Journal Of Applied Fluid Mechanics
Volume:4 Issue: 3, Jul-Aug 2011

In this study we will examine the applicability of the flow induced by a rotating disk in evaluating the performance of polymeric and surfactant additives in reducing skin friction drag and effect of viscosity on drag reduction capability of polymeric and surfactant solutions. It is shown that these additives can dramatically reduce friction drag provided that the flow is occurring under turbulent conditions while they have no effect on Taylor instabilities. Based on the experimental data, a drag reduction in the range of 10% can be achieved with the effect becoming more pronounced the higher the Reynolds number.

A steady two-dimensional MHD free convection and mass transfer flow past an inclined semi-infinite vertical surface in the presence of heat generation and a porous medium has been studied numerically. The governing partial differential equations are reduced to a system of ordinary differential equations by introducing similarity transformations. The non-linear similarity equations are solved numerically by applying the Runge-Kutta method of fourth order with shooting technique. The numerical results are presented graphically for different values of the parameters. Finally, the numerical values of the local skin-friction coefficient, local Nusselt number and Sherwood number are shown.

The steady, laminar axisymmetric convective heat and mass transfer in boundary layer flow over a vertical thin cylindrical configuration in the presence of significant surface heat and mass flux is studied theoretically and numerically. The governing boundary-layer equations for momentum, energy and species conservation are transformed from a set of partial differential equations in a (x,r) coordinate system to a system using a group of similarity transformations. The resulting equations are solved using the Network Simulation Method (NSM) for the buoyancy-assisted pure free convection and also the pure forced convection cases, wherein the effects of Schmidt number, Prandtl number and surface mass parameter on velocity, temperature and concentration distributions in the regime are presented graphically and discussed. For the buoyancy-assisted pure free convection case, non-dimensional velocity is found to increase with a rise in surface mass transfer (S) but decrease with increasing Prandtl number (Pr), particularly in the vicinity of the cylinder surface (small radial coordinate,). Dimensionless temperature decreases however with increasing S values from the cylinder surface into the free stream; with increasing Prandtl number, temperature is strongly reduced, with the most significant decrease at the cylinder surface. Dimensionless concentration is decreased continuously throughout the boundary layer regime with an increase in S; conversely is enhanced for all radial coordinate values with an increase in Prandtl number. For the pure forced convection case, velocity increases both with dimensionless axial coordinate and dimensionless radial coordinate but decays smoothly with increasing Prandtl number and increasing Schmidt number, from the cylinder surface to the edge of the boundary layer domain. The model finds applications in industrial metallurgical processes, thermal energy systems, polymer processing, etc.

Wall shear rate and its axial and azimuthal components were evaluated in stable Taylor vortices. The measurements were carried out in a broad interval of Taylor numbers (52-725) and several gap width (R1/R2 = 0.5 – 0.8) by two three-segment electrodiffusion probes and three single probes flush mounted in the wall of the outer fixed cylinder. The axial distribution of wall shear rate components was obtained by sweeping the vortices along the probes using a slow axial flow. The experimental results were verified by CFD simulations. The knowledge of local wall shear rates and its fluctuations is of primordial interest for industrial applications like tangential filtration, membrane reactors and bioreactors containing shear sensitive cells.

In this paper we studied a beta type Stirling machine. At first, we present the adopted theoretical quasi-stationary model. Then, we pass to the physical and geometrical presentation of this machine. The Latter was experimented according to two configurations: motor configuration and receiver configuration. For the first configuration, in order to improve the performances of the machine, we proceeded to the insulation of the machine hot room to reduce losses by radiation. For the second configuration, the machine is experimented as a heat pump and refrigerator. Comparisons between the theoretical and experimental results are also presented. We finally validated the results obtained by the model with experiments.

The objective of this study is to optimize experimental conditions of agitating a non-Newtonian liquid using experimental design methodology. The measurements of the temperatures have been carried out in a jacketed vessel equipped with Turbine impellers. The rheological properties of aqueous solutions of carboxymethylcellulose sodium salt had been studied using shear stress/shear rate data. The results of the experimental studies, concerning the effect of the diameter of the impeller, the impeller speed and baffled or unbaffled vessel on the overall heat transfer coefficient have been approximated in the form of equations. Based on the optimization criterion, an agitated vessel equipped with Flat Blade Disc Turbine (FBDT) of diameter ratio d/D = 0.6 and baffles is proposed as the most advantageous for heat transfer processes.

Experiments were performed at low Reynolds numbers in the range in the wake of a circular cylinder of diameter placed symmetrically between two parallel walls of height. 2D2C particle image velocimetry (PIV) was used to investigate the flow downstream the cylinder. In the unsteady flow regime downstream the cylinder, the detached primary vortices (Pi) interact with walls generating secondary ones (Pi’) and modify the cylinder wake dynamic. The kinematical properties (advection velocity, circulation, rotation kinetic energy, etc.) of the generated secondary vortices are studied and compared with the primary ones in order to show how the walls influence the von Kármán vortex street. The authors propose here a relation between the circulations and kinetic energies of primary and secondary vortices.

The enhanced heat transfer in the oscillatory flow of liquid metals between two thermally insulated infinite parallel plates, when a constant axial temperature gradient superimposed, is investigated. The fluid is set to oscillation by oscillating both the plates axially along with an axial oscillatory body force, having the same frequency as that of the plates. The effective average thermal diffusivity is calculated and the effect of oscillation of the plates and the oscillatory body force on the enhancement of heat transfer are discussed and compared.

A numerical study based on the Large Eddy Simulation (LES) methodology was made of mass transfer in locally forced turbulent separated and reattaching flow over a backward facing step. The local forcing was given to the flow by a sinusoidally blowing /suction of the fluid into a separated shear layer. The Reynolds number was fixed at 33000 and Schmidt number at 1. The forcing frequency was varied in the range 0 ≤ St ≤ 2, where St is the Strouhal number of forcing. The obtained results revealed the existence of an optimum forcing frequency value, St = 0.25, in terms of the reduced reattachment length. At this frequency the mass transfer is significantly enhanced in the recirculation zone. The influence of the frequency and the amplitude of forcing, in the maximum mass transfer positions and the maximum Sherwood number, are analyzed.

In this work we study the interaction of an axysymmetric thermal plume with a thermosiphon flow that surrounds it. The thermal plume is created by a circular disk heated by joule effect at constant temperature. The disk is placed at an open ended vertical cylinder on a quiet constant temperature. The internal wall of the cylinder heats up under the effect of thermal radiation emitted by the hot source. The confinement of the fluid causes, in the bottom part of the cylinder, an aspiration of the fresh air. It is a thermosiphon flow that comes to interact with the plume. By studying the average and fluctuating thermal fields it was found that the flow of the plume is strongly influenced by the presence of nearby walls. It was noted that the vertical transport becomes more intense and the structure of the flow becomes more turbulent. On the other hand, we attend a fast homogenization of the flow in the upper cylinder. To obtain more detailed information of this flow, we develops, during this study, a spectral analysis of the fluctuating thermal fields for the case of a plume evolving in unlimited and in an enclosed environment. The energy spectra study shows an important shift of the energy peaks toward the high frequencies under the effect of the thermosiphon. As destroying structures them on a big scale generated by the plume, the thermosiphon provokes a fast mixture of the fluid thus while leading to vortex of weaker size.

Rectangular S-duct diffusers are widely used in air-intake system of several military aircrafts. A well-designed diffusing duct should efficiently decelerate the incoming flow, over a wide range of incoming conditions, without the occurrence of streamwise separation. A short duct is desired because of space constraint and aircraft weight consideration, however this results in the formation of a secondary flow to the fluid within the boundary layer. The axial development of these secondary flows, in the form of counter rotating vortices at the duct exit is responsible for flow non-uniformity and flow separation at the engine face. Investigation on S-shaped diffusers reveals that the flow at the exit plane of diffusers is not uniform and hence offers an uneven impact loading to the downstream components of diffuser. Experiments are conducted with an S-shaped diffuser of rectangular cross-section at Re = 1.34105 to find out the effects of the corners (i.e. sharp 90º, 45º chamfered etc.) on its exit flow pattern. A ‘fishtail’ shaped submerged vortex generators (VG) are designed and introduced at different locations inside the diffusers in multiple numbers to control the secondary flow, thereby improving the exit flow pattern. It is found that the locations of the VG have a better influence on the flow pattern rather than the number of the VG used. The best combination examined in this study is a 45 chamfered duct with 33 VG fixed at the top and bottom of the duct inflexion plane. The results exhibit a marked improvement in the performance of S-duct diffusers. Coefficient of static pressure recovery (CSP) and coefficient of total pressure loss (CTL) for the best configuration are reported as 48.57% and 3.54% respectively. With the best configuration of VG, the distortion coefficient (DC60) is also reduced from 0.168 (in case of bare duct) to 0.141.

The problem of unsteady mixed convection heat and mass transfer near the stagnation point of a three-dimensional porous body in the presence of magnetic field, chemical reaction and heat source or sink is analyzed. An efficient, iterative, tri-diagonal implicit finite difference method is used to solve the transformed similarity equations in the boundary layer. Three cases were considered, namely, accelerating flow, decelerating flow and the steady-state case. The obtained results are presented in graphical and tabulated forms to illustrate the influence of the different physical parameters such as the magnetic field parameter, transpiration parameter, unsteadiness parameter, ratio of velocity gradients at the edge of the boundary layer parameter, heat generation/absorption parameter and the chemical reaction parameter on the velocity components in the x-and y- directions, temperature and concentration distributions, as well as the skin-friction coefficients and Nusselt and Sherwood numbers

Unsteady hydromagnetic Couette flow of a viscous incompressible electrically conducting fluid in a rotating system in the presence of a uniform transverse magnetic field is studied. The plates of the channel are considered porous and fluid flow within the channel is induced due to the impulsive movement of the upper plate of the channel. General solution of the governing equations is obtained which is valid for every value of time t. For small values of time t, the solution of the governing equations is obtained by Laplace transform technique. The expression for the shear stress at the stationary plate due to the primary and secondary flows is obtained in both the cases. It is found that the solution obtained by Laplace transform technique converges more rapidly than the general solution when time t is very small. Magnetic field retards the fluid flow in both the primary and secondary flow directions. Rotation retards primary flow whereas it accelerates secondary flow. There exists incipient flow reversal near the stationary plate on increasing rotation parameter K2. Suction accelerates primary flow whereas it retards secondary flow. Injection retards both the primary and secondary flows.

The effects of blade discharge angle on the performance of a standard industrial centrifugal oil pump of type 65Y60 were investigated experimentally as the pump handled both water and viscous oil. A one-dimensional hydraulic loss model was established to identify such effects mathematically. The effects have been estimated analytically by using the model at various viscosities. The results showed that the blade discharge angle has significant but equal influence on the head, shaft power and efficiency of the centrifugal oil pump at various viscosity conditions. For any viscosity, the total hydraulic loss in the impeller and volute rises with increasing blade exit angle. The diffusion loss in and behind the impellers as well as the friction loss in the volute are noticed in the pump, especially for highly viscous liquids. The hydraulic loss in the impellers is about 0.8-0.6 times the loss in the volute. In order to improve the pump performance, the hydraulic loss in the volute must be kept as small as possible.

The paper reports on the progress made in predicting large- and small-scale single and two-phase flows with heat transfer using the CMFD code TransAT. In the multi-phase context, the code uses the Level Set approach as the “Interface Tracking Method” of reference. The solver incorporates phase-change capabilities, surface tension and triple-line dynamics models, Marangoni effects, electric and magnetic fields, and a wall micro-film sub-grid scale model for lubrication. Complex 3D examples shown here were treated using a fully automatized version of the code, using the Immersed Surfaces Technique (IST) to map complex components into a simple rectangular Cartesian grid. It is shown that real coupled two-phase heat transfer (conjugate) problems are within reach of modern CMFD code using interface tracking, with relatively fast response times: 3D coupled two-phase flow heat transfer can run on a simple Linux PC cluster within 24 H time.

This work is aimed the evolution of the behaviour of a strongly contained wall fire in an enclosure during the post-flashover period. It has characterised the fire intensity decay up to extinction of a wall fire by lack of oxygen and the effects of a sudden door opening on the formation of an air gravity wave capable to bring the backdraft phenomenon. To better understand these two sequential fire scenarios, the study was divided into two complementary parts performed in the same laboratory scale experimental setup. The first part consists to stabilise a steady wall fire at the rear of the compartment and to follow its evolution when a door closes the aperture leaving only a thin slot opened to limit the air entering. It has been observed the formation of a ghosting flame moving through the compartment before dying at the aperture. By supposing the continuation of the fuel solid pyrolysis after flame extinction due to the radiation of the hot environment, fuel vapours continue to fill the compartment. The second part will study the effects of a reopening of the door. It has been observed and characterised the formation and the propagation of a gravity wave through the enclosure. This is representative of the development of the first stage of the backdraft. Tests are performed to measure the aerodynamic properties of the flow behaviour.

The paper deals with heat transfer by convection between impinging gas jets and solid surfaces. It considers both single and multiple jet systems. It emphasizes the flow and geometrical parameters as well as the environment conditions at which the jet emerges. In particular, it points out the effect of the jet tilting, thermal entrainment and jet confinement. ASN and ARN schemes are illustrated through industrial and aeronautical applications. Design correlations are proposed. Experimental data obtained from infrared thermography are compared to CFD simulations.