Influence Of Secondary Flow Research Articles

A Probabilistic Approach to Turbine Uncertainty

Abstract Efficiency is an essential metric for assessing turbine performance. Modern turbines rely heavily on numerical computational fluid dynamics (CFD) tools for design improvement. With more compact turbines leading to lower aspect ratio airfoils, the influence of secondary flows is significant on performance. Secondary flows and detached flows, in general, remain a challenge for commercial CFD solvers; hence, there is a need for high-fidelity experimental data to tune these solvers used by turbine designers. Efficiency measurements in engine-representative test rigs are challenging for multiple reasons; an inherent problem to any experiment is to remove the effects specific to the turbine rig. This problem is compounded by the narrow uncertainty band required to detect the incremental improvements achieved by turbine designers. Efficiency measurements carried out in engine-representative turbine rigs have traditionally relied upon assumptions such as constant gas properties and neglecting heat loss. This research presents an uncertainty framework that combines inputs from experiments and computational tools. This methodology allows quantifying uncertainty for high-fidelity efficiency data in engine-representative turbine facilities. This paper presents probabilistic sampling techniques to allow for uncertainty propagation. The effect of rig-specific effects, such as heat transfer and gas properties, on efficiency is demonstrated. Sources of uncertainty are identified, and a framework is presented which divides the sources into bias and stochastic. The framework allows the combination of experimental and numerical uncertainty. Gaussian regression models are developed to obtain speed-lines for the turbine map using the uncertainty of the measured efficiency.

Study on the effect of waveform film holes on film cooling efficiency

In order to improve the film cooling efficiency of the cooling gas on the hot side wall of the plate, the flow field, temperature field and the average film cooling efficiency of the cylindrical hole, rectangular hole and waveform hole near the wall were compared and analyzed by numerical calculation method. Compared with cylindrical holes, the average spanwise film cooling efficiency of rectangular hole is improved by 100%, and the average spanwise film cooling efficiency of waveform hole is improved by 130%. In addition, the effects of the amplitude A of the waveform hole and the initial phase ϕ on the cooling efficiency are compared and analyzed. The results show that: Within the scope of this study, when the blowing ratio M = 1, lifting the initial phase ϕ of the waveform hole can effectively improve the film cooling efficiency of the cooling gas on the wall. When the amplitude A of the waveform hole is raised, the film cooling efficiency of the cooling gas on the wall will first increase and then decrease, reaching the maximum at the amplitude A = 0.4D. At low blowing ratio, the geometric structure of waveform hole has little influence on the cooling effect of wall surface. Under high blowing ratio, when the geometric structure of waveform hole is changed, the influence of secondary flow on the average spanwise cooling efficiency of wall surface is great.

The Influence of Turbulence and Reynolds Number on Endwall Heat Transfer in a Vane Cascade

Abstract Endwall heat transfer measurements have been acquired in a vane cascade over a range of turbulence conditions and Reynolds numbers using an array of small commercial infrared (IR) cameras. The linear cascade was tested over five inlet turbulence conditions ranging from low turbulence (0.7%) to high turbulence (17.4%) and three exit chord Reynolds numbers ranging from 500,000 to 2,000,000. The small commercial IR cameras made by Therm-App have a resolution of 384 by 288 pixels and were connected to individual smartphones to record the images. The cascade was modified with small zinc selenide windows to provide IR access for the cameras. The five cameras were calibrated against a constant temperature test plate and the output images were adjusted for the fisheye effect and thermal droop at the edges. The large-scale low-speed cascade, used in the endwall heat transfer study, was configured in a four-vane three full passage arrangement. The vane design includes a large leading and aft loading. This same cascade has been used in the acquisition of vane surface heat transfer distributions, vane suction surface heat transfer visualizations, and vane surface film cooling distributions. This paper includes comparisons with two large eddy simulation calculations, which were conducted prior to the acquisition of the heat transfer data. The influence of the secondary flows on the endwall heat transfer distributions, including the leading edge horseshoe vortex system, is particularly visible at lower turbulence levels and lower Reynolds numbers. However, at higher turbulence levels, the influence of secondary flows is less visible but the influence of Reynolds number and turbulence on transition can be discerned.

Experimental and numerical simulation of the deposition characteristics of NH<SUB align="right">4Cl particles in 90° elbow

The deposition of NH4Cl particles in the equipment can cause potential safety hazards in petrochemical enterprises. In this study, a gas-solid-flow experimental equipment has been built to explore the characteristics of NH4Cl deposition with different humidity and fluid velocity. The results show that: the gas flow rate, relative humidity (RH), and duration have little influence on the deposition position of NH4Cl particles, but they affect the deposition amount of NH4Cl particles greatly; due to the formation of liquid droplets and the influence of secondary flow in the elbow, the deposition increases rapidly in the range of 45°-75° and reaches 0.57 g at 67.5°. After evaluating the experiment, a computational fluid dynamics (CFDs) coupled with DEM method is employed to analyse the flow parameters of particles. Numerical results of particle deposition show a good agreement with the corresponding experimental data. [Received: February 25, 2022; Accepted: July 6, 2022]

Influence of secondary flow with a Coanda surface on the direction of jets

Abstract We propose a method for fluidic thrust vectoring by studying the effect of excited secondary Coanda flow on the direction of jets. The primary flow is the steady continuous jet, while the the secondary flow is synthetic jet or continuous flow. In this experiment, speaker is used to adjusted the frequency and velocity amplitude of synthetic jets, while a blower is used to adjust the continuous jet and suction flow.We visualize and compare the primary flow under the influence of various secondary flows. Additionally, computational fluid dynamics is used to investigate the flow characteristics, including the deflection angle. The main results of this study is that both synthetic jets and continuous suction flow are capable of deflection.

Taylor-Aris methodology for the experimental determination of molecular diffusion coefficients: Tutorial with focus on large biomolecules

High-Performance Liquid Chromatography (HPLC) is a key technique in the evaluation of biopharmaceuticals. To improve the separation of biopharmaceuticals, it is crucial to improve the fundamental understanding of the parameters governing their band broadening behavior. This can be obtained by a detailed assessment of the individual contributions to their mass transfer. For this purpose, a precise knowledge of the molecular diffusion coefficient (Dm) of biopharmaceuticals is required. Only little experimental data is available for the Dm-values of biopharmaceuticals under HPLC relevant conditions. Furthermore, none of the available equations that can be used to calculate Dm-values, allows to account for any conformational changes that might occur.The Taylor-Aris method is a very simple and absolute method that is often employed to determine Dm-coefficients. The Taylor-Aris method measures the band broadening of an analyte in an open tube under laminar conditions, wherein (1) longitudinal diffusion can be ignored, (2) the sample is fully radially equilibrated and (3) the contribution of the extra-column variance to the total variance is negligible. Moreover, since the open tubes are typically coiled for practical reasons, (4) the influence of secondary flows on the band broadening should be insignificant.In this tutorial paper, the impact of the four conditions mentioned above on the accuracy of the obtained Dm values is revisited. For this purpose, Dm values are measured for two representative compounds (Bovine Serum Albumin and Thiourea), and the obtained values are compared with literature data and theoretical recommendations. Based on these observations, a set of ‘rules’ for accurate and fast Dm measurements is put forward. Finally, an Interactive Tool (IT), combining these rules in a comprehensive way, is introduced and can be used to set up TA experiments.

A SAR Micromixer for Water-Water Mixing: Design, Optimization, and Analysis

A numerical investigation of the mixing performance and fluid flow in a new split and recombine (SAR) Y−Uβ micromixer is presented in this work. A parameter called connecting angle βis varied from 0° to 90° to analyze the effect on the SAR process and mixing performance. Thenumerical data shows that the SAR process strongly depends on the connecting angle (β) and maximum efficiency (93%) can be achieved when the value of β is 45°. The Y−U45° the mixer also offers higher efficiency and lower pressure drop than a known SAR ‘H−C’ mixer irrespective of Reynolds numbers. The split and recombine process, the influence of secondary flow, and pressure drop characteristics at various Reynolds numbers are also studied. In addition, mixing effectiveness is also computed, and among all examined mixers, Y−U45° is by far the best performing one.

Influence of secondary flow on the thermal performance of exhaust gas recirculation (EGR) coolers

Soot particulate matter deposition on the tubes of exhaust gas recirculation (EGR) coolers would form an insulating deposit layer, which deteriorates the thermal efficiency of the cooler and increases the pressure drop substantially. The objective of this research is to examine the influence of using helical tubes instead of straight horizontal tubes on the development of deposit layer. An experimental setup has been designed and constructed to examine the thermal performance of different types of EGR coolers under both clean and fouling conditions. Two sets of experiments have been performed, i.e. with and without soot particles, and in each set three types of EGR coolers have been examined, namely straight tube cooler, and two helical tube coolers of 5 and 10 windings. The effectiveness of the helical tube cooler is highest in case of 10 windings and lowest in case of a straight tube cooler which is due to the increase in the heat transfer surface area. The thermal resistance of the fouling layer in case of a straight tube cooler and 10 windings cooler has stabilized, however the thermal resistance in case of 5 windings is not stable and it is increasing with time. It has been concluded that there is a cleaning action during the fouling experiment in case of the helical tube coolers, and that is due to the secondary flow, and the efficiency of cleaning is a function of the intensity of the secondary flow. The thermal performance of the 5 windings cooler is not stable due to the development of the fouling layers, while in case of the 10 windings is stable, and that means there is a critical number of windings for the helical tube EGR coolers above which fouling diminishes. This critical number of windings is a function of the gas speed, particles size, tube diameter, radius of curvature and the winding pitch. The secondary flow in helical tubes agitates the boundary layer and decreases the pressure drop, such that it is advised to use helical tube coolers with more windings from a thermal and a fluid mechanics point of view.

Study on performance of electrostatic precipitator under multi-physics coupling.

A wire-plate electrostatic precipitator (ESP) is developed to analyze the particle transport characteristics and the influence of various factors on the performance of ESP. Above all, an experimental device is built to measure the current density distribution of the plates and obtain good consistency with the numerical simulation results, taking the ESP model established by COMSOL/Multiphysics as the numerical simulating object. Firstly, the electric field is solved by finite element method(FEM) to obtain the potential and charge density distribution. Then, the influence of secondary flow on the main flow at different flow velocities is explored. Finally, multi-physics coupling calculations show the influence of dust particle properties, electrode configuration, and operating conditions on ESP performance. The study found that the particle diameter is positively correlated with its charge, force, and motion, and the relative permittivity of the particles affects the collecting efficiency by affecting its charge difficulty. The wire-to-wire spacing is not proportional to collecting efficiency, when the spacing is 80 mm, the efficiency and the corona current can be maximized. Average electric field strength, corona current density, and current density distribution standard deviation satisfy the cubic function relationship. In addition, the effect of airflow velocity on collecting efficiency and particle precipitation is revealed. It provides a valuable basis for design and performance optimization of ESP.

The method of averaging the gas flow parameters in turbomachines to evaluate their efficiency considering the velocity-field helicity

The methodology of averaging the nonuniform swirling flow in turbomachines has been developed. It is based on preserving the mass flow rate, the fluxes of total enthalpy and momentum (or entropy) with the addition of new invariant into the algorithm, i.e., the integral of the gas flow helicity. The dependence has been obtained and validated to specify the turbine stage efficiency calculation and to take into account the presence of large-scale vortex structures in the inter-blade channel of the turbomachine (the uncertainty of the influence of secondary flows on the value of losses does not exceed 0.2%). The methods of averaging the gas flow parameters, taking into account the contribution of secondary flows to the kinetic energy loss are presented. They show a 2.4% lower efficiency compared to the calculation previously used for the turbine stage of the Leningrad Polytechnic Institute.

Numerical Simulation of Transport and Deposition of Dust Particles in Human Tracheobronchial Airways

Dust is a common pollutant of the air we breath. If dust particles are inhaled and deposited in human airways, they can cause a variety of respiratory disorders. The inhaled dust particles motion in human airways goes along with the airflow. The transport process can be considered as a two-phase flow of a gas phase and a particle phase. In this study, we investigated the airflow and dust particles transport and deposition in human tracheobronchial airways using computational fluid dynamics (CFD) techniques. A steady simulation was performed in asymmetric tracheobronchial airway mode consisting of 19 outlets to observe the characteristics of airflow fields. The discrete phase model (DPM) was employed to predict the particle trajectories and deposition in the airway model. Deposition resulted from inertial impaction and gravitational sedimentation was considered. In the simulation, the airflow characteristics differences in the right and left bronchial trees were observed. The influence of secondary flow on dust particles motion was great. More dust particles were deposited in the right bronchial tree than in the left. The deposition fraction of dust particles in human tracheobronchial airways was high. This study can provide awareness on deposition of dust particles passing beyond the larynx and enhance prevention of their entry into the respiratory system. It can also contribute a convenient way on the location of deposition of particles of a given type in human respiratory tract to be used for respiratory disease preventions.

Influence of secondary flows on heat transfer from a localized heat source

Heat transfer from a localized heat source in a cylindrical fluid layer was studied experimentally. Experiments were performed for fluids with different values of the Prandtl number and for the Rayleigh number from 105 to 107. Particular attention has been paid to investigating the influence of small-scale convective structures in the boundary layer of a basic flow on heat transfer. The dependence of the Nusselt number on the Rayleigh number was obtained by means of temperature measurements. In addition, the mean velocity of convergent flow was estimated. It has been found that changes in the structure and intensity of secondary flows have a weak effect on heat transfer. At the same time, when the Rayleigh number increases, the basic flow velocity exhibits a linear growth.

The influence of secondary flow in a two-phase gas-solid system in straight channels with a non-circular cross-section

The paper considers two-phase gas-solid turbulent flow of pneumatic transport in straight horizontal channels with a non-circular cross-section. During turbulent flow, a specific flow phenomenon, known as secondary flow, occurs in these channels in the cross-sectional plane. The existence of strong temperature gradients in the cross-sectional plane of the channel or the cases of curved channels result in the appearance of the secondary flow of the first kind. However, in straight channels with a non-circular cross-section, in the developed turbulent flow mode, a secondary flow, known as Prandtl?s secondary flow of the second kind, is induced. The paper presents a numerical simulation of a developed two-phase turbulent flow by using the PHOENICS 3.3.1 software package. Reynolds stress model was used to model the turbulence. The paper provides the data on the changes in turbulent stresses in the channel cross-section as well as the velocities of solid particles transported along the channel.

Influence of secondary flows on the collection efficiency of a cylindrical electrostatic precipitator

Submicron bioaerosols collection with an axisymmetric wire/cylinder electrostatic collector was modelled in the laminar regime. The model was based on a particle population balance. In addition to the five dimensionless numbers identified in a previous paper, an extra dimensionless number which is the ratio of the electric charge scale acquired by convection to the total charge scale acquired by the particle was also revealed.The influence of recirculation characterising various flow regimes as shown in the aforementioned paper was evaluated. These severely disturb the spatial distribution of the particles. However, a comparison of the collection efficiency in these regimes showed that collection efficiencies do not depend on recirculation with high hydrodynamic Reynolds numbers and suffer an adverse effect below about 200. A critical analysis of a simplified criterion based on the Deutsch number for sizing wire/cylinder electrostatic precipitators was also performed.

Aerodynamics of long fibres settling in air at 10<Re<100

Abstract The aerodynamics of long aspect ratio nylon fibrous particles has been investigated experimentally whilst settling in air under super dilute conditions without any influence of secondary flows and at fibre Reynolds numbers of 10–100 based on fibre length. Measurement of the orientations and velocities of fibrous particles is undertaken by two-dimensional Particle Tracking Velocimetry (PTV), based on the two end-points. A statistical evaluation of fibres' mean vertical and horizontal components of settling velocities, angular velocity, orientation, number density is presented and used to assess particle aerodynamics.

PTV measurement of drag coefficient of fibrous particles with large aspect ratio

Abstract The aerodynamic behaviour of long aspect ratio nylon fibrous particles has been investigated experimentally while settling in air under super dilute conditions without any influence of secondary flows and at fibre Reynolds numbers of 0.5–2 based on fibre diameter. A method for laser-based measurement of the orientations and velocities of fibrous particles is also presented. The experimental apparatus employs a two-dimensional Particle Tracking Velocimetry (PTV) to calculate orientation and velocity based on the two end-points. The controlling length scale in the relationship between Reynolds number and drag coefficient was investigated and the equivalent diameter of settling fibre in air was reported. Finally the influence of volume fraction and fibre straightness were assessed.

Numerical simulation of the effects of secondary flow and boundary retention on contaminant dispersion

The secondary flow and dispersion of a tracer substance in a fluid flowing through a curved tube are studied numerically. The system consists of an infinitely long conduit defined by two concentric curved circular pipes. Thus the two phases correspond to a flowing fluid phase and the annular wall comprised of a stationary homogenous medium. The solute is soluble in the annular region and is assumed to satisfy a linear equilibrium relationship at the interface. Flow in the fluid phase is calculated using the finite difference scheme as described by Collins and Dennis [9]. The scheme is of second order accuracy with respect to grid sizes over the entire laminar flow region and reveals the asymptotic nature of the solution for large values of Dean number D. Further, a central difference scheme is used to solve the convection diffusion equation in both the phases with the condition that the injective solute is highly soluble in the wall layer according to a linear equilibrium relationship at the interface. Results for small Dean number are first verified with our earlier results (Jayaraman et al. [17]) which were based on Dean's solution [12,13]. The numerical calculations are then extended for larger Dean number and Schmidt number. The results are consistent with experimental results that the influence of secondary flows on dispersion is reduced if the tracer is very soluble in the wall. It is found that the effective longitudinal diffusivity falls below its straight tube value by an amount, which depends on the absorption coefficient, Dean number and the diffusivity in the wall. For large Dean number dispersion becomes negligible.

Numerical solution of 2D and 3D backward facing step flows

AbstractThe work deals with numerical modelling of flow through 2‐dimensional (2D) and 3‐dimensional (3D) backward facing step. In laminar case, we apply several higher order upwind and central discretizations and compare numerical results with measurements. The turbulent regime is considered in 2D as well as in 3D and influence of secondary flow is observed. Different modifications of low‐Re two equation turbulence models and an explicit algebraic Reynolds stress model (EARSM) are considered. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of secondary flow on diffusion with heterogeneous reaction

The concentration distribution in a Newtonian fluid for the secondary flow in a curved tube with bulk/wall reaction is obtained for a range of parameter values using an axially marching spectral finite difference scheme. The results predominantly illustrate that for fixed extent of secondary flows there is an improvement in the performance of the reactor as the effect of bulk and wall reaction mechanisms intensify in the system. For the same magnitude (moderate effects) of bulk and wall reaction mechanisms, there appears to be an increased conversion/performance in the homogeneous case. For small values of the reaction parameters, the improvement in the performance (3%) is more due to the wall reaction than bulk reaction. But, for moderate values of the non-dimensional reaction parameters (around 10), the enhanced performance (28%) is more due to the bulk reaction than wall reaction. As the effects of secondary flow dominate, there is an improvement in the performance of the reactor irrespective of the reactivity of the solute and the results agree with those of Mashelkar and Venkatasubramanian [AIChE J. 313 (1985) 440] for the case of non-absorbing walls. It is also observed that the approach of isoconcentration profile to the stream line pattern occurs at a lower value of the curvature parameter in the presence of wall reaction. The study is extended to examine the modifications in the tracer response measurements due to the power law rheology of the flowing fluid. The salient observation, irrespective of the reactivity of the solute, is that the performance of the reactor is improved in the case of pseudoplastic fluids compared to dilatant fluids. This improvement is further enhanced due to the presence of mild wall reaction.

Turbulent Flow and Endwall Heat Transfer Analysis in a 90 degree Turning Duct and Comparisons with Measured Data Part II: Influence of Secondary Flow, Vorticity, Turbulent Kinetic Energy, and Thermal Boundary Conditions on Endwall Heat Transfer

Turbulent Flow and Endwall Heat Transfer Analysis in a 90 degree Turning Duct and Comparisons with Measured Data Part II: Influence of Secondary Flow, Vorticity, Turbulent Kinetic Energy, and Thermal Boundary Conditions on Endwall Heat Transfer