ME 475/675 Introduction to Combustion Lecture 10

Announcements Midterm 1 HW 4 Due Friday, September 19, 2014 I’ll accept it on Monday, September 22, 2014 since that was the original due date Ch 2 (33, 35, 47, 50, 54, 63) I’m rethinking the Example/Homework assignments…

Problem 2.33 (Homework) Once more, repeat problem 2.30, but eliminate the unrealistic assumptions, i.e. allow for dissociation of the products and variable specific heats. Use HPFLAME (Appendix F), or other appropriate software. Compare and contrast the results of problems 2.30 to 2.33. Explain why they differ. 2.30 Determine the adiabatic flame temperature for constant- pressure combustion of a stoichiometric propane-air mixture assuming reactant at 298K, no dissociation of the products, and constant specific heats evaluated at 298K.

Problem 2.35 (homework) Repeat problem 2.30, but for constant-volume combustion. Also, determine the final pressures. Add, compare results with UVFLAME

Problem 2.47 (homework) Calculate the equilibrium composition for the reaction 𝐻 2 + 1 2 𝑂 2 ↔ 𝐻 2 𝑂 when the ratio of the number of moles of elemental hydrogen to elemental oxygen is unity. The temperature is 2000 K, and the pressure is 1 atm. Extra: What will happen to the amount of 𝐻 2 if the pressure is decreased? At what pressure will 𝜒 𝐻 2 =0.01?

Problem 2.50 (homework) Reformulate problem 2.47 to include the species OH, O, and H. Identify the number of equations and the number of unknowns. They should of course be equal. (Write a system of equations that can be used to solve for the unknowns). Do not solve your system.

Problem 2.54 (Homework) Consider the combustion of decane (C10H22) with air at an equivalence ratio of 1.25, pressure of 1 atm, and temperature of 2200 K. Estimate the mixture composition assuming no dissociation except for the water-gas shift equilibrium. Compare with results of TPEQUIL.

Problem 2.63 (Homework) A furnace uses preheated air to improve its fuel efficiency. Determine the adiabatic flame temperature when the furnace is operating at a mass air-fuel ratio of 16 for air preheated to 600 K. The fuel enters at 200 K. Assume the following simplified thermodynamic properties: Tref = 300 K, MWfuel = MWair = MWprod = 29 kg/kmol; cp,air = cp,prod = cp,fuel = 1200 J/kg-K; ℎ 𝑓,𝑎𝑖𝑟 𝑜 = ℎ 𝑓,𝑝𝑟𝑜𝑑 𝑜 =0; ℎ 𝑓,𝑓𝑢𝑒𝑙 𝑜 =4∗ 10 7 𝐽/𝑘𝑔

Computer Programs Provided by Book Publisher Described in Appendix F For “complex” reactions (11 product species) Fuel: CNHMOLNK Oxidizer: Air Download from web: www.mhhe.com/turns3e student edition Computer codes Access to TPEquil, HFFlame, UVFlame Extract All TPEQUIL (TP Equilibrium) Use to find Equilibrium composition and mixture properties Required input Fuel CNHMOLNK Temperature Pressure Equivalence ratio (with air) to determine initial number of moles of each atom

HPFLAME (HP Flame) Use to find Required Input Adiabatic flame temperature for constant pressure Required Input Fuel, equivalence ratio, enthalpy of reactants HR, pressure For constant pressure: HP = HR Find TAd In our examples we assume ideal combustion so we knew the product composition But this program calculates the more realistic equilibrium composition of the products from a (complex) equilibrium calculation (multiple equilibrium reactions) But this requires TProd = TAd, which we are trying to find! Requires program (not humans) to iterate