Thermodynamics

Chapter 15 | 783In the absence of any heat loss to the surroundings (Q 0),the temperature of the products will reach a maximum, whichis called the adiabatic flame temperature of the reaction.The adiabatic flame temperature of a steady-flow combustionprocess is determined from H prod H react ora N p 1h° f h h°2 p a N r 1h° f h h°2 rTaking the positive direction of heat transfer to be to thesystem, the entropy balance relation can be expressed for aclosed system or steady-flow combustion chamber asa Q kT k S gen S prod S reactFor an adiabatic process it reduces toS gen,adiabatic S prod S react 0The third law of thermodynamics states that the entropy ofa pure crystalline substance at absolute zero temperature iszero. The third law provides a common base for the entropyof all substances, and the entropy values relative to this baseare called the absolute entropy. The ideal-gas tables list theabsolute entropy values over a wide range of temperatures butat a fixed pressure of P 0 1 atm. Absolute entropy values atother pressures P for any temperature T are determined fromFor component i of an ideal-gas mixture, this relation can bewritten aswhere P i is the partial pressure, y i is the mole fraction of thecomponent, and P m is the total pressure of the mixture inatmospheres.The exergy destruction and the reversible work associatedwith a chemical reaction are determined fromands 1T, P2 s ° 1T, P0 2 R u ln P P 0si 1T, P i 2 s ° i 1T, P 0 2 R u ln y i P mP 0X destroyed W rev W act T 0 S genW rev a N r 1h° f h h° T 0 s 2 r a N p 1h° f h h° T 0 s 2 pWhen both the reactants and the products are at the temperatureof the surroundings T 0 , the reversible work can beexpressed in terms of the Gibbs functions asW rev a N r 1g ° f g T 0 g °2 r a N p 1g ° f g T 0 g °2 pREFERENCES AND SUGGESTED READINGS1. S. W. Angrist. Direct Energy Conversion. 4th ed. Boston:Allyn and Bacon, 1982.2. W. Z. Black and J. G. Hartley. Thermodynamics. NewYork: Harper & Row, 1985.3. I. Glassman. Combustion. New York: Academic Press,1977.4. R. Strehlow. Fundamentals of Combustion. Scranton,PA: International Textbook Co., 1968.5. K. Wark and D. E. Richards. Thermodynamics. 6th ed.New York: McGraw-Hill, 1999.PROBLEMS*Fuels and Combustion15–1C What are the approximate chemical compositions ofgasoline, diesel fuel, and natural gas?*Problems designated by a “C” are concept questions, and studentsare encouraged to answer them all. Problems designated by an “E”are in English units, and the SI users can ignore them. Problemswith a CD-EES icon are solved using EES, and complete solutionstogether with parametric studies are included on the enclosed DVD.Problems with a computer-EES icon are comprehensive in nature,and are intended to be solved with a computer, preferably using theEES software that accompanies this text.15–2C How does the presence of N 2 in air affect the outcomeof a combustion process?15–3C How does the presence of moisture in air affect theoutcome of a combustion process?15–4C What does the dew-point temperature of the productgases represent? How is it determined?15–5C Is the number of atoms of each element conservedduring a chemical reaction? How about the total number ofmoles?