Begell House Inc.
Computational Thermal Sciences: An International Journal
CTS
1940-2503
8
2
2016
NUMERICAL STUDIES OF THREE-DIMENSIONAL INSTABILITIES IN NEAR-WALL FLOW OVER A RAMP AT FREE-STREAM MACH NUMBER OF 5.4
117-125
Andrey V.
Novikov
Central Aerohydrodynamic Institute (TsAGI), Zhukovsky, Moscow region 140180, Russia
Propagation of three-dimensional (3D) disturbances through the 5.5° compression-ramp flow at the free-stream Mach
number 5.373 is simulated numerically. The disturbances are generated by a periodic suction–blowing actuator placed
on the wall. 3D Navier–Stokes equations are solved using the in-house parallel solver "HSFlow" on a fine grid. It
is shown that the selected forcing excites unstable disturbances relevant to the first and/or second mode instability depending on the actuator frequency. The instabilities evolving through the separation region exhibit nontrivial behavior, which is not captured by 2D simulations. The skin friction coefficient distributions show the presence of the transitional stage where a "young" turbulent wedge is formed.
NUMERICAL STUDY OF JET IMPINGING ON A ROTATING CYLINDER WITH HEAT TRANSFER
127-134
Azzeddine
Hammami
Faculty of Mechanical and Maritime Engineering, Mechanical Department, University of Sciences and Technology Oran, BP1505 El-Mnaouar, 31000 Oran, Algeria
Bachir
Imine
Aeronautical and Propulsive Systems Laboratory, USTO University, B.P. 1500 El Mnaouer
Oran, Algeria
Mustapha
Belkadi
Faculty of Mechanical and Maritime Engineering, Mechanical Department, University of Sciences and Technology Oran, BP1505 El-Mnaouar, 31000 Oran, Algeria
In this paper, a numerical modeling technique is used to study the phenomenon of a jet impinging on a cylindrical
surface at a constant temperature and rotating around its symmetrical axis. This phenomenon is encountered in a
milling cutter cooled by a cold air jet at low temperature. The cylinder is assumed to be at a constant temperature higher than the jet's temperature. Forced convection produced by the air jet at different Reynolds numbers induces the cooling of the cylinder. For this study, we adopted a mathematical model that will be resolved by the FLUENT software based on the finite volume method. The calculation code is validated with numerical and experimental results already available in the bibliography. Our results showed that the highest value of the Nusselt number occurs near the position of H/d = 6 for all different Reynolds number configurations; meanwhile the heat transfer increases with Reynolds number and
angular velocity.
IMPROVING ADIABATIC FILM-COOLING EFFECTIVENESS BY USING AN UPSTREAM PYRAMID
135-146
Zineb
Hammami
Département ELM, Institut de Maintenance et de Sécurité Industrielle, Université Oran 2,
Oran, Algeria
Zineddine Ahmed
Dellil
Déepartement ELM, Institut de Maintenance et de Sécurité Industrielle, Universitée Oran 2,
Oran, Algeria
Fadela
Nemdili
Laboratoire Aero Hydrodynamique Navale, (LAHN) USTO-MB, Oran, Algeria, Faculté de
Génie Mécanique, Université des Sciences et de la Technologie d'Oran, Mohamed Boudiaf,
BP1505 El-Mnaouar, 31000, Oran, Algeria
Abbes
Azzi
Laboratory of Naval Aero-Hydrodynamic, Faculty of Mechanical Engineering, Oran
University of Sciences and Technology, PO Box 1505, El-Mnaouar Oran, Algeria
As film cooling is an important and critical process for gas turbine applications, designers are always looking to increase the adiabatic film-cooling effectiveness. One possible solution is to modify the approaching boundary layer flow and its interaction with the film-cooling jets. Inspired by published research where an upstream ramp is added just before the cooling jet rows, this study presents a new design of the ramp still showing good cooling performances and fewer aerodynamic losses. In the new design concept, the ramp looks like an upstream pyramid centered exactly in the space between the two adjacent holes. With this design, it is expected that space between adjacent holes, which is less cooled by the jet, will be protected by the upstream pyramid and more lateral jet spreading can be realized. For comparison purposes, three geometrical configurations are considered, which are the baseline case, the case with an upstream ramp,
and finally the new case with an upstream pyramid. Computations, based on the ensemble-averaged Navier-Stokes equations solved by the realizable k−ε turbulence model and standard wall function, are used in the frame of the finite volume fluent computational fluid dynamic (CFD) code. For 12 computational cases, including the baseline case, centerline adiabatic film-cooling effectiveness as well as the laterally averaged adiabatic film-cooling effectiveness are presented and compared. Additionally, the surface distribution of the adiabatic film-cooling effectiveness is also presented. Lateral spreading is investigated by plotting lateral variation of the adiabatic film-cooling effectiveness at several longitudinal stations. Results obtained by the present computations show that the upstream ramp with a backward-facing step greatly increases surface adiabatic effectiveness. The laterally averaged adiabatic effectiveness with the ramp can be higher than
without the ramp by increasing lateral spreading of the coolant. For the case with l/d = 2.8, the new proposed geometry (upstream pyramid) still improves the thermal performances by less lateral spreading but with better pressure distribution, while for the case of l/d = 1.75, the pyramid case outperforms other cases close to the hole injection area (x/d < 10). Therefore, as a conclusion, the proposed geometry presents a good compromise between increasing the adiabatic film-cooling effectiveness and keeping the pressure losses to smaller levels.
INTEGRAL TRANSFORM SOLUTION FOR THERMALLY DEVELOPING SLIP-FLOW WITHIN ISOTHERMAL PARALLEL PLATES
147-161
Daniel J. N. M.
Chalhub
Group for Studies and Environmental Simulations in Reservoirs – GESAR, Department of Mechanical Engineering – PPGEM, Universidade do Estado do Rio de Janeiro – UERJ, Rua Fonseca Teles 121, Rio de Janeiro, RJ, 20940-903, Brazil
Leandro A.
Sphaier
Department of Mechanical Engineering – PGMEC, Universidade Federal Fluminense, Rua
Passo da Patria 156, bloco E, sala 216, Niteroi, RJ, 24210-240, Brazil
Leonardo
Alves
Departamento de Engenharia Mecânica - TEM
Universidade Federal Fluminense - UFF
This paper presents an analytical solution for an extended version of the Graetz problem for slip-flow in parallelplates channels. The problem formulation includes axial heat diffusion in a semi-infinite channel with a given inlet condition and isothermal walls. The solution methodology is based on the Generalized Integral Transform Technique, in which the sought temperature profile is written in terms of an orthogonal eigenfunction basis, stemming from a Sturm-Liouville type problem. Although the transformation of the original problem leads to a coupled ODE system, a closedform solution is obtained in terms of eigenvalues and eigenvectors of a matrix involving the ODE system's coupling coefficients. The solution is properly verified through comparisons with previous literature results and a numerical solution by finite differences. A convergence analysis of the results shows that better convergence rates are obtained for larger values of the Péclet and Knudsen numbers, even in the near-entrance region. Finally, a parametric analysis shows that the local Nusselt number increases with the Péclet number, and this increase is stronger for upstream positions.
Nevertheless, for larger values of the Knudsen number, the dependence of the Nusselt number values on the Péclet
number is shown to be weaker.
NATURAL CONVECTION IN TALL AND SHALLOW POROUS RECTANGULAR ENCLOSURES HEATED FROM BELOW
163-176
Zineddine
Alloui
Département du socle commun des Sciences et Technique, Faculté de Technologie, Université El-Hadj-Lakhdar Batna, 05000 Batna, Algeria
Patrick
Vasseur
Ecole Polytechnique, Université de Montréal, C.P. 6079, Succ. "Centre ville", Montréal,
Québec H3C 3A7, Canada
The Darcy model with the Boussinesq approximation is used to study both analytically and numerically natural convection in a porous medium saturated by a Newtonian fluid. The geometry considered is a rectangular cavity heated from below and cooled from above by a constant heat flux while the sidewalls are maintained adiabatic. The governing parameters for the problem are the thermal Darcy-Rayleigh number R and the aspect ratio of the cavity A. In the limit of extremely confined geometries the parallel flow approximation is used to predict the flow behavior in the case of a tall vertical layer (A >> 1) or shallow horizontal one (A << 1). For a tall cavity, the existence of multicellular flow patterns, consisting of m vertical cells, is predicted by the present analytical model. Also, for a shallow cavity, it is demonstrated
that the flow pattern can be either unicellular or multicellular. This is confirmed by the numerical results obtained by solving the full governing equations.
EFFECTS OF RADIATION, VISCOUS DISSIPATION, AND MAGNETIC FIELD ON NANOFLUID FLOW IN A SATURATED POROUS MEDIA WITH CONVECTIVE BOUNDARY CONDITION
177-191
Eshetu
Haile
Department of Mathematics, Bahir Dar University, Bahir Dar, Ethiopia
B.
Shankar
Department of Mathematics, Osmania University, Hyderabad 500 007, India
Effects of thermal radiation, viscous dissipation, and magnetic field on nanofluid flow in a saturated porous medium with convective boundary condition are clearly presented in this paper. The sheet is situated in the xz plane and y is measured normal to the surface directing to the positive y axis. A transverse external magnetic field is applied parallel to the y axis and the sheet is continuously stretching in the positive x axis. The governing boundary-layer equations of the problem are formulated and then transformed into a nonlinear system of ODEs. The resulting equations are then solved numerically by the Keller box method and effects of the pertinent parameters on velocity, temperature, concentration,
skin-friction coefficient, Nusselt number, and Sherwood number are mentioned and explained graphically and in tabular form. The results are in nice agreement with previously published results.
HYDRODYNAMICS AND HEAT TRANSFER ANALYSIS OF NANOFLUID FLOW IN A CIRCULAR MICROCHANNEL BY SIMULATIONS
193-208
K. Sunil
Arjun
Department of Mechanical Engineering, Indian School of Mines, Dhanbad-826004, India
K.
Rakesh
Department of Mechanical Engineering, Indian School of Mines, Dhanbad-826004, India
Water and its nanofluids with alumina (Al2O3) are used as the coolant fluid in a circular microchannel heat exchanger to justify the utility of a nanomaterial as a heat transfer enhancer using ANSYS Fluent 15.0. The 2D axis symmetric geometry with structured mesh and 100×18 nodes are used for single-phase flow with Al2O3 nanoparticles of 23 nm average diameter. Viscous laminar and standard k−ε models are used to predict the steady temperature in laminar and turbulent zones. The simulated heat transfer coefficient values in both laminar and turbulent zones have been compared with the published experimental values and very close agreement is observed statistically. Nanofluids increase the heat transfer coefficient by 15% and 12% in comparison to its base fluids in laminar and turbulent zones, respectively. The relation between heat transfer coefficient and thermal conductivity of nanofluids is proved. The entrance length for fully developed velocities and the increase in temperature depend on Re, with the latter also depending on Pe, but the temperature distribution is found to be independent of radial position even for very low Pe. The velocity contours at the outlet show that the wall effect penetrates more towards the center and the thickness of the zone with maximum velocity shrinks with increase in Re. With increase in Re, the temperature decreases and pressure drop increases. The velocity and wall and nanofluid temperatures calculated can also well predict the experimental data. The effect of Re, Pe, nanofluid concentrations, velocity, pressure, and temperature contours on the flow behavior of the microchannels was analyzed in laminar and turbulent cases.