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Heat release rate measurement in turbulent flames

Heat release rate measurement in turbulent flames

Heat release rate measurement in turbulent

Heat release rate measurement in turbulent flames BO Ayoola 1 , R Balachandran 2 , E Mastorakos 2 , CF Kaminski 1 1 Department of Chemical Engineering & 2 Department of Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA In this work, we report on the direct measurement of heat release rates via simultaneous laser-induced fluorescence of OH and CH2O radicals using planar laserinduced fluorescence (PLIF). The product of the two images is shown to correlate with the forward production rate of the HCO radical, which in turn has been found to correlate well with heat release rates in premixed hydrocarbon flames. Heat release rate measurements were also taken with OH* for comparisons with the results from the laser-based technique. The measurements were made in a lean turbulent premixed flame subject to acoustic forcing; this flame mimics the instabilities encountered in lean premixed pre-vaporized combustors (LPP). As the scheme is based on probing radical species that participate in the major heat release reactions, it is the closest nonintrusive measure of heat release rate currently available and thus presents a very useful diagnostic tool in combustion research. INTRODUCTION The reduction of emissions has been made paramount to the gas turbine industry since the introduction of government policies on pollution control. As a result, the LPP concept has been conceived for the reduction of NOx emissions in gas turbine combustors. The main advantage of this concept is the reduction of the operating temperature by the lean operation of the combustion process. At lower temperatures, the rate of the main NOx formation mechanism is reduced, as the mechanism requires a high activation energy. However, lean premixed flames are highly susceptible to combustion instabilities during lean operation. The instability is driven by the coupling of local heat release rates with acoustic eigenmodes within the combustor [1]. Although a lot of progress is being made in combustion instability research, the problem is far from being resolved. In ground-based gas turbines, the LPP concept has been successfully applied; however, in aero engines, the LPP concept has not yet been implemented because of the varying load conditions under which such engines are required to operate (e.g. during take-off, landing and operation at different altitudes). Combustion instabilities could in this case lead to engine failure with catastrophic consequences; hence, the investigation of the interaction between acoustics and heat release rate in lean pre-mixed flames is vital to the development of the next generation, ultra-low emission gas turbine combustors. Heat release rates are usually estimated using species chemiluminescence because chemiluminescence signals have been reported to correlate with fuel flow rate [2, 3]. However, it has been reported that chemiluminescence signals seem to be very sensitive in regions of high strain and curvature which dominate the flame front in highly turbulent flames. Laser based measurement techniques are ideal diagnostic tools in this context because they are species specific, fast and non-intrusive on the combustion system. One of the most powerful techniques is planar laser-induced fluorescence (PLIF) [4]. This technique is ideal for the determination of species concentrations in the combustion zone. From such measurements, flame properties such as temperature and heat release can be inferred with good precision and on timescales much smaller than characteristic timescales for the investigated flows. PLIF is especially useful for probing unsteady flames because it provides information 1

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