Increase In Average Nusselt Number Research Articles

Direct Numerical Simulation of Laminar Natural Convection in a Square Cavity at Different Inclination Angle

The effect of angle of rotation on laminar natural convection inside the square cavity have been observed in this research. It was assumed that left and right walls heated isothermally, whereas other two walls act as adiabatic. This problem was solved by assuming 2-D and by Direct Numerical Simulation (DNS) method using ANSYS Fluent 16.0. A series of DNS simulation were carried out for different inclination angle (𝜃 = 0°~90°) of the cavity at Ra = 103 & 104. It was observed that at average Nusselt number increase up to some value of angle of inclination after that it decrease though this variation is not significant.

Effect of Horizontal Spacing on Natural Convection to Power-Law Fluids from Two Horizontally Aligned Cylinders

This paper expounds the effect of horizontal spacing between two horizontally aligned circular cylinders on laminar natural convection to non-Newtonian power-law fluids. Numerical computations have been carried out for the following range of conditions: Grashof number (10 to 105), Prandtl number (0.71 to 100), horizontal spacing (0 to 20 times the cylinder diameter), and power-law index (0.4 to 1.6). The flow, pressure, and thermal fields are pictorially visualized in terms of streamlines, pressure contours, and temperature contours, respectively. The average Nusselt number shows a positive reliance on both Grashof and Prandtl numbers, whereas an adverse dependence is observed on power-law index. Irrespective of all other parameters, shear-thinning fluid behavior is seen to promote the convection, whereas shear-thickening behavior impedes it with reference to the Newtonian fluid. With decrease in the horizontal spacing, the average Nusselt number increases and attains a maximum value corresponding to the optimal spacing, however, it diminishes greatly with further reduction in spacing. Furthermore, the average Nusselt number is in excess of 21-89% at the optimal spacing compared to no-spacing. Finally, this paper is concluded with producing a new correlation for the average Nusselt number.

MHD natural convection and entropy generation in a grooved enclosure filled with nanofluid using two-component non-homogeneous model

In this paper, a computational study of natural convection in a grooved enclosure filled with water-based nanofluid in the presence of external magnetic field is numerically investigated. Two-component non-homogeneous model is introduced to develop the governing partial differential equations. Galerkin finite element method is used to solve the governing equations. The computation is carried out for a wide range of governing parameters such as Rayleigh number (103 ≤ Ra ≤ 106), magnetic field parameter (10 ≤ Ha ≤ 100) and volume fraction of nanoparticles (0% ≤ ϕ ≤ 5%) with fixed values of remaining parameters. A detailed parametric analysis is performed to show the effects of physical parameters on the fluid flow and temperature distributions within the enclosure via streamlines, isotherms, isoconcentrations, mid-sectional velocities, average Nusselt number and temperature, respectively. In addition, the entropy generation and Bejan number are also computed and discussed elaborately. The results of the current study are compared to those of previous numerical and experimental studies and found to be in rational agreements. The results ascertain that the average Nusselt number and entropy generation increase with rising Rayleigh number and nanoparticle volume fraction, whereas they decrease with increasing magnetic field strength. Moreover, it is found that the appropriate combination of governing parameters can maximize the heat transfer rate and minimize the entropy generation as well.

Numerical Investigation on Heat Transfer and Flow Characteristics of a Confined Circular Cylinder With Slit

Abstract Heat transfer and flow characteristics of channel-bounded circular cylinder with a slit vent parallel to the flow direction are numerically investigated using openfoam. The interesting feature of this configuration is the formation of the separation bubble behind the cylinder, which significantly alters the near wake characteristics. In this study, the emphasis is given to understand the effect of the slit on forced convection from the cylinder. Simulations were performed by varying the slit width from 0 to 0.25 (in steps of 0.05) for the range of Reynolds number (Re) 60–240. Re is defined based on the diameter of the cylinder (d) and centerline velocity (Uc) at the inlet of the channel. The influence of s/d and Re on the separation bubble, aerodynamic forces, and heat transfer characteristics are studied in detail. Results demonstrate that the slit can manipulate the flow to mitigate adverse effects of vortex shedding and thus can be used as a passive flow control technique. It was observed that the inclusion of the slit in the cylinder delays the onset of vortex shedding, and it also reduces the fluctuations in aerodynamic forces up to 99%. Compared with the solid cylinder, around 38% increase in vortex shedding frequency, a 16% reduction in drag, and a 10% increase in average Nusselt number is observed when the slit width is 0.25d. It was found that the introduction of slit vent in the cylinder not only enhances the heat transfer along with the reduction in expenditure of pressure loss across the cylinder but also suppresses the fluctuations in aerodynamic forces, which causes vortex-induced vibrations and thus improves structural stability and integrity.

An investigation on the influence of the shape of the vortex generator on fluid flow and turbulent heat transfer of hybrid nanofluid in a channel

The purpose of this study is to numerically investigate flow field and turbulent heat transfer of hybrid nanofluid, water–DWCNT–TiO2 in a two-dimensional rectangular channel with triangular and semicircular vortex generators with three different heights of 1, 2 and 3 mm. For numerically modeling this study, SIMPLEC algorithm was used. Therefore, initially, the geometry of channel and the grid points were generated, and then for solving the mathematical equations via finite volume method, ANSYS Fluent was used. The results indicate that with an increase in Reynolds number and volume fraction (φ) of nanoparticles, the average Nusselt number increases. Also, with an increase in the φ, the pressure loss in the channel increases. About the geometrical shape of the vortex generators, results indicate that with using the semicircular vortex generator, average Nusselt number is higher compared to that of triangular vortex generator. Also, by investigating the performance evaluation criteria of thermal–hydraulic, which is a criterion for comparing the positive effect of the Nusselt number increase with the negative effect of pressure drop, results indicate that this criterion is higher for the semicircular vortex generator.

Second law analysis of magneto-natural convection in a nanofluid filled wavy-hexagonal porous enclosure

Purpose Natural convection heat transfer analysis can be completed using entropy generation analysis. This study aims to accomplish both the natural convection heat transfer and entropy generation analyses for a hexagonal cavity loaded with Cu-H2O nanoliquid subjected to an oriented magnetic field. Design/methodology/approach Control volume-based finite element method is applied to solve the non-dimensional forms of governing equations and then, the entropy generation number is computed. Findings The results portray that both the average Nusselt and entropy generation numbers boost with increasing aspect ratio for each value of the undulation number, while both of them decrease with increasing the undulation number for each amplitude parameter. There is a maximum value for the entropy generation number at a specified value of Hartmann number. Also, there is a minimum value for the entropy generation number at a specified value of angle of the magnetic field. When the volume fraction of nanoparticles grows, the average Nusselt number increases and the entropy generation number declines. The entropy generation number attains to a maximum value at Ha = 14 for each value of aspect ratio. The average Nusselt number ascends 2.9 per cent and entropy generation number decreases 1.3 per cent for Ha = 0 when ϕ increases from 0 to 4 per cent. Originality/value A hexagonal enclosure (complex geometry), which has many industrial applications, is chosen in this study. Not only the characteristics of heat transfer are investigated but also entropy generation analysis is performed in this study. The ecological coefficient of performance for enclosures is calculated, too.

Enhancement of Heat Transfer in Perforated Circular Finned-Tube Heat Exchangers: A Numerical Investigation

The effects of employing perforations of circular shape in circular finned-tube heat exchangers (CFTHEs) are numerically investigated in the present study using Computational Fluid Dynamics (CFD) software. Simulations performed on the validated CFD model shows considerable increase in the Average Nusselt Number for perforated circular finned-tube heat exchangers (PCFTHEs) as compared to their solid counterpart, with only a slight increase in Euler Number for 10,000 ≤ Re ≤ 50,000. Diameter of perforations (dp ) is kept equal to 6mm and optimum range of number of perforations per fin (np ) by analysis is found to be 16 to 20. For this range, the performance of PCFTHEs improves with the increase in number of perforations per fin. The maximum increase in Average Nusselt Number is 24% for dp = 6mm and np = 20 at Re = 10,000. The minimum increase in Average Nusselt Number is 3% for dp = 6mm and np = 14 at Re = 40,000. This increase in Average Nusselt Number on introducing perforations is due to (i) flow of localized air jets through the perforations, and (ii) reduction in recirculation region caused by the separation of flow. This improvement in the performance of CFTHEs on introducing perforations is achieved with the maximum and minimum reduction in weight of 41% and 29% respectively.

Numerical evaluation of the thermal performance of a solar air heater equipped with two different types of baffles

AbstractIn the present study, the effect of utilizing two different types of baffles in the channel of the solar air heater is investigated numerically. The studied baffles include angled rectangular baffles and angled V‐shaped baffles, which are mounted on the bottom and top walls of the duct, respectively. Both considered baffles were evaluated separately which the studied parameter in each section was the angular position of baffles. Finally, the best‐obtained results of both sections were compared to each other. The results indicated that in the rectangular model by comparison between 90° model and no baffle, it was found that the pressure drop and average Nusselt number increase 316.67% and 148.15%, respectively at Reynolds number (Re) = 2000. Also, in V‐shaped angled baffles, the thermal efficiency of β = 90°, 60°, 45°, and 30° are 27%, 18%, 13% higher than no baffle channel at Re = 2000, respectively. Furthermore, at low Re (about Re below 300), utilizing baffles into the channel had no effect on the thermal efficiency of the system compared to the no baffle channel. However, at high Re, it was found that the highest thermal efficiency occurred in the model of rectangular baffles with an angle of 90°.

Numerical Study on Hydrothermal Performance Factor Using Jet impingement and Nanofluid

In this study, thermal-hydraulic performance of a confined slot jet impingement with Al2O3-water nanofluid has been numerically investigated over Reynolds number ranges of 100-1000. Two triangular ribs are mounted at a heated target wall; one rib located on the right side of the stagnation point and another one located on left side of the stagnation point. The governing momentum, continuity and energy equations in the body-fitted coordinates terms are solved using the finite volume method and determined iteratively based on SIMPLE algorithm. In this study, effects of Reynolds number, rib height and rib location on the thermal and flow characteristics have been displayed and discussed. Numerical results show an increase in the average Nusselt number and pressure drop when Reynolds number and rib height increases. In addition, the pressure drop and average Nusselt number increases with decrease the space between the stagnation point and rib. The maximum enhancement of the average Nusselt number is up to 39 % at Reynolds number of 1000, the rib height of 0.3, rib location of 2 and nanoparticles volume fraction of 4%. The best thermal-hydraulic performance of the impinging jet can be obtained when the rib height of 0.2 and rib location of 2 from the stagnation point with 4% nanoparticles volume fraction.

Effect of γ-Al2O3/Water Nanofluid on Natural Convection Heat Transfer of Corrugated ┐ Shaped Cavity: Study the Different Aspect Ratio of Grooves

In this paper, the effect of γ-Al2O3/water nanofluid on the flow pattern and natural convection heat transfer of corrugated ⅂ shaped cavity is investigated numerically by the Lattice Boltzmann Method (LBM). The effects of nanoparticles solid volume fraction (ϕ=0-0.006), Rayleigh number (Ra=103-106) and the aspect ratio of grooves cavity (h/H=0.05-0.15) on the streamlines, isotherms and averaged Nusselt number have been examined. In addition, a comparison between γ-Al2O3/water nanofluid and MWCNT-Fe3O4/Water Hybrid Nanofluid on the average Nusselt number are studied. The results showed that the heat transfer rate of γ-Al2O3/water nanofluid is higher than water. For all Rayleigh numbers and solid volume fraction of nanoparticles, increasing the height of grooves leads to an increment in average Nusselt number. Moreover, when the Rayleigh number reaches to 106, the average Nusselt number increases, which the domain mechanism of heat transfer, in this case, becomes convection. As to be expected, the influence of MWCNT-Fe3O4/Water Hybrid Nanofluid on heat transfer rate is higher than the γ-Al2O3/water nanofluid.

Experimental Study of Mixed Convection from Horizontal ‎Isothermal Elliptic Cylinders at Different Aspect Ratios

ABSTRACTMixed convective heat transfer from horizontal isothermal elliptic cylinder is experimentally studied. Three elliptic test cylinders with same perimeter and length are studied giving axis ratio (AR) equal 1, 0.709 and 0.496. The corresponding Gr for the three test sections is 9.8 × 105, 1.5 × 106, and 2.1 × 106, and Re varies from 111 to 1306. Consequently, the corresponding Ri is in range of 1 to 102. The angle of attack (ψ) is changed from 0° (assisting flow) to 180° (opposing flow) with an interval of 30°. New empirical correlation is obtained for the Nuavg as a function of the AR, Ri, and ψ. The experimental results are presented in the form of the average Nusselt number around the elliptic cylinder. It has been concluded that as the axis ratio decreases the averageNusselt number increases, in addition, increasing the angle of attack from 0° to 180° decreases the averageNusselt number considerably. New empirical correlations are obtained for the average Nusselt number as a function of the Richardson number and the attack angle.

Natural convection in inclined channel for air cooling of photovoltaic panels

Reducing the operating temperature of a photovoltaic (PV) module is an effective way to improve efficiency and prevent damage from overheating. The present paper focuses on the study of the effect of inclination on air natural convection in an asymmetrically heated channel (at uniform heat flux), in laminar regime. This configureuration models the passive and natural air-cooling of PV panels by inclined chimneys. The computational procedure solves the unsteady two-dimensional Navier-Stokes and energy equations by a finite volume approach while the projection method decouples the pressure from the velocity. The heat transfer and fluid flow are analyzed for a wide range of modified Rayleigh numbers varying from 102 to 105 and for inclination angles between 15o and 90o with respect to horizontal position. The results show that the mass flow rate and the average Nusselt number increase with the angle of inclination as well as with the modified Rayleigh number. However, a significant reduction in heat transfer rate and induced flow rate is observed for the low angles of inclination. To enhance the cooling of the PV panel, extensions are added at the inlet and outlet of the channel. The simulations show that only the downstream extensions of the channel are effective in improving the induced mass flow rate and therefore the convective heat transfer.

Hydrodynamic and thermal study of a trapezoidal cylinder placed in shear-thinning and shear-thickening non-Newtonian liquid flows

Hydrodynamic and thermal features of two-dimensional, incompressible shear-thinning and shear-thickening non-Newtonian power-law liquids under forced convection across a trapezoidal cylinder are the focus of this study, carried out at Reynolds number Re = 1–40, power-law index n = 0.4–1.8 and Prandtl number Pr = 50 using ANSYS Fluent. In this steady system, the drag coefficient is found to decrease, whereas the wake size and the average Nusselt number increase with the increase in Re. However, with the increase in n value, the average Nusselt number decreases. Similar to a square cylinder, compared to Newtonian liquids, shear-thinning liquids increases heat transfer and shear-thickening reduces it. The maximum increase in heat transfer for shear-thinning liquids compared to Newtonian liquids for a trapezoidal cylinder is approximately 25%, while the average heat transfer is somewhat lower for shear-thickening liquids than for Newtonian. The average Nusselt number is smaller for the trapezoidal cylinder than for a square cylinder in the parameter domain investigated; the largest difference is around 25%. A simple relationship for the average Nusselt number as a function of Re and n has been determined.

Effect of non-uniform asymmetric heating on the thermal and entropy generation characteristics for flow of Al2O3-water nanofluid in a micro-channel

Purpose The purpose of this study is to numerically analyze the thermal and entropy generation characteristics on two-dimensional, incompressible, laminar single-phase flow of Al2O3-water nanofluid in a micro-channel subjected to asymmetric sinusoidal wall heating with varying amplitude, length of fluctuation period and phase difference of applied heat flux for Reynolds number in the range of 25-1000. Design/methodology/approach The numerical computation is based on the Finite Element Method and the Lagrange finite element technique is used for approximating the flow variables within the computational domain. Findings The average Nusselt number increases with increasing Reynolds number (Re) for all the volume fractions of nanofluid. However, the total entropy generation decreases up to a critical value of Re and increases thereafter. Increase in volume fraction shifts the critical Re towards the lower Re regime. The average Nusselt number and total entropy generation increase with amplitude and length of fluctuation period of heat flux. The optimal choice of volume fraction for lesser entropy generation and higher heat transfer is found to be 3 per cent independent of the value of amplitude, length of fluctuation period and phase difference of the heat flux. Originality/value To the best of authors’ knowledge, the interplay of various parameters concerning non-uniform heating in achieving the maximum heat transfer with minimum irreversibility has not been investigated. Focusing on this agenda, the results of this study would benefit the industrial sector in achieving the maximum heat transfer at the cost of minimum irreversibilities with an optimal choice of inlet Reynolds number, volume fraction of nanofluid, amplitude, length of the period of fluctuation of heat flux and phase difference of applied heat flux.

Rayleigh-Bénard convection of non-Newtonian nanofluids considering Brownian motion and thermophoresis

In this study, the flow and heat transfer of non-Newtonian Al2O3-water nanofluid in Rayleigh-Bénard cavity are numerically studied. Power-law model is used to describe the non-Newtonian effect of nanofluids in which the flow behavior index depends on the nanoparticle volume fraction. We present a systematic study of the Al2O3-water nanofluid for Rayleigh number of 1000 ≤ Ra ≤ 500000, cavity aspect ratio of 0.25 ≤ AR ≤ 4.0 and nanoparticle volume fraction of 0% ≤ϕ≤ 3%. The flow pattern inside the cavity changes complicatedly as Ra increases and the heat transfer performance of the system is strongly affected by the flow pattern. The convectional structure is influenced by both of the shear-thinning effect and the variation of effective viscosity. The average Nusselt number changes non-monotonically because of the flow pattern change under the variation of Ra and n. The consideration of Brownian motion and thermophoresis could bring about 3% increase of average Nusselt number, and the value varies slightly with different AR and ϕ. The comparison between non-Newtonian and Newtonian models shows that noticeable underestimation of heat transfer performance could be resulted by Newtonian model. The average Nusselt number of non-Newtonian model is approximately 1.5 times to that of Newtonian model, and the deviation is more pronounced with the increase of particle volume fraction.

Interaction effects of an inclined magnetic field and nanofluid on forced convection heat transfer and flow irreversibility in a duct with an abrupt contraction

Abstract This research examines the interaction effects of an inclined magnetic field and nanofluid on forced convection heat transfer and flow irreversibility in a duct with an abrupt contraction. This contraction is created by an inclined forward facing step and is simulated using the Blocked region method. To analysis the flow irreversibility, the entropy generation number is calculated based on the second law of thermodynamics. The effects of Hartmann number H a = 0 - 100 , magnetic field inclination angle β = 0 Â ° - 90 Â ° and nanoparticles volume fraction ϕ = 0 - 0.06 on the values of friction coefficient, Nusselt number and entropy generation number are shown graphically. The results present that the Hartmann number and inclination angle of the magnetic field have the reverse effects on the heat transfer rate, whiles their influences on the flow irreversibility is significantly increasing. Also, total entropy generation number and average Nusselt number increase as the volume fraction of nanoparticles enhances from 0 to 0.06.

Numerical investigation of laminar compressible natural convection flow in asymmetrically and isothermally heated open-ended inclined channel

This study presents a numerical investigation of laminar compressible natural convection flow induced under high-temperature difference in an open-ended inclined parallel-walled channel heated isothermally and asymmetrically at the top wall (hot plate facing downwards) while the lower plate is considered adiabatic. The investigation is carried out with air (Pr=0.72) as the working fluid for a temperature difference of 110 °C from the ambient, over of range of modified Rayleigh number 6.2×101 to 1.6×104 and inclination of channel varying between 10 and 90° with respect to horizontal position. For the simulation, compressible governing equations without Boussinesq approximation is solved by employing new modified all-speed Roe scheme with matching preconditioning method, dual time-stepping technique and utilizing the modified Local One-dimensional Inviscid (LODI) relations suitable for compressible natural convection flow as non-reflecting boundary conditions at the inlet and outlet of the channel. Average Nusselt number based on inter-plate spacing increases with the increase of modified Rayleigh number and also with the increase of inclination from the horizontal position. This increase in average Nusselt number can be described by the variation of the level of thermal saturation inside the channel. Significant reduction in average Nusselt number is observed in the combined zone of lower angle of inclination and lower modified Rayleigh number. Visualization of fluid flow (temperature contour and velocity magnitude contour) indicates the existence of both fully developed flow and developing flow regime within the span of investigated modified Rayleigh number. Based on the investigation, a composite correlation between average Nusselt number and product of modified Rayleigh number and the sine of the angle of inclination considering as a single parameter is presented which can be conveniently used for designing of engineering application dealing with natural convection heat transfer under high-temperature difference.

Numerical Study of the Heat Transfer Rate of Nano-fluid Flow in a Channel with a Triangular Cross-section in the Presence of a Magnetic Field

In this study, nanofluid flow inside a triangular channel in the presence of a magnetic field with two phase model was analyzed. For this purpose, the external surface of the channel is heated or cooled with a specific heat transfer coefficient, and the effect of the changes in the Nusselt number as well as the pressure drop for variable parameters such as nanoparticle concentrations, magnetic field strength, and channel shape (relative to the circular mode) was investigated. In order to the Grid Independent Study, the Nusselt number for different sizes of grid has been evaluated. As a result, comparing the Nusselt number changes, the average is seen by changing the size of the networking. A grading with the number of 350,000 elements is suitable for simulating the corresponding problem. In order to confirm the validity of the results, simulation of Nusselt number changes during the channel was evaluated and compared with the results presented by Saeed et al. [23]. The error rate between the data presented in reference [23] and the simulation claims is not tangible, and therefore the model used for simulation has been approved. In this study, a triangular channel with a flow inside it is a nanofluid, is analyzed in the presence of a magnetic field in a smooth, completely two phases. For this purpose, the external surface of the channel with a certain heat transfer coefficient and the effect of changes in the thermal and fluid parameters with the change in the parameters involved in the problem have been observed. The results show that with increasing magnetic field, friction coefficient, in-channel velocity, heat transfer rate, average Nusselt number increase. In this study, for simulation we used computational fluid dynamics and limited volume method, and specifically using the Ansys-Fluent version 17 software. The problem is investigated in the form of a three dimensional, stable, single-phase and two-phase flow.

Analysis of fully developed flow and heat transfer through n-sided polygonal ducts with round corners using the Galerkin weighted residual method

The Galerkin weighted residuals method is extended solve the laminar, fully developed flow and heat transfer of Al2O3-water nanofluid inside polygonal ducts with round corners for the constant heat flux and uniform wall temperature boundary conditions. Using the method, semi-analytical, closed-form solutions are obtained for the friction coefficient and the Nusselt number in terms of the radius of the round corners for the triangular, rectangular, hexagonal, and octagonal ducts. The effects of varying the radius of the round corners and the volume fraction of the nanoparticles on the friction coefficient and the Nusselt number are analyzed. The results show that the friction factor and the average Nusselt number increase with increasing the radius of the round corners. The study indicates that the Galerkin weighted residuals method is an accurate and efficient technique to obtain closed-form solutions for the flow and temperature fields in ducts with complex cross sectional shapes.

Numerical investigation of mixed convection heat transfer of a nanofluid in a circular enclosure with a rotating inner cylinder

In the present paper, mixed convection heat transfer of water–Al2O3 nanofluid in the space between two cylinders is investigated numerically. The inner and outer cylinders are at Tc and Th temperatures, respectively. The forced and free convective heat transfers are due to the internal cylinder rotation and temperature difference between the two cylindrical surfaces, respectively. The effect of dimensionless parameters such as Rayleigh and Richardson numbers, the volume fraction of nanofluid, the eccentricity ratio and its angle on heat transfer ratio is analyzed. The governing equations are solved using a finite-difference method and SIMPLE algorithm. The results show that at eccentricity of e = 0.0 and 0.5 and within the entire range of Rayleigh number 103 ≤ Ra ≤ 105 and Richardson number 0.1 ≤ Ri ≤ 100, an increase in Rayleigh and Richardson numbers leads to an increase in average Nusselt number on the inner cylinder wall. But at eccentricity of e = 0.9, the average Nusselt number on the inner cylinder wall decreases with these dimensionless parameters. It is found that an increase in the volume fraction of the nanofluid results in an increase in average Nusselt number on the inner cylinder wall.