Turbulent Flow Region Research Articles

Experimental investigation on tetra-n-butyl-ammonium bromide clathrate hydrate slurry flows in a horizontal tube: Flow behavior and its rheological model

In the area of air conditioning, the tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is a promising high-density latent-heat carrying medium. In this article, flow properties of two types of TBAB CHS were experimentally investigated in a horizontal tube with parameters of pressure drop ΔP, flow velocity of CHS V, and solid mass fraction χ. First, the Bingham rheological characteristics of TBAB CHS were confirmed in the range of solid mass fraction of χ ≤ 22% for type A and χ ≤ 25% for type B, respectively. Then, the re-laminarization phenomenon and region of weakened flow resistance were found according to the variation of pressure drop ΔP affected by the velocity and solid mass fraction χ throughout the flow regions, including laminar, intermediate, and turbulent flows. The deduction of the critical flow velocity and region of entire laminar flow velocity were useful for latent-heat transportation applications. Finally, the criterial relations between Fanning friction factor f and Metzner-Reed Reynolds number ReM were obtained for laminar-flow and turbulent-flow regions, respectively.

Enhancement of heat transfer and hydraulic drag in gas-cooled helical channels with artificial roughness on convex wall

Abstract Results on experimental investigation of heat transfer enhancement in helical channels of rectangular cross-section in the hydrodynamically and thermally stabilized air flow are presented. The investigations have been carried out in the rough, with artificial three-dimensional pyramidal roughness (k/de ≈ 0.03) on the convex wall (surface), and in the smooth helical channels (relative curvature D/h ≈ 20 and relative width b/h ≈ 6) for a wide range of flow hydrodynamic parameter Re (103–2 × 105). The research has revealed that in the region of turbulent flow, heat transfer from convex surface of the rough channel more than twice exceeds heat transfer from convex surface of the smooth channel and reaches level of heat transfer from concave surface. Heat transfer from concave surfaces of the both rough and smooth helical channels is similar and is about 50% higher than of a straight flat channel. Heat transfer efficiency parameter St3/ξ for the rough channel is higher than for the smooth channel almost in the whole range of investigated Re, and the efficiency parameter η ((St/Stsm)/(ξ/ξsm)) is about 1.3. Correlations are proposed for calculation of stabilized heat transfer from convex and concave surfaces and average heat transfer and hydraulic drag in rough helical channels.

Heat transfer and flow region characteristics study in a non-annular channel between rotor and stator

This paper will present the results of the experimental investigation of heat transfer in a non-annular channel between rotor and stator similar to a real generator. Numerous experiments and numerical studies have examined flow and heat transfer characteristics of a fluid in an annulus with a rotating inner cylinder. In the current study, turbulent flow region and heat transfer characteristics have been studied in the air gap between the rotor and stator of a generator. The test rig has been built in a way which shows a very good agreement with the geometry of a real generator. The boundary condition supplies a non-homogenous heat flux through the passing air channel. The experimental devices and data acquisition method are carefully described in the paper. Surface-mounted thermocouples are located on the both stator and rotor surfaces and one slip ring transfers the collected temperature from rotor to the instrument display. The rotational speed of rotor is fixed at three under: 300rpm, 900 rpm and 1500 rpm. Based on these speeds and hydraulic diameter of the air gap, the Reynolds number has been considered in the range: 4000<Rez<30000. Heat transfer and pressure drop coefficients are deduced from the obtained data based on a theoretical investigation and are expressed as a formula containing effective Reynolds number. To confirm the results, a comparison is presented with Gazley?s (1985) data report. The presented method and established correlations can be applied to other electric machines having similar heat flow characteristics.

Correlation of Power Consumption for Several Kinds of Mixing Impellers

The authors reviewed the correlations of power consumption in unbaffled and baffled agitated vessels with several kinds of impellers, which were developed in a wide range of Reynolds numbers from laminar to turbulent flow regions. The power correlations were based on Kamei and Hiraoka's expressions for paddle and pitched paddle impellers. The calculated correlation values agreed well with experimental ones, and the correlations will be developed the other types of impellers.

Dissipation-scale fluctuations in the inner region of turbulent channel flow

The statistics of intense energy dissipation events in wall-bounded shear flows are studied using highly resolved direct numerical simulations of turbulent channel flow at three different friction Reynolds numbers. Distributions of the energy dissipation rate and local dissipation scales are computed at various distances from the channel walls, with an emphasis on the behavior of the statistics in the near-wall region. The dependence of characteristic mean and local dissipation scales on wall distance is also examined over the full channel height. Systematic variations in these statistics are found close to the walls due to the anisotropy generated by mean shear and coherent vortical structures. Results near the channel centerline are consistent with those found in homogeneous isotropic turbulence.

Small‐scale mixing at the turbulent/non‐turbulent interface in turbulent jets

AbstractThe numerical simulation of the turbulent mixing between the turbulent core flow region and the non‐turbulent irrotational ambient region requires a reliable modelling of the mass and momentum transfer in the intermittent zone. The dominant role of the small‐scale whirling motion in the turbulent entrainment process particularly challenges the subgrid‐scale (SGS) modelling in Large‐Eddy Simulation (LES). The present work carried out a Direct Numerical Simulation (DNS) of a round jet to analyze the scalar mixing conditions near the turbulent/non‐turbulent interface. The obtained statistical results widely confirm existing experimental and analytical findings made in previous studies on the momentum transfer. An a priori test of the DNS data reveals the limits of the standard Smagorinsky‐type SGS‐model for the eddy diffusivity in the description of the subgrid‐scale mixing near the interface. The extension to a dynamic SGS‐model using non‐constant, flow dependent model coefficients does not lead to a substantial improvement. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Volumetric reconstruction of the 3D boundary of stream tubes with general topology using tracer particles

A general method is proposed to reconstruct the volumetric interface between two fluid flows using tracer particles and 3D particle tracking techniques. The method relies on the fact that a homogeneous dispersion of tracer particles introduced in a stream tube remains confined in that tube so that the cloud of particles can be used to reconstruct the boundary of the flow covered by the stream tube. Thus it becomes possible to quantitatively determine the interface between laminar and turbulent flow regions in boundary or shear layers as well as the interface between two mixing fluids. Tracer particles, as opposed to dye tracers, have negligible diffusion and their position in the measurement volume can be precisely localized by means of 3D particle tracking methods. On the other hand, they provide a discrete representation of a continuous volume and the reconstruction of the interface cannot be implemented in a straight forward fashion. In this work, the problem of interface reconstruction, from a randomly scattered particle cloud, is addressed and two different reconstruction algorithms are proposed: one based on numeric diffusion and one based on Delaunay triangulation. The two methods are qualified and compared by means of numerical simulations using the Monte Carlo method. The simulations are used to estimate the accuracy of the method and to provide guidelines for the choice of parameter settings. Finally, results on the interface between two mixing fluids in a microfluidic mixer are shown. A resolution of 2.5 µm in the optical-axis direction, with a maximum estimated error of 5.5 µm in the three directions, was obtained.

Mean velocity and length-scales in the overlap region of wall-bounded turbulent flows

The length scales of the spectra and correlation functions of the velocity fluctuations in the overlap region of turbulent wall-bounded flows are analyzed. It is found that a mixing length based on the mean local shear works better as a normalization than the distance to the wall. To define an overlap range sufficiently long and independent of the Reynolds number to allow the two scalings to be tested, the classical asymptotic expansion of the mean shear is extended to include a near-wall virtual origin and a wake component. The result represents well the velocity profile in y+ > 70 and y/δ < 0.3 – 0.5 for Reτ≿2000, and the spectral scales for Reτ≿500. It is suggested that the scaling with a local mixing length could be interpreted as an indication that the size of the eddies is more related to the local shear time scale than to the interaction with the wall. It is also noted that the linearity of the mixing length is a more robust indicator of a logarithmic regime than those that rely on a zero virtual origin.

Heat and mass transfer analysis in air gap membrane distillation process for desalination

The air gap membrane distillation (AGMD) process was applied for water desalination. The main objective of the present work was to study the heat and mass transfer mechanism of the process. The experiments were performed on a flat sheet module using aqueous NaCl solutions as a feed. The membrane employed was hydrophobic PTFE of pore size 0.22 . A mathematical model is proposed to evaluate the membrane mass transfer coefficient, thermal boundary layers` heat transfer coefficients, membrane / liquid interface temperatures and the temperature polarization coefficients. The mass transfer model was validated by the experimentally and fitted well with the combined Knudsen and molecular diffusion mechanism. The mass transfer coefficient increased with an increase in feed bulk temperature. The experimental parameters such as, feed temperature, 313 to 333 K, feed velocity, 0.8 to 1.8 m/s (turbulent flow region) were analyzed. The permeation fluxes increased with feed temperature and velocity. The effect of feed bulk temperature on the boundary layers` heat transfer coefficients was shown and fairly discussed. The temperature polarization coefficient increased with feed velocity and decreased with temperature. The values obtained were 0.56 to 0.82, indicating the effective heat transfer of the system. The fouling was observed during the 90 h experimental run in the application of natural ground water and seawater. The time dependent fouling resistance can be added in the total transport resistance.

A predictive inner–outer model for streamwise turbulence statistics in wall-bounded flows

A model is proposed with which the statistics of the fluctuating streamwise velocity in the inner region of wall-bounded turbulent flows are predicted from a measured large-scale velocity signature from an outer position in the logarithmic region of the flow. Results, including spectra and all moments up to sixth order, are shown and compared to experimental data for zero-pressure-gradient flows over a large range of Reynolds numbers. The model uses universal time-series and constants that were empirically determined from zero-pressure-gradient boundary layer data. In order to test the applicability of these for other flows, the model is also applied to channel, pipe and adverse-pressure-gradient flows. The results support the concept of a universal inner region that is modified through a modulation and superposition of the large-scale outer motions, which are specific to the geometry or imposed streamwise pressure gradient acting on the flow.

Shear instability in the subsurface layer of a material in friction

The shear instability in subsurface layers of a material in friction has been investigated. As the friction surface is approached, several characteristic regions can be distinguished in the bulk of the metal: the region of plastic deformation and texturing (I), the region of severe fragmentation (II), the region of turbulent flow (III), and the region of laminar flow (IV). Regions I and II can be referred to as regions of conventional plastic deformation, whereas regions III and IV correspond to regions of the development of shear instability of the Kelvin-Helmholtz type at the shear boundary. The possibility of implementing this phenomenon within the hydrodynamic approach has been evaluated.

The calculation of the profile-linear average velocity in the transition region for ultrasonic heat meter based on the method of LES

The measurement accuracy of an ultrasonic heat meter depends on the relationship of the profile-linear average velocity. There are various methods for the calculation of the laminar and turbulence flow regions, but few methods for the transition region. At present, the traditional method to deal with the transition region is to adopt the relationship for the turbulent flow region. In this article, a simplified model of the pipe is used to study the characteristics of the transition flow with specific Reynolds number. The k − ɛ model and the Large Eddy Simulation (LES) model are, respectively, used to calculate the flow field of the transition region, and a comparison with the experiment results shows that the LES model is more effective than the k − ɛ model, it is also shown that there will be a large error if the relationship based on the turbulence flow is used to calculate the profile-linear average velocity relationship of the transition flow. The profile-linear average velocity for the Reynolds number ranging from 5 300 to 10 000 are calculated, and the relationship curve is obtained. The results of this article can be used to improve the measurement accuracy of ultrasonic heat meter and provide a theoretical basis for the research of the whole transition flow.

Pumping power of nanofluids in a flowing system

Nanofluids have the potential to increase thermal conductivities and heat transfer coefficients compared to their base fluids. However, the addition of nanoparticles to a fluid also increases the viscosity and therefore increases the power required to pump the fluid through the system. When the benefit of the increased heat transfer is larger than the penalty of the increased pumping power, the nanofluid has the potential for commercial viability. The pumping power for nanofluids has been considered previously for flow in straight tubes. In this study, the pumping power was measured for nanofluids flowing in a complete system including straight tubing, elbows, and expansions. The objective was to determine the significance of two-phase flow effects on system performance. Two types of nanofluids were used in this study: a water-based nanofluid containing 2.0–8.0 vol% of 40-nm alumina nanoparticles, and a 50/50 ethylene glycol/water mixture-based nanofluid containing 2.2 vol% of 29-nm SiC nanoparticles. All experiments were performed in the turbulent flow region in the entire test system simulating features typically found in heat exchanger systems. Experimental results were compared to the pumping power calculated from a mathematical model of the system to evaluate the system effects. The pumping power results were also combined with the heat transfer enhancement to evaluate the viability of the two nanofluids.

Comparison of Power Number for Paddle-Type Impellers by Three Methods

Power numbers in a baffled stirring tank with paddle-type impellers were measured over a range of Reynolds numbers, from laminar- to turbulent-flow regions. The impellers studied included a two-blade flat-paddle impeller, a 45° two-blade pitched-paddle impeller, a 45° four-blade pitched-blade turbine, and a two-stage pitched-blade turbine. The impeller-diameter-to-tank-diameter ratio was 0.5 to 0.6. The measured power numbers were compared with those derived from two other methods: prediction by using correlations and computational fluid dynamics (CFD). Power number correlations showed that the values estimated from the empirical correlations proposed by Nagata agreed closely with those measured in the laminar-flow region but deviated significantly in the transitional- and turbulent-flow regions. The power numbers derived from the correlation proposed by Kamei and Hiraoka agreed well with the experimental results in the laminar- and turbulent-flow regions. The power number in a stirring tank was also simulated by the CFD method. The numerical results showed satisfactory agreement with the experimental data over a wide range of Reynolds numbers.

Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube

An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6 mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2–10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%.

LIMITING MAXIMUM DRAG REDUCTION ASYMPTOTE FOR THE MOMENT COEFFICIENT OF AN ENCLOSED ROTATING DISK WITH FINE SPIRAL GROOVES

In this study, the limiting maximum drag reduction asymptote for the moment coefficient of an enclosed rotating disk with fine spiral grooves in turbulent flow region were obtained analytically. Analysis which were based on an assumption for a simple parabolic velocity distribution of turbulent pipe flow to represent relative tangential velocity, was carried out using momentum integral equations of the boundary layer. For a certain K- parameter the moment coefficient results agree well with experimental results for maximum drag reduction in an enclosed rotating disk with fine spiral grooves and drag reduction ratio approximately was 15 %. Additionally, the experimental results for drag reduction on a rotating disk can be explained well with the analytical results.

Experimental investigation of opposing mixed convection hear transfer in the vertical flat channel in a laminar–turbulent transition region

In this paper we present the results on experimental investigation of the local opposing mixed convection heat transfer in the vertical flat channel with symmetrical heating in a laminar–turbulent transition region. The experiments were performed in airflow ( p = 0.1–1.0 MPa) in the range of Re from 1.5 × 10 3 to 6.6 × 10 4 and Gr q up to 1 × 10 11 at the limiting condition q w 1 = q w 2 = const. The analysis of the results revealed significant increase in the heat transfer with increasing of air pressure ( Gr number). Also sharp increase in heat transfer was noticed in the region with vortex flow in comparison with the turbulent flow region.

Gas adsorption and Surface Diffusion on 5Ã… Microporous Adsorbent in Transition and Turbulent Flow Region

The flow of O2, H2 and CO2 was in transition regime at relatively lower ΔT and then became turbulent at relatively higher ΔT when passed through the 5Å microporous zeolite adsorbent. Adsorption and surface diffusivity of all the gases that are being studied in these regimes increased with increase in ΔT. For the temperature difference from 5 to 115 K, the range of adsorption of O2 was between 6.319?10-12 and 5.762?10-11 kg/s. The adsorption of H2 was between 4.4?10-13 and 3.88?10-12 kg/s for the temperature difference ranging from 1 to 20 K. The adsorption of CO2 was between 5.3?10-12 and 4.9?10-11 kg/s for the temperature difference ranging from 5 to 100 K. At pressure of 1 bar, CO2 adsorption was the highest at all ΔT, followed by O2 and H2. The surface diffusivity increases with increase in ΔT. At a particular ΔT, the diffusivity increases asymptotically regardless of the flow regions.

Wave-Packet Models for Large-Scale Mixing Noise

A wave-packet Ansatz is used to model jet noise generation by large-scale turbulence. In this approach, an equivalent source is defined based on the two-point space-time correlation of hydrodynamic pressure on a conical surface surrounding the jet plume. The surface is sufficiently near the turbulent flow region to be dominated by non-propagating hydrodynamic signatures of large-scale turbulent structures, yet sufficiently far that linear behavior can be assumed in extending the near-field pressure to the acoustic field. In the present study, a 78-microphone array was used to measure hydrodynamic pressure on the conical surface in order to identify parameters for the model and to validate the approach. Six microphones were distributed in the azimuthal direction at each of 13 axial locations spanning the first 8 jet diameters, allowing decomposition of azimuthal modes m = 0 and m = 1. We show that a source model based on a Gaussian correlation function provides a consistently good representation of the noise source attributed to large-scale structures. The results provide evidence that large-scale wave-like structures, known to dominate aft radiation at supersonic phase speeds, are also relevant at subsonic speeds.

Forced flow and convective melting heat transfer of clathrate hydrate slurry in tubes

In this study, we presented the forced flow and convective melting heat transfer characteristics of tetrabutylammonium bromide (TBAB) clathrate hydrate slurry (CHS) flowing through straight circular tubes. Pressure drops of TBAB CHS with the volume fraction in the range of 0–20.0 vol% were measured, and the influence of the volume fraction was analyzed. Power-law equation was used to describe the flow behavior of CHS, and the corresponding indicators including flow behavior index and fluid consistency coefficient indicated that TBAB CHS behaved as a pseudo plastic fluid. Based on the modified Reynolds number, it was found that the empirical correlation could be used to predict the flow friction factor of TBAB CHS. Heat transfer experiments were conducted to measure the local heat transfer coefficients of TBAB CHS flow under constant wall heat flux, which were found to be enhanced by the latent heat involved in the solid–liquid phase change. Local heat transfer coefficient decreased along the heating section until the volume fraction was smaller than a certain small value, then it increased in the following section due to the increase of Reynolds number. The effects of heating power, flow velocity, volume fraction on the local heat transfer coefficients were discussed. Correlations for predicting TBAB CHS heat transfer performance were proposed based on the experimental data in laminar, transitional and turbulent flow regions, respectively.