Non-uniform Velocity Research Articles

Experimental and Pore-scale Analysis of Flow and Thermal Fields in a Packed Bed Channel

ABSTRACT The study investigates experimentally and numerically the problem of turbulent convective heat transfer and pressure drop in a packed bed with internal heat generation. The main objective is to develop correlations for the heat transfer between the bed and coolant as well as the pressure drop in the system using detailed experimental and 3-dimensional pore-scale numerical simulation. The system analysed is a horizontal circular channel filled with different stainless steel spheres with diameters of 5.5 mm, 6.5 mm and 7.5 mm. Dry air is used as working fluid. The study is performed for turbulent flow regimes with different Reynolds number (based on the spheres diameter) in the range of 900-2600. The spheres are heated using an electromagnetic induction heating method. Based on the experimental data, empirical correlations for turbulent pressure drop and convective heat transfer coefficient are developed. Results of pore-scale analysis show the complexity of the flow domain, which the velocity experiences a considerable increase compared to inlet velocity and is more evident in narrow gaps between spheres. Moreover, numerical results indicate the coupling of temperature and velocity fields, which a non-uniform velocity filed leads to non-uniform temperature distributions of air and solid spheres in the packed bed.

The effect of opening windows on the airflow distribution inside naturally ventilated residential bedrooms with ceiling fans

Ceiling fans are the most common equipment in any household with electricity to induce a higher air movement since the potential of natural ventilation is limited. However, the higher airflow region is generally limited to the zone below the fan. The non-uniform distribution of airflow is also affected by the furniture layout and airflow from window openings. This study attempts to evaluate the effect of the different window-opening patterns on the airflow inside naturally ventilated bedrooms, having a ceiling fan for air movement with numerical simulations and on-site measurements. The airflow pattern created by a ceiling fan in a room with furniture is modelled and simulated with ANSYS Fluent 2019 R3. The results were validated with on-site measurements and compared with the literature. The air velocity was measured in bedrooms of three different sizes. It was observed that the opening of windows created a better distribution of air irrespective of room size. The non-uniformity of the air velocity is reduced from 76% to 39% with the opening of windows in the larger-sized room. The reduction in non-uniformity is influenced by the location of windows also. The practice of opening windows along with the induced air movement by ceiling fans results in a better distribution of air in the space. Practical application: People tend to depend mainly on ceiling fans even if windows are open in naturally ventilated rooms. A study of patterns of non-uniform distribution of airflow can help designers to improve comfort conditions by specifying the number and location of fans and windows while designing room furniture layout. This aids the building services engineers to provide thermal comfort without always depending on alternative active ventilation strategies.

Non-Hydrostatic Transitional Open-Channel Flows from a Supercritical to a Subcritical State

Abstract In this study, a depth-averaged numerical model was employed to investigate the two-dimensional flow features of transitional open-channel flows from a supercritical to a subcritical state. Compared to a shallow-water model, the proposed model incorporates supplementary terms to account for the effects of non-uniform velocity and non-hydrostatic pressure distributions. The model equation was solved numerically by means of the Adams–Bashforth–Moulton scheme. A wide variety of transitional open-channel flow problems such as hydraulic jumps was considered for assessing the suitability of the numerical model. The results of the model for the free-surface profile, pressure distribution, and characteristics of the first wave of an undular jump were compared with the experimental data, and the agreement was found to be satisfactory. Despite the effects of the three-dimensional characteristics of the flow and the bulking of the flow caused by air entrainment, the model performed reasonably well with respect to the simulations of the mean flow characteristics of the curvilinear turbulent flow problems. Furthermore, the results of this investigation confirmed that the model is more suitable for analyzing near-critical turbulent flow problems without cross-channel shock waves.

Velocity profiles in a water jet of a Pelton nozzle: CFD simulations on both 2D and 3D geometries

Hydropower is an important energy sector deployed all over the World with a significant share in the global electricity production. Due to the ongoing climate changes, the lack of water in summer periods compromises the performance of hydraulic machines, whose efficiency quickly decreases when they operate at strong part-load conditions.Among the most known hydraulic turbines, Pelton is suitable to be used with high geodetic heights and relatively low flow rates. When dealing with flow rates 30% lower than the Best Efficiency Point (BEP), the efficiency drastically decreases mainly due to the non-uniform velocity profiles of the water jets coming out from the nozzles. The quality of the jet is affected by the distributor geometry, bending pipes and spear valves with the respective supports located upstream.The aim of this work is to investigate different velocity profiles of the water jet coming out from a Pelton nozzle geometry by means of Computational Fluid Dynamics (CFD) simulations performed by ANSYS® Fluent solver. Four different spear valve angles were simulated on a 2D-axisymmetric geometry, starting from a reference case of = 50° with increasing steps of 5°, maintaining a fixed positioning of the spear valve that corresponds to a partial load operation of the machine.In addition, also a 3D simulation has been carried out and the streamlines along a symmetry plane are shown. The results of the 3D simulations were compared to the 2D-axisymmetric ones taking into account the aforementioned reference case, showing how this last simplification of the computational domain leads to almost the same macroscopic results.

Non-Newtonian fluid flow having fluid–particle interaction through a porous zone in a channel with permeable walls

Abstract The flow of Casson fluid streaming through a porous zone in a permeable channel with homogeneously distributed dust particles is studied. The flow is oscillatory and exposed to thermal radiation. With non-uniform temperature and velocity slip at the wall, the channel is put under transverse magnetic flux control. The variable separation approach is used to obtain the exact analytic solution for the flow fields of fluid and dust particles. Physical insights of collective effects of suction/injection, magnetic field, thermal radiation, and buoyance force on the fluid velocity, temperature, rate of heat transport and friction at the wall surface are examined and analysed graphically. Results reveal that surface friction increases on both channel plates as an injection on the hot plate rises.

Process simulation on atomization and evaporation of desulfurization wastewater and its application

With increasing social concern towards industrial wastewater treatment, desulfurization wastewater evaporation technology is attracting attention in theoretical investigation and showing competitive capability in practical applications by virtue of its low cost and high efficiency. In engineering practice, the selection of nozzle model, evaporation chamber size and flue gas source need to be carefully determined, ideally with comprehensive understanding of the process mechanism. Important factors to be considered include wastewater evaporation characteristics such as evaporation time, distance, and so on, which are difficult to be obtained directly through physical experiments. Numerical simulation can be utilized conveniently for the extraction of such data and further post analysis. In this study, a salt containing droplet evaporation model is combined with a two-way interphase coupling method to investigate the spray evaporation process. Numerical results show that by increasing the injection velocity and the liquid film thickness, and reducing the flue gas velocity, droplets were observed to be more dispersed in the flue, which in turn enhance the evaporability. The velocity difference between the gas-liquid phases is correlated with the gas-phase turbulent kinetic energy. Non-uniform inlet velocity can increase the flow turbulence intensity. This leads to enhanced droplet evaporation performance on one hand, but increased probability of the droplets hitting the wall on the other hand. From this study, we have also observed that the droplet evaporativity increases with the temperature. Finally, an optimal set of parameters are selected from the simulation exercises to achieve an optimal evaporation outcome. These parameters are introduced in large engineering design to guide the selection of the nozzle model and determine the evaporation chamber size.

Numerical modeling of an outdoor unit heat exchanger for residential heat pump systems with nonuniform airflow and refrigerant distribution

Multirow finned tube heat exchangers have been extensively used as an outdoor unit in heat pump applications. Several factors, including nonuniform airflow distribution, refrigerant circuitry arrangement, fin and tube configurations, and operating conditions, have a significant influence on its thermohydraulic performance. For accurate outdoor unit heat exchanger modeling and analysis, developing a robust and experimentally validated numerical model reflecting the influence of realistic nonuniform airflow and refrigerant distribution is necessary. This paper presents a general-purpose numerical model, implemented in MATLAB, for a multirow finned tube heat exchanger using a tube-by-tube modeling approach based on the log mean enthalpy difference method. State-of-the-art heat transfer and pressure drop correlations are used for each refrigerant-side's flow regime and airside fin configuration. An alternative iteration loop is also incorporated to solve the complex circuitry of the heat exchanger under wet conditions. Simulation results of the current model yield consistent and more real heat transfer, pressure drop, and temperature contours using nonuniform airflow velocity profiles than those of the simplified models that assumed uniform airflow. Therefore, higher accuracy of the performance evaluation for complex refrigerant circuitry was achieved. Furthermore, current model is able to identify the exit vapor quality that allows proper selection of tubes and mass flow rates to avoid unnecessary superheat or sub cooling. The model is also validated with available experimental data, and the maximum error is within ±10.0%.

Numerical Solution of Laminar Flow and Heat Transfer over a Flat Plate in a Nonuniform Stream.(Dept.M)

The laminar flow and heat transfer over an isothermal flat plate in a free stream with nonuniform velocity have been analysed, fully implicit numerical method (with a hybrid representation for the convective term normal to the streamwise direction) is used for solving the governing equations. Two practical cases are considered. In the first case, a flat plate is located in the laminar wake of an upstream plate. In the second case, a flat plate 18 inserted in the laminar shear layer joining a uniform stream and a quiescent fluid. The dependence of the boundary-layer and heat transfer on the lateral and the downstream shifts of the plate have been investigated. It was found that the nonuniformity of the free stream velocity causes reductions in both the wall shear stress and heat transfer relative to the corresponding values for uniform free stream. These relative reductions decrease with increasing downstream distance along the plate in the upper half of the wake. In the lower half of the wake, these Zeductions increase with distance along the plate. It was also found that the minimum values for drag and overall heat transfer rate at a certain streamwise location correspond to a slight downward shift of the plate from the wake centreline. In the case of shear layer, the relative reductions in the wall shear and heat transfer are more pronounced in the low velocity region. These reductions decrease with increasing downstream distance along the plate. It was found that both the drag and overall heat transfer rate decrease continuously with downstream shift of the plate.

A new method for analytical modeling of microfluidic extraction

Despite single-phase parallel flows for which analytical solutions are available for species conservation equation in its most general form, little progress has been made in analytical modeling of cross-stream diffusion in stratified multiphase flows. The main reason is that solute concentration and fluid properties are discontinuous at the interface of phases. In the present study, a new method of solution is presented for the analytical treatment of solute transport in stratified multiphase flows. The solution methodology starts with developing separate species conservation equations for the phases, which, upon non-dimensionalization, are replaced with a single equation for which series solutions are obtained utilizing the variational calculus. Using the method proposed, analytical solutions are obtained for mass transport in microfluidic two-phase extraction by taking the influences of the non-uniform velocity and the axial diffusion effects into account. The inclusion of the non-uniform velocity effects, which leads to 3D solutions, enables us to capture the heterogeneous transport of solutes, a phenomenon that is ignored by the available simple 2D solutions. The results indicate that axial diffusion in stratified multiphase flows is significantly more important than in single-phase flow and is not characterized solely by the Peclet number. It is also found that the only parameter controlling the length required for a complete extraction is the solvent-to-solution viscosity ratio.

Nanoparticles shape effects on thermal performance of Brinkman-type ferrofluid under heat injection/consumption and thermal radiation: A fractional model with non-singular kernel and non-uniform temperature and velocity conditions

The prime aim of this article is the construction of a fractional model to enhance the heat transfer rate for magnetohydrodynamic (MHD) convectional transport of ferrofluid by applying the time-fractional concept of Caputo-Fabrizio derivative on Brinkman-type fluid model. Kerosene oil and water are considered to serve as carrier fluids for iron oxide (Fe3O4) nanoparticles of various non-spherical shapes (brick, cylinder, platelet, and blade). The non-uniform time-dependent motion of an unbounded upward plate causes to generate the flow of ferrofluid. The supplementary physical phenomena such as ramped heating, thermal radiation, and heat injection/consumption are also analyzed. In addition, the effectiveness of using differently shaped Fe3O4 nanoparticles for heat transfer and ferrofluid flow is evaluated. The exact solutions for non-integer order modeled differential equations are computed by employing the Laplace transform method. The comparative graphical illustrations for ramped and constant (isothermal) velocity and temperature conditions are prepared to scrutinize the impacts of several associated thermal and physical quantities. Besides, the outcomes of numerical simulations of Nusselt number and shear stress are communicated through multiple tables for an in-depth analysis. It is perceived that blade and platelet shaped Fe3O4 nanoparticles are more effective to enhance the thermal efficiency of Brinkman-type ferrofluid in contrast to cylinder and brick shaped nanoparticles. It is interesting to report that an improvement of 13.80% in Nusselt number is observed for water based ferrofluid that significantly advances its thermal properties. The fractional parameter γ produces inverse variations in momentum boundary layer thickness for isothermal and ramped plate cases. Additionally, velocity and temperature curves for ramped plate case are always lower than those of isothermal plate case. This fact highlights the importance of plate ramping in heat and flow control problems.

Numerical investigation on optimization of thermal analysis due to immersion of hybrid nanostructures in a fluid of shear dependent viscosity using the finite element method

AbstractThis article considers the dispersion of hybrid and mono nanoparticles in a fluid with viscosity (Williamson) dependent on shear rate, over a heated surface moving with nonuniform velocity and exposed to a magnetic field in the presence of an applied current. Extensive modeling leads to complex coupled mathematical models that are solved numerically via the finite element method (FEM). Convergent simulations are run to investigate the role of parameters on the dynamics of flow fields. The magnetic field intensity plays a role in controlling the magnetohydrodynamic boundary layer thickness (BLT) and thermal radiation controls the thickness of thermal boundary layers (TBL). However, the magnetic field intensity is responsible for an increase in BLT. In contrast to this, thermal radiation plays a role in controlling the thickness of the TBL. The impact of shear rate dependent viscosity on velocity is remarkable for both fluids. The motion of both of the fluids slows down when viscosity varies as a function of shear rate. Viscosity depending on the shear rate has a significant impact on wall shear stress. It is observed from simulations that wall shear increases when the parameters appearing in the model for shear rate dependent viscosity are increased. However, this increase in wall shear stress associated with a hybrid nanofluid is greater than the increase in wall shear stress associated with a mono nanofluid.

Hydrodynamic characteristics of lateral withdrawal in a tidal river channel with saltwater intrusion

Saltwater intrusions occur widely in tidal river channels, significantly changing the vertical distribution of mainstream velocity and density (salinity). They are vital to the hydrodynamic conditions of lateral withdrawal. In this study, a 3-D numerical model was constructed and validated through experimental tests, and the hydrodynamic characteristics of lateral withdrawal under saltwater intrusion were investigated. Additionally, the vertical distribution effects of mainstream velocity, withdrawal velocity, and density stratification were analyzed. Under non-uniform mainstream velocity conditions, with larger surface velocity than bottom velocity, the division width of the upper layer is smaller, while the local secondary circulation is stronger than that of the lower layer. Secondary circulation is top-shaped and weakens as the withdrawal velocity increases. When salinity stratification exists, the division width of the lower layer decreases, and the secondary circulation is stronger than that of the upper layer; meanwhile, the vertical water exchange increases. The influence of mainstream velocity is greater than that of salinity stratification. Under these two factors, the sediment entering the intake increases, and submerged vane, sill, and weir measures should be adopted. This research represents an advance in lateral withdrawal hydrodynamic research and provides support for engineering design in estuarine areas.

Accurate Simulation of the Four Modes of Post-Die Extrudate Shape Distortion

Abstract A combined flow, thermal and structural analysis is employed to simulate post-die extrudate distortion in different profile dies. All four factors which can cause extrudate distortion, namely, nonuniform exit velocity distribution, extrudate shrinkage, extrudate draw down, and deformed shape of the calibrator or sizer profile, are simulated. To analyze the effect of exit velocity variation on extrudate distortion, the parameterized geometry of a simple profile die is optimized using an extrusion die optimization software. The simulation results presented for a bi-layer profile die successfully demonstrate how gradually changing profile shape in successive calibrators/sizers can be used to simplify the die design for extrusion of complex profiles. The predicted extrudate shape and layer structure for the bi-layer die are found to accurately match with those in the extruded product.

Numerical Study to Improve Fluid Flow Patterns in HRSG by using a Turning Vane and a Combination of using a Turning Vane with Transition Zone Geometry Modification

Since operated from 1995 until 2018, the total steam production of HRSG 1.1 has decreased by 6.34 % and exhaust gas outlet temperature increased by 40.86 °C. From previous studies on HRSG it can be assumed that flue gas flows at HRSG 1.1 has nonuniform velocity and temperature distribution. This research is carried out by modeling and simulating HRSG as porous media using Computational Fluid Dynamic (CFD) software. The simulation will be carried out on a 3D model with the steady-state condition, use energy model, a heat exchanger (HE) model, and standard k-ε as a turbulence viscous model. Energy absorption in HE will be modeled by an ungroup macro model with fixed inlet temperature and NTU model. Simulations in HRSG are implemented at a corner angle of 6 degrees (original), adding a turning vane on the inlet duct and combination adding a turning vane on inlet duct with 12 degrees corner angle on transition zone. Model validation was accomplished comparing simulation data to power plant data. The result shows adding a corner angle in the transition zone and installing a turning vane in the curve bend minimizes secondary flow in the transition zone. Heat absorption increases 0.008% and exhaust gas outlet temperature decrease 0.04% compared to the existing model.

Measurements of laser-imprint-induced shock velocity nonuniformities in plastic targets with the Nike KrF laser

We report results of direct-drive laser imprint experiments measuring velocity perturbation profiles of shock waves produced by the Nike krypton fluoride laser. A new high-resolution two-dimensional velocimeter system was successfully implemented on the Nike laser facility and used for sensitive optical measurements of the velocity perturbations. Planar polystyrene targets with and without a thin high-Z overcoat (400 Å Au or 600 Å Pd) were irradiated by four, eight, and sixteen Nike laser beams to examine laser imprint and its mitigation. The results from the uncoated targets showed that the shock velocity perturbations decreased with an increasing number of laser beams overlapped on target, precisely as anticipated by the beam averaging effect on laser imprint. In the experiment on the shocks driven in the high-Z coated targets, the shock velocity perturbations were further reduced by a factor of 2–6 compared to their counterparts in the uncoated experiment, with the amplitude of the velocity fluctuations measured as small as 20 m/s rms for shock velocities of 20 km/s. These experiments allowed more direct measurements of laser imprint effects without relying on the Rayleigh–Taylor hydrodynamic amplification, providing valuable quantitative data for calibrating radiation-hydrodynamic simulations of laser imprint.

Performance prediction of hydronic cooling coil with non-uniform air velocity (ASHRAE RP-1741)

Hydronic coils are often exposed to non-uniform air velocities due to a series of factors, including tight spacing between a fan and a coil, flow area contraction and expansion, existence of obstructions, abrupt change in airflow direction, or the combination of above factors. Although studies showed non-uniform air velocities could cause performance degradation for direct expansion (DX) cooling coils, efforts in quantifying this impact on hydronic cooling coils are limited. The objective of this paper is to develop and validate a section-by-section calculation procedure for cooling performance prediction of hydronic coils with non-uniform air velocities. As the first step, air velocities at the coil face in a hydronic fan coil unit (FCU) were measured using a vane-type anemometer. The coil was then discretized into multiple sections, with each section using the measured air velocity for heat and mass transfer analysis. The cooling performance prediction with non-uniform air velocities was validated against experimental results. The comparative analysis showed that capturing the impact of non-uniform air velocities decreased the normalized root mean square error (RMSE) from 39.9% to 8.1% for latent capacity prediction and 9.1% to 2.6% for total capacity prediction compared with the results assuming uniform air velocities.

Optimization of the PGV-1000MKO Steam Generator Using Computation Fluid Dynamics

Abstract This paper presents the work carried out in Experimental and design organization OKB “GIDROPRESS” to justify the separation characteristics of VVER-TOI steam generator (SG) using the STAR-CCM+ CFD-code. In the VVER-TOI project, new layout solutions were applied in the reactor plant as part of which the steam removal system from the SG was changed. Namely, in contrast to the VVER-1000 and VVER-1200 where the steam removal was organized through ten nozzles combined into a steam collector, in the VVER-TOI SG the steam removal was arranged through one nozzle located on the cold collector side. This change leads to the formation of a nonuniform velocity field in the separation volume, between the evaporation surface and the distribution perforated plate (DPP), and can lead to the excessive increase of steam humidity. To ensure the steam separation characteristics of a horizontal SG with one steam nozzle, it was proposed to create a nonuniform resistance on the way of steam motion from the evaporation surface into steam nozzle applying a nonuniform degree of the DPP perforation. Two computer models of the SG steam volume with different steam removal schemes (through one and ten nozzles) were developed, a set of studies on verification and validation was carried out and a set of calculations were performed. According to the results of the calculations, the necessity of introducing a nonuniform degree of the DPP perforation was justified. Further, to determine the nonuniform degree of DPP perforation, a set of optimization calculations of the SG steam volume with one steam removal nozzle was performed. The nonuniform degree of DPP perforation of the VVER-TOI SG was selected, which provide steam velocity distribution as close as possible to the SG with ten steam nozzles. To justify the chosen design, sensitivity analysis was also carried out according to the hole diameters tolerance and steam load profile.

Modeling and simulation based investigation of unsteady MHD radiative flow of rate type fluid; a comparative fractional analysis

In this work, a generalized unsteady magnetohydrodynamic transport of a rate type fluid near an unbounded upright plate is analyzed under Newtonian heating and non-uniform velocity conditions. The vertical plate is suspended in a porous medium and is encountering the radiation effects. Three different fractional models for Maxwell fluid are established by using the modern definitions of Atangana–Baleanu, Caputo–Fabrizio, and Caputo fractional operators. Triple fractional analysis is conducted to reach out to the solutions of consequent flow and energy equations. Laplace transform and Stehfest’s numerical algorithm are jointly applied to solve each fractional model. Shear stress and heat transfer rate are measured at the solid–fluid interface in the form of skin friction coefficient and Nusselt number respectively. The physical significance of associated parameters in velocity and energy boundary layers is investigated and graphs are provided to discuss the relevant physical arguments. A tabular analysis is performed to demonstrate the influence of physical parameters on shear stress and heat transfer rate. An empirical comparison between fractional and classical solutions indicates that fractional operators provide a better explanation of the physical features of the model. It is also analyzed that for Newtonian heating and non-uniform velocity conditions, the Atangana–Baleanu fractional operator is the finest fractional model to describe the memory effect of velocity and energy distribution. The velocity of a Maxwell fluid is always higher for constant velocity condition as compared to the ramped velocity condition. Furthermore, the heat transfer rate declines for increasing values of the fractional parameter α and a minimum value is witnessed for the classical model but, it follows an inverse trend when the radiation parameter Rd increases.

Expansion opacity in laboratory conditions

Radiation–matter interaction depends mainly on the state of matter (its density, temperature, etc.), and also on the radiation spectrum. The opacity of thick plasma also depends on plasma velocity—the Doppler effect shifts atomic lines. For the cases when there are many bound–bound transitions, i.e., the plenty of lines contribute to the opacity, the latter is enhanced when the plasma expands with a nonuniform velocity field. It is known as “expansion opacity” in the literature. Existing models are discrepant and predict diverse results in some cases. Here, we present a rigorous derivation of the effect and show that the effect is available for experimental study at modern laser facilities. The plasma created by a Cu target irradiated with an ∼100 J nanosecond laser pulse is rich in lines and has enough expansion velocity so that its opacity is increased in the spectral range ∼102−103 eV by the order of magnitude. The possible experimental measurement of the effect is briefly discussed.

Dynamics of stabilizer fins on the waterjet-propelled ship

An evaluation of the stabilizer fin's influence on the flow fields of a waterjet-propelled ship was studied by the Reynolds-Averaged Navier-Stokes (RANS) method. Model- and full-scale simulations of the unsteady multiphase flow were carried out both in the bare hull and the self-propulsion conditions. The reliability of grid arrangement and numerical methods was confirmed by the uncertainty analysis of the flow fields. The results indicated that the existence of stabilizer fins altered the turbulent boundary layer distribution around the hull and such disturbance extended to the capture areas of the waterjet's ingested flow. The discrepancy of the outer waterjet (OW) and the inner waterjet (IW) axial velocity distributions resulted in the difference of velocity non-uniformity in the pump inflow plane, and it also caused a relatively larger decrease in the duct efficiency of the OW. With respect to the scale effect of the flow fields, a relatively thinner boundary layer thickness and a higher non-uniformity of the internal flow were observed due to a higher turbulence in the full-scale simulation. It was noted that the model-scale computations underestimated the momentum velocity coefficients at station 1a, while they overestimated the duct efficiency in all cases, with or without the stabilizer fins.