1,720,984 research outputs found
Flow patterns and heat transfer around six in-line circular cylinders at low Reynolds number
No full textThe flow field and the heat transfer around six in-line iso-thermal circular cylinders have been studied by means of numerical simulations. Two values of the center to center spacing "" and 4d, where dis the cylinder diameter) at Reynolds number of 100 and Prandtl number of 0.7 have been investigated. Similarly to the in-line two cylinders configuration, in this range, a transition in the flow and in the heat transfer occurs. Two different flow patterns have been identified: the stable shear layer (SSL) mode and the shear layer secondary vortices (SLSV) mode, at 3.6 and 4 spacing ratios respectively. At "" the flow pattern causes the entrainment of cold fluid on the downstream cylinders enhancing the heat transfer. On the other hand, at "" two stable opposite shear layers prevent the cold fluid entrainment over the downstream cylinders reducing their heat exchange. The overall time average heat transfer of the array is enhanced up to 25% decreasing the spacing ratio from 4 to 3.6. Furthermore, it is found that the increased heat transfer is related to the phase shift between the Nusselt time series of successive cylinders
Buoyancy effect on the flow pattern and the thermal performance of an array of circular cylinders
No full textIn this paper, we found, by means of numerical simulations, a transition in the oscillatory character of the flow field for a particular combination of buoyancy and spacing in an array of six circular cylinders at a Reynolds number of 100 and Prandtl number of 0.7. The cylinders are isothermal and they are aligned with the earth acceleration (g). According to the array orientation, an aiding or an opposing buoyancy is considered. The effect of natural convection with respect to the forced convection is modulated with the Richardson number, Ri, ranging between-1 and 1. Two values of center-to-center spacing (s=3.6d-4d) are considered. The effects of buoyancy and spacing on the flow pattern in the near and far field are described. Several transitions in the flow patterns are found, and a parametric analysis of the dependence of the force coefficients and Nusselt number with respect to the Richardson number is reported. For Ri=-1, the change of spacing ratio from 3.6 to 4 induces a transition in the standard deviation of the force coefficients and heat flux. In fact, the transition occurs due to rearrangement of the nearfield flow in a more ordered wake pattern. Therefore, attention is focused on the influence of geometrical and buoyancy parameters on the heat and momentum exchange and their fluctuations. The available heat exchange models for cylinders array provide a not accurate prediction of the Nusselt number in the cases here studie
Simulazione diretta del flusso in un distributore oleodinamico mediante un metodo ai contorni immersi
No full textViene presentata l’analisi del flusso all’interno di una valvola distributrice 4/3 a centro chiuso mediante un codice, basato sul metodo dei contorni
immersi, che risolve le equazioni di Navier-Stokes mediante una DNS.
I risultati ottenuti sono presentati sia in termini di campi istantanei e medi, evidenziando la manifestazione dell’effetto Coanda alle piccole aperture,
sia in termini di parametri globali, quali il coefficiente di efflusso e di forza assiale.In this paper a directional valve (4\/3, closed center) is analyzed using a code based on the immersed boundary method and solving the Navier-. Stokes equations by a Direct Numerical Simulation (DNS).. The results are presented in terms of instantaneous and time-averaged fields, showing the Coanda effect for small valve openings, and global parameters,. such as the discharge coefficient and the flow force coefficient
Analysis of a directional hydraulic valve by a direct numerical simulation using an immersed-boundary method
No full textThe improvement of the hydraulic valves depends on the careful analysis
of the coherent structures driving the motion of the working fluid. In the
past those devices have been studied by experimental tests; during the last
15 years also several numerical works have been presented, solving the flow
on body-fitted computational grids by RANS methods.
In this study a different approach is proposed for the axisymmetric analysis
of a directional valve (4/3, closed centre): whereas the RANS techniques are
based on the time-averaged equations of the flow, in the present work the unsteady Navier-Stokes equations have been solved using the Direct Numerical
Simulation (DNS); the time evolution of the physics is simulated, providing
important details on the instantaneous structures of the flow, affecting the valve performance. Furthermore, while in the previous numerical studies the
computational domain has been discretized by conformal grids, in this case
the fluid-body interaction has been represented by an immersed-boundary
(IB) method on a Cartesian grid, more suitable for unsteady eddy-resolving
simulations, as DNS.
The analysis of the discharge coefficient and the flow forces for different openings s and pressure drops ∆p is presented in this paper. The behaviour of
those global parameters is justified also considering the time-averaged and
the instantaneous fields. For small openings and pressure drops the flow is
steady and attached to the wall of the discharge chamber on the side of the
restricted section. When s and ∆p are increased the jet separates at the restricted section and it re-attaches downstream (Coanda effect), keeping the
steady state. Finally, for large openings and pressure drops the flow becomes
strongly unsteady: it is organized like a free jet and is dominated by large
vortices
Wake interference effects on the heat transfer enhancement around a row of circular cylinders
No full textThe numerical simulations of the heat transfer around an array of isothermal circular cylinders immersed in a
stream has been carried out solving the two-dimensional Navier-Stokes equations. The cylinders have been
placed in a single row configuration aligned with the free stream velocity at Reynolds number 100 and Prandtl
number 0.7.
In Fig.1 it is shown the instantaneous temperature distribution for the case of six in-line circular cylinders at
spacing ratio (s/d) equal to 4 and 3.6, where s is the center-to-center cylinder spacing and d is the cylinder
diameter.
In the latter case, a transition in the flow patterns occurs with the flow organized in a vortex shedding
responsible for the entrainment of cold fluid in the gaps. This phenomenon makes stronger the thermal gradient
close to the cylinders leading the heat transfer enhancement with the Nusselt number 25 % higher respect to the
case at s/d=4.
Furthermore a frequency analysis of the time dependent Nusselt number, Nui, at the i-th cylinder, shows that
the main frequency is the same for all the cylinders.
We found evidences that the signature of the heat transfer enhancement could be related to the phase shift
between two successive cylinders (i+1- i), where the phase shift (i) is defined as the difference between the
phase of each main harmonic component of the Nui respect to the phase of the signal at the first cylinder
Simulazione diretta del Flusso in un Distributore Oleodinamico mediante un Metodo ai Contorni Immersi
No full textFlow transitions around in line cylinders
No full textThe flow around aligned cylinders is an archetype for several
industrial devices (rod structure of the nuclear reactors, compact heat
exchangers for electronic components, pin-fins heat exchangers for
micro-devices ) and environmental phenomena (diffusion process close to the
vegetation). Cylinders produce instabilities in the flow structures that are very
sensitive to the control parameters such as the inflow velocity, the spacing
between the cylinders and the fluid viscosity. The instabilities leads the
transport phenomena close to the cylinders and they affect the force, the
thermal balance at their surface and the diffusion process. The strong velocity
gradients in confined spaces make that the experimental analysis is difficult,
while the numerical simulation appears to be a promising tool for this purpose
Multiphase Rayleigh-Bénard convection
No full textNumerical simulations of two-phase Rayleigh-Bénard convection in a cylindrical cell with particles or vapor bubbles suspended in the fluid are described. The particles or bubbles are modeled as points, the Rayleigh number is 2×106 and the fluids considered are air, for the particle case, and saturated water for bubbles. It is shown that the presence of a second phase has a profound effect on the flow and heat transfer in the cell. The heat capacity of the particles and the latent heat of the liquid are used, in dimensionless form, as control parameters to modulate these effects. It is shown that, as these parameters are varied, the nature of the flow in the cell changes substantially, in some cases with adverse and in others beneficial effects on the Nusselt number. By the analysis of several aspects of the numerical results, a physical discussion of several mechanisms is provided
Influence of the Spool Velocity on The Performance of a Directional Hydraulic Valve
No full textIn this paper an accurate numerical method has been used to verify the influence of the spool velocity on the performance
of a directional hydraulic valve (4/3, closed center): the flow during the opening phase of the valve has been
solved by Direct Numerical Simulation (DNS), using an Immersed-Boundary (IB) technique.
The present results have been compared with the ones of a previous study, based on the same numerical method, but
with a stationary spool. The numerical comparisons prove that the "quasi-stationary" hypothesis is approximately correct
for present commercial devices, but it is not suitable for future high-speed valves. However it is shown that, even
inside the range of the spool velocities currently adopted, for small pressure drops Δp and small openings s more significant
differences arise on the axial forces
Undulatory theory of phonons on the nanofluid thermal conduction
No full textThe present paper proposes a theoretical model for the prediction of the specific heat capacity and conductivity of nanofluids. The main control parameters of the model are sound velocity and thermal expansion coefficient of liquid and solid phases. The model also includes the thickness of the liquid wrapping the particles, in which it is relevant the thermal diffusion. Theoretical predictions were tested against the experimental data about the heat transfer in a nanofluid made by aluminum oxide nanoparticles, stably suspended in tap water. Thermal conductivity and heat capacity linearly change as function of the nanoparticle concentration. The molecular boundary layer develops towards the nanoparticle with the thermal expansion coefficient that changes linearly from zero at the particle surface up to the bulk value. The specific heat capacity does not have relevant effects due to the temperature variations. The temperature increasing induces a slight enhancement of the thermal conductivity because of the base fluid contribution
- …
