A study of the effects of preheat and steam addition on the flame structure and NO formation in laminar counterflow flames
Abstract
An experimental and numerical study has been conducted to examine the effects of air-preheat and steam addition on flame structure and NO formation in laminar counterflow flames. The measurements for NO and major species were performed using a sampling technique and theoretical predictions were carried out using the OPPDIF code with GRI MECH 2.11. The major species predictions for diffusion and partially premixed laminar flames show good agreement with the measurements. For C2 species, the predictions showed significant over-estimate for all experimental conditions. The predictions for NO in the diffusion flames show good agreement with the measurements of the peak mole fraction. However, significant over-predictions were found on the fuel-rich side of the diffusion flames. The agreement improves with increasing levels of partial premixing and excellent match is obtained for the highest levels of partial premixing. Detailed effects of air-preheat and steam addition on the flames were studied using numerical simulations. Preheat of air increases CO concentration and decreases CO2 concentration by enhancing dissociation of CO 2. The NO formation increased by two-fold with an increase of air-preheat from 300 K to 560 K. This increase of NO is the result of the increase in the rate of prompt initiation reaction. Steam addition into diffusion and partially premixed flames causes an increase in OH radical concentrations and a decrease in H atom concentrations. The increased OH and decreased H atom concentration enhance CO oxidation. Steam addition significantly reduces NO formation by reducing CH concentrations in diffusion and partially premixed flames. The CH concentration decreases via direct combination with the abundant H2O with steam addition. State relationships independent of scalar dissipation rates were established from the measurements with air-preheat. The air-preheat increases scatter in the CO and H2 mole fraction profiles. Independent of the level of steam addition and scalar dissipation rates, the state relationships were obtained for CH4, O2 and N2. However, the state relationships for CO, CO2 and H2 must be obtained for each level of steam addition, but are still independent of the scalar dissipation rates.
Degree
Ph.D.
Advisors
Viskanta, Purdue University.
Subject Area
Aerospace materials|Chemical engineering|Environmental science|Mechanical engineering
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