In-cylinder flow–flame interactions in a production engine via combined endoscopic PIV and high-speed chemiluminescence imaging

Experiments were performed on the central pilot body (RPL-rich-pilot-lean) of Siemens prototype 4th generation DLE burner to investigate the flame behavior at atmospheric pressure condition when varying equivalence ratio, residence time and co-flow temperature. The flame at the RPL burner exit was investigated applying OH planar laser-induced fluorescence (PLIF) and high-speed chemiluminescence imaging. The results from chemiluminescence imaging and OH PLIF show that the size and shape of the flame are clearly affected by the variation in operating conditions. For both preheated and non-preheated co-flow cases, at lean equivalence ratios combustion starts early inside the burner and primary combustion comes to near completion inside the burner if residence time permits. For rich conditions, the unburnt fuel escapes out through the burner exit along with primary combustion products and combustion subsequently restarts downstream the burner at leaner condition and in a diffuse-like manner. For preheated co-flow, most of the operating conditions yield similar OH PLIF distributions and the flame is stabilizing at approximately the same spatial positions. It reveals the importance of the preheating co-flow for flame stabilization. Flame instabilities were observed and Proper Orthogonal Decomposition (POD) is applied to time resolved chemiluminescence data to demonstrate how the flame is oscillating. Preheating has strong influence on the oscillation frequency. Additionally, combustion emissions were analyzed to observe the effect on NOX level for variation in operating conditions.

Understanding thermoacoustic instabilities is essential for the reliable operation of gas turbine engines. To complicate this understanding, the extreme sensitivity of gas turbine combustors can lead to instability characteristics that differ across a fleet. The capability to monitor flame transfer functions in fielded engines would provide valuable data to improve this understanding and aid in gas turbine operability from R&D to field tuning. This paper presents a new experimental facility used to analyze performance of full-scale gas turbine fuel injector hardware at elevated pressure and temperature. It features a liquid cooled, fiber-coupled probe that provides direct optical access to the heat release zone for high-speed chemiluminescence measurements. The probe was designed with fielded applications in mind. In addition, the combustion chamber includes an acoustic sensor array and a large objective window for verification of the probe using high-speed chemiluminescence imaging. This work experimentally demonstrates the new setup under scaled engine conditions, with a focus on operational zones that yield interesting acoustic tones. Results include a demonstration of the probe, preliminary analysis of acoustic and high speed chemiluminescence data, and high speed chemiluminescence imaging. The novelty of this paper is the deployment of a new test platform that incorporates full-scale engine hardware and provides the ability to directly compare acoustic and heat release response in a high-temperature, high-pressure environment to determine the flame transfer functions. This work is a stepping-stone towards the development of an on-line flame transfer function measurement technique for production engines in the field.

<div class="section abstract"><div class="htmlview paragraph">Fuel consumption and NO<sub>x</sub> emissions of gasoline engines at part load can be significantly reduced by Controlled Auto-Ignition combustion concepts. However, the range of Gasoline Controlled Auto-Ignition (GCAI) operation is still limited by lacking combustion stability at low load and by high pressure-rise rates toward higher loads. Previous investigations indicate that the auto-ignition process is particularly determined by the thermodynamic state of the charge and by stratification effects of residual gas, temperature, and air-fuel ratio. However, little experimental data exist on the direct influence of mixture stratification on local ignition and heat-release rate (HRR) in direct-injection (DI) GCAI engines, because it is challenging to measure all the relevant charge and combustion parameters quasi-simultaneously with sufficient spatial/temporal resolution and precision.</div><div class="htmlview paragraph">In the present article, a newly designed laser-diagnostic approach is therefore presented, in which one-dimensional Spontaneous Raman Scattering (SRS) is combined with high-speed chemiluminescence imaging. SRS yields the composition of the charge along a line in terms of air-fuel ratio, exhaust-gas content, and temperature. Thereby, the mixture formation process is analyzed. Additionally, high-speed chemiluminescence imaging is conducted in the same experiment, in order to characterize the ignition and combustion process with relatively high temporal and spatial resolution. In particular, the influence of the three charge parameters, which are measured shortly before combustion, on the ignition process is investigated by exploiting the combined results of both optical techniques.</div><div class="htmlview paragraph">The results show that first auto-ignition in each cycle is basically determined by the gas-phase temperature, which depends on both the residual-gas distribution and local vaporization cooling. Thus, large-scale thermal stratification of the charge leads to a staged combustion process, which progresses from high-temperature to low-temperature regions. However, the progress of combustion and the resulting maximum heat-release rate also depend on the small-scale inhomogeneity in the mixture, which can be controlled by the fuel-injection timing. Furthermore, it is demonstrated that the stability of combustion phasing is dependent on the cyclic variability in the gas-exchange process.</div></div>

Natural gas is a major fuel source for many industrial and power-generation applications. The primary constituent of natural gas is methane (CH4), while smaller quantities of higher order hydrocarbons such as ethane (C2H6) and propane (C3H8) can also be present. Detailed understanding of natural gas combustion is important to obtain the highest possible combustion efficiency with minimal environmental impact in devices such as gas turbines and industrial furnaces. For a better understanding the combustion performance of natural gas, several important parameters to study are the flame temperature, heat release zone, flame front evolution, and laminar flame speed as a function of flame equivalence ratio. Spectrally and temporally resolved, high-speed chemiluminescence imaging can provide direct measurements of some of these parameters under controlled laboratory conditions. A series of experiments were performed on premixed methane/ethane-air flames at different equivalence ratios inside a closed flame speed vessel that allows the direct observation of the spherically expanding flame front. The vessel was filled with the mixtures of CH4 and C2H6 along with respective partial pressures of O2 and N2, to obtain the desired equivalence ratios at 1 atm initial pressure. A high-speed camera coupled with an image intensifier system was used to capture the chemiluminescence emitted by the excited hydroxyl (OH*) and methylidyne (CH*) radicals, which are two of the most important species present in the natural gas flames. The calculated laminar flame speeds for an 80/20 methane/ethane blend based on high-speed chemiluminescence images agreed well with the previously conducted Z-type schlieren imaging-based measurements. A high-pressure test, conducted at 5 atm initial pressure, produced wrinkles in the flame and decreased flame propagation rate. In comparison to the spherically expanding laminar flames, subsequent turbulent flame studies showed the sporadic nature of the flame resulting from multiple flame fronts that were evolved discontinuously and independently with the time. This paper documents some of the first results of quantitative spherical flame speed experiments using high-speed chemiluminescence imaging.

We present dynamics of ignition-stabilized combustion performed in two reactor configurations. The first configuration is a quartz flow channel incorporating a dielectric barrier discharge. The discharge is produced using 10\,ns duration and 23\,kV amplitude repetitive pulses. We observe filamentary plasma in air with gas temperatures below the methane auto-ignition temperature. Repetitive ignition is observed when a methane-air mixture flows through the discharge. From high-speed chemiluminescence imaging, we found that the repetitive ignition frequency depends on ignition-induced dynamics. High-speed images are processed to explore ignition dynamics. Flame front locations and absolute flame speed have been evaluated for various flow speeds and equivalence ratio conditions. The second reactor configuration has two co-axial channels. Four discharge configurations similar to the first setup are located in the outer channel, just below the interaction with the flow from the inner channel. A stable lean combustion is achieved at flow speeds above lean blowout limit.

Role of inertial forces in flame-flow interaction during premixed swirl flame flashback

Fundamental understanding of chemical kinetics pathways involved in ignition and flame propagation in hydrocarbon flames is essential for designing efficient and clean combustion devices for aerospace applications. In this study, spherically propagating methane-air and methane-ethane-air flames inside a constant-volume vessel were characterized using a species-specific, high-speed chemiluminescence diagnostic to reveal the position of the spatially and temporally resolved flame front and the primary combustion zone. The emission recorded by a high-speed camera with an image intensifier was used to investigate the effects of equivalence ratio on the chemiluminescence from electronically excited hydroxyl (OH*) and methylidyne (CH*) radicals. High-speed movies of OH*, CH*, and total broadband signals were recorded. The line-of-sight integrated images were Abel-inverted to obtain the spatially resolved two-dimensional flame structure and the temporal evolution of radical zone thickness. Features such as direct laminar flame-speed determination, flame wrinkling at elevated pressures, and the reaction zones of flames propagating under well-characterized initial turbulent conditions were all well-captured by the high-speed chemiluminescence imaging. In addition to visualizing the flame fronts and subsequent determination of the flame speed, this technique provides the simultaneous information of reactive chemical species that is not available using commonly used schlieren-based imaging methods.

Swirl-stabilized flame is commonly used in gas turbine combustor. Under the operation conditions of interests, such flame is in turbulent regime, exhibiting quasi-periodic dynamic characteristics with one or multiple oscillation modes. We have reported a strategy to investigate the dynamics of the flame. The measurement of the swirling flame in a gas turbine model combustor is carried out by simultaneous planar laser-induced fluorescence (PLIF) and high-speed chemiluminescence (CL) imaging of OH radical, as the former technique is capable of providing highly resolved transient structure of the reaction zone of the flame while the latter is a perfect indicator of the dynamic motion of the flame. The proper orthogonal decomposition (POD) method was applied to both sets of data to unveil the main dynamics mode of both observations. The evolution of the OH* CL mode coefficients provided the information of the oscillation frequency and the momentum of the flame when the PLIF image is taken. Furthermore, the connection of the OH PLIF and OH* CL measurements were investigated by extended proper orthogonal decomposition (EPOD) method. The EPOD analysis showed a strong correlation between the reaction zone at the PLIF plane and the total OH* CL of the flame, enabling the reconstruction of the evolution of the reaction zone. By the application of the strategy above, combustor was tested under different equivalent ratio, and the dynamics of the flame were compared.

Vitiation refers to the condition where the oxygen concentration in the air is reduced due to the mix of dilution gas. The vitiation effects on a premixed methane flame were investigated on a swirl-stabilized gas turbine model combustor under atmospheric pressure. The main purpose is to analyze the combustion stability and CO emission performance in vitiated air and compare the results with the flame without vitiation. The N2, CO2, and H2O (steam) were used as the dilution gas. Measurements were conducted in a combustor inlet temperature of 384 K and 484 K. The equivalence ratio was varied from stoichiometric conditions to the LBO (Lean Blowout) limits where the flame was physically blown out from the combustor. The chemical kinetics calculation was performed with Chemkin software to analyze the vitiation effects on the flame reaction zone. Based on the calculation results, the changes in the temperature gradient, CO concentration, and active radicals across the flame reaction zone were identified. The time-averaged CH chemiluminescence images were recorded and the results indicated the features of the flame shape and location. The CH signal intensity provided the information about the heat-release zone in the combustor. The combustion LBO limits were measured and the vitiation of CO2 and H2O were found to have a stronger impact to elevate the LBO limits than N2. Near the LBO limits, the instability of the flame reaction was revealed by the high-speed chemiluminescence imaging and the results were analyzed by FFT (Fast Fourier Transfer). CO emission was measured with a water-cooled probe which is located at the exit of the combustor. The combustion vitiation has been found to have the compression effect on the operation range for low CO emission. However, this compression effect could be compensated by improving the combustor inlet temperature.

Premixed jet flame behavior in a hot vitiated crossflow of lean combustion products

Combustion and emission characteristics of alcohol fuels in a CAI engine

Direct fuel injection into hot residual gases is an effective means to promote and control the autoignition timings during controlled autoignition combustion of gasoline engines. In order to understand better the underlying physical and chemical processes involved, a systematic experimental study was carried out on a single-cylinder engine with optical access by means of thermodynamic analysis, high-speed chemiluminescence imaging, and in-cylinder sampling-based gas chromatography and mass spectroscopy measurements. The relative effects of enthalpy of evaporation and gas expansion from direct liquid fuel injection were quantified. Alcohol fuels (methanol and ethanol), gasoline, and primary reference fuels were investigated and clarified. Chemiluminescence imaging and heat release analysis demonstrated the presence of fuel reforming reactions during the recompression stroke. In-cylinder gas speciation measurement showed that the fuel pyrolysis was the dominant process. In addition, exothermic fuel reformation reactions of alcohol resulted in an increase in the gas temperature and produced many intermediate species during negative valve overlap period. As a result, the start of main heat release process of alcohol fuel occurred earlier in the subsequent compression stroke.

Lean blowoff behavior of asymmetrically-fueled bluff body-stabilized flames

The competing chemical and physical effects of transient fuel enrichment on heavy knock in an optical spark ignition engine

Experimental and numerical studies were performed to understand the stabilization of lean premixed natural gas/air flames in a gas turbine model combustor which was equipped with a swirl burner, known as the CECOST burner, designed to replicate the flow and flame structures in an industrial gas turbine engine. The operability range, flame stabilization, and flashback were investigated employing simultaneous OH– and CH2O-PLIF, and high-speed chemiluminescence imaging. Large eddy simulation (LES) was carried out for analysis of the vortex breakdown structures under non-reacting conditions. It was found that the vortex breakdown structures under isothermal conditions were insensitive to the Reynolds number (Re) for Re ≥ 10000; however, the stability of the flames and operability range of the burner were highly sensitive to Re as well as to equivalence ratio (ϕ). The equivalence ratio was varied at various Reynolds numbers to observe different regimes of the flame ranging from the lean blowout (LBO) limit to the flashback limit. The LBO limit was found to be mainly a function of equivalence ratio while being nearly independent of the Reynolds number, whereas the occurrence of flashback showed distinct characteristics for different ranges of the Reynolds number. At low and moderate Reynolds numbers, (Re ≤ 17000), flashback occurred when increasing ϕ from lean towards stoichiometric conditions. The coupling between the flow field and heat release induces vortex breakdown in the mixing tube and initiates flashback. In contrast, at higher Reynolds numbers (Re > 17000) no flashback was observed even when ϕ was increased to stoichiometric conditions. At these conditions with high Re, the increase in the bulk flow velocity affects the vortex breakdown structure, pushing the vortex breakdown downstream, which in turn prevents the flame from flashing back into the mixing tube.