Effect Of Convection Research Articles

Unsteady nanofluid flow over a cone featuring mixed convection and variable viscosity

This article addresses unsteady nanofluid flow over a cone with MHD and mixed convection effects. Effects of variable viscosity and viscous dissipation are also considered. The resulting system of equations is tackled through the Homotopy Analysis Method (HAM). The impact of different influential variables on skin friction coefficient, heat and mass flux are discovered through numerical tables and graphs. It is noted that the surface drag force in x and y directions increases against the buoyancy force parameter. Also, it is observed that the tangential and azimuthal velocity decrease against the variable viscosity parameter. Furthermore, the temperature of fluid is observed to decay against the unsteady parameter but it increases against the Eckert number.

Enhanced scale-up performance on residence time distribution by integrated microcapillaries with high size uniformity

Continuous tubular flow reactors are widely used by virtue of their simple structures, controllable reaction conditions, enhanced heat and mass transfer, etc. Microreactors present specific microscale characteristics including laminar flow, reduced diffusion path, enhanced mixing degree and high surface area to volume ratio. Microcapillary reactors combined the advantages of continuous processing and microscale reactions. However, the scale-up of microreactions is still a very challenging task for the loss of enhanced transport characteristics in microscale. Residence time distribution (RTD) is a vital research method to investigate the flow behaviors of microreactors. Microcapillary films (MCFs) containing several parallel microcapillaries provide a strategy to multiplex the microreactions. Nevertheless, the low size uniformity of microcapillaries in MCF strips hinders their application in microreactions. We carried out Computational Fluid Dynamic (CFD) simulations with experimental RTD validation to investigate the effects of dimension, convection and diffusion on RTD and the relationship between the RTD diversity and the size uniformity of microcapillaries under fixed velocities and volumetric flow rates. MCFs with high size uniformity were manufactured by liquid-assisted extrusion, and the coefficient of variance of the mean residence times of the microcapillaries in the MCF strip was only 2.8%. Neutralization reaction also showed the consistence of reaction performances among microcapillaries. With the enhanced scale-up performance on RTD and microreactions, MCFs are expected to have broad applications in large-scale microreactions in chemical engineering industry.

Convection‐assisted intra‐vitreous drug delivery in human eye: An experimental investigation

AbstractWe report an in vitro transient experimental study of the human eye to determine the effects of convection on the transport of drugs from the point of injection to the retinal surface through the fluid in the vitrectomized eye. The human eye is modeled as a glass orb of realistic proportions, and water and silicone oil mimic the vitreous fluid. In a series of experiments, the dye that is used to mimic the drug is injected at two different spots in the vitreous chamber, and its transport through the vitreous fluid mimic to the retinal surface is recorded under conditions of simple diffusion and convection‐assisted diffusion. The drug concentrations at two different spots on the retinal surface at different times are monitored. It is shown that the drug reaches all parts of the retinal surface much faster during convection‐assisted diffusion than during pure diffusion, irrespective of where it is injected in the cavity. In a snapshot, it is seen that the dye concentration reaches at the target central retinal pigmented epithelium spot in just 12 min (0.2 h) with convection‐assisted diffusion as against 12 h in pure diffusion. The steady‐state average drug concentration at the target retinal region is seen to be 56.25 times greater for the convection‐assisted drug transport than for the pure diffusion case. The results show that applying mild heat to the retinal surface by methods such as laser irradiation can induce convection‐based transport of the drug in the vitreous fluid. This method makes the drug available much faster at the target retinal region than drug transport by pure diffusion. The faster transport of the drug within the cavity can expedite the drug's effect and prevent drug loss due to delays.

Finite element analysis on heat convection effect on air temperature distribution in a heated room

Using the finite element method to solve heat transfer problem in solid medium has been popular since last century, but heat convection effect in fluid medium makes the problem cumbersome to solve efficiently. Thermal transfer calculation of fluid medium is complicated, because solving heat convection and Navier-Stokes equation is hard. By making proper assumptions, intricate equations can be skipped, and it is possible to give rough estimate of heat transfer result through a simple and efficient calculation. Which is significant for design and verifiction of heating system for physicists and engineers. The study aims to solve the temperature distribution of a certain heat system, using finite elment method and with given assumpted air circumfluence. In the calculation, the room domain is subdivided into finite elments, and the boundary condition is given according to the heat influx and emmision, air circumfluence is introduced to simulate the effect of convection. The simplified model illustrated that the finite element method is effective in giving quantitative result of heat transfer calculation. It also qualitatively showcased the significance of heat convection, when the result is compared to calculation without any air flow.

Exergy Analysis of a Shell and Tube Energy Storage Unit with Different Inclination Angles

To optimize the utilization of solar energy in the latent heat thermal energy storage (LHTES) system, this study conducts exergy analysis on a paraffin-solar water shell and tube unit established in the literature to evaluate the effects of different inclination angles, inlet temperatures, original temperatures, and fluid flow rates on the exergy and exergy efficiency. Firstly, the thermodynamic characteristics of the water and the natural convection effects of the paraffin change with different inclination angles. When the inclination angle of the heat storage tank is less than 30°, the maximum exergy inlet rate rises from 0 to 144.6 W in a very short time, but it decreases to 65.7 W for an inclination angle of 60°. When the inclination angle is increased from 0° to 30°, the exergy efficiency rises from 86% to 89.7%, but it decreases from 94% to 89.9% with the inclination angle from 60° to 90°. Secondly, under the condition that the inclination angle of the energy storage unit is 60°, although increasing the inlet temperature of the solar water enhances the exergy inlet and storage and reduces the charging time, it increases the heat transfer temperature difference and the irreversible loss of the system, thus reducing the exergy efficiency. As the inlet water temperature is increased from 83 to 98 °C, the exergy efficiency decreases from 94.7% to 93.6%. Moreover, increasing the original temperature of the LHTES unit not only reduces the exergy inlet and storage rates but also decreases the available work capacity and exergy efficiency. Finally, increasing the inlet water flow rate increases the exergy inlet and storage rates slightly. The exergy efficiency decreases from 95.6% to 93.3% as the unit original temperature is increased from 15 to 30 °C, and it is enhanced from 94% to 94.6% as the inlet flow rate is increased from 0.085 to 0.34 kg/s with the unit inclination angle of 60°. It is found that arranging the shell and tube unit at an inclination angle is useful for improving the LHTES system’s thermal performance, and the exergy analysis conducted aims to reduce available energy dissipation and exergy loss in the thermal storage system. This study provides instructions for solar energy utilization and energy storage.

Stochastic Levenberg–Marquardt Neural Network Implementation for Analyzing the Convective Heat Transfer in a Wavy Fin

The present research examines the steady, one-dimensional thermal distribution and heat transfer of a wavy fin. This heat transfer analysis considers convective effects as well as temperature-dependent thermal conductivity. Furthermore, a novel implementation of a neural network with backpropagated Levenberg–Marquardt algorithm (NN-BLMA)-based machine learning intelligent strategies is provided to interpret the heat transfer analysis of a convective wavy fin. The non-linear ordinary differential equation (ODE) of the study problem is converted into its non-dimensional form using the similarity transformation technique. The dimensionless equation obtained is then numerically explored via the Runge–Kutta–Fehlberg scheme. A data set for varying the pertinent parameters is generated, and an artificial neural network model is designed to estimate the heat transfer behavior of the wavy fin. The effectiveness of the proposed NN-BLMA is subsequently endorsed by analyses using a regression model, mean square error, and histograms. The findings of comprehensive computational parametric studies illustrate that the presented technique, NN-BLMA is an effective convergent stochastic numerical solver employed for the heat transfer model of the convective wavy fin. The wavy fin’s temperature dispersion optimizes as the thermal conductivity parameter rises. Heat transfer rate is higher in wavy fin compared to rectangular fin.

Boundary layer and heat transfer analysis of mixed convective nanofluid flow capturing the aspects of nanoparticles over a needle

Since their significance in commercial and scientific procedures has been recognized, studies regarding boundary layer flow and heat transfer have increased. The goal of this research is to examine the effects of hydromagnetic forces, mixed convection, and joule heating on ethylene glycol-based nanofluids suspended above a thin vertical needle. For thermal conductivity and viscosity, both aggregation and non-aggregation models are used. Here, we employed the similarity transformation to turn partial differential equations (PDEs) into dimensionless ODEs. Skin friction coefficients, Nusselt numbers, velocities, and temperature profiles were computed over a wide range of parameter values. The velocity and temperature profiles generated by the models that account for aggregation are superior to those generated by the homogeneous model. Velocity profile increases with λ and ε and decreases with M and e. Thermal profile increases with ϕ,M,e and Ec and decreases against λ. Skin friction and Nusselt number increases with increasing values of ϕ and λ towards ε. When comparing the findings from the present research to those from the past, it is clear that there is a high level of concordance between the two. Data availability“Data will be available on demand from Bilal.Ali”.

Effect of external magnetic field on mass transport of reactive species in the reaction between nano zero valent iron and selenate anions in a simulated flue gas desulfurization wastewater

The mechanism involved in the reaction of regulated ions with nano Zero-Valent Iron (ZVI) in the presence of an applied weak magnetic field (WMF) was investigated. The WMF effect on mass transport of reacting ions and morphology and composition of the oxide layer in nano ZVI was elucidated using Electron Energy Loss Spectroscopy (EELS). The magnetic gradient force produced by ferromagnetic ZVI particles had a greater influence on the rate of selenate diffusion in solution than the Lorentz force due to the externally applied weak magnetic field. Moreover, the magnetic gradient force was suggested to suppress ZVI corrosion reactions. Despite changes in the iron oxide layer under the WMF influence, the rate of selenate reduction has not been significantly affected over short term lasting several hours. Most crucially, WMF had a negligible effect on the rate of mass transport of selenate anions, as compared to the dominant effect of forced convection.

A self-intensified inverted evaporation model constructed by high entropy spectrally selective absorber coating (SSAC) for brine water concentration

One strategy proposed before for applying spectrally selective absorber coating (SSAC) is to design separating structure for dividing the photo-thermal layer with steam generation layer. However, this separation concept faces paradox between salt-prevention ability and heat conduction loss, still has huge room for evaporation performance improvement. Herein, we designed an inverted evaporation model which horizontally deploys a high entropy spectrally selective absorber coating (SSAC) by adhering it upon upside of cloth that possessing strong water absorbing capacity. In the function of self-intensified convective airflow effect, this SSAC-assembled inverted evaporation model can achieve super high productivity (4.98 kg m−2 h−1). Resorting to hydromechanics-based analysis method comes to a conclusion that it probably the pressure difference that gives rise to promotion of steam generation, thus realizing the high-efficiency steam generation in inverted evaporation model. By jointly introducing the hanging structure for desalination, the concentrated brine water can be simultaneously collected through dripping from the bottom of the surface without the interface being solid-salt blocked. This work demonstrates the great potential of SSAC-based hanging and inverted evaporation structure for long-term practical desalination and promising application in separation industry.

Improvement of flow field uniformity and temperature field in gasoline engine catalytic converter

The uneven temperature field, flow field, and internal low temperature of the catalytic converter can cause substrate thermal deformation and catalytic ability decrease, which is not conducive to the service life of catalytic converter and control of automotive pollutants emission. A novel substrate structure with uniformly changes cell density and a new heater of three layers diffuser with optimized flow field uniformity are proposed. Their application in catalytic converter can effectively improve the above problems. Firstly, physical and mathematical models of the new catalytic converter were established, and the accuracy was verified by experiments. Next, the flow characteristics and catalytic performance was analyzed. Finally, the effects of different parameters on catalytic performance and temperature were analyzed. The results show that: (1) Compared with traditional catalytic converter, gas uniformity and NO conversion efficiency of the new catalytic converter are increased by 0.0885% and 13.66%, respectively, improving flow field uniformity and NO conversion. (2) The NO conversion efficiency increases and then decreases with adding of electric power, exhaust inlet temperature and exhaust inlet velocity; substrate temperature increases with adding of electric power and exhaust inlet temperature, but decreases with increase of exhaust velocity. During heating time, the NO conversion efficiency and substrate temperature first increase and then tend to stabilize. After heating is stopped, the NO conversion efficiency and substrate temperature will decrease. (3) The field synergy theory shows that synergy angle cosβ tends to − 1 with increase of electric power, which means convective heat transfer effect inside substrate becomes worse. (4) The grey relational analysis shows that exhaust inlet temperature is a key factor affecting the new catalytic converter performance.

Isotopic fractionation of chlorine and potassium during chloride sublimation under lunar conditions

The Moon is depleted in volatile elements and compounds, and lunar samples exhibit a wide range of Cl isotopic compositions, which is believed to result from the volatilization of metal chlorides (e.g., NaCl, KCl, and FeCl2). However, the Cl isotopic fractionation behavior during volatilization is not well constrained, particularly for metal chlorides. Furthermore, the effect of metal chloride evaporation on metal isotopes is poorly known. In the present study, we performed NaCl and KCl sublimation experiments to study Cl and K isotopic fractionations at temperatures ranging from 923 K to 1061 K and at pressures of 7×10–5 bar to 1 bar in an N2 atmosphere. The isotope fractionation factors of 37/35Cl(αgas–solid) from NaCl sublimation experiments are 0.9985±0.0002, 0.9958±0.0004, and 0.99807±0.00004 at 1, 10–2, and 7×10–5 bar, respectively. Those of 41/39K(αgas–solid) and 37/35Cl(αgas–solid) from KCl sublimation experiments are 0.99884±0.00004 and 0.9988±0.0003 at 1 bar, 0.9977±0.0002 and 0.9972±0.0003 at 10–2 bar, and 0.9989±0.0002 and 0.9989±0.0001 at 7×10–5 bar, respectively. Chlorine and K isotopes fractionate more at 10–2 bar than at 7×10–5 bar and 1 bar. The saturation index in all the sublimation experiments was >95%, which resulted in near-equilibrium isotopic fractionation at the sublimation interface. Therefore, the isotopic fractionation was controlled by mass transfer processes in the gas and solid phases. The isotopic fractionation at 10–2 bar was controlled by the chemical diffusion of sublimated gas in an N2 atmosphere with almost no convection effect, (i.e., Pe number close to zero), whereas the isotopic fractionation at 1 bar was suppressed by atmospheric convection with a turbulence factor of 0.4±0.1 (i.e., Pe number >1). The extremely high sublimation rate and the very slow diffusion in the sublimating solid at 7×10–5 bar suppressed isotopic fractionations. Based on our experimental results, calculations using Cl/K and Na/K in lunar materials reveal that degassing of KCl contributed very little (<0.2‰) to the K isotopic fractionation (>0.58‰) during lunar magma ocean degassing. The Cl isotopic fractionation factor from lunar samples is similar to our results at 10–2 bar. This similarity of Cl isotope fractionation indicates that there may have been a transient atmosphere above the lunar magma ocean.

Uniform stability for local discontinuous Galerkin methods with implicit-explicit Runge-Kutta time discretizations for linear convection-diffusion equation

In this paper, we consider the linear convection-diffusion equation in one dimension with periodic boundary conditions, and analyze the stability of fully discrete methods that are defined with local discontinuous Galerkin (LDG) methods in space and several implicit-explicit (IMEX) Runge-Kutta methods in time. By using the forward temporal differences and backward temporal differences, respectively, we establish two general frameworks of the energy-method based stability analysis. From here, the fully discrete schemes being considered are shown to have monotonicity stability, i.e. the L 2 L^2 norm of the numerical solution does not increase in time, under the time step condition τ ≤ F ( h / c , d / c 2 ) \tau \le \mathcal {F}(h/c, d/c^2) , with the convection coefficient c c , the diffusion coefficient d d , and the mesh size h h . The function F \mathcal {F} depends on the specific IMEX temporal method, the polynomial degree k k of the discrete space, and the mesh regularity parameter. Moreover, the time step condition becomes τ ≲ h / c \tau \lesssim h/c in the convection-dominated regime and it becomes τ ≲ d / c 2 \tau \lesssim d/c^2 in the diffusion-dominated regime. The result is improved for a first order IMEX-LDG method. To complement the theoretical analysis, numerical experiments are further carried out, leading to slightly stricter time step conditions that can be used by practitioners. Uniform stability with respect to the strength of the convection and diffusion effects can especially be relevant to guide the choice of time step sizes in practice, e.g. when the convection-diffusion equations are convection-dominated in some sub-regions.

Viscoelastic boundary layer analysis of constant surface temperature plate embedded in saturated porous media

AbstractMathematical models and numerical solutions of Williamson fluid flow under influences of various boundary conditions provide important support to experimental studies in the solar energy field. Therefore, the present study is concerned with the effects of forced convection of the viscoelastic boundary layer on a horizontal plate embedded in saturated porous media subjected to constant surface temperature. The study explores the profiles of shear stress, velocity, temperature, and heat transfer coefficient. The governing equations in nondimensional forms are obtained by using a model of Darcy–Forchheimer–Brinkman and finally are solved numerically by using bvp4c with MATLAB package. The results of the numerical solution show an insignificant rise in the distribution of the velocity boundary layer and shear stress profile as the Darcy parameter is increased, while a decrease in the temperature and Nusselt numbers are found. On the other hand, as the viscoelastic parameter is increased, the Darcy parameter shows a reverse response. Finally, insignificant increases in profiles of boundary layer velocity, temperature, shear stress, and Nusselt number are observed at high values of the Forchheimer number.

Elucidating the role of interlayer structure in the permeance-selectivity trade-off of ceramic nanofiltration membrane

Reasonable design of a rational interlayer in an asymmetric ceramic nanofiltration (NF) membrane facilitates water transport as well as solute retention. Nevertheless, the inherent relationship between the microstructure of the interlayer and the permselectivity in ceramic NF membrane remains to be unclear. In this study, a quantitative model is developed to predict the influence of the factors (i.e., particle diameter and layer thickness) controlling the structure of interlayer on the permselectivity by integrating the cake filtrated theory and Donnan-Steric Pore model with the Dielectric Exclusion model. The simulations demonstrate that interlayer thickness dominates the solute rejection rather than particle diameter that constitutes the interlayer when an appropriate top-layer structure is applied. However, the enhancement of water permeance requires simultaneous regulation of the microstructures of both interlayer and top layer. For negatively charged ceramic NF membranes, the comprehensive effect of diffusion, convection, and electro-migration is the transport mechanism of positively charged counter-ion (i.e., Na+), whereas the transport of negatively charged co-ion (i.e., SO42-) is merely dominated by diffusion. This study thereby elucidates the role of the interlayer in terms of improving the permeance-selectivity trade-off relationship, which is of great significance to the future development of a high-performance ceramic NF membrane.

Study of transient behaviors of wavefront error caused by natural convection for improving mirror seeing.

Astronomical seeing parameters calculated based on the Kolmogorov turbulence model cannot fully evaluate the effect of the natural convection (NC) above a solar telescope mirror on the image quality, as the convective air motions and temperature variations of the NC are significantly different from the Kolmogorov turbulence. In this work, a new method based on the transient behaviors and frequency characteristics of NC-related wavefront error (WFE) are investigated in detail and used to evaluate the image quality degradation caused by a heated telescope mirror, aiming to make up for the deficiency of astronomical seeing parameters with the conventional method in evaluating the image quality degradation. Transient computational fluid dynamics (CFD) simulations and WFE calculations based on discrete sampling and ray segmentation are performed to quantitatively evaluate the transient behaviors of the NC-related WFE. It clearly exhibits apparent oscillatory characteristics, which are coupled by main oscillation with low frequencies and minor oscillation with high frequencies. Moreover, the generation mechanisms of two types of oscillations are studied. The conspicuous oscillation frequencies of the main oscillation caused by heated telescope mirrors with varying dimensions are primarily lower than 1 Hz, suggesting that active optics may be adopted to correct the main oscillation of NC-related WFE while the adaptive optics may correct the minor oscillation. Furthermore, a mathematical relationship between WFE, temperature rise, and mirror diameter is derived, revealing a significant correlation between WFE and mirror diameter. Our work suggests the transient NC-related WFE should be considered as one of the critical supplements to the mirror seeing evaluation.

A practical design equation for accurate quantification of CRRT filter design factors and convection effects.

Considering the increasing clinical need for continuous renal replacement therapy (CRRT), further improvement in therapeutic efficacy has become an important focus for researchers. Here, we designed nine CRRT filters with various combinations of hollow fiber packing density (PD) and housing shape (effective hollow fiber length (L) and inner housing diameter (D) ratio (L/D ratio)) to evaluate the clearance of middle molecular uremic toxins (MMs) via simulation of an in vitro continuous veno-venous hemodialysis treatment model. We also used Doppler ultrasonography to measure the maximum internal filtration flow rate (QIF-Max) as an aid; this approach facilitated an exploration of the impacts of various design factors on convection effects, while revealing the mechanisms influencing MM removal performance. Furthermore, we constructed a multiple linear regression model of design factors and QIF-Max, then conducted experimental verification. Finally, we proposed an accurate and practical design equation to quantify the design factors influencing CRRT filters and convection effects: , where N/D2 and L/D affect QIF-Max by 15.0% and 85.0%, respectively. This design equation was able to effectively quantify the convection effects of CRRT filters with different design factors, thereby predicting MM removal performance; this convenient design equation can support the development of CRRT-related products.

High-performance GPU computing of phase-field lattice Boltzmann simulations for dendrite growth with natural convection

The effect of natural convection on dendrite morphology is investigated through three-dimensional large-scale phase-field lattice Boltzmann simulations using a block-structured adaptive mesh refinement scheme with the mother-leaf method in a parallel-GPU environment. The simulations confirmed that downward buoyancy enhances the growth of the primary and secondary arms, and upward buoyancy delays the growth of those arms. In addition, the effect of natural convection on the solidification morphologies gradually decreased as the primary arm tip reached the top of the computational domain and finally stopped. Furthermore, in the longer simulation under purely isothermal diffusive conditions, detachment of the secondary arms owing to curvature-driven fragmentation was observed. A large-scale non-isothermal dendrite growth simulation was also conducted, wherein it was observed that the tip growth rate of the primary arm was delayed, and the secondary arm spacing was larger than that in the isothermal condition.

Phase-field lattice Boltzmann simulation of three-dimensional settling dendrite with natural convection during nonisothermal solidification of binary alloy

Understanding the motion and growth behaviors of equiaxed dendrites during solidification is important for predicting macrosegregation. In this study, we develop a phase-field lattice Boltzmann (PF-LB) simulation method for the settling and growth of an equiaxed dendrite during the nonisothermal solidification of a binary alloy. The PF-LB computations are accelerated by employing parallel computation using multiple graphic processing units (GPUs) and the octree block-structured adaptive mesh refinement method, which incorporates multiple mesh and time increment methods. By using the developed method, we can simulate the three-dimensional long-distance settling dendrite while considering the effects of latent heat release and natural convection. From the simulation results, we confirm that the natural convection due to the high solute concentration around a dendrite reduces the settling velocity. In addition, we observe that the temperature increase owing to latent heat release slows dendrite growth, which in turn slightly slows the settling velocity. From these results, we confirm that the effects of latent heat release and natural convection are not negligible in the quantitative evaluation of settling dendrites.

A thermal modelling of triple tube heat exchanger using phase change material

This paper contains a numerical investigation comparing a series of data for the application of energy storage using a triple tube heat exchanger. The main objective is to study the effect of free convection for a uniform melting in a certain thickness of phase change material (PCM). In addition to that, a non-dimensionless parametric study has been conducted to evaluate the effect of thermal behaviour with the change of the parameters. In this study, the main focus has been kept on studying the effect of inlet temperature and inlet velocity of the heat transfer fluid (HTF) which is described by Stefan number (Ste) and Reynolds number (Re) respectively. The result obtained from the analysis showed that, inlet temperature has a direct impact on achieving uniform melting and lesser melting time however inlet velocity has no such effect on that due to dominant effect of free convection. The vertical orientation of the heat exchanger was also found helpful towards congruent melting as it is influencing the buoyancy force and created a motion in the PCM region.

Velocity discontinuity propagation model validation using an approximate point source in motion.

The classic geometric acoustic solution for propagation through a mean flow velocity discontinuity is evaluated experimentally using an approximate point source in motion. The geometric approximation of the pressure field in the frequency and time domains is, first, revisited for arbitrary subsonic flow and source motion along the flow axis. The derivation, computed via the method of stationary phase, shows the expected Doppler behavior of the radiated acoustic field due to the source motion acting in conjunction with the convective amplification effect for a stationary source in flow. The model is validated with a minimally intrusive, approximate point source of heat in the Quiet Flow Facility at the NASA Langley Research Center. Within the limitations of the experiment in this open-jet wind tunnel, data generally show agreement with the model across a range of flow speeds and microphone positions for a broad range of frequencies. Where disagreement between measurement and theory is noted, possible causes are discussed.