Fluid Dynamics

Willow wood presents a real interest for biomass pyrolysis due to its fast growth, renewability, and high energy density. Following the pyrolysis of willow wood in an inert atmosphere, a multi-fuel gaseous mixture was obtained, with the following composition (by volume): 38.20% CO/21.87% H2/17.44% CH4/1.15% O2/17.15% CO2/4.19% N2. The propagation of laminar premixed flames in these multifuel mixtures with air was investigated numerically for initial temperatures from 298 to 500 K, initial pressures from 1 to 20 bar, and fuel equivalence ratios between 0.60 and 2.00. The combustion of these gaseous mixtures as free laminar premixed flames was simulated by means of the INSFLA package and an extended reaction mechanism with 592 elementary reactions and 53 species. The modelling of the gas-phase combustion delivered several important parameters: the laminar burning velocities, Su, the maximum flame temperatures, Tfl,max, the flame front thicknesses, dfl, and the peak concentrations of t...

The paper reports results of the detailed chemical modelling of isobaric deflagrations in stoichiometric H2-N2O-N2 mixtures (N2 = 20 - 60 vol%) at various initial temperatures (300 - 500 K) and various initial pressures (1 - 10 bar): laminar burning velocities, and profiles of temperature, volumetric heat release rate and species concentrations across the flame front. The dependence of the laminar burning velocity on temperature and pressure is examined by empirical power laws, for restricted ranges of pressure (or temperature) variation. For each examined composition, the laminar burning velocity correlates well with the volumetric rate of heat release and with the sum of mass fractions of reactive species in the flame front.

In this study, experimental measurements in a spherical combustion bomb were performed in order to investigate the flame propagation in N2-diluted CH4-N2O mixtures with stoichiometric equivalence ratio, at several initial pressures (0.5–1.75 bar) and ambient initial temperatures. Methane was chosen as a test-fuel, since it is the main component of natural gas, a fuel often used as a substitute to gasoline in engines with internal combustion and industrial plants. The method approached in this study is based on a simple examination of the cubic law of pressure rise during the early (incipient) period of flame propagation. The incipient stage defined by a pressure rise equal or smaller than the initial pressure, was divided into short time intervals. The burnt mass fractions (obtained using three different Equations) and flame radii at various moments of the flame propagation in the course of the incipient stage were calculated. The cubic law coefficients and corresponding laminar bur...

In this study, the effect of oxygen in the shielding gas on the material flow behavior of the weld pool surface was discussed to clarify the dominant driving weld pool force in keyhole plasma arc welding (KPAW). To address this issue, the convection flow on the top surface of weld pool was observed using a high-speed video camera. The temperature distribution on the surface along keyhole wall was measured using the two-color pyrometry method to confirm the Marangoni force activity on the weld pool. The results show that the inclination angle of the keyhole wall (keyhole shape) increased especially near the top surface due to the decrease in the surface tension of weld pool through surface oxidation when a shielding gas of Ar + 0.5% O2 was used. Due to the change in the keyhole shape, the upward and backward shear force compositions created a large inclination angle at the top surface of the keyhole. From the temperature measurement results, the Marangoni force was found to alter the...

In this paper we have study the laminar flow of fluid in magnetic, viscous incompressible fluid flow between two parallel plates of a channel in which on is porous bounding wall and second is rigid bounding wall in the presence of magnetic field when the fluid is being withdrawn through both walls of a channel at same rate. A solution for the case of low Reynolds number Re (suction Reynolds number) and Magnetic field M is discussed. Expressions for the velocity components are obtained. The governing non-linear differential equations are solved using perturbation method analytically. Using matlab for the calculating the data of the graph. The graph of axial and radial velocity profiles have been drawn in different magnetic field and different height of the channel.

We present various numerical studies conducted with a novel pore-scale simulation technology called Direct Hydrodynamic (DHD) Simulation that can be used to study multiphase flow at various scales ranging from individual pore-scale events to complex scenarios like capillary de-saturation and relative permeability of digitized rock samples. DHD uses a diffuse interface description for fluid-fluid interfaces that is implemented via the density functional approach applied to the hydrodynamics of complex systems. In addition to mass and momentum balance, a full thermodynamic energy balance is considered. Hence the simulator inherently takes into consideration multi-phase and multi-component behavior and is suited for non-isothermal cases which allows the handling of many physical phenomena including multiphase compositional flows with phase transitions, different types of fluid-rock and fluid-fluid interactions (e.g. wettability and adsorption), and various types of fluid rheology. The ...

The study aims to determine the effect of nozzle groove on fluid flow through viscous 2D planar fluid. To fulfil the study’s aim, numerical method was adopted to introduce grooves of different dimensions from the nozzle exit. The study adopts SoldWork software was used to design nozzles and introduce groove shaped nozzles, each consisting of six different designs. The nozzle base model used in this study was similar to the one used in a previous study. The procedure was performed with different pressures (8, 10, and 12 bar) at the similar firefighting nozzle. The velocities contours were predicted based on the choice of nozzle section during the numerical stimulation. The results of present study demonstrated a new approach that can be used for increasing velocity at various types of modified nozzles through grooves at different pressures and locations. For grooves, dimensions 1×1 (mm) and location 15 mm at 8 bar, 10 bar and 12 bars showed no effect on velocity as it reduces velocit...

The purpose of this paper was to study the propagation of longitudinal waves in non-homogeneous four-parameter viscoelastic rods of arbitrary thickness. The rods were initially supposed to be unstressed and at rest. Apart from a sudden rising traction uniformly applied over the boundary of the opening and parallel to the faces of the plates, which is steadily maintained thereafter, the rods are otherwise free from loading. Methods for treating reflection at the free end of the finite rod and reflection and transmission at an interface between two media in the semi-infinite bi-viscoelastic rod are also presented. Asymptotic techniques are used throughout, and formal asymptotic wave-front expansions of the solution functions are obtained.

We present a realization of a fast interfacial Marangoni microswimmer by a half-spherical alginate capsule at the air–water interface, which diffusively releases water-soluble spreading molecules (weak surfactants such as polyethylene glycol (PEG)), which act as “fuel” by modulating the air–water interfacial tension. For a number of different fuels, we can observe symmetry breaking and spontaneous propulsion although the alginate particle and emission are isotropic. The propulsion mechanism is similar to soap or camphor boats, which are, however, typically asymmetric in shape or emission to select a swimming direction. We develop a theory of Marangoni boat propulsion starting from low Reynolds numbers by analyzing the coupled problems of surfactant diffusion and advection and fluid flow, which includes surfactant-induced fluid Marangoni flow, and surfactant adsorption at the air–water interface; we also include a possible evaporation of surfactant. The swimming velocity is determine...

We are interested in studying an unsteady fluid-structure interaction problem in a three-dimensional space. We consider a homogeneous Newtonian fluid which is modeled by the Navier-Stokes equations. Whereas the motion of the structure is described by the quasi-incompressible non-linear Saint Venant-Kirchhoff model. We establish the local in time existence and uniqueness of solution for this model. For this sake, first we rewrite the non-linearity of the elastodynamic equation in an explicit way. Then, a linearized problem is introduced in the Lagrangian reference configuration and we prove that it admits a unique solution. Based on the a priori estimates on the solution of this problem together with the fixed point theorem we prove that the non-linear problem admits a unique local in time solution. At last, by the inf-sup condition we reach to the existence of the fluid pressure.

Based upon high resolution LDA measurements over a range of momentum deficit thickness Reynolds numbers (Rθ=U∞θ/ν) from 1430 to 31 000, DeGraaff and Eaton [J. Fluid Mech. 422, 319 (2000)] propose a new mixed scaling for the near-wall region profile of the axial turbulent stress, u2¯. The present results support the validity of this scaling over an extended Reynolds number range 1000⩽Rθ⩽5×106.

Nano-particle image velocimetry (nPIV) uses evanescent-wave illumination to measure two tangential velocity components $U$ and $V$ averaged in a region next to the wall with submicron thickness. The illumination intensity decays exponentially with $z$, or normal to the wall, ...

Brownian fluctuations of colloidal tracers used in microscale velocimetry are isotropic in the bulk. In the near-wall region, however, such fluctuations are hindered, and the tracer has a greater probability of diffusing away from (\textit{vs.} towards) the wall. These anisotropic ...