Flame maximum attainable

We caution the reader that applying Carnot s analysis is based on the assumptions that the heat is available at temperature T and that the heat reservoir is infinite. This means that if we use the adiabatic flame temperature for T, we will end up with a maximum attainable efficiency, since the exchange of heat will inevitably lead to a reduction in the temperature of the reservoir. From our analysis, it is not clear whether we used an adiabatic flame temperature [11]. Note that the adiabatic temperature is the highest temperature that can be reached by the system if all the heat generated is used to elevate the flame temperature. However, we can safely state that at least 30% of the maximum work potential has been lost. We will return to this subtle point at a later stage, when we examine the combustion of natural gas. 

Table 3.4 Maximum flame temperatures attainable by using various fuel gases...

Current state-of-the-art in the understanding of these phenomena, as well as progress made in achieving empirical and quantitative descriptions of these combustion processes, are reviewed. The specific topics discussed are i) the maximum attainable turbulent flame speed in an obstacle array, ii) computer simulation of turbulent flame accelerations, iii) correlation between the detonation cell size and the dynamic parameters of fuel-air detonations, and iv) the transition from deflagration to detonation. Future directions in the investigation of these problems are also discussed. 

The diameter of the droplets produced by a pneumatic nebulizer varies from < 5 ixm to about 25 xm. The spray chamber allows droplets to reach the burner which can be vaporized and atomized in the flame. If the spray chamber prevents small droplets (diameter of about 10 tm or less) from entering the flame, sensitivity will be decreased. On the other hand, if large droplets (>10/x-m) reach the flame, the flame noise will increase and the temperature will decrease. From the total mass of the sample nebulized, the maximum useful amount of droplets is about 10%, which gives the limit for the maximum attainable efficiency of the nebulizer. However, the nebuliza-tion efficiency can be improved in various ways by altering the droplet size distribution. A bead or bar placed close to the orifice of the nebulizer, or a counter flow nebulizer can be used for this purpose (Figure 37). 

Meanwhile, a very important, new use for acetylene had been developed, oxy-acetylene welding and cutting. The oxy-acetylene became popular because it gave a temperature of 6,000 to 7,000° F. contrasted with 4,000, the maximum attainable with the oxy-hydrogen flame. This application stimulated the invention of a safe method of compression acetylene to facilitate its storage and shipment. The challenge was met by the Prest-O-Lite Company, who devised a method involving the compression of acetylene to 300 p.s.i. into a cylinder packed with a solid absorbent saturated with. acetone. 

Combustion. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. Atomized fuel mixed with enough air to form a close-to-stoichiometric mixture is continuously fed into a primary zone. There its heat of formation is released at flame temperatures deterruined by the pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system in terms of fuel consumed per unit of work output. Table 6 fists the net heat content of a number of typical gas turbine fuels. Net rather than gross heat content is a more significant measure because heat of vaporization of the water formed in combustion cannot be recovered in aircraft exhaust. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. Fuels that are liquid at normal atmospheric pressure and temperature are the most practical and widely used aircraft fuels kerosene, with a distillation range from 150 to 300 °C, is the best compromise to combine maximum mass —heat content with other desirable properties. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. 

Flame Temperature. The adiabatic flame temperature, or theoretical flame temperature, is the maximum temperature attained by the products when the reaction goes to completion and the heat fiberated during the reaction is used to raise the temperature of the products. Flame temperatures, as a function of the equivalence ratio, are usually calculated from thermodynamic data when a fuel is burned adiabaticaHy with air. To calculate the adiabatic flame temperature (AFT) without dissociation, for lean to stoichiometric mixtures, complete combustion is assumed. This implies that the products of combustion contain only carbon dioxide, water, nitrogen, oxygen, and sulfur dioxide. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

Burners. The ordinary Bunsen burner is widely employed for the attainment of moderately high temperatures. The maximum temperature is attained by adjusting the regulator so as to admit rather more air than is required to produce a non-luminous flame too much air gives a noisy flame, which is unsuitable. 

In flame extinction studies the maximum temperature is used often as the ordinate in bifurcation curves. In the counterflowing premixed flames we consider here, the maximum temperature is attained at the symmetry plane y = 0. Hence, it is natural to introduce the temperature at the first grid point along with the reciprocal of the strain rate or the equivalence ratio as the dependent variables in the normalization condition. In this way the block tridiagonal structure of the Jacobian can be maintained. The flnal form of the governing equations we solve is given by (2.8)-(2.18), (4.6) and the normalization condition... 

There is, of course, a chemical effect in carbon monoxide flames. This point was mentioned in the discussion of carbon monoxide explosion limits. Studies have shown that CO flame velocities increase appreciably when small amounts of hydrogen, hydrogen-containing fuels, or water are added. For 45% CO in air, the flame velocity passes through a maximum after approximately 5% by volume of water has been added. At this point, the flame velocity is 2.1 times the value with 0.7% H20 added. After the 5% maximum is attained a dilution effect begins to cause a decrease in flame speed. The effect and the maximum arise because a sufficient steady-state concentration of OH radicals must be established for the most effective explosive condition. 

In the above example of the combustion of carbon monoxide the time found experimentally in which the pressure reaches a maximum is 0.4 sec the combustion time of a single element according to an estimate based on the theory of flame propagation [11, 12], is less than 0.001 sec. The loss of heat in 0.4 sec is considerable the increase in pressure takes place so slowly that the state of the gas does not change adiabatically upon compression, and despite the compression each element cools after combustion. However, in 0.001 sec the loss of heat is negligibly small and each element in burning does attain the temperature Tp. 

The quantity r2 is related to the ignition time at the head. The total reaction time r3 of the entire gas mixture over the entire cross-section of the tube for spin detonation is determined by the time of turbulent propagation of the flame from the igniting heads over the entire cross-section. In a given tube this time decreases as the number of ignition points and the number of heads increases. As the limit is approached, when the number of heads decreases, the maximum magnitude of the reaction time r3 max is attained. The speed of turbulent propagation is proportional to the detonation speed, so that r3 max (the reaction time of the entire mixture at the limit) is proportional to the tube diameter d. 

When two or more gases interact with ever increasing velocity until a high maximum speed is attained, an explosion 8 is said to result. The velocity is many thousand times greater than of the slow, uniform propagation of flame dealt with in the previous section, and its accurate determination is a problem of considerable experimental difficulty. 

The drag coeflBcients for small particles have been tabulated (12) as a function of particle radius. By integrating the equation of motion, it can be shown that a particle moving in the flame as described would adjust to within 10% of the gas velocity in 20 mm if it had a radius of 1 X 10 m or of 50 mm if the radius were 10 X 10 m. These distances are consistent both with the particles attaining the maximum rather than the mean velocity in the burner tubes and with the production of 10-mm long vapor trails during evaporation. 

Further support for the attainment of a critical concentration of hydroperoxide prior to the passage of a cool flame at temperatures corresponding to the Lq and L, lobes has been obtained by Taylor [131], and more recently by Pollard and co-workers [68,132], who determined the maximum concentrations of tert-butyl hydroperoxide found during the cool-flame oxidation of isobutane. Again, the concentration of hydroperoxide increased prior to the cool flame and it was almost entirely consumed during its passage (Fig. 12). Also, in common with other hydrocarbon + oxygen systems, (e.g. refs. 55, 65, 78,133) the induction period to the first cool flame (r,) was related to the initial reactant pressure (po) by the expression... 

We saw in Section 9.6b that the highest attainable temperature in a combustion reaction— the adiabatic flame temperature—depends on the fuel-lo-air ratio, and we stated but did not prove that this upper temperature limit is a maximum when the fuel and oxygen are present in stoichiometric proportion. If the mixture is either rich (fuel in excess) or lean (O2 in excess), the adiabatic flame temperature decreases. 

Procedure Put 100 g of water in a beaker, place the beaker on the wire gauze on the tripod and measure the temperature. Exactly weigh the mass of the butane burner, turn it on and adjust the hottest burner flame, immediately position it under the beaker and start the stopwatch. After exactly 60 s, remove the burner, turn it off immediately and weigh the burner again. Record the maximum temperature attained by the water. 

The maximum temperature attainable in the combustor can be controlled by varying the air/fuel ratio. This is a unique feature of the catalytic combustor, since without a catalyst a flame can only be sustained in a narrow air/fuel ratio range. By using an appropriate catalyst, instead of operating at the typical temperature of conventional flame combustion (i.e. 1500°C), the combustor can operate under flameless conditions below 1300°C. It is well known that the... 

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