Outlet Velocity Profiles Research Articles

Numerical investigation of quenching technique for steel alloy hardening process using twins liquid jets

Quenching is one of the most common phenomena for hardening steel. The present study is focused on investigating the hardening of stainless-steel plates by single as well as twin liquid jets. To this end, flow interaction of twin jets on hot stainless-steel plate and consequent thermal characteristics (especially temperature profile) on the latter has been studied via commercial CFD software, ANSYS FLUENT 18. A steady-state, two- dimensional, and turbulent (standard k-epsilon) model has been developed by taking jet-based Reynolds number ranges between 3800 and 17,000 for the different nozzle to stainless steel plate distance, whereas the latter’s temperature is fixed for 700 °C in the beginning. The results were compared with the help of stagnation heat transfer coefficient, stagnation Nusselt number, and turbulence effect near hot plate for the different nozzle to plate distance. Fluid interaction from twin jets also affects the cooling phenomena strongly, especially at the stagnation point. The local variation of heat transfer at the stagnation point was qualitatively different for different nozzle outlet velocity profiles near the plate.

Turbulent Mixing Process of a Round Jet With Slot Lobes

Abstract Turbulent mixing in the near region of a round jet with three slot lobes is examined via mean velocity and turbulent statistics and structures at a Reynolds number of 15,000. The design utilizes separate flow motivations upstream of each geometric feature, deviating from conventional nozzles or orifice plates. Immediate outlet velocity profiles are heavily influenced by opposing pressure gradients between the neighboring round and slot streams. Spanwise mean velocity profiles reveal the majority of the convective exchange between a given slot and the round center occurs in the immediate near field, but has lasting effects on the axial centerline profiles downstream. This is also reflected by the velocity half-widths, exhibiting asymmetry across the entirety of available measurements. Centerline turbulence intensities exhibit strong and short-lived isotropy. The increasingly anisotropic intensities found downstream are lower than similar geometries from the literature, implying that mixing development is inhibited. Reynolds stresses at the round-slot interface are significantly smaller than the round-stagnant exchange, but achieve a symmetric condition at x/D ≅ 4. Two-point spatial correlations of the fluctuating streamwise velocity exhibit stronger dependence toward the axial centerline at the round-slot interface in comparison to the nominal round radius. In contrast, spanwise velocity fluctuations exhibit nearly identical, localized behaviors on each side of the jet. Corresponding differences in streamwise integral length scale peak in the range 1.0 ≤ x/D ≤ 1.5, and so too do the turbulent structures in this area, as a result of the collated jet geometry.

Thermal Hydraulics Analysis of the Distribution Zone in Small Modular Dual Fluid Reactor

The Small Modular Dual Fluid Reactor (SMDFR) is a novel molten salt reactor based on the dual fluid reactor concept, which employs molten salt as fuel and liquid lead/lead-bismuth eutectic (LBE) as coolant. A unique design of this reactor is the distribution zone, which locates under the core and joins the core region with the inlet pipes of molten salt and coolant. Since the distribution zone has a major influence on the heat removal capacity in the core region, the thermal hydraulics characteristics of the distribution zone have to be investigated. This paper focuses on the thermal hydraulics analysis of the distribution zone, which is conducted by the numerical simulation using COMSOL Multiphysics with the CFD (Computational Fluid Dynamics) module and the Heat Transfer module. The energy loss and heat exchange in the distribution zone are also quantitatively analyzed. The velocity and temperature distributions of both molten salt and coolant at the outlet of the distribution zone, as inlet of the core region, are produced. It can be observed that the outlet velocity profiles are proportional in magnitude to the inlet velocity ones with a similar shape. In addition, the results show that the heat transfer in the center region is enhanced due to the velocity distribution, which could compensate the power peak and flatten the temperature distribution for a higher power density.

Numerical simulation of an over-expanded supersonic and subsonic industrial nozzle flow relevant to flaring system

Flaring in oil and gas production is the controlled burning of unwanted exhaust gases to enhance safety. To improve flare combustion, gas flares are equipped with air nozzles that introduce extra oxygen and improve mixing in the combustion zone. These nozzles are operated in the subsonic, sonic, or supersonic regimes. In this paper, we are concerned with turbulence modeling of the jet flow exiting from a particular convergent–divergent nozzle used in flare systems. That nozzle has convergent and divergent sections that are connected via a throat section with a finite length and constant diameter. The Realizable k – ε and SST k – ω models are used to study the compressible flow within the nozzle. The velocity profiles, turbulent kinetic energy, Mach number profiles, and entrainment rate coefficients predicted by both turbulence models are compared for nozzle pressure ratios in the range 1.18 ≤ NPR ≤ 1.78. It is shown that both turbulence models predict nearly identical flow evolution along the nozzle. When the flow becomes supersonic, the shock surface, and consequently nozzle outlet velocity profiles, predicted by the SST k – ω model deviates slightly from the other model. The differences, however, become negligible a couple of diameters downstream of the nozzle outlet.

Multilevel design optimization of hydraulic turbines based on hierarchical metamodel-assisted evolutionary algorithms

In this paper, an efficient hydraulic optimization procedure is presented and applied to the design of hydraulic turbines. For computationally expensive industrial design optimization problems, an advanced optimization tool (EASY software) and a fast CFD evaluation tool are required. EASY optimization software is a Hierarchical Metamodel-Assisted Evolutionary Algorithm (HMAEA) that can be used in both single-(SOO) and multi-objective optimization (MOO) problems. In order to minimize the CFD solver calls during the optimization design, the MAEA rely on local metamodels, trained on the fly, that are used to identify the most promising members in each population and then only these are to be re-evaluated by the CPU costly CFD solver. For additional economy in the CPU cost, the hierarchical (two-level) optimization scheme is used in this paper, where at each level, a different evaluation tool, a low and a high fidelity specific software, can be linked. The low level utilizes a low-CPU cost and low-accuracy tool to explore the design space with a minimum impact to the wall clock time and the high level, using the high fidelity, high-CPU cost tool is used to exploit the information from the low level. For the applications presented in this paper, the high fidelity model is an incompressible Navier-Stokes equation solver and the low fidelity model is based on the solution of the incompressible Euler equations. In order to optimize the geometry of hydraulic machines, an in-house automatic geometry and mesh generation tool has been integrated in the optimization tool chain. In what follows, 2 three-objective design optimization problems of 3D Francis hydraulic turbines are presented. The optimization objective functions concern the ‘quality’ of the runner outlet velocity profile, the cavitation behavior and efficiency of the runner. The optimization results of the hydraulic turbine components along with the performance of the presented optimization procedure are shown in the paper.

An experimental study into the effect of injector pressure loss on self-sustained combustion instabilities in a swirled spray burner

Combustion instabilities depend on a variety of parameters and operating conditions. It is known, especially in the field of liquid rocket propulsion, that the pressure loss of an injector has an effect on its dynamics and on the coupling between the combustion chamber and the fuel manifold. However, its influence is not well documented in the technical literature dealing with gas turbine combustion dynamics. Effects of changes in this key design parameter are investigated in the present article by testing different swirlers at constant thermal power on a broad range of injection velocities in a well controlled laboratory scale single injector swirled combustor using liquid fuel. The objective is to study the impact of injection pressure losses on the occurrence and level of combustion instabilities by making use of a set of injectors having nearly the same outlet velocity profiles, the same swirl number and that establish flames that are essentially identical in shape. It is found that combustion oscillations appear on a wider range of operating conditions for injectors with the highest pressure loss, but that the pressure fluctuations caused by thermoacoustic oscillations are greatest when the injector head loss is low. Four types of instabilities coupled by two modes may be distinguished: the first group features a lower frequency, arises when the injector pressure loss is low and corresponds to a weakly coupled chamber-plenum mode. The second group appears in the form of a constant amplitude limit cycle, or as bursts at a slightly higher frequency and is coupled by a chamber mode. Spontaneous switching between these two types of instabilities is also observed in a narrow domain.

Effect of arteriovenous graft flow rate on vascular access hemodynamics in a novel modular anastomotic valve device.

Perturbed vascular access hemodynamics is considered a potential driver of intimal hyperplasia, the leading cause of vascular access failure. To improve vascular access patency, a modular anastomotic valve device has been designed to normalize venous flow between hemodialysis periods while providing normal vascular access during hemodialysis. The objective of this study was to quantify the effects of arteriovenous graft flow rate on modular anastomotic valve device vascular access hemodynamics under realistic hemodialysis conditions. Modular anastomotic valve device inlet and outlet flow conditions and velocity profiles were measured by ultrasound Doppler in a vascular access flow loop replicating arteriovenous graft flow rates of 800, 1000, and 1500 mL/min. Fluid-structure interaction simulations were performed to identify low wall shear stress regions on the vein wall and to characterize them in terms of temporal shear magnitude, oscillatory shear index, and relative residence time. The model was validated with respect to the Doppler measurements. The low wall shear stress region generated downstream of the anastomosis under low and moderate arteriovenous graft flow rates was eliminated under the highest arteriovenous graft flow rate. Increase in arteriovenous graft flow rate from 800 to 1500 mL/min resulted in a substantial increase in wall shear stress magnitude (27-fold increase in temporal shear magnitude), the elimination of wall shear stress bidirectionality (0.20-point reduction in oscillatory shear index), and a reduction in flow stagnation (98% decrease in relative residence time). While the results suggest the ability of high arteriovenous graft flow rates to protect the venous wall from intimal hyperplasia-prone hemodynamics, they indicate their adverse impact on the degree of venous hemodynamic abnormality.

Large-Eddy Simulation-Based Characterization of Suction and Oscillatory Blowing Fluidic Actuator

Unsteady turbulent flows inside a suction and oscillatory blowing actuator are simulated and characterized to provide better physical understanding of the complex actuator flows. Large-eddy simulation based upon a novel unstructured-grid technique is used to accurately calculate turbulent flows inside the actuator operating in still air. Simulations are performed for three inlet pressure conditions. Results show good agreement with the corresponding experimental measurements. The actuator is characterized by parameters such as pressure ratio, outlet velocity profile, oscillation frequency, and suction-to-total flow rates. The large-eddy simulation-generated data are used to develop a reduced-order model applicable to integrated simulation of aerodynamic flow control as an unsteady boundary condition. Dynamic mode decomposition is employed to obtain a lower-order representation of streamwise velocity at the actuator outlets. The frequency-optimized characteristics of dynamic mode decomposition enables to compactly represent the oscillatory turbulent outflows. Using the sparsity-promoting algorithm, an optimal representation of the actuator outflows is systematically derived. Using only two distinct dynamic modes, the sparsity-promoting variant of the standard dynamic mode decomposition algorithm adequately describes unsteady velocity fields at the actuator outlets.

The effect of water introduction rate and liquid saturation jumps on the performance of the flowing electrolyte – Direct methanol fuel cell

A single domain approach to resolving liquid saturation jumps and the water introduction rate within a flowing electrolyte - direct methanol fuel cell is presented. The derivation demonstrates the importance of retaining the porous property dependency on the capillary pressure gradient to obtain a liquid saturation jump. A proposed pulse function was shown to be a useful tool to approximating the macroscopic variations in porous properties across mating layers. The presented approach compared well against analytical solutions and experimental data. The overall performance of the fuel cell was shown to be rather insensitive to the choice of pulse diffusion index, suggesting that large scale simulations could reduce their computational cost with increased diffusion, without significantly affecting their predicted performance. Furthermore, the non-uniformity in the flowing electrolyte channel’s (FEC’s) liquid saturation distribution caused a non-uniform FEC outlet velocity profile. The higher FEC outlet velocity suggests that previous FE-DMFC models under-predicted the amount of methanol removal from the fuel cell.

The CFD Modeling of Heat Recovery Steam Generator Inlet Duct

The axial velocity and temperature distributions at the outlet section of inlet diffuser of Heat Recovery Steam Generator (HRSG) channel should be uniform as much as possible to avoid overheating of first rows of boiler heat exchangers tubes. Due to flow properties and angle of inlet diffuser, providing a uniform outlet velocity profile is impossible without using a correction device. A proposed design should be checked to satisfy the outlet velocity and temperature requirements. In current study, the abilities of computational fluid dynamics have been assessed to obtain the crucial profiles without the experimental difficulties. Regarding the special characteristics of flow and geometry, numerical solution may not be performed without taking some techniques into the CFD modeling. The actual HRSG inlet channel incorporates one perforated plate to correct the flow and three burner elements inside its wide-angle diffuser. Investigations have shown that the perforated plate and heat exchanger modules can be modeled by porous jump boundary condition and the burner elements by radiator faces respectively. Realizable k-e with non-equilibrium wall function seems to be the most optimum turbulence model for solution of the problem. KeywordsHRSG; Inlet Duct; Flow Correction; Diffuser; CFD

NUMERICAL ANALYSIS OF THE INFLUENCE OF DIESEL NOZZLE DESIGN ON INTERNAL FLOW CHARACTERISTICS FOR 2-VALVE DIESEL ENGINE APPLICATION

The current paper studies the potential effect of using tilted injectors on internal nozzle flow characteristics. This kind of injector has been commonly chosen for 2-valve diesel engines. In order to achieve similar spray targeting in a 4-valve configuration, the injector design needs to be modified, inducing a different angle on each of the nozzle holes with respect to the injector axis. To study the implications of this kind of nozzle design, a vertical and a tilted injector have been analyzed for two kinds of hole shape: tapered, which suppresses cavitation; and cylindrical, which is more prone to cavitating flow. A two-component surrogate model has been used to emulate the physical properties of a typical commercial diesel fuel. Full three-dimensional flow simulations have been performed, using a flash boiling model to capture the phase transition induced by cavitation. The simulations have been conducted at the full needle lift condition. The results show that there is a significant influence of hole angle on the mass flow distribution through the nozzle, as well as on the flow pattern and the outlet velocity profile, leading to an asymmetric spray distribution inside the combustion chamber. It is also demonstrated that cavitation formation is affected by the hole angle.

Experimental and numerical investigation of the tracer gas methodology in the case of a naturally cross-ventilated building

This paper presents the investigation of a naturally cross-ventilated building using both experimental and numerical methods with parameter being the incidence angle of the wind to the openings of the building. The experimental methodology calculates the air change rate based either on measurements of the inlet velocity profile, the outlet velocity profile or the descending rate of the tracer gas mass concentration using the decay method. The numerical investigation is based on the solution of the governing Reynolds averaged Navier–Stokes equations in their full three dimensional expression, focusing on the time dependent character of the induced flow field. The numerical results are compared with corresponding experimental data for the three aforementioned experimental methodologies in the case of a full-scale building inside a wind tunnel. The numerical investigation reveals that for large incidence angle the flow is governed by an unsteady character regarding especially the regions close to the inlet and outlet windows. It is concluded that velocity measurements in the inlet window are of high accuracy when the flow in this region has a steady character, whilst the decay method is depicting the effective air change rate in a control volume and not the intended one. Finally, conclusions about the air change rate spatial distribution in the control volume of the building are provided as a function of the incidence angle, revealing that its accuracy as a method for measuring the intended aeration rate depends significantly on the mixing of gas within the control volume of the building.

PIV MEASUREMENTS AND NUMERICAL SIMULATIONS OF FLOW IN CENTRIFUGAL PUMP IMPELLERS WITH SPLITTING VANES

PIV technology and numerical simulations are employed to study the flow in centrifugal pump impellers with splitting vanes under different working states. Through experimental tests and numerical simulations, the flow fields in impellers are obtained for various working states. The results exhibit some basic flow characteristics in impellers: the asymmetry of flow in impellers; backflow occurs in the inlet suction side of long vanes; high speed region exists in the immediate region behind the shroud of splitting vanes; the outlet velocity profile is impr oved by splitting vanes; relative velocity increases evidently with the increase of flow rate. All these flow phenomena directly affect the performance of pumps.

Influence of Compressor Exit Conditions on Diffuser Performance

Results are presented of an experimental investigation into the way in which compressor exit conditions influence the performance of two optimum combustor-dump diffusers. In an optimum system nearly all of the pressure rise occurs in a relatively long prediffuser which is designed to produce a symmetrical outlet velocity profile at the design flow split. One diffuser was tested downstream of a seven-stage axial flow compressor and, for comparative purposes, retested with fully developed flow at inlet. The second diffuser was tested downstream of an annular tandem cascade which could be sited at a number of positions relative to diffuser inlet. The overall performance, measured when the wakes from the outlet guide vanes had decayed considerably, was the same as that achieved with fully developed inflow.

An Initial Approach to the Design of Very Wide Angle Axisymmetric Diffusers With Gauzes to Achieve Uniform Outlet Velocity Profiles

A method is presented of designing very wide angle axisymmetric diffusing ducts which achieve uniform outlet velocity profiles. The aim was to concentrate the major part of the diffuser pressure rise on a small portion of the duct boundary with zero or favorable pressure gradients on the remainder of the boundary. With an optimum location and design of gauze the diffusers are capable of giving a very uniform outlet velocity profile in a short overall length, when working with thin inlet boundary layers and some, although recognizably moderate, pressure recovery. One diffuser tested (diffuser C) produced uniform outlet velocity profiles in approximately 1.5 inlet diameters compared with the 10 diameters required for a sudden enlargement having comparable performance.

On the residence-time distribution for a system with velocity profiles in its connections with the environment

A point-stimulus point-response function is defined that allows rigorous expressions to be written for the residence time distribution for a system where there are complicating factors such as inlet concentration profiles and inlet and outlet velocity profiles. This function is capable of being measured directly; the residence time distribution and other response functions may be calculated from it. It is not possible to calculate the residence time distribution from the results of experiments in which tracer is generated and/or measured in situ at locations were there are significant velocity profiles unless further information is available.

The effect of velocity profile decay on shear flow in diffusers

In turbulent flow, a velocity profile is not completely specified by shape alone. The rate at which the profile develops or decays, determined by its turbulence structure originating in its past history or method of generation, is of equal importance. The prediction of the behaviour of a shear flow, in say, a diffuser, has been made successfully for an equilibrium or slowly varying profile. If the entry profile is in a rapid state of decay then this instability may be the major factor influencing the outlet velocity profile shape, and the prediction of the profile will not be accurate. Some experimental results are presented for a two-dimensional diffuser illustrating the departures from theory caused by the decay of the inlet velocity profile.