Nadim El Wakil

Transfert de chaleur dans un tube vertical avec écoulement de convection mixte à débit variable

Abstract In this paper, we propose a numerical study on the mixed heat convection of a laminar flow inside a vertical pipe. A flow rate step is imposed at the inlet of the tube and a heat transfer coefficient is used to calculate the external heat convection. Both cases of positive and negative steps are considered. A strong difference in the thermal and the dynamic behaviour of the flow is observed between these two cases. Moreover, a non-dimensional study on the influence of some parameters like the thermal buoyancy coefficient, characterized by Ri∞ Re∞, and the external heat transfer, characterized by Bi∗/Pe (generalized Biot/Peclet numbers), has shown that, in the case of laminar mixed convection, their influence on both the inner flow and the wall heat transfer is great.

Hygrothermal and Acoustical Performance of Starch-Beet Pulp Composites for Building Thermal Insulation

This article deals with the elaboration and the characterization of an innovative 100% plant-based green composite made solely of beet pulp (BP) and potato starch (S). Using this type of material in insulation applications seems a good solution to reduce the CO2 gas emissions in building. The influence of the starch amount on composite characteristics was studied. Four mixtures were considered with different S/BP mass ratios (0.1, 0.2, 0.3 and 0.4). The physical properties of these materials were studied in terms of porosity, apparent and absolute densities, thermal conductivity, and hygric properties. The influence of humidity content on acoustical properties was studied as a function of frequency. Test results show a real impact of both starch and humidity contents on the hygrothermal and acoustical properties of the studied material due to the porosity. The composite with the lowest amount of starch (S/BP = 0.1) seems to be the optimal composition in terms of the hygrothermal and acoustical behaviors.

Non-Isothermal Fluid Flow in a Continuous Casting Tundish

Temperature variation phenomena observed inside the metallurgical tundish in the continuous casting is principally due to several factors, high temperature of the ladle metal jet, important thermal losses, heat transfer between molten steel and tundish walls and more still, in contact with the slag layer at bath surface and if necessary cooling or warming in refining composition operations. A numerical simulation using the CFD code “Fluent” is realised. Investigation of heat transfer—which has a great importance in the natural convection and the inclusions floating at slag surface—, is proposed. Results are presented for three arrangements of the tundish, where the interest and role of the control devices is improved.Copyright

SOME INVESTIGATIONS ON TRANSIENT MIXED CONVECTION

Fully developed mixed convection between parallel plates for transient laminar water flow is analyzed by making use of a finite difference method, in the case of a temperature step applied at one wall. Results show that during the transient state, both prescribed constant pressure drop and bulk velocity cannot be imposed. Velocity profiles are investigated. Comparisons of buoyancy, pressure and viscosity forces has been performed and compared to results obtained in steady state. They lead to the new definition of a transient buoyancy ratio RiRe(t). INTRODUCTION The results presented here are the second part of a more comprehensive investigation. The motivation for this investigation is threefold. iThe study of mixed convection within channels and ducts has historically received less attention than the analysis of pure forced or natural convective heat transfer, especially in unsteady states [Yan 1992, Lee 1994, Mai 1994, 1999, 2005, Nguyen 2003, 2004]. Nevertheless, these two extreme situations are in fact special cases of the more general formulation accounting for the combination of the buoyancy and imposed pressure gradient effects. In addition, the literature lacks a general criterion for the selection of the range of validity of these specific situations, in terms of the governing parameters or relevant dimensionless groups. iiResults obtained for transient mixed convection flows (for instance when a temperature step is imposed at one wall) show new properties compared to steady state and an assessment of their practical consequences is of interest, and raise a question as regard to the definition of the reference temperature in the buoyancy force. iiiPrevention of the occurence of excessive heat transfer in thermal systems as nuclear plants or oil and gas wells is more and more requested for safety and reliability [Bayazitoglu et al., 2007].