Laminar And Turbulent Flow In Pipes Pdf Creator
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- Modelling of droplet breakup and coalescence in an oil-water pipeline
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Modelling of droplet breakup and coalescence in an oil-water pipeline
Numeric and experimental analysis of the turbulent flow through a channel with baffle plates. Demartini; H. Vielmo; S. Brazil vielmoh mecanica. The present work presents the numeric and experimental analysis of the turbulent flow of air inside a channel of rectangular section, containing two rectangular baffle plates.
This is an important problem in the scope of heat exchangers where the characterization of the flow, pressure distribution, as well as the existence and the extension of possible recirculations need to be identified. The differential equations that describe the flow were integrated by the Finite Volumes Method, in two dimensions, employing the Fluent software with the k- e model to describe the turbulence.
The mesh is structured, with rectangular volumes. Several boundary conditions were explored, being the more realistic results obtained by prescribing the inlet velocity field and atmospheric pressure at the exit. The obtained results are compared with experimental data, being analyzed and commented the deviations.
The velocity field was measured with a hot wire anemometer, and the pressure field with an electronic manometer. The largest variations in the pressure and velocity fields occur in the regions near to the deflectors, as expected.
Keywords: Numerical simulation, experimental simulation, turbulence, fluent, hot wires. Banks of tubes are found in many industrial processes and in the nuclear industry, being the most common geometry used in heat exchangers. The heat is transferred from the fluid inside the tubes, to the turbulent flow outside them. Usually, a very complex flow is found there, where besides the turbulence, boundary layer separation and recirculating flows are found in the external flow. In shell-and-tube heat exchangers, the cross flow through the banks is obtained by means of baffle plates, responsible for changing the direction of the flow and for increasing the heat exchange time between fluid and the heated surfaces.
Baffles have also the purpose of increasing turbulence levels and, thus, heat exchange ratios Nusselt number - Nu. On the other side, boundary layer separation after the baffles may occur, as demonstrated in the old, but still very interesting results of flow visualization in models of heat exchangers and steam generators by Wiemer, This can be an additional and important source of disturbances in the flow, which can travel through the bank, influencing the tube bank and the baffles.
The characteristic value of the Strouhal number found was about 0. Important additional peak frequencies, appearing in spectra of tube wall pressure fluctuation, could not be associated neither to effects of pure cross flow through the bank nor to effects produced solely by the baffles. Numerical analysis of the laminar flow with heat transfer between parallel plates with baffles was performed by Kelkar and Patankar, Results show that the flow is characterized by strong deformations and large recirculation regions.
In general, Nusselt number and friction coefficient fR increase with the Reynolds number. Hwang et al. Numerical results show that the extension of the recirculation region upstream of the obstacle does not depend on its length in flow direction.
This does not happen in the region downstream of the obstacle, where the recirculation is strongly influenced by the length of the obstacle, decreasing when obstacle length is increased, but remaining constant when the flow on the upper side of the obstacle reattachs. De Zilwa et al. Turbulent flow simulations using k- e models showed to be very reliable when compared to experimental results, except near the walls in the recirculation regions, underestimating the reattachment location.
Measurements using LDA technique in the turbulent flow in a duct with several baffle plates were performed by Berner et al. Results showed that with a Reynolds number of 5. By increasing the Reynolds number to 1. Antoniou and Bergeles,, analyzed the flow over prisms with several aspect ratios using hot wire technique.
Characteristics of the turbulent flow with heat transfer in a rectangular duct with baffle plates were studied by Habib et al. The heat flux was uniform in both upper and lower walls. The experiment focused on the influence of Reynolds number and baffle height on the local and global heat transfer coefficients, and pressure drop measurements.
Large recirculation regions and velocity gradients were observed behind the baffles. Pressure drop increases more rapidly than the heat transfer coefficient with the Reynolds number. Li and Kottke ,, studied heat transfer and pressure drop in simulating models of shell-and-tube heat exchangers. Parameters of the experimental work were the Reynolds number and the distance between the baffles.
Results demonstrated that for a constant value of the Reynolds number, increasing the distance between the baffles increased the heat exchange coefficient and the pressure drop.
Behavior of the turbulent flow and the heat transfer on obstacles was also studied by Acharya et al. The purpose of the present work is to investigate the turbulent flow of air inside a channel of rectangular section, containing two rectangular baffle plates as presented in more detail in Demartini The characterization of the flow field in this channel intend to allow the comprehension of the behavior of dynamic loads, produced by the fluctuating velocity and pressure fields, and contributes to the characterization of heat transfer distribution in heat exchangers.
Test Section and Experimental Procedure. The test section, shown in Fig. Two baffle plates were placed on opposite channel walls. The flow rate, and thus the Reynolds number, was controlled by a gate valve.
Before the tube bank a Pitot tube was placed, at a fixed position to measure the reference velocity for the experiments. Pressure measurements were performed at the side wall of the channel. This velocity was determined with help of the fixed Pitot tube, as described above. Previous analysis of the behavior of the test section, by means of METRA accelerometers, and of the measurement technique, allowed to identify peaks in spectra due to resonances not related to the phenomena investigated.
For the determination of autospectral density functions, the sampling frequency was of 5 kHz, while the signals of the instruments were high pass filtered at 1 Hz, and low pass filtered at 2 kHz, to avoid effect of folding of frequencies higher than the cut off Nyquist frequency.
Presentation of the Problem Studied and its Numerical Approach. In this Chapter balance equations and the method employed to the solution of the problem investigated, including the necessary simplifications, are described. Turbulence Modelling. Reynolds-Averaged Navier-Stokes Equations are the governing equations for the problem analyzed momentum balance , with the continuity equation.
For a two-dimensional incompressible flow of a newtonian fluid, mass and momentum equation become, respectively. In eq. The classical approach is the use of Boussinesq hypothesis, relating Reynolds stresses and mean flow strain, through the eddy viscosity concept [Hinze, ].
In its general formulation, as proposed by Kolmogorov, Boussinesq hypothesis is written as. Successful turbulence models are those based on the eddy viscosity concept, which solve two scalar transport differential equations.
The most well known is the k- e model [Launder and Spalding, ]. The so called "standard" k- e model is a semi-empirical one, based on the conservation equation of the kinetic energy k and its dissipation rate e. The basis of the model is he Boussinesq's hypothesis, that the Reynolds stresses are proportional to the strain rate of the mean flow, by means of the eddy viscosity concept, thus. Balance equations for the kinetic energy k and its dissipation rate e for the model are, respectively.
In Eqs. The production term G k is modeled according to Boussinesq's hypothesis, by:. The presence of a wall influences the velocity field in its vicinity through the non slip boundary condition. Considering the effects of the wall for the standard k - e model, based on Launder and Spalding, , a "law of the wall" for the mean velocity distribution is given by.
Problem Definition and Boundary Conditions. The problem to be analyzed is the turbulent flow in a rectangular cross section duct were two baffles were placed, so as to simulate the conditions found in shell-and-tube heat exchangers, where flow and pressure distribution need to be known. A schematic view of the physical problem is shown in Fig. The total length of the channel is equivalent to 3. Therefore, no influence will result from the side walls, so that the flow can be considered as being two-dimensional.
Previous measurements of the flow velocity distribution in the outlet of the channel without any obstacle showed the validity of this hypothesis. In the entrance region, a velocity profile was prescribed, as shown in Fig. This velocity profile was obtained by means of hot-wire measurements. Kinetic energy of turbulence and dissipation rates are prescribed, respectively, as. For the upper and lower walls it is imposed.
Besides, non slip and impermeability boundary conditions are imposed at the walls. In the channel outlet it is prescribed the atmospheric pressure. Numerical Method. Structured meshes, with rectangular volumes were built and tested with the Fluent 5. Considering the characteristics of the flow, the Quick-scheme was applied to the interpolations Leonard, Mokhtari, , while a second-order upwind scheme was used for the pressure terms.
The mesh was generated by the pre-processor software Gambit 1. The mesh was refined at all solid boundaries, with volumes growing in geometrical progression with the increasing distance form the wall, as given by the expression. This expression is valid for the regions near the walls.
For the regions more distant from the walls, the mesh is uniform, as a first tentative. After importing the mesh from the preprocessor, additional refinements were performed, considering the geometry and features of the numerical solution of the problem. Figure 3 presents an example of the mesh used near the tip of a baffle plate, in the presence of flow separation. The mesh is too fine near the solid boundary so that graphic representation is beyond printer resolution.
This refinement was necessary to resolve the strong velocity and pressure gradients in that region. Velocity values are scaled to the velocity at the entrance reference velocity. The influence of the deformation of the flow field increases as the flow approaches the first baffle plate, increasing the velocity of the flow approaching the passage under the baffle. In the upper part of the channel, negative velocities indicate the presence of recirculation behind the first baffle.
In these locations, results show that as the flow approaches the second baffle, its velocity is reduced in the lower part of the channel, while in the upper part is increased. A comparison of numerical and experimental results of velocity profiles after the second baffle plate, near the channel outlet is given in Fig.
Head Loss In Pipe Pdf
In this study, single-phase heat transfer enhancement in internally finned tubes is investigated numerically. The influence of fin number, helix angle, fin height, fin width, and shape on the flow and heat transfer characteristics is studied. The research results indicate that the resistance coefficient and Nusselt number both increase with the increment of these parameters, among which the helix angle has the largest impact on the heat transfer enhancement. In addition, the shape of fins also has a small effect on the flow and heat transfer, and the heat transfer effect of triangular fins is the best. Single-phase convection heat transfer enhancement techniques are widely applied in industries such as petroleum, chemical engineering, etc. In recent years, its applications in the field of nuclear technology has been an engaging research area of heat transfer augmentation.
For a pipe or duct the characteristic length is the hydraulic diameter. The Reynolds Number for the flow in a duct or pipe can with the hydraulic diameter be expressed as. The Reynolds Number can be used to determine if flow is laminar, transient or turbulent. The flow is. In practice laminar flow is only actual for viscous fluids - like crude oil, fuel oil and other oils. A Newtonian fluid with a dynamic or absolute viscosity of 0.
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Created by Simpo PDF Creator Pro (unregistered version) Laminar flow, Flow Dynamics in Closed Conduit (Pipe Flow). 3 of Transitional/. Turbulent.
Laminar and turbulent flow
November 30, Journal article Open Access. Akeel M. Ali Morad ; Rafi M.
Neslihan Semerci. The pipe friction apparatus consists of a test pipe mounted vertically on the rig , a constant head tank, a flow control valve, an air-bleed valve, and two sets of manometers to measure the head losses in the pipe Figure. A set of two water-over-mercury manometers is used to measure large pressure differentials, and two water manometers are used to measure small pressure differentials.
In physics and engineering , fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids — liquids and gases. It has several subdisciplines, including aerodynamics the study of air and other gases in motion and hydrodynamics the study of liquids in motion. Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft , determining the mass flow rate of petroleum through pipelines , predicting weather patterns , understanding nebulae in interstellar space and modelling fission weapon detonation. Fluid dynamics offers a systematic structure—which underlies these practical disciplines —that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems.
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