Physical Submodels Research Articles

PeleMP: The Multiphysics Solver for the Combustion Pele Adaptive Mesh Refinement Code Suite

Abstract Combustion encompasses multiscale, multiphase reacting flow physics spanning a wide range of scales from the molecular scales, where chemical reactions occur, to the device scales, where the turbulent flow is affected by the geometry of the combustor. This scale disparity and the limited measurement capabilities from experiments make modeling combustion a significant challenge. Recent advancements in high-performance computing (HPC), particularly with the Department of Energy's Exascale Computing Project (ECP), have enabled high-fidelity simulations of practical applications to be performed. The major physics submodels, including chemical reactions, turbulence, sprays, soot, and thermal radiation, exhibit distinctive computational characteristics that need to be examined separately to ensure efficient utilization of computational resources. This paper presents the multiphysics solver for the Pele code suite, called PeleMP, which consists of models for spray, soot, and thermal radiation. The mathematical and algorithmic aspects of the model implementations are described in detail as well as the verification process. The computational performance of these models is benchmarked on multiple supercomputers, including Frontier, an exascale machine. Results are presented from production simulations of a turbulent sooting ethylene flame and a bluff-body swirl stabilized spray flame with sustainable aviation fuels to demonstrate the capability of the Pele codes for modeling practical combustion problems with multiphysics. This work is an important step toward the exascale computing era for high-fidelity combustion simulations providing physical insights and data for predictive modeling of real-world devices.

Physics-based reduced-order modeling of flash-boiling sprays in the context of internal combustion engines

Flash-boiling injection is one of the most effective ways to accomplish improved atomization compared to the high-pressure injection strategy. The tiny droplets formed via flash-boiling lead to fast fuel–air mixing and can subsequently improve combustion performance in engines. While most of the previous studies related to the topic focused on modeling flash-boiling sprays using three-dimensional (3D) computational fluid dynamics (CFD) techniques such as direct numerical simulations (DNS), large-eddy simulations (LES), and Reynolds-averaged Navier–Stokes (RANS) simulations, the present work introduces a reduced-order cross-sectionally averaged spray (CAS) model with significantly reduced computational cost. The proposed CAS model incorporates several physical submodels in flash-boiling sprays such as those for air entrainment, drag, superheated droplet evaporation, flash-boiling induced breakup, and aerodynamic breakup models. The CAS model is then applied to different fuels to investigate macroscopic spray characteristics such as liquid and vapor penetration lengths under flash-boiling conditions. It is found that the newly developed CAS model captures the trends in global flash-boiling spray characteristics reasonably well for different operating conditions and fuels. Moreover, the CAS model is shown to be faster by up to four orders of magnitude compared with simulations of 3D flash-boiling sprays. The novelty of the present work lies in providing a reduced-order flash-boiling spray model that offers cost-effective computational representations of complex phenomena. The proposed model can be very valuable for applications, such as experimental design, fuel screening, and creating digital twins, thus playing a crucial role in the advancement of research and development in the field of spray combustion.

Modeling of radiation heat transfer in dense-bed flows of solids in indirectly heated rotary kilns

• modification and implementation of a DOM-radiation model which combines all required physical submodels to simulate heat radiation to dense beds of particles. • Full time and space resolved simulation of thermal conversion of wet moving bulk in a large-scale rotary kiln reactor. • Analytical validation of model and justification with laboratory-scale experiments. • Efficiency optimization of the numerical solver for industrial-scale and long-time simulations on parallel supercomputers. • Parameter studies for large-scale reactors to investigate conversion rate and product quality. This work presents further development and validation of the Discrete Ordinates Model for thermal radiation implemented in OpenFOAM® for application to packed beds of particles. The radiation model is an important part of a more comprehensive model for simulating the thermochemical conversion of the discrete phase (here for instance wet biomass particles). The comprehensive Eulerian-Lagrangian model is part of a three-dimensional, time-resolved simulation of the essential chemical and physical processes occurring within and in-between particles in a moving bed. For the thermal treatment of solid particles, convective and radiative heat transfer couple the energy exchange between the reactor wall, gas- and disperse phase. The original implementation of the finite volume Discrete Ordinate Model (fvDOM) present in OpenFOAM® is valid for a dilute particulate phase and neglects the effect of local opacity due to the existence of individual particles. In the present application, a dense-packed bed of the particulate phase exists in the reactor and, therefore, this direction-based radiation model is adjusted for a computational cell with arbitrary particle volume fractions. To validate the results with the present thermal radiation model, a simple test case is first presented, where a bed of particles is heated from the top of the computational domain. A second test case relates to an experimentally investigated laboratory-scale reactor. The results of the improved fvDOM are compared to the original implementation in OpenFOAM® and the more simple and computationally cheap P-1 radiation model. In the presence of a packed bed, the P-1 model largely overpredicts the radiative heat transfer while the original fvDOM underpredicts the heat flux by about 15 % for the first test case. The new improved model delivers results within 1% deviation at the expense of maximum of 10 % increase in the computational time. Large-scale parallel simulations of real setups are crucial tools to predict the efficiency of a process and improve the operational parameters. To achieve the pre-defined product quality at minimum cost, an example of the implemented radiation model in thermochemical conversion of wet biomass in a rotary drum is been given and the importance of the radiation heat transport to the bulk is highlighted.

Climate Behaviour and Plant Heat Activity of a Citrus Tunnel Greenhouse: A Computational Fluid Dynamic Study

Response to the expanding demand for high-quality citrus saplings plants requires optimisation and a deep understanding of production climate behaviour. In this context, greenhouse production is the most used technique because it allows farmers to effectively monitor plant growth through production condition control, especially climatic parameters. The current work presents an analysis of climate behaviour and plant heat activity of a citrus sapling tunnel greenhouse in the middle region of Morocco. In this regard, a computational fluid dynamic (CFD) model was developed and validated with respect to temperature and relative humidity measured values. The specificity of this model is the inclusion of a new non-grey radiative and heat transfers physical sub-models to couple the convective and radiative exchanges at the plastic roof cover and crop level. The findings showed that using a green shade net increased the greenhouse shadow, and the layering of plastic and shade net significantly reduced solar radiation inside the greenhouse by 50%. Also, the greenhouse airflow speed was deficient; it cannot exceed 0.3 ms−1, hence the dominance of the chimney effect in heat transfer. Despite the previous results, analyses of greenhouse temperature and relative humidity fields clearly showed the greenhouse climate behaviour heterogeneity, where spatial greenhouse air temperature and relative humidity difference values reached a maximum of 29.7 °C and 23%, respectively. For citrus plants, heat activity results showed that a weak fraction (1.44%) of the short wavelength radiation is converted to latent heat, which explains the low plant transpiration under these conditions. While the convective currents are the primary source of temperature and relative humidity heterogeneity inside the greenhouse, the presence of crop rows tends to homogenise the climate inside the greenhouse. We also concluded the necessity of proper condensation modelling near ground surfaces and inside the crop, and the water vapour effect on climate determination.

Automatic management of electrical loads in more electric aircraft

PurposeThis paper aims to automatically generate load shedding sequences due to insufficient power supply, to ensure flight safety and complete flight task.Design/methodology/approachIn this paper, a power allocation and load management model, including logical and physical submodels of the distribution system, is first established according to different requirements of the loads in different flight phase and the current total power supply. Then, an optimal load management scheme based on an improved ant colony algorithm is proposed to automatically generate load shedding sequences for both safety-critical and nonsafety critical loads, to achieve a reliable and safe power supply.FindingsTo verify the efficiency and feasibility of the algorithm, the proposed method is verified in a virtual simulation platform. Simulation result illustrates that the proposed algorithm is efficient and feasible.Practical implicationsThe proposed method can provide guidance on load power supply when the civil aircraft is under abnormal power supply situation.Originality/valueAn optimal load management scheme is proposed by considering different requirements of the loads in different flight phase and the current total power supply.

Quantitative assessment of decadal water temperature changes in Lake Kasumigaura, a shallow turbid lake, using a one-dimensional model

To assess the decadal water temperature (WT) changes observed at the center of Lake Kasumigaura, a shallow turbid lake in Japan, we constructed a prediction model for WT. The thermal interactions among air, water, and sediment were simulated by a one-dimensional differential equation using hourly observations of meteorological and limnological parameters. The validated model showed good performance (i.e., <1 °C RMSE for the daily averaged WT) for 2012, 2013, and 2015, respectively, in both upper and lower water layers, which were attained by incorporating a physical sub-model regarding the surface water temperature (surface WT). The good description of seasonal, daily, and hourly changes in surface WT obtained with the sub-model was confirmed by the observed and simulated time series. We then quantified and evaluated the trends of the meteorological and limnological parameters influencing WT changes from 1979 to 2015 based on their effects on WT by using the prediction model with the increases in air temperature (AT), solar radiation (SR), wind velocity (WV), and turbidity during that period; then, WT in the former period (1979) was predicted according to the trends and compared with the measured WT. The observed WT changes during the period were quantitatively explained by the compound effects of the parameters' changes, i.e., the AT and SR raising the WT, and the WV and turbidity lowering the WT.

Dynamics of blast wave and fireball after hydrogen tank rupture in a fire in the open atmosphere

This paper describes a CFD model of the blast wave and fireball dynamics after high-pressure hydrogen tank rupture in a fire in the open atmosphere. Experiments on rupture in a fire of tanks with nominal working pressure 35 MPa and 70 MPa are used to validate the model and get insights into underlying physical phenomena. Parametric studies are performed to understand the effect of different physical sub-models, numerical methods and other model parameters, e.g. instantaneous or inertial tank opening, on the convergence of simulations and closer reproduction of experiments. The model reproduces experiments well using different turbulence (RNG, Smagorinski-Lilly) and combustion (EDC, FRC) sub-models. It is demonstrated that hydrogen combustion at the contact surface between heated by starting shock air and cooled by expansion hydrogen at the initial stage of the process affects the blast wave strength, i.e. the peak pressure of the leading front and the blast wave impulse.

Buoyancy effect in sooting laminar premixed ethylene flame

Polycyclic aromatic hydrocarbons (PAHs) are known as soot precursors, but their formation/consumption is not fully understood. A recent comprehensive experimental study of premixed laminar ethylene flame [23] investigated the transition from gas-phase to soot particles. The complex fluid dynamics of this system is taken into account to compare model predictions with experimental measurements and thus further validate a detailed kinetic mechanism of soot formation. The relatively low inlet velocity and the large distance between the burner and the stagnation plate lead to significant influence of buoyancy, which requires a 2-D simulation. The observed constricted (necking) flame structure can be reproduced only using a comprehensive 2-D simulation, which includes buoyancy effects, radiative heat losses, and thermal diffusion. Predicted axial gaseous and PAH species profiles obtained from the CRECK mechanism are in good agreement with the measurements, especially even-carbon-number aromatics. Reasonable agreement of the predicted soot volume fraction profiles is also observed. Additionally, simulation results from different literature kinetic mechanisms are also discussed to highlight similarities and differences. The largest discrepancies among the predictions of the mechanisms are observed for phenylacetylene, a key-species representing the first building block of PAHs synthesis in flames.A comprehensive analysis of relevant physical sub-models is also carried out in 2-D simulations. Additionally, predicted profiles from 2-D and 1-D simulations are compared. Following the literature, a 1-D simulation with imposed mass flux from the 2-D model was carried out to account for buoyancy effects. This approach provides an axial predicted flame temperature profile similar to the 2-D case. However, the predicted mole fraction profiles are quite different, especially for hydrogen and aromatics species because of the failure in accounting for the interplay of enhanced diffusion due to Soret effect, flame stretch, and large radial velocities in the proximity of the stagnation plane.

Numerical study on combustion characteristics of six-stroke-cycle gasoline compression ignition engine with continuously variable valve duration valve technology

Engine performance and emissions of a six-stroke gasoline compression ignition engine with a wide range of continuously variable valve duration control were numerically investigated at low engine load conditions. For the simulations, an in-house three-dimensional computational fluid dynamics code with high-fidelity physical sub-models was used, and the combustion and emission kinetics were computed using a reduced kinetics mechanism for a 14-component gasoline surrogate fuel. Variation of valve timing and duration was considered under both positive valve overlap and negative valve overlap including the rebreathing of intake valves via continuously variable valve duration control. Close attention was paid to understand the effects of two additional strokes of the engine cycle on the thermal and chemical conditions of charge mixtures that alter ignition, combustion and energy recovery processes. Double injections were found to be necessary to effectively utilize the additional two strokes for the combustion of overly mixed lean charge mixtures during the second power stroke. It was found that combustion phasing in both power strokes is effectively controlled by the intake valve closure timing. Engine operation under negative valve overlap condition tends to advance the ignition timing of the first power stroke but has minimal effect on the ignition timing of second power stroke. Re-breathing was found to be an effective way to control the ignition timing in second power stroke at a slight expense of the combustion efficiency. The operation of a six-stroke gasoline compression ignition engine could be successfully simulated. In addition, the operability range of the six-stroke gasoline compression ignition engine could be substantially extended by employing the continuously variable valve duration technique.

Turbocharger turbine rotor tip leakage loss and mass flow model valid up to extreme off-design conditions with high blade to jet speed ratio

Due to the power consumption restriction of the turbocharger compressor, common turbine maps are rather narrow. To extrapolate them, reliable physical submodels are needed that are valid for broad ranges. Plenty of research has been done referring to tip leakage losses in axial and traditional radial turbomachinery. However, less effort has been put into the tip leakage analysis of radial turbocharger turbines, whose characteristics including high rotational speed and geometry are rather different. Commonly developed tip leakage loss models in radial turbines are mainly based on correlations with the rotational speed, while in axial turbomachinery they are mainly based on blade loading assumptions. Wide range computational fluid dynamics (CFD) data of a medium sized automotive turbine have been used to analyze tip leakage mass flow under extremely diverse running conditions. To be able to fit a model in a broad range of the map, blade loading and rotational speed have to be considered. A novel tip clearance model has been derived from the Navier Stokes Equations. The model owns a dependency on the rotational speed and the blade loading. With this approach CFD data have been fitted in a very good quality to model the tip leakage mass flow rate and tip leakage losses.

Numerical investigation of soot formation from microgravity droplet combustion using heterogeneous chemistry

The use of isolated droplets as idealized systems is an established practice to get an insight on the physics of combustion, and an optimal test field to verify physical submodels. In this context, this work examines the dynamics of soot formation from the combustion of hydrocarbon liquid fuels in such conditions. A detailed, heterogeneous kinetic mechanism, describing aerosol and particle behavior through a discrete sectional approach is incorporated. The developed 1-dimensional model accounts for (i) non-luminous and luminous radiative heat losses, and (ii) incomplete thermal accommodation in the calculation of the thermophoretic flux. The combustion of droplets of n-heptane, i.e., the simplest representative species of real fuels, was investigated as test case; an upstream skeletal reduction of the kinetic mechanism was carried out to limit calculation times. After checking the performance of the reduced mechanism against gas-phase experimental data, the transient evolution of the system was analyzed through a comprehensive study, including fiber-suspended (D0 < 1 mm) as well as free (D0 > 1 mm) droplets.The different steps of soot evolution were quantified, and localized in the region between the flame front and the soot shell, where particle velocity is directed inwards because of thermophoresis, and residence times are much higher than what usually found in diffusion flames. As a result, growth, coalescence, and aggregation steps are significantly enhanced, and soot accumulates in the inner shell, with an evident modification of the particle size distribution, if compared to what observed in conventional combustion conditions. The model exhibits a satisfactory agreement with experimental data on flame temperature and position around the droplet, while for larger droplets an increasing sensitivity to the radiation model was observed. It is found that the latter has a significant impact on the production of soot, while scarcely affecting the location of the soot shell. On the other side, the inclusion of incomplete thermal accommodation in the thermophoretic law brought about more accurate predictions of both volume fractions and shell location, and highlighted the primary role of thermophoresis in these conditions, as already found in literature through more simplified approaches.

Laser-induced heating of dynamic particulate depositions in additive manufacturing

Many applications in additive manufacturing involve the dynamic deposition of powders and the in-flight heating of such material by a laser. In order to characterize such systems, the Discrete Element Method (DEM) is employed. Specifically, this paper focuses on the initial stages of this process by developing a modular discrete-element type multiphysics simulation method for the particle dynamics and heating by a laser and detailed thermal behavior. The objective is to provide researchers with a framework to construct computational tools for this growing industry. In order to achieve this, from a simulation standpoint, the overall particle-mixture system is constructed by coupling submodels for each primary physical process (dynamics and heating) together. An iterative staggering scheme is developed whereby, within every time step, each individual particle is solved “freezing” the state of the remaining multiparticle system. The state of the particle is then updated and the algorithm moves to the next particle in the system and the process is repeated. The overall process sweeps through the entire system repeatedly until convergence is achieved in an appropriate norm. As the system evolves, an error estimate dictates the time-step size that is needed to induce convergence to below an appropriate error level. Thus, the process can be considered as an implicit time-stepping scheme, which is combined with an (internal) iterative staggering process. In order to control rates of convergence within a time-step, the algorithm adjusts the time-step size. If the iterative process does not converge within a desired number of iterations, below an error tolerance, the time-step is reduced. The degree of time-step reduction is determined by utilizing an estimate of the spectral radius of the coupled system. Since the construction of model and solution process is modular, one can easily replace physical submodels with other choices, making it easy to numerically experiment with a variety of models. Qualitative and quantitative analyses are provided, as well as three-dimensional numerical examples. This paper addresses the first of two overall “macrostages” of a complex manufacturing process with DEM. The first stage, addressed in this paper, concentrates on dry-powder materials, which are of a discrete particulate character, using DEM to characterize the deposition and in-flight particle heating. Other papers of the author (Zohdi, 2010, 2013, 2014, 2015 [81,82], 2008) have addressed stage two, namely phase transformations, curing (cooling) and stress analysis, using more appropriate continuum and hybrid DEM-continuum approaches.

Collaboration of simulations and experiments for development and uncertainty quantification of a reduced char combustion model

Computational fluid dynamics (CFD) plays a decisive role in the development of cost-effective oxy-coal combustion technologies to improve process efficiency and to decrease pollutant emissions. The implementation of detailed physical models describing coal devolatilization, char oxidation, gas-phase reactions and pollutant formation ensures accurate CFD simulations of coal combustion but is still challenging for large-scale combustors due to the significant computational efforts required, especially in the framework of Large Eddy Simulation (LES). The development of reduced physics model with quantified model-form uncertainty is needed to overcome the challenges of performing LES of industrial coal-fired boilers. Reduced models must reproduce the main features of the detailed models and the capability of bridging scales and being predictive. A tight coupling of simulation and experiments is necessary to ensure predictivity with uncertainty quantification for a reduced model. This work proposes a combined experimental/numerical methodology that uses global sensitivity analysis to rank fundamental input parameters of a reduced char oxidation and gasification model describing reactions between char carbon and O2, CO2 and H2O reagents in both air and oxy-coal conditions. A careful evaluation of uncertainty in the data, in the model form and in the model parameters is performed. The reference dataset, consisting of the experiments carried out in a laminar entrained flow reactor operated by Sandia National Laboratories, has been exploited. The methodology is based on the use of so-called instrument models to include all the physical sub-models and the sources of uncertainty considered in the experiments and in the numerical simulations and affecting the main quantities of interest, e.g. reaction rates. The quantified uncertainty in the instrument models provides the range of uncertainty for the reduced char combustion model. Then, the reduced model with quantified uncertainty will be validated against the experimental data. The reduced model capability to address heterogeneous reaction at the particle surface, mass transport of species in particle boundary layer, pore diffusion and internal surface area changes will be assessed.

Experimental–numerical analysis of a biomass fueled microgeneration power-plant based on microturbine

Thermal use of biomass may have a significant role in the scheme of distributed power generation from renewable sources as it is intrinsically programmable. Moreover, if the furnace is designed for flexible fueling (multifueling), this feature gives maximum flexibility also from the standpoint of fuel availability and storage over the year. To fully exploit the fuel potential, the optimal size for these systems is in the lower range end (100 kW) for reasons mainly related to the fuel logistic chain. A Combined Heat and Power (CHP) configuration is furthermore important to balance the relatively low electric efficiency (in the order of 15% and less). In this paper a technology based on an Externally Fired Gas Turbine (EFGT) fed by woody biomass is demonstrated in the range of 70 kW electric power output. A multifuel prototype power-plant realized at the University of Rome Tor Vergata is described, and experimental data in terms of power, efficiency and fuel consumption are presented. To better understand the impact of fuel properties on power-plant performance, a model has also been developed, by means of physical submodels describing each component of the power-plant (biomass furnace, heat exchangers, compressor and turbine). The use of simple economic and management models is also discussed to better assess the economic sustainability of the solution depending on the characteristics of the fuel (fuel-end), matched with the utilization pattern (user-end), with special regard to thermal energy value over the year.

Three-dimensional vocal tract modeling of fricatives /s/ and /sh/ for post-glossectomy speakers

Production of fricatives involves a narrow supraglottal constriction along the vocal tract. Air flows through the constriction, and generates turbulent noise source(s) by impinging on some obstacles downstream. In post-glossectomy speakers, the production of /s/ and /sh/ is often problematic. It is mainly caused by the tongue surgery, which changes tongue properties such as volume, motility, and symmetry, preventing the tongue from creating proper constrictions. The purpose of this study was to gain some insights on how the vocal tracts of abnormal /s/ and /sh/ are shaped and what are their corresponding acoustic consequences. Based on cine magnetic resonance images, we built 3-D vocal tract models for /s/ and /sh/ from two post-glossectomy speakers (one with abnormal /s/ and the other with abnormal /sh/). Due to the missing part of the tongue, the reconstructed vocal tracts are asymmetric with either an air-flow bypass or a side branch formed near the constrictions. Two coupled physics submodels are included in the 3-D FEM acoustic simulation: incompressible potential flow for the mean air flow and aeroacoustics for the distributed noise sources. The resulting acoustic spectra and acoustic roles of air flow bypass or side branch will be discussed. [This study was supported by NIH R01CA133015.]

Effects of firing coal and biomass under oxy-fuel conditions in a power plant boiler using CFD modelling

One of the most promising technologies for carbon capture and storage (CCS) is oxy-fuel combustion. This study uses a commercial computational fluid dynamics (CFD) code to simulate the firing of coal and biomass under air and oxy-fuel conditions in an existing full-scale 500MWe coal-fired utility boiler. Results are presented for conventional air–coal combustion that corresponds well against available experimental data and an in-house empirical model. Maintaining the same thermal input and exit oxygen concentration, CFD was used as a predictive tool with standard physical submodels, to examine the effects of firing under air–biomass, oxy-coal and oxy-biomass conditions. The oxy-fuel conditions were investigated at oxygen concentrations of 25% and 30% by volume for a wet flue gas recycle. The effects of firing biomass in both air and oxy-fuel conditions are predicted to have a lower total heat transfer to the tube walls, with a lower furnace exit temperature in the boiler than the coal-fired cases. This may be attributed to the effects of large biomass particles, which have a lower total surface area and therefore causes a reduction in the radiative heat transfer to the tube walls as well as an increase in carbon in ash (CIA) predictions. For oxy-coal firing, the study suggested that the optimum oxygen concentration for heat transfer to be closely matched with air–coal, lies between 25% and 30%, but for oxy-biomass firing a value greater than 30% may be needed. This study highlights the possible impact of changing the fuel and combustion atmosphere on the heat transfer characteristics of an existing power plant boiler, underlining that minor redesign may be necessary when converting to biomass firing under air and oxy-fuel conditions.

Evaluation of the Performance of a Power Plant Boiler Firing Coal, Biomass and a Blend Under Oxy-fuel Conditions as a CO2 Capture Technique

The utilisation of biomass in coal-fired power plants can mitigate CO2 emissions, especially when combined with carbon capture and storage (CCS) technologies, such as oxy-fuel combustion. In this paper, a commercial computational fluid dynamics software was used, with small or no modifications to the physical submodels, to predict the performance of a 500 MWe sub-critical coal fired boiler under air and oxy-fuel conditions when firing coal, biomass, and a 20% biomass blend, for the same thermal input. The results suggest that for a wet recycle retrofit, the optimum oxygen concentration lies between the simulated range of 25 and 30%, where heat transfer characteristics of the air-fired design could be matched when firing either coal or a 20% biomass blend. However, for 100% biomass firing modifications of the firing arrangement may be necessary to achieve an output closer to the original design.

An Attempt at Large Eddy Simulation for Combustor Modeling

Large eddy simulation (LES) is a promising method for numerical simulation in combustion systems. A LES attempt in a model combustor has been made, and a few important issues related to grid size, inflow condition, wall boundary conditions, physical sub-models and data sampling are discussed. Some of the numerical results are presented and compared with a comprehensive experimental database, which indicates that LES can provide reasonable predictions for the mean axial velocity and temperature distributions inside the combustion chamber. However, in order to make LES a valuable and cost-effective tool in the development of advanced combustion systems, some fundamental questions remain to be addressed and more validation efforts are required. Moreover significant computing power is required for LES to capture both the high and low frequencies of interest in the present turbulent reacting flow.

Tyre friction behaviour under abrupt wheel torque transients on slippery road surfaces: experimental analysis and modelling

The paper shows that, during abrupt wheel torque transients for ice surface and low vehicle speeds, the tyre can develop significantly larger longitudinal force than the peak value of the tyre static curve. This so-called dynamic tyre friction potential (DTFP) effect has many influencing factors such as the rate of change of the wheel torque, the vehicle speed, and the tyre dwell time. The paper presents a detailed analysis of the DTFP behaviour based on the experimental data collected by using an in-wheel motor-based tyre test vehicle. The analysis results and an insight into the brush structure of a tyre model lead to the hypothesis that the different influencing factors may be predominantly explained by the bristle dwell time (BDT) effect. Following this hypothesis, the LuGre model of the tyre friction dynamics is extended with a physical BDT sub-model. The experimental validation results show that the proposed model can accurately capture the low-speed tyre–ice friction behaviour during abrupt wheel torque transients.

Large eddy simulation of oxy-coal combustion in an industrial combustion test facility

Abstract Oxy-fuel combustion is considered as one of the most promising technologies for carbon capture and storage (CCS). In this study, a commercial computational fluid dynamics (CFD) code has been employed for the simulation of an air-fired coal combustion and an oxy-fired coal combustion with recycled flue gas in a 1 MW th combustion test facility. Reynolds–averaged Navier–Stokes (RANS) solutions have been obtained for both cases. Results indicate that the CFD code with existing physical sub-models can provide a reasonable prediction for the air-fired combustion. However, the prediction for the oxy-fired case has not been as satisfactory as expected. In order to assess the impact of the turbulence treatment in CFD on the predictions, large eddy simulation (LES) has been performed for oxy-fired case and compared with the results from the RANS simulation and the available experimental data. Although the results suggest that LES can provide a more realistic prediction of the shape and the physical properties of the flame, there has not been significant improvement in the prediction of the temperature. In addition, the complexity of the problem requires more detailed experimental data for the validation of the LES. In order to improve the validity of numerical simulations for design purposes, further modelling improvements for oxy-coal combustion that are necessary for more accurate predictions are addressed. Based on this study, it is envisaged that the complexity in the oxy-coal combustion process requires more detailed analyses of the available physical sub-models.