A. Yu. Snegirev
Other affiliations: University of Central Lancashire
Bio: A. Yu. Snegirev is an academic researcher from Saint Petersburg State Polytechnic University. The author has contributed to research in topics: Combustion & Turbulent diffusion. The author has an hindex of 13, co-authored 27 publications receiving 452 citations. Previous affiliations of A. Yu. Snegirev include University of Central Lancashire.
TL;DR: In this paper, a Monte Carlo method for radiative heat transfer has been incorporated in CFD modeling of buoyant turbulent diffusion flames in stagnant air and in a cross-wind.
Abstract: A statistical (Monte Carlo) method for radiative heat transfer has been incorporated in CFD modeling of buoyant turbulent diffusion flames in stagnant air and in a cross-wind. The model and the computational tool have been developed and applied to simulate both burner flames with controlled fuel supply rate and in self-sustained pool fires with burning rates coupled with flame radiation. The gas–soot mixture was treated either as gray (using the effective absorption coefficient derived from total emissivity data or the Planck mean absorption coefficient) or as non-gray (using the weighed sum of gray gases model). The comparison of predicted radiative heat fluxes indicates applicability of the gray media assumption in modeling of thermal radiation in case of high soot content. The effect of turbulence-radiation interaction is approximately taken into account in calculation of radiation emission, which is corrected to allow for temperature self-correlation and absorption-temperature correlation. In modeling buoyant propane flames in still air above 0.3 m diameter burner, extensive comparison is presented of the predictions with the measurements of gas species concentrations, temperature, velocity and their turbulent fluctuations, and radiative heat fluxes obtained in flames with different heat release rates. Similar to previously published experimental data, the predicted burning rate of flames above the acetone pools exposed to flame radiation increases with the pool diameter and approaches a constant level for large pool sizes. The magnitude of predicted burning rates is shown to be in agreement with the reported measurements. Augmentation of burning rate of the pool fire in a cross-wind because of increased net radiative heat flux received by the fuel surface and non-monotonic dependence of burning rate on cross-wind velocity, subject to the pool diameter, is predicted. The statistical treatment of thermal radiation transfer has been found to be robust and computationally efficient.
TL;DR: In this paper, the experimental observations and modelling of buoyant whirling flames in a room-size enclosure are presented, and the periodic formation and destruction of whirling core and the increase of the time-averaged burning rate have been observed in the experiments.
Abstract: The experimental observations and modelling of buoyant whirling flames in a room-size enclosure are presented The periodic formation and destruction of whirling core, and the increase of the time-averaged burning rate have been observed in the experiments To interpret the mechanism of development of buoyant whirling flames, the concepts and results of existing theory of rotating flows have been used, and the conditions necessary for flame rotation to occur have been identified The CFD model is then discussed which is modified to represent the response of buoyant turbulent diffusion flame on the imposed circulation through decrease of turbulent mixing The model is first applied to simulate unconfined flames above a round fuel source approximating the pool fire studied in the experiments Elongation of whirling flames observed in published and our own experiments has been reproduced The predicted flame characteristics (radius of whirling core, angular velocity, swirl number) dependence on the magnitude of the imposed external circulation was found in qualitative agreement with the simplified theoretical model of whirling flow Also, the change of flame shape due to rotation resulted in reduced predicted radiative heat flux incident to fuel surface This indicates the need of additional physical mechanisms to explain and predict the experimentally observed increase in burning rate when the rotation occurs A possible mechanism, namely entrainment intensification of the air into the fuel rich region near the fuel surface, has been identified Finally, the CFD model is applied to simulate the experimentally studied whirling flames in the enclosure The simulation results recreated periodic precession, formation and destruction of the whirling flame as observed in the experiments The period of oscillations was found to decrease if the fuel supply rate increases
TL;DR: In this article, the activation energy, preexponential factor, and pre-exponential energy were obtained of a one-step pyrolysis reaction in supposition of a first-order reaction using simple mathematical fitting and an iso-conversion method.
Abstract: Using the methods of differential mass-spectrometric thermal analysis (DMSTA), thermogravimetric analysis (TGA), microscale combustion calorimetry (MCC), and fast pyrolysis (FP), thermal decomposition of high-molecular-weight polymethylmetacrylate (PMMA) has been investigated in the temperature range of 315÷500 °C. Based on these data, the kinetic parameters (the activation energy, the pre-exponential factor) were obtained of a one-step pyrolysis reaction in supposition of a first-order reaction using simple mathematical fitting and an iso-conversion method. Validity of the obtained kinetic parameters was verified by comparing the experimental data on dependence of the decomposition rate on temperature in the broad range of the heating rates with the results of simulating the above dependence, using these kinetic parameters. These parameters, obtained in the broad temperature range, may be further used in numerical simulation of PMMA combustion under fire conditions and for assessing the polymer’s flammability.
TL;DR: The Pyropolis model as mentioned in this paper is capable of predicting thermal decomposition of both charring and non-charring polymers; in case of charing polymers, material intumescence is assumed to be controlled by the amount of char produced in decomposition reactions.
Abstract: New comprehensive model, Pyropolis , aimed to predict performance of polymer composite materials exposed to radiative heating is presented, and a procedure to derive kinetic model of material thermal decomposition from either TGA or MCC measurements is introduced. In this procedure we do not pre-assume a kinetic function, but derive it from the measurements thereby achieving model validity in a wide range of heating rates. The Pyropolis model is capable of predicting thermal decomposition of both charring and non-charring polymers; in case of charring polymers, material intumescence is assumed to be controlled by the amount of char produced in decomposition reactions, given the intrinsic char porosity. Current version of the Pyropolis model has been calibrated and favorably validated for three types of flammable materials: non-charring polymer (high impact polystyrene), charring intumescent polymer (BPA polycarbonate), and the fiber-reinforced resin composite, all exposed to external heat flux. The model is demonstrated to be able of predicting test outcome (time to ignition, peak and average heat release rate) with a reasonable accuracy, provided material properties and test conditions are adequately identified. Sensitivity studies revealed the model components which have the most pronounced effect on the predictions. Sample surface emissivity, expansion propensity, and char layer conductivity are the key parameters controlling simulation results for charring polymers. For non-charring polymers, volumetric radiation absorption and availability of black coating film are important factors affecting the rate of virgin material gasification.
TL;DR: In this paper, the authors demonstrate inconsistency of the n-th order reaction assumption and reveal the autocatalytic behavior in thermal degradation of polyethylene, polystyrene and polycarbonate.
Abstract: Possibility of replicating polymer decomposition by a single global reaction greatly simplifies pyrolysis modeling. Apparent kinetic parameters are normally derived from the microscale experiments with linear heating program, and the n-th order reaction is routinely assumed thereby strongly affecting the numerical values of the kinetic parameters. In this work, we demonstrate inconsistency of the n-th order reaction assumption and reveal the autocatalytic behavior in thermal degradation of polyethylene, polystyrene and polycarbonate. The autocatalysis manifests itself in non-monotonicity of the conversion function, which markedly increases in a wide range of conversions. Although the iso-conversional approach makes it possible to explicitly recover the conversion function from the measurements, this option has not been used in most of the previous studies. Meanwhile, proper approximation of the experimentally derived conversion function results in excellent replication of the measured reaction rates, with the same kinetic parameters, in a range of the heating rates. Thus developed thermal decomposition kinetic models are provided in this paper for three kinds of polyethylene (LDPE, HDPE, and UHMWPE), seven kinds of polystyrene, polycarbonate, and two kinds of polymethylmethacrylate with different molecular weights. Although the pyrolysis of the polymers with different molecular weights proceeds differently, no systematic correlation of the pyrolysis characteristics (conversion-averaged apparent activation energy, heat of combustion, peak reaction rates and temperatures etc.) with the molecular weight has been observed for polystyrene. Peak reaction rates and temperatures varied in opposite directions for polyethylene and polymethylmethacrylate.
TL;DR: The main concepts of intumescence are reviewed in this article, highlighting the novelties as well as the most significant results achieved in the flame retardancy of polymeric materials in the last 10-15 years.
Abstract: The objective of current research on intumescent formulations is on consolidated approaches for conferring flame retardancy properties to polymers and polymer blends Numerous academic and industrial efforts have been carried out in the last fifteen years, by revisiting the traditional concept of intumescence on the basis of the new chemical synthesis or novel nano-technological developments The main concepts of intumescence are reviewed in this report, highlighting the novelties as well as the most significant results achieved in the flame retardancy of polymeric materials in the last 10–15 years Although the basic aspects of intumescence such as the chemical components, thermal and rheological aspects are well-known, the modeling and simulation of these systems are completely new and never reviewed Analogously, the traditional chemical compositions will be compared with the novel systems, most of them based on the nanotechnology and synergistic aspects Thus, the results collected up-to-now by using these new intumescent formulations will be dealt with the different polymer families The use of current intumescent coatings for metals, steel, wood and plastics as well as the application of novel intumescent coatings deposited on fabrics, films and foams through layer-by-layer assembly are reviewed Although the latter technique is not new, its use to confer flame retardancy properties to polymers is a recent development
TL;DR: The most recent developments in the modelling of heating and evaporation of fuel droplets, the results of which were published in 2014-2017, are reviewed, and the most important unsolved problems are identified.
Abstract: The most recent developments in the modelling of heating and evaporation of fuel droplets, the results of which were published in 2014–2017, are reviewed, and the most important unsolved problems are identified. Basic principles of power law and polynomial approximations and the heat balance method for modelling the heating of non-evaporating droplets are discussed. Several approaches to modelling the heating of evaporating droplets, predicting different heating and evaporation characteristics, are compared. New results in modelling heating and evaporation of spheroidal droplets are identified. Basic principles of the Discrete Component Model and its application to biodiesel fuel droplets are summarised. Main ideas of the Multi-dimensional Quasi-discrete Model and its applications to Diesel and gasoline fuel droplets are discussed. New developments in gas phase evaporation models for multi-component fuel droplets are presented. A self-consistent kinetic model for droplet heating and evaporation is described. New approaches to the estimation of the evaporation coefficient, including those taking into account quantum-chemical effects, are summarised. Among unsolved problems, the effects of non-spherical droplets, limitations of the ETC/ED model, effects of the interaction between droplets, effects of the moving interface due to evaporation, modelling of complex multi-component droplets, modelling of droplet heating and evaporation in near- and super-critical conditions, development of advanced kinetic and molecular dynamics models and effective approximation of the kinetic effects are discussed.
TL;DR: The interaction between turbulence and radiation (TRI) in reactive flows has been demonstrated experimentally, theoretically and numerically, and results from the highly nonlinear coupling between fluctuations of radiation intensity and fluctuations of temperature and chemical composition of the medium.
Abstract: The interaction between turbulence and radiation (TRI) in reactive flows has been demonstrated experimentally, theoretically and numerically, and results from the highly non-linear coupling between fluctuations of radiation intensity and fluctuations of temperature and chemical composition of the medium. The instantaneous and the time-averaged form of the radiative transfer equation (RTE) are presented, and the TRI effects resulting from time-averaging are discussed. Methods to account for TRI in practical calculations are surveyed, and works where such methods have been employed are reviewed. These include both decoupled and coupled fluid flow/radiative transfer calculations. It is shown that the solution of the RTE using instantaneous scalar data is the most accurate way to deal with TRI, but it is computationally prohibitive for coupled problems. Hence, this approach has been mainly used to calculate the radiation intensity along lines of sight. The generation of time series of instantaneous scalar data may be accomplished using stochastic or deterministic models, which are also surveyed. Coupled fluid flow/radiative transfer problems are generally solved using the time-averaged form of the RTE or the Monte Carlo method, and rely on the optically thin fluctuation approximation, which neglects the correlation between fluctuations of the absorption coefficient and fluctuations of the radiation intensity. Experimental data and numerical calculations demonstrate that turbulent fluctuations may significantly increase the mean spectral radiation intensity in both non-luminous and luminous flames. Turbulent fluctuations contribute to decrease the flame temperature below the level observed without fluctuations, particularly for optically thick flames. The net radiative power and the fraction of radiative heat loss increase due to TRI, particularly in the case of optically thin flames. Recent direct numerical simulations provide additional insight on the role of different correlations responsible for TRI, and on how they are influenced by the optical thickness of the medium.
TL;DR: The effects of temperature and gas flow rate in the conical spouted bed reactor on product yield and composition have been determined by using a spouting velocity from 1.25 to 3.5 times the minimum one.
Abstract: Continuous pyrolysis of polystyrene has been studied in a conical spouted bed reactor with the main aim of enhancing styrene monomer recovery. Thermal degradation in a thermogravimetric analyser was conducted as a preliminary study in order to apply this information in the pyrolysis in the conical spouted bed reactor. The effects of temperature and gas flow rate in the conical spouted bed reactor on product yield and composition have been determined in the 450–600 °C range by using a spouting velocity from 1.25 to 3.5 times the minimum one. Styrene yield is strongly influenced by both temperature and gas flow rate, with the maximum yield being 70.6 wt% at 500 °C and a gas velocity twice the minimum one.
TL;DR: In this article, the authors report the fabrication of poly (methyl methacrylate) (PMMA) foams with widely tunable cellular structures by using CO2 as the blowing agent.
Abstract: Polymer foams play an increasingly significant role in the field of thermal insulation due to their low thermal conductivities. As thermal insulation materials, their performances are primarily determined by the cellular structure. However, the correlation between the foam’s properties and its cellular structure is still not fully understood. This greatly limits the development of high-performance polymer foams with optimal cellular structures. Hereby, we report the fabrication of poly (methyl methacrylate) (PMMA) foams with widely tunable cellular structures by using CO2 as the blowing agent. In particular, the microcellular PMMA foam with a void fraction of 0.956 and with an average cell size of 4.7 μm was obtained, which, to the best of knowledge, is by far the largest void fraction of polymer foam with a cell size of less than 5 μm. The PMMA foam presents an excellent thermal-insulation behaviour with a thermal conductivity of as low as 29.9 mW/m K. Meanwhile, the PMMA foam exhibits excellent mechanical properties due to its extremely small cell sizes. Moreover, the dependences of thermal and mechanical properties on cellular structure are obtained by independently analyzing the effects of cell size and void fraction. All these results demonstrate a promising method to fabricate environmentally friendly and economical thermal insulation materials with improved thermal-insulation and compressive mechanical properties.