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Combustion Theory Premixed and Diffusion Flames Part (1)

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Combustion Theory Premixed and Diffusion Flames Part (1) Combustion Theory premixed and diffusion flames part (1) Overview of Combustion Ignition Three things must be present at the same time in order to produce fire: . Enough oxygen to provide combustion . Enough heat to raise the material temperature to its ignition temperature . Fuel or combustible material which produces high exothermic reaction to propagate heat to not-yet- burnt material nearby Activation energy Introduction Basic Flame Types: 5 Combustion Theory • Premixed flames: fuel and oxidizer are homogeneously mixed before reaction occurs. Laminar and turbulent premixed flames • Non premixed flames: fuel and oxidizer come into contact during combustion process. Laminar and turbulent diffusion flames 1. Laminar premixed flames Laminar premixed flames 7 A premixed flame is a self-sustaining propagation of a localized combustion zone at subsonic velocities (deflagration regime) The classical device to generate a laminar premixed flame is the Bunsen burner: Typical Bunsen burner flame Example: Typical Bunsen-burner CH4/Air flame 8 Outer diffusion flame Outer cone (luminous zone): reaction and heat transfer Preheating region containing fuel and air Inner cone (dark zone): fuel rich flame Typical Bunsen-burner flame is a dual flame: • a fuel-rich premixed inner flame • a diffusion outer flame: CO and H2 from inner flame encounter ambient air • Experimental evidence for the presence of a cool inner preheating region 9 Combustion Theory A wire to reveal the presence of a cool preheating region containing unburned CH4 and O2 A match in preheating region does not ignite until it is moved to the inner cone • Basic features of laminar premixed flames Fuel/Air Ratio Fuel lean Stochiometric Fuel rich Very fuel rich10 Combustion Theory Deep Violet Blue Green Yellow Flame colour, due to large due to large due to carbon concentrations concentrations particles i.e. colour of the of excited CH of C2 species outer cone radicals High-T burned gases usually show a reddish glow due to radiation from CO2 and H2O Flame characteristics for hydrocarbon-air stoichiometric mixtures • The flame is ~1 mm thick and moves at ~0.5 m/s • Pressure drop through the flame is very small: ~1 Pa • Temperature in reaction zone is ~2200-2600 K • Density ratio of reactants to products is ~7 • Kinematic balance for a steady oblique flame 11 Thermal expansionCombustion through Theory the flame front • Normal component of velocity vector u vn u vn b vn,b vn,u b • Tangential component of velocity vector vt,u vt,b • At steady-state the burning velocity equals the flow velocity of the unburnt mixture normal to the flame front SL,u vn,u vu sin 2- Turbulent flames • Most of combustion devices operate in turbulent 12 Combustion Theory flow regime, i.e. internal combustion or aircraft engines, industrial burners and furnaces. Laminar combustion applications are almost limited to candles, lighters and some domestic furnaces. Turbulence increases the mixing processes thus enhancing combustion. • Also combustion influences turbulence. Heat release due to combustion causes very strong flow accelerations through the flame front (flame- generated turbulence). Moreover, huge changes in kinematic viscosity associated with temperature changes may damp turbulence leading to flow re- laminarization Turbulence Turbulent shear flows . Structure of a one-dimensional premixed laminar flame. Structure of a one-dimensional non-premixed laminar flame. Here fuel and oxidizer streams are assumed to have the same temperature. LAMINAR PREMIXED FLAMES OVERVIEW Applications: .Heating appliances .Bunsen burners .Burner for glass product manufacturing Importance of studying laminar premixed flames: .Some burners use this type of flames as shown by examples above .Prerequisite to the study of turbulent premixed flames. Both have the same physical processes and many turbulent flame theories are based on underlying laminar flame structure. PHYSICAL DESCRIPTION Physical characteristics .Figure 8.2 shows typical flame temperature profile, mole fraction of reactants,R, and volumetric heat release, . Q .Velocity of reactants entering the flame, u = flame propagation velocity, SL .Products heated product density (b) < reactant density (u). Continuity requires that burned gas velicity, b >= unburned gas vel., u u u A = b b A (8.1) . For a typical hydrocarbon-air flame at Patm, u/b 7 considerable acceleration of the gas flow across the flame (b to u). A flame consists of 2 zones: .Preheat zone, where little heat is released .Reaction zone, where the bulk of chemical energy is released Reaction zone consists of 2 regions: .Thin region (less than a millimeter), where reactions are very fast .Wide region (several millimeters), where reactions are slow . In thin region (fast reaction zone), destruction of the fuel molecules and creation of many intermediate species occur. This region is dominated by bimolecular reactions to produce CO. Wide zone (slow reaction zone) is dominated by radical recombination reactions and final burnout of CO via CO + OH CO2 +H Flame colours in fast-reaction zone: . If air > stoichiometric proportions, excited CH radicals result in blue radiation. If air < stoichiometric proportions, the zone appears blue-green as a result of radiation from excited C2. .In both flame regions, OH radicals contribute to the visible radiation, and to a lesser degree due to reaction CO + O CO2 + h. .If the flame is fuel-rich (much less air), soot will form, with its consequent blackbody continuum radiation. Although soot radiation has its maximum intensity in the infrared (recall Wien’s law for blackbody radiation), the spectral sensitivity of the human eye causes us to see a bright yellow (near white) to dull orange emission, depending on the flame temperature Spectrum of flame colours Typical Laboratory Premixed Flames:- . The typical Bunsen-burner flame is a dual flame: a fuel rich premixed inner flame surrounded by a diffusion flame. illustrates a Bunsen burner. The diffusion flame results when CO and OH from the rich inner flame encounter the ambient air. The shape of the flame is determined by the combined effects of the velocity profile and heat losses to the tube wall. .For the flame to remain stationary, SL = normal component of u = u sin (8.2). Figure 8.3b illustrates vector diagram. Fuel & Advanced Combustion Lecture LAMINAR PREMIXED FLAMES PART (2) 27 SIMPLIFIED ANALYSIS Turns (2000) proposes simplified laminar flame speed and thickness on one-dimensional flame. Assumptions used: .One-dimensional, constant-area, steady flow. One-dimensional flat flame is shown in Figure 8.5. .Kinetic and potential energies, viscous shear work, and thermal radiation are all neglected. .The small pressure difference across the flame is neglected; thus, pressure is constant. .The diffusion of heat and mass are governed by Fourier's and Fick's laws respectively (laminar flow). .Binary diffusion is assumed. The Lewis number, Le, which expresses the ratio of thermal diffusivity, , to mass diffusivity, D, i.e., is unity, k Le DCD p k upC .The Cp mixture ≠ f(temperature, composition). This is equivalent to assuming that individual species specific heats are all equal and constant. .Fuel and oxidizer form products in a single-step exothermic reaction. Reaction is 1 kg fuel + kg oxidiser ( + 1)kg products .The oxidizer is present in stoichiometric or excess proportions; thus fuel is completely consumed at the flame. .For this simplified system, SL and found are 1/ 2 . m (8.20) S 21 F L u .and 2 u 1mF .or (8.21) 2 S L where is volumetric mass rate of fuel and is mF thermal diffusivity. Temperature profile is assumed linear from Tu to Tb over the small distance, as shown in Fig. 8.9. FACTORS INFLUENCING FLAME SPEED (SL) AND FLAME THICKNESS () 1. Temperature (Tu and Tb) .Temperature dependencies of SL and can be inferred from Eqns 8.20 and 8.21. Explicit dependencies is proposed by Turns as follows . kT () 0.75 1 (8.27) TTPu upCT() .where is thermal diffusivity, Tu is unburned gas temperature, TTT0.5 bu , Tb is burned gas temperature. where the exponent n is the overall reaction order, Ru = universal gas constant (J/kmol-K), EA = activation energy (J/kmol) .Combining above scalings yields and applying Eqs 8.20 and 8.21 .SL (8.29) . 0.375 nn / 2 EA (8.30 ( 2)/ 2) TTTPub exp 2RTub 0.375nn / 2EA / 2 TTPb exp 2RTub 8 .For hydrocarbons, n 2 and EA 1.67.10 J/kmol (40 kcal/gmol). Eqn 8.29 predicts SL to increase by factor of 3.64 when Tu is increased from 300 to 600K. Table 8.1 shows comparisons of SL and .The empirical SL correlation of Andrews and Bradley [19] for stoichiometric methane-air flames, -4 2 SL (cm/s) = 10 + 3.71.10 [Tu(K)] (8.31) which is shown in Fig. 8.13, along with data from several experimenters. .Using Eqn. 8.31, an increase in Tu from 300 K to 600 K results in SL increasing by a factor of 3.3, which compares quite favourably with our estimate of 3.64 (Table 8.1). .Table 8.1 Estimate of effects of Tu and Tb on SL and using Eq 8.29 and 8.30 Case A (ref) B C Tu (K) 300 600 300 Tb (K) 2,000 2,300 1,700 SL/SL,A 1 3.64 0.46 /A 1 0.65 1.95 .Case A: reference .Case C: Tb changes due to heat transfer or changing equivalent ratio, either lean or rich. .Case B: Tu changes due to preheating fuel Pressure (P) .From Eq. 8.29, if, again, n 2, SL f (P). .Experimental measurements generally show a negative dependence of pressure. Andrews and Bradley [19] found that -0.5 SL (cm/s) = 43[P (atm)] (8.32) fits their data for P > 5 atm for methane-air flames (Fig. 8.14). Equivalent Ratio () .Except for very rich mixtures, the primary effect of on SL for similar fuels is a result of how this parameter affects flame temperatures; thus, we would expect S L,max at a slightly rich mixture and fall off on either side as shown in Fig.
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