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Tutorial Sheet Combustion Fundamentals Third year mechanical Power, 2011

Mech. Power Eng. Dept. Dept

1- What are the conditions for the ignition of a fuel gas to occur? 2- Define the lower and higher flammability limits and what are the factors affecting the limits. 3- What are the different types of flames and what is the main factor affecting their speed? 4-

Discuss which flame of the th following is considered premixed or diffusion. - Port injection gasoline asoline engines engine - Diesel engines - Flame from a surface of a gasoline tank - Fire in a building - Bunsen burner - Candle

5- What are the reasons of flame quenching and what hat are the factors affecting flame quenching distance. 6- Employ correlation of Metghalchi and Keck to compare laminar flame speed of gasoline-air air mixtures with φ = 0.8 for 3 cases: a. At reference conditions of T = 298 K and P = 1 atm b. At conditions typical of a spark ignition engine operating at T = 685 K and P = 18.38 atm c. At same conditions as (b) (b) but with 15 percent (by mass) exhaust gas recirculation 7- A laminar premixed flame is required to be formed using Propane as a fuel from a 3 mm nozzle. The Propane gas is supplied from a 200 bar pressure bottle of 200 litres through a pressure regulator. regulator The bottle is connected to the nozzle by a 2 inches connecting pipe. What should be the regulator pressure in mm H2O. You may assume the temperature of the th gas remains constant at 300 K through the connecting pipe and the flame half cone angle is 20 degree. The propane laminar flame velocity is 44 cm/s

8- A full propane cylinder from a stove leaks contents of 1.02 lb (0.464 kg) into a 12’ x 14’ x 8’ (3.66 m x 4.27 m x 2.44 m) room at 20 ºC and 1 atm. After a long time, the fuel gas and the room air are well mixed. Is mixture in room flammable? 9-

A premixed propane-air mixture emerges from a round nozzle with a uniform velocity of 75 cm/s. The laminar flame speed of propane-air mixture is 35 cm/s. The flame is lit at the nozzle exit. What is the cone angle of this flame?

10- A methanol-air combustible mixture forms a combustible laminar premixed flame. The air supply orifice diameter is adjusted to provide a stoichiometric mixture. What happen if the air orifice diameter has been reduced to 80% and 40% of its original diameter. What will you notice on the flame? ( Assume the methanol is supplied through a pressure regulator that keeps a constant flow of methanol through the burner nozzle) 11- A stoichiometric methane air flame has a laminar burning velocity of 0.33 m/s and a temperature of 2200K. The reactants are initially at 300 K and 1 bar. Find the velocity of the combustion products relative to the flame front and the pressure change across the flame. Assume complete combustion and no dissociation. 12- A premixed laminar flame is stabilized in a one dimensional gas flow where the vertical velocity of the unburned mixture varies linearly with the horizontal distance according to the following equation: Vu=0.8+0.02x [m/s], for 0 < x < 20 mm Determine the flame shape and the distribution of the local angle of the flame surface from the vertical. Assume flame speed is independent of position and equal to 0.4 m/s. 13- Estimate the laminar flame speed of a stoichiometric propane-air mixture using the simplified-theory. Use the global one step reaction mechanism to estimate the mean reaction rate as follows:

14- The laminar burning velocity of a stoichiometric mixture of propane-air at 1 bar and 298 K is 0.4 m/s. Estimate the average reaction rate and the reaction zone thickness. Consider the thermal conductivity of the mixture is 0.026 W/mK at 298 K and increases at T0.5. Assume an appropriate average mixture temperature and ignition temperature. Calculate also the flame quench distance.

Formulae Sheet and Useful Data Simplified laminar flame model

α (T f − Tig ) ω  VL =    (Tig − To ) ρ u 

0.5

Metghalchi and Keck Correlation

 T S L = S L ,ref  u T  u ,ref

   

γ

β

 P  P  ref

  (1 − 2.1Ydil )  

S L ,ref = BM + B2 (φ − φM )

2

γ = 2.18 − 0.8(φ − 1) β = −0.16 + 0.22(φ − 1)