Internal Flow Configurations Research Articles

A Study and Optimization of the Unsteady Flow Characteristics in the Last Stage Impeller of a Small-Scale Multi-Stage Hydraulic Turbine

The demand for small-size multi-stage hydraulic turbines is experiencing rapid growth due to the ongoing efforts towards energy conservation and emission reduction. On account of their compact structural design, these turbines feature a more intricate internal flow configuration, rendering them prone to the creation of low-pressure zones, resulting in vapor–liquid two-phase flow, accompanied by the development of intense vibrations and noise, thereby adversely affecting the safety and stability of turbine operations. Concurrently, an innovative method for analyzing flow fields has been formulated combined with two-dimensional frequency domain visualization technology and proper orthogonal decomposition, serving to establish a diagnostic and optimization framework for the unsteady flow structures within rotating machinery by considering the features related to frequency distribution, spatial distribution, and energy contributions. It was found that there are two main unsteady flow structures which are the areas with high risks of vaporization under this study condition. According to the flow characteristics of the analysis, an optimization scheme was proposed to improve the two-phase flow problem in the secondary impeller, and the preliminary results were satisfactory.

Internal Flow Characteristics of Novel Turbine Performance Enhancement Method through Specific Fluid Injection

Gas turbine engines are an essential component for many industries, such as aerospace, marine propulsion, energy generation, etc., with modern engines capable of achieving high powers, efficiencies and reliability. However, these high performances are achieved on a narrow interval of working regimes; when operating at partial loads, a drastic decrease in performance is expected. In order to mitigate this drawback, a novel injection method has been proposed to improve axial turbine and gas turbine engine performance at these regimes. The method consists of the injection of a fluid in specific sections of turbine vanes to accelerate the flow, modify the velocity triangles and increase the generated power at partial loads. In this paper, the authors discuss the internal flow characteristics of the injection channels using numerical studies to determine the flow fields for different working regimes. The results show that the power generated by the rotor can be improved by 10% to 21% for different operating regimes without considering the internal geometries. The introduction of internal flow configurations led to smaller improvements in power generation, obtaining injection system pressure losses of between 10% and 20%. The paper concludes that the flow through channels is not uniform, with smaller dimensions of the internal geometries leading to higher pressures and an increased influence of the injection system.

Modeling of soft fluidic actuators using fluid–structure interaction simulations with underwater applications

Soft robots have been developed for a variety of applications including gripping, locomotion, wearables and medical devices. For the majority of soft robots, actuation is performed using pneumatics or hydraulics. Many previous works have addressed the modeling of these fluid-driven soft robots using static finite element simulations where the pressure inside the actuator is assumed to be constant and uniform. The assumption of constant internal pressure is a useful simplification but introduces significant errors during events such as pressurization, depressurization, and transient loads from a liquid environment. Applications that use soft actuators for locomotion or propulsion operate using a sequence of transient events, so accurate simulation of these events is critical to optimizing performance. To improve the simulation of soft fluidic actuators and enable the modeling of both internal and external fluid flow in underwater applications, this work describes a fully-coupled, three-dimensional fluid–structure interaction simulation approach, where the pressure and flow dynamics are explicitly solved. This approach provides a realistic simulation of soft actuators in fluid environments, and permits the optimization of transient responses, which may be due to a combination of environmental fluid loads and non-uniform pressurization. The proposed methods are demonstrated in a number of case studies and experiments for a range of actuation and both internal and external inlet flow configurations, including bending actuators, a soft robotic fish fin for propulsion, and experimental results of a bending actuator in a high-speed fluid, which correlate closely with simulations. The proposed approach is expected to assist in the design, modeling, and optimization of bioinspired soft robots in underwater applications.

Analytical study of fluid flow and thermal characteristics of flow in micro channel heat sinks with working fluid as water and alumina & water nanofluid

The electronic industry is looking at microchannel based MEMS technology to resolve the issue of increasing heat dissipation from electronic circuits. Micro-channels play an important role in development of small scale fluid flow devices. The internal flow configuration represents a convenient geometry for heat transfer between fluids used in chemical processing, environmental control, and energy conversion technologies. In order to fabricate such micro devices effectively, it is extremely important to understand the fundamental mechanisms involved in fluid flow and heat transfer characteristics in microchannels. This paper presents an analytical study of flow mechanism in microchannels, with a focus on effect of geometric dimensions and flow rate on the various flow parameters of a Rectangular microchannel. An attempt is also made to determine optimum microchannel dimensions based on the analysis. Basic flow equations are used for modelling the flow and determining various parameters. Further, the study analyses the effect of using Alumina & Water nanofluid on flow and thermal characteristics as compared to using water alone. The results observed show a deeper and wider microchannel would keep the pressure drop across the microchannel lower. Also using nanofluid as working fluid would result in lower pressure drop.

High-temperature thermochemical energy storage – heat transfer enhancements within reaction bed

Ca(OH)2/CaO reversible reaction system has high potentials to be used for high-temperature thermal energy storage. Endothermic dehydration of Ca(OH)2 and exothermic hydration of CaO can be carried out at high temperatures compatible with most of the concentrated solar power applications. One of the challenges in using Ca(OH)2/CaO is the slow rate of heat transfer inside the solid reaction beds. Heat transfer enhancement techniques are introduced in a circular fixed bed heated indirectly through heat exchanger walls. Two reactor configurations are studied: one in which the heat transfer fluid (HTF) flows through an outer annular shell and another in which the HTF flows through a central pipe. Dehydration and rehydration processes are simulated with and without fins attached to the heat exchanger wall extending into the reaction bed. Heat and mass transport equations coupled with reaction kinetics are solved numerically. The designed heat transfer enhancements (HTE) within the reaction beds resulted in a significant decrease in overall conversion time during the energy storage and discharge. It is shown that the reactor with an outer annular shell as the HTF channel has better performance compared to the reactor with internal pipe flow configuration for HTF.

Coupled external and internal flow simulation of a Liquid Fuelled Ramjet vehicle

For finalization of mission parameters of LFRJ-Technology Demonstrator vehicle with a belly-mounted twin intake configuration, CFD has been used extensively to obtain aerodynamic characteristics of the cruise vehicle as well as CV with booster. As the flow field is highly complex due to the asymmetric intake-body configuration, conventional engineering methods are inadequate and detailed CFD based methods have been introduced at early stage of vehicle design cycle. A coupled non-reacting external and internal flow through intakes, combustor and nozzle are solved at different flight conditions (Mach 1.8 to 2.5 and altitude 0.7 to 5 km at AOA=−5° to 5°) to obtain aerodynamic parameters. 3D RANS equations are solved along with SST k-ω turbulence model. Effects of grid and turbulence models are also studied. Utilizing CFD data, the external and internal flow configuration have been progressively evolved. CFD based methods have helped in generating aerodynamic characteristics of the vehicle which were very useful for mission study.

Numerical investigation of turbulent swirling flows in axisymmetric internal flow configurations

Several types of axisymmetric internal configurations were numerically simulated in order to investigate the development of turbulent swirling and non-swirling flows along the axial direction. The governing equations were the Navier–Stokes equations with swirl velocity, requiring the computation of all three velocity components. The turbulence model used in the present work was the quadratic k-ω formulation. Applying the above methods, both swirling and non-swirling flows in a pipe, in a typical Venturi tube and in an orifice were simulated and the results were analyzed in detail.

An Experimental/Numerical Investigation on Drag Reduction by Dimples

AbstractThe paper is concerned with experimental and numerical investigations of the turbulent flow over dimpled surfaces. Shallow dimples distributed regularly over the wall of a plane channel with large aspect ratio are used to study their effect on the skin‐friction drag. The resulting pressure drop in the channel was measured for smooth and dimpled walls. In addition to these investigations on internal flows, an external flow study was performed and boundary‐layer profiles were measured using a Pitot‐tube rake. Complementary to the measurements, direct numerical simulations for the internal flow configuration with and without dimples were carried out for two different grid resolutions and analyzed in detail. The objective was to clarify whether or not dimples cause reduction of the skin‐friction drag. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental analysis of internal gas flow configurations for a polymer electrolyte membrane fuel cell stack

The internal gas distribution system utilised for supplying fresh reactants and removing reaction products from the individual cells of a fuel cell stack can be designed in a parallel, a serial or a mixture of parallel and serial gas flow configuration. In order to investigate the interdependence between the internal stack gas distribution configuration and single cell as well as overall stack performance, a small laboratory-scale fuel cell stack consisting of identical unit cells was subject to operation with different gas distribution configurations and different operating parameters. The current/voltage characteristics measured with the different gas distribution configurations are analysed and compared on unit cell- as well as on stack-level. The results show the significant impact of the internal stack gas distribution system on operation and performance of the individual unit cells and the overall stack.

Drag reduction by dimples? – A complementary experimental/numerical investigation

The paper is concerned with an experimental and numerical investigation of the turbulent flow over dimpled surfaces. Shallow dimples distributed regularly over the wall of a plane channel with large aspect ratio are used to study their effect on the friction drag. The resulting pressure drop in the channel was measured for smooth and dimpled walls. In addition to these investigations on internal flows, an external flow study was performed and boundary-layer profiles were measured using a Pitot-tube rake. Complementary to the measurements, direct numerical simulations for the internal flow configuration with and without dimples were carried out for two different grid resolutions and analyzed in detail. The objective was to clarify whether or not dimples cause reduction of the skin-friction drag.

A combined time and frequency domain approach for acoustic resonance prediction

A general hybrid time- and frequency-domain methodology has been developed to identify acoustic resonance conditions of internal flow configurations. The acoustic modes are determined by imposing onto the flow a time-dependent excitation at several locations on the boundary. The resulting time-domain pressure responses, which are computed via an unsteady Favre–Reynolds averaged Navier–Stokes solver, are used to determine the frequency response function matrix of the fluid which can be considered to be a multiple-input multiple-output system. The main test case was selected to be a closed-end cylindrical duct for which the effect of different excitation techniques on the predicted acoustic modes is discussed in detail. The last test case deals with the acoustic characterization of a 2-D channel with symmetric bumps and an inlet flow velocity of 17 m s - 1 . It is shown that the methodology was suitable for identifying axial and transverse acoustic modes up to 3 kHz.

Short-scale break-up in unsteady interactive layers: local development of normal pressure gradients and vortex wind-up

Following the finite-time collapse of an unsteady interacting boundary layer (step 1), shortened length and time scales are examined here in the near-wall dynamics of transitional-turbulent boundary layers or during dynamic stall. The next two steps are described, in which (step 2) normal pressure gradients come into operation along with a continuing nonlinear critical-layer jump and then (step 3) vortex formation is induced typically. Normal pressure gradients enter in at least two ways, depending on the internal or external flow configuration. This yields for certain internal flows an extended KdV equation with an extra nonlinear integral contribution multiplied by a coefficient which is proportional to the normal rate of change of curvature of the velocity profile locally and whose sign turns out to be crucial. Positive values of the coefficient lead to a further finite-time singularity, while negative values produce a rapid secondary instability phenomenon. Zero values in contrast allow an interplay between solitary waves and wave packets to emerge at large scaled times, this interplay eventually returning the flow to its original, longer, interactive, boundary-layer scales but now coupled with multiple shorter-scale Euler regions. In external or quasi-external flows more generally an extended Benjamin–Ono equation holds instead, leading to a reversal in the roles of positive and negative values of the coefficient. The next step, 3, typically involves the strong wind-up of a local vortex, leading on to explosion or implosion of the vortex. Further discussion is also presented, including the three-dimensional setting, the computational implications, and experimental links.

Rotational compressible inverse design method for two-dimensional, internal flow configurations

The development of a rotational inviscid compressible inverse design method for two-dimensional internal flow configurations is described. Rotationality is due to incoming entropy gradient whereas total enthalpy is considered to be constant throughout the flowfield. The method is based on the potential function/stream function formulation. The Clebsch formulation is adopted to decompose the velocity vector into a potential and a rotational part. The physical space on which the boundaries of the flowfield are sought is mapped onto the (0, V7) space via a body-fitted coordinate transformation. A novel procedure based on differential geometry arguments is employed to derive the governing equation for the velocity. The velocity equation solved in conjunction with a transport equation for a thermal drift function provide the flowfield without any geometry feedback. An auxiliary orthogonal computational grid adapted to the solution is employed. Geometry is determined by integrating Frenet equations of the grid lines. Inverse calculation results are compared with results of direct reproduction calculations.

Grid generation for internal flow configurations

This paper is concerned with analysis and applications of geometry-grid generation methodologies for internal flow configurations. Weighted transfinite interpolation technique is combined with cubic splines and Bezier curves to accomplish well-distributed zonal grids. The concept of surface distribution mesh and volume distribution mesh is presented and its natural application to adaptive grids is developed. Grid refinement algorithms are developed using applications of elliptic grid systems. Various computational examples of practical interest are presented.

A Method of “Separation of Variables” for the Solution of Laminar Boundary-Layer Equations of Narrow-Channel Flows

A method of solution for laminar channel flow is established using the von Mises transformation followed by the “separation” of the main flow velocity into amplitude and profile functions. The known boundary conditions on the latter enable a generalized solution that yields, through a parametric relation between auxiliary parameters (the characteristics), a system of ordinary differential equations for the velocity amplitude and the pressure. The result is a powerful semi-analytical method which is very easy to implement for a variety of internal flow configurations. The method is, in essence, a downstream general solution which may be extended upstream to the singular limit of a uniform main flow velocity distribution at the channel entrance. The regular solution for uniform initial velocity distribution cannot thus be obtained. Comparisons with other solutions show only qualitative agreement, for reasons which are discussed, whereas agreement with experimental results, made in a separate publication, is remarkably good.

Note on the cancellation of contaminating noise in the measurement of turbulent wall pressure fluctuations

A method is presented for the cancellation of wide band contaminating noise occurring within internal flow configurations such as rectangular channels and pipes. Facility generated noise within these flow systems contaminates the turbulent wall pressure signature at low frequencies thus preventing the possible extraction of useful information. The proposed methodology utilizes the signals from two flush mounted wall pressure transducers. A first estimate for the one-point spectral density is obtained using a least mean square algorithm. A secondary correction to this estimate is obtained by taking advantage of the planar homogeneity of the turbulence. The application of the technique is demonstrated in a fully developed turbulent channel flow for which a more than 40 dB cancellation is obtained at low frequencies. In this low frequency range, the power spectral density is shown to have an approximate quadratic dependence, substantiating past theoretical predictions reported in the literature.

Locally analytic numerical method and application to viscous internal and external flows

A locally analytic numerical method is developed to predict the two-dimensional internal and external steady laminar flow of an incompressible viscous fluid. In this method, analytic solutions of locally linearized partial differential equations are incorporated into the numerical solution. This is accomplished by dividing the flow field into computational grid elements. In each individual element, the nonlinear convective terms of the Navier-Stokes equations are locally linearized, with analytic solutions then determined. The solution for the complete flow field is obtained by the assembly of these locally analytic solutions. The nonlinear character of the complete flow field is preserved as the flow is only locally linearized, i.e. independently linearized solutions are obtained in individual grid elements. This locally analytic numerical solution method is used to analyze the viscous flow in several internal and external flow configurations, with the prediction of flow development, reversal, separation, and reattachment demonstrated over a range of moderate values of the Reynolds number. In particular, three internal and one external flow configurations are investigated, with predictions obtained for entrance flow development in a straight channel, the flow through a sudden expansion, i.e. over a backward step, the flow in a diffuser, and the flow past a flat plate airfoil over a range of mean flow incidence angle values.

Study of Fully Developed Incompressible Flow in Curved Ducts, Using a Multi-Grid Technique

Fully developed flows inside curved ducts of rectangular as well as polar cross sections have been analyzed using the Navier-Stokes equations in terms of the axial velocity and vorticity and the cross-flow stream function. Numerical solutions of the three second-order coupled elliptic partial differential equations governing this flow have been obtained using efficient numerical schemes. For curved-duct flows, the similarity parameter of significance is the Dean number K, rather than the Reynolds number Re. Results have been obtained for curved ducts with square cross sections for K up to 900 which, in the present study, corresponds to Re = 9,000 for this internal flow configuration. The fine-grid calculations show that, for square cross-section ducts, Dean’s instability occurs at K ≈ 125 and, further, that this phenomenon does not disappear even for K = 900. In ducts of polar cross sections, which are geometrically more representative of turbomachinery cascade passages, the phenomenon of Dean’s instability is not seen to occur for K up to 600.

A Schwarz-Christoffel Method for Generating Two-Dimensional Flow Grids

A new coordinate generation technique, developed by Davis for external flows, is extended to allow for accurate grid generation for a variety of complex internal flow configurations. The approach is based on numerical integration of the Schwarz-Christoffel transformation for polygonal surfaces. It is shown to be second-order accurate with mesh size due to analytic treatment of boundary singularities. The method is flexible enough to allow for treatment of severe internal geometries, for a high degree of control of mesh spacing, and for generation of either orthogonal or nonorthogonal grids. In addition, this technique directly provides the two-dimensional incompressible potential flow solution for internal flows, as well as a simple expression for calculating the grid metric coefficients. Sample cases include symmetric and asymmetric channel, diffuser, and cascade flows.

Thermal convection and longitudinal macrosegregation in horizontal Bridgman crystal growth: I. Order of magnitude analysis

A wide class of new fluid phase transport problems has been raised as a consequence of the availability of microgravity conditions for solidification experiments. In this paper, we propose a parametric analysis of the influence of thermal buoyancy driven convection on the longitudinal macrosegregation of a solute during the solidification of a doped metal or semiconductor, in a simplified horizontal Bridgman crystal growth configuration. This study is performed according to the two following steps: (1) from an order of magnitude (OM) analysis of the general stationary transport equations, the extent δ̃ of the solute boundary layer in front of the growth interface is defined as a function of a proper set of non-dimensional numbers formed from the data of the problem; (2) the estimated δ̃ is introduced into a simple one-dimensional boundary layer model for non-stationary solute transport, from which the different macrosegregation situations are derived. Our analysis involves a ratio l s (flow length scale over cell dimension) which may be somewhat smaller that one, due to the internal flow configuration considered here. The precise determination of l s and the conversion of our OM results to quantitative practical laws are discussed in Part II.