Lower Stagnation Point Research Articles

Numerical Investigation of Ferrofluid Flow at Lower Stagnation Point over a Solid Sphere using Keller-Box Method

In this paper, ferrofluid flow at lower stagnation point on solid sphere is investigated theoretically by considering mixed convection boundary layer flow. The sphere surface is exposed to the magnetic field and thermal radiation by taking into account constant wall temperature boundary conditions. The discovery of the existing magnetic field near the surface while the ferrofluid flowing leads to the development of phenomenology called magnetohydrodynamic. The magnetite (Fe3O4) acts as nanoparticles dispersant and suspended in the water contained in ferrofluid are assumed as Newtonian fluid and behave as single-phase fluid flow is studied. These assumptions give physical insight into the behaviour of ferrofluid flow to be analysed and discussed. The Keller-box method is applied to solve the transformed partial differential equations numerically. The numerical results found the viscosity measured from magnetite (Fe3O4) volume fraction is the main element provided to the trend of the ferrofluid velocity flow. Besides, the ferrofluid temperature at lower stagnation point on sphere is proven influence the ferrofluid viscosity and change the velocity of ferrofluid flow.

Numerical Investigation at Lower Stagnation Point Flow Over a Horizontal Circular Cylinder of Brinkman-Viscoelastic Fluid

Investigations on the characteristics of fluid flow in manufacturing processes are essential since it will determine the quality of the end products. The flow might be involved whether the Newtonian (viscous) or non-Newtonian fluid moving over the different body depending on the process activities. Since the experimental works sometimes costly and hazardous, the study via mathematical approach is necessary to counter the limitations. Hence, this paper aims to investigate the flow at lower stagnation point over a horizontal circular cylinder on Brinkman Viscoelastic fluid embedded in porous medium. Mathematical model is constructed in terms of partial differential equations with some physical conditions to represent the condition of the problem. An appropriate non-dimensional variable is introduced to transform the model into the solvable system which is in less complexity, and then the system is solved using the Runge-Kutta-Fehlberg method. The numerical solutions for the temperature and velocity profiles as well as skin friction and heat transfer coefficient are computed and presented in graphical and tabular form. The feature of the flow and heat transfer characteristics for various values of mixed convection, Brinkman and viscoelastic parameter are analysed and discussed. This study has found that the incremented Brinkman and viscoelastic parameter have declined the fluid velocity while opposite trend is observed for temperature distribution. The theoretical results produced are relevance to researchers and engineers. It can be used for comparative purposes in data validation or experimentation study.

Magnetohydrodynamic Effects in Mixed Convection of Ferrofluid Flow at Lower Stagnation Point on Horizontal Circular Cylinder

In this paper, mixed convection of ferrofluid containing magnetite, Fe3O4 with ferroparticles suspended in water at the lower stagnation point on a horizontal circular cylinder is investigated. The partial differential equation which derived from the transformation of the dimensional governing equation and non-similarity transformation with the consideration of the effect of magnetohydrodynamic (MHD) are solved numerically by using Keller-box method. The influences of an external magnetic field on ferrofluid flow and heat transfer are then discussed. The results showed that the viscosity depends on the ferroparticle volume fraction and ferrofluid temperature. The uniform magnetic field that produced Lorentz force acts as a determiner of the trend of fluid movement and has the tendency to control the cooling rate of the surface.

Mixed convection over a horizontal circular cylinder embedded in porous medium immersed in a nanofluid with convective boundary conditions at lower stagnation point: A numerical solution

This study aims to examine the effect of governing parameters on the flow and heat transfer of the steady mixed convection flow embedded in porous medium with convective boundary conditions. The resulting system of nonlinear partial differential equations is solved numerically. The special case at the lower stagnation point of the cylinder is observed and the case where bottom surface of the cylinder is heated by convection from hot fluids is considered. Numerical solutions are obtained for the velocity, temperature and nanoparticle volume fraction profiles for two values of governing parameters namely convective parameter γ and Lewis number Le. It is found that as the convective parameter γ increases, velocity profile, temperature and nanoparticle volume fraction profile also increases.

Transient effects of fuel–air mixing in a partially-premixed turbulent swirl flame

Unsteady interactions of flow, fuel–air mixing and reaction in a lean partially-premixed turbulent swirl flame are investigated using simultaneous particle image velocimetry and planar laser-induced fluorescence of OH and acetone with a repetition rate of 10kHz. The flame is operated with methane and air in a gas turbine model combustor at ambient temperature and pressure. Transport and mixing of fuel is visualized by fluorescence of acetone, which is added with 9% by volume to the methane. The dominant unsteady flow structures are a precessing vortex core (PVC) in the shear layer of the inner recirculation zone and the lower stagnation point (LSP) at the flame root. The measurements show that fuel and air are largely separated at the inlet of the combustion chamber, but are then strongly mixed by the PVC. A well-mixed zone of unburned fuel and air is formed around the vortex, and then ignited by recirculating burned gas. At the flame root, the PVC induces periodic changes in the composition of the unburned gas, which varies between pure air and well-mixed fuel and air. Reaction is locally quenched at the LSP when fresh air without fuel is present, and re-ignition takes place when mixed fuel and air arrive at the boundary of recirculating burned gas. Generally, the results show that the enhancement of fuel–air mixing induced by the PVC contributes significantly to the stabilization of the flame, and that the flame dynamics can only be properly understood when an analysis of transient mixing mechanisms is included.

Non-Newtonian Mixed Convection Flow along an Isothermal Horizontal Circular Cylinder

The mixed convective laminar two-dimensional boundary-layer flow of non-Newtonian pseudo-plastic fluids is studied along an isothermal horizontal circular cylinder using a modified power-law viscosity model. In this model, there are no unrealistic limits of zero or infinite viscosity; consequently, no irremovable singularities are introduced into boundary-layer formulations for such fluids. Therefore, the boundary-layer equations can be solved numerically by using marching order implicit finite difference method with double sweep technique. Numerical results are presented for the case of shear-thinning fluids in terms of the fluid velocity and temperature distributions, shear stresses and rate of heat transfer in terms of the local skin-friction and local Nusselt number respectively. Here, it is found that heating the cylinder delays separation and if the cylinder is warm enough, suppress it completely. Cooling the cylinder brings the separation point nearer to the lower stagnation point and for a very cold cylinder there will not be boundary-layer on the cylinder.

Mixed convection boundary-layer flow near the lower stagnation point of a horizontal circular cylinder with a second-order wall velocity condition and a constant surface heat flux

Mixed convection boundary-layer flow near the lower stagnation point of a horizontal circular cylinder with a second-order wall velocity condition and a constant surface heat flux

Dynamics of lean blowout of a swirl-stabilized flame in a gas turbine model combustor

Lean blowout (LBO) of a partially premixed swirl flame is studied using chemiluminescence imaging and simultaneous stereo-PIV and OH-PLIF measurements at repetition rates up to 5 kHz. The flame, which is operated with methane and air in a gas turbine model combustor at atmospheric pressure, features a pronounced precessing vortex core (PVC) at the inner shear layer. In the first part of the study, the stabilization mechanism of the flame close to LBO is investigated. The fields of velocity and OH show that near LBO there are essentially two regions where reaction takes place, namely the helical zone along the PVC and the flame root around the lower stagnation point. The zone along the PVC is favorable to the flame due to low strain rates in the vortex center and accelerated mixing of burned and fresh gas. The flame root, which is located close to the nozzle exit, is characterized by an opposed flow of hot burned gas and relatively fuel-rich fresh gas. Due to the presence of high strain rates, the flame root is inherently unstable near LBO, featuring frequent extinction and reignition. The blowout process, discussed in the second part of the study, starts when the extinction of the flame root persists over a critical length of time. Subsequently, the reaction in the helical zone can no longer be sustained and the flame finally blows out. The results highlight the crucial role of the flame root, and suggest that well-aimed modifications of flow field or mixture fraction in this region might shift the LBO limit to leaner conditions.

Dye visualization near a three-dimensional stagnation point: application to the vortex breakdown bubble

An analytical model, based on the Fokker–Planck equation, is constructed of the dye visualization expected near a three-dimensional stagnation point in a swirling fluid flow. The model is found to predict dye traces that oscillate in density and position in the meridional plane in which swirling flows are typically visualized. Predictions based on the model are made for the steady vortex breakdown bubble in a torsionally driven cylinder and compared with computational fluid dynamics predictions and experimental observations. Previous experimental observations using tracer visualization techniques have suggested that even for low-Reynolds-number flows, the steady vortex breakdown bubble in a torsionally driven cylinder is not axisymmetric and has an inflow/outflow asymmetry at its tail. Recent numerical and theoretical studies show that the asymmetry of the vortex breakdown bubble, and consequently its open nature, can be explained by the very small imperfections that are present in any experimental rig. Distinct from this, here it is shown that even for a perfectly axisymmetric flow and breakdown bubble, the combined effect of dye diffusion and the inevitable small errors in the dye injection position lead to the false perception of an open bubble structure with folds near the lower stagnation point. Furthermore, the asymmetries in the predicted flow structures can be remarkably similar to those observed in flow observations and computational predictions with geometric asymmetries of the rig. Thus, when interpreting dye-visualization patterns in steady flow, even if axisymmetric flow can be achieved, it is important to take into account the relative diffusivity of the dye and the accuracy of its injection.

Experiments on fluid flow induced by melting around a migrating heat source

A series of melting experiments with a moving horizontal cylindrical heat source at constant surface heat flux have been performed. The heat source was designed in such a way that it could descend under its weight while melting the phase-change material (n-octadecane) surrounding it. The heat-source velocity was measured and the motion and shape of the solid–liquid interface were determined photographically. The effects of the surface heat flux, the density and initial position of the heat source, and the initial subcooling of the solid were investigated and are discussed. Conduction was found to be the dominant heat-transfer mechanism around the lower stagnation point and controlled the terminal velocity of the source. The fluid motion in the melt pool above the heat source was mainly induced by the descent of the source, while natural convection played only a relatively minor role in the motion.

Free convection about the lower stagnation point of a horizontal cylinder embedded in a porous medium with internal sources-sinks

Free convection about the lower stagnation point of a horizontal cylinder embedded in a porous medium with internal sources-sinks

Transient free convection from an isothermal horizontal circular cylinder

A numerical solution of the unsteady free convection on an isothermal horizontal circular cylinder whose temperature is suddenly increased is presented at large Grashof number. Solutions for simultaneous development of the momentum and thermal boundary-layers are obtained by using the local non-similarity solutions method, together with the difference-differential procedure. The transient approach of velocity and thermal fields to the ultimate steady state and the variations in time of wall shear and heat transfer coefficients were found. The results at the lower stagnation point of the cylinder for Prandtl number Pr=1.0 are seen to improve the previous study of Hermann [1].

Oscillation Effects upon Film Boiling from a Sphere

Heat transfer rates from a silver-plated copper sphere, 0.75 in. in diameter, were studied by high speed photography during oscillations of the sphere in saturated liquid nitrogen and Freon-11. The oscillation frequencies ranged from zero to 13 Hz, and the amplitude-to-diameter ratio varied from zero to 2.67. The sphere was supported by a thin-walled stainless steel tube and carried a thermocouple attached near the lower stagnation point. A Fastax WF-3 16mm movie camera was used at about 2000 frames/sec. The differences in the vapor removal process at lower and higher oscillation frequencies are discussed.

Laminar free convection near the lower stagnation point on an isothermal curved surface

This paper deals with some aspects of the three dimensional laminar free convection boundary layer near the stagnation point on a general curved isothermal surface, which is maintained at a temperature above the ambient temperature of the fluid. Thus the stagnation point is defined as the lowest (elliptic) point on the surface and such that the tangent plane at this point is horizontal. Boundary-layer equations are formulated and it is shown that the flow at the stagnation point depends on the ratio of the two principal radii of curvature at this point, the Prandtl number and the Grashof number. These equations are solved numerically for Prandtl number 0.72 and for various values of the ratio of the two principal radii of curvature. For the stagnation point flow there are two limiting cases, namely the flow at the lower stagnation line on a uniform horizontal cylinder and that at the lower stagnation point on a sphere. The numerical solutions for the sphere and cylinder are then used to develop an approximate method of solution for the stagnation point on a general curved surface; good agreement with the precise numerical solutions was obtained.

Ablation Behavior of Materials Subjected To Missile Re-Entry Heat Flux Rates

Abstract Hypersonic vehicles, during re-entry, are exposed to transient heating involving extremely high heat flux rates. These heat flux rates have been simulated in the laboratory with the plasma discharge from high-intensity, air-stabilized electric arcs of approximately 1000-kw power rating. This paper describes the behavior of samples of ten different materials tested in this facility, including four metals, three ceramics, two plastics, and graphite. Ablation rates, heat balances, and effective heats of ablation are presented. The influence of changes in the test environment upon material behavior is indicated. In the highest temperature test environment utilized, the plastic and graphite samples provided lower stagnation point weight loss rates during ablation than the metal and ceramic samples. 3.5.9