LES/CMC Simulations of Swirl-Stabilised Ethanol Spray Flames Approaching Blow-Off.

Large Eddy Simulations (LES) with the Conditional Moment Closure (CMC) combustion model of swirling ethanol spray flames have been performed in conditions close to blow-off for which a wide database of experimental measurements is available for both flame and spray characterization. The solution of CMC equations exploits a three-dimensional unstructured code with a first order closure for chemical source terms. It is shown that LES/CMC is able to properly capture the flame structure at different conditions and agrees reasonably well with the measurements both in terms of mean flame shape and dynamic behaviour of the flame evaluated in terms of local extinctions and statistics of the lift-off height. Experimental measurements of the overall (liquid plus gaseous) mixture fraction, performed using the Laser-Induced Breakdown Spectroscopy technique, are also included allowing further assessment and validation of the numerical method. The sensitivity of the simulation results to the various boundary conditions is discussed.Rolls-Royce Group, Engineering and Physical Sciences Research Council (Grant ID: EP/J021644/1

In situ, fast and non - perturbative diagnostics of combustion processes and its products using laser induced breakdown spectroscopy (LIBS)

Laser induced breakdown spectroscopy (LIBS) has attracted a lot of scientificinterest during the last two decades as it is generally considered to be an experimentallysimple and efficient laser-based technique which can perform real-time, qualitative andquantitative elemental analysis. The basic idea of LIBS is the creation of spark/plasmathrough tight focusing of a laser beam on the surface or into a sample, the subsequentexcitation and atomization of the species of the sample at the location where the spark isformed and the final detection and spectroscopic analysis of the emitted radiation fromthe decaying plasma. Seeing the numerous advantages holding the technique, LIBS hasbeen proposed for many practical, technical and technological applications in variousscientific areas. On the other hand, in the field of combustion, the proportion of fuel in acombustible mixture is of great importance as it strongly affects the efficiency of thechemical processes and the production of soot emissions. Therefore, there is acontinuously increasing need for the development of a rapid and non-perturbativediagnostic technique for the determination of the fuel content locally in the flamestructure with good spatial and temporal resolution.Ιn the present dissertation, LIBS technique which offers such advantages has beenapplied for combustion diagnostics purposes. During the experiments, laser systems withpulse duration in the scale of ns and fs have been applied as excitation sources, while thecombustible mixtures under investigation were hydrocarbon-air flames, of laminar andturbulent flow with simple and more complicated structures. From the LIBS spectra inflames of different compositions, it was exhibited that there is a strong dependence of theintensities of various spectral lines on the equivalence ratio, which demonstrates that theprecise determination of the amount of fuel can be performed. Also based on thiscorrelation, the determination of the equivalence ratio locally everywhere within theflame can be achieved giving useful information about its structure. Finally, a similardiagnostic technique has been employed. The dielectric breakdown is held using a sparkgenerator and the technique is called electrical Spark Induced Breakdown Spectroscopy(SIBS). The emitted light of the two plasmas induced by optical and electrical excitationwas collected and a comparative study was performed.Τα τελευταία χρόνια, η φασματοσκοπία πλάσματος επαγόμενο από λέιζερ (LIBS)έχει προσελκύσει μεγάλο ερευνητικό ενδιαφέρον καθώς αποτελεί μία πειραματικά απλήκαι αποτελεσματική τεχνική, η οποία παρέχει τη δυνατότητα λήψης μετρήσεων γιααπευθείας ποιοτική και ποσοτική στοιχειακή ανάλυση. Η τεχνική LIBS στηρίζεται στηδημιουργία σπινθήρα/πλάσματος μέσω ισχυρά εστιασμένης δέσμης λέιζερ στηνεπιφάνεια ή στο εσωτερικό του δείγματος, στην ακόλουθη διέγερση και ατομοποίησητων στοιχείων του στόχου και στην τελική καταγραφή και φασματοσκοπική ανάλυση τηςεκπεμπόμενης ακτινοβολίας του πλάσματος. Λόγω των πολλών πλεονεκτημάτων πουσυγκεντρώνει η τεχνική, το LIBS έχει προταθεί για πληθώρα πρακτικών, τεχνικών καιτεχνολογικών εφαρμογών σε ένα ευρύ φάσμα ερευνητικών πεδίων. Από την άλλη μεριά,στον τομέα της καύσης, η ποσότητα καυσίμου σε ένα εύφλεκτο μίγμα είναι αντικείμενομείζονος σημασίας καθώς επηρεάζει σημαντικά την απόδοση των χημικών διεργασιώνκαι την παραγωγή και εκπομπή ρύπων. Επομένως, δημιουργείται η ανάγκη ανάπτυξηςμίας γρήγορης και μη παρεμβατικής διαγνωστικής τεχνικής για τη μέτρηση τηςπεριεκτικότητας του καυσίμου τοπικά στη φλόγα με καλή τόσο χωρική όσο και χρονικήανάλυση.Στα πλαίσια της παρούσας διδακτορικής διατριβής, η τεχνική LIBS η οποίασυγκεντρώνει όλα αυτά τα πλεονεκτήματα χρησιμοποιήθηκε για αυτό το σκοπό. Κατά τηδιάρκεια των πειραμάτων, χρησιμοποιήθηκαν πηγές λέιζερ διάρκειας παλμών ns και fs,ενώ τα συστήματα καύσης που μελετήθηκαν ήταν φλόγες υδρογονανθράκων-αέρα,στρωτής και τυρβώδους ροής, απλής και συνθετότερης γεωμετρίας. Από τα LIBSφάσματα φλογών διαφορετικής σύστασης, προέκυψε λοιπόν ότι υπάρχει μία ισχυρήεξάρτηση μεταξύ των εντάσεων διαφόρων φασματικών γραμμών με το λόγοισοδυναμίας. Επομένως, μέσω της συσχέτισης αυτής μπορεί να επιτευχθεί με μεγάληακρίβεια τόσο η μέτρηση της περιεκτικότητα σε καύσιμο φλογών άγνωστης σύστασηςόπως επίσης και η μέτρηση της κατανομής του καυσίμου τοπικά μέσα σε όλη την έκτασητης φλόγας παρέχοντας σημαντικές πληροφορίες για την δομή της. Τέλος, εφαρμόστηκεμία παραπλήσια διαγνωστική τεχνική, κατά την οποία η διηλεκτρική κατάρρευση τουμέσου ήταν αποτέλεσμα ενός ηλεκτρικού σπινθήρα: electrical Spark Induced BreakdownSpectroscopy (SIBS) όπου και πραγματοποιήθηκε η συγκριτική μελέτη της ακτινοβολίαςτου πλάσματος επαγόμενο μέσω οπτικής και ηλεκτρικής διέγερσης

Optical investigation of prechamber combustion in an RCEM

In this study, detailed investigations of scavenged prechamber engine combustion are performed experimentally in a Rapid Compression Expansion Machine (RCEM), which allows optical access into the main chamber. OH* chemiluminescence measurements combined with pressure measurements are used to study the effect of varying ignition timing on combustion and cycle-to-cycle variations. The variation of ignition timing (pressure at ignition) showed an optimum ignition point for a given injection duration. Earlier ignition resulted in weaker but more reactive jets, coupled to increased cyclic variations. Later ignition did not significantly affect heat release rate, but increased cyclic variation

Numerical Simulations of Pre-Chamber Combustion in an Optically Accessible RCEM

In this work, numerical simulations of an automotive-sized scavenged pre-chamber mounted in an optically-accessible rapid compression-expansion machine (RCEM) have been carried out using two different turbulence models: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). The RANS approach is combined with the G-equation combustion model, whereas the LES approach is coupled with the flamelet generated manifold (FGM) model for partially-premixed combustion. Simulation results are compared with experimental data in terms of OH* chemiluminescence in the main chamber. Both RANS and LES results were found to qualitatively reproduce the main features observed experimentally in terms of spatial flame development. Simulation results are further analysed by means of early flame propagation within the pre-chamber (related to the fuel and turbulence intensity distributions) and the ignition process in the main chamber. During the turbulence jet ignition (TJI) process, the analysis of the LES progress variable variance reveals that during the intensive jet mixing the mixture in the main chamber is predominantly ignited by autoignition followed by a progressive transition to a deflagrative premixed flame propagation mode. For the lean fuel-air mixture considered (λ=2) the mixing of the additional fuel (previously injected into the pre-chamber) within the main chamber was found to play a major role on the ignition process.ISSN:0148-7191ISSN:2688-362

Numerical Study of Turbulence and Fuel-Air Mixing within a Scavenged Pre-Chamber Using RANS and LES

It is well-known that the spatial distribution of turbulence intensity and fuel concentration at spark-time play a pivotal role on the flame development within the pre-chamber in gas engines equipped with a scavenged pre-chamber. The combustion within the pre-chamber is in turn a determining factor in terms of combustion behaviour in the main chamber, and accordingly it influences the engine efficiency as well as pollutant emissions such as NOx and unburned hydrocarbons. This paper presents a numerical analysis of fuel concentration and turbulence distribution at spark time for an automotive-sized scavenged pre-chamber mounted at the head of a rapid compression-expansion machine (RCEM). Two different pre-chamber orifice orientations are considered: straight and tilted nozzles. The latter introduce a swirling flow within the pre-chamber. Simulations have been carried out using with two different turbulence models: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). Results of the RANS turbulence model have been compared with multi-cycle averaged LES results in order to assess the performance of the RANS model in predicting an accurate pre-chamber filling process until spark time. The orientation of the orifices was observed to have a profound impact on the spatial distribution of fuel concentration and turbulence intensity around the spark-plug. Overall, the RANS model employed was found to provide very good results.ISSN:0148-7191ISSN:2688-362

Flame-wall interaction modelling for pre-chamber combustion in lean burn gas engines

Lean burn combustion systems present a viable route to emissions reductions. Scavenged pre-chamber ignition (PCI) systems aim to address this challenge by creating favourable ignition conditions close to stoichiometry in the spark region. The lean main charge ignition is then delivered by flame jets propagating through the nozzles connecting the pre-chamber to the cylinder. However, when using pre-chambers in light-duty applications, several problems need to be overcome compared to conventional ignition systems. The interaction of the flame with the walls of the pre‐chamber is an important issue affecting operation of the PCI combustion system. The flame may quench near to the wall due to heat losses. This is more prominent in PCIs designed for light‐duty vehicles as the characteristic size of PCIs can be comparable with the flame quenching distance. Near wall quenching also affects the quality of the fuel mixture within the pre‐chamber due to the accumulation of unburned mixture in crevices near the spark housing and the gas valve outlet. This issue is crucial when looking at vehicle emissions. Finally, the thermal quenching effect can affect the main pre‐chamber operation. If the flame can propagate through the pre‐chamber nozzles without quenching then the mixture in the cylinder is ignited by the jet flame front. On the contrary, if the flame is quenched within the nozzles, the mixture in the cylinder is ignited by hot radicals injected which create distributed combustion microkernels downstream of the nozzles. Modelling of these phenomena is essential to the successful design of PCI systems. To simulate these effects, a novel phenomenological quenching model has been developed by Ricardo and implemented into the VECTIS CFD product to work with G‐equation combustion model. This paper illustrates the principles and applications of the developed model. Following initial verification, the model is applied to the analysis of a novel pre-chamber ignition system developed within Horizon 2020 GASON project and the results are compared with measurement data

Numerical study of fuel and turbulence distributions in an automotive-sized scavenged pre-chamber

This article presents a numerical study of the fuel and turbulence distributions in a pre-chamber at spark-time. The study has been conducted in the framework of the H2020 Gas-On project, dealing with the development of a lean-burn concept for an automotive-sized gas engine equipped with a scavenged pre-chamber. The test case considered studies a 7-hole pre-chamber with circumferentially-tilted orifices mounted on the cylinder head of a rapid compression-expansion machine (RCEM), consistent with the experimental test rig installed at ETH Zurich. An accurate description of turbulence and fuel distributions are key quantities determining the early flame development within the pre-chamber. Both quantities have an influence on the overall combustion characteristics and therefore on the engine performance. For this purpose, computational fluid dynamics (CFD) is employed to complement experimental investigations in terms of data completeness. The performance of the Reynolds-averaged Navier-Stokes (RANS)-based turbulence model is compared with large-eddy simulation (LES) through ensemble averaging of multiple LES realizations, in which the fuel injection rate evolution into the pre-chamber has been perturbed. Overall, RANS results show that the distributions of the turbulent kinetic energy and fuel concentration at spark-time agree well with the LES ensemble-averaged counterparts. This constitutes a prerequisite in view of the combustion phase and the accuracy reported provides further confidence in this regard

A laser-induced breakdown spectroscopy method to assess the stochasticity of plasma-flame transition in sprays

Abstract: An experimental approach is presented to evaluate the impact of plasma composition arising from pulse-to-pulse energy and mixture fluctuations on the non-resonant laser-induced ignition of sprays. This allows for spark events to be conditioned on the successful or failed establishment of a flame kernel, a phase dominated by plasma decomposition and recombination reactions and the on-set of combustion reactions, that is, independent of the subsequent flame growth phase controlled by propagation phenomena only, such as fuel availability and turbulent strain. For that, laser-induced breakdown spectroscopy of the spark-generated plasma is carried out, followed by OH ∗ high-speed imaging of the kernel. Exploratory experiments in spatially uniform and polydisperse kerosene droplet distributions in a jet suggest that the hydrogen concentration in the plasma deriving from the fuel dissociated by the spark is closely related to the generated OH ∗ radicals levels and, in turn, with the success of establishing a flame kernel. This suggests that the ignition process is heavily controlled by mixture fluctuations at the spark, inherent of spray flows. The instantaneous mixture at the spark is estimated with a stochastic model, with the probability density function of the equivalence ratio exhibiting values higher than twice the mean value, while the highest probability occurs at lean conditions between the gaseous equivalence ratio and the overall equivalence ratio. The findings corroborate insights on the early-phase ignition obtained from direct numerical simulations, and the framework paves the way for the development of smart online engine-health tools to assess relight capability in future aeroengines

Experimental and Numerical Analysis of Pre-Chamber Combustion Systems for Lean Burn Gas Engines

The current trend in automobiles is towards electrical vehicles, but for the most part these vehicles still require an internal combustion engine to provide additional range and flexibility. These engines are under stringent emissions regulations, in particular, for the reduction of CO2. Gas engines which run lean burn combustion systems provide a viable route to these emission reductions, however designing these engines to provide sustainable and controlled combustion under lean conditions at λ=2.0 is challenging. To address this challenge, it is possible to use a scavenged Pre-Chamber Ignition (PCI) system which can deliver favorable conditions for ignition close to the spark plug. The lean charge in the main combustion chamber is then ignited by flame jets emanating from the pre-chamber nozzles. Accurate prediction of flame kernel development and propagation is essential for the analysis of PCI systems. A modelling approach is proposed based on the Dynamic Discrete Particle Ignition Kernel model coupled with the G-equation combustion model. The model is validated for an air/methane academic benchmark. The approach is then applied to the investigation of performance of three pre-chamber designs developed within Horizon 2020 GASON project in conjunction with the experimental investigation of these pre-chambers mounted on Rapid Compression Expansion Machine (RCEM). The investigated pre-chamber designs vary with respect to the tangential nozzle angle and volume. The study focusses on a lean limit of the proposed system’s operation with the main charge at λ=2.0 and a variation of pre-chamber design and scavenging level. The comparison of the simulation results with the experimental observations demonstrates good accuracy of the developed model. In addition, the combined experimental and modelling provides insights into the effect of pre-chamber geometry on potential performance.ISSN:0148-7191ISSN:2688-362