Advanced thermodynamics for engineers part 2
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Advanced thermodynamics for engineers part 2
10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2a fuel as thermal energy). The most common combustion processes encountered in engineering are those which convert a hydrocarbon fuel (which might range from pure hydrogen to almost pure carbon, e.g. coal) into carbon dioxide and water. This combustion is usually performed using air because it is fr Advanced thermodynamics for engineers part 2eely available, although other oxidants can be used in special circumstances, e.g. rocket motors. The theory that will be developed here will be appliAdvanced thermodynamics for engineers part 2
cable to any mixture of fuel and oxidant and any ratio of components in the products; however, it will be described in terms of commonly available hyd10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2ailable at the beginning of combustion into products at the end of the process. This model presupposes that combustion is a process that can take place in only one direction and it ignores the true statistical nature of chemical change. Combustion is the combination of various atoms and molecules, a Advanced thermodynamics for engineers part 2nd takes place when (hey are close enough to interact, but there is also the possibility of atoms which have previously joined together to make a prodAdvanced thermodynamics for engineers part 2
uct molecule separating to form reactants again. The whole mixture is really taking part in a molecular ‘barn dance’ and the tempo of the dance is con10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2 reality a true combustion process is even more complex than this because the actual rate at which the reactions can occur is finite (even if extremely fast). This rate is the basic cause of some of the pollutants produced by engines, particularly NOX. In fact, in most combustion processes the situa Advanced thermodynamics for engineers part 2tion is even more complex because there is an additional factor affecting combustion, which is related to the rate at which the fuel and aứ can mix. TAdvanced thermodynamics for engineers part 2
hese ideas will be inưoduccd in Chapter 15. Hence, the approach to combustion in this chapter is a simplified one but, in reality, it gives a reasonab10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2o this simply by the introduction of additional equations: the basic approach is still valid.The manner in which combustion takes place is governed by the detailed design of the combustion system. The various different types of combustion process are listed in Table 10.1, and some examples arc given Advanced thermodynamics for engineers part 2 of where the processes might be found. There is an interdependence between thermodynamics and fluid mechanics in combustion, and this interaction isAdvanced thermodynamics for engineers part 2
the subject of current research. This book will concentrate on the thermo-Thermodynamics of combustion 183dynamics of combustion, both in equilibrium 10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2sses both with and without dissociation. It will be found that equilibrium thermodynamics enables a large number of calculations to be performed but, even with dissociation included, it does not allow the calculation of pollutants, the production of which are controlled both by mixing rates (fluid m Advanced thermodynamics for engineers part 2echanics) and reaction rates (thermodynamics).Table 10.1 Factors affecting combustion processesConditions of combustionClassificationExamplesTime depeAdvanced thermodynamics for engineers part 2
ndencesteady unsteadygas turbine combustion chamber, boilers peưol engine, diesel engineSpatial dependencezero-dimensional one-dimensional two-dimensi10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2 Bunsen burner general combustionMixing of initial reactantspremixed non-premixedpetrol, or spark ignition, engine diesel engine, gas turbine combustion chamberFlowlaminar turbulentspecial cases for measuring flame speed most real engine cases, boilersPhase of reactantssingle multiphasespark-ignited Advanced thermodynamics for engineers part 2 gas engines, peưol engines with fuel completely evaporated; gas-fired boilers diesel engines, gas turbines, coal- and oil-fired boilersReaction sitesAdvanced thermodynamics for engineers part 2
homogeneous heterogeneousspark-ignition engines diesel engines, gas turbines, coal fired boilersReaction rateequilibrium chemistry (infinite rate) fin10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2emissionsConvection conditionsnatural forcedBunsen flame, gas cooker, cenưal heating boiler gas turbine combustion chamber, large boilersCompressibilityincompressible compressiblefree flames engine flamesSpeed of combustiondeflagration detonationmost normal combustion processes ‘knock’ in spark igni Advanced thermodynamics for engineers part 2tion engines, explosions184 Thermodynamics of combustion10.1Simple chemistryCombustion is a chemical reaction and hence a knowledge of basic chemistryAdvanced thermodynamics for engineers part 2
is required before it can be analysed. An extremely simple reaction can be written as2CO + o2 <=> 2CO2-10.1This basically means that two molecules of10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2, whereas co2 is a triatomic gas. Equation (10.1) also indicates that two molecules of co2 will always break down into two molecules of co and one molecule of o2; this is signified by the symbol <=> which indicates that the processes can go in both dừections. It is conventional to refer to the mixtu Advanced thermodynamics for engineers part 2re to the left of the arrows as the reactants and that to the right as the products', this is because exothermic combustion (i.e. in which energy is rAdvanced thermodynamics for engineers part 2
eleased by the process) would require co and O2 to combine to give co2. Not all reactions are exothermic and the formation of NO during dissociation o10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2ctants combine to produce two molecules of products, hence there is not necessarily a balance in the number of molecules on either side of a chemical reaction. However, there is a balance in the number of atoms of each constituent in the equation and so mass is conserved.10.1.1FUELSHydrocarbon fuels Advanced thermodynamics for engineers part 2 are rarely single-component in nature due to the methods of formation of the raw material and its extraction from the ground. A typical barrel of cruAdvanced thermodynamics for engineers part 2
de oil contains a range of hydrocarbons, and these arc separated at a refinery; the oil might produce the constituents defined in Fig 10.1. None of th10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2el.Kerosene (paraffin.•vialion fact)Cases (batane, propane)diesel, heating oil)Fig. 10.1 Typical constituents of a barrel of crude oilCombustion of simple hydrocarbons fuels 185One fuel which approaches single-component composition is ‘natural gas’, which consists largely of methane (CHJ. Methane is Advanced thermodynamics for engineers part 2 the simplest member of a family of hydrocarbons referred to as paraffins or, more recently, alkanes which have a general formula CJi2n+2. The lower aAdvanced thermodynamics for engineers part 2
lkanes are methane (CH4), ethane (C2H6), propane (CjHjf) and butane (C4H10) etc. Two other alkanes that occur in discussion of liquid fuels are heptan10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2o them. However, it is possible to find hydrocarbons with less than 2n + 2 hydrogen atoms and these are referred to as unsaturated hydrocarbons. A simple unsaturated hydrocarbon is acetylene (C2H2), which belongs to a chemical family called alkenes. Some fuels contain other constituents in addition Advanced thermodynamics for engineers part 2to carbon and hydrogen. For example, the alcohols contain oxygen in the form of an OH radical. The chemical symbol for methanol is CH3OH, and that forAdvanced thermodynamics for engineers part 2
ethanol is C,H5OH; these are the alcohol equivalents of methane and ethane.Often fuels are described by a mass analysis which defines the proportion 10Thermodynamics of CombustionCombustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a Advanced thermodynamics for engineers part 2he liquid. Solid fuels, such as various coals, have a much higher carbon/hydrogen ratio but contain other constituents including oxygen and ash.Gọi ngay
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