Heat release from flame

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

In Figure 16.27, the flue gas is cooled to pinch temperature before being released to atmosphere. The heat released from the flue gas between pinch temperature and ambient is the stack loss. Thus in Figure 16.27, for a given grand composite curve and theoretical flame temperature, the heat from fuel and stack loss can be determined. 

The total amount of heat released from the flame front backward into the cooling combustion products is... 

Let us consider the symmetrical burning of a spherical droplet with the radius rp in surroundings without convection. Assume that there is an infinitely thin flame zone from the surface of the droplet to the radial distance rn [137], which is much larger than the radius of the droplet, rp. The heat released from the burning is conducted back to the surface to evaporate liquid fuel for combustion. Because the reaction is extremely fast, there exists no oxidant in the range of rp< r < m while no fuel vapor is available at r > rn. At a quasi steady state the mass flux through the spherical surface with the radius r (>rp), Mfv, can be obtained with Fick s law as... 

Because of the large amount of heat release from combustion, gas explosions always involve high temperature rise. For example, the maximum flame temperatures for hydrogen and methane are 2045°C and 1875°C, respectively.f l Even for weak deflagrations in fuel-lean mixtures near the LFL, the flame temperatures of hydrocarbons are in the range of 1300-1350°C (p. 330 in Ref9 (). This is why even weak deflagrations such as flash fires can cause severe burn injuries. 

In-flame temperature measurement provides useful insight into estimation of radiative heat release from flames, kinetics of the reaction, soot formation, and NO formation. In oxy-burner testing, thermocouple and suction pyrometer are extensively used to take point measurements within the flame. Recently, laser-based advanced diagnostic methods have been developed to map 2-D temperature field of the flame within the furnace [17]. 

Before a test is started, the coordinates of the flare and the radiometers (see Chapter 6) used to measure radiation are determined by utilizing a laser range finder to measure distances to three fixed objects with known coordinates and a technique called "triangulation." Multiple radiometers are used to measure various radiant fluxes simultaneously. A photo of the radiation measurement system is shown in Figure 28.12. The measured radiant fluxes, through sophisticated mathematical analysis, are used to determine the coordinates of the effective "epicenter(s)" of the flame, and the radiant fraction, which is defined as the fraction of heat release from combustion that is emitted as thermal radiation [43]. Solar radiation is subtracted from the radiation measurements as appropriate. 

Suppose the actual measured radiation level for this example is 1000 Btu/hr-ftt at a distance of 150 feet from the epicenter this would correspond to a radiant fraction of 0.26 (1000/3802). That is, 26% of the total heat released from this flame is transferred to the surroundings by thermal radiation. It should be mentioned that in this example we ignored the transmission loss of the radiation through the air. Recall that as a rule of thumb, 15-25% of the radiation is absorbed by the atmosphere over a distance of 500 feet. 

All standardized heating value analyses measirre the heat released from the fuel when bimied with enough air to fully oxidize the fuel (carbon forms carbon dioxide, hydrogen forms water, etc.), normalized by the mass of the fuel, not the mass of all combustion products. This value determines the amoimt of fuel reqirired to release a given amoimt of heat during combustion. It does not directly indicate the peak temperatnres of the resnlting flames. These temperatures play an important role in some... 

In flames only the net heat release is measured. This datum can be used in two different ways—in simple systems such as the hydrogen bromine flame chemical kinetic information can be inferred from thermal measurements, but in more complex flames heat release rates are useful primarily only as consistency checks. From the standpoint of chemistry, the most important physical process in the flame is diffusion, since it affects the composition. This will be discussed in more detail in the following section. 

The Ohio State University (OSU) calorimeter (12) differs from the Cone calorimeter ia that it is a tme adiabatic instmment which measures heat released dufing burning of polymers by measurement of the temperature of the exhaust gases. This test has been adopted by the Federal Aeronautics Administration (FAA) to test total and peak heat release of materials used ia the iateriors of commercial aircraft. The other principal heat release test ia use is the Factory Mutual flammabiHty apparatus (13,14). Unlike the Cone or OSU calorimeters this test allows the measurement of flame spread as weU as heat release and smoke. A unique feature is that it uses oxygen concentrations higher than ambient to simulate back radiation from the flames of a large-scale fire. 

Figure 4 illustrates the trend in adiabatic flame temperatures with heat of combustion as described. Also indicated is the consequence of another statistical result, ie, flames extinguish at a roughly common low limit (1200°C). This corresponds to heat-release density of ca 1.9 MJ/m (50 Btu/ft ) of fuel—air mixtures, or half that for the stoichiometric ratio. It also corresponds to flame temperature, as indicated, of ca 1220°C. Because these are statistical quantities, the same numerical values of flame temperature, low limit excess air, and so forth, can be expected to apply to coal—air mixtures and to fuels derived from coal (see Fuels, synthetic). 

Flame Temperature The heat released by the chemical reaction of fuel and oxidant heats the POC. Heat is transferred from the POC, primarily by radiation and convection, to the surroundings, and the resulting temperature in the reaction zone is the flame temperature. If there is no heat transfer to the surroundings, the flame temperature equals the theoretical, or adiabatic, flame temperature. 

Height of flame center above flare tip, m h = Height of flare tip above grade, m F = Fraction of heat release radiated from the flame m = Mass flaring rate, kg/s H = Lower heating value of the flare gas, MJ/kg r = Relative humidity, percent The following are the calculation steps ... 

Solid Fuel Flames. The flames from the combustion of solids such as coal and wood arc the result of a combination of processes including the burning of gases that have been released from the heated solid (devolatization) that burn in the gas phase as diffusion... 

Referring to Figure 7-72 at the calculated heat release, H., read the flame length, and refer to dimensional diagram for flame plume from a stack. Figure 7-73. 

Flames submitted to convective disturbances experience geometrical variations, which can in turn give rise to heat release unsteadiness. This process can be examined by considering different types of interactions between incident velocity or equivalence ratio modulations and combustion. The flame dynamics resulting from these interactions give rise to sound radiation and... 

Flame dynamics is intimately related to combustion instability and noise radiation. In this chapter, relationships between these different processes are described by making use of systematic experiments in which laminar flames respond to incident perturbations. The response to incoming disturbances is examined and expressions of the radiated pressure are compared with the measurements of heat release rate in the flame. The data indicate that flame dynamics determines the radiation of sound from flames. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects, usually treated separately, appear as manifestations of the same dynamical process. 

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