Search

The temporal development of the mixing field in a decaying jet (Re = 50,000) was quantified by measuring mole fraction and scalar dissipation rate (SDR) in a decaying, isothermal, turbulent gaseous jet. The 2D scalar field was measured by using planar laser induced fluorescence of acetone and, with appropriate image processing, this allowed estimation of the SDR using the two in-plane components within 16% error. The instantaneous and averaged distributions of the mole fraction are reported for downstream dimensionless distances up to 7 nozzle exit diameters and 35 exit flow timescales after end of injection. With advection of the last uniform exit concentration (UEC) profile core away from the nozzle exit, a region of weak concentration arises at the decaying jet's trailing edge. Estimates made in a Lagrangian frame of reference show that the trailing edge of the jet becomes leaner, after the end of injection (AEI), faster than in the steady state, confirming the existence of an ‘entrainment wave’. The normalised probability density functions of the 2D SDR at various stations and times AEI differ from a lognormal distribution at both low and high SDR values with negative skewness and positive excess kurtosis. A pseudo 3D SDR, made by including an estimate for the out of plane component, showed reduced departure from lognormal. The departure may be attributed to the disappearance of the strong shear layer associated with the absence of nozzle momentum AEI. To the authors’ knowledge, this study provides the first measurements of the SDR in a decaying, isothermal turbulent jet.

The effects of inert diluents in the fuel mixture of a model swirl stabilized gas turbine combustor, under thermoacoustically unstable limit cycle operation, were studied experimentally. The measurements included particle image velocimetry (PIV), high-speed CH* chemiluminescent imaging, and dynamic pressure signals. The paper focuses on the dynamic phenomenon of the period doubling bifurcation, which came about when the equivalence ratio ϕ of undiluted flames was enriched from 0.55 to 0.60 under constant Reynolds number (Re=18,000). The bifurcation featured an emergence of an aerodynamically related timescale in addition to the fundamental timescale that was induced from an unstable acoustic eigenmode. The aerodynamic timescale is introduced by azimuthal convection of a high-heat-release-rate region and is linked in the literature with a precessing motion of the recirculation zone. It was found that on increasing the diluent molar fraction up to 50%, the amplitude and the frequency of the limit cycle fundamental acoustic mode decreased. A mechanism to interpret this suppression is that increasing the diluent molar fraction of the fuel makes the flame more susceptible to quenching because the extinction strain rate of the mixture is decreased. The paper argues that the existence of inert diluents in the fuel can significantly alter the dynamic state of the combustor, because the anchoring locations of the flame greatly depend on the composition-sensitive extinction strain rate of the mixture.

The effects of inert diluents in the fuel mixture of a model swirl stabilized gas turbine combustor, under thermoacoustically unstable limit cycle operation, were studied experimentally. The measurements included Particle Image Velocimetry (PIV), high speed CH* chemiluminescent imaging and dynamic pressure signals. The paper focuses on the dynamic phenomenon of the period doubling bifurcation, which came about when the equivalence ratio (Ø) of undiluted flames was enriched from 0.55 to 0.60 under constant Reynolds number (Re). The bifurcation featured an emergence of an aerodynamically related timescale in addition to the fundamental timescale which was induced from an unstable acoustic eigenmode. The aerodynamic timescale is introduced by azimuthal convection of a high heat release rate region, and is linked in the literature with a precessing motion of the recirculation zone. Prior to the bifurcation, the flame anchored between the wall and the outer shear layers of the recirculation zone assuming a V-shape. The dynamics were attracted to a Period-1 limit cycle. Post the bifurcation the flame expanded in the inner shear layers of the recirculation zone assuming an M-shape, while the dynamics were attracted to a Period-2 limit cycle. The latter operational condition was subject to nitrogen dilution in order to parametrically increase the molar fraction of inert diluent in the fuel stream. It was found that on increasing the diluent molar fraction the amplitude and the frequency of the limit cycle fundamental acoustic mode decreased. Also, inert dilution suppressed the aerodynamic timescale and the flame assumed a V-shape again. A mechanism to interpret this mechanism is suggested. Increasing the diluent molar fraction of the fuel makes he flame susceptible to quenching because the extinction strain rate of the mixture decreased. Thus, the flow imposed strain rates quenched the flame when it attempted to anchor on the inner shear layers of the vortex breakdown induced recirculation zone. The inability of the flame to anchor on the shear layers suppressed the aerodynamic mode. The paper argues that the existence of inert diluents in the fuel can significantly alter the dynamic state of the combustor, since the anchoring locations of the flame greatly depend on the composition-sensitive extinction strain rate of the mixture.

Homogeneous Charge Compression Ignition (HCCI) engines deliver high thermal efficiency and, therefore, low CO 2 emissions, combined with low NO X and particulate emissions. However, HCCI operation is not possible at all conditions due to the inability to control the autoignition process and new understanding is required. A high-swirl low-compression-ratio, optically accessed engine that can produce overall fuel lean, axially stratified charge (richer fuel mixture close to the cylinder head was achieved using port injection against open valve and homogeneous mixture during injection against closed valve timing) was operated in HCCI mode without and with spark-assist mixture ignition. The present study investigates the differences in the HCCI autoignition process and the propagation of the autoignition front with homogeneous mixture or fuel charge stratification, internal Exhaust Gas Recirculation (iEGR) (introduced by utilizing different camshafts) and spark-assisted iEGR lean combustion. In order to visualize the HCCI process, chemiluminescence flame images, phase-locked to a specific crank angle, were acquired. In addition, time-resolved images of the developing autoignition flame front were captured. Proper Orthogonal Decomposition (POD) was applied to the acquired images to investigate the temporal and spatial repeatability of the autoignition front and compare these characteristics to the considered scenarios. The eigenvalues of the POD modes provided quantitative measure of the probability of the corresponding flame structures. The first POD mode showed higher probability of single autoignition sites originating from a particular location (depending on the scenario). However, the contribution from other modes cannot be neglected, which signified multiple locations of the single autoignition and also, multiple sites of self-ignition of the fuel-air mixture. It was found that increasing iEGR resulted in random combustion (multiple autoignition sites and fronts), which, however, became significantly non-random due to addition of spark-assisted ignition. It was identified in the POD analysis of the time-resolved flame images that the presence of inhomogeneity either in the temperature or the mixture fraction distribution increases the probability of random combustion during the very early stages of flame development. Thus, the fluctuations of heat release is higher during this period.

In the current paper, time resolved high speed optical Particle Image Velocimetry and CH* chemiluminescence measurements were performed, to study self-excited limit cycle combustion instabilities in a swirl stabilized model gas turbine combustor operating at atmospheric pressure with choked propane and air flow supplies. The combustor was operated under a constant Reynolds number (Re=22,000) and four equivalence ratios, namely ϕ = 0.50 for operation susceptible to extinction, and ϕ=0.55, ϕ=0.60 and ϕ=0.65 for operation under a thermoacoustically unstable combustion regime, to encounter two limit cycle dynamic states. The period-1 limit cycle was driven by thermoacoustic coupling between the acoustic and the thermal field at a fundamental timescale dictated by an acoustic eigenmode of the combustor. The period-2 limit cycle, further to the fundamental acoustic timescale featured a subharmonic aerodynamic signature in the heat release rate and dynamic pressure spectra caused by the helical coherent structure of a Precessing Vortex Core (PVC). Previous studies have shown that the PVC in the limit cycle regime may be suppressed by the temperature stratification at the inlet of the combustor. A mechanism is suggested to interpret the flame anchoring locations which effectively regulated whether PVC was excited or suppressed. It is showed that the conditions under which the flame attached to the centerbody and suppressed the PVC can be explained by the spatial distribution of the relative ratio of the flow imposed to the mixture extinction strain rate. The PVC was excited due to local extinction by aerodynamic straining at the inlet of the combustor, at the phase angle of maximum dynamic pressure. On increasing the equivalence ratio, the flame became robust to aerodynamic straining and flashed back at the phase angle of maximum dynamic pressure. The PVC was then suppressed due to the relative ratio of the flow imposed to the extinction strain rate, which allowed the establishment of swirl-damping temperature gradients at the combustor inlet. The paper underlines the importance of quantifying the relation between the flow imposed and extinction strain rate, as it largely dictates the eventual combustor limit cycle dynamic state and its resonant frequencies.

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, C∗2 and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

The current paper studies the thermoacoustically unstable combustion, under elevated mean pressure, of a commercial swirl stabilized gas turbine burner fitted with optically accessible windows. The considered measurements include particle image velocimetry (PIV), OH∗ chemiluminescence imaging, high speed broadband flame imaging and dynamic pressure signals. We study cases A and B, wherein natural gas flames at mean pressures equal to 3 bar and 6 bar delivered thermal loads equal to 335 kW and 685 kW respectively. The flow field demonstrated a typical vortex breakdown induced inner recirculation zone and a sudden step expansion induced outer recirculation zone. In case A, high amplitude dynamic pressure bursts were observed amidst a quiescent acoustic background. The flame was conical, it anchored on the shear layers of the recirculation zone and it periodically expanded in the outer recirculation zone (ORZ). In case B, the flame was consistently thermoacoustically unstable with seldom requiescent events, while at the same time expansion to the ORZ was suppressed. By applying Dynamic Mode Decomposition on high speed images of case A, it was showed that this expansion introduced an additional time scale, further to the fundamental acoustically related timescale. The superposition of two timescales over a turbulent background established an intermittent regime of thermoacoustic instabilities, wherein the dynamics transitioned between quiescent and fully oscillatory. A physical mechanism is suggested to explain the differences between the flame shapes on adjusting mean pressure. The mechanism considers that the premixture is characterized by a Lewis number lower than unity, the laminar flame speed increases on decreasing mean pressure and the flow imposed on the flame strain rate oscillated over a period of thermoacoustic instability. This combination resulted in oscillatory heat release rate, in the region of the outer shear layers. The phenomenon was more pronounced in case A than in case B, because flow dilatation imposed-strain rates are higher for the former than for the latter flame. The paper argues that for a given fuel, elevated mean pressure introduces time scales that can significantly affect the dynamic regime the combustor operates in.

A novel velocimetry method is proposed for point velocity measurement, which is based on tracking a laser-induced plasma in a flow. The plasma’s behaviour is first analysed spatially, temporally and spectrally in quiescent air. The dependence of this technique on the delay time between subsequent plasma images and the processing methods are described. It is found that, for optimized operation of the technique in a turbulent air jet (exit diameter 10.0 mm from a 480 mm long pipe; with an averaged velocity of 50 m/s at the jet exit resulting in Reynolds number of 34,000) with 100 µs time delay between plasma images, the systematic and random components of the velocity uncertainty are − 0.51 m/s and ± 3.6 m/s along the laser beam direction, and 1.25 m/s and ± 0.86 m/s along other directions perpendicular to the laser beam. These uncertainties are mainly caused by the asymmetric laser energy deposition during the formation of plasma, and the associated spatial resolution (in this realisation of the instrument) of 5 mm. The mean velocity measurements in the turbulent air jet flow are consistent with the reported flow behaviour in the literature for mean velocity: the turbulent intensity of axial velocity fluctuations is comparable to those in the literature but difference arises due to the limited spatial resolution. This velocimetry method is an alternative to traditional tracer-based velocimetry methods, because it does not require ‘seeding’ of particles or other substances in the flow. It also has the ability to measure local gas mixture composition, using laser-induced breakdown spectroscopy approach, simultaneously with flow velocity, but this aspect is not explored in the current study. Concept of Laser-Induced Plasma Image Velocimetry (LIPIV) applied to a turbulent jet flow. The LIPIV technique measures the flow velocity vector by using the temporal displacement of the laser-induced plasma in a flowing fluid. For this reason, two sequential images of the plasma with time delay Δt are used.

The present work investigates the effect of the injection dwell time and injection pressure on soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine operated under a double-injection scheme under low load and low engine speed conditions. The conducted experiments considered two different dwell times for three different injection pressures. The fuel quantity of the main injection was adjusted to maintain the same indicated mean effective pressure (IMEP) value among all cases considered. Findings were analyzed via means of pressure trace and apparent heat release rate (AHRR) analyses, as well as a series of optical diagnostics techniques, namely high-speed imaging and planar laser-induced incandescence (pLII). The combination of dwell time and injection pressure substantially affects charge reactivity and soot oxidation potential. The analysis suggests that a shorter dwell time combined with a higher injection pressure can lead to an enhanced potential for engine-out particulate reduction by creating an in-cylinder environment that promotes soot oxidation. Overall, results indicate that a close-coupled pilot and main injection scheme can reduce soot levels, albeit while increasing specific fuel consumption by up to 12% to maintain the same engine power output levels.

The interaction between droplet dispersion and evaporation in an acetone spray evaporating under ambient conditions is experimentally studied with an aim to understand the physics behind the spatial correlation between the local vapour mass fraction and droplets. The influence of gas-phase turbulence and droplet–gas slip velocity of such correlations is examined, while the focus is on the consequence of droplet clustering on collective evaporation of droplet clouds. Simultaneous and planar measurements of droplet size, velocity and number density, and vapour mass fraction around the droplets, were obtained by combining the interferometric laser imaging for droplet sizing and planar laser induced fluorescence techniques (Sahu et al., Exp. Fluids, vol. 55, 1673, 2014b, pp. 1–21). Comparison with droplet measurements in a non-evaporating water spray under the same flow conditions showed that droplet evaporation leads to higher fluctuations of droplet number density and velocity relative to the respective mean values. While the mean droplet–gas slip velocity was found to be negligibly small, the vaporization Damköhler number ($Da_{v}$) was approximately ‘one’, which means the droplet evaporation time and the characteristic time scale of large eddies are of the same order. Thus, the influence of the convective effect on droplet evaporation is not expected to be significant in comparison to the instantaneous fluctuations of slip velocity, which refers to the direct effect of turbulence. An overall linearly increasing trend was observed in the scatter plot of the instantaneous values of droplet number density ($N$) and vapour mass fraction ($Y_{F}$). Accordingly, the correlation coefficient of fluctuations of vapour mass fraction and droplet number density ($R_{n\ast y}$) was relatively high (${\approx}0.5$) implying moderately high correlation. However, considerable spread of the $N$ versus $Y_{F}$ scatter plot along both coordinates demonstrated the influence on droplet evaporation due to turbulent droplet dispersion, which leads to droplet clustering. The presence of droplet clustering was confirmed by the measurement of spatial correlation coefficient of the fluctuations of droplet number density for different size classes ($R_{n\ast n}$) and the radial distribution function (RDF) of the droplets. Also, the tendency of the droplets to form clusters was higher for the acetone spray than the water spray, indicating that droplet evaporation promoted droplet grouping in the spray. The instantaneous group evaporation number ($G$) was evaluated from the measured length scale of droplet clusters (by the RDF) and the average droplet size and spacing in instantaneous clusters. The mean value of $G$ suggests an internal group evaporation mode of the droplet clouds near the spray centre, while single droplet evaporation prevails near the spray boundary. However, the large fluctuations in the magnitude of instantaneous values of $G$ at all measurement locations implied temporal variations in the mode of droplet cloud evaporation.

This study reports on the use of fractal grids as a new type of turbulence generators in premixed combustion applications. Fractal grids produce turbulence fields which differ from those formed by regular turbulence generators such as perforated plates or meshes. Fractal grids generate high turbulence intensities over an extended region some distance downstream of the grid with a comparatively small pressure drop. Additionally, the integral scale of the flow does not change downstream of the grid. The extended region of high turbulence can also be optimized for the specific application at hand by changing certain parameters of the grid which makes it possible to design the downstream development of the turbulence field. Four space-filling fractal square grids were designed to independently vary the resulting turbulent field and a regular square mesh grid with similar turbulent intensity acted as a reference case. The structure of the resulting premixed V-shaped flames was investigated using Conditioned Particle Image Velocimetry (CPIV). At the same downstream position, flames in the turbulence field of fractal grids showed larger turbulent burning velocity compared to flames in regular grid generated turbulence. However, when compared for the same turbulence intensity, flames in fractal grid generated turbulence produced similar turbulent burning velocities compared to flames in regular grid generated turbulence. In particular, it could be shown that theories such as Taylor’s theory of turbulent diffusivity and Damkohler’s theory of premixed flame propagation, which were deduced from regular turbulence fields, adequately described the increase of effective flame surface area due to the increase in turbulence intensity. Using fractal grids allows the independent variation of the turbulent fluctuations, the integral length scale and the turbulent Reynolds number. An unexpected finding was that the burning velocity ratio, s t / s l was negligible influenced by the integral length scale. A correlation between the burning velocity ratio, s t / s l , and the normalized velocity fluctuations of the flow, u ′/ s l , showed a negligible influence of the integral scale on the turbulent burning velocity. A literature review revealed that the influence of the integral scale on the turbulent burning velocity is still unclear and further research is required. In this context, fractal grids are particularly helpful as they cover a wider range of integral length scales for sufficiently turbulent flows, u ′ ⩾ s l , compared to regular grids.

The liquid flow inside, and the induced air flow around, a falling droplet in stagnant air was numerically investigated using the volume of fluid method to describe the droplet interface. The droplet consisted of oil with the same surface tension and with viscosity as parameter. It was injected into stagnant air with an initial velocity of 1 m/s; therefore, the initial Weber (We = 0.14), Reynolds (Re = 141), and Bond (Bo = 2.4) numbers remained constant during the parametric study whilst the initial Capillary (Ca) and Ohnesorge (Oh) numbers varied by an order of magnitude from 0.46 to 4.6 and from 0.044 to 0.44, respectively. We examined the effect of viscosity on the flow inside, and around, the droplet as well as on the droplet deformation and its natural frequency. This investigation showed a strong dependence of the deformation with liquid viscosity. Specifically, the droplets achieved their final deformation in under-damped, for low viscosity, and in over-damped, for high viscosity, oscillation modes. After a critical time tcrit (or Recrit), the instantaneous air flow symmetry was disturbed, initially in the wake and soon after in the interior of the droplet and in the vortex shedding downstream of the droplet. The air flow in the wake region detached from the droplet surface and resulted in a wake which was approximately 1.5 times longer and wider than the wake behind a solid sphere at the same Re number at steady state conditions. A roller-vortex structure (called rollex) was established upon injection in the immediate wake of the droplet, forming the necessary kinematic link between the directions of the internal circulation in the droplet (Hill vortex) and of the external recirculating air flow in the droplet’s wake. The droplet drag coefficients were compared with corresponding values used in droplet breakup models: although, ultimately, the droplet drag coefficient converged to the values given by the models, the initial magnitudes after injection were incorrect.

[EN] This paper studies the effect of liquid viscosity on the atomisation regimes of initially spherical and non-spherical droplets and also kinematic characteristics of non-spherical droplets. The droplets consisted of water-glycerol solutions with viscosities ranging from 6.3 to 697 mPa s, and the initial aspect ratio was 1, The authors would like to acknowledge the financial support of Volvo Car Corporation (VCC), Gothenburg, Sweden and EPSRC grant EP/K019732/1.

In Gasoline Direct Injection engines, direct exposure of the injector to the flame can cause combustion products to accumulate on the nozzle, which can result in increased particulate emissions. This research observes the impact of injector fouling on particulate emissions and the associated injector spray pattern and shows how both can be reversed by utilising fuel detergency. For this purpose multi-hole injectors were deliberately fouled in a four-cylinder test engine with two different base fuels. During a four hour injector fouling cycle particulate numbers (PN) increased by up to two orders of magnitude. The drift could be reversed by switching to a fuel blend that contained a detergent additive. In addition, it was possible to completely avoid any PN increase, when the detergent containing fuel was used from the beginning of the test. Microscopy showed that increased injector fouling coincided with increased particulate emissions. Based on these results a selection of the injectors was installed in a laboratory injection chamber and the spray patterns were investigated with a high speed camera. Injectors corresponding to the largest PN drift produced the thinnest spray jets with the deepest penetration. These factors amplify the risk of wall wetting and provide an explanation for the increase of PN. The positive effect of the detergent was also reflected in the spray pattern analysis, which illustrates the potential benefits of such fuel additives.

Thermoacoustic instabilities under lean operation in gas turbine burners hinder the development of lean premixed combustion mode of operation, thus limiting the potential to decrease NO x emissions. A method to improve stability of lean combustion while maintaining low thermal NO x formation is to add hydrogen in typical gas turbine fuels such as natural gas. The present work examines the thermoacoustic dynamic characteristics of hydrogen-enriched methane blends in a swirl stabilized model gas turbine combustor. We blend CH4 with increasing H2 molar content (from 0% to 40%) at a global equivalence ratio of ϕ = 0.55 on a constant Reynolds number Re = 19000. The reference case of pure methane is susceptible to blow off at the same equivalence ratio. On increasing the hydrogen content at 10% H2, the flammability limits are extended. However, further increase of the H2 content leads to manifestation of random short bursts of dynamic pressure and heat release. Thermoacoustic dynamics are intermittently injected between a quiescent state and a regime of high amplitude oscillations. On further increasing H2 content, the dynamics are attracted towards a limit cycle; a fully established high amplitude regime, where no requiescence is observed. In this regime, heat release rate and dynamic pressure oscillate in phase and at the same frequency. Based on the observed dynamics, we seek a mixture property to characterize the dynamic state, the combustor operates in. We show that the extinction strain rate associated with each mixture can collapse the dynamic transitions from quiescent to intermittent instabilities and finally to fully established thermoacoustic oscillations. In each dynamic state, we examine the mechanism that affiliates coherent structures of the underlying thermodynamic flow field with the flame through the relation of the spatial distribution of the flow imposed strain rate over the extinction strain rate of each mixture. It is shown that the flame anchoring locations for a given mixture are dictated by the flow imposed strain rate and then that increased extinction resistance couples the flame with naturally excited to swirling flows helical instabilities, that may instigate mechanisms responsible for intermittent heat release rate bursts. Finally, the combustor demonstrates a limit cycle behaviour where the flow-imposed strain rate oscillates about the extinction strain rate along the inner shear layers causing local extinction of the flame at the root close to the centerbody.

Non-intrusive chemiluminescence measurements have been used as heat release rate and equivalence ratio indicators for gas turbine combustor active control. In the present study, measurements and modelling of OH*, CH(A)*, C 2 *, and CO 2 * chemiluminescence are used to examine chemiluminescence sensing of heat release rate and equivalence ratio in premixed counterflow methane – air flames with equivalence ratio from 0.6 to 1.3 and strain rate from 80 to 400 s -1 . Two spectrally resolved detecting optical systems were used to detect spatially-averaged (global) and spatially resolved (local) chemiluminescence characteristics in the reaction zone. A recently published reaction mechanism 1 for the chemiluminescence of the OH*, CH*, and C 2 * species is incorporated to GRI-Mech 3.0. The augmented mechanism is further validated against the experimental results of the present study and is used to predict the chemiluminescence characteristics of premixed counterflow methane – air flames. The mechanism includes OH* chemiluminescence formation paths from hydrogen reaction, which have not been evaluated before in premixed counterflow flames. The CHEMKIN based counterflow flame code, OPPDIF is employed to simulate the experiments. The calculated OH* and CH(A)* chemiluminescence agrees well with the experimental results measured by both optical methods. Both the experimental and numerical results demonstrate the ability of OH* and CH(A)* intensities to mark heat release rate in methane – air flames. Overall, CH* may be preferable for heat release rate sensing applications at elevated equivalence ratio and strain rate. For equivalence ratio sensing in methane combustion, the measured and simulated OH*/CH(A)* chemiluminescent intensity ratio is highly dependent on equivalence ratio and nearly independent of strain rate. Thus, this ratio can be used to monitor equivalence ratio. However, a non-monotonic behavior of the OH*/CH* ratio for very lean combustion (ER < 0.7) is observed, in agreement with previous studies. This behavior can be reproduced by the reaction mechanisms. The behavior of OH*/CH(A)* chemiluminescent intensity ratio for flames of methanepropane blends are also calculated with the detailed chemistry model. The addition of propane in methane modifies the behaviour of OH*/CH(A)* chemiluminescent intensity ratio dramatically. However, the numerical results suggest that the OH*/CH(A)* chemiluminescent intensity ratio is an indicator of equivalence ratio in lean methanepropane fuel blended flames.

The work investigates the effect of various post-injection strategies on the flame patterns in a Ricardo Hydra optical single-cylinder, light-duty diesel engine, operated in a partially premixed combustion mode, under low load (IMEP: ca. 2.3 bar), low speed (1200 rpm) conditions. The effect of post-injection fuel amount (12% and 24% of the total fuel quantity per cycle) and post-injection timing (0, 5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flame propagation is captured by means of high-speed flame natural luminosity imaging and of CH*, C2*, and OH* line-of-sight chemiluminescence measurements. Results suggest that post-injections suppress mixture reactivity but enhance oxidation, and that a larger amount of fuel and/or later post-injection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions, while post-injection close to the main combustion event appears to have a beneficial effect on the soot oxidation processes.

This paper describes the development and application of a novel optical technique for the measurement of liquid film thickness formed on surfaces during the impingement of automotive fuel sprays. The technique makes use of the change of the light scattering characteristics of a metal surface with known roughness, when liquid is deposited. Important advantages of the technique over previously established methods are the ability to measure the time-dependent spatial distribution of the liquid film without a need to add a fluorescent tracer to the liquid, while the measurement principle is not influenced by changes of the pressure and temperature of the liquid or the surrounding gas phase. Also, there is no need for non-fluorescing surrogate fuels. However, an in situ calibration of the dependence of signal intensity on liquid film thickness is required. The developed method can be applied to measure the time-dependent and two-dimensional distribution of the liquid fuel film thickness on the piston or the liner of gasoline direct injection (GDI) engines. The applicability of this technique was evaluated with impinging sprays of several linear alkanes and alcohols with different thermo-physical properties. The surface temperature of the impingement plate was controlled to simulate the range of piston surface temperatures inside a GDI engine. Two sets of liquid film thickness measurements were obtained. During the first set, the surface temperature of the plate was kept constant, while the spray of different fuels interacted with the surface. In the second set, the plate temperature was adjusted to match the boiling temperature of each fuel. In this way, the influence of the surface temperature on the liquid film created by the spray of different fuels and their evaporation characteristics could be demonstrated.

We quantify the temporal development of the mixing field of a starting jet by measuring the mixture fraction and the scalar dissipation rate and their statistics in an isothermal, impulsively started, gaseous jet. The scalar measurements are performed using planar laser induced fluorescence and, with appropriate processing of the resulting images, allow scalar dissipation rate measurements within 20%. The probability density functions of the mixture fraction, measured within a region of the order of 3 times the Batchelor length scale of the flow, are bimodal and skewed around a well-mixed radial location, which depends on the downstream distance and the time after the start of injection. The instantaneous distributions of the scalar dissipation rate reveal regions of high mixing at the jet periphery and at the developing vortex ring. The normalised probability density function (pdf) of the scalar dissipation rate at various flow positions and times after the start of injection has the same characteristic shape but differs from the usually suggested lognormal distribution at both low and high dissipation values; the same, also, holds true for the pdf conditioned on different values of the mixture fraction. The mean of the scalar dissipation rate conditional on mixture fraction shows a variation across the mixture fraction range, which differs between flow locations and times after the start of injection; however, at later times and for larger downstream distances the conditional mean between flow locations has similar distributions. Implications of the measurements for the auto-ignition of gaseous jets are examined and demonstrate that near the nozzle exit or at earlier times conditions are un-favourable for auto-ignition.

The measurement of the rate of fuel injection using a constant volume, fluid filled chamber and measuring the pressure change as a function of time due to the injected fluid (the so called “Zeuch” method) is an industry standard due to its simple theoretical underpinnings. Such a measurement device is useful to determine key timing and quantity parameters for injection system improvements to meet the evolving requirements of emissions, power and economy. This study aims to further the understanding of the nature of cavitation which could occur in the near nozzle region under these specific conditions of liquid into liquid injection using high pressure diesel injectors for heavy duty engines. The motivation for this work is to better understand the temporal signature of the pressure signals that arise in a typical injection cycle. A preliminary CFD study was performed, using OpenFOAM, with a transient (Large Eddy Simulation -LES), multiphase solver using the homogenous equilibrium model for the compressibility of the liquid/vapour. The nozzle body was modelled for simplicity without the nozzle needle using a nozzle hole of 200μm diameter and the body pressurised to values typical for common rail engines. Temperature effects were neglected and the wall condition assumed to be adiabatic. The chamber initial static pressure was varied between 10 and 50 bar to reflect typical testing conditions. Results indicate that vapour formation could occur in areas 10-30mm distant from the nozzle itself. The cavitation was initiated around 100 μs after the jet had started for low ΔP cases and followed the development period required for the formation of vortices associated with the vortex roll up of this jet. These vortices had localised sites, in their core region, below the vapour pressure and were convected downstream of their initial formation location. It was also found that vapour formation could occur at chamber static pressures up to 50 bar (the highest tested) due to cavitation in the shear layer and this vortex effect. The pressure signal received at the chamber would therefore be more difficult to interpret with additional error components.

Measured filtered density functions (FDFs) as well as assumed beta distribution model of mixture fraction and “subgrid” scale (SGS) scalar variance z′′2⎯⎯⎯⎯⎯⎯⎯, used typically in large eddy simulations, were studied by analysing experimental data, obtained from two-dimensional planar, laser induced fluorescence measurements in isothermal swirling turbulent flows at a constant Reynolds number of 29 000 for different swirl numbers (0.3, 0.58, and 1.07). Two-dimensional spatial filtering, by using a box filter, was performed in order to obtain the filtered variables, namely, resolved mean and “subgrid” scale scalar variance. These were used as inputs for assumed beta distribution of mixture fraction and top-hat FDF shape estimates. The presumed beta distribution model, top-hat FDF, and the measured filtered density functions were used to integrate a laminar flamelet solution in order to calculate the corresponding resolved temperature. The experimentally measured FDFs varied with the flow swirl number and both axial and radial positions in the flow. The FDFs were unimodal at flow regions with low SGS scalar variance, z′′2⎯⎯⎯⎯⎯⎯⎯< 0.01, and bimodal at regions with high SGS variance, z′′2⎯⎯⎯⎯⎯⎯⎯> 0.02. Bimodal FDF could be observed for a filter size of approximately 1.5-2 times the Batchelor scale. Unimodal FDF could be observed for a filter size as large as four times the Batchelor scale under well-mixed conditions. In addition, two common computational models (a gradient assumption and a scale similarity model) for the SGS scalar variance were used with the aim to evaluate their validity through comparison with the experimental data. It was found that the gradient assumption model performed generally better than the scale similarity one.

Schlieren [Schlieren and Shadowgraphy Techniques (McGraw-Hill, 2001); Optics of Flames (Butterworths, 1963)] is a non-intrusive technique that can be used to detect density variations in a medium, and thus, under constant pressure and mixture concentration conditions, measure whole-field temperature distributions. The objective of the current work was to design a schlieren system to measure line-of-sight (LOS)-averaged temperature distribution with the final aim to determine the temperature distribution inside the cylinder of internal combustion (IC) engines. In a preliminary step, we assess theoretically the errors arising from the data reduction used to determine temperature from a schlieren measurement and find that the total error, random and systematic, is less than 3% for typical conditions encountered in the present experiments. A Z-type, curved-mirror schlieren system was used to measure the temperature distribution from a hot air jet in an open air environment in order to evaluate the method. Using the Abel transform, the radial distribution of the temperature was reconstructed from the LOS measurements. There was good agreement in the peak temperature between the reconstructed schlieren and thermocouple measurements. Experiments were then conducted in a four-stroke, single-cylinder, optical spark ignition engine with a four-valve, pentroof-type cylinder head to measure the temperature distribution of the reaction zone of an iso-octane–air mixture. The engine optical windows were designed to produce parallel rays and allow accurate application of the technique. The feasibility of the method to measure temperature distributions in IC engines was evaluated with simulations of the deflection angle combined with equilibrium chemistry calculations that estimated the temperature of the reaction zone at the position of maximum ray deflection as recorded in a schlieren image. Further simulations showed that the effects of exhaust gas recirculation and air-to-fuel ratio on the schlieren images were minimal under engine conditions compared to the temperature effect. At 20 crank angle degrees before top dead center (i.e., 20 crank angle degrees after ignition timing), the measured temperature of the flame front was in agreement with the simulations (730–1320 K depending on the shape of the flame front). Furthermore, the schlieren images identified the presence of hot gases ahead of the reaction zone due to diffusion and showed that there were no hot spots in the unburned mixture.

Statistics of the alignment of fluid-dynamic principal strain-rates and the local flamelet-normal in a premixed turbulent V-flame (methane-air, Re t = 450, φ = 0.8) were measured experimentally using simultaneous stereoscopic particle image velocimetry (SPIV) and planar laser-induced fluorescence of OH (OH-PLIF). The use of a second OH-PLIF sheet, oriented in a crossed-plane imaging configuration enabled conditioning of the statistics with respect to through-plane flame orientation. The statistics show the geometric alignment changes significantly with the distance between the flame and the location where the strain-rate field is evaluated. It was observed that approximately 30 η upstream of the flame, the fluid-dynamic principal strain-rates show no preferential alignment with the flamelet. With increasing proximity to the flame, the most extensive principal strain-rate is observed to align preferentially perpendicular to the local flamelet-normal. In the immediate vicinity of the flame, where local fluid-dynamics are dominated by dilatation, the principal extensive strain-rate is observed to align preferentially parallel to the local flamelet-normal. The realignment of the principal strain-rates in the immediate vicinity of the flame is clearly the result of local flow acceleration caused by heat-release at the reaction zone. As the most extensive principal strain-rate tends to align preferentially perpendicular to the local flamelet-normal outside the region of heat-release, the data indicate that high scalar gradients observed ahead of the flamelet are produced by the local turbulent flow-field, rather than destroyed by it.

The operating range of lean-burn spark-ignition (SI) engines is limited by the cycle-to-cycle variability of the fuel concentration at or near the spark plug at ignition timing. An experimental investigation was undertaken to measure the temporal and spatial distribution of hydrocarbon (HC) concentration in a spark-ignition engine, using the infrared (IR) laser absorption at 3392 nm. The purposes were to establish whether there is a correlation between time-resolved HC measurements for a range of global air-to-fuel (A/F) ratios (A/F = 15.5–23) with the strength of the firing stroke and to establish how this varies with fuel port-injection strategies against either open (injection timing 30° crank angle (CA) after intake top dead center) or closed-valves (injection timing 180°CA after intake top dead center), respectively resulting in stratified and near-homogeneous charge distributions. The results showed that IR line-of-sight (LOS) averaged A/F ratio measurements yielded a good agreement with the global A/F ratio readings obtained by the linear air-to-fuel (LAF) zirconia-based sensor. Furthermore, the cyclic variability of the measurements of the fuel concentration increased with increasing A/F ratio. At A/F = 23, closed-valve injection strategy resulted in small spatial stratification of the fuel charge with an ensemble-averaged correlation coefficient of fluctuations of the IR LOS A/F ratios with fluctuations of peak in-cylinder pressure of 0.37. For open-valve injection strategy, which resulted in axial fuel mixture charge-stratification but no radial charge-stratification, a maximum correlation coefficient of the IR LOS A/F ratios with fluctuations of peak pressure of 0.34 was measured (at the measurement location closest to the spark). This correlation was reduced to 0.17 at locations furthest from the spark plug. Finally, results showed that at A/F = 23, fuel stratification can be used to control lean-burn SI combustion, while at A/F = 15.5, no significant difference was observed between the two injection timings. The consequences of these findings on the fuel distribution in the pentroof and the control of the lean limits of SI engines are discussed.