Physical processes, combustion

Third, design constraints are imposed by the requirement for controlled cooling rates for NO reduction. The 1.5—2 s residence time required increases furnace volume and surface area. The physical processes involved in NO control, including the kinetics of NO chemistry, radiative heat transfer and gas cooling rates, fluid dynamics and boundary layer effects in the boiler, and final combustion of fuel-rich MHD generator exhaust gases, must be considered. 

Capp, B. and Seebold, J. 1991. Detonation Experiments m an 18-mch Pipe. Paper presented at the 1991 Annual AlChE Meeting session of Fundamental Chemical and Physical Processes m Combustion and Incineration 1. Los Angeles, CA, November 17-22. American Institute of Chemical Engineers, New York, NY. 

A process that releases heat into the surroundings is called an exothermic process. Most common chemical reactions—and all combustions, such as those that power transport and heating—are exothermic (Fig. 6.8). Less familiar are chemical reactions that absorb heat from the surroundings. A process that absorbs heat is called an endothermic process (Fig. 6.9). A number of common physical processes are endothermic. For instance, vaporization is endothermic, because heat must be supplied to drive molecules of a liquid apart from one another. The dissolution of ammonium nitrate in water is endothermic in fact, this process is used in instant cold packs for sports injuries. 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

RADICALC Bozzelli, J. W. and Ritter, E. R. Chemical and Physical Processes in Combustion, p. 453. The Combustion Institute, Pittsburgh, PA, 1993. A computer code to calculate entropy and heat capacity contributions to transition states and radical species from changes in vibrational frequencies, barriers, moments of inertia, and internal rotations. 

Tolocka, M.P. and Miller, J.H. Production of polycyclic hydrocarbons from underventilated hydrocarbon diffusion flames, in Proceedings Chemical and Physical Processes in Combustion (Worcester, MA Combustion Institute/Eastern States section, October 16-18, 1995), pp. 253-256. 

In the past, studies on ACC have been motivated by undesirable combustor behaviors that include combustion instabilities [1-10], poor burning efficiency [8-13], limited operational range [8-10, 14], and excessive production of pollutants ]8, 12, 13, 15-17]. These studies have contributed greatly to the present understanding of fast-response ACC, but several technological challenges still remain before the ACC technique can be implemented to practical propulsion systems. One such challenge is the use of liquid fuel for control, and maximization of control efficiency via direct injection into the combustion chamber. Such a control has been difficult to obtain and the physical processes were not well understood. 

Bui, P.-A., D. G. Vlachos, and P. R. Westmoreland. 1997. Self-sustained oscillations in distributed flames modeled with detailed chemistry. Eastern States Section, Chemical and Physical Processes in Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 337-40. 

When aluminized AP composite propellant burns, a high mole fraction of aluminum oxide is produced as a combustion product, which generates visible smoke. If smoke has to be avoided, e. g. for miUtary purposes or a fireworks display, aluminum particles cannot be added as a component of an AP composite propellant In addition, a large amount of white smoke is produced even when non-aluminized AP composite propellants bum. This is because the combustion product HCl acts as a nucleus for moisture in the atmosphere and relatively large-sized water drops are formed as a fog or mist This physical process only occurs when the relative humidity in the atmosphere is above about 60%. If, however, the atmospheric temperature is below 260 K, white smoke is again formed because of the condensation of water vapor with HCl produced as combustion products. If the HCl smoke generated by AP combustion cannot be tolerated, the propellant should be replaced with a double-base propellant or the AP particles should be replaced with another... 

Spontaneous Ignition A material proceeding without constraint by internal impulse or outside energy to kindle or set fire quick or slow oxidation or combustion brought about by chemical, electrical, biological (bacterial) or physical processes (vibration, pressure, friction) without assistance of extraneous sources of heat (flame, sparks, hot or glowing bodies). 

Nevertheless, there are natural or economic limits to the amount of tailoring possible by use of chemical or physical processes applied to the products per se. There comes a time when these processes can no longer yield products that meet the exacting requirements of the commercial and consumer equipment in which they are to be used. Or there comes a time when these exacting requirements can be met more economically in other ways. Nor does the situation ever become static it continuously becomes more critical as technological improvements in combustion equipment (including the development of new types) intensify old problems or create new ones. 

In Diesel fuel combustion, the physical processes include metering and transportation of fuel and air into the combustion chamber, vaporization of fuel, mixing fuel vapor with air, and provision of an environment favorable for rapid chemical reaction. The chemical processes include self-ignition or autoignition of the fuel-air mixture, and extensive chemical reaction to liberate the potential energy in the fuel. The accomplishment of these processes is the basic consideration in the design of the Diesel engine. 

Air is used as a source of reactants for many chemical and physical processes the oxygen is used for combustion and respiration, and the nitrogen is used as a starting material for the production of ammonia. To treat these gases quantitatively, we need to know the composition of air and, in some applications, the partial pressures of the components. A certain sample, of dry air of total mass 1.00 g consists almost entirely of 0.76 g of nitrogen and 0.24 g of oxygen. Calculate the partial pressures of these gases when the total pressure is 1.00 atm. 

The enthalpy is a state function therefore the value of AH is independent of the path between given initial and final states. We saw an application of this approach in Section 6.12, where we calculated the enthalpy change for an overall physical process as the sum of the enthalpy changes for a series of two individual steps. The same rule applies to chemical reactions. In this context, the rule is known as Hess s law the overall reaction enthalpy is the sum of the reaction enthalpies of the steps into which the reaction can be divided. Hess s law applies even if the intermediate reactions or the overall reaction cannot actually be carried out. Provided the equation for each step balances and the individual equations add up to the equation for the reaction of interest, a reaction enthalpy can be calculated from any convenient sequence of reactions (Fig. 6.28). As we shall see, Hess s law also lets us use readily obtainable combustion data to compile information on a wide variety of reactions. 

The physical processes which occur here axe as follows the externally heated c-phase begins to burn, but it burns at a velocity exceeding the steady velocity and with a temperature of the combustion products which exceeds the calculated theoretical temperature. Such intensive combustion occurs... 

Zenin A. A. Fizicheskie protsessy pri gorenii i vzryve [Physical Processes in Combustion and Explosion]. Moscow Atomizdat, 215 p. (1980). 

The past decade has led to the detection of new carbon allotropes such as fullerenes26 and carbon nanotubes,27 28 in which the presence of five-mem-bered rings allows planar polycyclic aromatic hydrocarbons to fold into bent structures. One notes at the same time that these structures are not objects of controlled chemical synthesis but result from unse-lective physical processes such as laser ablation or discharge in a light arc.29 It should be noted, on the other hand, that, e.g., pyrolytic graphitization processes, incomplete combustion of hydrocarbon precursors yielding carbon black, and carbon fibers30 are all related to mechanisms of benzene formation and fusion to polycyclic aromatic hydrocarbons. 

Mechanisms of HMX The Interrelationship of Chemical and Physical Processes. Proceedings of the JANNAF Combustion and Hazards Meeting, Tucson, Arizona, 1998. 

The development of a detailed, microscopically based understanding of the chemistry of combustion represents a considerable challenge. This is because so many chemical and physical processes interact together to produce the phenomenon of combustion. 

The authors proposed a mechanism that accounts for the reduction in flammability properties, which depends on physical processes in the condensed phase rather than chemical reactions. Three factors are critical in determining the silica behavior during the combustion process the density and surface area of the additive, the melt viscosity of the polymer. The interplay between these factors can determine whether the silica will accumulate near the surface or sink through the polymer melt. Fumed silica and silica gel provide examples for the first case where the silica particles accumulated on the surface and formed an insulating layer that provide protection to the underlying polymer. This is in contrast to the other case where the fused silica particles sank through the polymer melt. 

Di Blasi, C. Modeling chemical and physical processes of wood and biomass pyrolysis. Progress in Energy and Combustion Science, 2008. 34, 47-90. 

As one can see from the above discussion, establishing correlation between bench-scale flammability and cable flame tests is a difficult task because of complicated chemical and physical processes involved in the burning and combustion of polymer materials and cables. Much research is still needed to understand fundamental processes governing the flaming combustion of wire and cable compounds in actual cable designs and cable bundles in a given environment. 

The next important step in the elucidation of the role of the thermal factor in the mechanism of the phenomena in question was studying the effect of the sample size on the characteristics of the autowave process. Can the self-sustained wave regime of conversion be made impossible by intensification of heat release at the expense of a decrease in the diameter of a cylindrical sample containing the reactant mixtures By analogy with combustion physics, the question of a critical sample size has been raised. 

A quantitative understanding of certain primary combustion phenomena, e.g., liquid fuel-droplet vaporization and burning, gas phase chemical reaction kinetics, radiation heat transfer from combustion products, and mixing of reactants and combustion products, is required to develop a rational approach for the effective utilization of synfuels in industrial boiler/furnace systems. Those processes are defined by the interaction of a number of mechanisms which are conveniently described in terms of physical and chemical related processes. The physical processes are ... 

The heat effects so far discussed have been for physical processes. Chemi reactions also are accompanied by the transfer of heat, by temperature cha during the course of reaction, or by both. These effects are manifestations of differences in molecular structure, and therefore in energy, of the products reactants. For example, the reactants in a combustion reaction possess grea ... 

Describe how to write combustion equations, and how to measure the heat changes caused by physical processes such as dissolving. 

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