Stelios Rigopoulos

Imperial College London | Imperial · Department of Mechanical Engineering

In this work, two soot models were implemented and used with the high-order spectral element code Nek5000, a moment-based model (MOMIC) and a comprehensive discretized sectional model (DSM). The integretion scheme and the numerical treatment of the transport equations of soot scalars is presented. Numerical issues that have emerged during the imple...

The objective of the present study is to investigate turbulence–coagulation interaction via direct numerical simulation (DNS) coupled with the population balance equation (PBE). Coagulation is an important process in several environmental and engineering applications involving turbulent flow, including soot formation, gas-phase synthesis of nanopar...

We combine extended proper orthogonal decomposition (EPOD) together with the Fukagata-Iwamoto-Kasagi (FIK) identity to quantify the role of individual coherent structures on the wall mass transfer in a turbulent pipe flow. Direct numerical simulation at a Reynolds number of 5300 (based on bulk velocity) is performed with the passive scalar released...

Aerosol dynamics plays an important role in the modelling of soot formation in combustion processes, as it is responsible for predicting the distribution of size and shape of soot particles. The distribution is required for the correct prediction of the rates of surface processes, such as growth and oxidation, and furthermore it is important on its...

The objective of this paper is to present a finite volume method for the discretisation of the population balance equation with coagulation, growth and nucleation that combines: (a) accurate prediction of the distribution with a small number of sections, (b) conservation of the first moment (or any other single moment) in a coagulation process, (c)...

In recent times, the LES-PBE-PDF framework has been developed to couple large eddy simulation (LES) and population balance models (PBE) for the description of soot formation in turbulent hydrocarbon flames. This approach is based on a modeled evolution equation for the LES-filtered probability density function (pdf) associated with the instantaneou...

Detailed mechanisms for kerosene surrogate fuels contain hundreds of species and thousands of reactions, indicating a necessity for reduced mechanisms. In this work, we employ a framework that combines Rate-Controlled Constrained Equilibrium (RCCE) with Computational Singular Perturbation (CSP) for systematic reduction based on timescale analysis,...

In this article, we combine the large eddy simulation (LES) concept with the population balance equation (PBE) for predicting, in a Eulerian fashion, the evolution of the soot particle size distribution in a turbulent non-premixed hydrocarbon flame. In order to resolve the interaction between turbulence and chemical reactions/soot formation, the tr...

Many chemical and environmental processes involve the formation of a polydispersed particulate phase in a turbulent carrier flow. Frequently, the immersed particles are characterized by an intrinsic property such as the particle size, and the distribution of this property across a sample population is taken as an indicator for the quality of the pa...

The development of reduced chemical mechanisms in a systematic way has emerged as a potential solution to the problem of incorporating the increasingly large chemical mechanisms into turbulent combustion CFD codes. In this work, a methodology is proposed for developing reduced mechanisms with Rate-Controlled Constrained Equilibrium (RCCE) via a Com...

In this work, a methodology for the tabulation of combustion mechanisms via Artificial Neural Networks (ANNs) is presented. The objective of the methodology is to train the ANN using samples generated via an abstract problem, such that they span the composition space of a family of combustion problems. The abstract problem in this case is an ensemb...

Kerosene is an important commercial fuel, due to its use in gas turbines. The numerical modelling of these systems employing direct integration for solving the source reaction term is extremely expensive, due to the fact that comprehensive kerosene mechanisms involve hundreds of species. The number of reactive scalars can be reduced by the employme...

Many engineering applications, such as the formation of soot in hydrocarbon combustion or the precipitation of nanoparticles from aqueous solutions, encompass a polydispersed particulate phase that is immersed in a reacting carrier flow. From a Eulerian perspective, the evolution of the particulate phase both in physical and in particle property sp...

A discretised population balance equation (PBE) is coupled with an in-house computational fluid dynamics (CFD) code in order to model soot formation in laminar diffusion flames. The unsteady Navier–Stokes, species and enthalpy transport equations and the spatially-distributed discretised PBE for the soot particles are solved in a coupled manner, to...

In this work, a discretised Population Balance Equation (PBE) model is coupled with a detailed in-house Computational Fluid Dynamics (CFD) code, to investigate soot formation in a laminar co-flow non-premixed ethylene flame. The unsteady Navier-Stokes, species conservation and enthalpy transport equations are solved in a segregated manner, combined...

This paper investigates the potential of the RCCE mechanism reduction approach for modelling turbulent flames within the framework of transported PDF methods. For this purpose, PDF simulations are performed with an RCCE-reduced mechanism via direct integration of the RCCE ODEs, without any tabulation, and comparison is made with both the experiment...

Numerical schemes for reacting flows typically invoke the method of fractional steps in order to isolate the chemical kinetics model from diffusion/convection phenomena. Here, the reaction fractional step requires the solution of a collection of independent ODE systems which may be severely stiff. Recently, researchers have begun to explore the hig...

In this work we propose a chemistry tabulation approach based on Rate-Controlled Constrained Equilibrium (RCCE) and Artificial Neural Networks (ANNs) and apply it to two non-premixed and non-piloted, CH4/H-2/N-2 turbulent flames (DLR-A and DLR-B, with Re = 15,200 and 22,800, respectively). The objective of this approach is to train the ANNs with an...

The present work shows results obtained from the incorporation of a soot model into a combined Large Eddy Simulation and Conditional Moment Closure approach to modelling turbulent non-premixed flames. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, where acetylene is emplo...

We demonstrate the use of adaptive chemistry in large scale turbulent combustion simulations, in this case a large eddy simulation (LES) of turbulent non-premixed ethylene flames. An additional transport equation for the mixture fraction is solved, enabling the chemistry to be treated using the methodology of conditional moment closure (CMC). In th...

The objective of this paper is the simulation of a turbulent flame by employing the Rate-Controlled Constrained Equilibrium (RCCE) approach for the chemistry reduction, and Large-Eddy Simulation (LES) coupled with Conditional Moment Closure (CMC) for the turbulence-chemistry interaction modelling. RCCE is a systematic method for mechanism reduction...

Processes involving particle formation in turbulent flows feature complex interactions between turbulence and the various physicochemical processes involved. An example of such a process is aerosol formation in a turbulent jet, a process investigated experimentally by Lesniewski and Friedlander [Proc. R. Soc. London, Ser. A454, 2477 (1998)]. Polydi...

Polydispersed particles in reactive flows is a wide subject area encompassing a range of dispersed flows with particles, droplets or bubbles that are created, transported and possibly interact within a reactive flow environment - typical examples include soot formation, aerosols, precipitation and spray combustion. One way to treat such problems is...

A framework for automated development of reduced mechanisms is presented by combining the methodologies of level of importance (LOI) and rate-controlled constrained equilibrium (RCCE). It is shown that these methods are complimentary, as they deal with different aspects of the overall reduction problem: LOI is a method of determining the species go...

Turbulent precipitation is a complex problem, whose mathematical description of precipitation requires a coupling of fluid dynamics with the population balance equation (PBE). In the case of turbulent flow, this coupling results in unclosed equations due to the nonlinear nature of precipitation kinetics. In this article, we present a methodology fo...

Turbulent reactive flows with particle formation, such as soot formation and precipitation, are characterized by complex interactions between turbulence, scalar transport, particle formation and particle transport and inter-particle events such as coagulation. The effect of formation, growth and coagulation on the particle size distribution (PSD) m...

The method of Rate-Controlled Constrained Equilibrium (RCCE) provides a general and physically sound mathematical description of the dynamics of a chemical system, of which several species are assumed to be kinetically controlled and others equilibrated. In this paper we present an RCCE study of premixed hydrogen–air and methanol–air flames for a w...

Turbulent combustion is the ultimate multiscale problem, with chemical reactions exhibiting time scales spanning more than ten orders of magnitude and turbulent motion, introducing further. space and time scales. The integration of the chemical kinetics equations is severely hampered by their excessive stiffness, resulting from the range of time sc...

The dynamical evolution of a chemically reacting system is governed by the equations of chemical kinetics, which exhibit a wide range of time scales thus giving rise to stiff equations. In the rate-controlled constrained equilibrium method (RCCE), the dynamical evolution of the system is governed by the kinetics of the species associated with the s...

Rate-controlled constrained equilibrium provides a sound framework for deriving and testing low-dimensional models of combustion chemistry, yet its application so far has been limited. The purpose of this paper is twofold: to propose a formulation of RCCE as a differential-algebraic problem and show how existing formulations such as the constraint...

New computational techniques for the analysis and design of systems for the manufacture of particulate crystals have become available, and the more complex precipitation processes whereby crystallization follows fast chemical reactions have also been analysed more deeply. This progress has been aided by the growing power of the population balance a...

According to the concept of rate-controlled constrained equilibrium (RCCE), combustion kinetics can be reduced into a set of constraints whose relaxation guides the evolution of the system towards equilibrium. In this work, a formulation of RCCE in terms of thermodynamically derived variables (constraint potentials) for a constant pressure–enthalpy...

The outcome of gas−liquid precipitation in industrial reactors, such as bubble columns, is determined by the interplay between multiphase fluid dynamics; gas−liquid reaction engineering; and crystallization mechanisms such as nucleation, growth, and agglomeration. In this work, a modeling approach that takes the above phenomena into account is prop...

The coupling of turbulent mixing and chemical phenomena lies at the heart of multiphase reaction engineering, but direct CFD approaches are usually confronted with excessive computational demands. In this hybrid approach, the quantification of mixing is accomplished through averaging the flow and concentration profiles resulting from a CFD flow fie...

A first-order finite-element scheme is proposed for solving the 1-D dynamic population balance equation. The model includes the kinetic processes of nucleation, growth, aggregation, and breakage in a spatially uniform system with or without flow. The numerical scheme features improved stability by taking into account the convective nature of the gr...

As a result of the complexity of activated sludge systems, most biological water treatment plants have been designed by scale-up, based upon the empirical knowledge gained from pilot plant experiments and simulations. Due to a lack of applicable design methodologies that are able to handle the complexity of the activated sludge process models, syst...

As a result of the complexity of activated sludge systems, most biological water treatment plants have been designed by scale-up, based upon the empirical knowledge gained from pilot plant experiments and simulations. Due to a lack of applicable design methodologies that are able to handle the complexity of the activated sludge process models, syst...

A superstructure-based stochastic reaction-separation network optimization methodology has been applied to investigate and optimize the activated sludge process for combined nitrogen and chemical oxygen demand removal. The method indicated a new concept for process design; instead of employing separate aerobic and anoxic reactors, it is suggested t...

This paper presents a dynamic model of a bubble column reactor with particle formation, accomplished by adopting a hybrid CFD-reaction engineering approach. CFD is employed for estimating the hydrodynamics and is based on the two-phase Eulerian–Eulerian viewpoint. The reaction engineering model links the penetration theory to a population balance t...

The objective of this paper is the simulation of a turbulent lifted flame via employing Rate-Controlled Constrained Equilibrium (RCCE) for the chemistry and a Large-Eddy-Simulation (LES)-based Conditional Moment Closure (CMC) approach for the flow field and turbulence-chemistry interaction modeling. RCCE is a systematic method for mechanism reducti...

Rate-Controlled Constrained Equilibrium provides a sound framework for deriving and testing reduced mech-anisms. The dynamical evolution of a chemical system is envisaged as a series of partial equilibrium states, parametrised by a set of variables that may be concentrations of individual species or linear combinations of them. So far, applications...