Radiative heat loss and NO x emission of turbulent jet flames in preheated air up to 1230 K

Abstract

Characteristics of flame radiation and NO x emission of turbulent jet flame of ethylene and methane were investigated in highly preheated airflow of up to 1230 K. The radiative heat loss of the jet flame was obtained by summation of the local flame radiation measured by a narrow-view radiative heat flux meter. Radiant fraction, the ratio of the radiative heat loss to the heat of combustion, increases with preheated air temperature and exceeds 0.6 at 1200 K for a C 2 H 4 jet. This ratio shows that flame radiation gives a significant impact on the flame temperature and consequent NO x emission. As air temperature is increased, the NO x emission for CH 4 rapidly increases, but it does not in the case of C 2 H 4 up to 1100 K. The NO x emission level is as low as 3 g/kg-fuel at 1100 K, without detectable soot in the exhaust gas. The low NO x emission of the sooty flame in high-temperature airflow is due to the radiative heat loss and competing oxidation of carbon or CO and nitrogen. The important role of the radiation of flames in high-temperature air is also understood by diluting the fuels. The radiant fraction of the C 2 H 4 jet changed from 0.6 to 0.1 by N 2 dilution. The flame temperature of the undiluted C 2 H 4 jet was 250 K lower than that of 1005 diluted C 2 H 4 due to radiative heat loss. The NO x emission index also increases with dilution, as much as 10 times of that of undiluted C 2 H 4 . NO x production rate is scaled with representative flame temperature, which estimates the flame temperature considering the radiative heat loss and the fuel dilution. Good correlations between NO x production rate and representative flame temperature were obtained, indicating that the radiative characteristic is dominant for NO x emission in a highly preheated airflow.