ME 475/675 Introduction to Combustion

1. ME 475/675 Introduction to Combustion Lecture 3

2. Thermodynamic Systems (reactors) • Closed systems • Open Steady State, Steady Flow (SSSF) Systems • How to find changes, and , for mixtures when temperatures and composition change due to reactions (not covered in Thermodynamics I) m, E Inlet i Outlet o Dm=DE=0

3. Calorific Equations of State for a pure substance For ideal gases • Differentials (small changes) • For ideal gas • = 0; • = 0; • Specific Heats, and [kJ/kg K] • Energy input to increase temperature of one kg of a substance by 1°C at constant volume or pressure • How are and measured? • Calculate • Molar based • = ; = w m, T m, T Q Q V = constant P = wg/A = constant

4. Molar Specific Heat Dependence on Temperature [K] • Monatomic molecules: Nearly independent of temperature • Only possess translational kinetic energy • Multi-Atomic molecules: Increase with temperature and number of molecules • Also possess rotational and vibrational kinetic energy

5. Specific Internal Energy and Enthalpy • Once and are known, specific enthalpy h(T) and internal energy u(T) can be calculated by integration • Primarily interested in changes, i.e. , • When composition does not change and are not important • Tabulated: Appendix A, pp. 687-699, for combustion gases • bookmark (show tables) • Curve fits, Page 702, for Fuels • Use in spreadsheets • = ; • =; =

6. Mixture Properties • Extensive Enthalpy • Specific Internal Energy • Use these relations to calculate mixture specific enthalpy and internal energy (per mass or mole) as functions of the properties of the individual components and their mass or molar fractions. • u and h depend on temperature, but not pressure

7. Standardized Enthalpy and Enthalpy of Formation • Needed to find and for chemically-reacting systems because energy is required to form and break chemical bonds • Not considered in Thermodynamics I • Standard Enthalpy at Temperature T = • Enthalpy of formation from “normally occurring elemental compounds,” at standard reference state: Tref = 298 K and P° = 1 atm • Sensible enthalpy change in going from Trefto T = • Normally-Occurring Elemental Compounds • Examples: O2, N2, C, He, H2 • Their enthalpy of formation at are defined to be = 0 • Use these compounds as bases to tabulate the energy to form other compounds

8. Standard Enthalpy of O atoms • To form 2O atoms from one O2 molecule requires 498,390 kJ/kmol of energy input to break O-O bond (initial and final T and P are same) • At 298K (1 mole) O2 + 498,390 kJ  (2 mole) O • for other compounds are in Appendices A and B, pp 687-702 • To find enthalpy of O at other temperatures use

9. Example: • Problem 2.14, p 71: Consider a stoichiometric mixture of isooctane and air. Calculate the enthalpy of the mixture at the standard-state temperature (298.15 K) on a per-kmol-of-fuel basis (kJ/kmolfuel), on a per-kmol-of-mixture basis (kJ/kmolmix), and on a per-mass-of-mixture basis (kJ/kgmix). • Find enthalpy at 298.15 K of different bases • Problem 2.15: Repeat for T = 500 K

10. Standard Enthalpy of Isooctane • Coefficients to from Page 702 • ; • Spreadsheet really helps this calculation

11. Enthalpy of Combustion (or reaction) Reactants 298.15 K, P = 1 atm Stoichiometric Products Complete Combustion CCO2 HH2O 298.15 K, 1 atm • How much energy can be released if product temperature and pressure are the same as those of the reactant? • Steady Flow Reactor