Mixed Convection of Hybrid Nanofluids in an Annulus

 In this study, mixed convection in an annulus formed by two horizontal isothermal cylinder surfaces and filled with hybrid nanofluids was examined with Galerkin weighted residual finite element method. The outer cylinder is rotating and inner cylinder is stationary. Influence of Rayleigh number, angular rotational speed of the outer cylinder, eccentricity of the inner cylinder, solid volume fractions of different nanoparticles (alumina, copper, hybrid particles between 0 and 0.02) on the fluid flow and heat transfer characteristics were analyzed. It was observed that average heat transfer enhances with Rayleigh number, solid volume fractions of nanoparticles and eccentricity ratio and reduces as the angular rotational speed of the outer cylinder increases. Adding nanoparticles was found to be advantageous for lower values of Rayleigh number and higher values of angular rotational speed. At the highest volume fraction of Cu nanoparticles, average Nusselt number increases by 31.75 % when the inner cylinder center moves in +y direction. Nanofluid with hybrid nanoparticles gives heat transfer rates which are higher than that of with alumina and lower than that of with copper nanoparticles for the same volume fraction

Natural convection in a trapezoidal cavity with an inner conductive object of different shapes and filled with nanofluids of different nanoparticle shapes

Natural convection in a trapezoidal cavity having different conductive obstacles which are portions of a full circular object and filled with different shaped nanoparticles (spherical, blade and cylindrical) was numerically investigated. The side walls of the trapezoidal cavity are kept at constant hot and cold temperatures, while the top and bottom walls are assumed to be adiabatic. The governing equations are solved with finite element method. The effects of the Rayleigh number (between 104 and 106), inclination angle of the side walls (between 0° and 20°), thermal conductivity ratio (between 0.01 and 100), solid volume fraction of the nanoparticles (between 0 and 0.04) and nanoparticle shape (spherical, blade and cylindrical) on the fluid flow and heat transfer characteristics were studied in detail. It was observed that the shape of the obstacle is very effective to change the heat transfer characteristic for lower values of Rayleigh number. Thermal conductivity ratio influences the heat transfer characteristics slightly. Averaged Nusselt number increases linearly with nanoparticle volume fraction and the slope of the curves is highest for the cylindrical nanoparticle and it is not affected by the obstacle types. Averaged Nusselt number enhancements are in the range of 13 and 16% when cylindrical nanoparticles are used instead of spherical ones with different obstacle shapes for thermal conductivity ratio of 0.01 and 100. Shiraz University 2017

AIAA JOURNAL

A nonlinear, low-order physics-based model for the dynamics of forced convection wall heat transfer in pulsating flow is formulated, based on the proper orthogonal decomposition technique. In a multivariate approach, proper orthogonal decomposition modes are constructed from computational fluid dynamics data for laminar flow and heat transfer over a flat plate in pulsating flow, spanning a range of pulsation frequencies and amplitudes. Then, the conservation equations for mass, momentum, and energy are projected onto the proper orthogonal decomposition modes, such that a system of ordinary differential equations for the modal amplitudes is obtained. The forcing at the inlet is written explicitly in the ordinary differential equations of the low-order model. The contribution of the nonvanishing pressure term resulting from the incompressible Navier-Stokes equation is included with a calibration method. The accuracy and stability of the low-order model are evaluated by comparison with computational fluid dynamics data. Possible applications of this heat source model to the computation of a describing function or the prediction of limit cycle amplitudes of thermoacoustic instabilities are discussed

INTERNATIONAL JOURNAL OF COMPUTATIONAL METHODS

In the present study, a novel approach based on Proper Orthogonal Decomposition (POD) and fuzzy clustering method is utilized to predict the flow field and heat transfer for the unsteady mixed convection in a square enclosure with two ventilation ports. An adiabatic thin fin is placed on the bottom wall of the cavity and all walls of the enclosure are kept at constant temperature. An oscillating velocity is imposed at the inlet port for a range of Strouhal numbers between 0.1 and 1. Reduced order models of the system are obtained with fuzzy-POD approach for Richardson number of 1 and 100. The estimation data set is obtained for Strouhal numbers 0.1 and 0.5, and the validation data set is obtained for Strouhal number of 0.25. A comparison of the modal coefficients obtained from the proposed approach compares well with the modal coefficients obtained by projecting the CFD data at Strouhal number of 0.25 onto the POD modes. The proposed approach is computationally efficient and the problem of numerical instability in the computation with the conventional Galerkin-POD approach can be circumvented

JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS

In this study, the problem of magnetohydrodynamics (MHD) mixed convection of lid-driven cavity with a triangular-wave shaped corrugated bottom wall filled with a non-Newtonian power-law fluid is numerically studied. The bottom corrugated wall of the cavity is heated and the top moving wall is kept at a constant lower temperature while the vertical walls of the enclosure are considered to be adiabatic. The governing equations are solved by the Galerkin weighted residual finite element formulation. The influence of the Richardson number (between 0.01 and 100), Hartmann number (between 0 and 50), inclination angle of the magnetic field (between 0 deg and 90 deg), and the power-law index (between 0.6 and 1.4) on the fluid flow and heat transfer characteristics are numerically investigated. It is observed that the effects of free convection are more pronounced for a shear-thinning fluid and the buoyancy force is weaker for the dilatant fluid flow compared to that of the Newtonian fluid. The averaged heat transfer decreases with increasing values of the Richardson number and enhancement is more effective for a shear-thickening fluid. At the highest value of the Hartmann number, the averaged heat transfer is the lowest for a pseudoplastic fluid. As the inclination angle of the magnetic field increases, the averaged Nusselt number generally enhances

EUROPEAN JOURNAL OF MECHANICS B-FLUIDS

In this study, numerical simulation of a thermo-acoustic heat engine is performed and a reduced order model of the system based on the Proper Orthogonal Decomposition method is obtained. The governing equations are solved with a finite volume-based solver. The reduced order model is constructed using the snapshots when the system reaches the limit cycle. A quadratic polynomial type ODE system with 11-modes is constructed from the Galerkin projection for the high fidelity CFD computations of the coupled thermo-acoustic system. (C) 2014 Elsevier Masson SAS. All rights reserved

INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER

In this study, numerical investigation of mixed convection in a square cavity filled with SiO2 nanofluid and volumetric heat generation is performed under the effect of an inner rotating cylinder and a flexible side wall. The top wall of the cavity is kept at constant cold temperature while the bottom wall is at hot temperature and the other walls of the cavity and the cylinder surface are assumed to be adiabatic. The finite element method is utilized to solve the governing equations. The Arbitrary Lagrangian-Eulerian method is used to describe the fluid motion with the flexible wall of the cavity in the fluid-structure interaction model. The effects of external Rayleigh number (between 10(3) and 5 x 10(6)), internal Rayleigh number (between 10(4) and 10(6)), Young's modulus of the flexible wall (between 5 x 10(2) and 10(6)), angular rotational speed of the cylinder (between - 2000 and 2000) and nano particle volume fraction (between 0 and 0.03) on the fluid flow and heat transfer are numerically studied for different solid nanoparticle shapes (spherical, cylindrical, brick and blade). It is observed that as the value of external Rayleigh number increases, internal Rayleigh number and elastic modulus of the flexible wall decrease, the local and averaged heat transfer enhances. The averaged heat transfer enhances with cylinder rotation in both directions for all nanoparticle types. Among all nanoparticle shapes, cylindrical ones show the best performance and spherical ones show the worst performance for heat transfer enhancement. (C) 2015 Elsevier Ltd. All rights reserved

Mixed convection in a lid-driven cavity filled with single and multiple-walled carbon nanotubes nanofluid having an inner elliptic obstacle

In this study, numerical analysis and optimization in a single and multiple walled carbon nanotube-water nanofluid filled lid driven cavity having an inner elliptic obstacle were performed by using finite element method and COBYLA optimization solver. The top wall is moving with constant speed and vertical walls are kept at constant temperatures. An optimal size of the inner elliptic obstacle was determined by using an optimization study to maximize the average heat transfer along the hot wall of the cavity. Numerical simulation was performed by for various values of Richardson numbers (between 0.05 and 50) and various solid particle volume fraction (between 0 and 0.06) for single and multiple-walled carbon nanotubes water nanofluid. A larger obstacle (higher values of radii in the major and minor axis) with lower values of Richardson number results in higher heat transfer rates. The average Nusselt number versus solid particle volume fraction shows a linear trend and the discrepancy between the average Nusselt number for the cavity with the optimized obstacle and other obstacles becomes higher with higher particle volume fraction. The average heat transfer enhances significantly which is about 120.20% for single wall carbon nanotube -water nanofluid at solid volume fraction of 0.06 when compared to pure water. The discrepancy between the average Nusselt number for single and multiple walled carbon nanotubes becomes higher for higher values of Richardson number and solid particle volume fraction. A polynomial type correlation was proposed for the average Nusselt number along the hot wall which is fifth order for Richardson number an first order for nanoparticle volume fraction. © 2019 Beihang Universit

PROGRESS IN COMPUTATIONAL FLUID DYNAMICS

In this study, a square cavity with two ventilation ports in the presence of an adiabatic fin placed on the bottom wall of the cavity is numerically analysed for the mixed convection case for a range of Richardson numbers (Ri = 0.1,1, 10, 30) and at Reynolds number of 300. The top and bottom walls of the cavity are kept at constant temperature while the verticals walls are assumed to be adiabatic. The effect of the fin height, inclination angle and Richardson number on the fluid flow and heat characteristics is numerically analysed. The results are presented in terms of streamlines, isotherm plots and averaged Nusselt number plots. It is observed that length and inclination angle of the fin significantly alter the streamlines and isotherms and hence the thermal performance of the system. For the best performance at different fin lengths, optimum inclination angle changes

EUROPEAN JOURNAL OF MECHANICS B-FLUIDS

In this study MHD flow in a lid driven nanofluid filled square cavity with a flexible side wall is numerically investigated. The top wall of the cavity is colder than the bottom wall and it moves in the +x direction with constant speed. Other walls of the cavity are insulated. The finite element formulation is utilized to solve the governing equations. The Arbitrary-Lagrangian-Eulerian method is used to describe the fluid motion with the flexible wall of the cavity in the fluid-structure interaction model. The influence of the Young's modulus of the flexible wall on the flow and heat transfer characteristics are numerically investigated for the following parameters: (10(4) N/m(2) <= E <= 2.5 x 10(5) N/m(2)), with a Richardson number of (0.01 <= Ri <= 5), a Hartmann number of (0 <= Ha <= 50) and a volume fraction of the solid particles given by (0 <= phi <= 0.04). The effect of Brownian motion on the effective thermal conductivity of the nanofluid is taken into account. Averaged heat transfer decreases with increasing Hartmann number and decreasing Richardson numbers. As the Young's modulus of the flexible wall decreases, the averaged heat transfer increases and 66.5% of the heat transfer enhancement is obtained for E = 10(4) N/m(2) compared with E = 2.5 x 10(5) N/m(2). An averaged heat transfer enhancement of 33.87% is obtained for a solid volume fraction of 4% compared to the base fluid for the fluid-structure model coupled with the magnetic field. (C) 2016 Elsevier Masson SAS. All rights reserved