Thermal conductivity variation on natural convection flow of water–alumina nanofluid in an annulus

This work is focused on the numerical modeling of steady laminar natural convection flow in an annulus filled with water–alumina nanofluid. The inner surface of the annulus is heated uniformly by a uniform heat flux q and the outer boundary is kept at a constant temperature T_c. Two thermal conductivity models namely, the Chon et al. model and the Maxwell Garnett model, are used to evaluate the heat transfer enhancement in the annulus. The governing equations are solved numerically subject to appropriate boundary conditions by a penalty finite-element method. A parametric study is conducted and a selective set of graphical results is presented and discussed to illustrate the effects of the presence of nanoparticles, the Prandtl number and the Grashof number on the flow and heat transfer characteristics for both nanofluid models. It is found that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by increasing the nanoparticles volume fraction and Prandtl number at moderate and large Grashof number using both models. However, for the Chon et al. model the greatest heat transfer rate is obtained.     

Analysis of Bejan’s heatlines on visualization of heat flow and thermal mixing in tilted square cavities

This article analyzes the detailed heat transfer phenomena during natural convection within tilted square cavities with isothermally cooled walls (BC and DA) and hot wall AB is parallel to the insulated wall CD. A penalty finite element analysis with bi-quadratic elements has been used to investigate the results in terms of streamlines, isotherms and heatlines. The present numerical procedure is performed over a wide range of parameters (10³ ? Ra ? 10⁵,0.015 ? Pr ? 1000,0° ? φ ? 90°). Secondary circulations cells are observed near corner regions of cavity for all φ’s at Pr = 0.015 with Ra = 10⁵. Two asymmetric flow circulation cells are found to occupy the entire cavity for φ = 15° at Pr = 0.7 and Pr = 1000 with Ra = 10⁵. Heatlines indicate that the cavity with inclination angle φ = 15° corresponds to large convective heat transfer from the wall AB to wall DA whereas the heat transfer to wall BC is maximum for φ = 75°. Heat transfer rates along the walls are obtained in terms of local and average Nusselt numbers and they are explained based on gradients of heatfunctions. Average Nusselt number distributions show that heat transfer rate along wall DA is larger for lower inclination angle (φ = 15°) whereas maximum heat transfer rate along wall BC occur for higher inclination angle (φ = 75°).     

Experimental determination of thermal conductivity and dynamic viscosity of Ag–MgO/water hybrid nanofluid

The main goal of this experimental work is to investigate the effect of nanoparticle volume fraction on thermal conductivity and dynamic viscosity of Ag–MgO/water hybrid nanofluid with the particle diameter of 40(MgO) and 25(Ag) nm and nanoparticle volume fraction (50% Ag and 50% MgO by volume) range between 0% and 2% and presenting new correlations. Several existing theoretical and empirical correlations for thermal conductivity (four correlations) and dynamic viscosity (five correlations) of nanofluids have been examined for their accuracy in predicting the value of thermodynamics properties by comparing the predicted values with experimental data. The examined correlations were found to present inaccuracies (under predictions) in the range of nanoparticle volume fraction under study. Predictions of the new developed correlations by comparing the predicted values with experimental data showed that the new correlations are within a very good accuracy.     

Molecular dynamics simulation of heat transfer with effects of fluid–lattice interactions

The nonequilibrium molecular dynamics simulation is employed to investigate the thermal properties of fluid confined in different FCC nanochannels. The results show that fluid in different lattice channels appears diverse wetting characteristics at low temperature. Based on wall parameters, a ratio is defined to describe the fluid–lattice interaction. Wall attraction, number of absorbed particles and thermal conductivity are increased as the increase of this ratio as well as the location of particles get closer to the wall. Thermal resistance exists along with the fluid–wall interface and loses the dominant of heat transport as the system temperature gets raised. At the same time, the thermal conductivity of nanoscale experiences unconventional increase. The fluid thermal properties are influenced both by wall–fluid interaction and temperature.     

Zinc oxide–silver/water hybrid nanofluid flow toward an off-centered rotating disk using temperature-dependent experimental-based thermal conductivity

Herein, the off-centered stagnation flow and heat transfer of zinc oxide–silver/water hybrid nanofluid over a rotating disk according to the mass-based algorithm is studied. It is assumed that the nanoparticles have a spherical shape. Also, the velocity slip between the base fluid and nanoparticles is negligible. The Prandtl number is kept constant at 6.2. In addition, it has been used an experimental relation for effective thermal conductivity which is a function of volume fraction and temperature. The governing partial differential equations are converted to dimensionless ordinary differential equation (ODE)s by the similarity transformation method. The simplified ODEs are solved numerically by the bvp4c function from MATLAB which is an efficient and reliable code according to the three-stage Lobatto IIIa formula. The influence of rotational parameters and both nanoparticles masses on the profiles and quantities of engineering interest are presented and discussed in detail. It is shown that the flow becomes complicated when there is a distance between the flow axis and the disk axis. Under determined conditions for a hybrid nanofluid with 30-g mass for both nanoparticles and 100-g mass for pure water, adding 30 g of the second nanoparticle's mass into the base fluid leads to enhance all hydrodynamic quantities of engineering interest by about 4.3%, while dispersing 30 g of the first nanoparticle's mass inside water results in decreasing the similarity temperature gradient at the surface about 3.6%. Also, when the disk rotates faster, the maximum radial velocity near the disk, s′(0) and f″(0) increases.     

Experimental investigation and development of new correlations for thermal conductivity of CuO/EG–water nanofluid

In the present study, the thermal conductivity of CuO/EG–water nanofluid in different solid concentrations and temperatures has been experimentally investigated. Using a two-step method, the nanofluid has been produced in different solid concentrations ranging from 0.1% to 2% and temperatures up to 50 °C. The thermal conductivity of the nanofluid has been experimentally measured using the KD2 Pro instrument. Based on the experimental data, new correlations for predicting the thermal conductivity of CuO/EG–water at different temperatures have been proposed. The results show that with the increase of the solid concentration, the thermal conductivity of the nanofluid increases. Furthermore, the thermal conductivity of the nanofluid increases while the temperature increases. This increase is by far more noticeable in higher solid concentrations compared with lower solid volume fraction. This means that it is the presence of nanoparticles in the base fluid that causes the increase of the effect of temperature on the thermal conductivity.     

Role of ‘Bejan’s heatlines’ in heat flow visualization and optimal thermal mixing for differentially heated square enclosures

Heat flow patterns in the presence of natural convection have been analyzed with Bejan’s heatlines concept. Momentum and energy transfer are characterized by streamfunctions and heatfunctions, respectively such that streamfunctions and heatfunctions satisfy the dimensionless forms of momentum and energy balance equations, respectively. Finite element method has been used to solve the velocity and thermal fields and the method has also been found robust to obtain the streamfunction and heatfunction accurately. The unique solution of heatfunctions for situations in differential heating is a strong function of Dirichlet boundary condition which has been obtained from average Nusselt numbers for hot or cold regimes. The physical significance of heatlines have been demonstrated for a comprehensive understanding of energy distribution and optimal thermal management via analyzing three cases. Case 1 involves the uniform and non-uniform heating of bottom wall with cooled side walls. The studies illustrate that the heat flow primarily occurs from the central regime of the bottom wall to a very small regime of the top portion of side walls. A large portion of central regime of cold side walls do not receive significant amount of heat. In order to maximize the thermal energy distribution, the distributed heating at the middle portions of the bottom and side walls have been considered in case 2 and heatlines clearly depict the distributions of heat from the hot walls to the large regimes of the cold wall. Further case 3 illustrates the enhanced heat flows in presence of heated bottom and left side walls. Heatline is found as an effective numerical tool to visualize energy distribution in order to establish a suitable heating strategy.     

A renovated Hamilton–Crosser model for the effective thermal conductivity of CNTs nanofluids

Thermal conductivity models for nanotube based nanofluids are less common than those for nanofluids containing spherical nanoparticles In this paper, a renovated Hamilton–Crosser model for the effective thermal conductivity of carbon nanotubes (CNTs) based nanofluids is proposed by simulating an equivalent anisotropic nanoparticle. The thermal conductivity of the anisotropic nanoparticle is deduced by analyzing the heat conduction process of a single CNT with an interfacial layer in an arbitrary direction. The present model which contains the effect of CNTs' diameter and aspect ratio as well as the interfacial layer was analyzed and validated with other models and available experimental data. The results show that the present model exhibits a more moderate variety rate driven by the diameter and aspect ratio of CNTs. And this characteristic results in a better adaptability than other models when compared with some available experimental results.     

Retrieval of thermal properties in a transient conduction–radiation problem with variable thermal conductivity

This article reports an inverse analysis of a transient conduction–radiation problem with variable thermal conductivity. Simultaneous retrieval of parameters is accomplished by minimizing the objective function represented by the square of the difference between the measured and the assumed temperature fields. The measured temperature field is calculated from the direct method involving the lattice Boltzmann method (LBM) and the finite volume method (FVM). In the direct method, the FVM is used to obtain the radiative information and the LBM is used to solve the energy equation. With perturbations imposed on the measured temperature data, minimization of the objective function is achieved with the help of the genetic algorithm (GA). The accuracies of the retrieved parameters have been studied for the effects of the genetic parameters such as the crossover and the mutation rates, the population size, the number of generations and the effect of noise on the measured temperature data. A good estimation of parameters has been obtained.     

Pulsating flow in circular pipes — the analysis of thermal stresses

Flow pulsation in externally heated pipes generates a pulsating temperature field, which, in turn results in oscillating thermal stresses across the pipe wall. In the present study, pulsating flow inside a circular pipe, which is externally heated, is considered. The flow and temperature fields are computed numerically using a control volume approach. The resulting thermal stresses across the pipe wall due to temperature variation in the transverse direction are computed. Pressure pulsation at the pipe inlet is employed to produce the flow pulsation. The simulations are extended to include different pipe lengths, pipe diameters and pipe thickness. It is found that pipe diameter has a significant effect on the effective stress levels; in which case, the amplitude of the oscillation in stress levels across the pipe wall reduces considerably with increasing pipe diameter. Moreover, the effect of Reynolds number is more pronounced at mid and outlet planes such that increasing Reynolds number amplifies the amplitude of stress levels in the pipe.     

Molecular dynamics simulation on the effect of water uptake on hydrogen bond network for OH− conduction in imidazolium-g-PPO membrane

OH⁻ conduction involved in the hydrophilic channel of anion exchange membrane strongly depends on the water uptake. To investigate the effect of water uptake on the hydrogen bond network for OH⁻ conduction, a series of molecular dynamics simulations based on all-atom force field were performed on the hydrated imidazolium-g-PPO membranes with different water uptakes. The systems were well verified by comparing the membrane density and OH⁻ conductivity with previous experiments. By means of local structural properties and pair-potential energy, reasonable hydrogen bond criteria were determined to describe the hydrogen bond network confined in the membrane. Increasing water uptake enhances the hydration structures of water and OH⁻, and facilitates the reorganization of the hydrogen bond network. Water and OH⁻ are nearly saturated with water when the water uptake reaches λ = 10, where well-connected hydrogen bond network is produced. Further increasing water uptake has much less contribution to improving the hydrogen bond network, but inevitably swells the membrane channel. This work provides a molecular-level insight into the effect of water uptake on the hydrogen bonding structures and dynamics of OH⁻ and water confined in the imidazolium-g-PPO membrane.     

Numerical solution for Sisko nanofluid flow through stretching surface in a Darcy–Forchheimer porous medium with thermal radiation

The heat transfer assessments in a Sisko nanofluid flow over a stretching surface in a Darcy–Forchheimer porous medium with heat generation and thermal radiation are studied. The numerical analysis technique is used to assess the governing nonlinear equations of the model. The influence of Forchheimer number, porosity, heat generation, radiation, and material parameters is examined. The outlines of Nusselt number and skin friction coefficient corresponding to pertinent parameters are revealed. The comparison of Nusselt number outlines of working fluid and Newtonian fluid is depicted. From the analysis, it has been examined that with the increase in Forchheimer number and material parameter values, heat transfer function decreases, whereas heat transfer characteristics of Sisko nanofluid increase with heat generation and material parameters. Moreover, working fluid velocity outlines depreciate when there is an increase in porosity parameter for both shear-thinning and shear-thickening. The comparison of this study with previous research has been conducted.     

Numerical simulation of turbulent flow in a Ranque–Hilsch vortex tube

The present work is about numerical simulations of the internal flow in a commercial model of a Ranque–Hilsch vortex tube (RHVT) operating in jet impingement. Simulation of the turbulent, compressible, high swirling flow was performed by both RANS and LES techniques. The effect of different turbulence closure models have been tested in RANS simulations using a first order closure RNG k–ε and, for the first time in this kind of flow, a second order RSM (Reynolds Stress differential Model) closure. RANS computations have been executed on an axis-symmetric two-dimensional mesh and results have been compared with LES ones, obtained over a three-dimensional computational grid. Smagorinsky sub-grid scale model was used in LES. All the calculations were performed using FLUENT^? 6.3.26. The use of a common code for the different simulations allowed the comparison of the performances of the different techniques and turbulence models, avoiding the introduction of other variables.In all the simulations performed, consistency with the real commercial vortex tube model in jet impingement operation has been followed by substituting an axial hot computational exit to the usual radial one. Comparison of the results between RANS simulations performed on both a traditional radial hot outlet computational domain and one with an axial hot outlet, demonstrates the suitability of the computational model adopted in this work, closer to the real geometry of the device, particularly in RANS RSM simulations. Results in different sections of the tube show significant differences in the velocity profiles, temperature profiles and secondary vortex structures, varying turbulence model.The accurate numerical simulation of the flow in a RHVT, resulting in an improved prediction capability of the kinematic and thermal properties of outgoing jets, could allow a correct estimation of the cooling performance of this device in jet impingement operation.     

Field synergy principle in forced convection of plane Couette–Poiseuille flows with effect of thermal asymmetry

The field synergy principle is employed to analyze convection heat transfer enhancement which can be achieved by reducing the included angle between the velocity vector and the temperature gradient (synergy angle). The present study is aimed to scrutinize the relationship of the synergy angle and the field synergy number with other pertinent parameters in forced convection of plane Couette–Poiseuille flows with asymmetric heat-flux wall boundary conditions. This type of problem arises in various engineering processes, such as in the operation of extruders and in various lubrication problems. The variation of the velocity vector is governed by the moving plate velocity while the temperature gradient is affected by both the moving plate velocity and the asymmetrical heat fluxes at the wall boundaries. Analytical solutions are obtained and the effects of thermal asymmetries under the imposition of isoflux at the walls in Couette–Poiseuille flows are analyzed by adopting field synergy principle. The variations of synergy angle with different boundary conditions and the relationship between the Nusselt number and the synergy (coordination) number, are compared and analyzed. The thermal condition at the wall boundary, the variations of the moving plate velocity and the Peclet number are the essential parameters in the synergetic behavior of the system.     

Flow and heat transfer in a micro-cylindrical gas–liquid Couette flow

Analytic solutions for the gas and liquid velocity and temperature distribution are determined for steady state one-dimensional microchannel cylindrical Couette flow between a shaft and a concentric cylinder. The solution is based on the continuum model and takes into consideration the velocity slip and temperature jump in the gaseous phase defined by the Knudsen number range of 0.001 < Kn < 0.1. The two fluids are assumed immiscible. The gas layer is adjacent to the shaft which rotates with angular velocity ω_s and is thermally insulated. The outer cylinder rotates with angular velocity ω_o and is maintained at uniform temperature. The governing parameters are identified and the effects of the Knudsen number and accommodation coefficients on the velocity and temperature profiles, reduction in the overall temperature rise due to the gas layer, the Nusselt number and shear reduction are examined. It was found that the required torque to rotate the liquid in the annular space is significantly reduced by introducing a thin gas layer adjacent to the shaft. Also, reduction in shaft temperature is enhanced through a combination of high energy accommodation coefficient and low momentum accommodation coefficients. Results also indicate that the gas layer becomes more effective in reducing the shaft temperature when the housing angular velocity is much larger than the shaft angular velocity.     

Nonlinear dynamic response of eccentrically stiffened FGM plate using Reddy’s TSDT in thermal environment

The nonlinear dynamics of an eccentrically stiffened functionally graded material (ES-FGM) plates resting on the elastic Pasternak foundations subjected to mechanical and thermal loads is considered in this article. The plates are reinforced by outside stiffeners with temperature-dependent material properties in two cases: uniform temperature rise and through the thickness temperature gradient. Both stiffeners and plate are deformed under temperature. Using Reddy’s third-order shear deformation plate theory, stress function, Galerkin and fourth-order Runge–Kutta methods, the effects of material and geometrical properties, temperature-dependent material properties, elastic foundations, and stiffeners on the nonlinear dynamic response of the ES-FGM plate in thermal environments are studied and discussed. Some obtained results are validated by comparing with those in the literature.     

On the onset of thermal convection in rotating nanofluid layer saturating a Darcy–Brinkman porous medium

In this paper, the effect of rotation on the onset of thermal convection in a horizontal layer of nanofluid saturated by a Darcy–Brinkman porous medium is considered. A linear stability analysis based upon normal mode is used to find solution of the fluid layer confined between two free boundaries. The onset criterion for stationary and oscillatory convection is derived analytically and graphically. The effects of the concentration Rayleigh number, Taylor number, Lewis number, Darcy number and modified diffusivity ratio on the stability of the system are investigated. The sufficient conditions for the non-existence of overstability are also derived.     

Investigation of buoyancy-driven flow and heat transfer in a trapezoidal cavity filled with water–Cu nanofluid

A numerical study is conducted to investigate the transport mechanism of free convection in a trapezoidal enclosure filled with water–Cu nanofluid. The horizontal walls of the enclosure are insulated while the inclined walls are kept at constant but different temperatures. The numerical approach is based on the finite element technique with Galerkin's weighted residual simulation. Solutions are obtained for a wide range of the aspect ratio (AR) and Prandtl number (Pr) with Rayleigh number (Ra = 10⁵) and solid volume fraction (? = 0.05). The streamlines, isotherm plots and the variation of the average Nusselt number at the left hot wall are presented and discussed. It is found that both AR and Pr affect the fluid flow and heat transfer in the enclosure. A correlation is also developed graphically for the average Nusselt number as a function of the Prandtl number as well as the cavity aspect ratio.     

Studying the effect of convergence parameter of CUSP’s scheme in 2D modeling of novel combination of two schemes in nucleating steam flow in cascade blades

In the present research, considering the importance of appropriate design of steam turbines, a combination of scalar and convective upwind split pressure (CUSP) with known value of z schemes is used for numerically modeling condensation of the 2D nucleating steam flow. Considering the importance of z parameter in the CUSP scheme, effects of several different values of this parameter on the modeling of steam flows are subjected to sensitivity analysis using the combination method. Results of the improved numerical method across sensitive nucleation condensation shock zone are in good agreement with experimental data. Furthermore, numerical errors are lower than those conventional methods by up to about 80% with the mass flow rate being well stable, which indicates better satisfaction of conservation laws, and revealing efficiency of the proposed novel combination method.     

Improvement of biodiesel’s policy in Thailand

Thailand’s economy is growing rapidly as seen in terms of GDP; according to Thailand is a developing country. So, in industrial sector and logistics are high cost. Around 36% of diesel is spent on the transportation sector, and the amount of oil imported is going up. The Thai government had launched a master plan to increase the stability of the energy situation.

This paper studied the biodiesel policy in Thailand. Although the Thai government had launched a plan 8 years back, the plan could not meet its expected target. The policy was then extended to a 15-year plan. The goal is changed, using more biodiesel: 492.75 million liters in 2008 compared with 1642.5 million liters in 2022. The problems are politics, lack of raw materials, standard of specifications, no clear subsidized policy, and farmers’ lack technology.