Wall Heat transfer correlation for rotary kiln with secondary air flow and recycled materials inlet

International audienceThe increasing use of the baffled-rotary kiln equipment in many innovative materials processing industrial applications suggests examining the heat transfer phenomena in order to improve the multi-phase flow modeling. Their development and use will be relevant for tackling the current energy issues. The heat transfer models available for the rotary kiln in the literature are, for now, not enough efficient for the baffled-rotary kiln case. The present paper is aimed at suggesting a wall heat transfer correlation for the rotary kilns with secondary inlet. The experimental thermal data acquired within large-scale rotary drum applied to the asphalt concrete materials production, are remained in order to give rise new issues. These latter results are connected to a visualization campaign performed at the pilot-scale in order to assess the transversal distribution of the granular materials. Their analysis suggest a more appropriate physical modeling of the wall heat transfer path leading to transform the classical correlation of type Nu=f(Re,Pr) in a new expression of type Nu=f(Re,St) based on a inventory corresponding to the hot and cold fluxes flowing within the baffled-rotary kiln. Thus, the introduction of the Stanton number (St) in the wall heat transfer correlation is compared with the experimental Nusselt numbers calculated from the inner heat transfer coefficients measured in the large-scale baffled-rotary kiln. This expression is found more convenient for the baffled-rotary kiln application

A sorting method to value recycled concrete

The purpose of this paper is to propose a sorting platform to increase the quality of recycled aggregates through the supplementary use of mineral processing techniques and additional sorting. The difficulty of liberation is discussed, and the methodology currently used in mineral processing is proposed. Jigs, hydrocyclones and sensor-based sorting are considered to have good performance in the sorting of adequately recycled aggregates. The new perspectives on sorting and liberation for recycling aggregates are discussed. The new process is presented based on the current process in recycling platforms, with supplementary sorting of the recycled concrete. The gain in density and the reduction in water absorption are studied. The relation between the water content and the density of aggregates is analysed for three quality levels of recycled aggregates. The gain in density and the reduction in water absorption were linked to the aggregate replacement rates. The reduction in transport and discharge costs due to the increased aggregate quality was linked to the aggregate replacement rate and distance to quarry. In our study, replacing a lower-quality aggregate with another of medium quality leads to an expected density gain of approximately 4%. Conversely, if replaced by superior quality aggregate, the expected gain will be 8.4%. As a consequence, a 34% reduction in water absorption could also be obtained. The cost reduction is exponential with the substitution rate of recycled materials. There is a decrease in transport costs if the quality of recycled aggregates increases. Conversely, if the reduction in transport costs when the quality of recycled aggregates increases is considered, the distance between the demolition site and quarry is not important

Caractérisation de l'hydrodynamique et du transfert de matière dans des installations miniaturisées (mini et micro-échelle)

Les travaux de recherche présentés dans ce manuscrit concernent la caractérisation de l hydrodynamique et de l intensification du transfert de matière dans des installations miniaturisées de tailles millimétriques et micrométriques. Ce sujet se situe à l interface entre le domaine de la microfluidique et celui du génie chimique où la diminution de la taille des procédés est un challenge à l heure actuelle. Deux cellules de travail ont fait l objet d études. La première est constituée d un réseau de minicanaux croisés, de section carrée et de côté égal à 1,5 mm. Des mesures de chutes de pression et de transfert de matière global à partir d une méthode électrochimique permettent une caractérisation globale du mini-réseau et une comparaison des résultats avec ceux obtenus dans les milieux poreux. L étude hydrodynamique locale permet une approche multi-échelles comprenant des mesures PIV et des mesures locales du gradient pariétal de vitesse à partir de microsondes électrochimiques affleurant en paroi. Une analyse spectrale des signaux électrochimiques a permis à la caractérisation des régimes d écoulement, la reconnaissance des structures tourbillonnaires et la mise en évidence des phénomènes d intermittence en proche paroi. Le second système, d échelle micrométrique, est constitué de deux microcanaux se croisant à angle droit gravés dans une cellule en plexiglass. Des mesures du gradient pariétal de vitesse à partir de la méthode électrochimiques sont comparées à des simulations numériques réalisées à l aide du logiciel Fluent et deux configurations d écoulement ont été testées dont l'une favorisant les opérations de mélange.The research works presented in this memory are devoted to the flow characterization and mass transfer enhancement in miniaturized systems of millimetre and micrometer in size. This topic is located at the interface between the microfluidic area and the chemical engineering for which the reduction in size of the processes is a challenge. The experimental study concern two: The first one is composed of a network of crossing minichannels of square section 1.5 mm in side. Pressure drops and global mass transfer measurements realized using an electrochemical technique allowed the global characterization of the mini-network and a comparison of the results with those obtained in porous media. The local hydrodynamics study allows a multi-scale approach involving PIV measurements and wall shear rate measurements from electrochemical microprobes flush-mounted to the wall. A spectral analysis of the electrochemical signals allows the flow regimes identification, recognition of the vortices structure and the study of intermittency phenomenon in the near wall location The second cell, at a micrometric scale, made in a plexiglass cell which is composed of two microchannels intersecting at right angle. Wall shear rate measurements from the electrochemical technique have been compared with numerical simulations using the Fluent software. Two flow configurations have been carried out in order to enhance mixing process. The impinging-flow is found to be favourable to mixing, due to the formation of instabilities and high shear stress conditions into the focusing zone of the fluid stream.NANTES-BU Sciences (441092104) / SudocNANTES-Ecole Centrale (441092306) / SudocSudocFranceF

A revisit of the electro-diffusional theory for the wall shear stress measurements

This article intends to revisit the electro-diffusional theory for the wall shear stress measurement from mass transfer probes of rectangular shape by considering the existence of two components of the wall shear rate (i.e., axial and transversal). General analytical formulas for the effective transfer length and the dimensionless mass transport coefficient were derived as a function of only two parameters: a dimensionless angle of the flow direction, relative to the leading edge of the probe, and the aspect ratio between the width and the length of the strip probe. The correctness of the analytical relations for arbitrary flow direction and the aspect ratio was confirmed by numerical solutions of the transport equation in the convective-diffusive regime. It has also been proved that the differences between the Lévêque solution and the general analytical formula exhibit a significant deviation for a specific range of parameters. In the case of the three-dimensional boundary layers, in addition to the magnitude of the wall shear stress, the direction of the fluid flow in the vicinity of the probe's surface is of paramount importance. Accordingly, a measurement methodology is proposed using two strip probes with different aspect ratios. The resulting equations required to quantify the magnitude of the wall shear rate vector and the dimensionless angle are also derived

Heat Transport in Rotating Annular Duct: A Short Review

Heat transport in rotating processes finds a wide range of application in which academic issues in the fluid mechanics and heat transfer areas are here reported. This paper discusses successive works from the seminal paper of Taylor (1923) to recent numerical results established from a broad range of methods such as DNS, LES, RANS or LB methods. The flow regimes identification is thus reported in Taylor–Couette geometry. The role of the axial flow rates in the apparition, stabilization and destruction of the large-scale of the turbulent structures is depicted in the case of Taylor–Couette–Poiseuille geometry. In a non-isothermal condition, a discussion is held on the various exponent values found in the scaling relationships relying on the Nusselt number as a function of the Rayleigh or Reynolds numbers according to the regimes of thermal convection

Heat Transport in Rotating Annular Duct: A Short Review

Heat transport in rotating processes finds a wide range of application in which academic issues in the fluid mechanics and heat transfer areas are here reported. This paper discusses successive works from the seminal paper of Taylor (1923) to recent numerical results established from a broad range of methods such as DNS, LES, RANS or LB methods. The flow regimes identification is thus reported in Taylor–Couette geometry. The role of the axial flow rates in the apparition, stabilization and destruction of the large-scale of the turbulent structures is depicted in the case of Taylor–Couette–Poiseuille geometry. In a non-isothermal condition, a discussion is held on the various exponent values found in the scaling relationships relying on the Nusselt number as a function of the Rayleigh or Reynolds numbers according to the regimes of thermal convection

Heat transfer rate within non-spherical thick grains

The prediction of the internal heat conduction into non-spherical thick grains constitutes a significant issue for physical modeling of a large variety of application involving convective exchanges between fluid and grains. In that context, the present paper deals with heat rate measurements of various sizes of particles, the thermal sensors being located at the interface fluid/grain and into the granular materials. Their shape is designed as cuboid in order to control the surface exchanges. In enclosed coneshaped apparatus, a sharp temperature gradient is ensured from a hot source releasing the air stream temperature equal to about 400°C. Two orientations of grain related to the air stream are considered: diagonally and straight arrangements. The thermal diffusivity of the grains and the Biot numbers are estimated from an analytical solution established for slab. The thermal kinetics evolution is correlated to the sample granular mass and its orientation dependency is demonstrated. Consequently, a generalized scaling law is proposed which is funded from the effective area of the heat transfer at the grain-scale, the dimensionless time being defined from the calculated diffusional coefficients

Récupération des pertes pariétales dans un four tournant

International audienceLes fours tournants, utilisés pour sécher et chauffer des matériaux granulaires, sont constitués d'un long tube cylindrique, légèrement incliné, en rotation : ce procédé de séchage est très répandu, simple et adapté au traitement de débits importants. La rotation du tube permet de mettre en mouvement les matériaux et de renouveler les surfaces d'échange. L'enceinte du four est chauffée à l'aide des gaz de combustion issus d'un brûleur. L'objectif de ce travail est la caractérisation expérimentale des transferts pariétaux de chaleur afin de valoriser cette chaleur fatale. Ce système de récupération, agissant comme un échangeur de chaleur, est appliqué au niveau de la paroi afin de minimiser et de valoriser les pertes thermiques du procédé. Une loi de Nusselt sur la paroi externe du four a été établie en fonction du Reynolds effectif