Laboratory of Research in Fluid Dynamics and Combustion Technologies

We give a brief overview of stabilized finite element methods and illustrate
\nthe developments applied to the advection-diffusion equation.

Debris flow is modelled using the equations governing the dynamics of a liquid-solid mixture. An upwind finite volume scheme is applied to solve the resulting differential equations in one dimension. These equations have a structure similar to those of the monophasic water flow, differing from them by the presence of some terms characteristic of the bifasic nature of the mixture, such as granular bed erosion velocity, sediment concentration, bed shear stress, etc. The model and the system of equations to be solved are presented with the description of the implementation of the upwind scheme for the resulting hyperbolic conservation system. The numerical method is first order in both space and time. The treatment of the source terms is specified in detail and some comparison with laboratory experiments are presented.

Optimization of the efficiency of an induction heating application is essential in order to improve both reliability and performance. For this purpose, multistranded cables with litz structure are often used in induction heating applications. This paper presents an analysis and optimization of the efficiency of induction heating systems focusing on the optimal copper volume of the winding with respect to different constraints. The analysis is based on the concept of a one-strand one-turn coil, which captures the dissipative effects of an induction heating system and reduces the number of variables of the analysis. An expression for the efficiency of the induction heating system is derived. It is found that, with the geometry and the other parameters of the system fixed, efficiency depends on the copper volume of the windings. In order to use this result to optimize the efficiency of an application, volume restrictions, the packing factor and the window utilization factor are also considered. The optimum frequency for an induction heating system is also studied in this study. An experimental verification for both planar and solenoidal cases is also presented.

Several alternative techniques have been proposed in the literature in order to avoid the CPU-intensive numerical integration of the thermochemical equations in the simulation of combustion processes. The present paper introduces a new approach, which is based on two artificial neural-network (ANN) paradigms, namely the self-organizing map (SOM) and the multilayer perceptron (MLP). The SOM is first employed for the automatic partitioning of the thermochemical space into subdomains. Then, a specialized MLP is trained in order to fit the thermochemical points belonging to a given subdomain. The presented strategy is tested on a partially stirred reactor (PaSR) with a reduced methane-air mechanism, and encouraging results are reported. The relatively modest CPU-time and memory requirements of the method make the SOM-MLP approach a promising technique for the inclusion of large chemical mechanisms in the context of complex applications, such as the multidimensional simulation of combustion.

An experimental and numerical study has been performed to improve the understanding of the air/liquid interaction in an air-blasted breaking water sheet. This research is focused in the near field close to the exit slit, because it is in this region where instabilities develop and grow, leading to the sheet breakup. In the experiments, several relevant parameters were measured including the sheet oscillation frequency and wavelength, as well as the droplet size distribution and the amplification growth rate. The flow was also investigated using linear instability theory. In the context of existing papers on instability analysis, the numerical part of this work presents two unique features. First, the air boundary layer is taken into account, and the effects of air and liquid viscosity are revealed. Second, the equations are solved for the same parameter values as those in the experiments, enabling a direct comparison between calculations and measurements; although qualitatively the behaviour of the measured variables is properly described, quantitative agreement is not satisfactory. Limitations of the instability analysis in describing this problem are discussed. From all the collected data, it is confirmed that the oscillation frequency strongly depends on the air speed due to the near-nozzle air/water interaction. The wave propagates with accelerating interface velocity which in our study ranges between the velocity of the water and twice that value, depending on the air velocity. For a fixed water velocity, the oscillation frequency varies linearly with the air velocity. This behaviour can only be explained if the air boundary layer is considered.

Abstract Tomato ( Lycopersicon esculentum L) fruits from plants grown hydroponically on polyacrylamide gel were obtained in order to assess any possible uptake of acrylamide monomer from the nutrient solution to the fruit during cultivation. Analysis of acrylamide in the gel itself involved aqueous extraction, bromination and then capillary GC determination with nitrogen‐specific detection. By standard addition a level of 0·18 ± 0·01 g kg −1 residual monomer was found to be present in a sample of gel used by an experimental horticultural station. Tomato fruits were analysed by extraction of the aqueous phase, bromination, silica‐gel cartridge clean‐up and capillary GC–MS determination by selected ion monitoring. The recovery of the method was 26–62% but losses throughout were compensated for by use of 2,3‐dibromo‐2‐dimethylpropionamide internal standard. No acrylamide monomer could be detected in tomato fruits from plants grown hydroponically on polyacrylamide gel at a limit of detection of 1 × 10 −6 g kg −1 , demonstrating that the monomer is not transferred from the growing medium into tomato fruits.

The energy management strategy plays a major role in hybrid platforms powered by fuel cells (FCs) and batteries. This paper presents an assessment of energy management focused on fuel economy and battery degradation. Particularly, a proposed heuristic strategy and the widely known equivalent consumption minimization strategy are compared with the optimal solution obtained offline via dynamic programming. The case study is based on a real FC hybrid vehicle. Accordingly, the powertrain model of the vehicle used for the simulations is validated experimentally, and the profile of the power demand is measured from the real application. The results show that the proposed strategy offers the same performance as the equivalent consumption minimization strategy when the battery degradation is prioritized, and in comparison with the optimal off-line solution, it can be seen that there is still margin for improvement in terms of battery degradation.

The effect of liquid compressibility on the dynamics of a single, spherical cavitating bubble is studied. While it is known that compressibility damps the amplitude of bubble rebounds, the extent to which this effect is accurately captured by weakly compressible versions of the Rayleigh-Plesset equation is unclear. To clarify this issue, partial differential equations governing conservation of mass, momentum, and energy are numerically solved both inside the bubble and in the surrounding compressible liquid. Radiated pressure waves originating at the unsteady bubble interface are directly captured. Results obtained with Rayleigh-Plesset type equations accounting for compressibility effects, proposed by Keller and Miksis [J. Acoust. Soc. Am. 68, 628-633 (1980)], Gilmore, and Tomita and Shima [Bull. JSME 20, 1453-1460 (1977)], are compared with those resulting from the full model. For strong collapses, the solution of the latter reveals that an important part of the energy concentrated during the collapse is used to generate an outgoing pressure wave. For the examples considered in this research, peak pressures are larger than those predicted by Rayleigh-Plesset type equations, whereas the amplitudes of the rebounds are smaller.

An inert dynamically passive scalar in a constant density fluid forced by a statistically homogeneous field of turbulence has been investigated using the results of a 256(3) grid direct numerical simulation. Mixing characteristics are characterized in terms of either principal curvatures or mean and Gauss curvatures. The most probable small-scale scalar geometries are flat and tilelike isosurfaces. Preliminary correlations between flow and scalar small-scale structures associate highly curved saddle points with large-strain regions and elliptic points with vorticity-dominated zones. The concavity of the scalar profiles along the isosurface normal coordinate xn correlates well with negative mean curvatures, Gauss curvatures displaying any sign, which correspond to scalar minima, tiles, or saddle points; on the other hand, convexity along xn is associated with positive mean curvatures, Gauss curvatures ranging from negative to positive signs, featuring maxima, tiles, or saddle points; inflection points along xn correlate well with small values of the mean curvature and zero or negative values of kg, corresponding to plane isosurfaces or saddle points with curvatures of equal and opposite signs. Small values of the scalar gradient are associated with elliptic points, either concave or convex (kg>0) , for both concave and convex scalar profiles along xn. Large values of the scalar gradient (or, equivalently, scalar fluctuation dissipation rates) are generally connected with small values of the Gauss curvature (either flat or moderate-curvature tilelike local geometries), with both concave and convex scalar profiles along xn equally probable. Vortical local flow structures correlate well with small and moderate values of the scalar gradient, while strain-dominated regions are associated with large values.

A boundary condition for lattice Boltzmann methods, based on the movement of information through Euler characteristic directions, is developed. With respect to the similar conditions used in finite-difference or finite-volume implementations, some corrections are needed to compensate the isothermal compressible nature of standard lattice Boltzmann methods for fluid flow. The results show that the proposed method for inlets and outlets is highly nonreflecting, and mass conserving.

This paper addresses the energy management of a standalone renewable energy system. The system is configured as a microgrid, including photovoltaic generation, a lead-acid battery as a short term energy storage system, hydrogen production, and several loads. In this microgrid, an energy management strategy has been incorporated that pursues several objectives. On the one hand, it aims to minimize the amount of energy cycled in the battery, in order to reduce the associated losses and battery size. On the other hand, it seeks to take advantage of the long-term surplus energy, producing hydrogen and extracting it from the system, to be used in a fuel cell hybrid electric vehicle. A crucial factor in this approach is to accommodate the energy consumption to the energy demand and to achieve this, a model predictive control (MPC) scheme is proposed. In this context, proper models for solar estimation, hydrogen production, and battery energy storage will be presented. Moreover, the controller is capable of advancing or delaying the deferrable loads from its prescheduled time. As a result, a stable and efficient supply with a relatively small battery is obtained. Finally, the proposed control scheme has been validated on a real case scenario.

Abstract The use of a colocated variable arrangement for the numerical solution of fluid flow is becoming more and more popular due to its coding simplicity. The inherent decoupling of the pressure and velocity fields in this arrangement can be handled via a special interpolation procedure for the calculation of the cell face velocity named pressure‐weighted interpolation method (PWIM) ( AIAA J. 1983; 21 (11):1525–1532). In this paper a discussion on the alternatives to extend PWIM to unsteady flows is presented along with a very simple criterion to ascertain if a given interpolation practice will produce steady results that are relaxation dependent or time step dependent. Following this criterion it will be shown that some prior schemes presented as time step independent are actually not, although by using special interpolations can be readily adapted to be. A systematic way of deriving different cell face velocity expressions will be presented and new formulae free of Δ t dependence will be derived. Several computational exercises will accompany the theoretical discussion to support our claims. Copyright © 2009 John Wiley & Sons, Ltd.

Some of the studies on the dynamics of cavitating bubbles often consider simplified submodels assuming uniform fluid properties within the gas bubbles, ignoring chemical reactions, or suppressing fluid transport phenomena across the bubble interface. Another group of works, to which the present contribution belongs, includes the radial dependence of the fluid variables. Important fluid processes that occur inside the gas bubble, such as chemical reactions, and across the bubble interface, such as heat and mass transfer phenomena, are here considered also. As a consequence, this model should yield more realistic results. In particular, it is found that water evaporation and condensation are fundamental transport phenomena in estimating the dissociation reactions of water into OH. The thermal and mass boundary layers and the radial variation of the chemical concentrations also seem essential for accurate predictions.

RNA-binding proteins (RBPs) govern a myriad of different essential processes in eukaryotic cells. Recent evidence reveals that apart from playing critical roles in RNA metabolism and RNA transport, RBPs perform a key function in plant adaptation to various environmental conditions. Long-distance RNA transport occurs in land plants through the phloem, a conducting tissue that integrates the wide range of signaling pathways required to regulate plant development and response to stress processes. The macromolecules in the phloem pathway vary greatly and include defense proteins, transcription factors, chaperones acting in long-distance trafficking, and RNAs (mRNAs, siRNAs, and miRNAs). How these RNA molecules translocate through the phloem is not well understood, but recent evidence indicates the presence of translocatable RBPs in the phloem, which act as potential components of long-distance RNA transport system. This review updates our knowledge on the characteristics and functions of RBPs present in the phloem.

An experimental study of the influence of atomizer dimensions and injection conditions on discharge coefficient and spray angle is presented. Simplex pressure-swirl nozzles atomizing heavy oil have been examined. Twenty nozzles of different geometries have been investigated, including orifice diameters down to 0.45 mm. Inviscid theory results or published correlations valid for larger atomizers do not seem to apply for those of smaller dimensions, for which viscous effects are thought to play a major role. This role is analyzed, and new correlations are proposed for discharge coefficient and spray cone angle.

ABSTRACT Particle formation and emission in the combustion of four (orujillo, eucalyptus, oak and chestnut tree) pulverized biomass fuels have been studied. The fuels have been burned in an entrained flow reactor, under controlled and realistic conditions. In all the cases, the final emission distributions contained at least two modes, one of them peaking at 30–200 nm, and the other being in the supermicron range. Alkali sulphates and chlorides account for most of the mass of the fine particles in all cases, while coarse particles essentially retain the inorganic matter properties of the original fuel. K2SO4 is experimentally found to start nucleation over 900°C for orujillo, while KCl is not observed at this temperature. Condensation of KCl on these nuclei is observed in a sample taken at 560°C and in a greater amount at 360°C. The same formation schema was found for the other biomass fuels; the much lower ash content of the latter is thought to cause a “delay” in the onset of these nucleation/condensation steps along the gas cooling process. In spite of these differences, the global similarities observed among the studied fuels confirm the generality of the fine particle formation process in biomass combustion.

A complete system of dynamical equations for the invariants of the velocity gradient, the strain rate, and the rate-of-rotation tensors is deduced for an incompressible flow. The equations for the velocity gradient invariants R and Q were first deduced by Cantwell [Phys. Fluids A 4, 782 (1992)] in terms of Hij, the tensor containing the anisotropic part of the pressure Hessian and the viscous diffusion term in the velocity gradient equation. These equations are extended here for the strain rate tensor invariants, RS and QS, and for the rate-of-rotation tensor invariant, QW, using HijS and HijW, the symmetric and the skew-symmetric parts of Hij, respectively. In order to obtain a complete system, an equation for the square of the vortex stretching vector, Vi≡Sijωj, is required. The resulting dynamical system of invariants is closed using a simple model for the velocity gradient evolution: an isotropic approximation for the pressure term and a linear model for the viscous diffusion term. The local topology and the resulting statistics implied by this model reproduce a number of trends similar to known results from numerical experiments for the small scales of turbulence.

The instability growth leading to a liquid sheet breakup has been studied with the objective of improving the understanding of the fundamental mechanisms of atomization. A three-dimensional Lagrangian code based on vortex dynamics methods has been implemented to track the air/liquid interfaces treated as inviscid vortex sheets. The results of these numerical simulations indicate a possible explanation for the presence of transverse and longitudinal filaments observed in liquid sheet air-assisted atomization experiments.

The ash behavior during suspension firing of 12 alternative solid biofuels, such as pectin waste, mash from a beer brewery, or waste from cigarette production have been studied and compared to wood and straw ash behavior. Laboratory suspension firing tests were performed on an entrained flow reactor and a swirl burner test rig, with special emphasis on the formation of fly ash and ash deposit. Thermodynamic equilibrium calculations were performed to support the interpretation of the experiments. To generalize the results of the combustion tests, the fuels are classified according to fuel ash analysis into three main groups depending upon their ash content of silica, alkali metal, and calcium and magnesium. To further detail the biomass classification, the relative molar ratio of Cl, S, and P to alkali were included. The study has led to knowledge on biomass fuel ash composition influence on ash transformation, ash deposit flux, and deposit chlorine content when biomass fuels are applied for suspension combustion.

In this work, we analyzed a variety of metrics derived from Laser Doppler Vibrometry (LDV) characterization of 3D printed rectangular prisms. The metrics of interest were natural frequencies and amplitudes of first and second bending vibration modes, equivalent elastic moduli, damping ratios and areas of transmissibility functions. To explore the influence of printing process parameters on the dynamical behavior and, therefore, on the aforementioned metrics, 48 different combinations were considered, including build orientation, raster angle, nozzle temperature, print speed and layer height as relevant parameters. Thus, 96 polylactic acid (PLA) rectangular prisms were fabricated and LDV characterization was carried out. Based on the equivalent elastic modulus metrics, it was possible to corroborate the influence of printing process parameters on the mechanical performance, being raster angle, build orientation and nozzle temperature the most influential parameters. Likewise, the analysis of damping ratios served to assess the degree of interfilament bonding of 3D printed rectangular prisms. Thus, rectangular prims that exhibited high damping ratios also showed evident lack of adhesion between deposited filaments. Low damping ratios and, therefore, superior interfilament bonding was connected with on edge build orientation, high nozzle temperature (220 °C) and low print speed (60 mm/s) for specimens fabricated using 0° as raster angle. The analyses of areas of transmissibility functions and amplitudes of vibration modes also confirmed a better transmission of excitation (i.e., larger areas and higher amplitudes) for optimally fabricated parts, that is, specimens featuring relatively high equivalent elastic moduli and low damping ratios. Moreover, the application of Classical Laminate Theory to establish the relationships between elastic modulus and damping ratio with raster angle confirmed low temperature (200 °C) and high print speed (120 mm/s) resulted in low elastic modulus a high damping ratios and, therefore, poor interfilament bonding. The present findings confirm LDV is a powerful technique in the characterization of additively manufactured products, being able to discriminate different mechanical behaviors as well as the degree of interfilament bonding. Despite of their simplicity, the metrics derived from LDV characterization represent an attractive tool for both research and industrial applications.