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5556 Background: Standard treatment for advanced ovarian cancer (aOC) includes primary cytoreductive surgery followed by chemotherapy (PDS > CTX). There is strong evidence that this strategy is accompanied by high risk for early failure in some patients (pts) but prospective multicenter data for this specific question are lacking. Methods: 64 AGO-sites prospectively registered pts with suspected aOC and collected following variables prior to start of therapy: age, Charlson Comorbidity Index (CCI), timed up and go test (TUG), ASA and ECOG performance score, weight, height, estimated ascites, albumin, creatinine, hemoglobin, leucocytes, platelets, CA125, patient reported outcome measures according to EORTC QLQ-C30 and OV28, hospital anxiety and depression score (HADS), physician-assessed suspected FIGO IV stage, abdominal pain requiring treatment, abdominal bloating, dyspnea and required palliative paracentesis (ascites, pleural effusions). Treatment followed according to investigator’s decision. Primary objective was to predict the unfavorable event of death or progression within 10 months after primary diagnosis. We applied univariate and multiple logistic regression with stepwise variable selection after multiple imputation of missing data in order to identify relevant predictors. Results: 223 pts with aOC FIGO IIIB to IVB and PDS were analyzed. Median age was 63 years (range 31-86). 52 (23.3%) pts experienced progression or death within 10 months from time of enrolment. In univariate regression, age (odds ratio (OR) 1.612 per 10 years), ASA (III vs I/II: OR 3.217), TUG (OR 1.087), body height (OR 0.541 per 10cm), ECOG (1 vs 0 OR 2.326, 2-3 vs 0 OR 4.102), estimated ascites ( > 500cc vs. none OR 2.811), paracentesis (OR 1.991), platelets (upper limit of normal (ULN) vs norm or < lower limit of normal (LLN) OR 1.998), albumin ( < LLN vs norm or > ULN OR 2.053), creatinine ( > ULN vs norm OR 3.969) and baseline QoL single-item subscales appetite loss (“very much” vs “not at all” OR 2.611), constipation (“quite a bit” vs “not at all” OR 4.903), and multi-item subscales global health status (OR 0.982 per 1 point) and nausea/vomiting (OR 1.013 per 1 point) were significant (p < 0.05). Multiple logistic regression identified age (OR 1.459 per 10 years), ASA (III vs I/II: OR 2.427), constipation (“quite a bit” vs “not at all” OR 3.786) and global health (OR: 0.985 per 1 point) as independent predictors of progression or death within 10 months after primary diagnosis (p < 0.05). Conclusions: A significant proportion of aOC pts undergoing PDS > CTX are at high-risk for early failure. The finding of independent risk factors including self-ratings of QoL at time of diagnosis should be confirmed and tested against other models to facilitate a more tailored treatment of high-risk pts and design of future trials in aOC. Clinical trial information: NCT02828618.

Here, outwardly propagating spherical hydrogen/air flames are examined theoretically and experimentally with respect to flame propagation speed and the onset of instabilities which develop due to thermal expansion and non-equal diffusivities. Instabilities increase the surface area of the spherical flame, and hence the flame propagation speed. The theory applied here accounts for both hydrodynamic and diffusive-thermal effects, incorporating temperature dependent transport coefficients. Experiments are performed in a spherical combustion chamber over a wide range of equivalence ratios (0.6–2.0), initial temperatures (298–423 K), and initial pressures (1 atm to 15 bar). The evolution of the flame propagation speed as a function of flame radius is compared to predictions from theory showing excellent agreement. Also the wrinkling of hydrogen/air flames is examined under increased pressure and temperature for various equivalence ratios. Critical flame radii, defined as the point of transition to cellular flames, are extracted from high-speed Schlieren flame imaging. Overall, the critical radius is found to decrease with increasing pressure. The predictions yield the growth rate of small disturbances and the critical flame radius. Experimental flame radii, as expected, are underpredicted by the theoretical findings. Experimental data are provided in the form of an approximation formula.

A skeletal chemical-kinetic mechanism for n-heptane cool flames is simplified to the maximum extent possible by introduction of steady-state approximations for intermediaries, following procedures ...

Kolmogorov introduced dissipative scales based on the mean dissipation $\langle {\it\varepsilon}\rangle$ and the viscosity ${\it\nu}$, namely the Kolmogorov length ${\it\eta}=({\it\nu}^{3}/\langle {\it\varepsilon}\rangle )^{1/4}$ and the velocity $u_{{\it\eta}}=({\it\nu}\langle {\it\varepsilon}\rangle )^{1/4}$. However, the existence of smaller scales has been discussed in the literature based on phenomenological intermittency models. Here, we introduce exact dissipative scales for the even-order longitudinal structure functions. The derivation is based on exact relations between even-order moments of the longitudinal velocity gradient $(\partial u_{1}/\partial x_{1})^{2m}$ and the dissipation $\langle {\it\varepsilon}^{m}\rangle$. We then find a new length scale ${\it\eta}_{C,m}=({\it\nu}^{3}/\langle {\it\varepsilon}^{m/2}\rangle ^{2/m})^{1/4}$ and $u_{C,m}=({\it\nu}\langle {\it\varepsilon}^{m/2}\rangle ^{2/m})^{1/4}$, i.e. the dissipative scales depend rather on the moments of the dissipation $\langle {\it\varepsilon}^{m/2}\rangle$ and thus the full probability density function (p.d.f.) $P({\it\varepsilon})$ instead of powers of the mean $\langle {\it\varepsilon}\rangle ^{m/2}$. The results presented here are exact for longitudinal even-ordered structure functions under the assumptions of (local) isotropy, (local) homogeneity and incompressibility, and we find them to hold empirically also for the mixed and transverse as well as odd orders. We use direct numerical simulations (DNS) with Reynolds numbers from $Re_{{\it\lambda}}=88$ up to $Re_{{\it\lambda}}=754$ to compare the different scalings. We find that indeed $P({\it\varepsilon})$ or, more precisely, the scaling of $\langle {\it\varepsilon}^{m/2}\rangle /\langle {\it\varepsilon}\rangle ^{m/2}$ as a function of the Reynolds number is a key parameter, as it determines the ratio ${\it\eta}_{C,m}/{\it\eta}$ as well as the scaling of the moments of the velocity gradient p.d.f. As ${\it\eta}_{C,m}$ is smaller than ${\it\eta}$, this leads to a modification of the estimate of grid points required for DNS.

Dimethyl ether has proven to be one of the most attractive alternatives to traditional fossil oil derived fuels for compression ignition engines. In this study, a skeletal mechanism consisting of 32 species and 49 elementary reactions, based on the detailed mechanism proposed by Fischer et al. [Int. J. Chem. Kinet. 32 (12) (2000) 713–740], is further reduced to a short 36-step mechanism describing first and second stage ignition as well as the rapid transition to final products. A global 4-step mechanism is derived by introducing steady state assumptions of intermediate species. An analytical solution for the ignition delay times of the first stage of ignition in the low temperature regime and the beginning of its transition to the intermediate temperature regime is presented. The important competition of the β -scission and the reaction with molecular oxygen of the hydroperoxy-methoxymethyl radical (C 2 OCH 2 O 2 H) is quantified by the introduction of a parameter β , related to the competition of chain-branching and chain-propagation. The calculated values agree very well with those of the 36-step mechanism. Also for the high temperature regime, an analytical solution is presented, which agrees well with the experiments and the values calculated with the 36-step mechanism.

The low temperature oxidation behavior of DME in a jet-stirred reactor is investigated. There have been numerous investigations on the low temperature reaction kinetics of DME. In this work, the focus is set on the instabilities that arise at very low temperatures (in the order of 580 K) in a jet-stirred reactor and a numerical analysis of the source of these oscillations is performed. A stability diagram was created where the parameters at which the oscillations were observed are displayed. The temperature oscillations at higher temperatures (around 1000 K) have been shown to be caused by ignition–extinction phenomena, whereas the oscillations observed at low temperatures are believed to be of thermokinetical origin, with the build-up of a semi-stable species (hydroperoxide-methylformate HPMF), its dissociation at increasing temperatures with the subsequent formation of large quantities of formaldehyde that increase the temperature giving rise to the first-stage ignition observed and at the same time act as inhibitor to the reaction. The continuous flow through the reactor brings the system back to its original state, and the process begins anew.

A turbulent flow in a wavy channel was investigated by tomographic particle-image velocimetry measurements and direct numerical simulations. To analyze the turbulent structures and their scaling behavior in a flow undergoing favorable and adverse pressure gradients, the streamline segmentation method proposed by Wang (J Fluid Mech 648:183–203, 2010) was employed. This method yields joint statistical information about velocity fluctuations and length scale distributions of non-overlapping structures within the flow. In particular, the joint statistical properties are notably influenced by the pressure distribution. Previous findings from flat channel flows and synthetic turbulence simulations concerning the normalized segment length distribution could be reproduced and therefore appear to be largely universal. However, the mean streamline segment length of accelerating and decelerating segments varies within one wavelength typically elongating segments of the type which corresponds to the local mean flow. Furthermore, the local pressure gradient was found to significantly impact local joint streamline segmentation statistics as a main influence on their inherent asymmetry.

A necessary condition for the accurate prediction of turbulent flows using large-eddy simulation (LES) is the correct representation of energy transfer between the different scales of turbulence in the LES. For scalar turbulence, transfer of energy between turbulent length scales is described by a transport equation for the second moment of the scalar increment. For homogeneous isotropic turbulence, the underlying equation is the well-known Yaglom equation. In the present work, we study the turbulent mixing of a passive scalar with an imposed mean gradient by homogeneous isotropic turbulence. Both direct numerical simulations (DNS) and LES are performed for this configuration at various Schmidt numbers, ranging from 0.11 to 5.56. As the assumptions made in the derivation of the Yaglom equation are violated for the case considered here, a generalised Yaglom equation accounting for anisotropic effects, induced by the mean gradient, is derived in this work. This equation can be interpreted as a scale-by-scal...

Experiments on the combustion of large n-heptane droplets, performed by the National Aeronautics and Space Administration in the International Space Station, revealed a second stage of continued quasi-steady burning, supported by low-temperature chemistry, that follows radiative extinction of the first stage of burning, which is supported by normal hot-flame chemistry. The second stage of combustion experienced diffusive extinction, after which a large vapour cloud was observed to form around the droplet. In the present work, a 770-step reduced chemical-kinetic mechanism and a new 62-step skeletal chemical-kinetic mechanism, developed as an extension of an earlier 56-step mechanism, are employed to calculate the droplet burning rates, flame structures, and extinction diameters for this cool-flame regime. The calculations are performed for quasi-steady burning with the mixture fraction as the independent variable, which is then related to the physical variables of droplet combustion. The predictions with t...

Based on a direct numerical simulation (DNS) of a temporally evolving mixing layer, we present a detailed study of the turbulent/non-turbulent (T/NT) interface that is defined using the two most common procedures in the literature, namely either a vorticity or a scalar criterion. The different detection approaches are examined qualitatively and quantitatively in terms of the interface position, conditional statistics and orientation of streamlines and vortex lines at the interface. Computing the probability density function (p.d.f.) of the mean location of the T/NT interface from vorticity and scalar allows a detailed comparison of the two methods, where we observe a very good agreement. Furthermore, conditional mean profiles of various quantities are evaluated. In particular, the position p.d.f.s for both criteria coincide and are found to follow a Gaussian distribution. The terms of the governing equations for vorticity and passive scalar are conditioned on the distance to the interface and analysed. At the interface, vortex stretching is negligible and the displacement of the vorticity interface is found to be determined by diffusion, analogous to the scalar interface. In addition, the orientation of vortex lines at the vorticity and the scalar based T/NT interface are analyzed. For both interfaces, vorticity lines are perpendicular to the normal vector of the interface, i.e. parallel to the interface isosurface.

This work focuses on the nonlinear control of instabilities in the combustion of a diluted dimethyl ether (DME) / air mixture under low temperature conditions. A nonlinear model predictive control (NMPC) is utilized to stabilize the combustion in simulation with the aim to specify minimum actuation dynamic requirements and develop control strategies for an existing experimental chemical reactor.

The combustion chemistry of the first stage ignition and chemistry/flow interactions are studied for dimethyl ether (DME) with a mathematical analysis of two systems: a plug flow reactor study is used to reduce the reaction chemistry systematically. A skeletal reaction mechanism for the low temperature chemistry of DME until the onset of ignition is derived on the basis of the detailed DME mechanism of the Lawrence Livermore National Laboratory – see Curran, Fischer and Dryer, Int. J. Chem. Kinetics, Vol. 32 (2000). It is shown that reasonably good results for ignition delay times can be reached using a simple system of three ordinary differential equations and that the resulting analytical solution depends only on two reaction rates and the initial fuel concentration. The stepwise reduction of the system based on assumptions yields an understanding on why these reactions are so important. Furthermore, the validation of the assumptions yields insight into the influence of the fuel and the oxygen concentra...

Using two-dimensional high-speed measurements of the mixture fraction $Z$ in a turbulent round jet with nozzle-based Reynolds numbers $R{e}_{0} $ between 3000 and 18 440, we investigate the scalar turbulent/non-turbulent (T/NT) interface of the flow. The mixture fraction steeply changes from $Z= 0$ to a final value which is typically larger than 0.1. Since combustion occurs in the vicinity of the stoichiometric mixture fraction, which is around $Z= 0. 06$ for typical fuel/air mixtures, it is expected to take place largely within the turbulent/non-turbulent interface. Therefore, deep understanding of this part of the flow is essential for an accurate modelling of turbulent non-premixed combustion. To this end, we use a composite model developed by Effelsberg & Peters (Combust. Flame, vol. 50, 1983, pp. 351–360) for the probability density function (p.d.f.) $P(Z)$ which takes into account the different contributions from the fully turbulent as well as the turbulent/non-turbulent interface part of the flow. A very good agreement between the measurements and the model is observed over a wide range of axial and radial locations as well as at varying intermittency factor $\gamma $ and shear. Furthermore, we observe a constant mean mixture fraction value in the fully turbulent region. The p.d.f. of this region is thus of non-marching character, which is attributed physically to the meandering nature of the fully turbulent core of the jet flow. Finally, the location and in particular the scaling of the thickness $\delta $ of the scalar turbulent/non-turbulent interface are investigated. We provide the first experimental results for the thickness of the interface over the above-mentioned Reynolds number range and observe $\delta / L\sim R{ e}_{\lambda }^{- 1} $, where $L$ is an integral length scale and $R{e}_{\lambda } $ the local Reynolds number based on the Taylor scale $\lambda $, meaning that $\delta \sim \lambda $. This result also supports the assumption often made in modelling of the stoichiometric scalar dissipation rate ${\chi }_{st} $ being a Reynolds-number-independent quantity.

Based on two large-eddy simulations (LES) of a non-reacting turbulent round jet with a nozzle based Reynolds number of 8,610 with the same configuration as the one that has recently been investigated experimentally (Gampert et al., 2012; J Fluid Mech, 2012; J Fluid Mech 724:337, 2013), we examine the scalar turbulent/non-turbulent (T/NT) interface layer in the mixture fraction field of the jet flow between ten and thirty nozzle diameters downstream. To this end, the LES—one with a coarse grid and one with a fine grid—are in a first step validated against the experimental data using the axial decay of the mean velocity and the mean mixture fraction as well as based on radial self-similar profiles of mean and root mean square values of these two quantities. Then, probability density functions (pdf) of the mixture fraction at various axial and radial positions are compared and the quality of the LES is discussed. In general, the LES results are consistent with the experimental data. However, in the flow region where the imprint of the T/NT interface layer is dominant in the mixture fraction pdf, discrepancies are observed. In a next step, statistics of the T/NT interface layer are studied, where a satisfactory agreement for the pdf of the location of the interface layer from the higher resolved LES with the experimental data is observed, while the one with the coarse grid exhibits considerable deviations. Finally, the mixture fraction profile across the interface is investigated where the same trend as for the pdf of the location is present. In particular, it is found that the sharp interface that is present in experimental studies (Gampert et al., J Fluid Mech, 2013; Westerweel et al., J Fluid Mech 631:199, 2009) is less distinct in the LES results and rather diffused in radial direction outside of the T/NT interface layer.

We present a detailed experimental investigation of conditional statistics obtained from dissipation elements based on the passive scalar field$\theta $and its instantaneous scalar dissipation rate$\chi $. Using high-frequency planar Rayleigh scattering measurements of propane discharging as a round turbulent jet into coflowing carbon dioxide, we acquire with Taylor’s hypothesis a highly resolved three-dimensional field of the propane mass fraction$\theta $. The Reynolds number (based on nozzle diameter and jet exit velocity) varies between 3000 and 8600. The experimental results for the joint probability density of the scalar difference$ \mathrm{\Delta} \theta $and the length$l$of dissipation elements resembles those previously obtained from direct numerical simulations of Wang & Peters (J. Fluid Mech., vol. 554, 2006, pp. 457–475). In addition, the normalized marginal probability density function$\tilde {P} (\tilde {l} )$of the length of dissipation elements follows closely the theoretical model derived by Wang & Peters (J. Fluid Mech., vol. 608, 2008, pp. 113–138). We also find that the mean linear distance${l}_{m} $between two extreme points of an element is of the order of the scalar Taylor microscale${\lambda }_{u} $. Furthermore, the conditional mean$\langle \mathrm{\Delta} \theta \vert l\rangle $scales with Kolmogorov’s$1/ 3$power law. The investigation of the orientation of long dissipation elements in the jet flow reveals a preferential alignment, perpendicular to the streamwise direction for long elements, while the orientation of short elements is close to isotropic. Following an approach proposed by Kholmyansky & Tsinober (Phys. Lett. A, vol. 373, 2009, pp. 2364–2367), we finally investigate the probability density function of the scalar increment$\delta \theta $in the streamwise direction, when strong dissipative events are either retained in or excluded from the measurement volume. In the present study, however, these events are related to maximum points of the scalar dissipation rate$\chi $together with their local extent. When these regions are excluded from the scalar field, we observe a tendency of the probability density function$P(\delta \theta (r))$towards a Gaussian bell-shaped curve.

Based on planar high-speed Rayleigh scattering measurements of the mixture fraction Z of propane discharging from a turbulent round jet into co-flowing carbon dioxide at nozzle-based Reynolds numbe...

Many direct numerical simulations of spherical hot spot auto-ignitions, with different fuels, have identified different auto-ignitive regimes. These range from benign auto-ignition, with pressure waves of small amplitude, to super-knock with the generation of damaging over-pressures. Results of such simulations are generalised diagrammatically, by plotting boundary values of ξ , the ratio of acoustic to auto-ignition velocity, against ɛ. This latter parameter is the residence time of the developing acoustic wave in the hot spot of radius r o , namely r o /a , normalised by the excitation time for the chemical heat release, τ e . This ratio controls the energy transfer into the developing acoustic front. A third relevant parameter involves the product of the activation temperature, E/R , for the auto-ignition delay time, τ i , normalised by the mixture temperature. T , the ratio, τ i / τ e , and the dimensionless hot spot temperature gradient, ( ∂ ln T / ∂ r ¯ ) , where r ¯ is a dimensionless radius. These parameters define the boundaries of regimes of thermal explosion, subsonic auto-ignition, developing detonations, and non-auto-ignitive deflagrations, in plots of ξ against ɛ.The regime of developing detonation forms a peninsula and contours, throughout the field. The product parameter ( E / R T ) ( τ i / τ e ) / ∂ ln T / ∂ r ¯ expresses the influences of hot spot temperature gradient and fuel characteristics, and a unique value of it might serve as a boundary between auto-ignitive and deflagrative regimes. Other combustion regimes can also be identified, including a mixed regime of both auto-ignitive and “normal” deflagrative burning. The paper explores the performances of a number of different engines in the regimes of controlled auto-ignition, normal combustion, combustion with mild knock and, ultimately, super-knock. The possible origins of hot spots are discussed and it is shown that the dissipation of turbulent energy alone is unlikely to lead directly to sufficiently energetic hot pots. The knocking characterisation of fuels also is discussed.

In this study we investigate the statistical properties of so called streamline segments in a turbulent channel flow with one plane and one wavy wall. We give a short overview of the concept of streamline segments and recent results in description and modeling in this field. We find that streamline segments in the vicinity of the wavy wall are significantly smaller on average than in the core region. However, normalizing the length distribution with the mean segment length leads to an almost perfect collapse of the pdfs. This quasi-universal behavior is further highlighted by the comparison to statistics of streamline segments in homogeneous isotropic turbulence. Finally, we investigate the kinematic behavior of streamline segments by means of conditional moments and show differences in scaling behavior compared to the classical structure function analysis.

The energy transfer between different scales of a passive scalar advected by homogeneous isotropic turbulence is studied by an exact generalized transport equation for the second moment of the scalar increment. This equation can be interpreted as a scale-by-scale energy budget equation, as it relates at a certain scale r terms representing the production, turbulent transport, diffusive transport and dissipation of scalar energy. These effects are analyzed by means of direct numerical simulation where each term is directly accessible. To this end, a variation of the Taylor micro-scale based Reynolds number between 88 and 754 is performed. Understanding the energy transport between scales is crucial for Large-Eddy Simulation (LES). For an analysis of the energy transfer in LES, a transport equation for the second moment of the filtered scalar increment is introduced. In this equation new terms appear due to the interaction between resolved and unresolved scales, which are analyzed in the context of an a priori and an a posteriori test. It is further shown that LES using an eddy viscosity approach is able to fulfill the correct inter-scale energy transport for the present configuration.

Journal of fluid mechanics 803, 250-274 (2016). doi:10.1017/jfm.2016.489, Published by Cambridge Univ. Press, Cambridge [u.a.]

The geometrical properties of streamline segments (Wang, 2010 [1] ) and their bounding surface (Schaefer et al., 2012 [2] ) in direct numerical simulations (DNS) of different types of turbulent flows at different Reynolds numbers are reviewed. Particular attention is paid to the geometrical relation of the bounding surface and local and global extrema of the instantaneous turbulent kinetic energy field. Also a previously derived model equation for the normalized probability density of the length of streamline segments is reviewed and compared with the new data. It is highlighted that the model is Reynolds number independent when normalized with the mean length of streamline segments yielding that the mean length l m plays a paramount role as the only relevant length scale in the pdf. Based on a local expansion of the field of the absolute value of the velocity u along the streamline coordinate a scaling of the mean size of extrema of u is derived which is then shown to scale with the mean length of streamline segments. It turns out that l m scales with the geometrical mean of the Kolmogorov scale η and the Taylor microscale λ so that l m ∝ ( η λ ) 1 / 2 . The new scaling is confirmed based on the DNS cases over a range of Taylor based Reynolds numbers of Re λ = 50 – 300 .

For better understanding of turbulence, the geometry of turbulent structures in turbulent jet flow should be analyzed. The aim of the present work was to experimentally verify the dissipation element theory on highly resolved two-dimensional measurements turbulent jets using Rayleigh scattering technique. The statistical analysis of the characteristic parameters of dissipation elements; namely the linear length connecting the extremal points and the absolute value of the scalar difference at these points, respectively was also investigated. Rayleigh scattering was used to topographically produce 2D images of turbulent mixing to obtain the concentration distribution of two gases in a turbulent shear flow. The scalar field obtained was subdivided into numerous finite size regions. In each of these regions local extremal points of the fluctuating scalar are determined via gradient trajectory method. Gradient trajectories starting from any point in the scalar field ϕ ( x , y ) in the directions of ascending and descending scalar gradients will always reach a minimum and a maximum point where ∇ ϕ = 0. The dissipation element has two extremal points (one maximal and one minimal) and two saddle points at the boundaries.

By calculating gradient trajectories in direction of ascending and descending scalar gradients a local maximum and a local minimum point is reached. Dissipation elements may then be defined as the spatial region from which the same pair of maximum and minimum points in a scalar field is reached. By exploring the two-point correlation of the scalar gradient along such trajectories it was found that for large elements the mean velocity increment scales linearly with the arclength distance along the trajectory. This is different from the classical Kolmogorov scaling and has consequences for the modeling of the length distribution of dissipation elements.

Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray/vaporization model and a model for fully developed turbulent flame front propagation. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, the spark plasma channel is stretched and wrinkled by the local turbulence, excessive stretching results in spark re-strikes, large variations occur in turbulence intensity and local equivalence ratio along the spark channel, and ignition occurs in localized regions along the spark channel (based upon a Karlovitz-number criteria). In this paper, SparkCIMM is enhanced by: (1) an extended flamelet model to predict localized ignition spots along the spark plasma channel, (2) a detailed chemical mechanism for gasoline surrogate oxidation, and (3) a formulation of early flame kernel propagation based on the G-equation theory that includes detailed chemistry and a local enthalpy flamelet model to consider turbulent enthalpy fluctuations. In agreement with new experimental data from broadband spark and hot soot luminosity imaging, the model establishes that ignition prefers to occur in fuel-rich regions along the spark channel. In this highly-turbulent highly-stratified environment, these ignition spots burn as quasi-laminar flame kernels. In this paper, the laminar burning velocities and flame thicknesses of these kernels are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelets over a wide range of stoichiometry and exhaust gas dilution. The criteria for flame propagation include chemical (cross-over temperature based) and turbulence (Karlovitz-number based) effects. Numerical simulations using ignition models of different physical complexity demonstrate the significance of turbulent mixture fraction and enthalpy fluctuations in the prediction of early flame front propagation. A third paper on SparkCIMM (companion paper to this one) focuses on the importance of molecular fuel properties and flame curvature on early flame propagation and compares computed flame propagation with high speed combustion imaging and computed heat release rates with cylinder pressure analysis. The goals of SparkCIMM development are to (a) enhance our fundamental understanding of ignition and combustion processes in highly-turbulent highly-stratified engine conditions, (b) incorporate that understanding into a physically-based submodel for RANS engine calculations that can be reliably used without modification for a wide range of conditions (i.e., homogeneous or stratified, low or high turbulence, low or high dilution), and (c) provide a submodel that can be incorporated into a future LES model for physically-based modeling of cycle-to-cycle variability in engines.

Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray and vaporization model and a model for fully developed turbulent combustion. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, large variations in turbulence intensity, equivalence ratio, and enthalpy along the stretched and wrinkled spark plasma channel favor localized ignition spot formations in rich-mixture regions. In combination with strong local flow velocity, multiple successful ignition events along the re-striking spark lead to early non-spherical turbulent flame fronts. In this paper, SparkCIMM is enhanced by: (1) criteria to capture localized flame extinction phenomena, (2) a formulation of early flame kernel propagation based on the G-equation theory that includes effects of non-unity Lewis numbers, and (3) an extended equation to compute turbulent burning velocities of stretched flames in stratified mixtures. Localized rich ignition along the spark leads to early flames, whose propagation is, due to initially small turbulent Damkohler numbers, significantly influenced by molecular fuel properties. The analysis reveals that non-unity Lewis number curvature effects, intensified by heavy dilution by exhaust gas recirculation, strongly affect the early flame-kernel development in spray-guided gasoline engines. In particular, these effects significantly bias the flammability limit of flame kernels towards rich-mixtures while inhibiting their propagation in lean regions. Favorable initial conditions for combustion are found in rich-mixture regions, albeit in the presence of substantial equivalence ratio fluctuations and scalar dissipation rates. This paper demonstrates that the full complexity of the model equations developed here is required to reproduce the characteristic experimental features (spark channel stretching, multiple re-strikes, localized flame kernel formation, and early turbulent flame front corrugation) of spray-guided ignition phenomena.

Fuels that are more resistant to autoignition allow more time for mixing before combustion occurs and help to reduce nitrogen oxides (NO x) and smoke in a diesel engine. However, hydrocarbon (HC) and carbon monoxide (CO) emissions are high at low loads because combustion is more likely to take place in lean mixture packets with better mixing caused by longer ignition delays. These problems can be significantly alleviated by managing the mixture strength by changing the injection pressure and the nozzle geometries. A single-cylinder diesel engine is run on a mixture of gasoline and diesel with a research octane number of 91, at different speeds, loads, and exhaust gas recirculation levels using three different nozzles. It is much easier to obtain low NO x and low smoke emissions with this fuel than with a European diesel fuel using the standard nozzle. Larger injector holes and lower injection pressures help to reduce the HC and CO emissions at low loads and also enable the gasoline-like fuel to run at a higher speed of 4000 r/min at a reasonably high load (indicated mean effective pressure) of 10 bar.

Subject of this work is a dynamic simulation strategy for PCCI combustion that can be used in closed-loop control development. A detailed multi-zone chemistry model for the high-pressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting stationary model is capable of describing PCCI combustion sufficiently well. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created. This paper summarizes an important outcome of the the Collaborative Research Centre "SFB 686 - Modellbasierte Regelung der homogenisierten Niedertemperatur-Verbrennung" at RWTH Aachen University and Bielefeld University, Germany.

The mechanisms related to the secondary splitting of zero-gradient points of scalar fields are analyzed using the two-dimensional case of a scalar extreme point lying in a region of local strain. The velocity field is assumed to resemble a stagnation-point flow, cf. Gibson (Phys Fluids 11:2305–2315, 1968), which is approximated using a Taylor expansion up to third order. The temporal evolution of the scalar field in the vicinity of the stagnation point is derived using a series expansion, and it is found that the splitting can only be explained when the third-order terms of the Taylor expansion of the flow field are included. The non-dimensional splitting time turns out to depend on three parameters, namely the local Peclet number Pe δ based on the initial size of the extreme point δ and two parameters which are measures of the rate of change of the local strain. For the limiting casePe δ → 0, the splitting time is found to be finite but Peclet-number independent, while for the case of Pe δ → ∞ it increases logarithmically with the Peclet number. The physical implications of the two-dimensional mathematical solution are discussed and compared with the splitting times obtained numerically from a Taylor–Green vortex.

Recent research in modern combustion technologies, like partial homogeneous charge compression ignition (PCCI), demonstrates the capability of reducing pollutant emissions, e.g. soot and NOX. In addition to this advantage, a possibility to reduce fuel consumption and noise production by model-based optimal control is presented in this paper. In order to understand the basic properties of the PCCI mode, process measurements were conducted using a slightly modified series diesel engine. Control variables are engine combustion parameters: the indicated mean effective pressure, the combustion average and the maximum gradient of the cylinder-pressure. Control inputs are the parameters: quantity of injected fuel, start of injection and the intake manifold fraction of recirculated exhaust gas. The process has very fast, almost proportional behaviour over the engine's working cycles. Focusing on the static behaviour of the process, a nonlinear neural network model is used for identification. Successive linearization of the nonlinear network is used to build an affine internal controller model for the actual operating point. The presented controller structure is able to consider constraints by individual formulation of the cost function. With this configuration the closed-loop process is able to track the combustion setpoints with high control quality with minimal possible fuel consumption and combustion noise.

A detailed chemistry mixed-mode flamelet model for the prediction of combustion in spark-assisted homogeneous charge compression ignition (HCCI) engines is presented in this paper. The complex phenomena of spark-channel processes (turbulent corrugation, multiple restrikes) and of early flame kernel propagation (localized flame kernel formation, non-spherical early flame shapes), both induced by spray-guided spark-ignition combustion initiation, are captured by the recently introduced SparkCIMM ignition model. In this paper, laminar burning velocities and flame thicknesses are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelet calculations to appropriately consider locally rich, highly diluted, and auto-igniting stratified mixtures. Flame extinction criterions are formulated and incorporated into an extended G -equation flame front tracking scheme. Auto-ignition processes in the unburnt mixture, controlled by the flame-induced pressure and temperature increase, are captured by a recently developed multi-zone flamelet model that accounts for detailed chemistry and effects of scalar mixing on turbulent combustion. The laminar burning velocity is shown to increase significantly as the flame propagates into the chemically reacting mixture within the first stage of auto-ignition. After the initiation of the thermal runaway, however, flame extinction occurs rapidly. These interactions of mixed-mode combustion processes are captured by the presented flamelet model. It is developed based on a time and length scale analysis, revealing a scale separation between the ignition delay of diffusion combustion and the flame time of flame propagation. The analysis of the mixture preparation process, along with the simulation of turbulent flame front propagation and its extinction, demonstrates that the prediction of combustion in spark-assisted HCCI engines requires the on hand comprehensive mixed-mode combustion model. A comparison of simulation results from this new model with data from experiments and combustion models of reduced physical complexity proofs its qualification.

The ignition delays of n -butanol, a potential bio-fuel candidate, have been determined in a high-pressure shock tube. Conditions behind the reflected shock are approximately between 10–42 bar and 770–1250 K. To our knowledge, the ignition delay measurements of butanol at these high pressures are the first of their kind. CH emission and pressure time histories have been probed to determine ignition delay times for all experiments. For stoichiometric fuel–air-mixtures the influence of the temperature and pressure has been characterized. Interestingly the experimental data deviate from the Arrhenius behavior for temperatures lower than 1000 K. This is in contrast to simulation results which have been obtained by employing the simulation tool CANTERA with different reaction mechanisms applying the typical assumption of isochoric conditions. It has been found out that a positive pressure and temperature gradient behind the reflected shock has a significant influence on the ignition delay below 1000 K causing a pronounced decrease in the ignition delay times. This change of the conditions behind the reflected shock is attributed to the shock attenuation and probably from pre-ignition. Including the measured pressure gradients and assuming an isentropic compression behind the reflected shock, the simulation data and the experimental results show a same trend in the temperature dependence of the ignition delay. Nevertheless, striking differences between experiment and simulation persist especially for higher pressures. By performing sensitivity analysis at different temperatures some critical reactions could be identified and their role under our experimental conditions is discussed. In summary it can be stated that the employed reaction mechanisms may not be fully applicable to high-pressure conditions and it seems plausible that the lack of more detailed low temperature fuel specific reactions could be the probable cause for the discrepancies which calls for detailed investigations at elevated pressures.

In order to study possibilities of Model Predictive Control (MPC) to low temperature combustion, the authors performed a numerical study of combustion with a lean highly diluted methane/air mixture in a perfectly stirred reactor using a detailed chemical kinetic model. Chosen conditions are the following: equivalence ratio 0.6, residence time of mixture in reactor 0.5 s; molar fraction of Nitrogen 0.9, temperature of incoming unburned gases and reactor 1100 K, and heat loss coefficient 2 × 10−3 cal/(cm2Ks). At these conditions strong oscillations are predicted in agreement with previous experimental findings of M. de Joannon et al. (2004). It is found that, at low temperatures (

A burner, which stabilizes near-one-dimensional low-temperature flames at atmospheric pressure, was designed to access the combustion regime near 1500 K for quantitative species diagnostics. Combustion temperatures between 1300 and 1800 K in argon-diluted methane-oxygen flames were achieved by preheating the burner and adapting the inert gas flow. Mass spectrometry with electron ionization was used to determine mole fractions profiles of reactants, products, and intermediates. Combustion parameters were varied including stoichiometry, diluent mole fraction and preheat temperature. Mole fraction profiles resemble those taken in regular premixed flat flames. A number of C-1- and C-2-intermediates as well as some oxygenated species were identified. Higher-mass species (m/z > 42) were not detected in the low-temperature methane-oxygen flames which contain 90% argon in the cold gases. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Direct Numerical Simulations (DNS) of Kolmogorov flows are performed at three different Reynolds numbers Re λ between 110 and 190 by imposing a mean velocity profile in y-direction of the form U(y) = F sin(y) in a periodic box of volume (2π)3. After a few integral times the turbulent flow turns out to be statistically steady. Profiles of mean quantities are then obtained by averaging over planes at constant y. Based on these profiles two different model equations for the mean dissipation e in the context of two-equation RANS (Reynolds Averaged Navier–Stokes) modelling of turbulence are compared to each other. The high Reynolds number version of the k-e-model (Jones and Launder, Int J Heat Mass Transfer 15:301–314, 1972), to be called the standard model and a new model by Menter et al. (2006), to be called the Menter–Egorov model, are tested against the DNS results. Both models are solved numerically and it is found that the standard model does not provide a steady solution for the present case, while the Menter–Egorov model does. In addition a fairly good quantitative agreement of the model solution and the DNS data is found for the averaged profiles of the kinetic energy k and the dissipation e. Furthermore, an analysis based on flow-inherent geometries, called dissipation elements (Wang and Peters, J Fluid Mech 608:113–138, 2008), is used to examine the Menter–Egorov e model equation. An expression for the evolution of e is derived by taking appropriate moments of the equation for the evolution of the probability density function (pdf) of the length of dissipation elements. A term-by-term comparison with the model equation allows a prediction of the constants, which with increasing Reynolds number approach the empirical values.

New combustion methods for engines have been recently researched very intensively. In diesel engines, the homogenisation of the air-fuel mixture by early fuel injection has significant effects on emission reduction. The paper presents a model-based optimal control strategy for premixed charge compression ignition (PCCI) low temperature combustion in diesel engines. In order to understand the basic properties of the PCCI mode, static and dynamic measurements were conducted using a real conventional diesel engine. The main inputs of the combustion process are the exhaust gas recirculation rate and injection parameters. Outputs are the indicated mean effective pressure and the fuel mass conversion balance point. The process has very fast, almost proportional dynamics over the engine's working cycles. Focusing on the static behaviour of the process, a nonlinear neural network model is used for identification. Successive linearisation of the nonlinear network is used as predictive controller model. The presented controller structure is able to consider constraints and can be computed very fast. Finally, the controller is validated under real time conditions by experimental tests at the engine test bench. Although the controller structure contains a model and a convex optimisation step with regards to constraints, its implementation is very simple, as no observer is used, and the linearised model consists of static gains only.

Subject of this work is the recently introduced extended Representative Interactive Flamelet (RIF) model for multiple injections. First, the two-dimensional laminar flamelet equations, which can describe the transfer of heat and mass between two-interacting mixture fields, are presented. This is followed by a description of the various mixture fraction and mixture fraction variance equations that are required for the RIF model extension accounting for multiple injection events. Finally, the modeling strategy for multiple injection events is described: Different phases of combustion and interaction between the mixture fields resulting from different injections are identified. Based on this, the extension of the RIF model to describe any number of injections is explained. Simulation results using the extended RIF model are compared against experimental data for a Common-Rail DI Diesel engine that was operated with three injection pulses. Simulated pressure curves, heat release rates, and pollutant emissions are found to be in good agreement with corresponding experimental data. For the pilot injection and the main or post injection, respectively, different ignition phenomena are pointed out and the influence of the scalar dissipation rate on these ignition phenomena is detailly investigated. 2009 SAE International.

The objective of this work is to present an innovative interactively coupled CFD-multi-zone approach. In a consistent manner, the approach combines detailed flow field information obtained from CFD with detailed chemical kinetics solved in a multi-zone model. Combustion and pollutant formation in an HCCI engine with recompressing VVA strategy are numerically investigated using the interactively coupled CFD-multi-zone approach. A surrogate fuel for gasoline is used in the simulation that consists of n-heptane (18% liquid volume fraction) and isooctane (82% liquid volume fraction). The underlying complete reaction mechanism comprises 482 elementary reactions and 115 chemical species. The interactively coupled CFD-multi-zone approach shows to be accurate enough to describe HCCI chemistry, and is at the same time economical enough to allow application in an industrial environment. For the test case investigated, the simulation results are compared to experimental data that has been obtained using real gasoline. The overall agreement between simulation and experiment is found to be very good.

Spherical flames of n -heptane, iso-octane, PRF 87 and gasoline/air mixtures are experimentally investigated to determine laminar burning velocities and Markstein lengths under engine-relevant conditions by using the constant volume bomb method. Data are obtained for an initial temperature of 373 K, equivalence ratios varying from ϕ = 0.7 to ϕ = 1.2 , and initial pressures from 10 to 25 bar. To track the flame front in the vessel a dark field He–Ne laser Schlieren measurement technique and digital image processing were used. The propagating speed with respect to the burned gases and the stretch rate are determined from the rate of change of the flame radius. The laminar burning velocities are obtained through a linear extrapolation to zero stretch. The experimentally determined Markstein numbers are compared to theoretical predictions. A reduced chemical kinetic mechanism for n -heptane and iso-octane was derived from the Lawrence Livermore comprehensive mechanisms. This mechanism was validated for ignition delay times and flame propagation at low and high pressures. In summary an overall good agreement with the various experimental data sets used in the validation was obtained.

Combustion modeling using the R epresentative I nteractive F lamelet (RIF) model has proven successful in predicting Diesel engine combustion. The RIF model was previously used for Diesel engine combustion processes with not more than two consecutive injections into the combustion chamber. In this study, the RIF model is extended allowing for any number of injection events. First, the two-dimensional laminar flamelet equations, which can describe the transfer of heat and mass between two-interacting mixture fields, are introduced. This is followed by a description of the various mixture fraction and mixture fraction variance equations that are required for the model extension accounting for multiple injection events. Finally, the modeling strategy for multiple injection events is derived: Different phases of combustion and interaction between the mixture fields resulting from different injections are identified. Based on this, the extension of the RIF model to describe any number of injections is put forward. Simulation results using the extended RIF model are compared against experimental data for a Common-Rail DI Diesel engine that was operated with three injection pulses. For the pilot injection and the main or post injection, respectively, different ignition phenomena are pointed out and the influence of the scalar dissipation rate on these ignition phenomena is investigated in detail.

Very rich premixed outward propagating spherical flames of n -heptane ( ϕ = 3.5) and iso -octane ( ϕ = 3.9) spherical flames were experimentally investigated under microgravity conditions at 420 K and initial pressures of up to 30 bar. Inert gases with significantly different molecular weights were used as diluents leading to mixtures with varying effective Lewis numbers. The experimental setup consisted of a spherical closed pressurized vessel that enabled optical access. The experiments were performed in a drop tower under microgravity conditions to prevent the influence of buoyancy on the expanding flames. A Schlieren measurement technique combined with a high-speed CCD camera was used to track the expanding flames. During the entire experiment, the flames were laminar and smooth with no wrinkles due to combustion instabilities observed. This allowed for accurate measurements of the propagation speed and its dependence on stretch, which was then compared to theoretical predictions based a hydrodynamic model for relatively thin flames. The experimental and theoretical results show good agreement with each other. Investigation of such rich mixtures that are close to the upper flammability limit has not been reported before.

Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular Jet-A1, Jet-A, and JP-8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of n-decane 80% and 1,2,4-trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene. A chemical-kinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemical-kinetic mechanisms for the components: n-decane and 1,2,4-trimethylbenzene. Improvements are made to the previously developed chemical-kinetic mechanism for n-decane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemical-kinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemical-kinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene.