# Ulrich Maas's research while affiliated with Karlsruhe Institute of Technology and other places

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## Publications (222)

While thermomechanical analyses in solid and combustion processes in the gas phase are intensively studied separately, their interaction is less investigated. On one hand, the combustion system can be affected by the solid as a result of, for example, the heat conduction. On the other hand, the high-temperature flame can induce the thermal load in...

A new model to couple the multiple mapping conditioning (MMC) mixing model with manifold based simplified chemistry is proposed. The concept is based on the fact that the system state can be locally confined to a manifold with lower dimension, which is embedded in the manifold with higher dimension. Therefore, in the framework of the MMC mixing mod...

This work reports on the development and experimental validation of a detailed reaction mechanism for the oxidation of polyoxymethylene dimethyl ethers (OMEn, n = 1-3). The validation is done by constant-volume chamber laminar flame speeds (393 K and 443 K, 1 to 5 bar, and equivalence ratio 0.8 to 1.6) and Rapid Compression Machine ignition delay t...

In the present study, reduced kinetics considering both differential diffusion effects and strong heat losses, based on the Reaction–Diffusion Manifold (REDIM) method, formulated and constructed in generalized coordi-nates, is proposed. The approach fully coupled to a CFD solver is applied to the side-wall quenching (SWQ) of a laminar premixed dime...

Understanding flame initiation is of great interests to combustion research. For induced ignition, a minimum ignition energy (MIE) must be provided by an external ignition source for the formation of a flame kernel. In this paper, the minimum ignition energy in lean primary reference fuel (PRF) /air mixtures is investigated by simulations and exper...

The accurate prediction of the NOx formation has gained great attention in view of clean combustion. In this direction, a reliable reduction technique for the chemical kinetics is important to capture the NOx formation accurately with reduced computational costs for the practical turbulent combustion processes. This work focuses on the hierarchical...

The generation of renewable energy through carbon-neutral fuels has been initiated by several international institutions. Hydrogen, ammonia and bio-methane are considered the available and most potential solutions for this transfer. However, the technologies for its reliable storage and optimized combustion are still under development or in the pro...

The generation of renewable energy through carbon-neutral fuels has been initiated by several international institutions. Hydrogen, ammonia and bio-methane are considered the available and most potential solutions for this transfer. However, the technologies for its reliable storage and optimized combustion are still under development or in the pro...

A chemical kinetic mechanism for ammonia decomposition and combustion from the CRECK modeling group is analyzed in this study with respect to the fast and slow chemical processes and the results are used to generate skeletal mechanisms for different applications. As examples two regimes of system parameters are chosen: values typical for engine exh...

Urea-water-solution (UWS) is often used as a source for the reduction agent ammonia for the selective catalytic reduction of nitric oxides in combustion after-treatments systems. In applications, however, some technical problems, for example the formation of solid residuals or incomplete decomposition of urea, appear. To solve these problems comple...

The Reaction-Diffusion Manifold (REDIM) method is a generic manifold based reduced kinetic model that has been applied to various combustion systems. In this work, it is extended and applied to combustion processes including charged species. In order to incorporate ions in the REDIM method, electrons and ions are accounted for in the coupling of ki...

In manifold-based reduced models, a manifold is often generated based on an a-priori identification. Subsequently, a successful reduced kinetic computation is largely dependent on the proper choice of the model assumptions. To relax this restriction, a model reduction on the fly technique is proposed for the Reaction-Diffusion Manifold (REDIM) meth...

We investigate a species and energy conversion process applied in an internal combustion engine. CO2 and CH2 is converted to synthesis gas (CO and H2). This reaction is endothermic and in addition requires high temperatures. The temperature is increased via compression and a parallel exothermic reaction. We investigate this process with the aim of...

Manifold based simplified chemistry is an efficient reduction technique for the chemical kinetics, which aims to reduce the computational effort in numerical simulations. While the concept of reduced chemistry has been used for decades and various models have been developed till now, their coupling with turbulent physical processes (e.g. mixing pro...

Detailed chemistry simulations of turbulent reacting flows involving combustion of hydrocarbons can easily exceed the available computational resources, depending on the dimensions of the chemical system. Previous work of the authors showed how the combination of the Eulerian Stochastic Fields (ESF) model with tabulated chemistry based on 2-dimensi...

In this work, a mathematical formulation for the calculation of the sensitivity of manifold based simplified chemistry with respect to the scalar gradient is derived. This methodology allows to answer how important the scalar gradient is for the Reaction-Diffusion Manifolds (REDIM) reduced chemistry and how sensitive the reduced scheme is with resp...

Ignition processes in laminar in counterflow configuration based on both detailed and reduced chemistry based on the Reaction-Diffusion Manifolds (REDIM) method are investigated. We choose in this work: i) ignition by hot gas and ii) spark ignition. The temporal development of thermo-kinetic quantities (e.g. temperatures and species concentrations)...

In the present work, the ignition of lean laminar premixed methane-air flames in a counterflow configuration is investigated using detailed numerical simulations. The dependence of the minimum energy necessary for a successful ignition on various parameters (spark geometry, spark duration, mixture composition, flow strain rates) is studied. A one-d...

This work focuses on the cryogenic impingement cooling performance depending on the given inflow conditions. An experimental setup is presented which allows a detailed observation of the time dependent temperature distribution in a solid body via a thermal imaging camera during the cooling process. The space- and time-dependent heat transport from...

Due to the importance of gas-phase chemical reaction kinetics in low-emission combustion, stirred tank reactors have been used for decades as an experimental tool to study high- and low-temperature oxidation. A Jet-Stirred Reactor (JSR) setup is valuable to determine the evolution of species mole fractions. For the accuracy of the experimental resu...

The aim of this work is to contribute to the better understanding of the oxidation process of light hydrocarbons, low calorific value gas and components of natural gas in the presence of inert gases and the influence of oxygen reduction in exhaust gas recirculation situations. The extinction behavior of laminar counterflow diffusion flames of metha...

The Eulerian stochastic fields (ESF) combustion model can be used in LES in order to evaluate the filtered density function to describe the process of turbulence–chemistry interaction. The method is typically computationally expensive, especially if detailed chemistry mechanisms involving hydrocarbons are used. In this work, expensive computations...

Spark ignition by low-energetic electrical discharges in combustible fuel/air mixtures is a significant safety risk in various industries. In the present paper, OH-LIF measurements and numerical simulations of the ignition and early flame propagation of hydrogen/air, propane/air and ethene/air mixtures are reported. The experiments, carried out at...

In this study, the dependence of minimum ignition energies (MIE) on ignition geometry, ignition source radius and mixture composition is investigated numerically for methane/air and iso-octane/air mixtures. Methane and iso-octane are both important hydrocarbon fuels, but differ strongly with respect to their Lewis numbers. Lean iso-octane air mixtu...

Side-wall quenching (SWQ) is one of the generic configurations for flame-wall interaction and has been widely investigated through simulations using detailed and reduced chemical kinetics. In all previous related studies using manifold-based reduced kinetic models, the reduced model equations are solved in thermokinetic coordinates. This study, by...

The aim of this work is to contribute to a better understanding of the oxidation characteristics of light C2 hydrocarbons (C2H2 , C2H4 , C2H6) in interaction with inert gases and the influence of oxygen reduction for example in exhaust gas recirculation and staged combustion situations. For these gas mixtures, the extinction strain rates (ESR) and...

In this work Reaction-Diffusion Manifolds (REDIMs) reduced chemistry is used in the simulation of turbulent non-premixed flames based on a transported-Probability Density Function (PDF) model. Differential molecular diffusion is applied in the generation of the manifolds. This is the first work to consider the gradients of the reduced variables as...

Head-on quenching is a canonical configuration for flame-wall interaction. In the present study, the transient process of a laminar premixed flame impinging on a wall is investigated for different strain rates, while previous studies with detailed chemistry and transport focused only on unstrained conditions. Increasing strain rate leads to a reduc...

Diethyl ether (DEE) is an interesting species for combustion for at least two reasons: On the one hand, it is used as a kind of ”worst case” reference substance for studies concerned with the prevention of accidental ignition events. On the other hand, it is also a candidate bio-fuel. For this reason, in this work, auto-ignition of two different /D...

The Global Quasi-Linearization (GQL) method for model reduction of chemical kinetics is applied to describe the very sensitive regime of the onset of the thermal-diffusion oscillations of the rich hydrogen-air flames. This type of flame propagation is characterized by a complex interaction of chemical reaction, ordinary diffusion, as well as therma...

We study the auto-ignition behavior of several natural gas surrogates; in particular, the influence of ethane and propane on the ignition delay time of methane is studied. A rapid compression machine was used to obtain experimental measurements of ignition delay times at two different compression pressures (10 and 15 bar) and a wide range of compre...

Chemical kinetics govern the dynamics of chemical systems leading towards chemical equilibrium. There are several general properties of the dynamics of chemical reactions such as the existence of disparate time scales and the fact that most time scales are dissipative. This causes a transient relaxation to lower dimensional attracting manifolds in...

Extinction limits are important quantities of counterflow diffusion flames. An accurate prediction of extinction limits is necessary for the design of engineering combustion devices involving flame quenching. In this work, the reaction–diffusion manifold (REDIM) reduced chemistry with a detailed transport model is applied for the numerical investig...

Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quen...

Cooling of machining operations by liquid nitrogen is a promising approach for reducing cutting temperatures, increasing tool life and improving the workpiece surface integrity. Unfortunately, the cooling fluid tends to evaporate within the supply channel. This induces process variations and hinders the use of nitrogen cooling in commercial applica...

In this work, the Reaction-Diffusion Manifold (REDIM) method, a method for model reduction, is applied to a premixed isooctane-air system with Flame-Wall-Interactions (FWI). In order to provide a highly accurate reduced kinetic model, a detailed model for the diffusive processes is applied and complex boundary conditions that account for heterogene...

Reduced models for the evaporation and decomposition of urea-water solution in exhaust gas systems are developed to improve computational efficiency for detailed simulations. The models describe a droplet or wall film of urea water solution in hot exhaust gas, which is simulated in detail including gas phase chemistry and an evaporation model for t...

For general reacting flows the numerical simulation faces two main challenges. One is the high dimensionality and stiffness of the governing conservation equations due to detailed chemistry, which can be solved by using simplified chemical kinetics. The other one is the difficulty of modeling the coupling of turbulence with thermo-chemical source t...

In this study, multi-dimensional molecular transport phenomena during Flame-Wall-Interactions (FWI) and their effects on model reduction strategies are investigated. In order to access the problem, the standard configurations of a two-dimensional Side-Wall Quenching (SWQ) flame and a one-dimensional Head-On Quenching (HOQ) flame are used and compar...

The dynamic behavior and structures of laminar counterflow diffusion methane flames under oscillatory strain rates are investigated based on both detailed and reduced chemistry. It is known both from experiment and numerical studies that flame can deviate from its quasi-steady manner for high frequencies, which makes the well-known steady laminar f...

This paper introduces a generic model for predicting the heat transfer during cryogenic cooling in machining processes in a comprehensive way. The model is based on a flow of coolant over a hot surface that is dominated by the Leidenfrost effect. To get an overall description of the problem, it is essential not only to consider the gas phase, but a...

In this work, the auto-ignition of two polyoxymethylene dimethyl ethers, namely OME2 and OME3 (CH3–O–CH2–On–CH3 with n=2 and n=3 respectively), is investigated in a rapid compression machine (RCM). Ignition delay times (IDT) of stoichiometric OME2/air and OME3/air mixtures were measured for pressures ranging from 3 to 10 bar, and temperatures rangi...

Combustion simulations involve the modeling of chemical kinetics, and due to the complexity of detailed mechanisms, chemistry reduction techniques are necessary. One model reduction strategy is the reaction-diffusion manifold (REDIM) method, and to obtain the REDIM, an evolution equation must be solved till its stationary solution and a gradient es...

During the last decades, tabulated chemistry approaches, like manifold-based concepts to implement model reduction, have become a widespread, promising and accurate method to take chemical reactions into account in computing reacting flows. However, there is a number of crucial issues concerning the generation and implementation of the tabulated ch...

In the context of explosion protection, very conservative safety factors need to be considered, e.g. in the design of electrical devices. This is due to standards which are mainly based on empirical data as opposed to a detailed knowledge of the underlying physiochemical processes. In this work, the early phase of ignition of burnable gas mixtures...

This paper investigates the concept of using piston engines as chemical reactors to convert CO2 to more useful substances. Conditions are presented under which significant conversions of CO2 and CH4 to synthesis gas (CO and H2) can be achieved in internal combustion engines (ICE). Numerical optimization of the process based on detailed chemistry is...

The selective catalytic reduction (SCR) methodology is notably recognized as the widely applied strategy for NOx control in exhaust after-treatment technologies. In real SCR systems, complex unsteady turbulent multi-phase flow phenomena including poly-dispersed AdBlue® spray evolve with a wide ranging relative velocity between the droplet phase and...

The selective catalytic reduction (SCR) methodology is notably recognized as the widely applied strategy for NOX control in exhaust after-treatment technologies. In real SCR systems, complex unsteady turbulent multi-phase flow phenomena including poly-dispersed AdBlue® spray evolve with a wide ranging relative velocity between the droplet phase and...

In this work, two different reduction methods for simplification of chemical kinetics, the Global Quasi-linearization (GQL) and the Directed Relation Graph (DRG), are implemented and compared for ignition of CH4/air homogeneous reacting system. The GQL approach is an automatic and scaling invariant method for a global analysis of the combustion sys...

Combustion simulations involve modeling of chemical kinetics, and due to the complexity of detailed mechanisms, chemistry reduction techniques are necessary. One model reduction strategy is the reaction-diffusion manifold (REDIM). To obtain the REDIM, an evolution equation must be solved till its stationary solution and a gradient estimation is nee...

During machining of Ti-6Al-4V, high thermal loads arise, which demand for advanced cooling concepts, such as the application of liquid nitrogen. An efficient approach to analyze the thermomechanical mechanisms which influence the tool life and the workpiece distortions is the usage of coupled numerical simulations.
In this work, the Finite-Element-...

Reaction-diffusion manifolds (REDIM) based on a simple gradient estimate are employed to simulate methane/air turbulent flames. Laminar validation is performed for non-premixed flames. A RANS/transported-PDF strategy for the simulation of turbulent reacting flows is presented and used to validated the proposed model. Results show that the simple gr...

The problem of reduced kinetic models for auto-ignition is revisited. The hydrogen-air system is considered in a wide range of system parameters and initial conditions. The Global Quasi-Linearization (GQL) method is implemented to study the slow/fast time scale separation. Accordingly, the rate equations can be split into evolution equations for th...

The problem of a reduced model formulation for combustion processes is discussed in the present study. For this purpose, different reduced model equations are proposed and applied for the transient regime of a head-on quenching flame. The so-called Reaction-Diffusion manifolds (REDIM) with different reduced model equations are used as an example. T...

Turbulent stratified combustion is often found in practical combustion devices, however, for large eddy simulations (LES) of it is still a challenge. In the present work, LES of the Darmstadt turbulent stratified flame (TSF) cases are conducted. In total, one isothermal flow case A-i2 and four reacting cases with different combinations of stratific...

Numerical simulation of turbulent reacting flow is still a challenging task. For the efficient computational simulation and applicability for technical systems, simplifications for both chemistry and turbulence are needed. However, both simplifications are typically treated separately, without considering the coupling between them. In manifold base...

Two advanced reduction methods for systems of chemical kinetics, Global Quasi-linearization (GQL) and Directed Relation Graph (DRG), are implemented to reduce a homogeneous combustion system. In this work, the auto-ignition process of H2-air system is considered for a wide range of system parameters and initial conditions. Results based on GQL and...

In this paper, the quenching of a laminar premixed flame at a cold inert wall is investigated. Transient system behavior of the quenching flame is studied both by using well known and widely used reaction mechanisms. Reduced models are developed for each particular mechanism. The different detailed mechanisms are applied and reduced by implementing...

Chemistry pre-tabulation is well suited to include information of detailed reaction kinetics at reasonable computational costs to allow for simulations of realistic devices. In order to evaluate its accuracy in the near wall region, a study with numerical simulations of flame–wall-interaction is performed in this work. A laminar side-wall-quenching...

A hybrid RANS/Transported-PDF model for the simulation of turbulent reacting flows based on automatically reduced mechanisms for the chemical kinetics (reaction-diffusion manifold, REDIM) is presented in this work. For modelling of turbulent reacting flows, chemistry is a key problem and affects largely the accuracy. The PDF method has been widely...

Rate constants of elementary reactions involving unimolecular steps can be calculated from molecular data in a most general way by solving appropriate master equations. The conventional numerical solution requires rather a fine discretization applied over a sufficiently large energy range to achieve a reasonable accuracy. This leads to linear but v...

A recently developed automatic reduction method for systems of chemical kinetics, the so-called Global Quasi-Linearization (GQL) method, is implemented to study and reduce homogeneous combustion system. The results of application of the GQL and the Quasi-Steady State Assumption (QSSA) are compared. A number of drawbacks of the QSSA are discussed, i...

Selective catalytic reduction using urea-water-solution to reduce emissions of NOx from diesel engines is commonly used in the automotive industry. For a high efficiency of this process, a good understanding of the formation of ammonia from urea-water-solution droplets is required. There are two main variants for the description of urea decompositi...

The usage of a methane-fueled homogeneously charged compression ignition (HCCI) engine process for producing base chemicals like ethylene and synthesis gas together with some work output is investigated. This polygeneration process is studied by numerical modeling, accompanied by rapid compression machine experiments. Studies include the seeding of...

The probability density function (PDF) model has been widely used to study turbulent reacting flows since the chemical source term is in a closed form, avoiding any modelling. However, the accuracy of PDF calculations largely depends on the accurate modelling of chemical reactions and on the mixing model. In this work, the PDF model for the simulat...

The reduction of chemical kinetics describing combustion processes remains one of the major topics in the combustion theory and its applications. Problems concerning the estimation of reaction mechanisms real dimension remain unsolved, this being a critical point in the development of reduction models. In this study, we suggest a combination of loc...

A numerical investigation is carried out to study ignition events of different premixed stoichiometric fuel/air mixtures by hot exhaust gas jets. The simulations are performed for premixed, stoichiometric hydrogen/air, ethylene/air and propane/air mixtures in configurations relevant to safety applications. The ignition events of the different fuel/...

This study presents the simulation and detailed analysis of the ignition of initially quiescent fuel/air mixtures by small, stationary, laser-heated spherical particles. Our simulations cover a wide parameter space by varying the kind of fuel, stoichiometry, heating rate, radical surface destruction efficiencies as well as particle size. The result...

The ignition of combustible/air mixtures by electrical discharges includes several physical and chemical processes. In process safety often the total available electrical energy is taken as a risk measure. However, to get a more detailed insight in the ignition process, also energy losses to the surrounding have to be considered. Additionally, for...

The focus of this study is the development of a robust and accurate algorithm for model reduction of chemical kinetics for near wall reacting flows. The so-called Reaction-Diffusion Manifold (REDIM) method is employed for this purpose. The problem statement represents a fundamental difficulty for manifolds based model reduction concepts, since it i...

In this work, we present a reduced model for treating chemical reactions in combustion simulations, with special attention on combustion in IC engines. The model which is based on low-dimensional manifolds in state space, is able to describe auto-ignition, burning in quasi homogeneous media as well as chemical reactions which are strongly coupled w...

A novel extension of a rapid compression machine (RCM), namely a Rapid Compression Expansion Machine (RCEM), is described and its use for studying chemical kinetics is demonstrated. Like conventional RCMs, the RCEM quickly compresses a fuel/air mixture by pushing a piston into a cylinder; the resulting high temperatures and pressures initiate chemi...