Adiabatic combustion process

For this adiabatic combustion process, the entropy production is equal to the entropy change of the process and is given by... 

For an adiabatic combustion process at constant pressure, the enthalpy stays constant. Figure 2 shows the exergy of the reactants, e, and the exergy of the reaction products, e , after this reaction has taken place. The loss in exergy is ... 

Evangelista et al. [120] and [121] illustrate this for an adiabatic combustion process, where the rate equation in terms of conversion is... 

Adiabatic flame temperatures agree with values measured by optical techniques, when the combustion is essentially complete and when losses are known to be relatively small. Calculated temperatures and gas compositions are thus extremely useful and essential for assessing the combustion process and predicting the effects of variations in process parameters (4). Advances in computational techniques have made flame temperature and equifibrium gas composition calculations, and the prediction of thermodynamic properties, routine for any fuel-oxidizer system for which the enthalpies and heats of formation are available or can be estimated. 

The principle of oxygen consumption is an empirical finding that the rate of heat release is proportional to the decrease in oxygen concentration in the combustion atmosphere [20, 21]. Thus, cone calorimeter heat release measurements do not require adiabaticity of reactions. Therefore, the combustion process can be carried out more openly, and reactions seen with the naked eye. The Cone calorimeter contains a load cell and can, thus, measure any property on a per mass lost basis. This permits... 

The air-intake used to induce air from the flight-altitude atmosphere plays an important role in determining the overall efficiency of ducted rockets. The air pressure built up by the shock wave determines the pressure in the ramburner. The temperature of the compressed air is also increased by the heating effect of the shock wave. The fuel-rich gaseous products formed in the gas generator burn with the pressurized and shock-wave heated air in the ramburner. The nozzle attached to the rear-end of the ramburner increases the flow velocity of the combustion products through an adiabatic expansion process. This adiabatic expansion process is equivalent to the expansion process of a rocket nozzle described in Section 1.2. 

This is frequently the required mode of operation for fast oxidation reactions because the heat release is too fast to provide efficient heat exchange. Most combustion processes are nearly adiabatic (your home furnace and your automobile engine), and many catalytic oxidation processes such as NH3 oxidation in HNO3 synthesis are nearly adiabatic. 

In the simple two-component system of PVC binder and oxidizer, the important propellant properties of specific impulse, density, adiabatic flame temperature, and burning rate increase with an increase in solids loading. This is shown in Figure 8, where theoretical calculated values of specific impulse, adiabatic flame temperature, and density are given for a range of oxidizer content for PVC plastisol propellants comprised of only binder and oxidizer. [Calculated values of specific impulse reported throughout this paper are for adiabatic combustion at a rocket chamber pressure of 1000 p.s.i.a. followed by isentropic expansion to 1 atm. pressure with the assumptions that during the expansion process chemical compo-... 

Assume that the combustion process occurs under well-mixed conditions. Use perfectly stirred reactor software together with the GRI-Mech mechanism (GRIM30. mec) to estimate the formation of NO in adiabatic combustion of CH4 with an excess-air ratio of 1.1... 

A process from E to A is an adiabatic process from supersonic conditions to subsonic conditions and is recognized as a shock wave. The entropy for this process increases from E to A, hence the reverse process from A directly to E entails an entropy decrease and is impossible. A strong deflagration, A to D, is therefore impossible except via C, a path involving an exothermic process from C to Z), followed by an endothermic process D to E. It seems unlikely that such a combustion process would be found in nature, although it is not impossible. 

Since we are located far from the boundary we may neglect the influence of heat transfer in the process. We equate the combustion temperature Tg with the so-called theoretical combustion temperature Tt, calculated from thermodynamic data under the assumption of adiabatic combustion. For more detail, see 1.4 and 1.5. 

Pure hydrogen gas at room pressure and temperature is adiabatically combusted with air. The combustion takes place with an amount of air that is 30% in excess of what is stoichiometrically required. Calculate the adiabatic flame temperature of the process, the work lost, and the thermo-dynamic efficiency of the process. Assume air to consist of a mixture of 79 mol% of N2 and 21 mol% of 02. 

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

To calculate the consumptions of exergy in the combustion process and the heat transfer process, it is supposed that the boiler may be separated into two distinct hypothetical entities an "adiabatic combustor" and a "heat exchanger." After determining the state of the combustion products, using an energy balance, the transport of exergy from the combustion process with product gases can be determined with the same procedures as above ... 

In technical combustion processes, this loss of exergy can be as high as 50%. If the combustion process is adiabatic, but at constant volume, then the internal energy stays constant. Usually the exergy losses are somewhat smaller than in the previous case (see Figure 3). 

Only the ideal cases of an isothermal-isobaric combustion process will be assumed. This combustion is superior to the usual isobaric-adiabatic process. Such an assumption can be verified more easily than in a normal combustion process, since in the cases studied here the chemical reactions take place at the surface of the oxygen carriers. 

Figure 13 shows the exergetic efficiency vs. the maximum temperature for different combustion processes with and without intermediate reactions. As a comparison, the adiabatic, isobaric... 

The idealization of the gas-turbine cycle (based on air, and called the Bray cycle) is shown on a PV diagram in Fig. 8.12. The compression step AB represented by an adiabatic, reversible (isentropic) path in which the press increases from PA (atmospheric pressure) to PB. The combustion process replaced by the constant-pressure addition of an amount of heat QBC. Work produced in the turbine as the result of isentropic expansion of the air to press... 

Most combustion reactions do not operate at stoichiometric or zero percent excess air. Incomplete combustion and higli carbon monoxide levels would result mider Uiis condition. If all Uie heat liberated by the reaction goes into heating up tlie products of combustion (tlie flue gas) die temperature acliieved is defined as Uie flame temperature. If die reacUon process is conducted adiabatically, with no heat transfer loss to die surroundings, the final temperature achieved in die flue gas is defined as die adiabatic flame temperature. If die combustion process is conducted widi theoretical or stoichiometric air (0% e.xcess air), die resulting temperature is defined as die theoretical flame temperature. (Theoretical or stoichiometric air is defined as diat exact amount of air required die completely react widi the compound to produce o. idized end products. Any air in excess of this stoichiometric amount is defined as excess air.)... 

With adiabatic combustion, departure from a complete control of m by the gas-phase reaction can occur only if the derivation of equation (5-75) becomes invalid. There are two ways in which this can happen essentially, the value of m calculated on the basis of gas-phase control may become either too low or too high to be consistent with all aspects of the problem. If the gas-phase reaction is the only rate process—for example, if the condensed phase is inert and maintains interfacial equilibrium—then m may become arbitrarily small without encountering an inconsistency. However, if a finite-rate process occurs at the interface or in the condensed phase, then a difficulty arises if the value of m calculated with gas-phase control is decreased below a critical value. To see this, consider equation (6) or equation (29). As the value of m obtained from the gas-phase analysis decreases (for example, as a consequence of a decreased reaction rate in the gas), the interface temperature 7], calculated from equation (6) or equation (29), also decreases. According to equation (37), this decreases t. Eventually, at a sufficiently low value of m, the calculated value of T- corresponds to Tj- = 0, As this condition is approached, the gas-phase solution approaches one in which dT/dx = 0 at x = 0, and the reaction zone moves to an infinite distance from the interface. The interface thus becomes adiabatic, and the gas-phase processes are separated from the interface and condensed-phase processes. 

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