Alvaro Valencia

Heat transfer in square cavities with partially active vertical walls

Natural convection of air in square cavities with half-active and half-insulated vertical walls is numerically investigated for Rayleigh numbers of 103–107. This problem is related to applications in solar collection and cooling of electronic components. Five different relative positions of the active zones are considered. While circulation depends strongly on the total exit length downstream of the active zones, heat transfer depends less on this parameter. Significant conduction effects occur even at a Rayleigh number of 105. Expressions for average Nusselt number in the five situations and hints on how to generalize heat transfer results to intermediate cases are given.

Blood Flow Dynamics in Saccular Aneurysm Models of the Basilar Artery

Blood flow dynamics under physiologically realistic pulsatile conditions plays an important role in the growth, rupture, and surgical treatment of intracranial aneurysms. The temporal and spatial variations of wall pressure and wall shear stress in the aneurysm are hypothesized to be correlated with its continuous expansion and eventual rupture. In addition, the assessment of the velocity field in the aneurysm dome and neck is important for the correct placement of endovascular coils. This paper describes the flow dynamics in two representative models of a terminal aneurysm of the basilar artery under Newtonian and non-Newtonian fluid assumptions, and compares their hemodynamics with that of a healthy basilar artery. Virtual aneurysm models are investigated numerically, with geometric features defined by beta = 0 deg and beta = 23.2 deg, where beta is the tilt angle of the aneurysm dome with respect to the basilar artery. The intra-aneurysmal pulsatile flow shows complex ring vortex structures for beta = 0 deg and single recirculation regions for beta = 23.2 deg during both systole and diastole. The pressure and shear stress on the aneurysm wall exhibit large temporal and spatial variations for both models. When compared to a non-Newtonian fluid, the symmetric aneurysm model (beta = 0 deg) exhibits a more unstable Newtonian flow dynamics, although with a lower peak wall shear stress than the asymmetric model (beta = 23.2 deg). The non-Newtonian fluid assumption yields more stable flows than a Newtonian fluid, for the same inlet flow rate. Both fluid modeling assumptions, however, lead to asymmetric oscillatory flows inside the aneurysm dome.

Turbulent unsteady flow and heat transfer in channels with periodically mounted square bars

Abstract A numerical investigation was conducted to analyze the unsteady turbulent flowfield and heat transfer characteristics in a channel with streamwise periodically mounted square bars arranged side-by-side to the approaching flow. The transverse separation distance between the bars is varied, whereas the bar height to channel height ( d / H ) are 0.152 and 0.2, the Reynolds number Re based on channel height is 2×10 4 and the periodicity length is 2H. The channel walls are subjected to a constant wall temperature. The k – e turbulence model was used in conjunction with the Reynolds-averaged momentum and energy equations for the simulations. A finite volume technique is applied with a fine grid and time resolution. Complex periodic vortex shedding develops in the channel due the interaction between the two streamwise periodically mounted square bars. Results show that the unsteady flow behavior, pressure drop and heat transfer are strongly dependent of the transverse separation distance of the bars.

Numerical simulation of gas bubbles formation at a submerged orifice in a liquid

We numerically study the growth, rise, and interaction with the upper air-water interface of bubbles generated forcing air through a submerged orifice in a cylindrical vessel with polymeric surface containing quiescent water. The simulations were carried out using the volume of fluid (VOF) technique implemented in the commercial solver Fluent. We study the influence of numerical parameters as grid size, and physical parameters as orifice diameter and gas inlet velocity on bubble size and velocity. The first bubble is the slowest and the second is the fastest

Heat transfer enhancement due to self-sustained oscillating transverse vortices in channels with periodically mounted rectangular bars

Abstract Numerical investigations of flow structure and heat transfer in a channel with periodically mounted transverse vortex generators (bars) have been conducted in the Reynolds number range of steady laminar to oscillatory transitional flow. The unsteady Navier–Stokes equations and the energy equation have been solved by a finite volume code. Due to the periodic geometry, instability leads at relatively low Reynolds number to self-sustained oscillations and vortex shedding from the bars. At Reynolds number Re = 150, the flow structure is already time periodic. With increasing Reynolds number their amplitudes and number of frequencies increase. The data for heat transfer and flow losses are presented for different periodicity lengths in a Reynolds number range of 100–400.

TURBULENT FLOW AND HEAT TRANSFER IN A CHANNEL WITH A SQUARE BAR DETACHED FROM THE WALL

Computations are conducted to study the heat transfer and friction in a channel with a mounted square bar of different sizes detached from the channel wall. The Reynolds number (Re) based on channel height ranges from 10 4 to 105, whereas the bar height to channel height (d H) varies from 0.15 to 0.35. The channel walls are subjected to a constant wall temperature. The standard k- epsilon turbulence model and a modified version were used in conjunction with the Reynolds-averaged momentum and energy equations for the simulations and were compared thereafter. A finite volume technique with staggered grids combined with the SIMPLEC algorithm is applied. Results show that the local and global Nusselt numbers on the channel walls are strongly modified by the unsteady vortex shedding induced by the bar. The displacement of the bar from the channel axis toward the wall did not cause an increase in the global heat transfer coefficient on the channel walls compared with the bar centered in the channel.

Unsteady flow and heat transfer in plane channels with spatially periodic vortex generators

Abstract The flow structure and heat transfer in a plane channel with periodically placed vortex generators of different forms have been investigated in the Reynolds number range corresponding to unsteady laminar and transitional flow. Numerical results from four different configurations are reported: a pair of square bars, a rectangular bar, and two different baffle arrangements. The heat transfer and pressure drop are strongly dependent on the geometry used. A wide range of geometric parameters have been computed to cover the different possibilities. The unsteady Navier–Stokes equations and the energy equation have been solved by a finite-volume code with staggered grids combined with SIMPLEC pressure correction. The velocity and temperature fields were computed. Results for the same pumping power show heat transfer enhancement by a factor larger than 3.5 in the best cases.

Fluid dynamics of submerged gas injection into liquid in a model of copper converter

We have numerically studied the fluid dynamic effects on water in a converter-shaped vessel of air injection from a submerged tuyere. The time dependent and three dimensional simulations of the bi-phase system were carried out using the volume of fluid (VOF) and the standard κ-e turbulence models implemented in the commercial solver Fluent. Experimental observation of the phenomena were carried out in a water tank. We have also calculated the standing waves on the surface of a liquid solving the wave equation for potential flow. We study the influence of air inlet velocity on bath dynamic which produced the most favorable results with respect to good mixing in the bath, and minimum splashing

Heat transfer enhancement in a channel with a built-in square cylinder

Abstract A two dimensional numerical investigation of the unsteady laminar flow pattern and forced convective heat transfer in a channel with a built-in square cylinder is presented. The channel in the entrance region has a length to plate spacing of ten. The computations were made for several Reynolds number and two square cylinder sizes. Hydrodynamic behavior and heat transfer results are obtained by solution of the complete Navier-Stokes and energy equation. The results show that these flow exhibits laminar self-sustained oscillations for Reynolds numbers above the critical one. This study shows that oscillatory separated flows result in a significant heat transfer enhancement but also in a significant pressure drop increase.

Comparison between computational fluid dynamics, fluid–structure interaction and computational structural dynamics predictions of flow-induced wall mechanics in an anatomically realistic cerebral aneurysm model

Haemodynamically induced stress plays an important role in the progression and rupture of cerebral aneurysms. The current work describes computational fluid dynamics (CFD), fluid–structure interaction (FSI) and computational structural dynamics (CSD) simulations in an anatomically realistic model of a carotid artery with two saccular cerebral aneurysms in the ophthalmic region. The model was obtained from three-dimensional (3D) rotational angiographic imaging data. CFD and FSI were studied under a physiologically representative waveform of inflow. The arterial wall was assumed elastic or hyperelastic, as a 3D solid or as a shell depending on the type of modelling used. The flow was assumed to be laminar, non-Newtonian and incompressible. The CFD, FSI and CSD models were solved with the finite elements package ADINA. Predictions of velocity field and wall shear stress (WSS) on the aneurysms made using CFD and FSI were compared. The CSD model of the aneurysms using complete geometry was compared with isolated aneurysm models. Additionally, the effects of hypertensive pressure on CSD aneurysm models are also reported. The vortex structure, WSS, effective stress, strain and displacement of the aneurysm walls showed differences, depending on the type of modelling used.