Convective Transport Phenomena Research Articles

Numerical simulation of entropy generation in thermo-magnetic convection in an inverted T-shaped porous enclosure under thermal radiation

PurposeThermo-magnetic convective flow analysis under the impact of thermal radiation for heat and entropy generation phenomena is an active research field for understanding the efficiency of thermodynamic systems in various engineering sectors. This study aims to examine the characteristics of convective heat transport and entropy generation within an inverted T-shaped porous enclosure saturated with a hybrid nanofluid under the influence of thermal radiation and magnetic field.Design/methodology/approachThe mathematical model incorporates the Darcy-Forchheimer-Brinkmann model and considers thermal radiation in the energy balance equation. The complete mathematical model has been numerically simulated through the penalty finite element approach at varying values of flow parameters, such as Rayleigh number (Ra), Hartmann number (Ha), Darcy number (Da), radiation parameter (Rd) and porosity value (e). Furthermore, the graphical results for energy variation have been monitored through the energy-flux vector, whereas the entropy generation along with its individual components, namely, entropy generation due to heat transfer, fluid friction and magnetic field, are also presented. Furthermore, the results of the Bejan number for each component are also discussed in detail. Additionally, the concept of ecological coefficient of performance (ECOP) has also been included to analyse the thermal efficiency of the model.FindingsThe graphical analysis of results indicates that higher values of Ra, Da, e and Rd enhance the convective heat transport and entropy generation phenomena more rapidly. However, increasing Ha values have a detrimental effect due to the increasing impact of magnetic forces. Furthermore, the ECOP result suggests that the rising value of Da, e and Rd at smaller Ra show a maximum thermal efficiency of the mathematical model, which further declines as the Ra increases. Conversely, the thermal efficiency of the model improves with increasing Ha value, showing an opposite trend in ECOP.Practical implicationsSuch complex porous enclosures have practical applications in engineering and science, including areas like solar power collectors, heat exchangers and electronic equipment. Furthermore, the present study of entropy generation would play a vital role in optimizing system performance, improving energy efficiency and promoting sustainable engineering practices during the natural convection process.Originality/valueTo the best of the authors’ knowledge, this study is the first ever attempted detailed investigation of heat transfer and entropy generation phenomena flow parameter ranges in an inverted T-shaped porous enclosure under a uniform magnetic field and thermal radiation.

Combined effects of Joule heating and binary chemical reaction of MHD Williamson nanofluid on Darcy–Forchheimer porous medium past unsteady stretching cylinder

In this study, the convective transport phenomena of Williamson nanofluid flow with variable thermal conductivity on Darcy–Forchheimer porous medium past unsteady stretching cylindrical sheet is presented. The conservation laws of mass, momentum, energy and concentration are used to formulate the governing boundary value problem of the flow regime. The flow is expressed with a couple of nonlinear partial differential equations. An appropriate similarity transformation and techniques are used to transform the couple of partial differential equations into a system of initial value problems. The system of initial value problems is then solved numerically using the Runge–Kutta fourth order with the shooting technique. The Python programming tool is used to carry out the computation. The influence of unsteady parameter, thermal and concentration Biot numbers, thermal conductivity parameter and other parameters on velocity, temperature and concentration profiles of the nanofluid are examined. The responses of the three transfer rates (i.e momentum, mass and heat) to many pertinent parameters are also investigated. Most flows in industries are in a controlled manner hence the cylindrical geometry is used to study the flow regime. The results of the study show that the convective mass transfer is initiated with a rise in thermal and concentration Biot numbers whereas the convective heat transfer is motivated and demotivated with a rise in thermal and concentration Biot numbers, respectively. Larger values of the unsteady parameter on the other hand creates substantial wall friction to the motion of the nanofluid. Further, the numerical method used is found in excellent agreement with some earlier works under common assumptions.

Convective heat transfer enhancement in an inverted T-shaped porous enclosure through vertical varying circular cylinder

This research aims to enhance convective thermal transport in an inverted T-shaped porous enclosure filled with a water-based hybrid nanofluid, incorporating a circular cylinder at different vertical locations (Case C1-no cylinder, Case C1-cylinder at (0.5, 0.25), Case C2-cylinder at (0.5, 0.5), Case C3-cylinder at (0.5, 0.75)). The numerical investigation employs the penalty finite element technique to simulate the Darcy–Brinkmann–Forchheimer-based mathematical model. Moreover, the complete results of streamlines, isotherms, mean Nusselt number (Num ), and thermal enhancement percentage ( En % ) are analyzed at the broad range of flow parameters, including Rayleigh number ( Ra = ( 10 3 − 10 6 ) ), Darcy number ( Da = ( 10 − 5 − 10 − 2 ) ), and porosity value ( ϵ = ( 0.1 − 0.9 ) ). Initial comparative investigations of different configurations (C0–C3) at selected Ra values reveal that case C1 exhibits significant potential for enhancing convective heat transport phenomena. Consequently, only case C1 physical domain has been explicitly analyzed for convective heat and fluid flow characteristics at the selected range of flow parameters. Further, it is analyzed that the increasing range of Ra, Da, and ϵ enhances the convective heat and fluid flow phenomena. Furthermore, the comparative study of Num and En % for case C1 against the simple case C0 reveals substantial improvements as Ra, Da, and ϵ increase. The En % for Da and Ra reaches up to 97%, while a maximum of 25% improvement is observed with varying values of ϵ. These findings highlight the promising opportunities to optimize convective thermal transport in the investigated system, mainly through adopting case C1.

Indirect urea electrooxidation by nickel(III) in alkaline medium: From kinetic and mechanism to reactor modeling

AbstractThis study consists in the determination of two kinetic laws of urea oxidation (UO) and electrooxidation (UEO) in alkaline media on nickel(III). Two kinds of active sites were examined, the first one derived from a Ni(OH)2 powder and the second from a massive nickel electrode. Partial orders of nickel(III) (two for UO and five UEO) enable to conclude about (i) the urea adsorption on two nickel(III) sites and (ii) that a multistep oxidation of urea occurs involving five nickel(III) site electroregenerations. A multipathway mechanism is also proposed to explain UEO facilitated by the nickel(III)/nickel(II) mediation system, and to predict the by‐products' formation previously identified (). At last, a model combining the UEO kinetic law previously established, with diffusive and convective transport phenomena was developed. A consistent correlation (maximum deviation of 6%) between laboratory electrolysis results with the model' predictions was obtained under different operating conditions, enabling the validation of this model.

THERMO-FLUIDIC CONVECTIVE FLOW STUDY OF HYBRID NANOFLUID IN AN INVERTED T-SHAPED POROUS ENCLOSURE UNDER UNIFORMLY ACTING MAGNETIC FIELD

This research work attempted to analyze the impact of a magnetic field having various inclination angles on the hydrothermal convective flow and heat transport phenomena in an inverted T-shaped porous enclosure saturated with water-based hybrid nanofluid. Firstly, a comparative study for accessing the significance and applicability of the hybrid nanofluid over simple fluid (water) is accomplished at various Rayleigh numbers. Furthermore, the generalized Darcy-Forchheimer-Brinkmann model having magnetic forces is numerically simulated through the penalty finite element approach. The complete study covers the parametric studies in the presence and absence of magnetic field for parameters, including the broad-ranging values of Hartmann number (Ha), inclination angles of magnetic field (ζ), Rayleigh number (Ra), porosity (∈), and Darcy number (Da). Moreover, results obtained from the parametric studies are graphically demonstrated through the isotherms, streamlines, and the variation of mean Nusselt number plots (Nu<sub>m</sub>). The results variation suggests that the heat transfer rate is getting more robust with Da, Ra, and ∈, though the appearance of the magnetic field restricts the fluid flows and heat transport. Additionally, the increasing angle supports the heat transfer process till ζ = 30 up to some extent, and further, it deteriorates till ζ = 90. Thus, a proper inclination and the magnitude of the magnetic field may be a better choice to control the flow regime and the thermal transport phenomena inside the porous domain.

Impact of different arrangements of heated elliptical body, fins and differential heater in MHD convective transport phenomena of inclined cavity utilizing hybrid nanoliquid: Artificial neutral network prediction

In the current scenario, the impact of different arrangements of solid heated elliptical body, discrete heater (LH/H), array of fins with different sizes (1×2, 2×1, 2×2), conductivity ratio (r), magnetic field (Hartmann number, Ha), angle of inclination (β), nanoparticle volume fraction (χ) and Rayleigh number (Ra) is investigated for natural transport phenomena in the cavity containing hybrid nanoliquid. The cavity consists of three equally spaced fins on one wall and heated elliptical body inside the cavity. The homogeneous hybrid nanoliquid (Al2O3-Cu/water) model using experimental data is utilized and results are evaluated for different controlling parameters: Rayleigh numbers (Ra=103 to 106), Hartmann number (Ha=0 to 100), discrete heater (LH/H=0.1 to 0.9), angle of elliptical body (ω=0° to 180°) with three different positions (Center, POS I and II), angle of inclination (β=0 to π), thermal conductivity ratio (r=0.01–100), nanoparticle volume fraction (χ=0 to 0.02) using Galerkin Finite Element Method (GFEM). The mesh grid independency test is also performed for different combinations of meshes and a refined mesh with optimal degrees of freedom (DoFs=190275) has been considered for entire convection range (Ra=103−106).The impact of controlling parameters on average Nusselt number evaluated from the numerical heat transfer analysis has been compared and predicted using feed forward neural network model. The Levenberg-Marquardt back propagation algorithm is used for ANN training based on limited simulated data with optimal number of neurons and average R2 of 0.99988. The quadratic regression model, which is also determined using the MATLAB built-in subroutine ’fitlm’, demonstrates the greatest dependency of Nusselt number on nanoparticle concentration.

Low Reynolds number MHD mixed convection of nanofluid in a corner heated grooved cavity

This study aims to explore mixed magnetohydrodynamic convective transport phenomena in a grooved channel geometry using CuO-H2O nano-fluid flow under low fluid velocity situation. Convective dynamics in such complex geometry have not been studied in detail considering multiphysical scenarios. The fluid enters through the enclosure in cold conditions having temperature Tc and all the walls of the cavity are insulated, except the corner heated with Th temperature. The numerical technique is adopted to solve two-dimensional continuity, momentum, and energy equations. A parametric study is carried out here in terms of Rayleigh number (Ra), Reynolds number (Re), Hartmann number (Ha), and nano-particle concentration (φ). The observations are highlighted in the form of isotherms, streamline plots, and average Nusselt number (Nu) to study the variation of Ra, Re, Ha, and φ on the thermo-fluid coupled process. The current study reveals that for the specified geometry, the change of Ra, Re, and Ha significantly affects the flow pattern along with related heat transfer features as compared to φ. The analysis implied that the optimum heat-transfer performance (Nu = 7.58) is obtained under conditions of Ra = 105, Re = 30, and Ha = 0. The flow separation process becomes more complex and is dictated by the magnetic field intensity.

Assessment of thermal behavior of nanofluid flow in a wavy walled cavity in presence of sliding motion and magnetic field

In this study, magnetohydrodynamic mixed convective heat transport phenomena of CuO-water nanofluid in a double lid-driven cavity having a heated wavy bottom wall is investigated numerically. The cavity in consideration has moving vertical walls, assumed adiabatic. The wavy bottom wall is kept at a high temperature and the top wall is exposed with ambient temperature, respectively. The dimensionless governing equations are discretized and solved using the finite element approach. The impacts of various governing parameters such as the Rayleigh number (Ra = 103–106), Hartmann number (Ha = 0–70), Reynolds number (Re = 10–500), and nanoparticle concentration (φ = 0–3%) are examined and visualized by illustrations of local distributions of streamlines and isotherms. A correlation representing the relationship of the average Nusselt number, Nu in terms of all the examined parameters has been presented. The numerical simulations reveal that the convective heat transfer rate within the wavy enclosure is maximum for higher values of Reynold’s number and nanoparticle concentration and lower values of Hartmann number. Comparisons with other similar published numerical investigations show good agreement with the current outcomes.

Thermo-fluidic transport process in a double-driven cavity with triangular adiabatic obstacles

The present study aims to explore the mixed convective magnetohydrodynamic transport phenomena of nanofluid flow in a differentially heated double-driven-cavity with adiabatic equilateral triangular obstacles inside. The top and bottom walls of the upper and lower cavity are insulated and translating into opposite directions. COMSOL Multiphysics software has been used for conducting the numerical study. The cases of different obstacles size are analyzed. A CuO-water-based nanofluid is filled inside the complex double-driven cavity. A numerical approach is adopted to solve the coupled governing equations using the finite Galerkin method. The convective phenomena and associated heat transfer characteristics are examined systematically for a range of control parameters like Rayleigh numbers (103–105), Hartmann number (0–100), and nano-fluid volume fraction (0–0.05), and the size of the equilateral triangular adiabatic block (0, 0.2 and 0.4). The illustrations of the local distribution of streamlines, isotherms are presented along with the average Nusselt number, Nu. This study shows that the presence of adiabatic blocks not only controls the flow characteristics but also improves the heat transfer rates. Both the characteristics are further strongly influenced by the involved flow controlling parameters and their ranges. The study also illustrates the importance of the size of the adiabatic blocks on heat transfer.

Effect of multibanded magnetic field on convective heat transport in linearly heated porous systems filled with hybrid nanofluid

The paper attempts to enhance the control of convective transport phenomena in magnetothermal devices applying a technique of multibanded magnetic field. For this demonstration, a typical cavity-like thermal system is considered involving linear heating, porous substance, hybrid nanofluid, and magnetic field. Four identical bands of magnetic fields are applied horizontally with uniform inactive zones between the bands. The transport equations of the coupled multiphysics evolving from the thermal buoyancy (due to linear heating at one sidewall and isothermal cooling at the opposite sidewall), filled porous medium, spatially intermittently active magnetic fields, and the engineered working fluid of Cu–Al2O3/water hybrid nanofluid are solved by an indigenously developed computing code. The study is conducted using the pertinent dimensionless parameters for the following ranges: Darcy–Rayleigh number (Ram = 1–104), Darcy number (Da = 10−5 − 10−1), Hartmann number (Ha = 0–70), and concentration of hybrid nanoparticles ϕ (= 0–2%). The convective phenomena are analyzed using the heatlines (for heat transport), streamlines (flow pattern), isotherms (static temperature), and the average Nusselt number (for heat transfer). The outcomes of this technique of multibanded magnetic field are rigorously compared with other established application methods of magnetic fields. It establishes different local behaviors along with an improved heat transfer. Heatline visualization reveals the definite portraits of heat flow paths depending upon parametric values. Furthermore, the presence of linear heating is in particular treated to explore the insight of linear heating (that featuring multiple heating and cooling zones along with the linear heater), utilizing the local Nusselt number and heatlines. One of the important advantages of this new technique is it is more energy-efficient particularly for the square or shallow cavity. The multibanded magnetic field shows a promising technique for the control of convective transport phenomena involving coupled multiphysics used during sophisticated applications (such as materials processing, biomedical applications, etc.).

The influence of turbulent transport in reactive processes: A combined numerical and experimental investigation in a Taylor-Couette reactor

Turbulent reactive flows are ubiquitous in industrial processes. Decoupling transport effects from intrinsic chemical reactions requires an in-depth understanding of fluid flow physics; computational fluid dynamics (CFD) methods have been widely used for this purpose. Most CFD simulations of reactive liquid-phase flows, where the Schmidt numbers are large, rely on isotropic eddy viscosity models. However, the assumption of turbulent isotropy in most stirred reactors and wall-bounded flows is fundamentally incorrect and leads to erroneous results. Here, we apply a systematic CFD approach to simulate liquid-phase diffusive and convective transport phenomena that occur in a Taylor-Couette (TC) reactor. We resolve the turbulent flow by extracting statistics from large eddy simulation which is used to tune the anisotropic Reynolds stress model. In addition, we conducted a series of turbulent precipitation and mixing studies in a TC reactor that was designed and fabricated in-house. The numerical model is successfully validated against a published torque correlation and it is found to accurately describe the advection and diffusion of chemical species. The validated model is then used to demonstrate key flow properties in the reactor. We define new local turbulent Peclét numbers to characterize the relative increase in diffusivity from turbulent advection and observe a 29% increase in the turbulent contribution as Reynolds number is doubled. Both reactive simulations and experiments show an increase in overall reaction rates with increased turbulence. The results from reactive simulations provide a deeper understanding of flow-kinetics interactions at turbulent conditions.

Time step estimates for Lorentz force and thermal driven convective transport

Most physical models for fluid flow and heat transfer obey partial differential equations depending on time and space. Unsteady numerical simulations consist of a considerable number of time steps for which the stability requirement of the explicit time integration has been investigated, while, there is not a suitable method to determine the time step in the computation for fully implicit time scheme and requires a lot of trial and error process. In this paper, a direct approach to estimate the time step in unsteady computation using finite volume method is presented based on characteristic time scale of laminar convective transport phenomena. As an example, this approach is described with buoyant convection in a rectangular pool which is heated due to an electric current generated by an externally applied electric potential difference across the two vertical walls, then, Lorentz force driven convection is considered also. Our theoretical predictions are conducted through full numerical simulations. It is found that the actual time steps in numerical experiments are in good agreement with the scaling analytical predictions. The characteristic time step is deduced and predicted also, not assumed randomly, which provides a basis for the recommendation of the time step for fully implicit time scheme in the finite volume method. This is a key feature that differs from the existing numerical methods.

The extended Graetz problem for micro-slit geometries; analytical coupling of rarefaction, axial conduction and viscous dissipation

In order to support the recent MEMS and Lab-on-a-chip technologies, we studied heat transport in micro-scale slit channel gas flows. Since the micro convection transport phenomena diverges from conventional macro-scale transport due to rarefaction, axial conduction and viscous heating, an accurate understanding requires a complete coupling of these effects. For such cases, we studied heat transfer in hydrodynamically developed, thermally developing gas flows in micro-slits at various flow conditions. The analytical solution of the energy equation considered both the heat conduction in the axial direction and heat dissipation of viscous forces. Furthermore, updated boundary conditions of velocity slip and temperature jump were applied based on Knudsen number of flow in order to account for the non-equilibrium gas dynamics. Local Nusselt number (Nu) values were calculated as a function of Peclet (Pe), Knudsen (Kn) and Brinkman (Br) numbers which were selected carefully according to possible micro-flow cases. Strong variation of Nu in thermal development length was found to dominate heat transfer behavior of micro-slits with short heating lengths for early slip flow regime. For this instance, influence of axial conduction and viscous dissipation was equally important. On the other hand, high Kn slip flow suppressed the axial conduction while viscous heating in a small surface-gas temperature difference case mostly determined the fully developed Nu and average heat transfer behavior as a function of Kn value.

Modeling transport phenomena of ice slurry in an ice forming unit

In this article, multiphase convective and solidification transport phenomena of ice slurry is investigated by developing a one-domain macroscopic model to simulate its formation in a rectangular ice forming unit. Convection, sedimentation, interfacial drag, permeability, remelting and viscosity variations are incorporated into this model through the appropriate governing multiphase transport equations. Validation studies with literature data are performed to determine the most suitable drag law by comparing the position of the interface between the coherent and non-coherent zones. After establishing modified Stokes' Law as the best-suited drag model, solidification study of an aqueous ammonium chloride solution is performed. The results for the evolution of ice fraction, species distribution, temperature profile and multiphase velocity field are presented. Solid fraction gradient, due to the generation of ice particles, has a major influence on the density gradient leading to a counter-clockwise flow current resulting in the homogenization of temperature and species in the generated ice slurry.

Experimental study of formation and development of coherent vortical structures in pulsed turbulent impinging jet

Coherent structures play a crucial role in enhancement of convective transport phenomena. An experimental investigation was carried out to study the effect of jet flow oscillation on the characteristics of vortex structures in a turbulent confined circular impinging jet. Effects of key parameters such as frequency of pulsation and type of excitation on the development of shear layer coherent vortical structures in time and space and their dynamics were studied using the smoke-wire technique and high speed photography. Results of this study show that flow pulsation in a circular impinging jet leads to the periodic formation of coherent vortical structures which are larger than those in steady impinging jets. Pulsation frequency has significant effects on the formation and size of structures and consequently their dynamics. At higher frequencies, vortex structures are formed in a regular inline pattern and as a result the mixing between the jet fluid and stagnant fluid is reduced. For the step (rectangular) signal shape larger vortices are created in comparison with those in sinusoidal one. The results of flow visualization show that enhanced mixing and transport rates will be achieved for turbulent impinging jets that excited with “step” signal shape.

Mechanistic modeling of vacuum UV advanced oxidation process in an annular photoreactor

A novel mechanistic model that describes the vacuum UV advanced oxidation process in an annular photoreactor initiated by 172 nm and 185 nm (in combination with 253.7 nm, with and without exogenous H2O2) is presented in this paper. The model was developed from first principles by incorporating the vacuum UV-AOP kinetics into the theoretical framework of in-series continuous flow stirred tank reactors. After conducting a sensitivity analysis, model predictions were compared against experiments conducted under a variety of conditions: (a) photo-induced formation of hydrogen peroxide by water photolysis at 172 nm (for both air- and oxygen-saturated conditions); (b) photo-induced formation of hydrogen peroxide by water photolysis at 185 + 253.7 nm (in the presence of formic acid, with and without the initial addition of hydrogen peroxide); (c) direct photolysis of hydrogen peroxide by 253.7 nm; (d) degradation of formic acid by 185 + 253.7 nm (with and without initial addition of hydrogen peroxide); and (e) degradation of formic acid by 253.7 nm (with the addition of exogenous hydrogen peroxide). In all cases, the model was able to accurately predict the time-dependent profiles of hydrogen peroxide and formic acid concentrations. Two newly recognized aspects associated with water photolysis were identified through the use of the validated model. Firstly, unlike the 185 nm and 253.7 nm cases, water photolysis by the 172 nm wavelength revealed a depth of photoactive water layer an order of magnitude greater (∼230–390 μm, depending on the specific operating conditions) than the 1-log photon penetration layer (∼18 μm). To further investigate this potentially very important finding, a computational fluid dynamics model was set up to assess the role of transport mechanisms and species distributions within the photoreactor annulus. The model confirmed that short-lived hydroxyl radicals were present at a radial distance far beyond the ∼18 μm photon penetration layer. Secondly, kinetic simulations showed that the higher penetration depth of hydroxyl radicals was not caused by diffusive or convective transport phenomena but rather the effect of non-linear behavior of the complex reaction kinetics involved in the process.

A novel center-of-mass method to measure fluid velocity with MRI

Major obstacles in effective local drug delivery to a target site include non-invasive measurement of the concentration distribution after local administration. Herein, the development of a non-invasive in vitro magnetic resonance imaging (MRI) method is described to quantitatively study the distribution of drug surrogate and to measure fluid flow velocity in the region of interest (ROI). Dynamic contrast enhanced MRI was used to study diffusion–convection transport phenomena of a magnetic resonance contrast agent, Gd-DTPA, in 1% agarose gel. The relationship between the concentration of Gd-DTPA and T1 relaxation time was determined using an inversion recovery MRI technique. The concentration distribution of Gd-DTPA images was estimated from a calibration curve relating T1 relaxation time and the concentration of Gd-DTPA. Using the estimated concentration profiles, center-of-mass points were calculated in a series of time points in order to determine fluid flow velocity, which correlated well with the real volumetric flow velocity at early time points. In this study, we developed a method to analyze MR images quantitatively and to determine fluid flow velocity through a tissue in vivo.

Ultrasonic Intensification of Grape Stalk Convective Drying: Kinetic and Energy Efficiency

Grape stalk is a by-product of the winemaking process with a high antioxidant content. Drying is a necessary stage before antioxidant extraction, which may affect not only kinetic and energy efficiency but also product quality. Coupling non-thermal technologies, such as power ultrasound, to convective drying is considered a strategy for process intensification in order to prevent certain drawbacks of conventional technologies. In this work, the use of power ultrasound in the convective drying of grape stalk was tested in order to estimate its influence on kinetic and energy efficiency. For this purpose, convective drying kinetics of grape stalk were carried out with and without power ultrasound application (21.8 kHz, at two ultrasonic power levels: 45 and 90 W). In addition, the inlet and outlet air temperatures of the drying chamber were monitored. The drying process was modeled considering heat and mass transfer phenomena jointly with the total energy consumption and the energy efficiency of the system. Power ultrasound application involved a shortening of grape stalk drying time, which was dependent on the drying air temperature (40 and 60°C) and the ultrasonic power applied (45 and 90 W). The modeling showed the increase in diffusion and convective heat transport phenomena produced by ultrasound application, despite grape stalk being a low-porosity product and, therefore, only slightly sensitive to ultrasonic effects. In addition, it was also highlighted that ultrasound application increased the energy efficiency during the drying of grape stalk.

Energy efficiency in process industry – High-efficiency vortex (HEV) multifunctional heat exchanger

In the process industry, vortex generators are being increasingly incorporated in modern multifunctional heat exchangers/reactors to enhance heat and mass transfer and thus increase energy efficiency. Longitudinal and transverse pressure-driven vortices and shear-instability-driven flow structures generated by flow separation behind the vortex generators play a crucial role in convective transport phenomena. The purpose of this work is to demonstrate experimentally the effects of hydrodynamics on the transfer processes accompanying such flows. The high-efficiency vortex (HEV) is an innovative static mixer and a low energy consumption heat exchanger designed to exploit these types of vortices. Heat transfer results obtained in turbulent flow with embedded vorticity in this multifunctional heat exchanger are compared with numerical results in the literature. Both numerical and experimental results confirm the high energy efficiency of the HEV static mixer flow.

Thermal Transport Characteristics of Mixed Pressure and Electro-Osmotically Driven Flow in Micro- and Nanochannels With Joule Heating

This study investigates convective transport phenomena of combined electro-osmotic and pressure-driven flow in a microchannel subject to constant surface heat flux, with Joule heating effect taken into account. The governing system of equations includes the electric potential field, flow field, and energy equations. Analytical solutions are obtained for constant fluid properties, while numerical solutions are presented for variable fluid properties. For constant properties, the problem is found to be governed by three ratios: the length scale ratio (the ratio of Debye length to half channel height), the velocity scale ratio (the ratio of pressure-driven velocity to electro-osmotic velocity), and the ratio of Joule heating to surface heat flux. A small length scale ratio corresponds to a microchannel, while finite length scale ratio represents a nanochannel. For electro-osmotic flow only, the momentum transport is solely a function of the length scale ratio. For combined electro-osmotic and pressure-driven flow, the velocity profile and therefore the friction factor depend on both the length scale ratio and the velocity scale ratio. Assuming a thermally fully developed flow, analytical expressions for the normalized temperature profile and Nusselt number are developed. The representative results for the friction factor, normalized temperature profile, and Nusselt number are illustrated for some typical values of the three ratios. For purely electro-osmotic flow, it is found that the Nusselt number increases with decreasing ε, approaching the value for slug flow as the length scale ratio approaches zero. For mixed flow with a given length scale ratio, the results show that the Nusselt number decreases with the velocity scale ratio, approaching the classical Poiseuille flow as the velocity scale ratio approaches infinite. When the effects of variable fluid properties are included in the analysis, numerical solutions are generated to explore the influence of thermal conductivity and viscosity variations with local temperature on the hydrodynamic and thermal characteristics of the fluid. These temperature-dependent property variations would initially develop pressure-driven flow, and correspondingly the dimensionless velocity and volume flow rate increase to account for such variations. The friction factor reduces considerably with viscosity variation, while the Nusselt number increases gently. Although the influence of thermal conductivity variation on the hydrodynamic characteristics is not impressive, it has certain impact on the heat transfer results; more specifically, increasing the conductivity variation will produce a sensible increase in Nusselt number but a small decrease in the normalized temperature.