Dimensional Turbulent Flow Research Articles

Effect of Design Parameters and Support Conditions on Natural Frequency of Pipe Excited by a Turbulent Internal Flow

In this study, the effect of design parameters such as pipe diameter, pipe wall thickness, pipe material and the effect of fluid velocity on the natural frequency of fluid-structure interaction in straight pipe conveying fully developed turbulent flow were investigate numerically,analytically and experimentally. Also the effect of support conditions, simply-simply and clamped-clamped was investigated. Experimentally, pipe vibrations were characterized by accelerometer mounted on the pipe wall. The natural frequencies of vibration were analyzed by using Fast Fourier Transformer (FFT). Five test sections of two different pipe diameters of 76.2 mm and 50.8 mm with two pipe thicknesses of 3.7 mm and 2.4 mm and two pipe materials,stainless steel and polyvinyl chloride PVC in the range of Reynolds numbers from 4*104 to 5*105 were studied. Mathematically, the governing continuity and momentum equations were solved numerically by using the finite volume method to compute the characteristics of two dimensional turbulent flow. The dynamics of a pipe conveying fluid was described by the Transfer Matrix Method (TMM) which is provides a numerical technique for solving the equations of pipe vibrations for simply-simply and clamped supports. The results showed that,the natural frequencies increase with pipe diameter increase and the natural frequencies slightly increases with pipe wall thickness increase. Also, the natural frequencies in clamped-clamped supported pipe are higher than those in simply-simply supported pipe.

Influence of Internal Fins and Nanoparticles on Heat Transfer Enhancement Through a Parabolic Trough Solar Collector

Current article presents a numerical investigation for three dimensional turbulent flow using the MCRT method to simulate solar radiations as a heat flux and the finite volume method in Ansys Fluent to simulate the equations. The fluid in absorber is a synthetic oil having eight hundred index mixed with Aluminium Oxide nanoparticles to enhance the heat transfer process. The study comparing different cases, including different numbers of longitudinal fins assembled inside the absorber with two arrangements and three particular Reynolds number in addition to the testing concentration ratio of the heat flux using the MCRT method. The impact of using three different numbers of fins (2, 4 and 6) at two different arrangements ("a" and "b") were presented. The three Reynolds number used was (18600, 23000 and 28000). Two Rim angles of the collector are tested (80o) and (120o), results showed that (120º) was achieved high outlet temperature with a concentration ratio (CR=82.47). The effects on outlet temperature, friction factor, Nusselt number, and then on the performance evaluation criterion (PEC) of the absorber tube were displayed. The results of the study proved that using nanofluid and finned absorber can increase the absorber hydrothermal performance and using (6) fins of arrangement "b" shows better performance in terms of heat transfer than arrangement "a".

Level set-based topology optimization for two dimensional turbulent flow using an immersed boundary method

This paper presents a topology optimization method for two dimensional turbulent flow based on the Reynolds-averaged Navier-Stokes (RANS) equations using a level set boundary expression and the immersed boundary method (IBM). In this study, two-equation turbulence models, the k-ϵ and the k-ω, are considered. In our proposed method, the level set method is used for capturing the exact fluid-solid interfaces. Additionally, the no-slip boundary condition along the fluid-solid interfaces is imposed explicitly using the IBM during the optimization process. Based on the information of the exact boundary position, the interpolated velocity and pressure values within the viscous sublayer are estimated using the standard wall function. From the above formulations, we construct a topology optimization method for the total pressure drop minimization problems considering two dimensional turbulent flows, under the frozen turbulence assumption. We provide numerical examples to confirm the validity and utility of the proposed method.

Numerical study on turbulent characteristics of wall jet in a quiescent environment over a plate in motion

The present article reports the numerical analysis of a two dimensional turbulent wall jet flow over a horizontal moving plate in a quiescent environment. The simulation is done using the low-Reynolds number turbulence model proposed by Yang and Shih [21]. The turbulent characteristics are observed for various velocity ratio of plate with respect to jet (Up ) from keeping Reynolds number at jet inlet to be constant at 15000. The Reynolds averaged Navier Stokes (RANS) equations are discretized using the finite volume method and solved in steady state condition. The pressure correction technique applied to couple the velocity and pressure terms is SIMPLEC (semi-implicit pressure linked equation-consistent). The variation of jet half-width and the decaying local maximum velocity (Umax ) for the bottom plate moving with high velocity (Up > 1) shows dominancy of plate motion over the jet stream in the flow field. The velocity contour diagram shows the velocity distribution in the flow field. The Reynolds stresses induced in the wall proximate regions are directly proportional to the relative motion of the flow field but the magnitude of Reynolds stress away from wall decreases by increasing the plate velocity. The plate motion significantly affects the turbulence characteristics and the flow field in the viscous laminar zone of the wall region.

An Urban Lagrangian Stochastic Dispersion Model for Simulating Traffic Particulate-Matter Concentration Fields

The accumulated particulate matter concentration at a given vertical column due to traffic sources in urban area has many important consequences. This task, however, imposes a major challenge, since the problem of realistic pollutant dispersion in an urban environment is a very demanding task, both theoretically and computationally. This is mainly due to the highly inhomogeneous three dimensional turbulent flow regime in the urban canopy roughness sublayer, which is far from “local equilibrium” between shear production and dissipation. We present here a mass-consistent urban Lagrangian stochastic model for pollutants dispersion, where the flow field is modeled using a hybrid approach by which we model the surface layer based on the typical turbulent scales, both of the canopy and in the surface layer inertial sub-layer. In particular it relies on representing the canopy aerodynamically as a porous medium by spatial averaging the equations of motion, with the assumption that the canopy is laterally uniform on a scale much larger than the buildings but smaller than the urban block/neighbourhood, i.e., at the sub-urban-block scale. Choosing the spatial representative averaging volume allows the averaged variables to reflect the characteristic vertical heterogeneity of the canopy but to smooth out smaller scale spatial fluctuations caused as air flows in between the buildings. This modeling approach serves as the base for a realistic and efficient methodology for the calculation of the accumulated concentration from multiple traffic sources for any vertical column in the urban area. The existence of multiple traffic sources impose further difficulty since the computational effort required is very demanding for practical uses. Therefore, footprint analysis screening was introduced to identify the relevant part of the urban area which contributes to the chosen column. All the traffic sources in this footprint area where merged into several areal sources, further used for the evaluation of the concentration profile. This methodology was implemented for four cases in the Tel Aviv metropolitan area based on several selected summer climatological scenarios. We present different typical behaviors, demonstrating combination of source structure, urban morphology, flow characteristics, and the resultant dispersion pattern in each case.

Computational analysis of heat transfer due to turbulent annular jet impingement

Present paper deals with the numerical study of fluid flow and heat transfer due to an annular turbulent jet impingement on a flat heated plate. The geometrical configurations of annular jet was selected such a way that the mass and momentums efflux at the annular jet exit will be same as a circular jet having same inner diameter at a particular Reynolds number. Three dimensional turbulent flow field was resolved using Transition SST Model by solving the mass, momentum and energy equation with SIMPLE algorithm. A highly refined mesh was used for numerical computation after validation of present model with established results of same geometrical configuration. The flow structure and heat transfer characteristic of the annular jet was compared with the conventional circular jet and also with an established two dimensional study of same configurations and reported in this paper. It was observed that that two dimensional studies on annular jet under predicts heat transfer characteristics.

Heat transfer intensification in a LFR unit considering exergy analysis of radiative and convective mechanism

In current solar unit, mirrors with involve of parabolic secondary reflector has been employed to enhance the absorbed radiation flux. The optical efficiency with involve of modified secondary reflector augments about 21%. The absorbed flux profile was added in energy equations as source term and three dimensional turbulent flows have been analyzed. Bottom of the pipe facing with mirrors has greatest radiation flux and its value is 7500 W/m2. The top point of pipe has higher radiation flux than left and right sides because of presence of secondary reflector. Water as base fluid inside the pipe was mixed with Al2O3. Besides, install of fins has been considered as second approach to help the thermal performance. With increase of fin length and Re, swirl flow enhances and superior cooling obtains which offers thinner boundary layer. At highest values of other factors, growth of Re leads to 0.43% reduction in temperature while rise of Tin makes temperature to augment about 9.97%. The value of ηex for Tin = 323.15 is 97.43 times greater than that of Tin = 298.15 when Re = 8000 and t = 0. With select of highest values of Re and lowest values of Tin, the finned tube (t = 3 mm) has greatest thermal efficiency.

Heat transfer optimization through new form of pin type of finned tube heat exchangers using the exergy and energy analysis

Abstract In the present work, computational fluid dynamics method is used to simulate the three dimensional turbulent thermal energy transfer and fluid flow through the new types of the finned tube heat exchangers. The realizable turbulent kinetic energy - turbulent kinetic energy dissipation model is selected as the turbulence model because it has the best results compared to the other turbulence models with the experimental data. After the simulation the energy and exergy analysis are performed and a multi-objective model is defined to obtain the optimal and critical values of the operational and geometrical parameters. Furthermore, equations are suggested for estimation the three objective parameters i.e. thermal effectiveness, dimensionless exergy loss and dimensionless work. Results also indicate, the dimensionless exergy loss is optimum at Re=8333 and Pr=10.63. Furthermore, it was found, the optimum value for the transverse pitch is equal to 3.52, when the objective parameters are the thermal effectiveness and the dimensionless exergy loss.

Effect of winglet vortex generators configuration on thermal performance of a heated rectangular channel

In present paper, a three dimensional turbulent flow; and heat transfer inside a rectangular channel equipped with array of winglet vortex generators has been investigated experimentally and numerically. Three pairs array of winglet vortex generators of different geometrical configuration as rectangular, triangular, semi circle and parabolic are considered. The array of winglet vortex generators is formed and distributed on the bottom hot surface with a facility for changing the angle of attack from (0° to 60°). ANSYS Fluent Code (15.0) based on a finite volume method is used to get the numerical results while a k-w turbulence model is used to model the turbulent . The Reynolds number range is from 10000 to 50,000 under a constant heat flux boundary condition. The spin wise between the vortex generators are set to 1 cm and 1.5 cm while the stream wise between the vortex are 9cm,12cm and 18 cm respectively. The results indicated that the rectangular winglet vortex generators array indicated the optimum thermal performance with considerable penalty of the pressure drop while the parabolic vortex generator stated the minimum performance among the considered vortex generators shapes. The overall efficiency for rectangular, triangular, semi circle and parabolic winglet vortex generators is enhanced by (166%), (130%), (165%) and (121%) respectively as compared with plane channel.

Diffusion of ellipsoids in laboratory two-dimensional turbulent flow

We report on the transport properties and orientational dynamics of ellipsoidal objects advected by laboratory two-dimensional turbulence. It is found that ellipsoids of different sizes have preferential direction of transport, either along their major axes or minor axes. The two components of the ellipsoid diffusion coefficient depend on the ratio of the length of the ellipsoids along major axes aa to the turbulence forcing scale Lf. Large ellipsoids (aa > Lf) diffuse faster in the direction parallel to their major axes. In contrast, small ellipsoids diffuse faster in the direction transverse to their major axes. We study this transition vs the ratio aa/Lf and relate it to the coupling between translational and rotational motion of anisotropic objects. The features of the turbulent transport of ellipsoids can be understood by considering the interaction of these anisotropic objects with the underlying structure of two dimensional turbulent flows made of meandering coherent bundles.

Numerical Investigation of Three-dimensional Turbulent Wind Flow around Two Square Buildings with Hip Roofs

This computational work aims at studying the three – dimensional turbulent wind flow around two square buildings with pyramid roofs of trivial architectural forms for an arbitrary geographical location. The overall investigation substantially reduces to the fundamental problem of an external three – dimensional turbulent flow field past a mounted obstacle of predefined shape. The novelty of this research is that the independency of the numerical solution for any possible distribution of mesh points was demonstrated in a theoretical manner without the necessity of changing the original grid with simultaneous repetition of the computational process.

Numerical Resolution of Fluid Dynamic Problems Using a Saddle Point Variational Formulation

Variational methods are useful for finding numerical solutions of differential equations, which are the corresponding Euler-Lagrange equations to the stationary condition of the functional. Usually the functional is a maximum or a minimum with respect to some function, but in some cases the functional is a saddle point. In this work a saddle point variational formulation is proposed to solve fluid dynamic problems, and the saddle point is found through an iterative method using the optimization software GAMS. Two case studies are solved to show the applicability of the proposed method, one for a single fluid in a two dimensional laminar flow in a pipe and another for a one dimensional turbulent flow for gas-liquid column.Key Words: variational formulation, saddle point, fluid dynamics.

Solution of three-dimensional temperature and turbulent velocity field in continuously cast steel billets with electromagnetic stirring by a meshless method

This paper demonstrates the development of meshless Local Radial Basis Function Collocation Method (LRBFCM) for solution of three dimensional (3D) turbulent molten steel flow and solidification under the influence of electromagnetic stirring (EMS) and its application to continuous casting process of steel billets. A mixture continuum approach is used to formulate the coupled set of macroscopic equations for mass, momentum, energy, turbulent kinetic energy, and dissipation rate in Cartesian coordinates. The mushy zone is treated as a Darcy porous media. The explicit Chorin fractional step method is used to resolve the pressure-velocity coupling. The LRBFCM uses explicit time discretization and spatial discretization with shape-adaptive multiquadrics radial basis functions collocation on non-uniform seven-noded influence domains, and displacement-adaptive upwind scheme. The Lorentz force due to EMS is provided by a one-way coupling by Elmer software. The advantages of the meshless method are trivial adaptation of the nodal densities, simple upgrade to 3D from previous two-dimensional models, and high accuracy. A study on the influence of changing the EMS parameters on the calculated temperature and velocity fields is performed. The paper confirms the ability of LRBFCM meshless solution of a realistic 3D multiphysics industrial problem with complicated swirling flow pattern.

Turbulent heat transfer optimization for solar air heater with variation method based on exergy destruction minimization principle

In this paper, a heat transfer optimization approach is brought in with focusing on exergy destruction to properly deal with the trade-off between irreversibility of heat transfer process and pump power consumption in turbulent flow for solar air heater. Thus, the exergy destruction minimization principle is mathematically extended to three dimensional turbulent flow and governing equations are derived with variation method. Successively, the optimized flow field for solar air heater is obtained at Re = 12,000 by applying SST k–ω turbulence model. Furthermore, with detail analysis of velocity and temperature distribution, it is found that the longitudinal swirl flow is generated through this optimization approach. Besides, within the scope of this study, the total heat transfer exergy destruction and average absorber temperature are maximally decreased by about 65% and 30 K respectively in the test section compared with the plain duct. And the maximum Nusselt number and friction factor are increased to 1.81 and 3.13 times over plain duct respectively. These results indicate that the optimization approach is effective for improving the thermal-hydraulic performance of solar air heater. Finally, the inclined vortex plate is designed to realize this kind of optimized flow field successfully. This work will promote technique developments of solar air heater.

Numerical Modeling for Novel Solar Air Heater Utilizing Wax Paraffin-PCM

A mathematical model to analysis three–dimensional forced convection turbulent flow in a novel solar air heater integrated with multiple rectangular capsules filled by paraffin wax-based on phase change material PCM was implemented. The investigations were performed under three airflow speed of (0.6, 1.2, and 1.8) kg/min and average solar flux of 625 W/m2. The results revealed that the delaying melting time and also lower the melting temperature of PCM by increasing airflow speed during the charging process. As well as, the freezing period is dependent on the airflow speed by inverse relation. Also, the data results represent that the useful energy rate and thermal storage efficiency were a strong dependence on the airflow speed. Moreover, it can be detected that the optimal freezing time and the air temperature rise of the heater were reached about 210 minutes with (12 – 1.5 °C), 150 minutes with (7.5 – 1.4°C), and 120 minutes with (5.5 – 1.5 °C), at airflow speed of 0.6, 1.2, and 1.8 kg/min, respectively, which can be used at night to supply some applications by thermal energy such as heating buildings and drying agricultural crops.

Three dimensional cyclonic turbulent flow structures at various geometries, inlet-outlet orientations and operating conditions

Flow structure inside a chamber greatly determines the process performances. Therefore, the flow structure inside a chamber are often constructed in such a way as an effort to obtain equipment performances in accordance with the expectations. This study explored flow structure inside several chamber geometries and operating conditions. Three types of chamber, namely; GTC, DTC and TJC were set as the investigated chambers. The Computational Fluid Dynamics technique, supported by some experimental data from the literature, is used as an investigation method. The RANS based models, under Ansys-Fluent software were used in this numerical investigation. Simulation results revealed that the flow structures of GTC and DTC are predominantly created by spiral and vortex patterns. The vortex stabilizer diameter in the GTC affects the vortex pattern, velocity profile and pressure drop. The flow structure of DTC presents the most complex behavior. The flow structure inside TJC, in the case of unconfined outlet boundary, is characterized by the helical and wavy jet pattern. This structure is determined by the initial tangential intensity (IIT) and the inlet aspect ratio (RIA). The structures of vortex, helical, and wavy axial flow are properly constructed and visualized in this paper. There is no a turbulence model which is always superior to the other models, consistently. The standard k-ε model exhibits the realistic and robust performances among all of investigatied cases.

Effect of rotational speed on the hydraulic performance for flow conduits of shaft extension tubular pumping system

In order to investigate the effect of rotational speed on hydraulic performance of inlet and outlet conduits for S-shaped shaft extension tubular pumping system, the three dimensional steady turbulent flow in pumping system was simulated using computational fluid dynamics technology. The experiment results are compared with the CFD results. The hydraulic performance parameters of inlet and outlet conduits were analyzed quantitatively considering the hydraulic interaction of conduits and impeller. The change trend of relationship curves between hydraulic loss ratio value and flow rate coefficient is same in different rotational speed. With the increase of flow rate coefficient, the hydraulic loss ratio value increases gradually, the hydraulic efficiency increases firstly then decreases gradually, and the average velocity circulation of outlet section increases firstly then decreases gradually in the same rotational speed. With the increase of rotational speed, the average velocity circulation increases gradually in the same flow rate coefficient. With the increase of flow rate coefficient, the relative swirl angle decreases firstly then increases in different rotational speed. With the increase of rotational speed, the static pressure ratio of outlet conduit increases gradually, and the distribution of drift angle in inlet section of outlet conduit is same. There is no linear functional relationship between the square of hydraulic loss and flow rate for outlet conduit.

Studies on effusion cooling: Impact of geometric parameters on cooling effectiveness and coolant consumption

This study is focused on the impact of certain important geometric parameters on cooling effectiveness and coolant consumption for effusion cooling of aircraft combustor liner. The three dimensional turbulent flow field in a domain representing the combustor with several rows of effusion coolant injection is considered for the analysis. The geometric parameters considered are: angle of injection of the coolant, axial and transverse pitch of the injection holes, hole spacing and hole diameter. Also, based on the analysis of the temperature field within the chamber, a novel concept of ‘variable hole diameter’ has been introduced to reduce coolant consumption. A symmetric 3D computational model including the combustion chamber, coolant chamber and the effusion plate was used for the study. Conjugate heat transfer was modeled between the effusion-cooled wall and the two chambers. A detailed mass flow rate analysis has been performed for the various cases in order to study the impact of parameters on coolant consumption. The proposed approach of using an effusion plate with variable hole diameters is found to be effective in reducing the net coolant consumption significantly while maintaining a given level of cooling effectiveness.

Numerical Investigations on Heat Flow of Nanofluids in Ribs Tube Configurations

Abstract
 In this paper presents two dimensional turbulent flow of different nanofluids and ribs configuration in a circular tube have been numerically investigation using FLUENT 6.3.26. Two samples of CuO and, ZnO nanoparticles with 2% v/v concentration and 40 nm as nanoparticle diameter combined with trapezoidalribs with aspect ratio of p/d=5.72 in a constant tube surface heat flux were conducted for simulation. The results showed that heat flow as Nusselt number for all cases raises with Reynolds number and volume fraction of nanofluid, likewise the results also reveal that ZnO with volume fractions of 2% in trapezoidal ribs offered highest Nusselt number at Reynolds number of Re= 30000.
 Keyword: Heat transfer augmentation, Friction factor, Thermal performance factor, Ribs configuration, CFD simulation, Fluent.

Experimental validation of heat/mass transfer analogy for two-dimensional laminar and turbulent boundary layers

Heat and mass transfer from a solid surface into a fluid stream are both diffusion driven processes. Their constant property mathematical model equations are identical for identical flow and equivalent boundary conditions, when Lewis number is equal to unity. Problems with engineering applications can be transformed from one domain to the other for practical and economic advantages. Mass transfer measurement using naphthalene sublimation can be faster, of higher resolution, economical, and more accurate than a direct heat transfer measurement. The diffusion rates by heat in air and naphthalene in air are quantitatively different. Hence, the advantages of naphthalene sublimation measurement are justified when the heat/mass transfer analogy is experimentally and/or analytically verified and an analogy factor (F=Nu/Sh) is determined.Heat/mass transfer analogy is experimentally verified in this study for two dimensional laminar and turbulent boundary layer flows over a flat plate. A thermal boundary layer technique is used to measure local heat transfer coefficient (Nu) and a naphthalene sublimation measurement is used to evaluate local mass transfer coefficient (Sh) for two similar plates subjected to the same boundary layer flow. The heat/mass transfer analogy factor is calculated using the measured Nu and Sh. It is found to agree with the analytical prediction of 0.677 within 2% for the laminar case and is found to be constant at 0.667 for the turbulent flow.