3 (a) Auto-ignition can be defined as the spontaneous ignition of a fuel-air mixture without the presence of an energy source. With reference to specific reactions taking place in hydrogen-air combustion explain what is meant by the terms chain branching reactions and termination reactions. How does the domination of chain branching reactions over termination reactions lead to the auto-ignition of a hydrogen-air mixture?

(b) How do the minimum ignition energy (MIE) and the quench distance (QD) of hydrogen compare with those of methane in air? Briefly discuss the implications of these differences for the use of hydrogen as a replacement for natural gas (which is mainly methane) in the domestic gas supply system.

(c) Define the meaning of the burning velocity for a laminar flame. How are laminar burning velocities (LBV) and turbulent burning velocities related?

(d) Briefly describe how the heat flux method is used to obtain laminar burning velocities for gas-air mixtures using a flat stable flame set-up. Include in your answer a discussion of the limitations of the method.

(e) Design a premixed laminar flame burner (fuel and air flow rates and number of burner holes) for operation using liquid petroleum gas (LPG) with a thermal output of 10 kW that must not have a risk of flash back into the upstream premixing passage. Use the data provided below, and you may assume in your calculations an overall efficiency of 95% and a calorific value (CV) for LPG of 45.5 MJ kg-1 . The density of the mixture can be taken to be 1.2 kg m-3 . Explain your reasons for any further assumptions made.