Flow Field Behavior Research Articles

Experimental study of boiling heat transfer in a microchannel with nucleated-shape columnar micro-pin-fins

The boiling regime and flow field patters of boiling heat transfer of FC-72 dielectric fluid in a microchannel are experimentally studied. The inner surface of the test section of the microchannel is covered with nucleated columnar fins of height 100 μm. Each fin is nucleated with the core diameter of 60 μm and opening of 45 μm, which the fin openings is aligned opposite to the flow direction. The experiments are performed for two saturation temperatures of Tsat = 35 °C and Tsat = 50 °C. The heat flux and mass velocity are in the range of 0.67–4.5 W/cm2, and 94–138 kg/m2 s, respectively. The results are reported for the on boiling temperature difference and pressure drop. The flow-field behavior of formation of bubbles at the nucleated fins, single-pore nucleation, and boiling flow regime are addressed. The results reveal that the increase in the mass velocity increases the subcooled temperature difference. The flow-behavior images in the test-section show that cell flow, block flow, droplet/block flow, droplet flow, and annular flow patterns can be formed by the increase of the surface heat flux. The on boiling phase change induces about three-fold of pressure-drop in the microchannel. Besides, the rise of the working fluid saturation temperature raises the pressure-drop.

Entropy generation and exergy destruction in condensing steam flow through turbine blade with surface roughness

In the steam turbine, the wetness loss due to vapor condensation is one of the most crucial losses at low-pressure stage. This study focused on entropy generation and exergy destruction of condensing steam flow in turbine blade with the roughness. The governing equations including entropy transport equation combined with condensation model, transition SST model and roughness correlation were established and verified by experiments and theory. Flow field behaviors, such as wetness fraction, intermittency and turbulent viscosity distributions, controlled by the deviation angle were obtained to evaluate effects of back pressure ratio and surface roughness. The mass-averaged wetness fraction at outlet was also extracted considering the influence of uneven mass flux. Finally, each part of entropy generation derived from viscous, heat conduction, phase change and aerodynamic losses and exergy destruction ratio were analyzed. Research shows that roughness plays an important part in the intermittency and turbulent viscosity. The mass-averaged wetness fraction at outlet sharply drops with back pressure ratio but slightly decreases with the roughness. With the roughness rising or back pressure dropping, the entropy generation grows resulting in more exergy destruction. The maximum value of the total entropy generation is 84.520 J·kg−1·K−1, corresponding exergy destruction is 25.187 kJ·kg−1 and exergy destruction ratio is 4.43%.

Numerical Investigation of Flow Field Behaviour and Pressure Fluctuations within an Axial Flow Pump under Transient Flow Pattern Based on CFD Analysis Method

In this present work, CFD numerical method is applied to analyses the flow field in an axial flow pump qualitative and quantitative analyses. Qualitative analysis for these parameters comprise static pressure variations, dynamic pressure variations, velocity magnitude, turbulent kinetic energy, shear stress. Quantitative analysis including the pressure fluctuations in frequency domain analysis under different operation conditions. Also, sliding mesh method and turbulence model type k-epsilon are used. Various monitoring points are stalled in order to analyses pressure fluctuation mechanism in the impeller blade. The numerical results revealed that the flow field for pressure and velocity are increase start from the suction side of the pump to discharge side. Also, the results found that the high pressure occurs at the discharge side along the axial direction of the impeller. The maximum value of pressure fluctuations is occurred at tip blade region due to high interaction flow at this particular area. Moreover, the pressure decreases as flow rate in the pump increases. Additionally, the results shown that the pressure fluctuations have four peaks and four valleys the similar impeller blades number. Furthermore, there are different positive and negative pressure regions, the negative pressure area occurs due to lower pressure zone at inlet impeller area and hence which can lead to cause occurrence of cavitation in this specific area. The current numerical demonstration results can help the researches for further axial flow pump design.

Navier’s Slip Effect on Mixed Convection Flow of Non-Newtonian Nanofluid: Buongiorno’s Model with Passive Control Approach

This article is devoted to the study of Navier’s slip effect on MHD mixed convection boundary layer flow of non-Newtonian nanofluid over a stretching surface. The flow is induced due to the continuously stretching surface. The mathematical modeling of the flow situation, is governed by Buongiorno’s nanofluid model with passively controlled nanoparticle boundary condition. To analyze the flow field behavior, the similarity transformation approach is adopted. The approximate similar solution of the problem is obtained using finite element method. Effects of active flow parameters on the quantities of engineering interests viz. skin friction coefficient, Nusselt number and flow variables viz velocity, temperature and nanoparticle concentration, are presented graphically. The Numerical results of the present investigation are validated with earlier published results. The findings suggest that the velocity slip plays a vital role in skin friction coefficient and Nusselt number of non-Newtonian nanofluid. It has been observed that the Brownian diffusion does not have very promising impact on Nusselt number as compared to results obtained in previous studies with active control of wall nanoparticles. The potential application of present investigation are in various industrial manufacturing processes such as cooling and/or drying of textile and paper, rolling sheet drawn from a die, manufacturing of crystalline materials, polymeric sheets, glass sheets, etc.

CFD modeling of liquid film reversal of two-phase flow in vertical pipes

The phenomenon of two-phase flows is characterized by its wide range of presence in nature and industrial applications. In gas and oil production, the simultaneous two-phase flow of gases and liquids often occurs in gas wells. The produced gases flow rate decreases over the years until reaching a critical flow rate when they lose their ability to lift the associated liquids upward and the liquid film reverses direction, which triggers liquid loading. Liquid loading causes the produced liquids to accumulate in the bottom of the wellbore, which causes a high back pressure that reduces the well production rate till production is ceased eventually. The critical gas velocity exists in the churn flow regime which is mainly characterized by the oscillatory behavior of the liquids flow field. This study employs CFD techniques to model the churn flow in a 3-inch-diameter vertical pipe near the critical gas flow rate for different liquid flow rates. This work utilizes the two-fluid Eulerian model along with the RNG (Re-Normalization Group) k-ε turbulence model to investigate the behavior of the flow field in a two-dimensional axisymmetric computational domain. Simulations were carried out using the commercial software ANSYS Fluent 18.2 with air and water as the two working fluids. The model results showed a good agreement with the experimental data and proved the mesh and time independence of the model. Oscillatory behaviors of the liquid film flow rate, shear stress, and pressure were observed along with the formation of interfacial waves. Detailed information about the velocity, shear stress, and pressure behaviors is presented. Accordingly, recommendations are suggested for future considerations.

Pulsatile flow in a constricted channel: flow behaviour and equilibrium states

Results are reported from linear and nonlinear stability analyses of pulsatile flow in a straight channel with a smooth constriction with 50 % occlusion. Physical mechanisms based on the energy variation of the two-dimensional modes are proposed to elucidate the equilibrium states and changes in the flow-field behaviour. When investigating how the energy of the two-dimensional modes varied with Reynolds number, an energy minimum was observed before the primary instability, so that it marked the Reynolds number value at which the flow field started to exhibit one pulse front per cycle downstream of the constriction. In addition, this flow is characterized by a bistable system in two dimensions, whose equilibrium states are related to the energy level of the two-dimensional mode. The energy difference between the equilibrium states surprisingly decreases from the lower to the higher Reynolds number, which indicates that the system should converge to a unique solution at high Reynolds number. Regarding the flow’s nature, the equilibrium state with higher energy presented a vortex pair soon after the constriction, with vorticity opposite to that of the base flow separating shear layers.

Numerical investigation of the isothermal flow field and particle deposition behaviour in a rotating fluidized bed solar receiver

A numerical analysis of the isothermal flow field within a directly irradiated Rotating Fluidized Bed Receiver (RFBR), is presented to provide a systematic assessment of the influence of key receiver control parameters, namely fluidized bed rotational speed and radial fluidizing gas velocity, on the flow field inside the receiver and particle deposition onto the receiver window. To achieve these aims, a Computational Fluid Dynamics (CFD) model of the RFBR was developed and coupled with Discrete Phase Model (DPM) to analyse the fluid flow and particle trajectory in the receiver cavity due to systematic variations in the key control parameters. The fluid flow modelling approach was partially verified by comparing the numerical predictions with previously published experimental flow measurements in a rotating vortex flow device that is geometrically similar to the RFBR. Using the reported modelling approach, the sensitivity of the flow field and particle deposition to the variations in the key control parameters was determined. Flow features and physical mechanisms linked to particle deposition onto the receiver window were identified with the view to better understand the operation of the RFBR and determine suitable operating regimes that achieve a low risk of particle deposition.

X-ray tomography and modelling study on the mechanical behaviour and performance of metal foam flow-fields for polymer electrolyte fuel cells

Porous metal foams have been used as alternative flow-fields in proton exchange membrane fuel cells (PEMFCs), exhibiting improved performance compared to conventional ‘land and channel’ designs. In the current work, the mechanical behaviour of PEMFCs using metal foam flow-fields is investigated across different length scales using a combination of electrochemical testing, X-ray computed tomography (CT), compression tests, and finite element analysis (FEA) numerical modelling.Fuel cell peak power was seen to improve by 42% when foam compression was increased from 20% to 70% due to a reduction in the interfacial contact resistance between the foam and GDL. X-ray CT scans at varying compression levels reveal high levels of interaction between the metal foam and gas diffusion layer (GDL), with foam ligaments penetrating over 50% of the GDL thickness under 25% cell compression. The interfacial contact area between the foam and GDL were seen to be 10 times higher than between the foam and a stainless-steel plate. Modelling results demonstrate highly uniform contact pressure distribution across the cell due to plastic deformation of the foam. The effect of stack over-tightening and operating conditions are investigated, demonstrating only small changes in load distribution when paired with a suitable sealing gasket material.

Poboljšanje efikasnosti sedimentacionih rezervoara u uklanjanju taloga korišćenjem različitog ulaznog i izlaznog položaja

Sedimentation by gravity is the oldest water treatment. Inlet and outlet of rectangular sedimentation tank are often located in the middle of the tank. The present investigation studies the effect of changing inlet and outlet position of rectangular sedimentation tank on removal efficiency using Computational Fluid Dynamics method. Two different configurations are proposed and they have been tested with varying particles diameters and concentration. Numerical model ability to describe flow field behaviour inside the tank is confirmed by an experimentation data. Results show that inlet and outlet position influence the flow field and removal efficiency of sedimentation tanks, especially for the case of fine particles (50 and 120µm).

Investigation of gas–liquid two‐phase flow field behavior based on computational fluid dynamics in a water‐sparged aerocyclone

AbstractThe water‐sparged aerocyclone (WSA) is a new type of high efficient gas–liquid mass transfer equipment. The focus of this paper is to investigate the gas–liquid interaction, the distribution pattern of the flow field, and the distribution characteristics of turbulent kinetic energy in the coupled space of WSA. The Reynolds stress model and the multiphase flow model of volume of fluid were employed to simulate the distribution of three‐dimensional flow field in WSA, simultaneously. In present work, the numerical simulation results match well with the measured pressure drop data. The simulation results show that the symmetry of the velocity distribution of the coupled field is weakened, and the decay rate of turbulent kinetic energy is raised with the increase of gas inlet velocity. The maximum value of the axial velocity occurs a phenomenon of metastasis from the vicinity of the wall to the center of the exhaust pipe along the radial distribution. The radial velocity reaches a maximum near the center of the coupling field. The tangential velocity distribution has peak areas near the WSA wall and near the central exhaust pipe, respectively. The research results can provide the basis for the optimal design and industrial scale‐up of WSA.

Investigations on flowfield behavior and resistance backpressure characteristics of supersonic cascade with boundary layer suction

A numerical study aiming at the enhancement of the operating range of supersonic cascade via boundary layer suction is presented. The purpose of this paper is to improve the maximum backpressure ratio of the flowfield by suction control and to learn the flowfield behavior and characteristics with different suction control parameters. An S-shaped supersonic cascade developed and tested at DLR (DLR-PAV-1.5) that operates with a relative entrance Mach number of 1.5 is selected in this paper. A two-dimensional multiblock NS solver has been successfully applied to the cascade flow which is validated using the data of the experiment of Schreiber. An in-deep numerical study has been carried out to investigate the effects of suction locations, widths and angles by comparing the operating range and aerodynamic performance parameters of the supersonic cascade. The results indicate that the maximum backpressure ratio can be enhanced by 17.4% with a loss of 3% mainstream mass flow by suction slot in 60% or 70% length of the axial chord. As the width of the suction slot increases to 7 mm, the maximum backpressure ratio is enhanced by 24%, and the total pressure recovery coefficient is improved to 95.3%. The range of suction slot angle that can effectively improve the maximum backpressure ratio is 60–90° and −90 to −60°. This paper provides guidance for the supersonic cascade to improve its performance parameters through suction control.

Large Eddy Simulation on the Two‐Phase Flow in a Water Model of Continuous Casting Strand with Gas Injection

The Large Eddy Simulation (LES) turbulent model coupled with VOF multiphase model and Lagrangian Discrete Phase Model (DPM) is applied to investigate the gas–liquid two‐phase flow and bubble distribution. The drag force, lift force, virtual mass force, and pressure gradient force are incorporated in the current developed model, and the lift force is added via user defined subroutine. The influence of the drag force, the lift force, bubble size, and gas flow rate on the distribution of bubbles and flow field are investigated. The PIV measurements are employed to validate the mathematical model. The results show that the gas injection can affect the flow field and bubble behavior directly. The impingement point on the narrow face and vortex core of the upper recirculation zone moves upward with the increase of gas flow rate. The drag force has distinct influences on the bubble distribution and flow field. The interaction induced by the lift force cannot be ignored. Without the consideration of the lift force, a larger flow velocity will be predicted. Moreover, the lift force has significant effect on the bubble distribution at mold thickness direction, especially the larger bubbles.

Numerical investigation of thermal characteristics of spray cooling with minimum quantity lubrication in milling process

• A numerical model is developed to examine the cooling performance associated with spray cooling with MQL in milling process. • The model mimics the motion of cutting workpiece as well as the rotation of tool. • The flow and heat transfer characteristics, and droplet behavior for different spray angles are investigated. This study explores the cooling performance associated with spray cooling with minimum quantity lubrication (MQL) in milling of Ti 6Al 4V using tungsten carbide (WC) cutting tool. Vegetable oil is used as a cutting fluid, and it impacts the cutting zone. A numerical model using sliding mesh and discrete phase model (DPM) is developed to calculate the flow field and the temperature distributions in both WC and Ti 6Al 4V subjected to conjugated heat transfer. The effect of spray angle relative to the feed direction on cooling performance is examined by analyzing the flow field and droplet behavior when nozzle is oriented to the tool or tool flute. The result shows that the temperature of both WC and Ti 6Al 4V for 0 and 90° of spray angle is lower than that for 45° of spray angle. However, 90° of spray angle is not suitable for lubrication purpose in milling process since MQL droplets hardly penetrate into the cutting zone.

Numerical simulations of oil flow inside a gearbox by Smoothed Particle Hydrodynamics (SPH) method

Abstract In this paper, numerical simulations of oil flow inside a gearbox are presented and compared to experimental particle image velocimetry (PIV) results. Instead of a traditional grid-based finite volume method, a mesh-free Smoothed Particle Hydrodynamics (SPH) method is employed. A multi-phase SPH formulation is utilized to resolve the complex multiphase fluid flow. A total of nine simulations are carried out for three oil levels and three Reynolds numbers, to investigate the flow field behavior and to compare with the experimental results. The aeration effect is first considered to qualitatively analyze the quantity and size of bubbles generated due to the rotation of the gears. Furthermore, the velocity field and velocity profile beneath the oil surface are comprehensively analyzed. Our simulation results exhibit physically consistent behavior of the oil flow and good agreement is achieved compared to the experimental results. Flow structures, splashing and recirculation areas are very well captured by the simulation. However, some discrepancies of the velocity field between the numerical and experimental results are also observed, and discussed.

Detailed parametric investigations on drag and heat flux reduction induced by a combinational spike and opposing jet concept in hypersonic flows

In the current study, the flow field behavior induced by a novel combinational opposing jet and spike concept has been investigated numerically. A comprehensive analysis on the selections of the turbulence model and the height of the first grid point off the wall (Δx) has been conducted firstly. Then, this paper mainly focuses on the influences of the nozzle diameter (d0), the length-to-diameter ratio of the aerospike (L/D) and the jet pressure ratio (PR) on the flow field structures, the aerodynamic drag and heat properties. The results show that when d0 is 2 mm, adding the opposing jet to the single spiked blunt body has no help in modifying the high drag and heat environment. However, the combinatorial thermal protection system has a great contribution to reduce the drag and heat when d0 is 4 mm, with 45% and 38% drop on the wall Stanton number (St) and drag coefficient (Cd) respectively. The values of the wall heat flux, the static pressure and the drag coefficient decrease apparently when L/D increases from 1.0 to 1.5, while the drop rates of these indexes seem not so obvious when L/D rises from 1.5 to 2.0. Meanwhile, the introduction of the opposing jet creates a recirculation zone upstream the spike head, and there is a recirculation zone emerges downstream the aerodisk as well when L/D increases to 2.0. Both of the recirculation zones have alleviated the high temperature the aerodisk sustained significantly.

Effect of turbulence models' choice on the aero-thermal flow numerical validations within a ribbed trailing edge geometry

In this paper, the commercial ANSYS Fluent software was used to achieve a numerical survey of the effect of five (05) turbulence models’ formulation on the aero-thermal computational fluid dynamics validation of two 30:1 scaled models reproducing an original internal ribbed trailing edge prototype. Tests were conducted for stationary and rotation conditions, for Re = 10,000–40,000 and Ro = 0–0.23. Particle image velocimetry and thermochromic liquid crystal experimental data were employed to check the consistence computational fluid dynamics results qualitatively and quantitatively, aerodynamically and thermally, for various working conditions. Numerical predictions revealed that the choice of the turbulence model affects the accuracy of results. Concerning the shear stress transport k-w model, limiters defined in the eddy viscosity formulation induce a surplus estimation of the turbulence kinetic energy ( k) which leads to noticeable discrepancies in terms of velocity profiles and recirculation zones. Also, numerical calculations confirmed former experimental assumptions concerning origins of the aerodynamic structures and heat transfer's features, especially, those related to the increase of the cooling temperature balance efficiency, the appearance/disappearance of the horseshoe structures within the trailing edge region and velocities/boundary layers’ profile variations. The obtained results assist the understanding and the forecast of the flow field behavior, throughout the design process, by the assessment of the aerodynamic and thermal performances within the considered blade's cooling system.

Toward Investigation of External Oil Flow From a Journal Bearing in an Epicyclic Gearbox

High loads and bearing life requirements make journal bearings the preferred choice for use in high-power, planetary gearboxes in jet engines. With the planet gears rotating about their own axis and orbiting around the sun gear, centrifugal forces generated by both motions interact with each other and create complex kinematic conditions. This paper presents a literature and state-of-the-art knowledge review to identify existing work performed on cases similar to external journal bearing oil flow. In order to numerically investigate external journal bearing oil flow, an approach to decompose an actual journal bearing into simplified models is proposed. Preliminary modeling considerations are discussed. The findings and conclusions are used to create a three-dimensional (3D), two-component computational fluid dynamics (CFD) sector model with rotationally periodic boundaries of the most simplistic approximation of an actual journal bearing: a nonorbiting representation, rotating about its own axis, with a circumferentially constant, i.e., concentric, lubricating gap. In order to track the phase interface between the oil and the air, the volume of fluid (VoF) method is used. External journal bearing oil flow is simulated with a number of different mesh densities. Two different operating temperatures, representing low and high viscosity oil, are used to assess the effect on the external flow field behavior. In order to achieve the future objective of creating a design tool for routine use, key areas are identified in which further progress is required.

A microfluidic device for study of the effect of tumor vascular structures on the flow field and HepG2 cellular flow behaviors

To build a microfluidic device with various morphological features of the tumor vasculature for study of the effects of tumor vascular structures on the flow field and tumor cellular flow behaviors. The designed microfluidic device was able to approximatively simulate the in vivo structures of tumor vessels and the flow within it. In this models, the influences of the angle of bifurcation, the number of branches, and the narrow channels on the flow field and the influence of vorticity on the retention of HepG2 cells were significant. Additionally, shear stress below physiological conditions of blood circulation has considerable effect on the formation of the lumen-like structures (LLSs) of HepG2 cells. These results can provide some data and reference in the understanding of the interaction between hemorheological properties and tumor vascular structures in solid tumors.

Quantifying the influence of oscillatory flow disturbances on blood flow

Pulsatile blood flow is renowned for inducing localised flow disturbances that are characterised by significant oscillations. These flow disturbances are recognised to have a physiologic significance within the cardiovascular system, particularly with respect to their proatherogenic expression on endothelial cells. Flow disturbances also impart significant influence on the mechanics of cardiovascular flow and are formally shown in the present study to be coupled to the period-average behaviour of a pulsatile flow field, causing it to be misrepresented by its steady-equivalent; which is often used in models of atherogenesis and hence limits their reliability. It is demonstrated that a measure of the localised influence of flow disturbances on the period-average flow can be realised by the relative significance of their kinetic energy, which is quantified by the introduced Oscillatory Kinetic Energy Index (OKEI). A specific measure of direction-reversing oscillations is also developed with the introduction of the Oscillatory Flow Index (OFI), which is an extension of the wall-bound Oscillatory Shear Index (OSI) onto flow and wall spaces. A case study of a human carotid artery is made; wherein pulsatile flow is studied relative to its steady-equivalent state. The introduced indices are demonstrated to collectively identify oscillatory flow disturbances within the flow; quantifying their spatial distribution and influence on the flow field.

Computational modelling and analysis of heat and mass transfer in MHD flow past the upper part of a paraboloid of revolution

The flow and heat transfer nature of MHD dissipative flow past the upper part of a non-melting paraboloid of revolution with non-uniform heat source/sink, thermophoresis and Brownian moment effects are investigated numerically. The buoyancy induced on the flow is considered in such way that the surface is neither vertical/horizontal nor wedge/cone. The equations governing the flow are solved numerically by employing the RKF integration scheme after suitable transformations. Graphical results are obtained to discuss the behaviour of flow, temperature and concentration fields at various parameters of interest. The reduced wall friction, local Nusselt and Sherwood numbers are computed. It is found that the non-uniform heat source/sink parameters regulate the thermal boundary layer. Thermophoresis and Brownian moment parameters act as controlling parameters of heat and mass transfer rate.