Uniform Velocity Distribution Research Articles

Improved design of Wells turbine for wave energy conversion using entropy generation

In this paper, a comparison of total entropy generation between a suggested design and a constant chord Wells turbine is presented. A variable chord rotor Wells turbine design is suggested in order to have a more uniform axial velocity distribution at blades leading edge in comparing conventional design to improve turbine efficiency. Each model is analysed numerically with respect to the total entropy generation due to viscous dissipation around rotor blades. Simulations have been performed by numerical solving of the steady, incompressible, three-dimensional Reynolds-averaged Navier–Stokes together with RNG k–e turbulence model equations in a non-inertial reference frame rotating with the turbine rotor. A comparison of the computed results with the available experimental data exhibits agreement in terms of efficiency, torque and input coefficients at different flow rates. Finally, the detailed comparisons of the results are done between the suggested design and the conventional Wells turbine, demonstrating a 26.02 % average decrease in entropy generation throughout the full operating range, hence supporting the superiority of the new design.

Experimental Investigation of the Full Flow Field in a Molten Salt Pump by Particle Image Velocimetry

Particle image velocimetry (PIV) technology was used to study steady and unsteady internal flow fields in a molten salt pump under both internal and external synchronization modes. The velocity fields in the suction chamber, impeller passage and volute were analyzed at different flow rates. The velocity distribution uniformity, velocity weighted average divergent flow angle, and circumferential component of absolute velocity were calculated on the basis of the obtained flow fields. The research is meaningful to the development of molten salt pumps, and the experimental method serves as a reference to similar rotating fluid machinery.

Supersonic Flows in Channels and Jets when Applied to the Problems of Designing Aerodynamic Windows of Gas Lasers

Consideration is given to issues associated with the development of aids for mathematical modeling of steadystate supersonic perfect compressible gas flows in aerodynamic windows of gas lasers. A comparison is made of flow patterns in supersonic jets, which appear when a uniform velocity distribution and a velocity distribution corresponding to the free vortex are realized on the nozzle section. Aids for designing channels are developed on the basis of solution of a sequence of primal problems. The calculation results show the high efficiency of the developed software as applied to solution of problems associated with the contouring of channels and nozzles and with the control over supersonic jet flows.

The Flutter Shutter Code Calculator

The goal of the utter shutter is to make uniform motion blur invertible, by a \uttering shutter that opens and closes on a sequence of well chosen sub-intervals of the exposure time interval. In other words, the photon ux is modulated according to a well chosen sequence called utter shutter code. This article provides a numerical method that computes optimal utter shutter codes in terms of mean square error (MSE). We assume that the observed objects follow a known (or learned) random velocity distribution. In this paper, Gaussian and uniform velocity distributions are considered. Snapshots are also optimized taking the velocity distribution into account. For each velocity distribution, the gain of the optimal utter shutter code with respect to the optimal snapshot in terms of MSE is computed. This symmetric optimization of the utter

The effect of the way of setting boundary conditions on the results of modeling a flow at the flameholder outlet

The influence of different algorithms of generating inlet turbulent fluctuations on the results of calculating the flow parameters downstream the flameholder is discussed in the paper. Large eddy simulation is used on a turbulence model with a subgrid Smagorinsky-Lilli model. Propane-air mixture with a temperature of 300 K in a square-section channel is investigated. A bluff body with a base of an equilateral triangle with the side length equal to 25 mm is placed inside the channel. The mass-averaged inlet velocity is 10 m/s. Several ways of setting inlet boundary conditions are considered. Two cases without turbulence at the inlet (uniform velocity distribution across the inlet section and pipe velocity profile) and two cases with artificially modeled turbulence (vortex method and spectral turbulence synthesizer) are analyzed. The values of flow intensity upstream the stabilizer, the distribution of flow velocity and kinetic energy in longitudinal and cross sections downstream the stabilizer have been obtained. The graphs of the velocity fluctuation power density spectrum are also presented.

A novel method for inlet duct geometry improvement of heat recovery steam generators

The geometry of inlet duct of HRSGs strongly affects flow uniformity and plays an important role in an increase in the efficiency of combined cycle power plants (CCPPs) where HRSG is the main component. Therefore, using some remedies to create a uniform flow distribution in HRSG inlet duct seems to be required in order to avoid reducing the effective surface temperature. In this research paper, part elimination and lattice search (PELS) method is presented to improve the inlet duct geometry. This is an innovative method which improves the inlet duct shape due to the amelioration of the flow quality. Uniformity of the velocity profile is considered at the exit section by quantifiable performance parameters of Root Mean Square (RMS). The RMS difference of local velocity from average velocity is used to evaluate the uniformity of the flow. To apply this method, a standard inlet duct of a 5 MW HRSG is considered and improvements for this case are addressed. The terms of uniform velocity and temperature distribution along the HRSG is numerically solved and explained. Finally, the results are reported in order to compare the standard inlet duct and the improved one. The comparison shows that the flow uniformity at the duct exit section in the modified model has been enhanced.

Influence of stator vane number on performance of the axial-flow pump

The interplay of impeller and stator is investigated for an axial-flow pump. Three stator vane numbers of 5, 7 and 9 are devised to match the same impeller. The renormalization group k-e turbulence model is used to simulate three-dimensional flows in three pumps with different vane numbers. Axial velocity distributions at impeller outlet and stator outlet are comparatively analyzed. Experiments assess operation performance of the three pumps. Vibration parameters and static pressure fluctuations are measured as well. It is indicated that the influence of vane number on both pump head and pump efficiency is insignificant. Large stator vane number contributes to the improvement of the uniformity of axial velocity distribution at impeller outlet. At stator outlet, similar tendency is revealed. Severe vibration occurs near the outlet bend of the pump, as is particularly remarkable at vane number of 9. For the three cases, blade passing frequency and its harmonics are predominant in the frequency spectra of pressure fluctuations. As flow rate increases from 0.8Q to 1.0Q, high-frequency pressure fluctuations are suppressed considerably at vane number of 9, while the other two cases also manifest a decline in overall pressure fluctuation amplitude. The 7-vane case is the most preferable one among the three cases in terms of both pump performance and pressure fluctuation between the impeller and the stator.

Transverse jet-cavity interactions with the influence of an impinging shock

For high-speed air breathing engines, fuel injection and subsequent mixing with air is paramount for combustion. The high freestream velocity poses a great challenge to efficient mixing both in macroscale and microscale. Utilising cavities downstream of fuel injection locations, as a means to hold the flow and stabilise the combustion, is one mechanism which has attracted much attention, requiring further research to study the unsteady flow features and interactions occurring within the cavity. In this study we combine the transverse jet injection upstream of a cavity with an impinging shock to see how this interaction influences the cavity flow, since impinging shocks have been shown to enhance mixing of transverse jets. Utilising qualitative and quantitative methods: schlieren, oilflow, PIV, and PSP the induced flowfield is analysed. The impinging shock lifts the shear layer over the cavity and combined with the instabilities generated by the transverse jet creates a highly complicated flowfield with numerous vertical structures. The interaction between the oblique shock and the jet leads to a relatively uniform velocity distribution within the cavity.

Designing of Sub-entry Nozzle for Casting Defect-free Steel

Production of defect-free steel is a continuous demand for each & every steel maker. The design of refractories used in steel making process has a vital role on this. Manufacturing of cleaner steel is mostly attributed to the continuous casting process of the steel plant. Defects in steel mainly come from the non-metallic inclusions present in steel that are incorporated during various steel making process. During continuous casting, the main focus is to remove these inclusions to cast defect free steel. The flow pattern of molten steel in the mould plays a vital role in the removal of these impurities. The turbulence of the flowing steel in the mould should be such that more and more impurities will be floated at the meniscus of the mould and they will be captured by the casting powder. But, this turbulence should not be higher enough to capture the particles of casting powder inside the molten steel. So, in order to optimize the turbulent intensity and to get uniform distribution of temperature and velocity in the mould, the design of Sub-entry nozzle (SEN) is very crucial. SEN is used to guide the flow of steel from tundish to mould during continuous casting. The bore configuration and outlet design of SEN along with its immersed depth in the mould determine the flow pattern of steel inside the mould, which is the key factor to produce cleaner steel. This paper focuses on the different designing aspects of SEN to get an optimized and uniform flow of steel in the mould. The designing parameters include bore diameter, bore configuration, port shape, port angle, port dimension, number of ports and immersed depth of SEN inside the mould. Design optimization is done to improve the steel quality by removing the impurities.

Velocity Distribution inside a Laminated‐Sheet Microchannel Reactor

AbstractIn a laminated‐sheet microchannel reactor, several microchannel sheets with the same or different structures are mutually laminated together. The effect of microchannel and manifold structure as well as the number of laminated sheets on the velocity distribution among microchannels in each sheet with the same structure is investigated. Results indicate that a large microchannel length, a high‐aspect‐ratio microchannel, and centrosymmetric manifold structure are favorable for a relatively uniform velocity distribution in each sheet. Considering the centrosymmetric manifold structure, a shorter distance from inlet and outlet to microchannels in direction of the microchannel width and a longer distance to the microchannel array in direction of the microchannel length can contribute to a more uniform velocity distribution. The laminated‐sheet number has only a minor impact on the velocity distribution among microchannels in each sheet.

The Research of the Characteristics of Secondary Equipment Device and Airflow Jet

In large space and span building, secondary airflow with nozzle's jet could be used to improve the uniformity of temperature and velocity distribution of indoor thermal environment, as well as the occupant's comfort in the occupied zone. Therefore, it is very important to reveal the characteristics of the secondary airflow equipment with nozzle's jet. In this paper, the characteristics of secondary airflow equipment and its air jet were researched through experiment. The experiment results on secondary airflow equipment performance showed that air volume, air static pressure, power and sound pressure level of the equipment would increase along with the increase of rotation speed under the same air nozzle diameters. And the results on air jet track illustrated that when the horizontal distance increased, the axis velocity of secondary airflow decreased both in isothermal jet and non-isothermal jet conditions. Through this experiment, the characteristic parameters of the secondary airflow equipment were provided for actual construction design.

Design of Mini Wind Tunnel Based on Coanda Effect

An experimental investigation and CFD treatment were employed to design mini-wind tunnel based on Coanda effect for model tests and basic research. The inlet source flow is efficiently creating smooth steady airflow with acceptable noise, achieving the possibility of placing the test target closer to the source of flow with reasonable estimates of turbulence intensity. The design aims at achieving flow uniformity in the working section midplane, preventing separation in the contraction and minimizing the boundary–layer thickness. Intensive measurements after construction demonstrate the significance of the design process and validate the CFD predictions. The results are represented in graphic form to indicate the aspects of the contraction ratio. The numerical and experimental results show the uniformity of velocity distribution inside the working section. Tracing of separation and backflow is crucial allowing a variety of realistic demonstrations to be performed. The numerical solution provides a powerful tool to demonstrate the rate of boundary–layer growth inside the working section and validate against the empirical correlations with insignificant wall–friction drag. Assessment study to address large–scale wind tunnel based on coanda effect would be considered.

2D-PIV Experimental Study on the Air Distribution with Natural Convection Effect of Passengers in an Air Cabin Mockup

Many previous studies have mentioned the critical effect of natural convection caused by passengers heat transfer on the air distribution inside the air cabins, but No study focused on it to make a further experimental or numerical analysis. In fact, the air flow field inside the air cabin with narrow interior space and passengers seated intensively is the result of interaction between natural convection from the passengers and forced convection from the supply air diffusers, including the air jet which plays the most essential part on the flow pattern. This study has measured the air flow jet in a 7-row cabin mockup with 2D-PIV (two dimension Particle Image Velocity) measurement system, and to make comparison of the air jet under isothermal and cooling conditions to analyze qualitatively the impact of natural convection inside the cabin mockup, which is to increase the air jet entrainment and weaken the attaching effect which can also enhance the velocity distribution uniformity. With the variety of air flow volume in a reasonable range of design parameters, we measured the different air velocity fields to quantify the effects of natural convection on the air jet. It can be concluded that the air jet decay rate becomes slower with the enhancement of natural convection.

Advantageous swirling flow in 45° end-to-side anastomosis

The effects of swirling flow on the flow field in 45° end-to-side anastomosis are experimentally investigated using a particle image velocimetry technique to reveal fluid dynamic advantages of swirling flow in the vascular graft. Non-swirling Poiseuille inlet flow unnecessarily induces pathological hemodynamic features, such as high wall shear stress (WSS) at the ‘bed’ side and large flow separation at the ‘toe’ side. The introduction of swirling flow is found to equalize the asymmetric WSS distribution and reduces the peak magnitude of WSS. In particular, the intermediate swirling intensity of S = 0.45 induces the most uniform axial velocity and WSS distributions compared with weaker or stronger swirling flows, which addresses the importance of proper selection of swirling intensity in the vascular graft to obtain optimum flow fields at the host vessel. In addition, swirling flow reduces the size of flow separation because it disturbs the formation of Dean-type vortices in secondary flow and inhibits secondary flow collision. The beneficial fluid dynamic features of swirling flow obtained in this study are helpful for designing better vascular graft suppressing pathological hemodynamic features in the recipient host vessel.

Optimization of Parallel and Serpentine Configurations for Polymer Electrolyte Membrane Fuel Cells

AbstractA network‐based optimization model was developed to optimize the channel dimensions of flow fields in order to achieve a uniform flow distribution and improve the performance of polymer electrolyte membrane (PEM) fuel cells. Different flow field configurations, including parallel, parallel‐in‐series, and serpentine, were investigated using the present optimization model. Two cases, with and without considering reactant consumption, were compared to show the effect of including reactant consumption on the flow field designs. The results demonstrated that the optimized designs significantly improved the flow velocity distribution in all the configurations investigated. The optimized designs with consideration of reactant consumption exhibited more uniform flow velocity distribution when the entire fuel cell unit was considered. Additionally, the performances of PEM fuel cells for the conventional and optimized flow field designs were studied with a three‐dimensional, two‐phase fuel cell simulation model, and the computational results showed that the optimized designs with considering reactant consumption produced the highest maximum power density for each configuration investigated. These results show that the network‐based model is capable of optimizing various flow field configurations with flexibility and indicate the importance of considering reactant consumption in the optimization model.

Adiabatic contraction revisited: Implications for primordial black holes

We simulate the adiabatic contraction of a dark matter (DM) distribution during the process of the star formation, paying particular attention to the phase space distribution of the DM particles after the contraction. Assuming the initial uniform density and Maxwellian distribution of DM velocities, we find that the number $n(r)$ of DM particles within the radius $r$ scales like $n(r) \propto r^{1.5}$, leading to the DM density profile $\rho\propto r^{-1.5}$, in agreement with the Liouville theorem and previous numerical studies. At the same time, the number of DM particles $\nu(r)$ with periastra smaller than $r$ is parametrically larger, $\nu(r) \propto r$, implying that many particles contributing at any given moment into the density $\rho(r)$ at small $r$ have very elongated orbits and spend most of their time at distances larger than $r$. This has implications for the capture of DM by stars in the process of their formation. As a concrete example we consider the case of primordial black holes (PBH). We show that accounting for very eccentric orbits boosts the amount of captured PBH by a factor of up to $2\times 10^3$ depending on the PBH mass, improving correspondingly the previously derived constraints on the PBH abundance.

Discriminating between the nearfield and the farfield of acoustic transducers.

Measurements of the performance of acoustic transducers, as well as ordinary measurements made with the same, may require discriminating between the farfield, where the field is spherically divergent, and the complementary nearfield, where the field structure is more complicated. The problem is addressed for a planar circular piston projector, with uniform normal velocity distribution, mounted in an infinite planar rigid baffle. The inward-extrapolated farfield pressure amplitude pf is compared with the exact nearfield pressure amplitude pn, modeled by the Rayleigh integral, through the error 20 log |pf /pn|. Three sets of computations are performed for a piston with wavenumber-radius product ka = 10: normalized pressure amplitudes and error versus range at angles corresponding to beam pattern losses of 0, 10, 20, and 30 dB; error versus angle at three ranges, a(2)/λ, πa(2)/λ, and 10a(2)/λ, where λ is the wavelength; and range versus angle for each of two inward-bounded errors, 1 and 0.3 dB. By reciprocity, the results apply equally to the case of a baffled circular piston receiver with uniform sensitivity over the active surface. It is proposed that proximity criteria for measurements of fields associated with circular pistons be established by like modeling, and that a quality factor be assigned to measurements on the basis of computed errors.

Generalized Analytical Solutions for Alternate and Sequent Depths in Rectangular Open Channels: Sine Form

AbstractTwo novel generalized analytical solutions of nondimensional specific energy and specific force equations for flow in rectangular open channels are obtained, which enable direct and explicit calculation of the alternate and sequent depths for the given nondimensional specific energy and specific force, respectively. Hydrostatic pressure and uniform velocity distributions across flow depth have been assumed, and friction loss in the flow has been neglected, while deriving the new solutions. Each generalized solution yields only three particular solutions out of which only two solutions giving positive values for the depth of flow are of practical significance. The particular solutions/equations of practical significance pertain to the alternate depths and sequent depths in the cases of nondimensional specific energy and specific force, respectively, and are distinguishable for subcritical and supercritical flow. The new solutions are compact (handy), computationally simple, and useful to the field ...

Reconstructing transient acoustic radiation from an arbitrary object with a uniform surface velocity distribution.

This paper presents the general formulations for reconstructing the transient acoustic field generated by an arbitrary object with a uniformly distributed surface velocity in free space. These formulations are derived from the Kirchhoff-Helmholtz integral theory that correlates the transient acoustic pressure at any field point to those on the source surface. For a class of acoustic radiation problems involving an arbitrarily oscillating object with a uniformly distributed surface velocity, for example, a loudspeaker membrane, the normal surface velocity is frequency dependent but is spatially invariant. Accordingly, the surface acoustic pressure is expressible as the product of the surface velocity and the quantity that can be solved explicitly by using the Kirchhoff-Helmholtz integral equation. This surface acoustic pressure can be correlated to the field acoustic pressure using the Kirchhoff-Helmholtz integral formulation. Consequently, it is possible to use nearfield acoustic holography to reconstruct acoustic quantities in entire three-dimensional space based on a single set of acoustic pressure measurements taken in the near field of the target object. Examples of applying these formulations to reconstructing the transient acoustic pressure fields produced by various arbitrary objects are demonstrated.

An investigation into the performance of fabric diffusers used in stratum ventilation

Fabric terminals made of polymer were found to potentially provide a clean, healthy and comfortable indoor environment. In this paper, the performance of fabric diffusers under stratum ventilation is investigated. A total of nine cases, eight of which with fabric diffusers and the other one with a double deflection grille, are conducted in an experimental classroom. The performance of fabric diffusers under stratum ventilation is studied by analyzing the velocity and temperature distributions, the effective draft temperature, the air diffusion performance index and the thermal sensation and comfort feedback. The experimental results show that fabric diffusers perform better under a higher airflow rate. Compared with double deflection grilles, fabric diffusers were able to provide a relatively uniform velocity and temperature distribution for most of the areas in a room and a more comfortable thermal environment. It performs better in air diffusion than double deflection grilles. Therefore, fabric diffusers can be used as air terminals for stratum ventilation.