Numerical Investigation Of Flow Research Articles

Enhancement of flow boiling heat transfer in microchannel using micro-fin and micro-cavity surfaces

• A numerical investigation into the flow boiling process in a microchannel with a micro-structured surface was performed. • Micro-fin, micro-cavity, and smooth surfaces with varying wettabilities were investigated. • The enhancing effects and detailed mechanisms of micro-structured surfaces were revealed. Micro-structured surfaces have a significant impact on the flow boiling process in microchannels, but few numerical studies have been carried out due to their complex nature. In this study, the numerical investigation of flow boiling on micro-fin, micro-cavity, and smooth surfaces in a microchannel was conducted, with water serving as the working fluid. The volume-of-fluid (VOF) method, the phase change model, and solid-fluid thermal coupling were adopted in an OpenFOAM solver to perform the computation. The enhancing effects and detailed mechanisms of the micro-fin and micro-cavity surfaces on the heat transfer process are discussed, and the influences of wettability on these surfaces are investigated. With a contact angle of 60°, the heat transfer coefficient of the micro-fin surface was 61.92% larger, and the overall thermal resistance was 36.64% lower than that of a smooth surface, respectively. The confined bubbles on the micro-fin surface had a much smaller dryout area on the heated wall due to the capillary wetting effect. Moreover, the micro-fin surface with the rising nucleate bubbles can induce vortexes, which strengthens the convective heat transfer. As for the micro-cavity surface, it had a moderate heat transfer enhancement with a 17.16% larger heat transfer coefficient and a 13.55% lower thermal resistance when compared with a smooth surface. When the wettability of a heating surface is enhanced, the dryout area is minimized. Thus, the heat transfer performance of the smooth and micro-cavity surfaces is enhanced. The enhancement resulting from modified surface wettability has less of an effect on the micro-fin surface because the micro-fin array serves a similar function to minimize the dryout area.

Numerical Investigation of Flow and Heat Transfer over a Shallow Cavity: Effect of Cavity Height Ratio

Flow over shallow cavities is used to model the flow field and heat transfer in a solar collector and a variety of engineering applications. Many studies have been conducted to demonstrate the effect of cavity aspect ratio (AR), but very few studies have been carried out to investigate the effect of cavity height ratio (HR) on shallow cavity flow behavior. In this paper, flow field structure and heat transfer within the 3-D shallow cavity are obtained numerically for two height ratio categories: HR = 0.0, 0.25, 0.5, 0.75, and 1.0 and HR = 1.25, 1.5, 1.75, 2.0, 2.25, and 2.5. The governing equations, continuity, momentum, and energy are solved numerically and using the standard (K-ε) turbulence model. ANSYS FLUENT 14 CFD code is used to perform the numerical simulation based on the finite volume method. In this study, the cavity aspect ratio, AR = 5.0, and Reynolds number, Re = 3 × 105, parameters are fixed. The cavity’s bottom wall is heated with a constant and uniform heat flux (q = 740 W/m2), while the other walls are assumed to be adiabatic. For the current Reynolds number and cavity geometry, a single vortex structure (recirculation region) is formed and occupies most of the cavity volume. The shape and location of the vortex differ according to the height ratio. A reverse velocity profile across the recirculation region near the cavity’s bottom wall is shown at all cavity height ratios. Streamlines and temperature contours on the plane of symmetry and cavity bottom wall are displayed. Local static pressure coefficient and Nusselt number profiles are obtained along the cavity’s bottom wall, and the average Nusselt number for various height ratios is established. The cavity height ratio (HR) is an important geometry parameter in shallow cavities, and it plays a significant role in the cavity flow behavior and heat transfer characteristics. The results indicate interesting flow dynamics based on height ratio (HR), which includes a minimal value in average Nusselt number for HR ≈ 1.75 and spatial transitions in local Nusselt number distribution along the bottom wall for different HRs.

Experimental and Numerical Investigations of Flow over and under Weir-Culverts with a Downstream Ramp

AbstractA series of experimental and numerical studies were conducted to investigate the hydrodynamics of weir-culverts with a downstream ramp in submerged-flow condition. A comparison of the numer...

Numerical investigation of flow around a structure using Navier-slip boundary conditions

PurposeComplicated motion of vortex is frequently observed in the wake of islands. These kinds of swirling fluid cause the trap of sediments or pollutants, subsequently inducing the dead zone, odor or poor water quality. Therefore, the understanding of flow past a circular cylinder is significant in predicting water quality and positioning the immersed structures. This study aims to investigate the flow properties around a structure using Navier-slip boundary conditions.Design/methodology/approachBoundary conditions are a major factor affecting the flow pattern because the magnitude of flow detachment on a surface can redistribute the tangential stress on the wall. Therefore, the authors performed an analysis of laminar flow passing through a circular structure to investigate the effect of boundary conditions on the flow pattern.FindingsThe authors examined the relationship between the partial-slip boundary conditions and the flow behavior at low Reynolds number past a circular cylinder considering velocity and vorticity distributions behind the cylinder, lift coefficient and Strouhal number. The amplitude of lift coefficient by the partial slip condition had relatively small value compared with that of no-slip condition, as the wall shear stress acting on the cylinder became smaller by the velocity along the cylinder surface. The frequency of the asymmetrical vortex formation with partial slip velocity was increased compared with no-slip case due to the intrinsic inertial effect of Navier-slip condition.Originality/valueThe ability to engineer slip could have dramatic influences on flow, as the viscous dominated motion can lead to large pressure drops and large axial dispersion. By the slip length control, no-slip, partial-slip and free-slip boundary conditions are tunable, and the velocity distributions at the wall, vortex formation and wake pattern including the amplitude of lift coefficient and frequency were significantly affected by slip length parameter.

Experimental and Numerical Investigation of Flow in ‎Hydraulic Elbows

Elbow fittings are common in hydraulic and pipeline systems. These components cause a significant ‎pressure drop in the total pressure of a system. The banjo elbow is advantageous in areas low to the ‎ground and where flexible connection angles are needed. However, this elbow yields a larger pressure ‎drop than a standard elbow. Additionally, the position of the internal bolt in the banjo elbow cannot be ‎determined prior to installation, which corresponds to a wide range of possible pressure drop. In this ‎study, the pressure drop through a 3/8” banjo elbow is investigated for different positions of the internal ‎bolt, experimentally and numerically. Experiments and simulations were carried out on hydraulic oil with ‎four different Reynolds numbers ranging from 3111 to 6222 and at nine bolt connection angles ranging ‎from 0° to 60°. Experiments were repeated with the standard elbow of the same size to compare the ‎pressure drops to those of the banjo elbow. Pressure was measured at both the inlets and outlets of the ‎elbows. The results suggest that the connection angle of the internal bolt is an important factor in the ‎pressure drop and minor head loss through a banjo elbow. For Reynolds numbers of 3111 and 6222, an ‎improvement in minor head loss by 33% and 58%, respectively, was achieved by adjusting the ‎connection angle of the internal bolt in the banjo elbows.

Experimental and Numerical Investigation of Flow and Alignment Behavior of Waste Tire-Derived Graphene Nanoplatelets in PA66 Matrix during Melt-Mixing and Injection.

Homogeneous dispersion of graphene into thermoplastic polymer matrices during melt-mixing is still challenging due to its agglomeration and weak interfacial interactions with the selected polymer matrix. In this study, an ideal dispersion of graphene within the PA66 matrix was achieved under high shear rates by thermokinetic mixing. The flow direction of graphene was monitored by the developed numerical methodology with a combination of its rheological behaviors. Graphene nanoplatelets (GNP) produced from waste-tire by upcycling and recycling techniques having high oxygen surface functional groups were used to increase the compatibility with PA66 chains. This study revealed that GNP addition increased the crystallization temperature of nanocomposites since it acted as both a nucleating and reinforcing agent. Tensile strength and modulus of PA66 nanocomposites were improved at 30% and 42%, respectively, by the addition of 0.3 wt% GNP. Flexural strength and modulus were reached at 20% and 43%, respectively. In addition, the flow model, which simulates the injection molding process of PA66 resin with different GNP loadings considering the rheological behavior and alignment characteristics of GNP, served as a tool to describe the mechanical performance of these developed GNP based nanocomposites.

Experimental and Numerical Investigation of Flow and Heat Transfer in Stationary Two-Pass Rectangular Duct (AR = 1:2) With Continuous and Broken V-Shaped Ribs

Abstract The combined experimental and large eddy simulations (LES) were performed in the stationary two-pass duct of aspect ratio (AR) 1:2. The experiments were conducted with three different rib arrangements, namely, 60 deg V, 60 deg V–IV, and broken 60 deg V–IV ribs, and the analysis was carried out with Reynolds numbers of 45,000, 60,000, and 75,000. The infrared thermography (IRT) technique is employed to obtain the local temperature distribution on heated smooth and ribbed surfaces. In all ribbed cases, the copper ribs are glued to the heated surface with a fixed rib height-to-hydraulic diameter (e/Dh) ratio of 0.125 and the rib pitch-to-height ratio (P/e) of 10 and 5 for continuous and broken ribs, respectively. In addition, the LES turbulence model was adopted for carrying out a simulation to understand the flow and heat transfer behavior in ducts populated with all three V-shaped ribs. The comparison of the time-averaged thermal fields generated using computations has been made with experimentally measured Nusselt numbers, friction factors, thermal performance factor (TPF), and Reynolds analogy performance parameter (RAPP) for all cases. The overall thermal performance factor was found to be quantitatively within 8.0–10.66% between experimental and numerical results. Among all the cases, the 60 deg V–IV ribbed duct provides the best TPF and RAPP than the other two ribbed ducts, whereas the smooth duct shows poor TPF.

Numerical investigation of flow over tandem and side-by-side cylinders

Numerical investigation of flow over tandem and side-by-side cylinders

Experimental and Numerical Investigations of Flow over a Pitching Airfoil in Periodic Flow

Experimental and Numerical Investigations of Flow over a Pitching Airfoil in Periodic Flow

Numerical Investigation of the Streamlining of Nozzle Arrays Equipped with Grid Valves

The purpose of one of the stages of this research is to develop the technique for the numerical investigation of the flow of the nozzle array equipped with the grid valve for its further use for the solution of the problem relating to the optimal design of the flow passage in cogeneration turbines. The development of this technique was divided into two parts, in particular the simulation of the two-dimensional or flat flow and the simulation of the three-dimensional flow. This research paper gives some data obtained for the first part of the developed technique, i. e. the verification data obtained by the comparison of the results of numerical and experimental investigations of the two-dimensional flow of the nozzle array with the grid valve. The Fluent software package was used for the simulation and computation of the flow of operating fluid. Computational domains restricted by one interblade channel were constructed for a different degree of the opening of the nozzle closure diaphragm. The grids were constructed for computational domains. The computations were performed to define the effect of the used turbulence model on the computation results. The best coincidence with experimental data was obtained when using the k–ω SST model. The computations done allowed us to obtain the flow patterns in the interblade channel and behind it and the distribution of the coefficients of the loss of kinetic energy over the grid front for a different level of the opening of closure diaphragms at the inlet to the nozzle ring. The obtained research data allow us to draw the following conclusions: the selected computation parameters enable a qualitative reflection of physical phenomena in the flow both for subsonic and supersonic flows of the cascade; the computation coefficients of the loss of kinetic energy showed a satisfactory coincidence with experimental data and it is indicative of that the development of the technique for the numerical investigation of the flow of nozzle arrays with grid valves is possible in principle when using the Fluent software package. The data obtained through this research will be used for the development of the technique for the numerical investigation of the three-dimensional flow of nozzle arrays with grid valves.

Numerical investigation of flow and heat transfer between concentric cylinders with slit wall

This paper investigates the flow field distribution and heat transfer characteristics of turbulent flow between concentric cylinders with slit wall in the outer cylinder. The slit wall cross sections are triangle, rectangle and trapezoid respectively. The governing equations are established by the PANS model which is modified from the standard k - ε model. The rotating Reynolds number ranges from 2336 to 4682 and axial Reynolds number ranges from 161 to 645, the influence of slit wall cross section on the formation and intensity of small vortex in slit region is analyzed. The results show that the slit wall geometry has a significant influence on the flow field distribution and its heat transfer process, the main vortex in annular region and small scale vortex in slit region has a tendency to merge together in the model with trapezoid cross section, resulting in more intense mixing of fluid with different temperature, therefore, the slit model with trapezoid cross section shows the best heat transfer performance among three models. The present analysis results may be applied for the structural design and optimization of many related rotating machinery.

Numerical investigation of flow and heat transfer in axially-finned in-line tube banks

ABSTRACT Numerical analysis of heat and fluid flow over tube bank heat exchangers with axial fins is performed in this work. Finite volume method is used to solve the governing equations numerically. Simulations are carried out in the range of 400–1500 Reynolds number. To ensure the numerical model reliability, unfinned in-line tube bank is simulated and compared with results from the literature. It is shown that the results are in accordance with the experimental studies in the literature. Heat transfer and pressure loss computations are carried out for four different axial and transversal pitch ratios; namely, and , and , and , and . Air and water are used as working fluids for determined mass flow rates where Prandtl numbers are 0.7 and 7, respectively. Moreover, velocity streamlines and temperature contours along the flow field are given. Effects of axial () and transversal () pitch ratios and fin lengths on the average Nusselt number and friction factor are also presented in graphical forms. It is found that the maximum average Nusselt number is observed at Re = 1500 for the pitch ratios of and where fin length equals to 2D mm. Besides, adding axial-fins to in-line tube bank results in a decrement in friction factors.

Numerical investigation of flow boiling characteristics of water in a rectangular microchannel

In the case of flow boiling, the prediction of vapour fraction in the horizontal microchannel is a severe issue using the numerical technique. Two-dimensional numerical simulation was carried out for the flow boiling in microchannels with different boiling models (VOF, MM and EM). This study is one of the first studies that report a numerical assessment of these three models. Numerical simulations have done with water as a working fluid. The different mass flow rates (1.586 × 10−6 kg−1, 2.541 × 10−6 kg s−1, 3.112 × 10−6 kg s−1) and different heat fluxes (300 kW m−2, 400 kW m−2, 500 kW m−2) with different flow boiling models are used. The vapour fraction estimation was done by image processing in MATLAB program and compared with various mass flow rate and heat flux. It well validated with the published literature. An onset of nucleate boiling point position is estimated with same time step, and uncertainties of the numerical simulation were less than 2.5% at the lowest mass flow rate. The result shows that the vapour fraction in the microchannel increases with an increase in mass flow rate and heat flux. Similarly, the model’s heat transfer rate compared with the same mass flow rate and heat flux. The mixture model is best to estimate of vapour fraction compared to other models. The estimated vapour fraction values are 0.2950, 0.1848 and 0.1726 for MM, VOF and EM respectively. The heat transfer coefficient value for the mixture model is 20.381 kW m−2. Its value was very higher compared to other models because of the increase in fluid temperature difference at constant heat flux. This comparison can be used to provide design guidelines by selecting proper model for simulation work and minimize the complexity and wastage time.

Numerical investigation of flow boiling in manifold microchannel-based heat exchangers

Microchannel heat sinks have the potential to achieve the physical limit of phase change heat transfer. With advances in nano-/microscale fabrication techniques, important progress on microchannel heat exchangers has been made in recent years. However, the two-phase flow evolution and its influence on flow boiling at microscale remains ambiguous. Detailed information on transport process in microchannel and underlying mechanism are required for the fully understanding and further optimization of the microchannel heat sink. Here, we constructed a three-dimensional manifold microchannel-based heat exchanger model to study the two-phase flow pattern and heat transfer performance using the Phase-field method. The dynamics of vapor bubbles in microchannels of 80 μm, 150 μm, and 300 μm in depth was carefully investigated with respect to Reynolds number and thermal boundary conditions. Parameters and underlying mechanism that influence the two-phase flow evolution and heat transfer performance in the manifold microchannels are systematically analyzed. Finally, mechanism of heat transfer crisis and critical heat flux (CHF) in microchannel is studied, and strategies to optimize the flow field to delay the CHF are discussed. Our study provides detailed information on the transport process at microscale and offers opportunities for the design and fabrication of next generation microscale heat sinks.

Numerical investigation of flow and heat transfer in rotating trapezoidal channel with lateral slots and dimple structure

High-efficiency trailing edge cooling technology is essential for the safe and stable operation of turbine blades. In this paper, trapezoidal trailing edge channels with lateral slots are modeled, and a variety of dimples with different sizes and arrangement densities are placed on the end walls. Flow characteristics, wall heat transfer distributions and overall heat transfer performance under stationary and rotating states are numerically investigated and effects of dimple arrangement, rotation number (Ro) and channel orientation (β) are comprehensively analyzed. The results show that, under rotating state, the Coriolis force leads to the flow separation, while dimples promote the generation and development of three-dimensional spatial vortices, and effectively suppresses the expansion of separation area. Dimples greatly improve the rotating cooling performance, and there exists a critical point of arrangement density, in which the maximum heat transfer level is obtained. A 108.2% improvement in relative Nusselt number of trailing surface can be achieved by increasing rotation number in dimple channels. And the operation condition of Ro = 0.8 and β = 15° is recommended for the cooling of trapezoidal trailing edge. Compared with rotation number, the channel orientation has a stronger effect on the flow and heat transfer performances.

Experimental and Numerical Investigation of Flow Over Ogee Spillway

Ogee type spillway is one of the most preferred sluice types due to its functional suitability and high safety factor. It is used for controlling the flow rates and water levels in reservoirs, such as lowering the water level in emergency situations, maintaining normal river functions and discharging excess water. The main aim of this study is to investigate the flow over ogee type spillway by performing experiments in an open channel flume in the laboratory and simulating with numerical model. The numerical model having the same dimensions with the physical model is modeled with two different programs of ANSYS-Fluent and OpenFOAM. The flow depths of the models were measured at four points, H1, H2, H3, H4. In the numerical analysis, two different turbulence models, K-e and K-ω SST turbulence model were used in order to investigate the accuracy of the turbulence models in the open channel. According to the results, R2 values, obtained from ANSYS- Fluent for the each measurement points where H1, H2, H3, H4, are 0.9776, 0.9859, 0.9701, 0.9916 and obtained from OpenFOAM for the each measurement points 0.9920, 0.9687, 0.9855, 0.9926 respectively. The findings show that the numerical tools have been sufficiently developed to simulate flow depths and water surface profiles.

Combined numerical and experimental study of microstructure and permeability in porous granular media

Abstract. Fluid flow on different scales is of interest for several Earth science disciplines like petrophysics, hydrogeology and volcanology. To parameterize fluid flow in large-scale numerical simulations (e.g. groundwater and volcanic systems), flow properties on the microscale need to be considered. For this purpose experimental and numerical investigations of flow through porous media over a wide range of porosities are necessary. In the present study we sinter glass bead media with various porosities and measure the permeability experimentally. The microstructure, namely effective porosity and effective specific surface, is investigated using image processing. We determine flow properties like tortuosity and permeability using numerical simulations. We test different parameterizations for isotropic low-porosity media on their potential to predict permeability by comparing their estimations to computed and experimentally measured values.

Numerical investigation of flow boiling of refrigerant-based nanofluids and proposing correlations for heat transfer

The numerical simulation of subcooled flow boiling of R-113 working fluid has been done for two different nanofluids (R-113/Al2O3, R-113/TiO2) under different volume concentrations (0.5%, 1%, and 3%). The numerical results were compared with experimental results obtained by previous researchers, and the comparison shows that the numerical results are in good accordance. Nucleation site density, bubble departure frequency, and bubble departure diameter, which are three key parameters, are investigated in this study. The results express that these three parameters have the highest variation at low Reynolds numbers. The influence of different nanoparticles concentrations on the variation of the heat transfer coefficient is studied. The results indicate there is an insignificant difference between the effect of 1% and 3% concentrations on the heat transfer coefficient that means an increase of nanoparticles more than 1% concentration cannot improve heat transfer. The effect of different non-drag forces such as lubrication force, turbulent dispersion force, and lift force is also studied. Two correlations are proposed for predicting the convective heat transfer coefficient.

Numerical Investigation of Flow around Two Tandem Identical Trapezoidal Cylinders

The trapezoidal beam structure is ubiquitous in giant engineering equipment, while their aerodynamic characteristics have not been clearly understood. Numerical simulation method was adopted to investigate the flow around two tandem identical trapezoidal cylinders. The study was conducted using a Reynolds number of 2.2 × 104, and with a spacing ratio varying from 0.5 to 10. The incompressible two-dimensional finite volume method was used for solving Reynolds-Averaged Navier–Stokes (RANS) equations with realizablek−εmodel. The effects of cylinder geometry and spacing between the cylinders on aerodynamic characteristics, unsteady flow patterns, time-averaged flow characteristics, and flow instability was studied. The results show that the flow around the two tandem trapezoidal cylinders is highly dependent on the spacing ratio. The flow modes can be classified into: extended-body regime (Mode I,S∗ ≤ 1), reattachment regime (Mode II, 2 ≤ S∗ ≤ 3), and binary regime (Mode III,S∗ ≥ 4). We explored their respective flow characteristics and distinctions through the force/pressure coefficients, time-average streamwise velocity, and the flow field evolution.

Numerical investigation of flow and heat transfer characteristics of subcooled boiling in a single rod channel with/without spacer grid

A multiphase flow numerical model based on Eulerian is built to simulate the characteristics of subcooled boiling. The thermodynamic performance in a single rod channel with/without spacer grid are obtained by numerical simulation, and the influence of spacer grid on flow and heat transfer characteristics of subcooled boiling is studied. The model is verified by comparing with the experimental results. After passing through the spacer grid, the high-temperature zone departures from the surface of fuel rod moves towards the cold wall. This impact extends to a certain downstream distance, and then it gradually declines. The influence of different components in spacer grid on the pressure drop is detailed. The pressure drop with spacer grid is 5% higher than that without spacer grid, within the allowable design limitation. The maximum value of section average secondary flow can reach 13.5% of the inlet velocity, indicating that the mixing vanes have stronger ability to produce transverse flow. The influence length of spacer grid on heat flux partitioning is about 27Dh. The effect of spacer grid on the wall heat flux partitioning mainly lies in the decrease of the evaporation heat flux, benefited to reduce the risk of nuclear reactor approaching DNB.