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See detailEvaporation Rates and Bénard-Marangoni Supercriticality Levels for Liquid Layers Under an Inert Gas Flow
Machrafi, Hatim ULg; Sadoun, Nacer; Rednikov, Alexey et al

in Microgravity Science and Technology (2013)

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See detailTime-dependent Marangoni-Bénard instability of an evaporating binary-liquid layer including gas transients
Machrafi, Hatim ULg; Rednikov, A.; Colinet, P. et al

in Physics of Fluids (2013), 25(8),

We are here concerned with Bénard instabilities in a horizontal layer of a binary liquid, considering as a working example the case of an aqueous solution of ethanol with a mass fraction of 0.1. Both the ... [more ▼]

We are here concerned with Bénard instabilities in a horizontal layer of a binary liquid, considering as a working example the case of an aqueous solution of ethanol with a mass fraction of 0.1. Both the solvent and the solute evaporate into air (the latter being insoluble in the liquid). The system is externally constrained by imposing fixed "ambient" pressure, humidity, and temperature values at a certain effective transfer distance above the liquid-gas interface, while the ambient temperature is also imposed at the impermeable rigid bottom of the liquid layer. Fully transient and horizontally homogeneous solutions for the reference state, resulting from an instantaneous exposure of the liquid layer to ambient air, are first calculated. Then, the linear stability of these solutions is studied using the frozen-time approach, leading to critical (monotonic marginal stability) curves in the parameter plane spanned by the liquid layer thickness and the elapsed time after initial contact. This is achieved for different ratios of the liquid and gas thicknesses, and in particular yields critical times after which instability sets in (for given thicknesses of both phases). Conversely, the analysis also predicts a critical thickness of the liquid layer below which no instability ever occurs. The nature of such critical thickness is explained in detail in terms of mass fraction profiles in both phases, as it indeed appears that the most important mechanism for instability onset is the solutal Marangoni one. Importantly, as compared to the result obtained previously under the quasi-steady assumption in the gas phase [H. Machrafi, A. Rednikov, P. Colinet, and P. C. Dauby, Eur. Phys. J. Spec. Top.192, 71 (2011)]10.1140/epjst/e2011-01361-y, it is shown that relaxing this assumption may yield essentially lower values of the critical liquid thickness, especially for large gas-to-liquid thickness ratios. A good-working analytical model is developed for the description of such delicate transient effects in the gas. The analysis reveals that the system considered in this paper is generally highly unstable, the instability setting in even for very small times and liquid thicknesses. © 2013 AIP Publishing LLC. [less ▲]

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See detailEvaporation in motion
Machrafi, Hatim ULg; Rednikov, Alexey; Colinet, Pierre et al

Article for general public (2012)

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See detailSolutal Marangoni instability in a binary liquid layer evaporating into air: the importance of transients in the gas for highly unstable cases
Machrafi, Hatim ULg; Rednikov, Alexey; Colinet, Pierre et al

in Bulletin of the American Physical Society, Vol. 57, n°17 (2012) (2012)

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See detailSolutal Marangoni instability of binary mixtures evaporating into air: an analytical model describing highly unstable cases
Machrafi, Hatim ULg; Rednikov, Alexey; Colinet, Pierre et al

in Book of abstracts of the Seventh International Symposium on TWO-PHASE SYSTEMS FOR GROUND AND SPACE APPLICATIONS (2012)

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See detailExtremely Unstable Evaporative Bénard-Marangoni Systems: the Role of Transients in the Gas
Rednikov, Alexey; Machrafi, Hatim ULg; Dauby, Pierre ULg et al

in Book of Conference Abstracts of the International Marangoni Association, 6 (2012)

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See detailGreen Energy and Technology
Machrafi, Hatim ULg

Book published by Bentham Science Publishers (2012)

This new book presents recent developments, state-of-the-art and progresses in the field of energy where efforts are done in order to improve the usability of energy systems, reducing their environmental ... [more ▼]

This new book presents recent developments, state-of-the-art and progresses in the field of energy where efforts are done in order to improve the usability of energy systems, reducing their environmental impact. The book aims at providing researchers, academics, engineers and advanced students information and points of discussion, a platform for future improvements in green energy. Both theoretical and applied aspects are treated in this book. Many illustrations and mathematical equations as well as practical on-the-field applications are incorporated. This book aims at contributing to the increasing interest in reducing the environmental impact of energy as well as its further development. Three parts are considered. The first part treats different energy applications and the efforts that are done in order to improve their impact on the environment. It deals with bio-energy, well-to-wheel analyses and heavy oils. Concerning bioenergy, the costs are evaluated under strict regulations imposed by a climate stabilization target, where a certain case is taken as example. The well-to-wheel energy analyses take into consideration a comprehensive study on greenhouse gases and certain criteria that concern the emissions in the case of hydrogen based fuel-cells. The second part deals with different kinds of renewable energy sources. The chapters discuss bioenergy, wind energy and solar energy. The use of biofuels is critically discussed. A real application and evaluation of wind energy is presented, by means of designing and optimalizing a wind power system. A detailed discussion about the generation of electricity by means of different kinds of solar energy is performed. The third part puts emphasis on alternative energy processes for transport utilities. In this part, is discussed, amongst others, the Homogeneous Charge Compression Ignition combustion mode. Advantages and inconveniences are proposed. Wide and comprehensive studies on this combustion mode are presented. Both a large synthesis of available work and experimental results are used in an effort to discuss these advantages and inconveniences in order to propose possible solutions. Some examples are treated elaborately. Hydrogen-based combustion is discussed and its efficiency evaluated in detail. Elaborate studies and examples give a rather complete vision of the probable use of hydrogen in internal combustion engines and the different aspects that are to be examined are treated in detail. The commercialization of fuel-cells are investigated, giving trends and possible developments that can be envisaged. Finally, a philosophy of controlling the auto-ignition in an HCCI engine is presented. This philosophy is somewhat general and the outlines can also be used for the abovementioned transportation modes. A rather general overview is given in this book, starting from different visions on reducing the impact of energy on the environment (green energy) and continuing on how this can be achieved (green technology). The different results and the available literature that is treated in various chapters in this book show that many researchers are investigating the efficient use of energy whilst looking for ways to decrease its impact on the environment. This shows the importance of such research and the work that remains to be done. [less ▲]

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See detailPhilosophy for controlling auto-ignition in an HCCI engine
Machrafi, Hatim ULg

in Machrafi, Hatim (Ed.) Green Energy and Technology (2012)

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See detailBénard instabilities in a binary-liquid layer evaporating into an inert gas: stability of quasi-stationary and time-dependent reference profiles
Machrafi, Hatim ULg; Rednikov, Alexey; Colinet et al

in European Physical Journal. Special Topics (2011), 192(1), 71-81

This study treats an evaporating horizontal binary-liquid layer in contact with the air with an imposed transfer distance. The liquid is an aqueous solution of ethanol (10 % wt). Due to evaporation, the ... [more ▼]

This study treats an evaporating horizontal binary-liquid layer in contact with the air with an imposed transfer distance. The liquid is an aqueous solution of ethanol (10 % wt). Due to evaporation, the ethanol mass fraction can change and a cooling occurs at the liquid-gas interface. This can trigger solutal and thermal Rayleigh-B´enard-Marangoni instabilities in the system, the modes of which corresponding to an undeformable interface form the subject of the present work. The decrease of the liquid-layer thickness is assumed to be slow on the diffusive time scales (quasi-stationarity). First we analyse the stability of quasistationary reference profiles for a model case within which the mass fraction of ethanol is assumed to be fixed at the bottom of the liquid. Then this consideration is generalized by letting the diffusive reference profile for the mass fraction in the liquid be transient (starting from a uniform state), while following the frozen-time approach for perturbations. The critical liquid thickness below which the system is stable at all times quite expectedly corresponds to the one obtained for the quasi-stationary profile. As a next step, a more realistic, zero-flux condition is used at the bottom in lieu of the fixed-concentration one. The critical thickness is found not to change much between these two cases. At larger thicknesses, the critical time at which the instability first appears proves, as can be expected, to be independent of the type of the concentration condition at the bottom. It is shown that solvent (water) evaporation plays a stabilizing role as compared to the case of a non-volatile solvent. At last, an effective approximate Pearson-like model is invoked making use in particular of the fact that the solutal Marangoni is by far the strongest as an instability mechanism here. [less ▲]

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See detailCO2 valorization by means of dielectric barrier discharge
Machrafi, Hatim ULg

in Journal of Physics: Conference Series (2011), 275(1, art_number=012016),

As atmospheric pollution is causing several environmental problems it is incumbent to reduce the impact of pollution on the environment. One particular problem is the production of CO2 by many transport ... [more ▼]

As atmospheric pollution is causing several environmental problems it is incumbent to reduce the impact of pollution on the environment. One particular problem is the production of CO2 by many transport and industrial applications. Instead of stocking CO2 and instead of being a product, it can be used as a source. The case considered is the CO2 reformation of methane producing hydrogen and CO. It is an endothermic reaction, for which the activation barrier needs to be surpassed. This can be done efficiently by the method of Dielectric Barrier Discharge. The process relies on the collision of electrons, which are accelerated under an electrical field that is created in the discharge area. This leads to the formation of reactive species, which facilitate the abovementioned reaction. This study is performed using a Matlab program with the Reaction Engineering module in COMSOL (with an incorporated kinetic mechanism) in order to model the discharge phase. Then COMSOL (continuity and Navier-Stokes equations) is used to model the flow in the post-discharge phase. The results showed that both a 2D and 3D model can be used to model the chemical-plasma process. These methods need strongly reduced kinetic mechanism, which in some cases can cause loss of precision. [less ▲]

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See detailAn extended thermodynamic model of transient heat conduction at sub-continuum scales
Lebon, Georgy ULg; Machrafi, Hatim ULg; Grmela, M. et al

in Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences (2011), 467

A thermodynamic description of transient heat conduction at small length and timescales is proposed. It is based on extended irreversible thermodynamics and the main feature of this formalism is to ... [more ▼]

A thermodynamic description of transient heat conduction at small length and timescales is proposed. It is based on extended irreversible thermodynamics and the main feature of this formalism is to elevate the heat flux vector to the status of independent variable at the same level as the classical variable, the temperature. The present model assumes the coexistence of two kinds of heat carriers: diffusive and ballistic phonons. The behaviour of the diffusive phonons is governed by a Cattaneo-type equation to take into account the high-frequency phenomena generally present at nanoscales. To include non-local effects that are dominant in nanostructures, it is assumed that the ballistic carriers are obeying a Guyer–Krumhansl relation. The model is applied to the problem of transient heat conduction through a thin nanofilm. The numerical results are compared with those provided by Fourier, Cattaneo and other recent models. [less ▲]

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See detailCO2 reforming of methane: valorizing CO2 by means of Dielectric Barrier Discharge
Machrafi, Hatim ULg; Cavadias, Simeon; Amouroux, Jacques

in IOP Conference Series: Materials Science and Engineering (2011), 19(1), 012006

The impact of pollution on the environment is causing several problems that are to be reduced as much as possible. One important example is the production of CO2 that is emitted by many transport and ... [more ▼]

The impact of pollution on the environment is causing several problems that are to be reduced as much as possible. One important example is the production of CO2 that is emitted by many transport and industrial applications. An interesting solution is to view CO2 as a source instead of a product that can be stocked. The case considered in this work is the CO2 reformation of methane producing hydrogen and CO. It is an endothermic reaction, for which the activition barrier needs to be overcome. The method of Dielectric Barrier Discharge can do this efficiently. The process relies on the collision of electrons, which are accelerated under an electrical field that is created in the discharge area. This leads to the formation of reactive species, which facilitate the abovementioned reaction. The determination of the electron density is performed by PLASIMO. The study is subsequently continued using the Reaction Engineering module in COMSOL (with an incorporated kinetic mechanism) in order to model the discharge phase. Then COMSOL (continuity and Navier-Stokes equations) is used to model the flow in the post-discharge phase. The results showed that both a 2D and 3D model can be used to model the chemical-plasma process. These methods need strongly reduced kinetic mechanism, which in some cases can cause loss of precision. It is also observed that the present experimental set-up that is modeled needs to be improved. A suggestion is made. [less ▲]

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See detailInstability analysis of a horizontal evaporating binary liquid layer with transient reference profiles
Machrafi, Hatim ULg; Alexey, Rednikov; Colinet, Pierre et al

(2011)

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See detailThe Chemistry in HCCI Combustion and the Use of Reduced Kinetic Mechanisms for Controlling Strategies
Machrafi, Hatim ULg

in Lackner, M.; Winter, F.; Agarwal, A. K. (Eds.) Handbook of Combustion (2010)

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See detailDiesel and Gasoline for HCCI Combustion
Machrafi, Hatim ULg

in Acosta, M. J. (Ed.) Advances in Energy Research (2010)

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See detailExperimental validation of concentration profiles in an HCCI engine, modelled by a multi-component kinetic mechanism: Outline for auto-ignition and emission control
Machrafi, Hatim ULg

in Energy Conversion and Management (2010), 51(10), 2025-2031

In order to contribute to the auto-ignition and emission control for Homogeneous Charge Compression Ignition (HCCI), a kinetic multi-component mechanism, containing 62 reactions and 49 species for ... [more ▼]

In order to contribute to the auto-ignition and emission control for Homogeneous Charge Compression Ignition (HCCI), a kinetic multi-component mechanism, containing 62 reactions and 49 species for mixtures of n-heptane, iso-octane and toluene is validated in this work, comparing for the concentration profiles of the fuel, the total hydrocarbons, O-2, CO2, CO, acetaldehyde and iso-butene. These species are sampled during the combustion and quantified. For these measurements an automotive exhaust analyser, a gas chromatograph, coupled to a mass spectrometer and a flame ionisation detector are used, depending on the species to be measured. The fuel, total hydrocarbons, O-2, CO2, iso-butene and acetaldehyde showed a satisfactory quantitative agreement between the mechanism and the experiments. Both the experiments and the modelling results showed the same formation behaviour of the different species. An example is shown of how such a validated mechanism can provide for a set of information of the behaviour of the auto-ignition process and the emission control as a function of engine parameters. (C) 2010 Elsevier Ltd. All rights reserved. [less ▲]

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See detailUsing time-dependent reference profiles for an instability analysis of an evaporating binary liquid layer
Machrafi, Hatim ULg; Rednikov, Alexey; Colinet, Pierre et al

Poster (2010)

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See detailCO2 valorization by means of dielectric barrier discharge
Machrafi, Hatim ULg; Cavadias, S.; Amouroux, J.

(2010)

In order to reduce atmospheric pollution by CO2 (by transport and industrial applications), CO2 can be used as a source, instead of stocking it as a product. The case considered is the reaction between ... [more ▼]

In order to reduce atmospheric pollution by CO2 (by transport and industrial applications), CO2 can be used as a source, instead of stocking it as a product. The case considered is the reaction between CO2 and methane producing hydrogen: CO2 + CH4 2 CO + 2 H2 H° = 258.9 kJ/mole. The activation barrier of this endothermic reaction needs to be surpassed. The method of Dielectric Barrier Discharge (DBD) can be used, needing less energy than heating or catalytic pyrolysis. The process relies on the collision of electrons (forming reactive species), accelerated under an electrical field in the discharge area, facilitating the abovementioned reaction. The first step of the simulation in this work presents a 1D model in Matlab (Poisson equation, conservation of electrons and ions), obtaining an average electron and ion density. This is then introduced in a kinetic model of CH4/CO2 dissociation via electron collisions in the discharge region (Reaction Engineering module in COMSOL). Then COMSOL (continuity and Navier- Stokes equations) is used to model the flow in the post-discharge phase (reactions between vibrationally excited CO2 and CH4). Preliminary calculations for a plasma reactor model show that when the reactor wall is heated at 1000K, without electrical discharge, the decomposition of CO2 is 7.5%. With electrical discharge, the same decomposition is attained at a lower temperature, namely 880K. [less ▲]

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See detailSimulation of the formation of methanol from CO2 and H2, in electrocatalytic plasma chemical reactor
Machrafi, Hatim ULg; Cavadias, Simeon; Amouroux, Jacques

(2010)

In order to reduce atmospheric pollution by CO2 (by transport and industrial applications), CO2 can be used as a source, instead of stocking it as a product. The case considered is the reaction between ... [more ▼]

In order to reduce atmospheric pollution by CO2 (by transport and industrial applications), CO2 can be used as a source, instead of stocking it as a product. The case considered is the reaction between CO2 and H2 producing methanol: The reaction for the formation of methanol: CO2 + 3H2  CH3OH + H2O is strongly exothermique with an activation barrier needed to be surpassed and proceeds at high pressure and temperature with the use of a catalyst. The use of a non-thermal plasma namely a Dielectric Barrier Discharge (DBD) can generate energetic electrons which can initiate, excitation reactions (vibrational, electronic), dissociation, formation of radicals and ionization. Thus the activation of reactants (CO2 and H2) by the plasma can dramatically enhance the conversion of CO2. The aim of this work is to simulate this reaction associating electrical discharges and heterogeneous catalysis. The first step of the simulation in this work presents a 1D model in Matlab (Poisson equation, conservation of electrons and ions), obtaining an average electron and ion density. This electron density is then introduced in a gas phase kinetic model for the vibrational excitation and dissociation of H2/CO2 via electron collisions in the discharge region. The adsorption of radicals, produced in the gas phase, in the catalyst, is simulated using the commercial code COMSOL. The parameters studied in this work are the input power of the electrical discharge and the temperature of the catalyst on the conversion rate of CO2. [less ▲]

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