High Reynolds Number Conditions Research Articles

Numerical investigation of coupled heat transfer and flow characteristics in helical coil heat exchanger for mine water waste heat recovery

To efficiently recover waste heat from mine water, this study numerically investigates the flow and heat transfer performance of a tube-shell coupled helical coil heat exchanger (HCHE), under high Reynolds number conditions. The effects of inlet water temperature, mass flow rate (m), tube diameter (d), and helix pitch (P) on the tube side, shell side, and overall performance are analyzed. Performance metrics include friction factor (f), Nusselt number (Nu), overall heat transfer coefficient (U), heat transfer rate, and the Performance Evaluation Criteria (PEC) number. Specifically, this study focuses on detailed observation and analysis of the shell-side flow and temperature fields. Findings reveal that enlarging d from 30 mm to 50 mm results in a 32.55% decrease in U, whereas increasing P from 50 mm to 200 mm boosts U by 8.02%. The highest PEC number of HCHE is 2.43, observed in conditions of P = 50 mm, d = 30 mm, and mc = 4 × 103 kg/h. Correlations for fc, Nuc, and Nush are proposed based on simulation results. This study provides a theoretical basis for the design and structural optimization of HCHE in the mine water waste heat recovery applications.

Measurement of interfacial mass transfer of single bubbles rising in homogeneous turbulence

Accurate quantification of the mass transfer coefficient kl of bubbles under high Reynolds number conditions has been in desperate need for decades to provide insights into its fundamental physics and reach a general mass transfer model. In this paper, a state-of-the-art SI-LIF technique is developed to quantify kl indirectly based on the law of conservation of mass. The reconstruction algorithms of the required physical quantities are validated numerically, suggesting an overall uncertainty of less than ± 5 % in the estimation of kl. We perform mass transfer experiments of single oxygen bubbles with deq = 1.5 mm rising in both quiescent water and homogeneous turbulence, with turbulent Reynolds number ReT ranging from 0 to 1114. The measured kl under turbulent conditions are independent of ReT, and all collapse to the analytical solution of potential flow approximation, revealing that the flow in the immediate vicinity of the bubble interface remains undisturbed by the fluctuating liquid motions across all the turbulence settings examined in this paper.

Advanced Modeling of Flow and Heat Transfer in Rotating Disk Cavities Using Open-Source Computational Fluid Dynamics

Abstract Code_Saturne, an open-source computational fluid dynamics (CFD) code, has been applied to a range of problems related to turbomachinery internal air systems. These include a closed rotor–stator disk cavity, a co-rotating disk cavity with radial outflow and a co-rotating disk cavity with axial throughflow. Unsteady Reynolds-averaged Navier–Stokes (RANS) simulations and large eddy simulations (LES) are compared with experimental data and previous direct numerical simulation and LES results. The results demonstrate Code_Saturne's capabilities for predicting flow and heat transfer inside rotating disk cavities. The Boussinesq approximation was implemented for modeling centrifugally buoyant flow and heat transfer in the rotating cavity with axial throughflow. This is validated using recent experimental data and CFD results. Good agreement is found between LES and RANS modeling in some cases, but for the axial throughflow cases, advantages of LES compared to URANS are significant for a high Reynolds number condition. The wall-modeled large eddy simulation (WMLES) method is recommended for balancing computational accuracy and cost in engineering applications.

The effects of imposed swirling flow on the transport and deposition of particulate pollutants in the 90° industrial duct bends: An Eulerian-Lagrangian approach

Duct bend is one of the important parts of ventilation and dust removal systems, and particles deposited in curved ducts can reduce system efficiency or cause erosion on the bend wall. To investigate whether particle deposition is affected by imposed swirl on fluid flow, this article combines the RSM turbulence model and the Discrete Phase Model (DPM) to predict the deposition efficiency of particles in the bend under high Reynolds number conditions. The results show that the imposed swirl flow modifies the secondary flow initially dominated by the pressure gradient caused by the curvature effect. With the gradual increase of the swirl number ( S n), the deposition efficiency of the particles gradually decreased. However, when the swirl number is low ( S n ≤ 0.17), particles with smaller Stokes numbers are more susceptible to the intensity of turbulence. The higher the turbulence intensity near the wall, the easier it is for low inertia particles ( St ≤ 0.456) to deposit. The higher swirl intensity dominates the centrifugal force, reduces the turbulent intensity in the central region of the duct, improves the stability of the airflow and makes it easier for particles with larger inertia ( St ≥ 0.811) to pass through the bend.

Impact of high-speed turbidity currents on offshore spanning pipelines

Pipelines in the deep sea are at risk of damage due to submarine landslides, which can result in the loss of costly infrastructure and pollution stemming from hydrocarbon leaks. Submarine landslides can exhibit a wide range of flow characteristics, which in turn can affect how they interact with pipelines. Researchers have previously focused on pipelines impacted by submarine debris and/or mud flows described by non-Newtonian fluid rheological models and the laminar model. However, the impact of larger-scale and higher-speed submarine turbidity currents described by turbulence models on pipelines has been overlooked. In this study, we address this gap by utilizing a more accurate turbulence simulation method, namely, the large eddy simulation (LES) method, to analyze the effect of submarine turbidity currents on fixed spanning pipelines, and we validate the effectiveness of the proposed method via typical circular cylinder flow experiments and numerical simulations. We find that the lift force on the pipeline impacted by submarine turbidity currents under high-Reynolds number (Re) conditions is particularly significant relative to debris and/or mud flows under low-Re conditions. In parallel, the vortex shedding frequency increases with increasing Re, and the Strouhal number basically remains unchanged and ranges from 0.2–0.25 at 1,112 ≤ Re ≤ 333,559. Furthermore, the vortex structure and its arrangement behind the spanning pipeline become irregular with increasing Re, forming a turbulent vortex street, which reveals the mechanism of pipeline vibration. Finally, a methodology for predicting characteristic drag force and lift force coefficients is established for submarine pipeline design.

Miniaturized metachronal magnetic artificial cilia

Biological cilia, hairlike organelles on cell surfaces, often exhibit collective wavelike motion known as metachrony, which helps generating fluid flow. Inspired by nature, researchers have developed artificial cilia as microfluidic actuators, exploring several methods to mimic the metachrony. However, reported methods are difficult to miniaturize because they require either control of individual cilia properties or the generation of a complex external magnetic field. We introduce a concept that generates metachronal motion of magnetic artificial cilia (MAC), even though the MAC are all identical, and the applied external magnetic field is uniform. This is achieved by integrating a paramagnetic substructure in the substrate underneath the MAC. Uniquely, we can create both symplectic and antiplectic metachrony by changing the relative positions of MAC and substructure. We demonstrate the flow generation of the two metachronal motions in both high and low Reynolds number conditions. Our research marks a significant milestone by breaking the size limitation barrier in metachronal artificial cilia. This achievement not only showcases the potential of nature-inspired engineering but also opens up a host of exciting opportunities for designing and optimizing microsystems with enhanced fluid manipulation capabilities.

Flow field modeling of airfoil based on convolutional neural networks from transform domain perspective

For complex flow problems such as wall-bounded turbulence with multi-scale and strongly nonlinear characteristics, the non-orthogonal and non-uniform meshes commonly used in numerical simulation limit the direct use of Convolutional Neural Networks (CNNs). The flow field is usually projected onto a uniform Cartesian mesh to use the convolution operation, which inevitably leads to the problem of missing information about the turbulent flow field in the near-wall region. To address this limitation, a method is proposed for flow field modeling using CNN from the perspective of the transform domain. Specifically, the flow field is transformed from the non-uniform physical plane to the uniform computational plane. From the view of the transform domain, the flow governing equations will naturally generate variables used to characterize the geometry through the coordinate transformation without the need to design specific features. Moreover, since the computational plane is uniform and orthogonal, the convolution can be used directly without interpolation preprocessing. For the case of shape-based flow field modeling of the 2D airfoil and 3D wing under high Reynolds number conditions, the velocity and pressure of the turbulent boundary layer can be predicted very well by this method, even in the discontinuous shock wave region. In addition, the pros and cons of two different but common modeling frameworks represented by CNN and proper orthogonal decomposition (POD) are presented. The modeling error of CNN is one order of magnitude smaller than that of POD, and the relative error of the integral force of CNN is less than 1%. This work shows that the proposed method has significant advantages of high accuracy and interpretability, and provides an efficient solution for parameter space research.

Multi-objective performance optimization of target surface of bionic blue whale-skin impinged by array jet

Jet impingement cooling technology is usually applied to combustor liner with large heat load. With the continuous improvement of high capacity and low emission of heavy-duty gas turbines,the research on low resistance and high cooling efficiency cooling technology is particularly important. To meet these urgent needs, based on the blue whale skin structure, a new array impingement target surface cooling structure was proposed from the perspective of bionics. The flow and heat transfer characteristics of the array impingement bionic target surface cooling structure were numerically simulated under high Reynolds number (Re) and high Heat flux density (q) conditions. The influence of groove depth (H, 1 to 3 mm), radius ratio (R/H, 1 to 3) and spacing ratio (P/H, 10 to 20) on the cooling performance of impingement target surface was analyzed. Multi-objective performance optimization of the bionic structure was carried out by using the second-order response surface approximation model (RSM) and the second-generation non-inferior sequencing genetic algorithm (NSGA-II), and the performance of the optimized structure was tested. The results show that the above optimization method is accurate and effective, and the optimal bionic structure is H = 1.212 mm, R/H = 1.132, P/H = 12.078. Compared with the plane target surface, the average Nusselt number (Nuave), flow friction factor (f), and comprehensive thermal coefficient (F) of the optimal bionic groove structure increased by 14.5%, 1.1% and 14.1%，respectively. The structure and multi-objective optimization method proposed in this paper can provide a certain reference for the design of new efficient cooling structure of heavy-duty gas turbines combustor liner.

Numerical simulation of flow resistance characteristics in a rod bundle channel under vertical and inclined conditions

Numerical simulation of flow resistance characteristics in a 3 × 3 rod bundle channel under vertical and inclined conditions is carried out. Compared with experimental results, it is verified that Reynolds stress model with enhanced wall treatment can accurately calculate flow parameters in rod bundle. Under vertical conditions, flow boundary layer at high Reynolds number is relatively thinner and radial velocity vector at interface of each sub-channel is basically parallel to dividing line or opposite direction. Mainstream velocity distribution in channel under heating conditions is more uniform and radial secondary flow is more likely to occur than that under adiabatic conditions. Axial velocity near wall is a dominated factor to determine wall shear stress. Under inclined conditions, deviation of overall flow velocity distribution is more obvious and relative intensity of natural convection is more significant at low Reynolds number conditions compared with high Reynolds number conditions. Main mechanism of increase in frictional resistance due to inclination is that natural convection caused by thermal stratification changes flow velocity distribution of each sub-channel. Additionally, natural convection increases radial secondary flow area, broadens wall shear stress distribution and also increases average wall shear stress of cross section.

High subsonic and low reynolds number airfoil optimization of high-altitude propeller

This paper presents an airfoil optimization at high subsonic and low Reynolds number condition, to improve the efficiency of an E387-based HALE UAV propeller at cruising condition. The optimization design method is based on surrogate model coupled with validated RANS simulation. The optimal airfoil shape is flat and sharp leading-edged, which remarkably differs from the original and resembles a very thin airfoil in reversed configuration. Significant increment of lift-to-drag ratio and aerodynamical independency of Mach and Reynolds number were observed due to expanded supersonic flow region, weakened shock wave, and considerably reduced flow separation. By applying the optimized airfoil to propeller outboard, the new propeller’s efficiency increased more than 2%, while maintained required thrust, as proved by both Blade Element Theory and CFD simulation.

On the Vaporization Rate and Flame Shape of Nonspherical Droplets

Abstract Motivated by the study of spray combustion, this work addresses the combustion of nonspherical droplets. The combustion of spray is usually understood through the theory of droplet combustion, and improving this latter theory is the narrow aim of this work. This work uses perturbation theory to derive a novel model for the vaporization of nonspherical droplets. Compared to previous efforts in this area, the work uses a physics-based approach by incorporating ideas from the asymptotic analysis of Taylor and Acrivos (1964, “On the Deformation and Drag of a Falling Viscous Drop at Low Reynolds Number,” J. Fluid Mech. 18(3), pp. 466–476). The perturbation strategy expands the droplet shape using spherical harmonics, and the theory characterizes the shape of the droplet via the Weber number. The introduction of this parameter is key as it is a parameter that can be easily measured in experiments, and thus it can be used to connect the theoretical results with application. The perturbation analysis is performed based around the classical solution of spherical droplet combustion in quiescent flow. The theory indicates that the effect of droplet deformation can be accounted for by a correction to the droplet combustion rate that is a simple polynomial function of the droplet Weber number. Results are compared to existing literature, and it confirms the established trend that deformed droplets vaporize faster than spherical droplets. Analysis of the flame shape reveals that the flame remains nearly spherical; however, the mean flame standoff changes with droplet shape. The extension of the theory to high Reynolds number conditions is briefly discussed.

Modeling high-speed gas–particle flows relevant to spacecraft landings

The interactions between rocket exhaust plumes and the surface of extraterrestrial bodies during spacecraft landings involve complex multiphase flow dynamics that pose significant risk to space exploration missions. The two-phase flow is characterized by high Reynolds and Mach number conditions with particle concentrations ranging from dilute to close-packing. Low atmospheric pressure and gravity typically encountered in landing environments combined with reduced optical access by the granular material pose significant challenges for experimental investigations. Consequently, numerical modeling is expected to play an increasingly important role for future missions. This article presents a review and perspectives on modeling high-speed disperse two-phase flows relevant to plume–surface interactions (PSI). We present an overview of existing drag laws, with origins from 18th-century cannon fire experiments and new insights from particle-resolved numerical simulations. While the focus here is on multiphase flows relevant to PSI, much of the same physics are shared by other compressible gas–particle flows, such as coal-dust explosions, volcanic eruptions, and detonation of solid material.

A spectral element-based phase field method for incompressible two-phase flows

In this paper, we propose a spectral element-based phase field method by solving the Navier–Stokes/Cahn–Hilliard equations for incompressible two-phase flows. The high-order differential nonlinear term of the Cahn–Hilliard equation poses a great challenge for obtaining numerical solutions, so the Newton–Raphson method is adopted to tackle this conundrum. Furthermore, we employ the time-stepping scheme to decouple the Navier–Stokes equations to favor the computations with large density and viscosity contrast, in terms of both stability and convergence efficiency. In addition, the continuum surface tension model is used to account for the tangential component of the interfacial force, and thus our numerical method has the ability to simulate thermocapillary flows. We present four examples to demonstrate the interface capture accuracy of the proposed method. The shape of the rotating Zalesak's disk is accurately preserved by the present method even for two periods, which implies less dissipation and higher accuracy at long time numerical simulation. It is also noted that the present method with fourth-order element can achieve similar accuracy with the consistent scheme by evaluating the convective fluxes with the fifth-order weighted essentially non-oscillatory scheme. Moreover, the proposed method appears to comply well with mass conservation. And the results of thermocapillary flow test show good agreement with theoretical prediction. Finally, the rising bubble and Rayleigh–Taylor instability are considered to verify the proposed method for complex changes in interfacial topology, as well as its performance under large density and viscosity contrasts and high Reynolds number conditions.

Numerical simulation of flooding characteristics in steam-water countercurrent inside an inclined pipe under condensation conditions

In this paper, a three-dimensional CFD model, which is established on VOF (Volume of Fluid) model, Lee model together with CSF (Continuum Surface Force) model and RNG k − ɛ turbulence model, is adopted for the condensation heat and mass transfer of steam in an inclined circular tube with the occurrence of flooding and liquid bridge. The entire process of liquid bridge formation is depicted with a three-dimensional space-time plot of the wave amplitude of the interfacial wave with time and axial distance. The effects of different pipe inclination angles and different contact angles on the flow behavior and heat transfer characteristics are investigated and the results demonstrate that under high liquid film Reynolds number and condensation conditions, as the inclination angle increases, the liquid bridge begins to form at a position which moves upward along the pipe. The reason is that as the inclination angle increases, the liquid film is more uniformly distributed in the radial direction, and a vortex symmetrical about the axis is formed on the leeward side of the flooding wave.

Design of compressor impeller under high Reynolds number conditions

PurposeA parametric method for designing the hub, casing and blades of the miniature centrifugal compressor impeller was developed. The relationship model of the size, aerodynamic and performance parameters of the centrifugal impeller was established. Based on the selected design parameters, the miniature centrifugal-type impeller was designed, and the work efficiency was calculated.Design/methodology/approachIn this study, a micro-centrifugal compressor impeller with a diameter of less than 25 mm was designed. A parametric design method was developed, and the functional relationship between the geometric and gas fluidity parameters was established.FindingsThe results of this study showed that the performance parameters of the designed micro-centrifugal impeller satisfied the design requirements. The proposed method is useful as a reference for designing and analysing compressor impellers under high Reynolds number conditions.Originality/valueA parametric design method was developed, and the functional relationship between the geometric and gas fluidity parameters was established. Under the Reynolds number conditions, the flow characteristics of the gas in the compressor were analysed; the shear-stress transport turbulence equation was solved using the finite volume method. In addition, the effects of the Reynolds number on the velocity, pressure, mass flow and efficiency of the micro-scale centrifugal compressor were evaluated. The results showed that the performance parameters of the designed micro-centrifugal impeller satisfied the design requirements. The proposed method is useful as a reference for designing and analysing compressor impellers under high Reynolds number conditions.

Experimental investigation on cryogenic chilldown performance under high-Reynolds number condition and using interior micro-fin structure

Future aerospace vehicle requires a fast propellant fueling and a rapid launching. In the present study, a high-pressure chilldown test platform is established, and two different tube structures, including a straight tube and an interior micro-fin tube, are tested under different inlet Reynolds number (Re) conditions. The test results show that for a typical chilldown case through the straight tube, different boiling mechanisms successively dominate the fluid-wall heat transfer, and inverse annular flow film boiling occupies the maximum proportion of wall temperature decrease. Increasing inlet flow rate could accelerate the chilldown rate, and the time cost in Re=45,000 case is only about 15% of that in Re=5,000 case. For the proposed interior micro-fin tube, the test results verify its excellent performance in improving chilldown rate. Under Re=5,000 condition, the time cost for micro-fin tube case is only about 70 s, which is far less than the time cost by a straight tube case. This astonishing performance is mainly because the micro-fin structure induces a radial velocity component in the inverse annular vapor film. The radial velocity strike acts on the liquid-vapor interface, yielding an interface fluctuation or even a liquid column breakup. Therefore, the liquid phase could contact the tube warm wall earlier, and then a sustained high heat flux gives rise to significant temperature decrease effect throughout the whole chilldown process.

Stereoscopic Micro PIV Investigation of Velocity Boundary Layer Near Piston Top of a Tumble Enhanced SI IC Engine

Stereoscopic Micro PIV Investigation of Velocity Boundary Layer Near Piston Top of a Tumble Enhanced SI IC Engine

Macroscopic turbulent flow via hard sphere potential

In recent works, we proposed a hypothesis that the turbulence in gases could be produced by particles interacting via a potential and examined the proposed mechanics of turbulence formation in a simple model of two particles for a variety of different potentials. In this work, we use the same hypothesis to develop new fluid mechanics equations that model turbulent gas flow on a macroscopic scale. The main difference between our approach and the conventional formalism is that we avoid replacing the potential interaction between particles with the Boltzmann collision integral. Due to this difference, the velocity moment closure, which we implement for the shear stress and heat flux, relies upon the high Reynolds number condition rather than the Newton law of viscosity and the Fourier law of heat conduction. The resulting system of equations of fluid mechanics differs considerably from the standard Euler and Navier–Stokes equations. A numerical simulation of our system shows that a laminar Bernoulli jet of an argon-like hard sphere gas in a straight pipe rapidly becomes a turbulent flow. The time-averaged Fourier spectra of the kinetic energy of this flow exhibit Kolmogorov’s negative five-thirds power decay rate.

Using a Modified Turian-Yuan Model to Enhance Heterogeneous Resistance in Municipal Sludge Transportation Pipeline.

Based on the Turian–Yuan heterogeneous resistance model, the simulation results of three urban sludge pipelines with a volumetric concentration of 2.38, 3.94, and 5.39% were analyzed. The reasons for the large deviation of the simulation results under high Reynolds number conditions were also analyzed. The results showed that the deviation of the simulation was mainly caused by the difference between the sludge volumetric concentration (CV), the settlement resistance coefficient (CD), and the values of the two parameters in the Turian–Yuan heterogeneous resistance model. Consequently, it was necessary to optimize the index m1 of CV and the index m2 of CD. Taking mean square deviation as the objective function, using Matlab programming, the abovementioned two indexes were optimized by the simulated annealing algorithm. The optimized index m1 of CV was 0.887, and the index m2 of CD was −0.162. Hence, a modified Turian–Yuan heterogeneous resistance model was obtained. The model verified that the minimum value of the regression coefficient, R2, of the simulated value reached 0.9701, proving that, the model can be used to simulate the heterogeneous resistance of urban sludge pipeline transportation.

An adaptive plate at flapping wing’s trailing edge in promoting energy extraction performance

As is known, the maximum thickness position plays a key role in determining the energy extraction properties for flapping wing airfoils. Previous work demonstrated that the energy extraction efficiency might increase when we move the maximum thickness position towards the leading-edge direction. Inspired from this phenomenon, we propose in this work the addition of a flat plate at the tail of the flapping wings. Under two different Reynolds numbers (Re = 1100 and Re = 5×105), the effect of adding a rigid flat plate or a flexible flat plate to the trailing edge of the flapping wing on the flapping wing’s energy efficiency was investigated. The corresponding results were also compared with that of the original flapping wing. Results show that the addition of a rigid flat plate is sufficient for the amelioration of the flapping wing’s energy extraction efficiency in low Reynolds number cases. At high Reynolds number cases, the flapping wing with flexible flat plate trailing edge can maintain a relatively high energy extraction efficiency over a wide range of frequency, while the flapping wing with rigid flat plate trailing edge can only improve the energy extraction efficiency in the low frequency region. Therefore, we propose the application of flapping wings with flexible flat plate trailing edge for increasing the energy extraction efficiency under high Reynolds number conditions.