Turbulent Kinetic Energy Distribution Research Articles

Numerical investigation of heat transfer enhancement of graphene nanofluid in turbulent flow conditions using the Lagrangian two-phase model

This paper aims to introduce one of the parameters which contribute to heat transfer enhancement of graphene nanofluid to seek the optimal heat transfer system. Numerical analysis has been performed on the turbulent heat transfer of the nanofluid in a horizonal stainless steel tube which is subjected to a constant heat flux at its over surface. To model turbulent flow, the finite volume method with realizable k-ε model and Wolfstein model is employed to calculate the momentum, continuity, energy conservation, and turbulent equation of continuous phase in two-dimension domains (Axially symmetric flow). To assure its reliability, wall functions are compared with the result of numerical analysis, and it gained its reliability. Lagrangian two-phase model is employed to solve the mass, momentum, energy conservation of dispersed phase and model the physical interaction between two phases. In this model, phase interaction occurs to predict the influence of one phase upon the other phase within the two-phase simulation. This paper mainly focuses on this phase interaction as parameter which contributes to heat transfer enhancement since many other papers on this topic use a single-phase flow model and fail to focus on this. The results of the two-phase model are compared with those of the single-phase model. As a result, the Nusselt number (Nu) of the graphene nanofluid is higher than the base fluid and increased as the flow rate as other investigation have shown. Finally, some results reveal changed property, such as velocity distribution, turbulent kinetic energy distribution and it is found that this change influences on heat transfer enhancement. Moreover, the insight that nanoparticle gathering is the factor of this change is also obtained.

ラグランジュ二相モデルを用いた乱流条件下におけるグラフェンナノ流体の熱伝達促進に関する数値解析

This paper aims to introduce one of the parameters which contribute to heat transfer enhancement of graphene nanofluid to seek the optimal heat transfer system. Numerical analysis has been performed on the turbulent heat transfer of the nanofluid in a horizonal stainless steel tube which is subjected to a constant heat flux at its over surface. To model turbulent flow, the finite volume method with realizable k-εmodel and Wolfstein model is employed to calculate the momentum, continuity, energy conservation, and turbulent equation of continuous phase in two-dimension domains （Axially symmetric flow）. To assure its reliability, wall functions are compared with the result of numerical analysis, and it gained its reliability. Lagrangian two-phase model is employed to solve the mass, momentum, energy conservation of dispersed phase and model the physical interaction between two phases. In this model, phase interaction occurs to predict the influence of one phase upon the other phase within the two-phase simulation. This paper mainly focuses on this phase interaction as parameter which contributes to heat transfer enhancement since many other papers on this topic use a single-phase flow model and fail to focus on this. The results of the two-phase model are compared with those of the single-phase model and experimental results. Finally, some results reveal changed property, such as velocity distribution, turbulent kinetic energy distribution and it is found that this change influences on heat transfer enhancement. Moreover, the insight that nanoparticle gathering is the factor of this change is also obtained.

Performance Simulation of a Modified Geothermal Grain Dryer Based at Menengai Well 3 in Kenya

Geothermal energy can be effectively utilized for grain drying to reduce carbon emissions and also cut operational costs associated with conventional methods. The main challenges encountered in the use of the geothermal grain dryer, such as in Menengai, Kenya, include uneven grain drying and long throughput times. Grains near the hot air inlet dry at a faster rate compared to those near the exhaust end. Therefore, the grains must be recirculated within the dryer to achieve uniform moisture distribution. Grain recirculation is energy-intensive as it utilizes electricity running the elevator motors in addition to the suction pump. A Computational Fluid Dynamics (CFD) model was developed to study the airflow pattern and its impact on drying of maize. The model was simulated in ANSYS 21 and validated using experimental data. Finite volume discretization method was employed for meshing. Pressure-based segregated solver was used in the Computational Fluid Dynamics (CFD) simulation. Also, K-Omega turbulent model was used for enhancing wall treatment. The findings indicate that non-uniform hot air distribution across the grain buffer section causes uneven drying. Introducing filleted flow-guides results in a relatively uniform velocity, temperature, and turbulence kinetic energy distribution across the dryer. The average velocity and temperature magnitudes in lower compartments increased by 153.3% and 0.25% respectively for the improved dryer. In the upper compartments, the velocity and temperature increase were 176.5% and 0.22% respectively.

Effect of transverse magnetic field on magnetogasdynamic flow and heat transfer characteristics in insulated / conductive circular tubes

The regulating effect of magnetic field on magnetogasdynamic flow and heat transfer characteristics in circular tubes has important applications in many fields, but there is still a lack of relevant basic research. Considering the conductivity of the tube wall and the insufficient development of turbulence, the physical model and mathematical model of magnetogasdynamic flow in a circular tube under a given transverse magnetic field are constructed, and the numerical algorithm is designed within a theoretical framework of the finite volume method. The effect of factors including Hartman number (<i>Ha</i>) and wall conductivity ratio (<i>C</i>) on the flow and heat transfer characteristics are obtained through analyzing the distributions of velocity, turbulent kinetic energy, and temperature. Furthermore, the regulation mechanism of the transverse magnetic field is discussed by analyzing the spatial distribution of induced current, electromagnetic force and Joule heat. The results show that the distribution of velocity and the distribution of turbulent kinetic energy in the circular tube under a given transverse magnetic field are both anisotropic. The turbulent kinetic energy near the Hartmann boundary layer is much lower than that near the Roberts boundary layer, and the anisotropic distribution of velocity and turbulent kinetic energy become more and more evident with the increase of <i>Ha</i> and the extension of the flow. The transverse magnetic field has a suppression effect on the heat transfer in the tube. For different values of <i>C</i>, the average Nusselt number (<inline-formula><tex-math id="M1">\begin{document}$ \overline {Nu} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M1.png"/></alternatives></inline-formula>) shows a first-decreasing-and-then-increasing trend with <i>Ha</i> increasing, that is, there is a “saturation effect” in heat transfer suppression. When the wall conductivity is small (<i>C</i> <inline-formula><tex-math id="Z-20220808124116">\begin{document}$\leqslant $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_Z-20220808124116.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_Z-20220808124116.png"/></alternatives></inline-formula> 0.67), the change of <inline-formula><tex-math id="M2">\begin{document}$ \overline {Nu} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M2.png"/></alternatives></inline-formula> under the condition of conductive wall is basically consistent with that of an insulating wall. However, when <i>C</i> exceeds a certain value (<i>C</i> <inline-formula><tex-math id="Z-20220808124049">\begin{document}$\geqslant $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_Z-20220808124049.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_Z-20220808124049.png"/></alternatives></inline-formula> 66.67), the <inline-formula><tex-math id="M3">\begin{document}$ \overline {Nu} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M3.png"/></alternatives></inline-formula> under the condition of small <i>Ha</i> increases in comparison with that of the insulating wall, while the <inline-formula><tex-math id="M4">\begin{document}$ \overline {Nu} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M4.png"/></alternatives></inline-formula> decreases under the condition of large <i>Ha</i> . The change of flow characteristics in the circular tube results from the variation of electromagnetic force under the coupling of magnetic field and fluid, while the change of heat transfer characteristics originates from the coupling effect of the suppression of turbulence and the Joule heating. When <i>Ha</i> is small, the suppression effect of the magnetic field on turbulence is dominant, and the <inline-formula><tex-math id="M5">\begin{document}$ \overline {Nu} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M5.png"/></alternatives></inline-formula> decreases with the increase of <i>Ha</i>. When <i>Ha</i> exceeds a certain value (<i>Ha </i><inline-formula><tex-math id="Z-20220808123523">\begin{document}$\geqslant $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_Z-20220808123523.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_Z-20220808123523.png"/></alternatives></inline-formula> 222), the large accumulation of Joule heat in the circular tube enhances the heat transfer, resulting in the increase of the <inline-formula><tex-math id="M6">\begin{document}$ \overline {Nu} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20220051_M6.png"/></alternatives></inline-formula> with the continuous increase of <i>Ha</i>.

Analysis of Turbulent Flow Structure with Its Fluvial Processes around Mid-Channel Bar

Researchers have recognized that the successive growth of mid-channel bar deposits can be entertained as the raison d’être for the initiation of the braiding process, which is closely interlinked with the growth, decay, and vertical distribution of fluvial turbulent kinetic energy (TKE). Thus, focused analysis on the underlying mechanics of turbulent flow structures in the proximity of a bar deposit occurring in the middle of the channel can afford crucial scientific clues for insight into the initiating fluvial processes that give rise to braiding. In the study reported herein, a physical model of a mid-channel bar is constructed in an experimental flume to analyze the turbulence parameters in a region close to the bar. Notably, the flow velocity plays an important role in understanding the flow behavior in the scour-hole location in the upstream flow divergence zone as well as near the downstream zone of flow convergence in a mid-channel bar. Therefore, the fluctuating components of turbulent flow velocity are herein discussed and analyzed for the regions located close to the bar. In the present study, the impact of the mid-channel bar, as well as its growth in turbulent flow, on higher-order velocity fluctuation moments are investigated. For near-bed locations, the results show the dominance of ejection events in upstream zones and the dominance of sweep events at locations downstream of the mid-channel bar. In scour-hole sections, the negative value of the stream-wise flux of turbulent kinetic energy and the positive value of the vertical flux of turbulent kinetic energy indicate energy transport in downward and forward directions, respectively. The downward and forward energy transport processes lead to scouring at these locations. The maximum turbulent production rate occurs in the wake region of the bar. The high rate of turbulence production has occurred in that region, which can be ascribed to the process of shedding turbulent vortices. The results show that the impact of the presence of the bar is mainly restricted to the lower layers of flow. The turbulent dissipation rate monotonically decreases with an increase in the vertical distance from the bed. The turbulent production rate first increases and then decreases with successive increases in the vertical distance from the bed. The paper concludes with suggestions for the future potential use of the present research for the practical purpose of examining braid bar occurrences in alluvial rivers to develop an appropriate response through training measures.

Influence of the contraction angle on the injection of large coal particles

The CFD–DEM coupling numerical calculation and pneumatic conveying experiment were carried out on the injection of large coal particles. The influence of contraction angle on the flow field and coal particles was studied by the distribution of turbulent kinetic energy, instantaneous distribution of particles and trajectories of particles. The performance of injection was studied by injected particles, remaining particles and mass flux of particles. The results show that the center of turbulence is located in the top of the mixing section and constriction section. The particle collision frequency increases with the increase of the contraction angle, and the conveying state of coal particles changes from uniform flow to bottom flow. When the contraction angle is 12.5°, the injector has the maximum efficiency. Since the range of mass flux is less than 1%, 10°–15° can be regarded as the optimal contraction angle.

Experimental and CFD Characterization of a Double-Orifice Synthetic Jet Actuator for Flow Control

The paper presents a joint experimental and numerical characterization of double-orifice synthetic jet actuators for flow control. Hot-wire measurements of the flow field generated by the device into a quiescent air environment were collected. The actuation frequency was systematically varied to obtain the frequency response of the actuator; its coupled resonance frequencies were detected and the velocity amplitude was measured. Direct numerical simulations (DNS) of the flow field generated by the device were subsequently carried out at the actuation frequency maximizing the jet output. The results of a fine-meshed parametric analysis are outlined to discuss the effect of the distance between the orifices: time-averaged flow fields show that an intense jet interaction occurs for small values of the orifice spacing-to-diameter ratio; phase-averaged velocity and turbulent kinetic energy distributions allow to describe the vortex motion and merging. A novel classification of the main regions of dual synthetic jets is proposed, based on the time- and phase-averaged flow behaviour both in the near field, where two distinct jets converge, and in the far field, where an unique jet is detected. The use of three-dimensional DNS also allows to investigate the vortex merging for low values of the jet spacing. The work is intended to provide guidelines for the design of synthetic jet arrays for separation control and impinging configurations.

Large-eddy simulation of a reacting swirling flow in a model combustion chamber

We perform Large-eddy simulations of a non-premixed swirling flame in a model of a combustion chamber with a swirling air bulk flow at Re = 15000 and a central pilot low-velocity jet with methane using the Flamelet-generated manifold model. The unsteady behaviour of this regime is well reproduced based on the flame dynamics. The distribution of turbulent kinetic energy suggests the presence of intensive vortical structures typical of high-swirl flows similar to the precessing vortex core.

Influence of pressure on the wake of a hydrophobic circular cylinder for a flow regime with Reynolds number of 2.2×105

Vortex flow structures in a turbulent wake behind a circular Teflon cylinder immersed in an incoming flow with a change in pressure for the Reynolds number Re = 2.2×105 have been experimentally studied using a two-dimensional image (2D-PIV) of particles in a closed-circuit water tunnel. The obtained results are presented in the form of time-averaged velocity fields, Reynolds stresses, and distributions of turbulent kinetic energy. The flow data showed that the size of the wake flow region, Reynolds stresses and turbulent kinetic energy change depending on the pressure in the flow. As a result of a 20% reduction in pressure, the size of the vortex zone in the wake increases by about 20%.

Effect of Mold Width on the Flow Field in a Slab Continuous-Casting Mold with High-Temperature Velocity Measurement and Numerical Simulation

In this paper, the effects of the width of the mold on the surface velocity, flow field pattern, turbulent kinetic energy distribution, and surface-level fluctuation in the mold were studied with measurement of the flow velocity near the surface of the mold at high temperature with the rod deflection method and numerical calculation with the standard k-ε model coupled with the discrete-phase model (DPM) model for automobile exposed panel production. Under the conditions of low fixed steel throughput of 2.2 ton/min, a nozzle immersion depth of 140 mm, and an argon gas flow rate of 4 L/min, as the width of the mold increases from 880 mm to 1050 mm and 1300 mm, the flow velocity near the surface of the mold decreases. The flow direction changes from the positive velocity with the mold widths of 880 mm and 1050 mm to the unstable velocity with the mold width of 1300 mm. The calculated results are in good agreement with the measured results. The turbulent kinetic energy near the submerged entry nozzle (SEN) gradually increases, and the risk of slag entrainment increases. Under the conditions of high fixed steel throughput of 3.5 ton/min, the SEN immersion depth of 160 mm, and the argon gas flow rate of 10 L/min, as the width of the mold increases from 1600 mm to 1800 mm and 2000 mm, the velocity near the mold surface decreases. The flow velocity at 1/4 of the surface of the mold is positive with the mold width of 1600 mm, while the velocities are negative with the widths of 1800 mm and 2000 mm. The calculated results are basically consistent with the measured results. The high turbulent kinetic energy area near the nozzle expands to a narrow wall, and the risk of slag entrainment is significantly increased. In both cases of low and high fixed steel throughput, the change rules of the flow field in the mold with the width are basically the same. The argon gas flow rate and the immersion depth of SEN should be adjusted reasonably to optimize the flow field in the mold with different widths under the same fixed steel throughput in the practical production.

Comparative analysis of transient valve-induced flow characteristics between opening and closing processes

Transient control of process valves, including opening and closing processes, is consistently encountered in many fluid transportation and control industries. During opening and closing processes, valve-induced transient flow presents different unstable flow characteristics. This transient valve-induced unstable flow that develops along the pipeline can cause violent pressure and velocity fluctuations that considerably influence accurate flow measurement downstream. In this paper, gate valve-induced flow characteristics during opening and closing processes were comparatively studied. An experimental system was developed to monitor the downstream pressure along the pipeline, and corresponding transient numerical simulations were performed on opening and closing processes using a user-defined function and dynamic grid technology. The pressure distributions along the pipeline's downstream area during valve opening and closing processes were investigated to verify the accuracy of the numerical simulation. The mechanism of transient flow difference under the same valve opening during opening and closing processes was determined to be a hysteresis effect. The jet flow intensity under a small valve opening in the opening process was greater than that in the closing process, and the difference in flow field under the 50% valve opening was the largest. Moreover, the velocity and turbulent kinetic energy distributions in different downstream cross-sections during valve opening and closing processes were comparatively analyzed. The change rate of the maximum turbulent kinetic energy was introduced to further analyze the different effects of opening and closing processes on the transient flow stability downstream of the valve. Results showed that the flow stability between 40% and 50% valve opening was the worst irrespective of the adjustment process, that is, a large pipeline distance was required to stabilize this transient flow. This study helps in understanding transient valve-induced flow characteristics in fluid transportation pipelines and provides guidance for accurate flow metering industrial applications.

A study on diurnal microclimate hysteresis and plant morphology of a Buxus sempervirens using PIV, infrared thermography, and X-ray imaging

Plants modify the climate and provide natural cooling through transpiration. However, plant response is not only dependent on the atmospheric evaporative demand due to the combined effects of wind speed, air temperature, humidity, and solar radiation, but is also dependent on the water transport within the plant leaf-xylem-root system. These interactions result in a dynamic response of the plant where transpiration hysteresis can influence the cooling provided by the plant. Therefore, a detailed understanding of such dynamics is key to the development of appropriate mitigation strategies and numerical models. In this study, we unveil the diurnal dynamics of the microclimate of a Buxus sempervirens plant using multiple high-resolution non-intrusive imaging techniques. The wake flow field is measured using stereoscopic particle image velocimetry, the spatiotemporal leaf temperature history is obtained using infrared thermography, and additionally, the plant porosity is obtained using X-ray tomography. We find that the wake velocity statistics are not directly linked with the distribution of the porosity but depends mainly on the geometry of the plant foliage which generates the shear flow. The interaction between the shear regions and the upstream boundary layer profile is seen to have a dominant effect on the wake turbulent kinetic energy distribution. Furthermore, the leaf area density distribution has a direct impact on the short-wave radiative heat flux absorption inside the foliage where 50% of the radiation is absorbed in the top 20% of the foliage. This localized radiation absorption results in a high local leaf and air temperature. Furthermore, a comparison of the diurnal variation of leaf temperature and the net plant transpiration rate enabled us to quantify the diurnal hysteresis resulting from the stomatal response lag. The day of this plant is seen to comprise of four stages of climatic conditions: no-cooling, high-cooling, equilibrium, and decaying-cooling stages.

Turbulent Characteristics and Air Entrainment Patterns in Breaking Surge Waves

Breaking surge waves are highly turbulent three-dimensional (3D) flows, which occur when the water flow encounters a sudden change in depth or velocity. The 3D turbulent structures across a breaking surge are induced by the velocity gradient across the surge and phase discontinuity at the front. This paper examined the turbulent structures in breaking surge waves with Froude numbers of 1.71 and 2.13 by investigating the air entrainment and perturbation patterns across the surge front. A combination of the Volume Of Fluid (VOF) method and Large Eddy Simulation (LES) was utilized to capture air entrainment and turbulent structures simultaneously. The 3D nature of the vortical structures was simulated by implementing a spanwise periodic boundary. The water surface perturbation and air concentration profiles were extracted, and the averaged air concentration profiles obtained from the numerical simulations were consistent with laboratory observations reported in the literature. The linkage between turbulent kinetic energy distribution and air entrainment was also explored in this paper. Finally, using quadrant analysis and the Q-criterion, this paper examined the role of the spanwise perturbations in the development of turbulent structures in the surge front.

Engineering characterization of the novel Bach impeller for bioprocessing applications requiring low power inputs

The choice of impeller design in stirred-tank reactors (STRs) affects important process characteristics such as the suspension of particulates, mixing of nutrients or gasses, and the distribution of turbulent kinetic energy and its dissipation rate. A novel impeller, the Bach impeller, has been characterized in this work and its novel geometry and mechanism of action are discussed in detail. Experimental measurements of the power consumption, mixing time, and microparticles suspension dynamics were carried out at the 1 L scale in an unbaffled reactor and the performance compared with two conventional up-pumping axial flow impellers, namely the 3-Blade segment (3BS, also called “elephant ear”) and marine impeller. In addition, the impeller flow number and pumping efficiency were obtained from Particle Image Velocimetry experiments. The Bach impeller exhibited a low power number of NP=0.36 and showed it can generate a uniform suspension of microcarriers even at relatively low impeller speeds (from N=25–30 rpm onwards). The impeller was found to effectively mix the reactor volume within tM=25 s at N>50 rpm (i.e. NtM=37±5) at a higher clearance when it was mounted closer to the free surface, C/T>0.5, while minimally affecting its suspension capabilities. Lastly, the Bach impeller displaced more fluid than the marine impeller and roughly an equal amount to the 3BS impeller, suggesting that the novel impeller can successfully be used for STR processes in the (bio)chemical engineering industry where low power inputs, but high mixing and particle suspension efficiencies are sought.

Large eddy simulation of boundary-layer turbulence over the heterogeneous surface in the source region of the Yellow River

Abstract. Lake breezes are proved by downdrafts and the divergence flows of zonal wind in the source region of the Yellow River (SRYR) in the daytime based on ERA-Interim reanalysis data. In order to depict the effect of the circulations induced by surface anomaly heating (patches) on the boundary-layer turbulence, the UK Met Office Large Eddy Model was used to produce a set of 1D strip-like surface heat flux distributions based on observations, which were obtained by a field campaign in the Ngoring Lake basin in the summer of 2012. The simulations show that for the cases without background wind, patch-induced circulations (SCs) promote the growth of convective boundary layer (CBL), enhance the turbulent kinetic energy (TKE), and then modify the spatial distribution of TKE. Based on phase-averaged analysis, which separates the attribution from the SCs and the background turbulence, the SCs contribute no more than 10 % to the vertical turbulent intensity, but their contributions to the heat flux can be up to 80 %. The thermal internal boundary layer (TIBL) reduces the wind speed and forms the stable stratification, which produces the obvious change of turbulent momentum flux and heat flux over the heterogeneous surfaces. The increased downdrafts, which mainly occur over the lake patches, carry more warm, dry air down from the free atmosphere. The background wind inhibits the SCs and the development of the CBL; it also weakens the patch-induced turbulent intensity, heat flux, and convective intensity.

Enhancement of turbulent liquid-liquid dispersions in a round-bottom stirred tank equipped by a flexible impeller

Stirred tanks have been widely used for liquid-liquid mixing in the chemical industry. In the present work, the influences of flexible impeller on the hydrodynamic characteristics and droplet size distribution of oil in water dispersions were investigated using experimental and numerical approaches. The one-way Fluid Structural Interaction (FSI) analysis was carried out to model the geometry of the flexible impeller used in CFD simulations. Computational fluid dynamics (CFD) was coupled with the population balance model (PBM) to simulate the liquid-liquid dispersion process in the stirred tank. The numerical model was validated by the experimental results. Compared with the rigid impeller, the flexible impeller generates a more uniform distribution of turbulent kinetic energy and turbulent dissipate rate in the stirred tank. Compared with the case of rigid impeller, the region indicating the trailing vortices behind the impeller blades was significantly reduced by the flexible impeller. It was found the flexible impeller with a smaller elastic modulus can result in narrower droplet size distributions in turbulent dispersion process. Its peak value of the number fraction frequency was in the range of 244-308μm while that was 308-388μm for the case of rigid impeller. It was found that most of the dispersed phase droplets occur in the turbulent inertial subrange regime. Related correlations between the Sauter mean diameter and local turbulent field were also obtained. The shape factor accounting for the effects of the impeller shape on the Sauter mean diameter was found to be dependent on the material property of the impeller blades, such as the Poisson's ratio and elastic modulus.

Heat and Flow Analysis of Different Piston Bowl Geometries in a Diesel Engine

In this study, heat and flow in a four cylinder direct injection diesel engine combustion chamber has been simulated by using AVL Fire ESE Diesel. In order to clarify the effect of the combustion chamber geometry on the flow area prop-erties, the bowl geometric shape of two different pistons, one of the combustion chamber as a standard, was taken into account in the flow analysis. Standard combustion chamber and modified combustion chamber geometries have been compared. The simulation results showed that the bowl shapes of the combustion chambers are quite effective on temperature-spray droplet distribution, turbu-lence kinetic energy distributions, turbulence velocity distributions and laminar flame speed distributions at the end of the compression stroke. The fuel reaches the cylinder wall more easily and then, the temperature distribution in the cham-ber is lower as a result of evaporation of the fuel with modified combustion chamber. Average turbulent kinetic energy value in the MCC type combustion chamber is 10.53 m²/s², in the standard combustion chamber type combustion chamber this value is 8.35 m²/s² at 720° CA. Turbulence velocity distribution is spread over a wider area in the modified combustion chamber geometry. As a re-sult of the large area of turbulence, the laminar flame velocity has also increased in this chamber geometry.

Permeability Characteristics of Water-Sand Seepage in Fracture by Experiment

Studies on the seepage characteristics of water-sand cracks are of great significance to reveal the mechanisms of water and sand inrush. Using a self-made water-sand fracture seepage test instrument, a water-sand seepage test was carried out, and the permeability of water and sand in the fracture was determined. The hysteresis characteristics of water-sand flow in the fracture were obtained after the required permeability was attained. The results show that the hysteresis curve changes from type I to type IV with the increase in sand particle size and concentration. The hysteresis parameters are described by the maximum hysteresis Gp ∗ and the hysteresis area S, both of which show an increasing trend with the increase in sand particle size and concentration; however, this increase is not synchronous. The average velocity and turbulent kinetic energy distribution of the water-sand fluid on the fracture cross section are greatly affected by the particle size and concentration of the volume of sand. This study can provide a reference for further study of water inrush from a shallow coal seam. Through simulation, it is found that the particle size has a great influence on the seepage velocity, and the influence near the side wall surface is greater than that in the middle position.

Hydraulic characteristics of countercurrent jets on adverse-sloped beds

Abstract The counter-current jet (CCJ) acts like a reverse surface jet layer covering the free jump surface and has potential applications in the energy dissipation of hydraulic engineering. The present study investigated the hydraulic characteristics of CCJs on adverse-sloped beds. The results showed that, compared to the horizontal bed, the slope didn't increase the energy dissipation rate of CCJ but reduced the return flow length and upstream depth. The velocity distribution along the depth was divided into the boundary layer region, mixing region, and reverse surface jet region. The velocity distribution in the boundary layer region and the mixing region was similar to the classical wall jet. The jet Froude number and the bed slope had no significant effect on the turbulence intensity distribution and turbulence kinetic energy (TKE) distribution of CCJ. The distribution of TKE was similar to that of a submerged jump. The maximum absolute turbulence intensity appeared at exactly half of the maximum velocity. The maximum TKE appeared at the mixing region. Besides, empirical formulas for estimating length scales and maximum velocity attenuation are proposed. The results could provide a reference for the potential application of CCJ in energy dissipation in hydraulic engineering.

Numerical simulation on hydraulic performance and tip leakage vortex of a slanted axial-flow pump with different blade angles

The blade angle has a great effect on hydraulic performance and internal flow field for axial-flow pumps. This research investigated the effect of the blade angle on hydraulic performance and tip leakage vortex (TLV) of a slanted axial-flow pump. The hydraulic performance and the TLV are compared with different setting angles. The dimensionless turbulence kinetic energy (TKE) is used to investigate the TLV. A novel variable fv is utilized to analyze the relation among the TLV, strain tensor and vorticity tensor. The proper orthogonal decomposition (POD) method is used to analyze TLV structure. The results show that with the increase of the blade angle, the pump head is getting larger, the flow rate of the best efficiency moves to be larger, and both the primary TLV (P-TLV) and the secondary TLV (S-TLV) are getting stronger. The P-TLV often exists in the outer edge of TKE distribution and S-TLVs often exist in the largest value area of TKE. This phenomenon is more evident with blade angle increasing. Through POD method, it shows that the first six modes contain more than 90% of TKE. The reason why the TKE value near the region of S-TLV is high is that the tip leakage flow is a kind of jet-like flow with high kinetic energy. The main structure of the P-TLV is shown in modes 4−6, resulting in a reflux zone but not with the highest TKE.