An arrangement of ideal zones with shifting boundaries as a way to model mixing processes in unsteady stirring conditions in agitated vessels

This paper investigates the way modelling mixing phenomena occur in unsteady stirring conditions in agitated vessels. In particular, a new model of torus reactor including a well-mixed zone and a transport zone is proposed. The originality of the arrangement of ideal reactors developed here lies in the time-dependent location of the boundaries between the two zones. This concept is applied to model the positive influence of unsteady stirring conditions on homogenization process: the model avoids a mass balance discontinuity when the transition from steady to unsteady stirring conditions is performed.To ascertain the reliability of the model proposed, experimental runs with highly viscous fluids have been carried out in an agitated tank. The impeller used was a non-standard helical ribbon impeller, fitted with an anchor at the bottom. The degree of homogeneity in the tank was observed using a conductivity method after a tracer injection.It is shown that for a given agitated fluid and mixing system, model parameters are easy to estimate and that modelling results are in close agreement with experimental ones. Moreover, it would appear that this model allows the easy derivation of a control law, which is a great advantage when optimizing the dynamics of a mixing process.     

Measurement and Modelling of NO Fluxes on Maize and Wheat Crops During their Growing Seasons: Effect of Crop Management

Fertilized agricultural soils are a significant source of NO, a gas involved in tropospheric ozone formation. The aims of the research reported here were to measure NO fluxes over the length of the growing season of wheat and maize crops, and to build a model of soil NO emissions from arable land. Field experiments were carried out on a 1-ha field divided into two parts. The first one was cropped with wheat and harvested in late July, 2002, whereas the second part was sown with maize and harvested in October. The wheat and maize received 130 kg N ha⁻¹ and 140 kg N ha⁻¹, respectively. For each crop, NO fluxes were measured during 10 months every 2 weeks using manual closed chambers, and continuously with a wind tunnel immediately after nitrogen fertilization. Fertilizer application significantly affected NO emissions: the largest NO emissions were recorded a few days after nitrogen application. This delay depended on the kinetics of nitrogen incorporation in the soil, as influenced by rainfall. The emissions measured on the maize field (2.6% of the fertilizer amount applied) were more important than those on the wheat field (1.0% of the fertilizer amount applied), owing to differences in timing of nitrogen application, with respect to climate and crop growth. Relationships between soil nitrification rate and NO emission obtained from laboratory incubations, and experimental data appeared useful and relevant to predict NO emissions at the field-scale.     

High porous yttria-stabilized zirconia with aligned pore channels: Morphology directionality influence on heat transfer

High porous yttria stabilized zirconia with unidirectionally aligned channels is used in engineering applications with extremely low thermal conductivity. This property is strongly influenced by microstructure features such as pore volume fraction, pore size distribution, random porous microstructure and pore morphology directionality. Although several models are reported in the available literature, but their analytical formulas are formalised for homogeneous structures or they are based on proportion between solid and fluid phases. These differences from real microstructures cause significant computational errors especially when thermal conductivity changes as the function of the measurement direction (parallel or perpendicular). In this context, the application of an intermingled fractal unit's procedure capable of reproducing porous microstructure as well as predicting thermal conductivity has been proposed. The results are in agreement with experimental ones measured for parallel and perpendicular directions and suggest improving the formalisation of fractal modelling in order to obtain an instrument of microstructure design.     

Frozen mirabelle plum drying: Kinetics, modelling and impact on biochemical properties

The aim of this research was to study and model the kinetics of the hot air drying of frozen mirabelle plums. Effects of temperature (50-85 °C), air velocity (0.6-1.2 m/s) and a pre-treatment (ascorbic acid + sucrose-saturated solution) were investigated. The sorption isotherm and heat of sorption of mirabelle plum were mathematically described respectively by the Guggenheim, Anderson and de Boer (GAB) model and a new one. Depending on temperature and air velocities, the effective diffusivity and the energy of activation were respectively in the range of 8-16.8 × 10⁻¹⁰ m² s⁻¹ and 43.7-55.14 kJ mol⁻¹. To establish the model best adapted to describe this process the fit quality of five models (Newton, page, modified page, logarithmic and diffusion) were evaluated and compared. The diffusion model led to low values of reduced χ-square, mean square root and mean relative deviation modulus with the highest linear regression coefficient. This model was validated in a large range of operating conditions and represents an excellent tool to predict the duration of this process. The pre-treatment preserves biochemical properties of the dried mirabelle plums (reducing sugar content, total phenolic compounds and anti-oxidant activity).     

Prediction of pyrolysis of pistachio shells based on its components hemicellulose, cellulose and lignin

The objective of this contribution is to describe thermal degradation of pistachio shells by a detailed reaction mechanism. Pistachio shells are assumed to be composed of hemicellulose, cellulose and lignin of which degradation is described by relevant kinetics based on experimental data. The mechanism yields a detailed composition of product gases, and therefore, is well-suited to predict evolution of both thermal decomposition and products. Thermal degradation is described by a system of coupled differential conservation equations for mass, momentum, species, and energy for a pistachio shell particle. The relevant conservation equations are discretised by the Finite Volume Method (FVM) and solved within the conversion module of the Discrete Particle Method (DPM). A comparison between experimental data and predicted results yielded good agreement.     

Gas and solid behaviours during defluidisation of Geldart-A particles

Bed collapsing experiments were carried out in a cold-air transparent column 192 mm in diameter and 2 m high. Typical Fluid Catalytic Cracking (FCC) catalyst with a mean particle size of 76 μm and a density of 1400 kg/m³ was used. Both single and double-drainage protocols were tested. The local pressure drop and bed surface collapse height were acquired throughout the bed settling.Typical results were found regarding dense phase voidage of a fluidised bed and the bed surface collapse velocity. In addition, bubble fraction was calculated based on the collapse curve.Experimental results showed that windbox effect is significantly reduced compared to previous works since the volume of air within the windbox was reduced. The comparison of single/double-drainage protocols revealed a new period in the defluidisation of Geldart-A particles concerning gas compressibility. Through the temporal analysis of local pressure drop, the progress of the solid sedimentation front from bottom to top was determined, analysed and modelled.     

Experimental investigations and modelling of the ball motion in planetary ball mills

Planetary ball mills feature attractive properties, like the possibility of dry or wet operation, straightforward handling, cleanability and moderate costs. Consequently they are very well suited for lab scale process development in diverse industries, including pharmaceuticals and new materials. A number of questions still remain unanswered regarding this mill type. These include the stress conditions as well as transfer of the grinding results to other types of mills with free moving balls, such as stirred media mills, which can be built in large scales and operated continuously.In order to measure the ball motion and, thus, the stress conditions, a planetary ball mill was equipped with a high speed video camera, so that the grinding ball motion during the comminution process can be recorded and analysed. The influence of important process parameters on the ball motion pattern was assessed in this study, namely speed ratio, ball filling ratio and friction conditions, the latter by applying different mill feeds. The experimental results show considerable influences of the ball filling ratio and friction conditions. The measured ball motion patterns differ significantly from ball trajectories which were calculated using kinetic equations proposed in older publications.In addition to the measurements the ball motion was simulated using a three dimensional Discrete Element Model (DEM). An attempt was made to account for mill feed via altered friction coefficients. Correlations of the DEM results and experimental findings at different operating conditions show a good agreement. Based on simulation data the frequency distribution of the stress energies in the mill could be calculated and compared for different operating conditions.     

异步化汽轮发电机数学模型

本文简要叙述了异步化同步发电机的应用前景。文章的重点是讨论不同转子绕组布置的异步化汽轮发电机数学模型的建立，为含异步化汽轮发电机电力系统的仿真研究提供基础。     

A Parametric Study of the Gas Flow Patterns and Drying Performance of Co-current Spray Dryer: Results of a Computational Fluid Dynamics Study

A computational fluid dynamic study was carried out to investigate airflow pattern, temperature, and humidity profile at different levels in the drying chamber. Good agreement was obtained with published experimental data. The effects of operating pressure, heat loss from the chamber wall and inlet air conditions on the gas flow pattern, droplet trajectories, and overall dryer performance also were investigated. Results are presented and discussed in terms of the gas velocity, temperature, and humidity profiles within the chambers. The volumetric evaporation values, heat transfer intensity, and thermal energy consumption per unit evaporation rate were computed and compared for drying of a 42.5% solids solution in a spray chamber 2.215 m in diameter with a cylindrical top section 2.005 m high and a bottom cone 1.725 m high. Wall regions subject to formation of undesirable deposits are also identified.