Circular Duct Research Articles

Study on supercritical methane flow and heat transfer performance in a 180-degree curved duct

ABSTRACT In this paper, an investigation into flow and heat transfer performance of supercritical methane (S-CH4) in a 180-degree curved circular duct has been numerically carried out. The mathematical description of energy equation induced by the Dean vortices under the Cartesian-coordinate is first deduced. After validating the dynamic computational fluid dynamics model and method against the public experimental data, the effect of mass flux on the thermodynamics properties of S-CH4 is revealed. The calculation results show that due to the centrifugal force, the low temperature of S-CH4 gathers near the outer wall generatrix. Meanwhile, owing to the existence of multiple Dean vortices, all thermophysical parameters on the 90° cross-section are symmetrically concave along the vertical axis. The core position of multiple Dean vortices inside the curved duct is closer to the inner wall generatrix, which makes the velocity fluctuation greater. The maximum value of circumferential heat transfer coefficient on different cross-sections differs, and the non-uniform flow development process occurs inside the curved duct. Compared to the straight duct, when the mass fluxes are respectively 300 kg/m2 · s and 600 kg/m2 · s, the magnitude of increase in heat transfer coefficient of curved duct presents 18.8% and 23.5%. In addition, the forced convection caused by the secondary flow inside curved duct is so strong that the natural convection by the gravity could be neglected. The research outcome is of vital importance for the optimization design of liquefied natural gas vaporizer.

Numerical investigation on the role of a mixer on spray impingement and mixing in channel cross-stream airflow

The present study numerically investigates the influence of introducing a spin-type mixer and different angular orientations of the mixer blades on the spray-wall interaction and mixing, following cross-stream injection of a pulsed spray into airflow in a circular duct. This is relevant to the Selective Catalytic Reduction system in diesel engines for exhaust gas after-treatment. The spin-type static mixer is located downstream of the injector and generates a swirling airflow in the duct. All simulations were carried out using ANSYS Fluent V18.0. The standard k–ω model is used to simulate the turbulent continuous phase flow, while the discrete phase model is employed to track the spray droplets. The Taylor Analogy Breakup and Kuhnke wall film models are adopted to model droplet breakup and wall-film formation, respectively. First, the swirling airflow characteristics without spray injection are validated against in-house particle image velocimetry measurements. Second, the spray computations are compared with the experiment. Overall, good agreement between simulation and experiment is achieved. Furthermore, the choice of water and urea water solution injection liquid on the in-channel spray characteristics is also studied. The main focus of the present work is on the study of the influence of spin mixer clocking on the post-impingement spray evolution, droplet redistribution and mixing, and wall-film characteristics. The results show that the choice of the angular orientation of the mixer governs the extent of droplet deposition and splashing on the mixer blades and, as a result, strongly influences the spatial uniformity of droplets and ammonia species at the channel exit.

Effect of duct geometry on thermal management of PCM – Liquid hybrid PV panel

In this study, a computational fluid dynamics (CFD) model of a PV panel with different cooling technologies is performed. The basic setup consists of a hybrid PV-PCM system, with absorber plate of 0.25m breadth, 0.5m length and 5mm thickness. The study includes the design and simulation of a three-dimensional photo-voltaic model integrated with a water channel and PCM. A commercial CFD tool ANSYS Fluent is used to analyse the thermal transport. The melting of the PCM is simulated using enthalpy – porosity method. Effect of shape and orientation of duct cross-section on the performance of the system is investigated. The cross section dimensions are chosen so as to maintain negligible difference in the volume on PCM. The Reynolds number in each model is maintained constant (Re = 1500) to have similar flow characteristics, so that only the effect of shape on heat transfer can be analysed. Additionally, a PCM-PV model with circular cross-section duct was modelled and compared to triangular, square, rectangular and ellipse cross-section. From the results obtained, it was found that there was a minimum temperature drop of 20oC between simple PV model and PV-PCM model. Triangular shapes had highest thermal efficiencies, with an average of 46%, Square duct showed better electrical efficiency, with an average of 13.25% with same pressure drop as circular ducts. Orientation of duct showed a huge impact on thermal and electrical efficiencies of rectangular channel.

Speed distribution in the exhaust diffuser of the air fan

A perfect connection between the column and the fan is that which ensures an air inlet in the fan, evenly distributed, over the entire surface of the suction mouth and an air outlet from the fan outlet made in a way that allows the full use of developed pressure. For both suction and exhaust, fans must be equipped with a device/diffuser. When the fan discharges freely into the atmosphere without any connection, a loss equivalent to 50% of the average dynamic pressure at the discharge port occurs. If the fan discharges into a speaker, the loss depends on its angle. At a peak angle of 30° corresponds to a loss of ≈ 25% of the average dynamic pressure in the discharge mouth, and to reduce air vortices the speakers must be built at an angle of inclination to vertical or horizontal between 12- 15°, in order to reduce the aerodynamic resistances. The paper will present the speed field distribution of an axial fan located on a circular duct, provided on the air discharge side with a diffuser with a length of 1.5 m, at an angle of inclination to the vertical or horizontal of 12°.

FULLY DEVELOPED TEMPERATURE PROFILE FOR A POWER-LAW LAMINAR NANOFLUID FLOW IN A CIRCULAR DUCT SUBJECT TO A UNIFORM HEAT FLUX

In the present work, we develop a theoretical analysis of a fully developed laminar nanofluid flow in a circular duct in steady state, subject to a uniform heat flux condition. The rheological behavior of the base fluid is described by the power-law model and the nanofluid properties such as dynamic viscosity, thermal conductivity, density and specific heat are assuming as constants for a fixed value of the volumetric fraction. An analytical solution can be obtained by the mean temperature concept. The Nusselt number was defined and depends on the dimensionless parameter of the viscosity and the power-law index. The main results reveals that, for a maximum value of the volumetric fraction, the Nusselt number decreases drastically for shear-thinning base fluids, for shear-thickenning base fluids tends asymptotically to a constant value. For an increase in the volumetric fraction value, the dimensionless velocity profiles shown lower values for shear-thinning base fluids and the dimensionless temperature profiles showed a greater difference between the dimensionless wall temperature and the nanofluid stream temperature.

Validity of performance factors used in recent studies on heat transfer enhancement by surface modification or insert devices

Based on literature survey, the performance factors were used in 62 articles which have been published in 2021 in International Journal of Heat and Mass Transfer, International communications in Heat and Mass Transfer, International Journal of Thermal Sciences and Applied Thermal Engineering, to evaluate the heat transfer enhancement by geometry modification or insertion of tapes, wires or turbulence promoters under identical pumping power. However, the validity of the performance factors was not examined yet. The heat transfer rate in a periodic fully developed region of a circular duct with periodic annular baffle plates was numerically obtained. Comparing the heat transfer rate with that of a circular smooth duct, the validity of the performance factors used in previous studies was examined. It was found that the performance factors used in the previous studies are not valid for the evaluation of heat transfer enhancement. Furthermore, this paper shows that the direct comparison of heat transfer rates without the assumptions of equal surface area and the equal temperature difference is suitable for the evaluation of heat transfer enhancement.

Hydro-thermal characteristics of laminar flow through a square duct having transverse ribs and helical screw-tape inserts

The hydro-thermal characteristics of transverse ribs combined with helical screw tape inserts in square duct are numerically investigated and compared the numerical findings with very little available experimental results. Moreover, the experimental data is available only with circular duct. No other cross-sectioned duct has ever been investigated. The motivation and novelty of the present work is to fill this hiatus. Finite volume method-based solver, ANSYS Fluent has been used for simulation. The uniform heat flux boundary condition has been imposed and a wide range of Prandtl number with five different fluids has been used for this investigation. This is never ever possible in a single experimental programme. Only numerical simulation can do the job and this is the inherent beauty of the present investigation. The results of experimental and computational findings proved that the present configuration produces good mixing and enhanced boundary layer separation. The results of the effect of rib height, rib pitch and screw tape parameter on Nusselt number and friction factor are presented. The numerical investigation has shown that Servotherm oil is the best fluid. The Nusselt number is increasing with increase in rib height and Reynolds number and it is decreasing with increase in rib pitch and screw tape parameter value. Also, the friction factor is increasing with increase in rib height and Reynolds number but the increment in Nusselt number is larger than the increment in friction factor. Therefore, the present fin geometry is recommended for industrial use.

Designing a Facility for Measuring Higher-Order Acoustic Modes in Uniform Heated Flows

An experimental facility was recently developed at the Georgia Tech Research Institute to measure the acoustic scattering coefficients of a test article under mean flow, high-temperature, high-amplitude, and higher-order mode propagation conditions. The experimental facility acts like a two-sided impedance tube, in which the measurement section is a circular duct with 16 acoustic drivers and 16 microphones mounted on each side of the test article. Such a facility can be configured for grazing and bias flow measurements of acoustic liners with sound traveling in and against the direction of the flow. The new facility is designed to maximize the accuracy of acoustic measurements of higher-order modes in heated flows. The influence of experimental uncertainties on the acoustic measurements is analyzed using a Monte Carlo method. Measurements of half-wave resonators in quiescent air and heated flows demonstrate that the experimental uncertainty decreases as the absorption coefficient increases. High-sensitivity microphones produce extremely low uncertainty, and even lower-sensitivity pressure transducers are sufficiently accurate for many applications. This paper is intended to serve as a reference for designing and analyzing an experimental facility to measure the acoustic scattering coefficients in heated-flow and higher-order mode propagation conditions.

Analytical solution for thermally developing laminar core-annular flow with two immiscible liquids considering axial diffusion in semi-infinite ducts

Flows involving core-annular immiscible liquids provide a wide range of useful engineering applications, such as transportation of heavy oils and cooling processes. The present article considers an analytical solution for the internal convection heat transfer in hydrodynamically developed and thermally developing laminar core-annular liquid–liquid immiscible flows in circular ducts. The axial diffusion is considered in the model and the problem is investigated for semi-infinite domain under convective boundary condition on duct walls and prescribed inlet flow temperature. The steady-state problem is solved through the generalized integral transform technique (GITT) with a single domain approach, in which, the coupled ODE system arising from the GITT is analytically solved using the matrix exponential method. Moreover, integral balance is used to compute the Nusselt number. The convergence process of the local Nusselt number is shown in different flow regions and the results are presented for multiple flow parameters. The effects of the boundary conditions, flow properties, and other parameters are discussed. An analysis of axial diffusion effects is performed, which confirms that it can interfere with the local Nusselt number, especially for lower Péclet numbers.

Numerical analysis of convective heat transfer on straight finned tube with different nano fluids

The major idea of this effort is to study and analyse heat transfer through a straight finned duct with different nanofluids. Dynamics and simulations for fluid flow at the start of fully developed laminar flow with nanofluids in a circular duct with and without fins under constant wall temperature conditions are performed by using fluent commercial software. Heat transfer behaviours of the duct are investigated for constant wall temperature conditions for oxides of nanofluid. The thermal properties of nanofluid are considered from the literature. Commercial software ANSYS 2018 was used to compute the velocity and thermal analysis between the inlet and outlet of the tube. The finite volume method will be used to discretize the whole domain in quadratic elements considered with the turbulence model. Turbulence differential equations system coupled with the heat convection. The flow and pressure, the temperature in the exit will be calculated as a function of initial conditions in the entrance of the duct. The fluid properties such as conductivity and viscosity of the base fluid are taken as temperature independent. The enhancement in heat transfer rate are presented graphically with the effect of effects of nanoparticle volume fraction, nanoparticle size.

A Comparison of Circular Duct and Real Hexagonal Duct Results Using Hydraulic Diameter

To see whether the turbulent flow correlations derived for circular ducts can be used for hexagonal cross-sectional ducts using hydraulic diameter, turbulent flow in hexagonal ducts is numerically investigated under constant wall temperature boundary condition using ANSYS Fluent 17.0 software. Investigated parameters are the Reynolds number between 10×103Re50×103 and side angle of the duct varying between 30o and 90o. Standard k-ε model is used as turbulence model. General expressions are proposed for fully developed dimensionless heat transfer coefficient Nusselt number and fully developed Darcy friction factor in terms of Reynolds number and side angle for hexagonal-shaped cross-sectional duct. Results show that side angle of hexagonal duct affects the pressure drop along duct and heat transfer coefficient in duct. Results point out that regular hexagonal duct, =60o, gives minimum pressure drop and maximum Nusselt number. It is concluded that correlations given in the literature for circular ducts in turbulent flow can give 14% higher dimensionless heat transfer coefficient, Nusselt number, than that of actual hexagonal duct flow.

Sound Reduction of Ventilation Ducts through Walls: Experimental Results and Updated Models

Ventilation ducts can have a negative effect on the sound reduction index between two rooms if they pass through the dividing structure without treatments. The overall sound reduction of a ventilation duct is dependent on several factors including the transmission loss when sound is breaking in and out from the duct. This study aims to model the sound reduction of a combined system with a separating wall and a ventilation duct through it. Three walls, characterized according to ISO 717-1, are combined with three different ventilation ducts, two circular and one rectangular with different dimensions. Laboratory measurement data are used to determine the sound reduction of the different configurations and the type of treatments needed for each configuration. A proposed model with existing theory for describing sound transmission losses of circular and rectangular ventilation ducts predicts the shape of the measurement data for many frequency bands. A new theory part is developed through an iterative process for circular ducts, which is based on measurements with previous methods and studies as a guide because the existing prediction scheme is somewhat perplexing. For rectangular ducts, the existing theory has been updated to better match measurement data. The application of the proposed theory and model in this article shows similar results when compared to measurements. The difference in weighted sound reduction index between developed theories and measurement data is 0–1 dB for every configuration.

Active control behavior on the flow pattern in a circular duct

The current analysis aims to review the effectiveness of the flow regulations at Mach 1.87, a considerably large Mach number. The duct radius is11 mm, and the diameter ratio concerning the exit of the nozzle is 2.2. Tests were done at different expansion levels for eight different duct sizes. Here we are pondering in a situation where there is a considerable variation of pressure is noticed. The flow control was located at a PCD of 13 mm from the axis. The control jets of 0.001 m diameter producing a Mach unity jet, used as energetic flow control. For shorter tube sections at smaller NPRs, the pressure undertakes a minimal value. Soon After, with an increase of the tube size and expansion levels, a continuing fall in pressure owing to the decline in the level of adverse pressure and shock waves intensity. Flow fluctuations are witnessed for various duct sizes. The duct size represents a vital part while ascertaining the flow advancement in the duct. Relatively high pressure is seen for duct length L = 132 mm and L = 220 mm for nozzle pressure ratios three and five. The pressure becomes modest due to the effect of duct size. For NPR = 7, an increase in pressure is restricted by the separated zone, and atmospheric restrictions are almost accomplished. For the 10D duct, the realization of the ambient condition is additionally enhanced near the upstream.

Analytical Solution of Non-Newtonian Nanofluid Flows Within Circular Duct under Convective Boundary Condition

At the outset, this work aims to carried out an analytical investigation of forced convection by establishing a laminar flow into circular duct under convective boundary conditions of the third type for non-Newtonian nanofluid, the fluid containing TiO2 uniformly dispersed in aqueous solution with 0.5wt% of CMC solutions (Carboxymethyl Cellulose) is used as working fluid. The viscous dissipation effects are taken into account, the employed methodology is based on a combination of the Ritz variational approach with the Laplace transformation technique, so the power-law fluids flow model is used to describe the non-Newtonian fluid behavior. The effect of dimensionless parameters such as Biot (Bi), Brinkman (Br), Peclet (Pe) numbers, power-law index (n), and nanoparticles concentration (φ) on the temperature distribution contours and on the examined local Nusselt number. Our results have been compared with those found in the literature in particular the cases of base fluids (φ=0) with and without viscous dissipation effects.

Comparative analysis and research on heat transfer enhancement of fin tube-paraffin

In order to improve the heat transfer between the air and paraffin phase change heat storage, the circular fins are arranged on the smooth circular heat storage air duct to enhance the heat transfer between the air and paraffin, and study the effects of different fin thickness, fin space and the number of heat storage air ducts on the heat storage and release process of paraffin. In this research work, the heat transfer process is studied by numerical work.The model of the heat storage device is established by using Fluent software in computational fluid dynamics (CFD)simulation software, and make simulation calculation for different fin thickness, fin space and the numbers of duct.The results of simulation calculation show that the paraffin melting degree and average temperature of paraffin regenerator with fin thickness of 15 mm is higher than that with fin thickness of 10 mm.The paraffin melting degree and average paraffin temperature of paraffin regenerator with fin space of 25 mm are highest.At the paraffin regenerator with ducts number of 16, the melting degree, average temperature and flow channel of paraffin are highest, which are 92.42%,319.64 K and 323.71 K respectively.The Nusselt number and paraffin melting degree of the best regenerator are fitted with the empirical formula by the method of curve fitting.And the curve shows that when the number of air ducts is 18, the melting degree of paraffin reaches the maximum.The heat released by the best combination regenerator can maintain the indoor heating for eight hours at night.

Interaction of mixed convection with non-gray gas radiation in a partially heated horizontal pipe: entropy generation analysis

PurposeThis paper aims to numerically analyze the effect of non-gray gas radiation on mixed convection in a horizontal circular duct with isothermal partial heating from the sidewall. The influence of heater location on heat transfer, fluid flow and entropy generation is given and discussed in this study.Design/methodology/approachThe numerical computation of heat transfer and fluid flow has been developed by the commercial finite element software COMSOL Multiphysics. Radiation code is developed based on the T10 Ray-Tracing method, and the radiative properties of the medium are computed based on the statistical narrow band correlated-k model.FindingsThe obtained results depicted that the radiation considerably contributes to the temperature homogenization of the gas. The findings highlight the impact of the heater location on swirling flow. It is also shown that the laterally heating process provides better energy efficiency than heating from the top of the enclosure.Originality/valueThis study is performed to improve heat transfer and to minimize entropy generation. Therefore, it is conceivable to improve the model design of industrial applications.

Heat and Mass Transfer from Moist Air Flowing inside Cold Horizontal Circular Duct I-Theoretical Study. (Dept. M)

Heat and Mass Transfer from Moist Air Flowing inside Cold Horizontal Circular Duct I-Theoretical Study. (Dept. M)

Study of Parameters Affecting Thermal Drying Process. (Dept. M)

Parameters affecting drying process has been studied experimentally as it associated with heat and mass transfer between porous bed to be dried and air which flows through the porous bed. To perform this study, a test rig is designed and constructed as it consists of vertical circular duct packed with wet porous bed which is made from clay balls of 20 mm in diameter. Drying process occurs as a result of forcing hot air vertically to pass through the wet porous bed, evaporation of moisture is existed in the bed due to the simultaneous heat and mass transfer. The studied operating parameters are inlet air temperature, inlet air mass flux, porous bed depth and porosity of balls. The average heat and mass transfer coefficients are estimated for flow of hot air through the wet porous bed. The results indicated that, the drying rate is directly proportional to inlet air temperature, mass flux of air, dimensionless bed depth and ball porosity. So, Drying process can be achieved in lower time as increasing both air mass flux and or inlet air temperature and decreasing bed depth. Also, heat transfer coefficient and mass transfer coefficient increase with increasing inlet air temperature, inlet air mass flux, and ball porosity, but they decrease with increasing bed depth. Comparison between the obtained experimental results and the previous work gave the same trend.

Heat and Mass Transfer from Moist Air Flowing inside Cold Horizontal Circular Duct II- Experimental Study. (Dept. M)

Heat and Mass Transfer from Moist Air Flowing inside Cold Horizontal Circular Duct II- Experimental Study. (Dept. M)

Effect of Cavities in Suddenly Expanded Flow at Supersonic Mach Number

The contribution from the base drag due to the sub-atmospheric pressure is significant. It can be more than two-thirds of the net drag. There is a need to increase the base pressure and hence decrease the base drag. This research examines the effect of Mach Number on base pressure. To accomplish this objective, it controls the efficacy in an enlarged duct computed by the numerical approach using Computational Fluid Dynamics (CFD) Analysis. This experiment was carried out by considering the expansion level and the aspect cavity ratio. The computational fluid dynamics method is used to model supersonic motion with the sudden expansion, and a convergent-divergent nozzle is used. The Mach number is 1.74 for the present study, and the area ratio is 2.56. The L/D ratio varied from 2, 4, 6, 8, and 10, and the simulated nozzle pressure ratio ranged from 3 to 11. The two-dimensional planar design used commercial software from ANSYS. The airflow from a Mach 1.74 convergent-divergent axi-symmetric nozzle expanded suddenly into circular ducts of diameters 17 and 24.5 mm with and without annular rectangular cavities. The diameter of the duct is taken D=17mm and D=24.5mm. The C-D nozzle was developed and modeled in the present study: K-ε standard wall function turbulence model was used with the commercial computational fluid dynamics (CFD) and validated. The result indicates that the base pressure is impacted by the expansion level, the enlarged duct size, and the passage’s area ratio.