Guy Lauriat

Forced convection cooling enhancement by use of porous materials

Abstract Results are presented for laminar forced convection cooling of heat generating blocks mounted on a wall in a parallel plate channel. The effect on heat transfer of insertion of a porous matrix between the blocks is considered. The flow in the porous medium is modeled using the Brinkman–Forchheimer extended Darcy model. The mass, momentum and energy equations are solved numerically by a control-volume-based procedure. The local Nusselt number at the walls of the blocks, the mean Nusselt numbers and the maximum temperature in the blocks are examined for a wide range of Darcy number and thermal conductivity ratio. The computations are first conducted for a single block, then for evenly mounted blocks. The results show that the insertion of a porous material between the blocks may enhance the heat transfer rate on the vertical sides of the blocks. Although the porous matrix reduces the heat transfer coefficient on the horizontal face, significant increases in the mean Nusselt number (up to 50%) are predicted and the maximum temperatures within the heated blocks are reduced in comparison with the pure fluid case.

Large eddy simulation of turbulent flow in a rotating pipe

Large eddy simulation (LES) of fully developed, incompressible turbulent channel flows are presented for stationary and rotating pipes. A dynamic model and the Smagorinsky model were used and compared with DNS results. The 3-D governing equations written in a cylindrical coordinate system were solved by a finite difference method, second-order accurate in space and in time. The features of the flows at two Reynolds numbers and for various rotation speeds are discussed and compared to available data of literature.

Thermoconvective instabilities in a narrow horizontal air-filled annulus

Abstract This paper reports a numerical investigation of natural convection flows between horizontal concentric annuli with the inner cylinder isothermally heated. Thermal and hydrodynamic instabilities for air-filled annuli of small radius ratios are discussed. At fairly low Rayleigh numbers (Ra⩽3000), thermal instabilities develop at the upper part of the annulus as steady cells. The results show the existence of an imperfect bifurcation, which could explain the discrepancies between the solutions reported previously in the literature. Locations of the bifurcation points are determined numerically for various radius ratios. At higher Rayleigh numbers, unsteady hydrodynamic instabilities are demonstrated in the vertical portions of an annulus with a radius ratio R =1.14. It is also shown that a reverse transition from multicellular to unicellular base flow patterns occurs when further increasing the Rayleigh number.

Stability analysis of natural convective flows in horizontal annuli: Effects of the axial and radial aspect ratios

Linear stability analyses for two-dimensional natural convection in horizontal air-filled annuli are performed for three-dimensional perturbations and radius ratios in the range 1.2⩽R⩽3. Flow transitions from moderate to large-gap annuli, which have not been reported before, are thoroughly investigated. As a result, stability diagrams are obtained for finite and for infinite length annuli. The leading disturbances and threshold values are found to agree well with experimental data and three-dimensional numerical solutions. Three-dimensional simulations were also carried out to examine the influence on the flow stability of no-slip boundary conditions at the end-walls.

Heat Transfer in Nanofluids 2012

1 School of Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854-8058, USA 2Dipartimento di Ingegneria Aerospaziale e Meccanica, Seconda Universita degli Studi di Napoli, Viale Beneduce 10, Via Roma 29, 81031 Aversa, Italy 3 Laboratory of Thermodynamics in Emerging Technologies, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland 4Department of Mechanical Engineering, University of California, Riverside, A363 Bourns Hall, Riverside, CA 92521-0425, USA 5Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong

Natural Convection in a Narrow Horizontal Annulus: The Effects of Thermal and Hydrodynamic Instabilities

Multicellular natural convective flows in narrow horizontal air-filled concentric annuli are considered numerically. The results show that the multiplicity of the multicellular upper flows reported in the literature can be credited to the existence of an imperfect bifurcation with two stable branches. The emergence and extinction of the buoyancy-driven cells have been proved to be identical on both branches. The appearance of another secondary flow, the origin of which is purely hydrodynamic and located within the crescent base flow at the vertical portions of the annulus, has also been evidenced at moderate values of the Rayleigh number. As Ra is increased a reverse transition from a multicellular structure to a unicellular pattern occurs through a gradual decrease in the number of cell

A new analytical solution for heat transfer in the entrance region of ducts: hydrodynamically developed flows of power-law fluids with constant wall temperature

Heat transfer by forced convection in the thermal entrance of flat ducts and circular pipes is investigated for constant surface temperatures and hydrodynamically developed flows. A new technique, based on separation of variables and spectral decomposition of the eigenfunction in polynomial form, is introduced to solve the problem for viscous fluids. Application of the present method is discussed for Newtonian and power-law fluids.

Numerical study of steady forced convection in a grooved annulus using a design of experiments

Numerical simulations have been performed to study the flow and heat transfer characteristics in the annular space between an inner smooth-cylinder rotating at constant angular velocity and an outer stationary grooved one. A first series of calculations were conducted to determine the ranges of Taylor number and longitudinal aspect ratio for which a two-dimensional modeling can be considered as valid. To this end, a map showing the domain of validity of two dimensional computations is presented. The assumption of circumferential periodicity of the flow is then justified in the range of Taylor numbers investigated. A number of simulations were carried out to investigate the effects of the geometrical parameters describing the cross section of the grooved annulus for various Taylor numbers. To end, correlations describing the influences on the main parameters both on the friction factor and Nusselt numbers are derived by using a design of experiments.

Amélioration du transfert thermique par un dépôt poreux sur la paroi d'un échangeur tubulaire

A study of flow regime and heat transfer in an annular heat exchanger partially filled with a porous medium is presented in this work. Constant heat flux and constant wall temperature boundary conditions on the inner cylinder are considered, while the outer cylinder is assumed adiabatic. The study is for both the thermal entry region and the thermally fully developed region. The flow in the porous region is modelled either by the Darcy-Brinkman equation for which an exact solution is developed or by the Darcy-Brinkman-Forchheimer equation in order to take into account inertial effects. For this case a numerical solution based on a control volume method is discussed. The results emphasize the effect of the porous layer attached to the inner cylinder on the thermal development length and heat transfer rate. It is shown that the porous substrate reduces the thermal entry length. When the effective thermal conductivity of the saturated porous medium is of the order of the fluid thermal conductivity, the local Nusselt does not vary monotonically with the thickness of the substrate. However, the use of a porous matrix always leads to an increase in the heat transfer rate provided its thermophysical properties and thickness are well chosen.

Boundary conditions for gas flow problems from anisotropic scattering kernels

The paper presents an interface model for gas flowing through a channel constituted of anisotropic wall surfaces. Using anisotropic scattering kernels and Chapman Enskog phase density, the boundary conditions (BCs) for velocity, temperature, and discontinuities including velocity slip and temperature jump at the wall are obtained. Two scattering kernels, Dadzie and Meolans (DM) kernel, and generalized anisotropic Cercignani-Lampis (ACL) are examined in the present paper, yielding simple BCs at the wall fluid interface. With these two kernels, we rigorously recover the analytical expression for orientation dependent slip shown in our previous works [Pham et al., Phys. Rev. E 86, 051201 (2012) and To et al., J. Heat Transfer 137, 091002 (2015)] which is in good agreement with molecular dynamics simulation results. More important, our models include both thermal transpiration effect and new equations for the temperature jump. While the same expression depending on the two tangential accommodation coefficients is obtained for slip velocity, the DM and ACL temperature equations are significantly different. The derived BC equations associated with these two kernels are of interest for the gas simulations since they are able to capture the direction dependent slip behavior of anisotropic interfaces.