Unsteady Convective Heat Transfer Research Articles

Numerical investigation of fluid flow and heat transfer of a nanofluid around two circular cylinders arranged in tandem in horizontal duct

In this study, two-dimensional unsteady laminar convective heat transfer from two isothermal cylinders arranged in tandem in horizontal duct has been numerically investigated. The numerical study is performed by solving the governing equations (continuity and momentum) and energy equations using a finite volume-based code ANSYS Fluent. The present study was conducted using nano sized copper particles suspended in water. The Prandtl number Pr of the resulting suspension being equal to 7. The range of solid volume fractions studied is and the flow Reynolds number is equal to 100.The spacing ratio of the two cylinders was varied from 2 to 5 while the blockage ratio was kept constant β=0.25. A detailed discussion has been provided regarding the variation of flow structure and the characteristics of flow, including total drag and lift coefficients, as a function of the dimensionless parametersIn order to visualize the flow and heat transport, streamlines, vorticity contours and isotherms were generated. In addition, the local and average Nusselt numbers for the upstream and downstream cylinders are calculated. The results demonstrate that the force coefficients, the wake structure behind the cylinders, and the Nusselt number are significantly influenced by the value of the spacing ratio and the volume fractions of nanoparticles. The results demonstrate a significant enhancement in heat transfer efficiency relative to the base fluid. The aforementioned improvement is more pronounced in the case of higher gap ratios and higher nanoparticle volume fractions.

Natural convective heat transfer of Al2O3-Cu/water hybrid nanofluid in a rectotrapezoidal enclosure under the influence of periodic magnetic field

This study seeks to examine quantitatively the two-dimensional unsteady free convective heat transfer enhancement of Al2O3-Cu/H2O hybrid nanofluid within a rectotrapezoidal-shaped structure. The lower wall is both uniformly and non-uniformly heated with different thermal boundary conditions, the top horizontal wall is adiabatic, and the vertical and inclined walls are assumed chilly. The enclosure is penetrated by a periodic magnetic field. A set of variable transformations is used to render the governing equations dimensionless, which makes it possible to identify the model parameters that control it. The finite element method is then used to numerically solve these equations. To verify the code's numerical precision, comparisons with formerly available works are performed and, a high level of agreement is obtained among the results. The generated outcomes are shown for the principal parameters that control the flow in the context of streamlines, heat lines, isotherms, average Nusselt number, friction factor, and thermal efficiency index. The findings demonstrated that the efficiency index, friction factor, and heat transmission are all considerably influenced by the Rayleigh number (Ra). The lowest Ra yields the maximum thermal efficiency. The results also revealed that the hybrid nanoparticle volume fraction and period of the magnetic field amplified the heat transfer rate and efficiency index due to the increased pumping force. It is observed that when the percentage of the mixing ratios for Cu nanoparticles rises from 0% to 100%, the heat transport in Al2O3-Cu/H2O hybrid nanofluid increases for a fixed volume fraction of hybrid nanoparticles. As compared to parabolic and sinusoidal boundary conditions, the uniform temperature boundary setting for Ra = 105 exhibits the highest average rate of heat transport of 10.04. The average Nusselt number increases by 25.12% at Ra = 103 with a 3% volume fraction of hybrid nanofluid as compared to pure water and the mixing ratios of Al2O3:Cu = 0:300 gives the highest increased rate of 16.30%. The results further illustrate that Al2O3-Cu/H2O hybrid nanofluid has the highest heat transfer rate compared to the pure water and Al2O3– H2O nanofluid.

Optimization and sensitivity analysis of hydromagnetic convective heat transfer in a square cavity filled with a porous medium saturated by Ag-MgO/water hybrid nanofluid using response surface methodology

A computational exploration is presented to emphasis on optimization and sensitivity inspection of the unsteady free convective heat transfer flow of hybrid nanofluid in a square cavity under multidirectional periodic magnetic field with aluminum foam porous medium. The vertical walls of the cavity are maintained at constant low temperature whereas the constant as well as the variable thermal conditions on the bottom wall is considered and the top wall is considered as adiabatic. The finite element technique has been applied to solve the governing equations after changing them into dimensionless form. In the numerical simulations, the Ag-MgO/water hybrid nanofluid has been engaged to gain insight into the flow and thermal arenas. The obtained outcomes are exhibited in terms of streamlines, isotherms and average Nusselt number as well as plotting the response surface and contours. The average Nu is displayed in line graphs, response surface and contours graphs as well as tabular form for different flow parameters. Response surface methodology is applied to achieve an optimization procedure to search for the best situation in which the highest heat transfer rate occurs when the distinct model parameters namely Darcy number, porosity and hybrid nanoparticles volume fraction are treated as the sensitivity of the parameters. A correlation equation between the output response and input variables has been derived with the help of response surface methodology. The magnetic field and its path regime the flow decoration of hybrid nanofluid suggestively. The Ag and Ag-MgO nanomaterial with water flow arenas are compared. While considering Ag-H2O nanofluid, the average Nu increases 23.97 % at Ra = 107 with ϕhnp = 0.02 whereas the corresponding increases is 25.68 % if the Ag-MgO/H2O hybrid nanofluid is taken into account. The average Nu upsurges suggestively, as hybrid nanoparticles volume fraction, period number, magnetic field leaning angle, Darcy number and Rayleigh number rises but porosity and Hartmann number have inverse effect. The outcomes showed that the average Nu is more sensitive to the hybrid nanoparticles volume fraction rather than the porosity or Darcy number. Also, it is discovered that the optimum condition arises at Da = 0.001, ε = 0.2 and ϕhnp = 0.02 with the final Nuave = 2358.21.

Approximate Analytical Solution of the Influences of Magnetic Field and Chemical Reaction on Unsteady Convective Heat and Mass Transfer of Air, Water, and Electrolyte Fluids Subject to Newtonian Heating in a Porous Medium

This paper addresses the unsteady hydrodynamic convective heat and mass transfer of three fluids namely air, water, and electrolyte solution past an impulsively started vertical surface with Newtonian heating in a porous medium under the influences of magnetic field and chemical reaction. Suitable dimensionless parameters are used to transform the flow equations and the approximate analytic method employed to solve the flow problem. The results are illustrated graphically for the velocity, temperature, and concentration profiles. Though, low Prandtl numbers produce high-thermal boundary layer thickness, however, as a novelty, the presence of the magnetic field delayed the convection motion hence, the thermal boundary layer thickness is greater for water with high Pr = 7.0 as compared to air with low Pr = 0.71 and electrolyte solution with low Pr = 1.0. Practically, water with a high-Prandtl number can effectively absorb and release heat. This makes water useful in applications such as geothermal heat pumps and solar thermal collectors, industrial processes such as chemical reactions, distillation, and drying, and in oceanography in predicting the movement and behavior of ocean currents, which in turn can impact weather patterns and climate. Another major observation from the study is that the rate of cooling associated with air, water, or electrolyte impacts differently on the product being cooled.

Heat and Mass Transfer on Unsteady MHD Chemically Reacting Rotating Flow of Jeffrey Fluid Past an Inclined Plates under the Impact of Hall Current, Diffusion Thermo and Radiation Absorption

An analytical solution for two dimensional unsteady MHD free convective double diffusive heat and mass transfer rotating flow of viscous incompressible optically thin non Newtonian fluid (jeffrey fluid) past a semi-infinite porous plate in the presence of generation of heat absorption with hall current, radiation absorption and Dufour effects are presented in this paper. Lorentz force is applied perpendicular direction to the plate with a first-order chemical reaction. The governing PDEs transformed into ODEs by applying the perturbation technique. The effects of various physical parameters like Hall current, heat absorption, radiation parameter, radiation absorption parameter, Grashof number, modified Grashof number, Dufour effect, magnetic field parameter, inclined angle, chemical reaction parameter, etc., are studied with graphically. Temperature profile is increased by raising the values of Dufour effect, radiation absorption parameter and reverse effect of the heat absorption parameter and also concentration profile distribution is downfall by raising the values of Schmited number, chemical reaction parameter. Another significant finding of the current study is that the numerical data are displayed with the use of Matlab programmes to investigate the outcomes of the skin friction solution. the rate of heat and mass transfer at the wall rising under the influence of Dufour effect. Tables, graphs, and reports can be used to display the impact of all pertinent parameters.

Effects of Suction and Lorentz Force on an Unsteady MHD Convective Mass and Heat Transfer Nanofluid Flow upon a Vertical Permeable Sheet

Effects of Suction and Lorentz Force on an Unsteady MHD Convective Mass and Heat Transfer Nanofluid Flow upon a Vertical Permeable Sheet

Correction: Alshehri et al. Unsteady Convective MHD Flow and Heat Transfer of a Viscous Nanofluid across a Porous Stretching/Shrinking Surface: Existence of Multiple Solutions. Crystals 2021, 11, 1359

We ought to correct the misprinted terms in our published paper ‘Unsteady Convective MHD Flow and Heat Transfer of a Viscous Nanofluid across a Porous Stretching/Shrinking Surface: Existence of Multiple Solutions’ [Crystals2021, 11, 1359] [...]

Stability Analysis of Unsteady Mixed Convection Flow Near the Stagnation Point with Buoyancy Effect

This numerical study investigates the behaviour of the unsteady mixed convection flow and heat transfer near the stagnation region past a vertical plate in Al2O3-Cu/H2O hybrid nanofluids. By choosing an appropriate similarity transformation, an ordinary differential equations system is obtained, hence scrutinized via the bvp4c package embedded in the MATLAB program. The results show that as the nanoparticle volume fraction and unsteadiness parameter increase, the fluid's skin friction coefficient also increases. Also, an increase in the nanoparticle volume fraction and unsteadiness parameter has broadened the range of solutions hence, delayed the progress of boundary layer separation. In addition, the presence of the unsteadiness parameter provides a significant result in the thermal system. The first solution is declared stable by the stability analysis.

N-Side Cell-Based Smoothed Finite Element Method for Incompressible Flow with Heat Transfer Problems

In this paper, an n-sided cell-based smoothed finite element method (nCS-FEM) is presented to solve the convective heat transfer in incompressible viscous fluid flow. The Characteristic-based-split (CBS) scheme is used to stabilize the spatial and pressure oscillations of CS-FEM that occur during the process of solving incompressible Navier-Stokes equations. Meanwhile, the characteristic-Galerkin method in CBS is adopted as the convective stabilization in the heat transfer equation. In order to verify the proposed method, typical numerical examples are used to test the computational accuracy of CS-FEM. Meanwhile, the investigation of the robustness of highly distorted element is conducted in a natural convection test case. The calculation results of numerical examples show that CS-FEM is qualified to solve the steady and unsteady convective heat transfer problems for incompressible laminar flow with high reliability. The flow across tube banks with complex geometry in the heat sinks is also simulated to prove the capabilities of the proposed method. Well agreed comparison with the results of CFD software, Fluent, proves that the present method can effectively simulate realistic and complex flow with heat transfer problems.

Numerical analysis of unsteady mixed convection heat transfer characteristics of nanofluids confined within a porous lid-driven L-shaped cavity

The numerical analysis of unsteady combined convective heat transfer properties of kerosene-alumina nanofluid restricted in an L-shaped porous lid-driven cavity has been examined in this research. Fluid flow governing equations and conjugate heat transport along with the corresponding boundary conditions are used to express the physical problem mathematically. Firstly, boundary conditions and governing equations are transformed into non-dimensional forms using a series of appropriate transformations. Later on, the transformed non-dimensional equations are solved by utilizing the Finite Element Method based on the Galerkin residuals. A comparison with earlier available work is shown and turned out to be a great agreement. Numerical solutions are acquired in a wide variety of governing variables. The governing variables in this study are the Darcy number ( Da ), Reynolds number ( Re ), Richardson number ( Ri ), solid volume fraction (δ), time parameter (τ), etc. The numerical findings of flow and thermal fields are represented graphically in respect of isotherms, streamlines, and average Nusselt number at the hot surface of the cavity. The Re and Da are found to have an extensive impact on the isotherms and streamlines in the cavity, and also in determining the average heat transmission rate.

Hall Current and Soret Effects on Unsteady MHD Rotating Flow of Second-Grade Fluid through Porous Media under the Influences of Thermal Radiation and Chemical Reactions

The unsteady MHD free convection heat and mass transfer flow of a viscous, incompressible, and electrically conducting fluid passing through a vertical plate embedded in a porous medium in the presence of chemical reactions and thermal radiation is investigated. The effects of the Hall current, rotation and Soret are studied. Using the perturbation approach, one can obtain an accurate analytical solution to the governing equations for the fluid velocity, fluid temperature, and species concentration, provided that the initial and boundary conditions are acceptable. It is possible to obtain expressions for the shear stress, rate of heat transfer, and rate of mass transfer for both plates with the ramping temperature and isothermal conditions. On the one hand, the numerical values of the primary and secondary fluid velocities, fluid temperature, and species concentration are presented graphically. On the other hand, the numerical values of the shear stress and rate of mass transfer for the plate are presented in tabular form for various values of the relevant flow parameters. These values are given for a range of pertinent flow parameters. It was determined that an increase in the Hall and Soret parameters over the whole fluid area leads to a corresponding increase in the resulting velocity. The resultant velocity continually climbs to a high level due to the contributions of the thermal and solute buoyancy forces. Lowering the heat source parameter reduces the temperature distribution, resulting in a lower overall temperature. When there is a rise in the chemical reaction parameter over the whole fluid area, there is a corresponding decrease in the concentration. The concentration buoyancy force, Hall current, and Prandtl number reduce the skin friction. On the other hand, the permeability of the porous medium, rotation, chemical reaction, the Soret number, thermal buoyancy force, and mass diffusion all have the opposite effects on the skin friction.

Numerical and asymptotic solutions of the unsteady mixed convection boundary-layer flow over an expanding/contracting vertical cylinder

PurposeThe purpose of this study is to obtain both the numerical and asymptotic solutions of the unsteady mixed convection flow and heat transfer over an expanding or contracting cylinder which is placed vertically.Design/methodology/approachSolutions of the governing ordinary (similarity) differential equations for the fluid flow and temperature field are obtained using the function bvp4c from MATLAB. The problem involves the Prandtl number σ, the mixed convection parameter λ and unsteadiness parameter S that characterize an expanding or contracting cylinder. This solution approach is capable of producing multiple solutions once the necessary assumptions are provided.FindingsIt is found that solutions exist for all negative values of S, expanding cylinder, and only for small positive values of S, contracting cylinder. Further, more than one solution is observed; numerical computation shows that the critical point of S becomes singular as λ approaching zero. For the case of expanding cylinder, the mixed convection parameter has a significant effect on both the flow and heat transfer characteristics. Asymptotic analysis also shows that when σ is large, dual solutions exist for some values of S and λ.Originality/valueThe present results are new and original for the study of the unsteady mixed convection flow and heat transfer over an expanding/contracting cylinder where numerical solutions are obtained for representative values of the involved parameters. Asymptotic solutions for large λ and large σ are derived.

Hall, Soret, and rotational effects on unsteady MHD rotating flow of a second-grade fluid through a porous medium in the presence of chemical reaction and aligned magnetic field

The effects of Hall current, rotation and Soret effects on an unsteady MHD free convection heat and mass transfer flow of a viscous, incompressible and electrically conducting fluid past an inclined plate embedded in a porous medium in the presence of chemical reaction and an aligned magnetic field are investigated. An exact analytical solution of the governing equations for fluid velocity, fluid temperature, and species concentration subject to appropriate initial and boundary conditions is obtained using the perturbation technique. Expressions for shear stress, rate of heat transfer and rate of mass transfer at the plate are derived for both ramped temperature and isothermal plates. The numerical values of primary and secondary fluid velocities, fluid temperature and species concentration are displayed graphically, whereas those of shear stress and rate of mass transfer at the plate are presented in tabular form for various values of pertinent flow parameters. It is concluded that, the resultant velocity is increased with an increasing in Hall and Soret parameters throughout fluid region. The thermal and solutal buoyancy forces contribute to the resultant velocity ever-increasing to high. The temperature distribution is trim downs through an increasing in heat source parameter. The concentration is reduced with an increase in the chemical reaction parameter in the entire fluid region. Concentration buoyancy force, hall current, and Prandtl number have tendency to reduce in skin friction whereas permeability of the porous medium, Rotation, chemical reaction, Soret number, thermal buoyancy force and mass diffusion have reverse effect on it.

A global thermal node model for no-load indirect solar dryer

ABSTRACT This paper presents a global thermal node model for predicting heat transfer in no-load indirect solar dryer. The overall influence of the interaction of the indirect solar dryer with the uppermost soil layer is considered. The mathematical model describing the solar drying system with a double-pass air-circulation collector is derived. Based on the Moore and Fisch approach, the soil surface fluxes are calculated using the surface layer similarity theory. The drying air temperature distribution based on dynamic descriptions of the drying environment is studied. The unsteady convection heat transfer results low thermal efficiency of the dryer at low air speed. The linear increase of the inner air speed from to makes the maximum efficiency increases of . When the offsetting air speed undergoes a convex quadratic increase, the average value of the energy efficiency in this case increases of compared to the scenario of constant air speed.

Unsteady natural convection of nano-encapsulated phase change materials (NEPCMs) inside a random porous medium considering local thermal non-equilibrium condition

A local thermal non-equilibrium analysis is undertaken to simulate unsteady free convective heat transfer inside a square enclosure filled with a random porous medium. There is a time-varying temperature profile at the hot wall. The fluid is a suspension comprising nano-encapsulated phase change materials (NEPCM) particles dispersed in water while aluminum foam, copper foam, or glass balls are considered for the solid matrix. It is found that when the hot wall temperature reaches its minimum value during one oscillation period, most of the cavity is occupied by the NEPCM particles in the solidious phase. For the metallic foams, the heat transfer of the medium is substantially higher than the NEPCM particles. Meanwhile, the Nusselt number of the glass balls is much lower than the suspension. A rise in the dimensionless frequency or amplitude of the temperature profile as well as approaching the dimensionless fusion temperature to 0.5 elevates Nusselt number of the suspension. However, no general conclusion can be drawn for their consequences on the heat transfer in the solid matrix. Among the current alterable parameters, only the contributions of the non-dimensional interface heat transfer coefficient and the non-dimensional amplitude of the temperature profile are found significant.

Development of turbulent heat flux model for unsteady forced convective heat transfer of small-to-medium Prandtl-number fluids based on deep learning

Turbulent heat flux (THF) models are used for the closure of the THF term when solving the steady/unsteady Reynolds-averaged scalar transport equation to simulate the turbulent heat transfer in industry. It is known that the simple gradient diffusion hypothesis (SGDH) has deficiencies under complex conditions with flow separations. To develop a more general THF model, this paper firstly establishes a high-fidelity database of forced convective heat transfer passing a circular cylinder under different Prandtl number (Pr) conditions at the Reynolds number (Re) of 500 via the direct numerical simulations. Proper orthogonal decomposition method is then employed for the triple decomposition on unsteady turbulent flow with the first two orders of eigenmodes reconstructing the large-scale field and the remaining reconstructing the turbulent field. Then, architectures using different neural network structures based on tensor basis neural network (TBNN), including MLP-TBNN-THF which adopts the multilayer perceptron (MLP) and ResNet-TBNN-THF that uses the residual network (ResNet), are constructed to predict normalized THF from large-scale flow features and Pr. Posterior tests are carried out on different Prs and three different Res: 500, 5000 and 16900 using the well-trained TBNN-THF models to evaluate their performance and generalization capability. Most models proposed in this paper predict the heat transfer more accurately than the SGDH model, even when extended to conditions out of the range trained. The failure of the isotropic assumption of SGDH model is observed in most regions. In that case, it is vitally necessary to use a model like proposed in this paper to simulate the THF term more accurately. In conclusion, the THF model, which is appropriate for complex working conditions and fluids with small-to-medium range of Pr, obtained by deep learning method in this paper, is helpful to improve the prediction accuracy of temperature field or concentration field by steady or unsteady Reynolds-averaged Naiver-Stokes approach in engineering. Besides, the current framework can be generalized to scalar-flux modeling under other conditions with the supplement of more databases in the future.

Unsteady forced convection flow and heat transfer past a blunt headed semicircular cylinder at low Reynolds numbers

A numerical analysis is performed to study the forced convective fluid flow and heat transfer past an unconfined semicircular cylinder. The two-dimensional simulations are carried out for Reynolds number ranging 10 ≤ Re ≤ 200 and operating fluid is air (Pr = 0.71) for Newtonian constant property fluid. Continuity, Navier-Stokes, and energy equations with appropriate boundary conditions are solved using computational fluid dynamics solver Ansys Fluent. Various parameters flow such as lift, drag, skin friction coefficient, pressure coefficient, Nusselt (Nu) number, Strouhal (St) number, and vortex strength are calculated. The transition from steady to time periodic flow occurs between Re = 60 and 80. The effect of Reynolds number on heat transfer is discussed. Finally, a developed correlation of Nu and St numbers are presented.

Effect of Heat Generation/Absorption and Radiation on Two-Dimensional Unsteady Magnetohydrodynamic Heat and Mass Transfer Nanofluid Boundary Layer Flow Over a Permeable Shrinking Sheet

The objective of this research is to look into the impact of heat generation/absorption and radiation on twodimensional unsteady magnetohydrodynamic forced convection heat and mass transfer nanofluid boundary layer flow over a permeable shrinking sheet. Similarity transformations are used to translate the governing time-dependent nonlinear PDEs into nonlinear ODEs. The Keller box method is used to solve the transformed equations. The effect of non-dimensional variables is studied and graphically illustrated on velocity, temperature, and concentration. The friction factor, Nusselt, and Sherwood numbers are also investigated and arranged for different values of Unsteadiness parameter, effective Prandtl number, thermophoresis parameter, magnetic field, Grashof number, Modified Grashof number, Lewis number, Heat Source/Sink, and Suction in tabular form. The velocity profiles for dual solutions have improved as the Thermal Grashof number and Modified Grashof number have amplified, but the temperature and concentration profiles have shown the reverse pattern. Temperature profiles are increasing as improving the Heat source/sink. The temperature and concentration profiles decline in the first and opposite pattern in the second solution with improved values of the magnetic field. The findings of this study are very well-acknowledged with current research.

Thermal analysis of simultaneously evolving laminar pulsatile flow through two large parallel plates with time-dependent heat flux boundary conditions

Understanding unsteady convective heat transfer (UCHT) behavior of simultaneously evolving laminar pulsatile flow between two large parallel plates exposed to time-dependent thermal boundary conditions is the goal of this research. The time-dependent sinusoidal velocity waveform is applied at the entrance of the two large parallel plates exposed to time-dependent sinusoidal heat flux boundary conditions (TSHBC). The finite volume technique is used in the present work. The current results have been compared to the literature data under the same operating conditions and found to be in good concurrence. The influence of various parameters, for instance, amplitude, frequency, phase difference, heat flux ratio, and the Reynolds number on the transient Nusselt number, is investigated. An improvement in energy transfer is observed when the pulsatile flow between two large parallel plates with TSHBC than the non-pulsatile flow with a fixed heat flux type boundary. Furthermore, the energy transfer improvement is observed once the velocity and heat flux waveform frequency is equal. The applicability of the present work could be found in the solar heating system, cooling of electronic devices, etc., whereby a more robust understanding of the unsteady convective heat transfer could facilitate the design and develop an efficient system.

Effectiveness of magnetize flow on nanofluid via unsteady natural convection inside an inclined U-shaped cavity with discrete heating

This analysis investigates the unsteady MHD free convection flow and heat transfer in an inclined U-shaped cavity filled with Cu-water nanofluid. Both the right and left walls of the cavity and the internal walls of the cavity are supposed to be adiabatic, while the upper horizontal walls of the cavity are kept at a lower temperature. A heat source is located on the bottom horizontal wall with its position that alters the left vertical wall. The dimensionless governing equations are solved using the Successive Under-Relaxation technique. The investigation is achieved by controlling the impact of a set of pertinent parameters, namely; the size and position of the heat element (B = 0.3–0.6, D = 0.3–0.6), Hartman number (Ha = 0–50), nanoparticle volume fraction (ϕ = 0.0–0.07), heat source generation/ absorption (Q = −4.0–2.0), and aspect ratio (AR = 0.2–0.6). The results show that the mean Nu declines with heat source length and Hartmann number, whereas it augments with dimensionless heat source location. Furthermore, the mean Nu rises in parallel with the solid volume fraction of nanoparticles and decreases with aspect ratio. In addition, the increasing of aspect ratio reduces the convection mode compared to conduction heat transfer.