Modélisation des transferts thermiques couplés à l'aide de la méthode de Boltzmann sur réseau

Dans cette communication, nous présentons un schéma numérique hybride pour simuler l'interaction entre la convection naturelle et le rayonnement surfacique dans une cavité rectangulaire différentiellement chauffée. Les parois verticales de la cavité sont adiabatiques, tandis que ses parois horizontales supérieure et inférieure sont respectivement froide et chaude. Les vitesses sont déterminées par la méthode de Boltzmann sur réseau et l'équation de l'énergie est discrétisée à l'aide de la méthode des volumes finis. Les systèmes algébriques obtenus sont résolus par la méthode des gradients conjugués. Seul le rayonnement surfacique est pris en compte et les surfaces radiatives sont supposées grises et isotropes en émission/réflexion

Simulation numérique des transferts thermiques dans une serre agricole chauffée par un bloc solide isotherme

International audienceLes effets provoqués par le transfert thermique radiatif sur la distribution de température, l'écoulement d'air et le transfert de chaleur dans une serre contenant un bloc solide carré, isotherme et chaud, sont étudiés numériquement. Les équations différentielles gouvernant le système sont discrétisées à l'aide d'une méthode des volumes finis et le couplage pression-vitesse est traité par l'algorithme SIMPLER. Les systèmes algébriques obtenus sont résolus par la méthode des gradients conjugués. La serre est supposée de rapport de forme A=2, et les résultats sont présentés en termes d'isothermes, de lignes de courant et de nombre de Nusselt pour des nombres de Rayleigh compris entre 103 et 106

The enerMENA Meteorological Network – Solar Radiation Measurements in the MENA Region

International audienceFor solar resource assessment of solar power plants and adjustment of satellite data, high accuracy measurement data of irradiance and ancillary meteorological data is needed. For the MENA region (Middle East and Northern Africa), which is of high importance for concentrating solar power applications, so far merely 2 publicly available ground measurement stations existed (BSRN network). This gap has been filled by ten stations in Morocco, Algeria, Tunisia, Egypt and Jordan. In this publication the data quality is analyzed by evaluating data completeness and the cleanliness of irradiance sensors in comparison for all of the stations. The pyrheliometers have an average cleanliness of 99.2 % for week-daily cleaning. This is a 5 times higher effort than for Rotating Shadowband Irradiometer (RSI) stations which even have a slightly higher average cleanliness of 99.3 % for weekly cleaning. Furthermore, RSI stations show a data completeness of 99.4 % compared to 93.6 % at the stations equipped with thermal sensors. The results of this analysis are used to derive conclusions concerning instrument choice and are hence also applicable to other solar radiation measurements outside the enerMENA network. It turns out that RSIs are the more reliable and robust choice in cases of high soiling, rare station visits for cleaning and maintenance, as usual in desert sites. Furthermore, annual direct normal and global horizontal irradiation as well as average meteorological parameters are calculated for all of the stations

A Novel Method for Automatic Real-Time Monitoring of Mirror Soiling Rates

The effect of soiling of solar concentrators influences the output of solar thermal power plants significantly. The literature values estimate the loss in specular reflectivity due to dust on the mirrors to be as high as 14% or even 26% after a few months. It varies significantly with location. Given the fact that these numbers are quite high and very broadly spread in the literature, project planners and plant operators demand better methods to measure soiling loads in running power plants as well as in resource assessment measurement campaigns. In this paper we introduce a new measurement setup named “TraCS” (Tracking Cleanliness Sensor) that allows for an easier, supposedly more precise and more cost effective measurement of the soiling level of solar mirrors. It will assist power plant operators and maintenance teams to optimize their cleaning cycles based on a real time measurement. At the same time the setup paves the way to including cleanliness as a standard parameter in solar resource assessment. It enables more sophisticated research on the most important weather parameters influencing the soiling rates at different plant sites and on upcoming new coating materials

Coupling of Lattice Boltzmann Equation and Finite Volume Method to Simulate Heat Transfer in a Square Cavity

International audienceThe objective of this paper is to assess the effectiveness of the coupled Lattice Boltzmann Equation (LBE) and finite volume method strategy for the simulation of the interaction between thermal radiation and laminar natural convection in a differentially heated square cavity. The vertical walls of the cavity are adiabatic, while its top and bottom walls are cold and hot, respectively. The air velocity is determined by the lattice Boltzmann equation and the energy equation is discretized by using a finite volume method. The resulting systems of discretized equations have been solved by an iterative procedure based on a preconditioned conjugate gradient method. Only the surface radiation is taken into account and the walls of the enclosure are assumed to be diffuse-grey. The achieved simulations have shown that the coupling between the lattice Boltzmann equation and the finite volume method gives excellent results. It was also observed that the surface radiation standardizes the temperature inside the cavity and causes a considerable increase of the heat transfer

The Uncertainty of the HelioClim-3 DNI Data Under Moroccan Climate

The purpose of this study is to evaluate the uncertainty of the DNI satellite-derived data from HelioClim-3 against those measured at ground level by calculating the classical statistical performance indicators for different time resolutions (hourly, daily and monthly). Correlations between the errors and the Aerosol optical depth (AOD) data measured by an AERONET sun photometer station at the University of Oujda were performed. Results show that the DNI data derived from Helioclim-3 can be considered as acceptable for satellite data with an RMSE of 19.7 % and a bias of 7.9 % for the hourly data. HelioClim-3 systematically over-estimates the daily DNI sums for the investigated site. The correlation to AOD at 550 nm can only be one of the reasons for the deviation. Corrections or site adaptation could help to increase the accuracy of the data base for that pixel

Lattice Boltzmann simulation of surface radiation and natural convection in a square cavity with an inner cylinder

International audienc

Numerical study of natural convection in a square cavity containing a cylinder using the lattice Boltzmann method

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Computation of coupled surface radiation and natural convection in an inclined ``T'' form cavity

International audienceThe present paper is concerned with computation of the radiation-natural convection interactions in an inclined ``T'' form cavity. The cavity contains two symmetrically identical isothermal blocks and is vented by two opening located in a vertical median axis at the top and the bottom parts of the cavity. Calculations are made by using a finite volume method and an efficient numerical procedure is introduced for calculating the view factors, with shadow effects included. Effects of Rayleigh number Ra and inclination angle phi are investigated for Pr = 0.71 in presence and in absence of the radiation exchange. Results are reported in terms of isotherms, streamlines, local and average Nusselt numbers and mass flow rate. In light of the obtained results, we can conclude that the heat transfer decreases with increasing phi. In addition, the increase of Ra and the taking into account of the radiation exchange produce a considerable increase in the heat transfer. (C) 2010 Elsevier Ltd. All rights reserved