Equivalence Ratio Measurements Research Articles

Femtosecond laser-induced plasma spectroscopy for combustion diagnostics in premixed ammonia/air flames.

Ammonia (NH3) is a sustainable fuel with excellent emission characteristics. Hence, it is important to develop diagnostic techniques for NH3 combustion. In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) was performed in premixed NH3/air flames. The plasma emission spectra induced by the femtosecond laser in the flame and the chemiluminescence spectra of the flame itself were both measured. Through calibration, we found that the spectral intensity ratio of NH (336 nm)/N2 (337nm) could be used for equivalence ratio measurements in NH3 combustion flow fields. This work is the first attempt at using a femtosecond laser-based technique for NH3 combustion diagnostics.

Fluorescence characteristics of the fuel tracer 1-methylnaphthalene for the investigation of equivalence ratio and temperature in an oxygen-containing environment

In this work, the fluorescence properties of the fuel tracer 1-methylnaphthalene (1-MN) are investigated in a calibration flow cell at elevated temperatures and pressures. This fuel tracer is well suited to investigate mixture formation in diesel engine applications, as 1-MN is a natural diesel fuel component. A tracer-LIF (laser-induced fluorescence) concept is developed that enables temperature measurements using a two-color approach in oxygen-containing gas mixtures. Furthermore, the potential for simultaneous measurement of equivalence ratio (fuel–air ratio, FAR) is evaluated. First, a picosecond Nd:YLF-laser at 263 nm in combination with a spectrograph and a streak unit was used to investigate the spectral fluorescence emission and fluorescence lifetimes. In addition, a nanosecond Nd:YAG-laser at 266 nm was used for fluorescence calibration with regard to equivalence ratio and temperature. All measurements were performed in an oxygen-containing environment at different equivalence ratios, temperatures up to 800 K and pressures up to 2.5 MPa. The dependency of the fluorescence emission on equivalence ratio was studied for varied fuel amount and air concentrations. The calibration data form the basis for investigations of fuel distribution and temperature under realistic engine conditions in an oxygen-containing environment.

Spark-induced breakdown spectroscopy for fuel-air equivalence ratio measurements at internal combustion engine-relevant conditions

Spark-induced Breakdown Spectroscopy was used for local fuel-air equivalence ratio measurements of premixed methane-air mixtures by a discharge from an inductive ignition system in a constant volume cell. Elemental emissions of hydrogen Balmer-alpha line (Hα), nitrogen (N746) and oxygen (O777) were experimentally observed in the vicinity of the electrode gap, using a lens-coupled spectrometer and an intensified camera. After optimization of the gating strategy, the spectral emission features, i.e. line intensity ratios and peaks width (full width at half maximum) ratios were analyzed and their relation with the local fuel-air equivalence ratio at the spark plug was discussed, for operating pressures ranging from 10 to 20 bar.Results showed that a change in pressure and temperature did not affect the spectral emissions' atomic peaks ratios, as long as the density was kept constant. For tests at constant temperature and at constant pressure, the peaks intensity ratios showed a linear correlation with local fuel-air equivalence ratio, while peaks width did not show noticeable changes. When pressure was varied, while keeping temperature and fuel-air equivalence ratio constant, a different behavior was observed: peaks intensity ratio were only slightly affected by pressure change, more prominently at fuel-rich conditions, while a peak broadening was clearly recognizable, as the peak width linearly increased with pressure.Moreover, an additional optical setup involving a fiber-optic spark plug was implemented with the aim to adapt the proposed diagnostic tool to engines in operando. Comparisons with the lens setup showed that the developed technique is promising as a compact and versatile tool for applications involving local fuel-air equivalence ratio measurements at different ambient conditions.

Instantaneous one-dimensional equivalence ratio measurements in methane/air mixtures using femtosecond laser-induced plasma spectroscopy.

Equivalence ratio is one of the most significant parameters in combustion flow fields. In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) technique for instantaneous one-dimensional local equivalence ratio measurements were performed. By measuring the spatially resolved spectra of FLIPS, we found that the spectral peak area ratios of CH (431 nm)/N2 (337 nm), CH (431 nm)/N2 (357 nm), and CH (431 nm)/O (777 nm) can be utilized to achieve one-dimensional local equivalence ratio measurements. Among them, the CH peak at ~431 nm and the O peak at ~777 nm are strong enough to be used to achieve single-shot measurements, which is important to turbulent flow fields. Furthermore, systematic experiments were performed by using FLIPS in both laminar and turbulent flow fields. The FLIPS technique features the abilities of instantaneous one-dimensional quantitative measurements, high spatial resolution, and no Bremsstrahlung interference.

Lean partially premixed turbulent flame equivalence ratio measurements using laser-induced breakdown spectroscopy

The creation of a more stable flame along with the extension of flammability limits under lean mixture combustion was the main motivation to develop a new burner design, which has been investigated in this research. The current burner configuration was utilized to create a wide range of higher turbulent intensities and to produce different degrees of mixture inhomogeneity, which acted to promote minimum pollution, highest performance and higher flame stability. The burner stability assessment was investigated using two types of fuel: natural gas (NG) and liquefied petroleum gas (LPG). They were tested under different degrees of partial premixing, and two turbulence generator disks for lean mixture at an equivalence ratio of φ = 0.8 were used. Following this, the Laser Induced Breakdown Spectroscopy (LIBS) technique was utilized to characterize and quantify the impact of changing the disk slit diameter on the distributions profiles of equivalence ratio or mixture fraction for a NG/air partially premixed flame. A series of homogeneous NG/air mixtures with different equivalence ratios were used to obtain the correlations between the measured emission lines of LIBS spectra and the global flame equivalence ratio. Consequently, the emission spectral lines ratios of H/N, H/O and C/N + O were utilized to predict the equivalence ratio distributions. The results demonstrated that for all of the mixing lengths, NG/air mixture with larger disk generator diameter yielded the maximum burner stability, whilst the LPG/air mixture with a larger disk generator diameter resulted in the minimum burner stability. Furthermore, the flame associated with the larger disk slit diameter had a uniform local equivalence ratio distribution and lower RMS fluctuation profiles of equivalence ratio in comparison to the lower disk slit diameter.

High-Speed Rainbow Schlieren Deflectometry of n-Heptane Sprays Using a Common Rail Diesel Injector

More precise measurements of the fuel injection process can enable better combustion control and more accurate predictions resulting in a reduction of fuel consumption and toxic emissions. Many of the current methods researchers are using to investigate the transient liquid fuel sprays are limited by cross-sensitivity when studying regions with both liquid and vapor phases present (i.e., upstream of the liquid length). The quantitative rainbow schlieren technique has been demonstrated in the past for gaseous fuel jets and is being developed here to enable study of the spray near the injector. In this work, an optically accessible constant pressure flow rig (CPFR) and a modern common rail diesel injector are used to obtain high-speed images of vaporizing fuel sprays at elevated ambient temperatures and pressures. Quantitative results of full-field equivalence ratio measurements are presented as well as more traditional measurements such as vapor penetration and angle for a single condition (13 bar, 180 °C normal air) using n-heptane injected through a single hole (0.1 mm diameter) common rail fuel injector at 1000 bar fuel injection pressure. This work serves as a proof of concept for the rainbow schlieren technique being applied to vaporizing fuel sprays, and full details of the image-processing routine are provided. The ability of the imaging technique combined with the constant pressure flow rig make this approach ideal for generating large data sets in short periods of time for a wide range of operating conditions.

Equivalence Ratio Measurement in Fuel Rich Combustion by Laser-Induced Breakdown Spectroscopy

Equivalence Ratio Measurement in Fuel Rich Combustion by Laser-Induced Breakdown Spectroscopy

Impact of plasma dynamics on equivalence ratio measurements by laser-induced breakdown spectroscopy

The systematic errors introduced by triggering a USB spectrometer for laser-induced breakdown spectroscopy equivalence ratio measurements are studied. We analyze the temporal behavior of laser-induced plasma in a nonreacting methane/air mixture and investigate the influence of the dynamics on equivalence ratio measurements with gated and ungated detection. For use of gated detectors, optimal delay times were found to be between 500 and 2000 ns to allow effective suppression of interferences while maintaining sufficient signal-to-noise levels. Good precision was found for short and long exposure time intervals when an intensified CCD camera was employed. On the other hand, the jitter of an externally triggered ungated spectrometer leads to high uncertainties. Running the ungated spectrometer freely, the single-shot uncertainty can be reduced by more than 1 order of magnitude.

Porous Inserts for Passive Control of Noise and Thermo-Acoustic Instabilities in LDI Combustion

In this study, ring-shaped porous inserts made from high-strength, temperature-resistant ceramic materials are utilized within the combustor to passively reduce noise and thermo-acoustic instabilities in lean direct injection (LDI) combustion. Inserts of 85% porosity; pore density of 18 pores per cm; and converging, diverging, and hyperbolic configurations are considered. Kerosene fuel atomized by an air-blast fuel injector is introduced in the swirl-stabilized combustor operated at atmospheric pressure. For a fixed equivalence ratio, emissions and acoustics measurements are obtained at different heat release rates and atomizing air to liquid mass ratios. Results show negligible effects of porous inserts on NOx and CO emissions at the combustor exit. Porous inserts are, however, very effective in suppressing both noise and thermo-acoustic instabilities for a range of operating conditions. The measured trends in acoustic absorption are predicted using a simplified thin-layer model of the porous material, and an important flow mechanism to explain the observed trends is discussed.

Combustion Diagnostics with Femtosecond Laser Radiation

In the present work, the potential of Laser Induced Breakdown Spectroscopy employing femtosecond laser pulses (fs-LIBS) for fuel-air equivalence ratio measurements in premixed methane-air and propane-air flames is presented. A Ti-Sapphire laser system (100 fs, 10 Hz, 800 nm) was used as an excitation source for the plasma creation, while a spectrometer was employed to record the plasma emission spectra. The concentration of the investigated methane-air and propane-air mixtures were expressed by the fuel mole fraction Xfuel and varied from only air - Xfuel=0 (ϕ=0) to only fuel - Xfuel=1 (ϕ=∞). The spectral characteristics of the fs-LIBS spectra are discussed, while the time and energy dependence of the main spectral features are presented. Moreover, from the analysis of fs-LIBS spectra collected at different fuel mole fractions Xfuel, it was found that the fuel variations could be very well correlated with the variation of the intensity of some spectral lines and/or their ratios. The prepared calibration curves of the fuel mole fraction Xfuel versus the atomic line total intensity ratios (Hα 656.3 nm and O (I) 777 nm) and molecular lines total intensity ratios (C2 516.5 nm and CN 388.3 nm) suggest the high potential of using fs-LIBS for the determination of the local fuel concentration and its temporal variations.

Modeling the Ignitability of a Pilot Injection for a Diesel Primary Reference Fuel: Impact of Injection Pressure, Ambient Temperature and Injected Mass

In this paper, we studied the accuracy of computational modeling of the ignition of a pilot injection in the Sandia National Laboratories (SNL) light-duty optical engine facility, using the physical properties of a cetane/iso-cetane Diesel Primary Reference Fuel (DPRF) mixture and the reaction kinetics of a well-validated mechanism for primary reference fuels. Local fuel-air equivalence ratio measurements from fuel tracer based planar laser-induced fluorescence (PLIF) experiments were used to compare the mixture formation predictions with KIVA-ERC-based simulations. The effects of variations in injection mass from 1 mg to 4 mg, in-cylinder swirl ratio, and near-TDC temperatures on non-combusting mixture preparation were analyzed, to assess the accuracy of the model in capturing average jet behavior, despite its inability to model the nonnegligible jet-by-jet variations seen in the experiments. Fired simulations were able to capture well the measured ignitability trends at the different injection conditions tested, but showed some deviations in the minimum temperature needed for robust ignition, pointing out the need for further work to focus on achieving fully comprehensive modeling with detailed chemical kinetics of the DPRF58 mixture and a full engine geometry representation.

Fuel concentration imaging inside an optically accessible diesel engine using 1-methylnaphthalene planar laser-induced fluorescence

The performance and emissions of modern automotive diesel engines are highly dependent on the application of pilot injection technology. This technology also appears well suited for application to low-temperature combustion strategies. In this study, the first results of a new quantitative planar laser-induced fluorescence equivalence ratio measurement technique of pilot injections inside an optically accessible diesel engine are presented using 1-methylnaphthalene as a tracer in a mixture of the diesel primary reference fuels, n-hexadecane (cetane) and 2,2,4,4,6,8,8-heptamethylnonane (iso-cetane). This combination overcomes the shortcomings of mismatched fuel volatility and density associated with commonly used toluene/n-heptane/iso-octane planar laser-induced fluorescence techniques. A tracer characterization in a flow cell and a calibration in the internal combustion engine are performed. The internal combustion engine measurements illustrate the mixture formation process for a pilot injection. Even at low injection mass of 3 mg, a strong penetration of the pilot is observed; fuel hits the piston bowl wall and is redirected upward to the cylinder head. Small amounts of fuel are also found to have penetrated into the bottom of the piston bowl. At top dead center, the pilot injection is still not completely homogeneously distributed in the piston bowl, and local equivalence ratios of Φ > 1 are found in the bowl.

On the ignition and flame development in a spray-guided direct-injection spark-ignition engine

High-speed fuel, flow, and flame imaging are combined with spark discharge measurements to investigate the causes of rare misfires and partial burns in a spray-guided spark-ignited direct-injection (SG-SIDI) engine over a range of nitrogen dilution levels (0–26% by volume). Planar laser induced fluorescence (PLIF) of biacetyl is combined with planar particle image velocimetry (PIV) to provide quantitative measurements of equivalence ratio and flow velocity within the tumble plane of an optical engine. Mie scattering images used for PIV are also used to identify the enflamed region to resolve the flame development. Engine parameters were selected to mimic low-load idle operating conditions with stratified fuel injection, which provided stable engine performance with the occurrence of rare misfire and partial burn cycles. Nitrogen dilution was introduced into the intake air, thereby displacing the oxygen, which destabilized combustion and increased the occurrence of poor burning cycles. Spark measurements revealed that all cycles exhibited sufficient spark energy and duration for successful ignition. High-speed PLIF, PIV, and Mie scattering images were utilized to analyze the spatial and temporal evolution of the fuel distribution and flow velocity on flame kernel development to better understand the nature of poor burning cycles at each dilution level. The images revealed that all cycles exhibited a flammable mixture near the spark plug at spark timing and a flame kernel was present for all cycles, but the flame failed to develop for misfire and partial burn cycles. Improper flame development was caused by slow flame propagation which prevented the flame from consuming the bulk of the fuel mixture within the piston bowl, which was a crucial step to achieve further combustion. The mechanisms identified in this work that caused slower flame development are: (1) lean mixtures, (2) external dilution, and (3) convection velocities that impede transport of the flame into the fuel mixture.

A comparison of multivariate LIBS and chemiluminescence-based local equivalence ratio measurements in premixed atmospheric methane–air flames

In the present work, a comparative study of fuel–air equivalence ratio measurements in premixed atmospheric methane–air flames with ungated laser-induced breakdown spectroscopy (LIBS) and natural chemiluminescence emission spectroscopy is presented. With ungated LIBS, the strong elastically scattered laser light greatly reduces the measurement dynamic range and sensitivity. For effectively suppressing the elastically scattered laser light in ungated LIBS detection, a polarizer and a notch filter in the signal collection system were tested. It was observed that the polarizer performed better compared to the notch filter in suppressing the elastically scattered laser light. The LIBS spectra collected at ten different equivalence ratios ranging from 0.74 to 1.42 were used to develop the equivalence ratio calibration model using atomic line peak intensity ratios (for H656nm, N500nm+, and O777nm) as well as broad spectral features with a multivariate, partial least squares regression (PLS-R) approach. The developed calibration models were tested with an unknown data set comprising LIBS spectra taken at similar experimental conditions used in developing the calibration model. The LIBS-based prediction results were also compared with chemiluminescence-based predictions from the same experimental setup. It was found that the LIBS-based calibration provides a more robust measurement approach with better predictions of equivalence ratios compared to the chemiluminescence-based calibration.

Comparison of Quantitative In-Cylinder Equivalence Ratio Measurements with CFD Predictions for a Light Duty Low Temperature Combustion Diesel Engine

Comparison of Quantitative In-Cylinder Equivalence Ratio Measurements with CFD Predictions for a Light Duty Low Temperature Combustion Diesel Engine

Laser induced breakdown spectroscopy for local equivalence ratio measurement of kerosene/air mixture at elevated pressure

The quantitative local equivalence ratio measurements of premixed kerosene/air gas at elevated pressure by the laser induced breakdown spectroscopy (LIBS) technique were preformed in a high temperature test cell, which was designed and manufactured for simulated supersonic combustion state. According to in-situ experimental conditions in supersonic combustion, the correlations between the values of emission intensity ratio of H/O, H/N and the equivalence ratio of premixed kerosene/air mixture in different gas pressures was established. The dependence of the FWHM of Hα on pressure of the mixture was also measured. Therefore, the laser diagnostic method for quantitative local equivalence ratio measurements of kerosene/air mixture by the LIBS technique in supersonic combustion was developed in laboratory. The fundamental data was prepared for on-line quantitative local equivalence ratio measurements of kerosene/air mixture in supersonic combustion experiments.

Measurement of Equivalence Ratio in a Light-Duty Low Temperature Combustion Diesel Engine by Planar Laser Induced Fluorescence of a Fuel Tracer

Measurement of Equivalence Ratio in a Light-Duty Low Temperature Combustion Diesel Engine by Planar Laser Induced Fluorescence of a Fuel Tracer

Experimental study of signal trapping of OH laser induced fluorescence and chemiluminescence in flames

The absorption of OH∗ chemiluminescence and laser-induced fluorescence (LIF) in the exhaust gas of confined premixed laminar CH4/air flames at atmospheric pressure was investigated. One flame was used as source and a second as absorber. OH LIF was excited in the ν″=0→ν′=1 band of the A–X electronic system around ≈283 nm and spectrally resolved detected in the (0,0) and (1,1) vibrational bands around 305–320 nm. For OH∗ chemiluminescence, spectrally resolved detection was performed in the wavelength range 280–340 nm. For an absorption path of 54 mm and at T≈2000 K, signal trapping on the order of 10–40% was observed. Signal trapping was most pronounced in the (0,0) band, as expected from the thermal population distribution of OH in the electronic ground state. The spectral distribution of the signals and the wavelength dependence of the signal trapping are addressed in this paper. Implications from the results with respect to detection strategies and chemiluminescence-based equivalence ratio measurements are discussed.

Feasibility of laser-induced plasma spectroscopy for measurements of equivalence ratio in high-pressure conditions

In this paper, experimental results obtained with laser-induced plasma spectroscopy to retrieve local compositions are presented for an ambient pressure up to 5.0 MPa in a still cell. Well-controlled mixtures of gases are introduced and plasma is obtained with the fundamental emission of a pulsed Nd:YAG laser. Simultaneously, plasma shape and spectrally resolved data are taken with a temporal resolution down to 2 ns. First, the temporal evolutions of a high-pressure nitrogen plasma are analyzed as function of spark energy. It is shown that plasma changes orientation from an elongated shape parallel to the laser line to a perpendicular one in a very short time. Results are reported for both spatial and spectral variations. Afterward, the effects of increased carbon concentration are discussed in both shape and spectra. It is seen that strong intensity due to the atomic carbon emissions appear for the high-pressure case. From those experiments, calibration strategies are proposed to get equivalence ratio under high-pressure conditions with a ratio of carbon versus nitrogen and oxygen. The delay between plasma and measurements is set to 2,000 ns and the signal is integrated for 5,000 ns, so as to yield a good signal to noise ratio and a good sensitivity of the technique to changes in mixture fraction. Calibration curves are reported for equivalence ratio up to 1.00 and for pressure from 1.0 to 5.0 MPa. It is shown that typical uncertainties are limited to 7.5% regardless the equivalence ratio in a single shot approach using a spectral fit procedure, whereas it accounts to two times more in a more classical peak ratio approach. Increasing the pressure tends to increase the precision as lower pressure had higher uncertainties.

Real-time equivalence ratio measurements in gas turbine combustors with a near-infrared diode laser sensor

A rapid (2kHz) near-infrared tunable diode laser (TDL) absorption sensor is developed for real-time measurements of methane concentration (and thus equivalence ratio) in gas turbine combustors. The non-intrusive sensor is based on fixed-wavelength laser absorption of methane near 1.65μm using a single fiber-coupled diode laser. This CH4 absorption wavelength is free of interference from the absorption of other species in air. Wavelength modulation spectroscopy (WMS) is combined with second-harmonic detection to improve the sensor sensitivity and accuracy. WMS-1f signal is used to normalize the 2f signal to remove the need for calibration and account for the laser transmission variations in gas turbine combustors. The near-IR TDL sensor is first calibrated in a static cell with CH4 and N2 mixture to determine the line-strength and find the laser setpoint. Test with a gas turbine combustor operating at atmospheric pressure is then conducted to demonstrate the sensor accuracy and response time. The TDL sensor is used to characterize equivalence ratio fluctuations in the combustor before the fuel/air mixtures enter the combustion chamber. The gain between heat release and equivalence ratio fluctuation is measured when the fuel flow rate is modulated at different levels. The combustion test results demonstrate the utility of the TDL sensor as a useful diagnostic tool to study flame characteristics and evaluate different combustor designs.