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The use of nanoparticles to improve the thermal efficiency of PTCs has become a popular area of research. In this study, we present a comparative analysis of the effects of nanoparticles on the thermal performance of PTCs using water and thermal oil as heat transfer fluids. To obtain accurate temperature estimates, we developed a mathematical model that considers heat transfer in both axial and radial directions. The model includes a two-phase flow homogenous model for the liquid phase, as well as nanoparticle concentration, to achieve the best temperature estimation. We validated our model using experimental data and found that the maximum relative error was 2.3% for water and a lower error of 1.5% for thermal oil. Our analysis of thermal efficiency and exergetic efficiency showed that the addition of Al2O3 nanoparticles led to greater improvements in the thermal efficiency for water, even at the same range of nanoparticle concentration. Furthermore, our results demonstrated that exergetic efficiency was generally higher for water than for thermal oil. Overall, this study highlights the potential benefits of using nanoparticles to enhance the thermal efficiency of PTCs. Our findings could help guide the design and optimization of PTC systems in various applications.

Amorphous alloys have high strength, but softening can lead to room temperature brittleness, limiting their applications. Typically, soft means low hardness and yield strength, and the lower the yield stress, the higher the fracture toughness. Two amorphous/crystalline multilayer samples with submicron single layer thickness were prepared. The crystalline layer of one sample is composed of highly-textured (111) nanocrystalline grains, and the other sample is untextured. Indentation experiments proved that the samples were plastically deformed, while the amorphous layer did not form shear bands with decreased layer thickness. The thickness reduction of the amorphous layer in the sample with highly-textured crystalline layers is greater than in the sample with untextured crystalline layers. The strain-hardening ability of the textured crystalline layer is higher, while the corresponding amorphous layer is deformed more severely with free volume annihilation and greater flow with more compact atoms. The amorphous layer corresponding to the untextured crystalline layer absorbs dislocations, and activates shear transformation zones (STZs), increasing the free volume near the interface. The annihilation of the free volume is less than the sample with highly-textured crystalline layers. • The amorphous/crystalline multilayer samples with a single layer thickness of submicron have achieved plastic co-deformation. • The interaction and thinning phenomena during the deformation of amorphous/crystalline were studied by indentation method. • The highly-textured crystalline layer is more conducive to the strain hardening of the amorphous layer. [ABSTRACT FROM AUTHOR]

Most of the energy needed by the world has still been provided by fossil fuels. Energy costs are increasing day by day due to pandemics, wars, natural disasters, etc. Therefore, the need for energy-efficient systems is increasing. This study recovered the waste heat of a natural gas-fired boiler with a flue gas condenser. First, the condenser heat transfer surface area was determined by analytical calculations. Then, the final design was obtained by CFD analysis to make homogeneous fluid distribution on the shell and tube sides. Finally, performance tests were carried out on the condenser. According to the test results, the analytical calculations were confirmed by 94%. In addition, in the final design, the homogeneity of the flow on the shell side has been proven by the thermal camera images. [ABSTRACT FROM AUTHOR]

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Drift-flux model is a simple and reliable tool for void fraction prediction in two-phase flows. This work examines its performance in gas-liquid flow of small bubbles (<1 mm) at low void fractions (<10  −1 ) that resembles bubbly flow in human bloodstream during Decompression Sickness and can be also found in other two-phase applications such as flow boiling in macro-channels. Drift-flux model predictions are compared with experimental data measured in co-current upward bubbly flow for varying gas/liquid flow properties. Water and blood simulant are used as test liquids, while bubble size is controlled using prescribed surfactant (SDS) concentrations. Homogeneous flow model, which is a sub-case of drift-flux model, predicts adequately void fraction in water and blood simulant when the mean bubble size is lower than 200 μm. For larger bubbles in water (mean size up to 800 μm) and in blood simulant (mean size up to 350 μm), the performance of thirteen drift-flux models from literature is examined. Ten of them concern specifically bubbly flow, whereas three models are applicable to a wide range of flow conditions. To our knowledge, this is the first time that drift-flux models are tested at combined low void fractions (between ∼10 −3 and ∼10 −1 ) and sub-millimeter bubble sizes, so the determination of drift-flux parameters (distribution parameter, C 0 and drift velocity, U gm ) is elaborated. On this account, a bubble size dependent correlation is proposed for U gm determination. [ABSTRACT FROM AUTHOR]

The deposition mechanisms of metallic glass particles impacting a substrate at high velocity (385–485 m/s) and temperatures near and above the glass transition are studied using finite element modeling. The deformation mechanisms of the metallic glass particles in these conditions are extremely dependent on their Reynolds number only leading to deposition and bonding at high Reynolds number. Unlike early works, this study includes the homogenous flow deformation under Newtonian and non-Newtonian regime modeled using the constitutive equations of the free-volume model. The computed results are compared against experimental data of metallic glass coatings build-up by Cold Gas Spray. A critical value of the Reynolds number is found by both experiments and simulation, showing that it is a useful parameter to control the activation of viscoplastic deformation and bonding of metallic glass particles. Interestingly, this work demonstrates that deposition of metallic glass particles is governed by a cooperative movement of the liquid instead of a simple shear instability effect at the particle-substrate interface unlike polycrystalline metals. [ABSTRACT FROM AUTHOR]

The room temperature mechanical behavior of amorphous Ni–P thin films deposited on Ni substrate under tension was systematically investigated. Due to the effect of substrate confinement, unexpected homogeneous plastic flow occurred in the film simultaneously with cracking or shear banding, which co-contributed to the plastic deformation. The film thickness gradually reduced with increasing tensile strain and the maximum reduction increased up to 69% in 2.6 μm-thick film. Such a severe plastic deformation leads to significant structure change in the film where free volume annihilation dominates, evidently by almost 100% reduction in the total relaxation enthalpy in the deformed film, accompanied by the densification and hardening. The stress required for shear band and crack propagation in Ni–P film was analyzed on the basis of Griffith's criterion. A deformation map is proposed to account for co-existence of homogeneous flow and localized plastic deformation (shear banding or cracking), as well as the transition of deformation modes. [ABSTRACT FROM AUTHOR]

In this paper, we describe a mathematical model and a numerical simulation method for the condenser component of a novel two-phase thermosiphon cooling system for power electronics applications. The condenser consists of a set of roll-bonded vertically mounted fins among which air flows by either natural or forced convection. In order to deepen the understanding of the mechanisms that determine the performance of the condenser and to facilitate the further optimization of its industrial design, a multiscale approach is developed to reduce as much as possible the complexity of the simulation code while maintaining reasonable predictive accuracy. To this end, heat diffusion in the fins and its convective transport in air are modeled as 2D processes while the flow of the two-phase coolant within the fins is modeled as a 1D network of pipes. For the numerical solution of the resulting equations, a Dual Mixed-Finite Volume scheme with Exponential Fitting stabilization is used for 2D heat diffusion and convection while a Primal Mixed Finite Element discretization method with upwind stabilization is used for the 1D coolant flow. The mathematical model and the numerical method are validated through extensive simulations of realistic device structures which prove to be in excellent agreement with available experimental data. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we reported the thermoplastic deformation and micro/nano forming ability of a Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass (BMG) in the supercooled liquid region (SCLR) at strain rates ranging from 1×10–4 to 3×10–2 s−1. It is found that the deformation behavior of Au-based BMG has strong dependences on strain rate and temperature applied. The strain-rate sensitivity value, obtained by the strain-rate change method, approaches an unity (Newtonian flow) at low strain rates of 10–4 s−1, but it decreases with increasing strain rate to ∼0.3 (non-Newtonian flow) at 10–2 s−1. The homogeneous flow behavior is observed from the compressive test results. As the result of homogeneous flow, the deformability with a high compressive strain of ∼97% is achieved. The excellent micro/nano-patterning obtained at 430K is also demonstrated for imprinted stripes with a feature size of ∼156nm, signifying the good formability of this alloy in SCLR. [Copyright &y& Elsevier]

This paper reports the results of compression tests conducted in situ in a transmission electron microscope of taper-free nanosized pillars of Al86Ni9Y5 metallic glass with diameters ranging between 110 and 900nm. A critical size was found, below which the deformation mode changes from intermittent shear banding to homogeneous flow. The threshold size for the transition from brittle to ductile behavior is 300nm in Al86Ni9Y5. Such a high plasticity has not been found previously in metallic glasses of these dimensions. [ABSTRACT FROM AUTHOR]

Abstract: The influence of alcohol concentration on the gas void fraction in open tube and annular gap bubble columns has been investigated using a vertical column with an internal diameter of 0.102m, containing a range of concentric inner tubes, which formed an annular gap; the inner tubes had diameter ratios from 0.25 to 0.69. Gas (air) superficial velocities in the range 0.014–0.200m/s were investigated. Tap water and aqueous solutions of ethanol and isopropanol, with concentrations in the range 8–300ppm by mass, were used as the working liquids. Radial profiles of the local void fraction were obtained using a four-point conductivity probe and were cross-sectionally averaged to give mean values that were within 12% of the volume-averaged gas void fractions obtained from changes in the aerated level. The presence of alcohol inhibited the coalescence between the bubbles and consequently increased the mean gas void fraction at a given gas superficial velocity in both the open tube and the annular gap bubble columns. This effect also extended the range of homogeneous bubbly flow and delayed the transition to heterogeneous flow. Moreover, isopropanol results gave slightly higher mean void fractions compared to those for ethanol at the same mass fraction, due to their increased carbon chain length. It was shown that the void fraction profiles in the annular gap bubble column were far from uniform, leading to lower mean void fractions than were obtained in an open tube for the same gas superficial velocity and liquid composition. [Copyright &y& Elsevier]

Abstract: The homogeneous flow exhibits strain softening for Ti40Zr25Ni8Cu9Be18 bulk metallic glass, which is related to increase in free volume concentration based on free volume model. When metallic glass is pre-annealed above Tg before deformation, the trend of strain softening becomes slow with pre-annealing time, indicating that the enthalpy recovery contributes to strain softening, because the enthalpy of metallic glass will recover towards equilibrium value above Tg, and leads to increase in free volume concentration. So the strain softening for Ti40Zr25Ni8Cu9Be18 bulk metallic glass is related to enthalpy recovery. [Copyright &y& Elsevier]

Abstract: Limiting current measurements were made at the electrode surfaces fixed flush with the inner wall of a helically coiled circular tube. The measured limiting current facilitated the computation of wall–liquid mass transfer coefficients. The mass transfer data were obtained for the flow of electrolyte through the helical coil in the absence and presence of packed solids. The mass transfer coefficient was found to decrease with increasing p/D ratio and approached minimum for straight tube case. The mass transfer data were correlated in terms of Colburn factor jD, expressed as a function of Reynolds number Re and torsion number Tn obtained as . Analysis of the experimental data obtained in case of packed coiled tubes of circular cros- section revealed that the effect of p/D ratio on pressure drop was observed to be insignificant. The effect of pitch on mass transfer coefficient in case of packed helical coils was found to be marginal. [Copyright &y& Elsevier]

Abstract: This paper presents the effects of various geometries of helical capillary tubes on the flow characteristics of alternative refrigerants flowing through adiabatic helical capillary tubes. The theoretical model is based on the conservation of mass, energy and momentum of fluids in the capillary tube. The two-phase flow model developed was based on a homogenous flow assumption. The model was validated by comparing it with the experimental data of published in literature for R-22, particularly various pairs of refrigerants. It was found conventional refrigerants had lower capillary lengths than alternative refrigerants. For all pairs, the numerical results showed that the traditional refrigerants consistently gave lower pressure drops for both single-phase and two-phase flows, which resulted in longer tube lengths. The results show that coil diameter variation (less than 300mm) for helical capillary tube geometries affected the length of helical capillary tubes. However, pitch variation (more than 300mm) had no significant effect on the length of helical capillary tubes. This adiabatic helical capillary tube model can be used to integrate system models working with alternative refrigerants for design and optimisation. [ABSTRACT FROM AUTHOR]

The non-linear vibration of a squeeze film damper (SFD) supported rotor assembly is closely linked to the presence of air bubbles in the lubricant, due to cavitation, where the discrete gas phase influences the squeeze film pressure profile and gives rise to a non-linear stiffness force. The aim of this paper is to assess the ability of a computational model for homogeneous bubbly oil to predict the influence of air bubbles on the film reaction forces under various operating parameters. The numerical model which considers the solubility of gas and the growth of gas bubbles was developed using the finite-element software, FEMLAB™. Parametric studies of eccentricity ratio, whirling frequency, and supply pressure were conducted to evaluate the influences of air bubbles on the pressure field and hence the squeeze film forces. Results show that an increase in eccentricity ratio and whirling frequency enhances the growth of air content in lubricant and hence increases the radial (stiffness) to tangential (damping) force ratio in an SFD, whereas an opposite effect is gained by applying higher supply pressure. Compared with the classical theoretical half-film model predictions, the bubbly oil model provides a more realistic estimation with respect to different damper operating conditions. From the simulation findings, it can be concluded that the homogeneous two-phase flow model reasonably describes the bubbly oil behaviour in SFDs and effectively shows the rise in stiffness force due to the growth of air bubbles. The homogeneous model can be easily applied to the well-established lubrication equation and solved with efficiency. However, any possible interfacial effects between the liquid and gas phases are inevitably concealed. The success of the current model allows its subsequent coupling with a structural rotor to form a multi-disciplinary model for unbalance analysis. [ABSTRACT FROM AUTHOR]

Abstract: Using an analogy between thermal conductivity of porous media and viscosity in two-phase flow, new definitions for two-phase viscosity are proposed. These new definitions satisfy the following two conditions: namely (i) the two-phase viscosity is equal to the liquid viscosity at the mass quality=0% and (ii) the two-phase viscosity is equal to the gas viscosity at the mass quality=100%. These new definitions can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach. These new models are assessed using published experimental data of two-phase frictional pressure gradient in circular pipes, minichannels and microchannels in the form of Fanning friction factor (f m) versus Reynolds number (Re m). The published data include different working fluids such as R-12, R-22, argon (R740), R717, R134a, R410A and propane (R290) at different diameters and different saturation temperatures. Models are assessed on the basis minimizing the root mean square error (e RMS). It is shown that these new definitions of two-phase viscosity can be used to analyze the experimental data of two-phase frictional pressure gradient in circular pipes, minichannels and microchannels using simple friction models. [Copyright &y& Elsevier]

Abstract: Flow characteristics of a non-adiabatic capillary tube in a transcritical CO2 heat pump cycle have been investigated employing homogeneous flow model. The model is based on fundamental equations of mass, energy and momentum which are solved simultaneously through an iterative process. Chosen empirical correlations are used for calculating viscosity and friction factor. Single and two phase heat transfer coefficients are estimated through appropriate empirical correlations. Sub-critical and super-critical thermodynamic and transport properties of CO2 are calculated employing precision property subroutines. Three different cases are considered to optimise location of the heat exchange process: only single phase flow region, only two phase flow region, and both single and two phase flow regions. Heat transfer is more prominent in single phase region due to larger temperature difference between suction line fluid and capillary tube fluid. Relatively superior performance is obtained when the heat exchanger is placed in the single phase region. Chances of condensation are lower as refrigerant quality is relatively higher at the time of inception of vaporisation. Design and simulation are employed to investigate the refrigerant flow behaviour inside the non-adiabatic capillary tube. [Copyright &y& Elsevier]

The rotation of an inertialess ellipsoidal particle in a Newtonian fluid has been firstly analyzed by Jeffery [1]. He found that in the shear flow the particle rotates such that the end of its axis of symmetry describes a closed periodic orbit. The aim of this paper is to discuss the effects of particle inertia on the rotary motion of a fiber in uniform flow fields. We recall the equations of motion and the constitutive equation for the hydrodynamic moment in the case of a slender fiber. These equations are solved numerically for several flow fields. We demonstrate that for the plane flow fields with dominant vorticity (elliptic and rotational flows) the effect of inertia is the slow particle drift toward the flow plane. [ABSTRACT FROM AUTHOR]

Abstract: An assessment of second-order closure models in the prediction of stratified homogeneous shear flows has been conducted. The emphasis is placed on analyzing the effects of three modeled dissipation rate equations on the prediction of equilibrium states. It is found that second-order closure models are unable to reproduce the main features of stratified homogeneous shear flow. In particular, the models considered cannot predict an important experimental observation, namely, the existence of a stationary gradient Richardson number, Rs, below which the turbulent kinetic energy, K, shows exponential growth and, above which, K, decays exponentially. This result is partly due to deficiencies in the modeled dissipation rate equation. It is shown, herein, that the performance of second-order closure models will substantially improve if stationary conditions of the turbulent kinetic energy K and the dissipation rate ε are satisfied. [Copyright &y& Elsevier]

• The conservation equations are established for the natural circulation system. • A set of constitutive models are obtained to solve thermal hydraulic parameters. • The modular modeling is universal for systems with different structures. • Mass flow increases with heat flux increasing and inlet subcooling decreasing. • The influence of phase change depends on the structural characteristics. In this paper, the conservation equations of mass, momentum and energy are established for the natural circulation system. A complete set of constitutive models for solving the steady-state thermal hydraulic parameters are obtained including heat conduction, condenser, pipe, pressurizer, system flow and other auxiliary modules. For calculating natural circulation systems with different structures conveniently, the modular modeling method is adopted. By comparing with the data of different experiments, it proves that modular modeling is universal for systems with different structures, and the correctness of the process of model establishment and model solution is also proved. It is found that the mass flow would increase with the increase of heat flux and the decrease of inlet subcooling of test section. The influence of phase change on the mass flow depended on the structural characteristics of the system. The appearance of two-phase not only increases the driving force of natural circulation system, but also increases the flow resistance. When the increase of driving force is dominant, the appearance of two-phase will further increase the mass flow. Otherwise, the two-phase flow will hinder the increase of mass flow and may even cause its decrease. For the structural design of natural circulation system, the introduction of large form resistance should be avoided as far as possible so as to avoid the establishment of natural circulation process hindered by two-phase flow. The steady-state model of natural circulation system established in this paper has laid a good foundation for the following transient studies and is of great significance. [ABSTRACT FROM AUTHOR]