Heat release rate measurement in turbulent flames

Transcription

1 Heat release rate measurement in turbulent flames BO Ayoola, R Balachandran, E Mastorakos, CF Kaminski Department of Chemical Engineering & Department of Engineering, University of Cambridge, Pembroke Street, Cambridge, CB 3RA In this work, we report on the direct measurement of heat release rates via simultaneous laser-induced fluorescence of OH and CH O 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 []. 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 [, 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

2 in dimensions in contrast to traditional point and spatially averaged measurement techniques such as chemiluminescence. In order to measure heat release rates, we employ an experimental scheme that involves the simultaneous PLIF of the OH radical and the CH O radical [5, 6]. It has been shown that the product of these PLIF images generates an image that is well correlated with the instantaneous distribution of heat release rate. With a reliable indicator of heat release rate, flame-acoustic interactions can be thoroughly investigated to provide vital information for the prediction and control of the unsteady phenomenon typical of LPP combustors. This work describes the laser-based technique of measuring spatially resolved heat release rates in a model combustor, which mimics the unsteady phenomenon encountered in LPP combustors by the acoustic excitation of the inlet flow of premixed reactants. This paper is divided into four main sections. In the first section, the theoretical aspects of the PLIF measurements are discussed and the heat release rate measurement technique, using the simultaneous PLIF of OH and CH O, is described. In the second section, the experimental details of the measurements including the laser facility and the combustor are discussed and in the third section, the heat release rate results obtained are presented. The paper is then concluded in the fourth section. EXPERIMENTAL TECHNIQUE Laser-induced fluorescence (LIF) LIF is the most widely used laser-based diagnostic technique for investigating combustion and it is the method employed in this research work. It is capable of high species detection sensitivity and concentrations below 00 ppm can be routinely detected. A great number of chemical intermediates formed in flames are detectable using LIF and in turbulent combustion applications, measurements can be performed at much faster timescales than the characteristic flow timescales. This technique has been extensively reviewed by Eckbreth [7], Kohse-Höinghaus [8] and Hanson et al [9]. In LIF, molecules are excited from a ground state to a higher energy level by absorption of light. A number of these excited molecules relax back to the ground state by emitting spontaneous fluorescence which is subsequently collected as signal. The principles of LIF can be described using a rate equation approach. Figure below illustrates a simplified energy level diagram to show the energy transfer processes that occur when a molecule is excited through an allowed transition. P W i b A Q Figure. Schematic energy level diagram for the LIF process.

3 There are five energy transfer mechanisms that can take place when a molecule is excited ( E ). The molecule can return to level by laser-induced stimulated emission at a rate b ; be photoionized to higher energy levels at a rate W i ; predissociate into its constituent atoms at a rate P; be quenched by inelastic collisions with other molecules resulting in energy transfer to other ro-vibrational states at a rate Q return to level by spontaneous emission (fluorescence) of photons at rate A. The last of these processes constitutes the LIF signal. The rate equations governing the time dependent population N, N of the ground state (level ) and the excited state (level ) are given by dn dt dn dt = N b + N + + () ( b A Q) = N b N b + A + Q + P + ) () ( W i where N i is the number density in level i, A ij is the spontaneous emission coefficient from i to j, b ij is the stimulated emission rate from i to j, P is the predissociation rate, W i is the photoionization rate. I υ b ij = Bij where Bij is the Einstein coefficient for stimulated absorption, c is the c speed of sound and I ν is the laser spectral irradiance in Wcm - cm. P and W i can often be ignored because most excited states are neither predissociative nor will they photoionize readily. By assuming that no chemical reaction occurs during the measurement and steady state is reached during the excitation pulse, an expression can be obtained for the fluorescence signal power, F. Integrating equation () and substituting for N and b ij in equation () gives: F Ω = hυ V 4π c 0 B A N (3) υ B + B I sat + I υ where the saturation irradiance I ν sat is defined as ( A + Q ) υ I sat B + B c (4) and h is Planck s constant, v is the frequency of the emitted fluorescence, Ω is the fluorescence collection solid angle, V c is the fluorescence collection volume [7]. 3

4 υ It is seen from (3) that for I ν >> I sat, the emitted fluorescence signal is υ independent of the laser power used. For I υ >> I sat, the fluorescence is linearly proportional to the laser irradiance and (3) simplifies to F hυ Ω A 0 = Vc N BI υ. (5) c 4π A + Q The term A/(A+Q) in (5) is defined as the fluorescence quantum efficiency, φ fe. In atmospheric pressure flames, A << Q and only a fraction of the excited molecules are detected as most of them are collisionally quenched. The quenching term Q is often difficult to estimate because it incorporates the collisional cross sections of every chemical species that the excited molecules can collide with. Theoretical advances in recent years have led to accurate predictions of Q for a number of flames and this approach is generally used for concentration measurements today, [0, ]. Thus, an avenue is provided through which the concentration of certain reacting species can be obtained to yield spatially resolved flame properties such as number density, pressure, heat release rate and temperature. Heat release rate measurements (HR) Heat release rates are usually estimated using chemiluminescence radiation emitted from chemically excited species such as OH*, CH*, C * and CO * [, 3]. However, this technique has been reported to be insensitive to high strain and curvature which dominates highly turbulent flames [, 3]. It is also a line of sight technique; hence, spatial features of the flame front are not clearly resolved. Here, we employ an experimental scheme that correlates the concentration of the HCO radical with local heat release rate in a premixed hydrocarbon flame; a technique developed by Najm et al. [6]. The major reason for this correlation is the direct dependence of the production of HCO on formaldehyde, CH O, which is produced in the breakdown of CH 3 ; a reaction which is a major contributor to local heat release rate [4]. However, the direct PLIF measurement of the HCO radical is experimentally difficult due to the relatively short lifetimes of the HCO fluorescence and its fast reaction rates. An alternative is to obtain a signal proportional to the forward reaction rate of the reaction: CH O + OH HCO + HO (6) using the pixel-by-pixel product of simultaneous OH and CH O PLIF measurements. Single-shot D measurements of this reaction rate are feasible since signals from OH and CH O are considerably stronger than from HCO. Since this reaction rate correlates well with heat release rate, the product of the OH and CH O LIF signals can be related to the local heat release rate [5, 6]. The LIF signals are proportional to the species concentrations but also depend on temperature through the Boltzmann fraction population and collisional quenching [7]. Therefore, the product of simultaneously recorded LIF signals, F of CH O and OH can therefore be expressed as F CH O x F OH α f(t, ξ) [CH O][OH] = HCO Reaction Rate (7) 4

5 where T is temperature and ξ is equivalence ratio. Over a limited range of ξ, transition lines are selected such that f(t, ξ) varies similarly with the forward reaction rate of reaction k(t) in equation (6). By doing this, the direct pixel-by-pixel product of the simultaneous PLIF images obtained generates an image of HCO concentration which correlates well with local heat release rate. Fayoux et al [5] employed this measurement scheme for heat release rate measurements in laminar counter-flow flames and successfully compared the results with theoretical calculations. EXPERIMENTAL APPARATUS Combustor The experiments were carried out in an enclosed 0kW, laboratory scale, bluff body combustor operating on premixed ethylene, C H 4 and air. The flow through the combustor is highly turbulent with a Reynolds no. of 5,000 and bulk velocity of 0m/s. The diameter of the bluff body used in this investigation was 5 mm resulting in a 50% blockage ratio at the burner lip. In order to impose controlled acoustic oscillations on the flame, acoustic drivers, mounted diametrically opposite each other on the circumference of the burner plenum, were used. This enables the combustor to mimic the phenomena that occur in large-scale, industrial LP combustors; hence, it provides a controlled system in which to study the response of flames to acoustic oscillations under well-defined conditions. An image of the bluff body stabilized combustor exit and an image showing the burner while stabilizing a lean premixed flame is shown Fig. below. Laser set-up The laser system consists of a cluster of Nd:YAG lasers (Continuum Surelite), dye lasers (Sirah Cobra-Stretch) and high-resolution double-pulsed ICCD cameras (Lavision Nanostar). The camera used for imaging OH fluorescence was fitted with a UV f/4.5 Nikkor camera lens and UG and WG 305 schott glass filters. Since each camera has the capability of taking images in quick succession (double exposure), a second image of OH chemiluminescence was taken after the OH PLIF image was recorded as chemiluminescence can also be measured through the same optical set-up. The CH O camera was fitted with an f/. Nikkor camera lens with UG 375 and 550 nm short-pass filters. For OH LIF, the frequency-doubled output from one dye laser was tuned near 83 nm to pump the Q (5) transition of the A Σ - X Π (, 0) band, and OH fluorescence from the (0, 0) and (, ) bands was measured. The frequency-doubled output from the second dye laser was tuned to ~ pump the ~ A A X A 4 band of CH 0 O near 353 nm. The laser energies used were 5 mj for the OH laser pulse and mj for the CH O laser pulse. The laser pulses were expanded into planar laser sheets 50 mm high and 0 µm thick using a combination of plano-convex lenses (f = 5mm) and spherical lenses (f = 50 mm). A schematic representation of the laser imaging facility is also shown in Fig. below. 5

6 (a) (b) (c) Figure. (a) An image of the bluff body stabilized combustor, (b) an image of the combustor operating with a lean premixed ethylene-air flame. The border demarcates the region investigated with PLIF and (c) a schematic representation of the laser imaging facility. RESULTS AND DISCUSSION The fluorescence images were processed as follows. Firstly, we subtract a background image, which is taken in the absence of the PLIF signal. Secondly, both fluorescence images are overlapped on a pixel-by-pixel basis. In order to do this, a target image was aligned in the measurement plane defined by the laser sheets and in the field of view of both cameras. From the coordinates of the target images, polynomial warping coefficients were extracted and used to geometrically transform one of the images to overlap with the other. The precision of the image matching was in the sub-pixel range. Finally, beam profiles of each beam, recorded in homogeneous vapours of premixed bi-acetyl and air were used to correct for laser sheet inhomogeneities. After these corrections are made, the pixel-by-pixel product of the simultaneous OH and CH O images is calculated to generate an image which correlates with heat release rate. Fig. 3 shows an instantaneous and averaged images of OH chemiluminescence, OH PLIF, CH O PLIF and the corresponding HR image from the unforced bluff body stabilized combustor at a bulk velocity of 0 m/s and an equivalence ratio, φ =

7 (a) (b) OH* OH PLIF CH O PLIF HR Figure 3. (a) Instantaneous and (b) an average of 75 images of OH*, OH PLIF, CH O PLIF and heat release rate, HR from the unforced bluff body stabilized flame at a bulk velocity of 0 m/s and φ = 0.6. From the instantaneous PLIF images, the spatial structure of the flame front and the distribution of OH, CH O and heat release rate, HR can be clearly seen. The main advantage of the laser-based technique is the ability to spatially visualize the radical species reaction between OH and CH O to give HCO; a species, produced only in the flame front, with a concentration that correlates well with local heat release rates. In the instantaneous OH* chemiluminescence image, the spatial structure of the flame front is not clearly resolved as chemiluminescence radiation is collected from every part of the flame including the flame front. The flame response is investigated by phase-locking the lasers to the input signal to the acoustic drivers. Measurements can be taken at any phase angle throughout the acoustic cycle by varying the delay between the signal to the acoustic drivers and the laser trigger. In this work, the acoustic forcing signal was sinusoidal at a frequency of 60 Hz and the cycle was divided into phases, 0.5 ms apart. At each phase, 75 simultaneous PLIF images of OH and CH O were taken and processed to obtain the corresponding HR images. The instantaneous HR images were subsequently phase averaged to obtain the periodic response of the heat release rate from the flame to the acoustic forcing. Fig. 4 shows a series of phase averaged HR images with the flame perturbed with a high amplitude signal at a frequency of 60 Hz. 7

8 c d Figure 4. Phase averaged images of heat release rate, HR from the forced bluff body stabilized flame at a frequency of 60 Hz and high amplitude, bulk velocity of 0 m/s and φ = 0.6. From image 5 in fig. 4, the effect of the acoustic perturbation manifests as a pair of counter-rotating vortices, which starts at the burner lip and shapes the flame front as they propagate downstream. As the vortex propagates through the flame, heat release rate decreases in regions of high strain and curvature (c), but increases due to the re-circulation of reactants into the hot products (d). From image 6 9, the flame front is straight once again, but the re-circulation of reactants by the tail end of the vortices is still visible. From image 9, the flame front returns to its original location as another pair of vortices starts to form at the bluff body. The periodic nature of the heat release response of the flame to the acoustic forcing can be seen from these phase averaged HR images and the spatial distribution of heat release rate around the counter-rotating vortices is also clearly resolved. The periodic response is more evident from the profile of the spatial average of the phase averaged HR images, over a cycle of period t p. This is shown in fig. 5 below. The result from OH chemiluminescence measurements (OH*) taken simultaneously is also shown for comparison. 6 9 Figure 5. Magnitude of fluctuation of heat-release about its mean value over a cycle of period t p, measured using HR and OH* normalised with their respective mean value. Forcing at high amplitude with frequency of 60 Hz, bulk velocity of 0 m/s, and φ =

9 The magnitude of the fluctuation of heat release rate measured with HR and OH* about its mean value show similar sinusoidal trends in accordance with the forcing signal; however, the magnitude of the fluctuation with HR is larger than that with measured with OH*. This could be due to the insensitivity of OH* in regions of high strain and curvature []. The spatial distribution of heat release rate measured with HR also enables the structural resolution of the flame front through the cycle. A series of similar measurements were carried out for several forcing amplitudes and frequencies. The heat release rates obtained will be analysed to build a library of transfer functions between the heat release response of the flame to acoustic excitation. These transforms are vital to the study, prediction and control of combustion instabilities in LPP combustors. CONCLUSIONS A laser-based scheme to measure spatially resolved heat release rate in turbulent premixed hydrocarbon flames is discussed. This scheme involves probing the OH and CH O radicals as they combine, in a highly exothermic reaction, to form HCO and H O. Using planar laser-induced fluorescence, the simultaneous concentrations of the reacting species are obtained and combined on a pixel-by-pixel basis to generate an image of the HCO radical which correlates well with local heat release rate. The technique was applied in a highly turbulent bluff body stabilized flame subjected to acoustic forcing to mimic the instability phenomenon encountered in LPP combustors. The instantaneous heat release rate images show the spatial distribution of heat release rates at the flame front. However, in the OH chemiluminescence measurements, the flame front is not clearly resolved as signals are collected from different regions of the flame. From the phase averaged heat release rate images, the periodic flame response to the acoustic excitation can be resolved. The flame front is shaped by the counterrotating vortices as they propagate from the burner lip along the flame front. The spatial distribution of heat release rate as the flame front is wrinkled can clearly be seen. The spatially averaged heat release rates obtained from HR and OH* both show cyclic variations in accordance with the sinusoidal forcing signal although the magnitude of the fluctuation is larger for HR than for OH*. This technique will be used to further investigate the origin of the instability phenomenon and the results will be used to develop and test theoretical models employed in the prediction and control of combustion instabilities. 9

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