Experimental Investigation of High Temperature Reaction Kinetics of Hydrogen and Air in Turbulent, Supersonic, Combusting Flows

Nigil Satish Jeyashekar, Aeroacoustics Engineering, National Center for Physical Acoustics, University of Mississippi, 1, Coliseum Drive, UNIVERSITY, MS 38677 and John M. Seiner, Associate Director of Applied Research and Research Professor of Aeroacoustics Engineering, National Center for Physical Acoustics, University of Mississippi, 1, Coliseum Drive, University, MS 38677.

The equilibrium chemical reaction for hydrogen-air reaction at stoichiometric condition is:

2 H2 + [O2 + 3.76 N2] <=> 2 H2O + 3.76 H2

At high temperatures, coupled with supersonic flow conditions and heavy turbulence, the reaction is no longer in equilibrium. The chemistry of the process is best described by a series of elementary reactions. Each of these reactions takes place at a rate given by the Arrhenius equation:

k Reaction rate constant (cm3mol-1s-1).

A Arrhenius constant ((cm3mol-1)n-1s-1).

E Activation energy KJmol-1.

T Temperature (K).

n Order of the reaction.

The hydrogen-air reaction mechanism is best described by a series of 25 elementary reactions. At supersonic speeds, mixing of fuel and air streams is created by turbulence. However, chemical reaction time is rapid compared to the mixing time, causing reaction to occur before complete mixing has taken place. This leads to fuel wastage and low combustion efficiency. The reaction rate constant is indicative of the chemical reaction time and is in turn a measure of the degree of mixing. Turbulent combusting conditions have temperatures much different from the adiabatic/theoretical flame temperatures. Comparison of the experimentally obtained reaction rate constants, via temperature measurements, with those of rate constants from adiabatic conditions indicates the departure from equilibrium.

Measurements were made in a Mach 2 supersonic plume, with the central core of air, heated to a temperature sufficient for auto-ignition of the fuel, which is a co-flow of hydrogen. Combustion occurs in a shear layer around the central supersonic core. The situation applies to after burning rockets which employ hydrogen as its fuel. Temperature and number density measurements have been made at five downstream locations in the combusting shear layer of the jet. This is used to qualitatively comment on the degree of mixing as an effect of turbulence and the extent of combustion. Kinetic studies will help in future study to implement design modification of the supersonic nozzle, re-circulate hydrogen to improve mixing, combustion efficiency and study finite-rate chemistry effects, for combustors in automotive and aviation applications. Griffiths, J. F. and Barnard, J. A. High Temperature Flame Chemistry in Flame and Combustion (3rd edition), 94-123, Blackie Academic and Professional, New York.

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