difficult to make inside flame environments, where density and temperature fluctuate rapidly. Better methods of measuring pressure in biological systems, such as inside hlood vessels, would have major benehts in diagnosing heart disease and improving health. 

Temperature fluctuations actually do exert an indirect influence on atomic absorption and fluorescence measurements in several ways. An increase in temperature usually increases the efficiency of the atomization process and hence increases the total number of atoms in the vapor. In addition, line broadening and a decrease in peak height occur because the atomic particles travel at greater rates, which enhances the Doppler effect. Finally, temperature variations influence the degree of ionization of the analyte and thus the concentration of nonionized analyte on which the analysis is usually based (see page 246). Because of these effects, reasonable control of the flame temperature is also required for quantitative absorption and fluorescence measurements. 

As it is seen in Fig. 19.1, the test sample damage takes place on the 15th sec, which is evidenced by temperature fluctuations on curve 1. The filled sample (contains 15% of the hydrophilic filler) maintains the integrity up to 50 sec the sparking is observed at combustion that is, probably, related to water injection into the combustion zone. Besides, when the flame source is eliminated, the sample self-extinguishes for 2-3 sec. 

The experimental design chosen sets a limit on the amount of information that can possibly be extracted from the data obtained. For instance, when measuring the temperature of a flame, fluctuations in the ambient temperature of the room make no difference since, as we say, the temperature fluctuation of the room is below the sensitivity of the instrument. An analogous situation is typical in kinetic modeling where the existence or nonexistence of some reactions cannot be established even in principle within the framework of possible experiments. 

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

The physics and modeling of turbulent flows are affected by combustion through the production of density variations, buoyancy effects, dilation due to heat release, molecular transport, and instabiUty (1,2,3,5,8). Consequently, the conservation equations need to be modified to take these effects into account. This modification is achieved by the use of statistical quantities in the conservation equations. For example, because of the variations and fluctuations in the density that occur in turbulent combustion flows, density weighted mean values, or Favre mean values, are used for velocity components, mass fractions, enthalpy, and temperature. The turbulent diffusion flame can also be treated in terms of a probabiUty distribution function (pdf), the shape of which is assumed to be known a priori (1). 

Model of ID dissipation spectrum from Pope [19] (line) and measured, noise-corrected spectrum of the square of the radial gradient of fluctuating temperature in a CH4/I-I2/N2 jet flame (Re = 15,200) (symbols). Each spectrum is normalized by its maximum value. The arrow indicates the 2% level, which corresponds to the normalized wavenumber k = 1 according to the model spectrum. (From Barlow, R.S., Proc. Combust. Inst., 31, 49,2007. With permission.)... 

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. 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

Coppalle, A., and D. Joyeux. 1994. Temperature and soot volume fraction in turbulent diffusion flames Measurements of mean and fluctuating values. Combustion Flame 96 275-85. 

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