5th Asia-Pacific Conference on Combustion, The University of Adelaide, Adelaide, Australia 17-20 July 2005 Effect of Reynolds number on the Spatial Distribution of OH and Formaldehyde in Jet Flames in a Heated and Diluted Co-low P. R. Medwell, P. A. M. Kalt and B. B. Dally School of Mechanical Engineering The University of Adelaide, S.A. 5005, AUSTRALIA Abstract The spatial distribution of OH, formaldehyde (H 2 CO) and tem- perature imaged by laser diagnostic techniques are presented. The measurements are of nonpremixed jet flames in a hot and highly diluted co-flow. These conditions emulate those of Mod- erate and Intense Low Oxygen Dilution (MILD) combustion. This paper presents some results on the effect of O 2 co-flow level and jet Reynolds number on the structure of the flames for various fuels (each diluted by hydrogen 1:1 vol/vol). A reduction of O 2 levels is shown to lead to a substantial sup- pression of OH and a drop in peak temperatures. For strained flames, increased dilution of O 2 also leads to reduction of H 2 CO. Reynolds number effects on the radial profiles of the flame species considered are shown to be minor compared to O 2 levels. The drop of the peak temperature in a low O 2 co-flow leads to a reduction of thermal gradients and hence a laminari- sation of these flames. Strain rate does have a significant effect on the relative levels of H 2 CO however, with levels increasing markedly with higher Reynolds number. 1 Introduction Moderate and Intense Low Oxygen Dilution (MILD) combus- tion is a combustion mode of growing significance offering both high thermal efficiency and low pollutant levels, especially NO x . This desirable combination of combustion features is achieved by simultaneously reducing the oxygen concentration and increasing the temperature of the combustion air. MILD combustion is typically characterised by a distributed re- action zone and flat thermal field. Despite the air preheating used in MILD combustion, peak flame temperatures are signif- icantly lower than those encountered in conventional combus- tion, thereby reducing NO x production. Under certain operat- ing conditions, the MILD combustion process may result in no visible or audible flame. Based on this observation it is also referred to as flameless oxidation (FLOX ® ) [9]. The MILD combustion technology has been successfully ap- plied in several industrial situations [10], and has the potential for introduction into numerous other applications [1]. To date however, implementation has been impeded by a lack of funda- mental understanding of the establishment and structure of this combustion regime. The current project aims to examine the structure of the reaction zone by using laser diagnostic techniques to instantaneously and simultaneously image the distribution of temperature and concentration of the two key flame intermediates hydroxyl rad- ical (OH) and formaldehyde (H 2 CO). Under conventional com- bustion conditions OH has long been used as a flame front marker, whereas under MILD conditions Dally [2] has shown that OH concentrations are comparatively minor, and H 2 CO ap- pears to dominate. Moreover, the product of [OH] and [H 2 CO] has been suggested as an indicator of the formyl (HCO) radical, which is closely related to the heat release rate [5]. Fuel jet (ID 4.6mm) Co-flow (ID 82mm) Porous bed Perforated plate Figure 1: Cross-sectional diagram of experimental burner The depleted O 2 oxidant at elevated temperatures necessary for MILD combustion is typically realised by the recirculation of hot exhaust gases. This may be achieved either internally or externally with regard to the combustor. The complex interac- tions within such a system make it unsuitable for a fundamental study of the reaction zone. Instead, an experimental burner is used to emulate MILD combustion under controlled conditions, enabling a range of combustion parameters to be varied inde- pendently, and decoupling the flow from the chemical kinetics. The key parameters investigated in this work are the effect of the oxidant stream O 2 level and the jet Reynolds number on the establishment and structure of the MILD combustion regime. 2 Experimental Setup The burner to achieve MILD combustion conditions consists of a central insulated fuel jet (4.6mm) within an annular co-flow (82mm) of hot exhaust products from a porous bed burner mounted upstream of the jet exit plane, shown in Figure 1. The O 2 level of the co-flow is dictated by the excess oxygen level of the secondary porous burner. For the experiments, the secondary burner fuel flowrate was kept constant, and the ra- tio of the co-flow air/nitrogen was varied to give excess oxygen levels between 3% and 12%. This ensured that the co-flow tem- perature and exit velocity was constant at 1100K and 2.3m/s for different O 2 levels, giving a bulk Reynolds number of ∼1300. The fuels used in the jet are; natural gas, ethylene and LPG. All flames presented in the current paper are diluted with hydrogen in equal parts (volumetric basis) with the main constituent fuel to reduce soot interferences on the detected signal. Jet Reynolds numbers (based on jet diameter and mean exit velocity) are var- ied between 5,000 and 20,000. Within this paper, the co-flow O 2 level was either 3% or 9% (mole basis). Measurements were all taken 30mm downstream from the jet exit plane. 381