Correlations of high-pressure lean methane and syngas turbulent burning velocities: Effects of turbulent Reynolds, Damkohler, and Karlovitz numbers

This paper investigates correlations of high-pressure turbulent burning velocities (S-T) using our recent S-T measurements of lean methane and syngas spherical flames at constant elevated pressures (p) and constant turbulent Reynolds numbers (Re-T equivalent to u' L-I/v), where u', L-I, and v are the r.m.s. turbulent fluctuation velocity, the integral length scale of turbulence, and the kinematic viscosity of reactants, respectively. Such constant constraints are achieved by applying a very large high-pressure, dual-chamber explosion facility that is capable of controlling the product of u'L-I in proportion to the decreasing v due to the increase of p. We have found that, contrary to popular scenario for S-T enhancement with increasing p at any fixed u', S-T actually decreases similarly as laminar burning velocities (S-L) with increasing p in minus exponential manners when values of ReT are kept constant. Moreover, S-T increases noticeably with increasing ReT varying from 6700 to 14,200 at any constant p ranging from 1 atm to 10 atm. It is found that a better correlation for the normalization of S-T is a power-law relation of S-T/u' = aDa(b), where Da = (L-I/u')(S-L/delta(F)) is the turbulent Damkohler number, delta(F) approximate to alpha/S-L is the laminar flame thickness, and a is the thermal diffusivity of unburned mixture. Thus, the very scattering S-T data for each of lean methane and syngas mixtures can be merged on their S-T/u' vs. Da curves with very small data fluctuations. For lean methane flames with the Lewis number (Le) approximate to 1, S-T/u' 0.1 2Da(0.5) supporting a distributed reaction zone model anticipated by Ronney (1995), while for lean syngas flames with Le approximate to 0.76 << 1, S-T/u' 0.52Da(0.25) supporting a theory predicted by Zimont (1979). A simple physical mechanism is proposed in attempt to explain what causes the aforesaid discrepancy on the power-law constants. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.