Energy valorization of industrial biomass: Using a batch frying process for sewage sludge

Issu de : WasteEng 08 - 2nd International conference on engineering for waste valorisation, Patras, GREECE, June 3-5, 2008International audienceThis paper studies the energy valorization of sewage sludge using a batch fry–drying process. Drying processes was carried out by emerging the cylindrical samples of the sewage sludge in the preheated recycled cooking oil. Experimental frying curves for different conditions were determined. Calorific values for the fried sewage sludge were hence determined to be around 24 MJ kg−1, showing the auto-combustion potential of the fried sludge. A one-dimensional model allowing for the prediction of the water removal during frying was developed. Another water replacement model for oil intake in the fried sewage sludge was also developed. Typical frying curves were obtained and validated against the experimental data

A Model to Predict the Heat Transfer Coefficient at the Casting-Die Interface for the High Pressure Die Casting Process

A model is presented for the prediction of the heat transfer coefficient (HTC) at the casting-die interface as a function of time for the high pressure die casting process. Contact geometry and interface characteristics are included in the model through die surface roughness, the mean trapped air layer between the casting and the die, the parameters of area density and the radius of contact spots. The density and the radius of contact spots are integrated into a classical thermal flux tube theory in order to calculate HTC at the casting-die interface. The time dependence of the HTC is derived in terms of the degradation of contact between the casting and the die that occurs during solidification. The calculated HTC is found to agree well with the experimentally determined results for different casting conditions. The presented model provides a valuable tool to predict the effect of various casting process parameters, die surface roughness, casting quality and thickness on the HTC during the high pressure die casting process

Estudio de la resistencia a fatiga térmica en moldes de inyección de aluminio utilizando aceros de alta conductividad térmica

La mayoría de los mecanismos de fallo que determinan la durabilidad de un molde de inyección de aluminio a alta presión están relacionados con las solicitaciones termo-mecánicas de la superficie. En cada ciclo, la superficie de la herramienta está expuest a a elevadas variaciones de temperatura y cargas mecánicas que ocasionan la pérdida de propiedades superficiales (dureza, resistencia, etc.). La variación de las propiedades superficiales y las tensiones termo-mecánicas generadas en los ciclos de inyección inducen la aparición de grietas. A fin de retardar la aparición de estas grietas, se ha desarrollado una nueva generación de aceros de herramienta de trabajo en caliente denominados HTCS® (High Thermal Conductivity Steels) caracterizados por poseer elevados valores de difusividad térmica combinados con elevadas propiedades termo-mecánicas, comparados con los aceros de trabajo en caliente convencionales. Estas propiedades, sobre todo la difusividad térmica elevada, tienen su principal efecto en la reducción de las tensiones superficiales generadas en las herramientas debido a los gradientes térmicos durante el servicio, Esta reducción de tensiones se traduce en un aumento de la vida útil del molde de inyección. En este trabajo se evalúa el efecto de la conductividad térmica del acero de herramientas en la aparición y propagación de grietas por fatiga térmica . Para ello, se han fabricado dos moldes experimenta les de inyección de aluminio a alta presión con la misma geometría y dureza. Un molde se ha fabricado con acero de trabajo en caliente convencional (AISI Hll) y el otro molde con acero de alta conductividad térmica (HTCS®). Se ha realizado el seguimiento de la aparición y propagación de las grietas por fatiga térmica en los dos moldes durante el servicio. Para poder investigar y entender mejor la relación entre las diferencias térmicas y las tensiones superficiales generadas en los dos moldes experimenta les, se ha utilizado un modelo simple de Simulación por Elementos Finitos representativo de una sección del molde real.Postprint (published version

Interfacial Heat Transfer in Die Casting of Light Alloys

In die casting, heat transfer is controlled by the properties of the casting-die interface, due to the high thermal diffusivity and conductivity of the die. Higher heat transfer rates result in higher productivity in the die casting process along with finer microstructure with superior part qualities. In die casting processes, the use of numerical simulation to predict the mould filling, solidification, and the distribution of temperature in the die has become an important development in foundry technology and cast product developments. The effectiveness of the simulation is dependent on the accuracy of the heat transfer data at the metal-mould boundary used in the simulation software. For these reasons, accurate knowledge about the heat transfer between the casting and the die during the die casting process has become an important issue for most researchers in this field. This investigation studies the interfacial heat transfer during the casting and solidification of light alloys in Gravity (GDC) and High Pressure (HPDC) Die Casting processes. Suitable experimental methods and sensors have been developed that enable the measurement of temperatures, in-cavity pressure and a range of process parameters. Heat Transfer Gauges have been manufactured, accurately calibrated and integrated into the dies for both investigated processes. These gauges measure the casting surface temperature with a pyrometric chain and the temperature through the die wall at different depths with very thin K-type thermocouples. Extensive industrial trials were performed using Al-7Si-0.3Mg, A-9Si- 3Cu and AZ91 D alloys. From the temperature measurements, the Heat Transfer Coefficient (HTC) and heat flux density (q) at the casting-die interface have been evaluated using an inverse method. The obtained results have been correlated with the process parameters and to the microstructure of the castings for some selected cases. In GDC, the results show that the applied coating on the die surface governs the heat transfer during solidification of the casting through its low effective thermal conductivity. The effective thermal conductivity of the coating is determined by the high level of porosity in its structure. For this reason, its chemical composition has a very limited influence on the peak value of heat transfer. However, the coating composition does influence the variation of the heat transfer coefficient with time during solidification. The variation of HTC at the castingdie interface has been divided into three main stages according to the conditions of contact during casting and solidification: liquid-solid, semisolid-solid and solid-solid contact. The HTC at each of these stages is dominated by different process parameters. A secondary peak in the evolution of the HTC has been observed at the end of the semisolid-solid stage. This secondary peak was attributed to the exudation of solute rich eutectic liquid onto the surface region of the casting enabling fresh liquid contact with the mould coating. In HPDC the value of the HTC is much larger than that determined in GDC ((105 Wm-2K-1 compared to103 Wm-2K-1 in GDC). The variation of HTC with time is also very rapid since the heat flux density is much greater than that in GDC. The rapid variation of the HTC is attributed to the degradation of contact between the casting and the die occurring during solidification. The second stage velocity and the impact pressure caused by the sudden deceleration of the piston as the cavity was filled have a significant influence on the interfacial heat transfer during HPDC. This impact pressure is different from the intensification pressure which is applied during the third stage of the casting process. The values of the peak heat transfer coefficient showed no dependence on intensification pressure in our trials. Analytical models have been developed to predict the thermal contact resistance (TCR) at the liquid-solid interface in general. Contact topography and interface characteristics are included in the model through die surface roughness and the mean trapped air layer between the casting and the die. The mean trapped air layer is determined from the mechanisms of contact at the liquid-solid interface. Two different contact mechanisms have been considered; liquid-porous solid and liquid-non porous solid contact. In the liquid-non porous case, the air is trapped and compressed inside the microcavities. However, when the solid surface is porous, the air is not trapped and can escape through the pores. In this situation, the contact conditions are determined by the pressure, surface tension of the liquid and the surface roughness characteristics of the die or coating. The proposed models determine the radius and the density of the microcontact points for a given condition of contact. The density and the radius of contact spots have been integrated into a classical thermal flux tube theory in order to calculate HTC at the casting-die interface. The models have been applied to the casting-die interface in GDC and HPDC. The calculated TCR is found to agree well with the experimentally determined results. The models provide valuable information about the role of the casting-die interface in heat transfer during GDC and HPDC processes. The results show that the effect of the measured parameters such as pressure and roughness on the HTC at the casting-die depends on the mechanisms of contact. For HPDC, the evolution of the HTC with time has been associated with the presence of a rapidly solidified region on the casting surface which forms early during solidification due to the high thermal gradient. However, in GDC the relatively low thermal gradients are associated with microstructures more consistent with the formation of a mushy zone across the casting

The impact of the casting thickness on the interfacial heat transfer and solidification of the casting during permanent mold casting of an A356 alloy

International audienceMany important quality indicators for components manufactured using permanent mould casting, such as the presence of shrinkage porosity, microstructure features, dimensional stability, cycle time and filling mechanisms are controlled directly or indirectly by the heat transfer mechanisms linking the casting to the mould. While interfacial heat transfer in permanent mould casting has been significantly investigated and widely reported in the literature, the geometrical dependence of heat transfer parameters has not been studied or reported in detail. Understanding this dependency is very important as the same cast component most often is constituted by different sections and geometrical variations. In this paper, experimental methods and analytical correlations have been developed and presented that enable an accurate determination of the time dependent interfacial heat flux density and heat transfer coefficient at the casting-mould interface. The variation of these parameters is investigated and analysed for three different casting sections and two types of thermal barrier coatings

Heat transfer rates during fry-drying of sewage sludge

Contenu dans : Proceedings of AFSIA 2009 European drying conferenceInternational audienc

A predictive model for the evolution of the thermal conductance at the casting–die interfaces in high pressure die casting

International audienceAn analytical model is proposed to predict the time varying thermal conductance at the casting–die interface during solidification of light alloys during High pressure Die Casting. Details of the topography of the interface between the casting and the die are included in the model through the inclusion of solid surface roughness parameters and the mean trapped air layer at the interface. The transitory phase of the interfacial thermal conductance has been related to the degradation of contact as solidification progresses through the casting thickness. The modelled time varying thermal conductance showed very good agreement with experimentally determined values for different alloy compositions and casting geometries. The analysis shows that the parameters that govern the thermal conductance are different for the first stage of contact compared to the second stage of contact when the alloy begins to solidify