Ibrahim Hassan

Film Cooling Effectiveness for an Advanced-Louver Cooling Scheme for Gas Turbines

A novel film-cooling scheme for high temperature gas turbine applications was introduced in this paper. Compared with the traditional circular hole, the new scheme combines both the advantages of traditional film cooling with those of impingement cooling. The hole that transports coolant fluid from the inside to the outside of the blade is designed in such a way that the coolant must go through a bend before exiting the blade, thus impinging on the blade material. This scheme is expected to produce the greatest coverage on the blade with the least amount of mixing and least possible amount of coolant. A benchmark case of a traditional circular hole in a crossflow, the fundamental problem of film cooling, was employed to validate the present methodology with the jet liftoff effect clearly captured in the simulation. Turbulence was modeled using four different turbulence models, namely, k-e (including its three variants), k-w, Reynolds-stress, and Spalart-Allmaras with different wall treatments. It was found that the proposed cooling scheme can prevent the jets from penetrating into the mainstream much better and provide more uniform protection on the surface, indicating that the proposed scheme yields superior performance.

Experimental Analysis of Microchannel Entrance Length Characteristics Using Microparticle Image Velocimetry

The study of the entrance region of microchannels and microdevices is limited, yet important, since the effect on the flow field and heat transfer mechanisms is significant. An experimental study has been carried out to explore the laminar hydrodynamic development length in the entrance region of adiabatic square microchannels. Flow field measurements are acquired through the use of microparticle image velocimetry (micro-PIV), a nonintrusive particle tracking and flow observation technique. With the application of micro-PIV, entrance length flow field data are obtained for three different microchannel hydraulic diameters of 500 μm, 200 μm, and 100 μm, all of which have cross-sectional aspect ratios of 1. The working fluid is distilled water, and velocity profile data are acquired over a laminar Reynolds number range from 0.5 to 200. The test-sections were designed as to provide a sharp-edged microchannel inlet from a very large reservoir at least 100 times wider and higher than the microchannel hydraulic diameter. Also, all microchannels have a length-to-diameter ratio of at least 100 to assure fully developed flow at the channel exit. The micro-PIV procedure is validated in the fully developed region with comparison to Navier–Stokes momentum equations. Good agreement was found with comparison to conventional entrance length correlations for ducts or parallel plates, depending on the Reynolds range, and minimal influence of dimensional scaling between the investigated microchannels was observed. New entrance length correlations are proposed, which account for both creeping and high laminar Reynolds number flows. These correlations are unique in predicting the entrance length in microchannels and will aid in the design of future microfluidic devices.

Evaluation of shear stress accumulation on blood components in normal and dysfunctional bileaflet mechanical heart valves using smoothed particle hydrodynamics

Evaluating shear induced hemodynamic complications is one of the major concerns in design of the mechanical heart valves (MHVs). The monitoring of these events relies on both numerical simulations and experimental measurements. Currently, numerical approaches are mainly based on a combined Eulerian-Lagrangian approach. A more straightforward evaluation can be based on the Lagrangian analysis of the whole blood. As a consequence, Lagrangian meshfree methods are more adapted to such evaluation. In this study, smoothed particle hydrodynamics (SPH), a fully meshfree particle method originated to simulate compressible astrophysical flows, is applied to study the flow through a normal and a dysfunctional bileaflet mechanical heart valves (BMHVs). The SPH results are compared with the reference data. The accumulation of shear stress patterns on blood components illustrates the important role played by non-physiological flow patterns and mainly vortical structures in this issue. The statistical distribution of particles with respect to shear stress loading history provides important information regarding the relative number of blood components that can be damaged. This can be used as a measure of the response of blood components to the presence of the valve implant or any implantable medical device. This work presents the first attempt to simulate pulsatile flow through BMHVs using SPH method.

Numerical and experimental investigations of pulsatile blood flow pattern through a dysfunctional mechanical heart valve

Around 250,000 heart valve replacements are performed every year around the world. Due their higher durability, approximately 2/3 of these replacements use mechanical prosthetic heart valves (mainly bileaflet valves). Although very efficient, these valves can be subject to valve leaflet malfunctions. These malfunctions are usually the consequence of pannus ingrowth and/or thrombus formation and represent serious and potentially fatal complications. Hence, it is important to investigate the flow field downstream of a dysfunctional mechanical heart valve to better understand its impact on blood components (red blood cells, platelets and coagulation factors) and to improve the current diagnosis techniques. Therefore, the objective of this study will be to numerically and experimentally investigate the pulsatile turbulent flow downstream of a dysfunctional bileaflet mechanical heart valve in terms of velocity field, vortex formation and potential negative effect on blood components. The results show that the flow downstream of a dysfunctional valve was characterized by abnormally elevated velocities and shear stresses as well as large scale vortices. These characteristics can predispose to blood components damage. Furthermore, valve malfunction led to an underestimation of maximal transvalvular pressure gradient, using Doppler echocardiography, when compared to numerical results. This could be explained by the shifting of the maximal velocity towards the normally functioning leaflet. As a consequence, clinicians should try, when possible, to check the maximal velocity position not only at the central orifice but also through the lateral orifices. Finding the maximal velocity in the lateral orifice could be an indication of valve dysfunction.

Numerical Investigation of Multistage Viscous Micropump Configurations

The viscous micropump consists of a cylinder placed eccentrically inside a microchannel, where the rotor axis is perpendicular to the channel axis. When the cylinder rotates, a net force is transferred to the fluid because of the unequal shear stresses on the upper and lower surfaces of the rotor. Consequently, this causes the surrounding fluid in the channel to displace toward the microchannel outlet. The simplicity of the viscous micropump renders it ideal for micropumping; however, previous studies have shown that its performance is still less than what is required for various applications. The performance of the viscous micropump, in terms of flow rate and pressure capabilities, may be enhanced by implementing more than one rotor into the configuration either horizontally or vertically oriented relative to each other. This is analogous to connecting multiple pumps in parallel or in series. The present study will numerically investigate the performance of various configurations of the viscous micropumps with multiple rotors, namely, the dual-horizontal rotor, triple-horizontal rotor, symmetrical dual-vertical rotor, and eight-shaped dual-vertical rotor. The development of drag-and-lift forces with time, as well as the viscous resisting torque on the cylinders were studied. In addition, the corresponding drag, lift, and moment coefficients were calculated. The flow pattern and pressure distribution on the cylinders’ surfaces are also included in the study. Results show that the symmetrical dual-vertical rotor configuration yields the best efficiency and generates the highest flow rate. The steady-state performance of the single-stage micropump was compared to the available experimental and numerical data and found to be in very good agreement. This work provides a foundation for future research on the subject of fluid phenomena in viscous micropumps.

Experimental Investigation on the Onset of Gas Entrainment from a Stratified Two-Phase Region Through Multiple Branches Mounted on a Curved Surface

An experimental investigation has been carried out to simulate the onset of gas entrainment phenomenon from a stratified region through branches located on a semicircular wall configuration, in close dimensional resemblance with a Canada Deuterium and Uranium (CANDU) header-feeder system. New experimental data for the onset of gas entrainment was developed during single and multiple discharge from an air/water stratified region over a wide range of Froude numbers (0 to 100), in order to thoroughly understand the onset of gas entrainment phenomenon. It was found that the critical height at the onset of gas entrainment (single or simultaneous) was a function of the corresponding Froude number of each branch, the vertical distance between the centerlines of the branches (for multiple discharge), the hydraulic resistance of the discharging lines, as well as the orientation of the branches and their diameter with respect to the main header. Concerning multiple discharge comparisons, at intermediate Fr values (1 10) there is convergence. The present data are necessary in validating future analytical and numerical models of the onset of gas entrainment for a curved geometry, particularly at low Froude numbers.

Flow through a defective mechanical heart valve: A steady flow analysis

Approximately 250,000 valve replacement operations occur annually around the world and more than two thirds of these operations use mechanical heart valves (MHV). These valves are subject to complications such: pannus and/or thrombus formation. Another potential complication is a malfunction in one of the valve leaflets. Although the occurrence of such malfunctions is low, they are life-threatening events that require emergency surgery. It is, therefore, important to develop parameters that will allow an early non-invasive diagnosis of such valve malfunction. In the present study, we performed numerical simulations of the flow through a defective mechanical valve under several flow and malfunction severity conditions. Our results show that the flow upstream and downstream of the defective valve is highly influenced by malfunction severity and this resulted in a misleading improvement in the correlation between simulated Doppler echocardiographic and catheter transvalvular pressure gradients. In this study, we were also able to propose and test two potential non-invasive parameters, using Doppler echocardiography and phase contrast magnetic resonance imaging, for an early detection of mechanical heart valve malfunction. Finally, we showed that valve malfunction has a significant impact on platelet activation and therefore on thrombus formation.

DSMC Simulation of Subsonic Flows in Parallel and Series Microchannels

Flows in uniform, parallel, and series microchannels have been investigated using the direct simulation Monte Carlo (DSMC) method. For the uniform microchannel cases, at higher pressure ratio, mixed Kn-regime flows were observed, where the Knudsen number (Kn) varies from below 0.1 to above 0.1. Also, the higher pressure ratio makes the flow accelerate more as the flow develops through the uniform microchannel. In order to examine the heat transfer characteristics between the wall and the bulk flow, a linear temperature distribution was imposed on the wall. Most of the wall heat flux occurs within the channel entrance region while it remains a constant with a slight magnitude along the rest of the channel wall. For the series microchannel cases, the computational domain was established by adding three surfaces and excluding one region from the rectangular domain. Diffuse effects were observed near the interface of the two segments, where the flow upstream the interface can be either heated or cooled by the flow downstream depending on their temperature difference. In addition, the effect of the gas species was investigated by conducting the simulation using helium and argon respectively. It can be found that the speed of the gas with lighter molecular mass is much higher than that of the heavier gas. The computational domain of the parallel microchannel was established similarly to that of the series microchannel. Under a certain pressure ratio, more pressure drop occurs in the parallel parts as the gap height increases. The recirculation phenomenon was observed after the gap wall between the two parallel parts and was evaluated quantitatively in the present study by defining a parameter called the developing coefficient. The gap height between the two parallel parts has only slight effect of the flow development.

An advanced impingement/film cooling scheme for gas turbines – numerical study

Purpose – The present study aims to conduct a numerical investigation of a novel film cooling scheme combining in‐hole impingement cooling and flow turbulators with traditional downstream film cooling, and was originally proposed by Pratt & Whitney Canada for high temperature gas turbine applications.Design/methodology/approach – Steady‐state simulations were performed and the flow was considered incompressible and turbulent. The CFD package FLUENT 6.1 was used to solve the Navier‐Stokes equations numerically, and the preprocessor, Gambit, was used to generate the required grid.Findings – It was determined that the proposed scheme geometry can prevent coolant lift‐off much better than standard round holes, since the cooling jet remains attached to the surface at much higher blowing rates, indicating a superior performance for the proposed scheme.Research limitations/implications – The present study was concerned only with the downstream effectiveness aspect of performance. The performance related to the h...

Smoothed particle hydrodynamics method applied to pulsatile flow inside a rigid two-dimensional model of left heart cavity

This paper aims to extend the application of smoothed particle hydrodynamics (SPH), a meshfree particle method, to simulate flow inside a model of the hearts left ventricle (LV). This work is considered the first attempt to simulate flow inside a heart cavity using a meshfree particle method. Simulating this kind of flow, characterized by high pulsatility and moderate Reynolds number using SPH is challenging. As a consequence, validation of the computational code using benchmark cases is required prior to simulating the flow inside a model of the LV. In this work, this is accomplished by simulating an unsteady oscillating flow (pressure amplitude: A = 2500 N ∕ m(3) and Womersley number: W(o)  = 16) and the steady lid-driven cavity flow (Re = 3200, 5000). The results are compared against analytical solutions and reference data to assess convergence. Then, both benchmark cases are combined and a pulsatile jet in a cavity is simulated and the results are compared with the finite volume method. Here, an approach to deal with inflow and outflow boundary conditions is introduced. Finally, pulsatile inlet flow in a rigid model of the LV is simulated. The results demonstrate the ability of SPH to model complex cardiovascular flows and to track the history of fluid properties. Some interesting features of SPH are also demonstrated in this study, including the relation between particle resolution and sound speed to control compressibility effects and also order of convergence in SPH simulations, which is consistently demonstrated to be between first-order and second-order at the moderate Reynolds numbers investigated.