Effects Of Variable Properties Research Articles

Insight into the peristaltic motion through a tapered channel with Newton’s cooling subject to viscous dissipation, Lorentz force, and velocity slip

Peristalsis has gained significant attention due to its numerous applications in the medical field, engineering, and manufacturing industries. Therefore, the current work intends to look into the effects of variable liquid properties on the magnetohydrodynamics of peristaltic flow exhibited by viscous fluid through a tapered channel. The viscosity of the liquid differs over the thickness of the channel, and temperature-dependent thermal conductivity is considered. The constitutive relation for energy is formulated with the addition of viscous dissipation and heat generation/absorption. The assumption of velocity slip along with the convective boundary condition energizes the thermal system as well as the flow phenomena. The mathematical formulation is established on the grounds of low Reynolds number and long wavelength approximations. Perturbation solution were obtained for the resulting non-linear differential equations of momentum and energy for small values of variable viscosity and variable thermal conductivity. The effects of various relevant parameters on flow properties were investigated through graphical analysis. The results show that the maximum velocity does not occur in the middle of the tapered channel, but moves toward the upper wall with the increase in the variable viscosity difference between the walls. The application of viscosity is essential in many engineering and industrial processes.

The effects of property variation on the dripping behaviour of polymers during UL94 test simulated by particle finite element method

Abstract The dripping behaviour of polymers is often observed experimentally through the UL94 flammability standard test. In this work, polymeric dripping under fire is investigated numerically using particle finite element method. A parametric analysis was carried out to observe the influence of a single property on overall dripping behaviour via a UL94 vertical test model. Surrogates and property ranges were defined for variation of the following parameters: glass transition temperature (T g), melting temperature (T m), decomposition temperature (T d), density (ρ), specific heat capacity (Cp), apparent effective heat of combustion of the volatiles, char yield (µ), thermal conductivity (k), and viscosity (η). Polyamide, poly(ether ether ketone), poly(methyl methacrylate), and polysulfone were used as benchmarks. Simulated results showed that specific heat capacity, thermal conductivity, and char yield allied with viscosity were the properties that most influenced dripping behaviour (starting time and occurrence).

Impact of variable properties in natural convection airflows within simple and adiabatically-extended vertical tubes

Influence of variable thermophysical properties is investigated in airflows established by natural convection inside vertical tubes. Systematic numerical results are obtained for covering the entire range laminar to turbulent (modified Rayleigh number ranging 10−3–109), for both isothermal and isoflux heating conditions. Particular attention is devoted to the effects of variable properties of air. Own functions are used to take into account the variation of thermophysical properties as a function of temperature. Results are compared to those computed assuming the Boussinesq approach. For isoflux heating, it is proven that the disruptive effect consisting of the emerging of a critical heat flux and the consequent thermal crisis phenomenon, appear in the regarded cylindrical geometry. An analytical estimation of the critical heat flux is developed for vertical tubes, valid for the laminar regime, and an overall correlation is proposed for the laminar-turbulent range. The impact of the height and the widening of a funnel-shaped adiabatic extension is explored for selected values of the modified Rayleigh number, and the conditions under which its inclusion is beneficial to postpone the appearance of the crisis phenomenon, are determined.

Effects of CO2 variable thermophysical properties and phase behavior on CO2 geological storage: A numerical case study

CO2 injection into the reservoir can effectively reduce greenhouse gas emissions. The thermophysical parameters and phase behavior of CO2 are highly sensitive to pressure and temperature, resulting in drastic change in wellbore and reservoir, and thus affecting the CO2 storage effect. To study the impact of CO2 thermophysical properties and phase behavior on CO2 geological storage, we established a fully coupled 1D+3D wellbore coupled 3D reservoir model. Based on the analysis of non–isothermal flow characteristics, CO2 thermophysical properties and phase behavior, the influence of engineering parameters on the key factors of CO2 geological storage is discussed. The findings indicate that the density and thermal capacity of CO2 are the most important thermophysical parameters that affect wellhead pressure and bottomhole temperature respectively. The efficiency of CO2 storage is greatly influenced by the density and viscosity of CO2, more so than any other thermophysical parameters. CO2 phase behavior and permeability change increase CO2 seepage capacity by 3.50 % and 8.79 % respectively. Engineering parameters will affect the thermophysical properties and permeability of bottom–hole CO2, thereby affecting the injectability and storage efficiency of CO2. This research can offer suggestions and theoretical groundwork for the improved design of CO2 geological storage.

Effect of nucleation frequency and compressible buffer volume on the thermofluidic characteristics for flow boiling through a microchannel heat sink

A 1-D transient model of flow boiling in a rectangular microchannel is developed to assess the pressure drop and heat transfer characteristics, particularly emphasizing the impacts of nucleation frequency and compressible buffer volume. The combined influence of nucleation frequency with channel height, inlet velocity and heat flux is also studied. The increased nucleation frequency reduces the pressure drop amplitude and enhances the heat transfer coefficient. As the buffer volume is increased, the pressure drop and heat transfer coefficient are reduced due to the predominant occurrence of reverse flow in the channel. The amplitude and duration of reverse flow are identified as two important key parameters. As the reverse flow strongly affects the thermal-hydraulics of the channel, a sensitivity analysis is performed to assess the variation of these two key parameters over a range of operating conditions. The variation of amplitude and duration of reverse flow strongly depends on the trends of channel pressure drop, which is the consequence of the competing effects of bubble growth rate and reverse flow. Lastly, the effect of variable vapor properties on pressure drop and heat transfer characteristics of the channel has been discussed.

Study of the geometry and structure of a thermoelectric leg with variable material properties and side heat dissipation based on thermodynamic, economic, and environmental analysis

Thermoelectric generator (TEG) without moving parts, refrigerants, and direct carbon emissions is the most promising way to reduce carbon dioxide. The economy and thermal efficiency of TEG are seriously inhibited by geometry and structure, while existing studies have not investigated the effect of geometric and structure on output characteristics when the model considers the effect of side leg dissipation and variable properties. This paper proposes a one-dimensional model considering the effect of Peltier heat, conductive heat, Joule heat, Thomson heat, side leg dissipation, and temperature dependence for variable and conventional geometric and structures for 8 different materials. The effects of operation parameters, geometry parameters, and structures on thermodynamic, economic, and environmental performances are studied. The higher hot source temperature and shorter leg length benefit comprehensive performance. Increasing side leg dissipation decreases efficiency. When the fin parameter (L(heP/λA)0.5) is below a specific value, increasing side leg dissipation can promote the output power, economic and environmental performance. Compared to the traditional structure, the output power and efficiency of new configurations are increased by 1.01–3.59 and 1.03–4.57 times, respectively; The cost and CO2 emission reduce by 1.0–72.1% for low hot source temperature and figure of merit (ZT) material.

Investigation of flow and heat transfer characteristics of supercritical argon in conical Hampson heat exchangers for a J-T cryocooler

The Hampson-type heat exchanger is critical for the rapid cooling of a conical J-T cryocooler, and its internal flow and heat transfer characteristics directly affect the performance of the conical cryocooler. The flow and heat transfer characteristics of supercritical argon are studied using a numerical model of a conical helical tube in this work. The simulated results show that supercritical argon flow in the conical helical tube is mainly influenced by the combined effect of variable thermophysical properties and the secondary flow produced by centrifugal force. The buoyancy has little impact on heat transfer. The heat transfer coefficient reaches its maximum at the pseudo-critical region. A more significant heat transfer coefficient and faster heat exchanger cooling rate can be achieved by increasing the mass flow rate and maintaining the fluid in the tube close to the pseudo-critical region. At a cone angle of 50°, the maximum total heat exchange quantity is observed. With a greater temperature drop per unit length, a stronger heat transfer effect can be achieved close to the outlet of the tube. Based on the numerical analyses, a new correlation for heat transfer is proposed.

Functionally graded cylinders: Vibration analysis

AbstractThe problems of steady‐state and free oscillations of functionally graded hollow cylinders are considered in the present article, with the cylinders' properties considered changing along the radial or longitudinal coordinates. The stress–strain state was described using the model of an isotropic body with the variable Lamé parameters and density. The cylinders were assumed to be subjected to the normal and tangential loads. In order to obtain the numerical problem solutions, the finite element method was employed. The effect of variable properties on the amplitude–frequency characteristics and the resonant frequencies values for various cylinders types is estimated. Some features, which can be used in solving new inverse coefficient problems on material properties identification based on acoustic sounding data, are revealed.

Scrutinization of Solar Thermal Energy and Variable Thermophysical Properties Effects on Non-Newtonian Nanofluid Flow

Nanofluids generate high values of convection heat transfer coefficients, low specific heat, and density, which improve the solar thermal energy performance by making it work effectively. By utilizing nanotechnology and solar thermal radiation, the modern world is moving in the direction of new technologies. Therefore, this research is communicated to explore the significance of solar thermal energy, variable properties on non-Newtonian nanofluid flow. However, to exemplify the fluid transport features of the Casson nanofluid (CF), the Buongiorno nanofluid model was utilized. Also, the Lie-group technique is used in the framework to develop similarity variables that will be used to reduce the number of independent variables in partial differential equations (PDEs) and is solved numerically by using the weighted residual Galerkin method (WRGM). The graphical findings revealed that when the variable viscosity parameter is increased, the fluid temperature decreases, while the presence of the solar radiation parameter has the opposite impact. Additionally, when the non-Newtonian parameter approaches infinity, the Casson fluid obeys the viscosity law. The report of this study will be of benefit to thermal and chemical engineering for nanotechnology advancement. KEYWORD: Solar Thermal Energy, Nanofluids, Non-Newtonian, weighted residual Galerkin method (WRGM).

Direct numerical simulations of cooling flow and heat transfer in supercritical CO2 Brayton cycle coupled with solar energy

Direct numerical simulation was used to study supercritical carbon dioxide’s flow and heat transfer mechanisms under cooling conditions in the supercritical CO2 Brayton Cycle coupled with solar energy. This paper aims to investigate the effects of property variation, buoyancy, and thermal deceleration under different flow conditions. Momentum balance analysis is used to quantify changes caused by different mechanisms and to describe the flow and turbulence evolution characteristics. In forced convection, due to the dominant thermal deceleration effect, the pressure gradient decreases across the entire cross-section, resulting in the deceleration of the fluid. In upward flow, buoyancy plays a dominant role and acts as a resistance, causing the pressure gradient to increase significantly. As a result, when the core fluid accelerates, the velocity of the near-wall fluid slows significantly, enhancing heat transfer, and the Nusselt number increases by up to 33.21% compared to forced convection. In downward flow, buoyancy is strong, the pressure gradient is negative, and the flow is entirely driven by buoyancy. External and structural buoyancy effects contribute to flow laminarization, leading to heat transfer deterioration. With increasing heat flux, Nu decreases more significantly, with a maximum reduction of 62.94%, and begins to recover after reaching its lowest point. Importantly, in the recovery stage, the structural buoyancy effect triggers turbulence recovery. The recovery of shear production of turbulent kinetic energy is later than that of buoyancy production and can only occur with a strong M−shaped velocity distortion. The FIK identity decomposition shows that the heat transfer deterioration is related to flow laminarization and inhomogeneous effects. Heat transfer recovery is purely due to the increase in turbulence components.

Direct numerical simulation of thermal stratification of supercritical water in a horizontal channel

A direct numerical simulation of water at a supercritical pressure is carried out to study the fundamental characteristics of turbulence and heat transfer in the flow between two parallel plates. The top and bottom plates are differentially heated to achieve stable thermal stratification. Both forced and mixed convection are studied to investigate buoyancy and variable property effects in a stably-stratified horizontal flow. The simulation results show that the variable property effect causes turbulence deterioration and flow laminarization near the cold wall while increasing the turbulence stress intensity near the hot wall. The direct buoyancy effect weakens the turbulence development throughout the channel. The indirect buoyancy effect strongly enhances flow laminarization near the cold wall and it increases turbulence near the hot wall. However, the net buoyancy effect near the hot wall is insignificant due to a cancelling of the competing direct and indirect buoyancy effects. The turbulence behaviour near the hot wall is dominated by the variable property effect. The heat flux transferred through the wall under the condition of fixed wall temperature is strongly reduced by buoyancy, while it is less sensitive to property variation.

Evaporating Rayleigh–Bénard convection: prediction of interface temperature and global heat transfer modulation

We propose an analytical model to estimate the interface temperature $\varTheta _{\varGamma }$ and the Nusselt number $Nu$ for an evaporating two-layer Rayleigh–Bénard configuration in statistically stationary conditions. The model is based on three assumptions: (i) the Oberbeck–Boussinesq approximation can be applied to the liquid phase, while the gas thermophysical properties are generic functions of thermodynamic pressure, local temperature and vapour composition, (ii) the Grossmann–Lohse theory for thermal convection can be applied to the liquid and gas layers separately and (iii) the vapour content in the gas can be taken as the mean value at the gas–liquid interface. We validate this setting using direct numerical simulations in a parameter space composed of the Rayleigh number ( $10^6\leq Ra\leq 10^8$ ) and the temperature differential ( $0.05\leq \varepsilon \leq 0.20$ ), which modulates the variation of state variables in the gas layer. To better disentangle the variable property effects on $\varTheta _\varGamma$ and $Nu$ , simulations are performed in two conditions. First, we consider the case of uniform gas properties except for the gas density and gas–liquid diffusion coefficient. Second, we include the variation of specific heat capacity, dynamic viscosity and thermal conductivity using realistic equations of state. Irrespective of the employed setting, the proposed model agrees very well with the numerical simulations over the entire range of $Ra$ – $\varepsilon$ investigated.

Towards seismic performance quantification of viscous damped steel structure: Site-specific hazard analysis and response prediction

With an emphasis on predictable performance, the paramount importance of performance-based earthquake engineering (PBEE) in quantifying earthquake risks and facilitating the better-informed design of the built environment to achieve earthquake resilience has been widely acknowledged. And the uses of damping systems, i.e., structures incorporated with supplemental damping devices, have been recognized as an effective structural development to increase the resilience of structures to earthquakes. This paper presents a uniform hazard spectrum (UHS) based site-specific ground motion selection procedure, to implement the PBEE framework to relate earthquake hazard to structural performance for structures with variable dynamic properties. This ground motion selection procedure accounts for the effect of variable structural dynamic properties on hazard characterization for structures with damping systems. A paradigm building site, on which the site-specific hazard from the probabilistic seismic hazard analysis (PSHA) is consistent with the risk-targeted hazard from the design maps of ASCE/SEI 7-10, is selected for the implementation of the procedure. Three suites of 40 ground motions representing various hazard intensities at the building site are selected for time history dynamic analysis of a prototype structure with nonlinear viscous damping system. Evaluated is the probability distribution of major engineering demand parameters (EDPs) including story drift, residual story drift, floor velocity, and floor acceleration. The results demonstrated that the major EDPs of a building structure with supplemental nonlinear viscous dampers at a paradigm site can be estimated using the lognormal probability distribution, and the proposed UHS-based site-specific ground motion selection procedure is critical to the performance assessment of structures with variable dynamic properties in the PBEE framework.

Three-dimensional exact solutions of double-coated structure with arbitrary thickness under normal point load

Explicit closed expressions of stresses and displacements of an adherend and an adhesive provide researchers and designers with insight into the effects of variable parameter properties on structural performance. Through potential theory method, the Green's functions of a double-coated structure under normal point load are solved using the mirror image method, where the mirror image rule satisfies the recursion law of the Fibonacci sequence. The stress distribution of the multilayer structure under the action of point load is presented in the form of an infinite series with a fast convergence rate. A guideline for the design of coating thickness and material parameters to reduce interfacial stress is developed (reduce adhesive thickness, reduce elastic modulus of adherend, increase elastic modulus of adhesive). The accuracy of the exact solution is demonstrated through comparison with contact analytical solutions. In addition, the critical load of interface shear delamination failure is predicted. The stresses and displacements of the adherend and the adhesive are presented with the solutions, which can serve as a reference for researchers and designers.

A design model for spiral wound heat exchangers which accounts for property variation, longitudinal heat conduction, and heat-in-leak

• A combined analytical-numerical design model for cryogenic SWHE is developed. • Analytical sizing module finds the optimum configuration of SWHE. • The effects of property variation, LHC, and heat-in-leak are investigated. • LHC impact is negligible compared to property variation and heat-in-leak. • A sensitivity analysis on the input constraint is performed. Spiral Wound Heat Exchangers (SWHEs) can be a good and reliable choice for serving as the main heat exchanger in the cryogenic gas liquefaction cycles using throttling process such as the Linde-Hampson cycle. In this application, a high pressure hot gas stream has to be cooled down via transferring heat to a low pressure cold gas in a high-effectiveness heat exchanger. Present research has been organized to develop a design model which is able to find the optimum SWHE configuration for gas liquefaction cycles. The model has to be capable of considering the important physical phenomena in the cryogenic temperature ranges, including variation of fluid properties with temperature, longitudinal heat conduction (LHC) through separating walls, and heat-in-leak. Therefore, a combined analytical-numerical design model has been developed in the present study. The design model is essentially composed of two stand-alone modules, i.e. an analytical sizing module and a numerical performance evaluation module. To develop the design model, first, the geometrical characteristics of SWHE are studied and primary geometrical parameters are recognized. Then, the analytical sizing module is developed which is able to determine the optimum values of primary geometrical parameters of SWHE minimizing the heat transfer surface area based on the input design requirements. Afterwards, a 1D finite difference numerical module is presented in which important physical phenomena can be considered in the evaluation of the thermo-hydraulic performance of all types of counter flow or parallel flow heat exchangers as well as SWHEs. Finally, the two modules are combined to construct a design model. Two examples in which the low pressure superheated vapor helium is intended to cool down the high pressure supercritical helium flowing inside the tubes are also provided to demonstrate the applicability of the proposed combined model, and also, to elucidate its sensitivity to the input constraints. It was concluded that while variations of fluid properties and the external heat fluxes due to the radiation and convection have significant influence on the performance of cryogenic SWHE, the LHC is of minor importance and can be neglected. Moreover, decreasing the shell-side allowable pressure drop from 10 kPa to 2 kPa, causes the heat exchanger length and its outside diameter to increase by 79% and 48%, respectively.

Numerical Analysis of the Mixed Flow of a Non-Newtonian Fluid over a Stretching Sheet with Thermal Radiation

A mathematical model is elaborated for the laminar flow of an Eyring-Powell fluid over a stretching sheet. The considered non-Newtonian fluid has Prandtl number larger than one. The effects of variable fluid properties and heat generation/absorption are also discussed. The balance equations for fluid flow are reduced to a set of ordinary differential equations through a similarity transformation and solved numerically using a Chebyshev spectral scheme. The effect of various parameters on the rate of heat transfer in the thermal boundary regime is investigated, i.e., thermal conductivity, the heat generation/absorption ratio and the mixed convection parameter. Good agreement appears to exist between theoretical predictions and the existing published results.

Effects of variable thermophysical properties of water on the heat extraction of an enhanced geothermal system: A numerical case study

The variation of water thermophysical properties in deep high-temperature enhanced geothermal systems (EGS) is dramatic, which seriously affects the thermal performance of the system. While most previous researches focus on the variation of water density and viscosity with temperature in the reservoir, conversely the pressure effect and other water thermophysical properties are ignored. In this paper, a fully coupled wellbore-reservoir model is presented to explore the influences of the water thermophysical properties varied with temperature and pressure on the system's thermal performance. The model is validated against the 2D single-fracture model analytical solution and field data. The influence of different engineering methods (open-hole length, production pressure) on the system's thermal performance is explored according to the fluid flow and heat transfer mechanisms and thermophysical property analysis. Results show that the pressure difference decreases by 17.66 MPa when the open hole length increase from 20 m to 60 m. Increasing the production pressure would increase the system pressure loss and decreases the system thermal power due to the reduction of water heat capacity. These results provide significant references for the optimization design of the EGS.

Effects of Variable Properties on the Convective Flow of Water near Its Density Extremum in an Inclined Enclosure with Entropy Generation

The aim of the current numerical simulation was to understand the effects of the temperature-dependent properties of viscosity and thermal conductivity on the buoyant convection of cold water around its density extremum in a tilting chamber. The equations for thermal conductivity and viscosity were derived based on the reference temperature parameter. The entropy generation and cup mixing temperature were also calculated. The governing mathematical model was solved by the finite-volume-based iterative technique. The obtained results were deliberated for several values of inclination angles and for the density reversal parameter for variable viscosity and thermal conductivity cases. It was detected that density inversion has a strong effect on stream and thermal patterns through the development of a bi-cellular arrangement due to density inversion.

Exploring effects of property variation on fragmentation of metal rings using a simple model

A simple finite element implementation of the Mott model for fragmentation of a thin walled ring has been implemented and used to explore the effect of local variations in fracture strain around the ring. The model has successfully reproduced the fragment size distributions previously reported, which follow a characteristic “Mott distribution” form, providing sufficient (1000) simulations are run. It has been shown that this form is retained even when there are large differences in the random distribution of fracture strains or a different choice of function used to describe the fracture strain scatter. In these cases, the strain rate has a much stronger effect than fracture strain distribution the on the average fragment size and fragment distribution. However, for cases where there are a small number of local defects that strongly reduce the fracture strain at certain locations around the ring, the predicted fragment size distribution develops a bimodal character. This is also the case for large but gradual variations in fracture strain with position around the ring. The results have implications for cases where a small number of large pre-existing defects exist, or processing has led to macrozones in the microstructure. The utility of a simple fast running model to study these cases is discussed.

Entropy analysis with the Cattaneo–Christov heat flux model for the Powell–Eyring nanofluid flow over a stretching surface

The recent trend of research in fluids' transport along with heat transfer processes is focused on entropy minimization in thermal systems. This leads to the introduction of nanofluids as operating fluids, use of non-Newtonian fluid models, the type, shape, and sizes of nanoparticles, and the effect of variable thermophysical properties. In the present research, a numerical investigation is carried out for the entropy generation in a time-dependent, thermodynamically stable Powell–Eyring nanofluid flow over a flat, porous and non-linear horizontal stretching surface. The Cattaneo–Christov model for the heat flux is assumed for the thermal boundary layer analysis. The effects of the uniform thermal radiation, nanoparticle's shape and the boundary slip is also included in the mathematical model. The governing equations are abridged under the assumptions of the usual boundary layer flow and the Roseland approximations. Partial differential equations are first transformed to a system of ordinary differential equations by using appropriate similarity transformations. The Keller box scheme is then adopted to find the approximate solutions for the reduced ordinary differential equations. Two different classes of nanofluids, copper-water () and titanium-water () are considered for our analysis. Numerical results are presented graphically to study the effect of various thermophysical parameters. Numerical results show material parameters, velocity slip, nanoparticle concentration and fluid injection increase the thermal boundary layer thickness and reduce the rate of heat transfer at the surface. Moreover, increasing the strength of these reduces fluid movement within the boundary layer and increases the overall entropy of the system.