Variable Flux Research Articles

Influence of upstream and downstream flow on different packed bed configurations in phase change process

One approach to storing latent heat in large quantities is through PBSS (Packed bed storage system) with spheres filled with PCM (Phase Change Materials). In these storage systems, an HTF (heat transfer fluid) flows over the spheres. So, both the sphere wall temperature and the heat flux can vary depending on the geometric conditions of the bed of spheres, the PCM thermodynamic conditions, and the HTF thermal conditions. Thus, this work aims to verify the influence of the thermal conditions of the external flow on the PBSS filled with PCM in the melting process. Cases with variable heat flux as well as uniform temperature and uniform heat flux conditions were tested. Three sphere positions, 4 sphere spacing ratios (blockage ratio), and three temperature differences between HTF and PCM phase change temperature were also analyzed, for two external flow directions (upstream and downstream), totaling 74 different configurations. The study found that the uniform temperature condition resulted in a 33 % faster total melting time than the uniform heat flow condition. Additionally, the upstream flow showed a total melting time that was approximately 24 % shorter than the flows in the opposite direction. The relationship between the total melting time and the temperature differences was found to be non-linear. Moreover, the blockage ratio showed a non-linear dependence with total melting time. The configuration with a blockage ratio of 1.125 demonstrated the shortest total melting time, with values on average 22.4 % shorter than the blocking ratio of 0.550, which had the longest melting times. These results demonstrate that optimizing external flow thermal conditions is crucial for improving the performance of PBSS with PCM-filled spheres in large-scale latent heat storage systems.

A Low-Cost Programmable Reversing Flow Column Apparatus for Investigating Mixing Zones.

This note describes the development and testing of a novel, programmable reversing flow 1D (R1D) experimental column apparatus designed to investigate reaction, sorption, and transport of solutes in aquifers within dynamic reversing flow zones where waters with different chemistries mix. The motivation for constructing this apparatus was to understand the roles of mixing and reaction on arsenic discharging through a tidally fluctuating riverbank. The apparatus can simulate complex transient flux schedules similar to natural flow regimes The apparatus uses an Arduino microcontroller to control flux magnitude through two peristaltic pumps. Solenoid valves control flow direction from two separate reservoirs. In-line probes continually measure effluent electrical conductance, pH, oxidation-reduction potential, and temperature. To understand how sensitive physical solute transport is to deviations from the real hydrograph of the tidally fluctuating river, two experiments were performed using: (1) a simpler constant magnitude, reversing flux direction schedule (RCF); and (2) a more environmentally relevant variable magnitude, reversing flux direction schedule (RVF). Wherein, flux magnitude was ramped up and down according to a sine wave. Modeled breakthrough curves of chloride yielded nearly identical dispersivities under both flow regimes. For the RVF experiment, Peclet numbers captured the transition between diffusion and dispersion dominated transport in the intertidal interval. Therefore, the apparatus accurately simulated conservative, environmentally relevant mixing under transient, variable flux flow regimes. Accurately generating variable flux reversing flow regimes is important to simulate the interaction between flow velocity and chemical reactions where Brownian diffusion of solutes to solid-phase reaction sites is kinetically limited.

Extending aquatic spectral information with the first radiometric IR-B field observations.

Planetary radiometric observations enable remote sensing of biogeochemical parameters to describe spatiotemporal variability in aquatic ecosystems. For approximately the last half century, the science of aquatic radiometry has established a knowledge base using primarily, but not exclusively, visible wavelengths. Scientific subdisciplines supporting aquatic radiometry have evolved hardware, software, and procedures to maximize competency for exploiting visible wavelength information. This perspective culminates with the science requirement that visible spectral resolution must be continually increased to extract more information. Other sources of information, meanwhile, remain underexploited, particularly information from nonvisible wavelengths. Herein, absolute radiometry is used to evaluate spectral limits for deriving and exploiting aquatic data products, specifically the normalized water-leaving radiance, , and its derivative products. Radiometric observations presented herein are quality assured for individual wavebands, and spectral verification is conducted by analyzing celestial radiometric results, comparing agreement of above- and in-water observations at applicable wavelengths, and evaluating consistency with bio-optical models and optical theory. The results presented include the first absolute radiometric field observations of within the IR-B spectral domain (i.e. spanning 1400-3000 nm), which indicate that IR-B signals confer greater and more variable flux than formerly ascribed. Black-pixel processing, a routine correction in satellite and in situ aquatic radiometry wherein a spectrum is offset corrected relative to a nonvisible waveband (often IR-B or a shorter legacy waveband) set to a null value, is shown to degrade aquatic spectra and derived biogeochemical parameters.

Quantifying hinge torque in self-folding, shape memory polymer origami under varying thermal loads

Self-folding origami actuated by thermally responsive shape memory polymers (SMPs) presents a promising mechanism for the deployment of space structures. Pre-strained SMP sheets shrink when heated. The SMP sheets are patterned with black ink hinges that heat locally via a surface heat flux when exposed to infrared (IR) light. The localized heating induces gradients in shrinking, or shape recovery, which in turn produces folding. However, the thermal and mechanical loads applied to the structure during deployment pose unique challenges to the application of SMP as actuators. Herein, we investigate the effects of thermal loads (variable IR flux and convection) and folding resistance (mass moment of inertia) on self-folding behavior of polystyrene (PS) SMP sheets. We analyze digital video recordings to determine the transient bending angle (the angle traversed by the folding face of the SMP sheet) and hinge torque. It is observed that higher heat fluxes and reduced convection lead to higher maximum bending angles, lower fold start times, and higher hinge torques. We found that the measured hinge torque increases with increasing folding resistance. In addition, we highlight the significance of convective heat transfer by demonstrating self-folding under vacuum at IR flux intensities as low as 2,622 W/m2 at a wavelength of 980 nm and identify a minimum surface heat flux required to induce self-folding in the presence of convection. Comparisons to dynamic mechanical analysis (DMA) and a thermal finite element analysis (FEA) offer insight to the initiation of folding under varying thermal loads. These results provide a necessary understanding of the effects of varying thermal loads on the behavior of self-folding origami for deployable space structures.

An enhanced semi-analytical estimation of tool-chip interface temperature in metal cutting

An accurate estimation of the temperature distribution on tool surfaces is of great industrial importance; without it, a reliable prediction of tool wear in machining, especially thermally-induced wear mechanisms such as dissolution-diffusion and oxidation, is deemed impossible. This has promoted the development of semi-analytical models for simulation of the tool-chip interface temperature, which are less time-intensive and reasonably accurate.This study aims to present an enhanced prediction of the tool-chip interface temperature within the context of the available semi-analytical solutions of the heat conduction-advection problem with a moving heat source. A novel approach is presented to obtain the variable heat flux along the tool-chip interface based on a non-uniform contribution of generated heat in the sticking and sliding zones during chip flow. The capability of the enhanced model to simulate the temperature distribution is demonstrated for machining C45 and C50 plain carbon steels using uncoated carbide tools. The predictions are validated against the results of experimental orthogonal cutting tests for the same cutting conditions. A comparative analysis is then performed to underline the importance of incorporating the variable heat flux for reliable predictions of the maximum interface temperature and its location on the rake face. The outlook for future developments is also highlighted.

Mathematical modeling of viscoelastic fluid flow across a nonlinear stretching surface in porous media with varying magnetic field

We investigate the cross-diffusion on MHD mixed convective thermo-solutal transport in viscoelastic fluid from a nonlinear stretching sheet. The novelty of the work is the analytical evaluation of the nonlinear stretching sheet in a porous medium with a variable magnetic parameter and radiative flux. Also, thermo-solutal transport of a mixed convective flow is analyzed with Dufour and Soret effects in the presence of the chemical reaction, heat source/sink. The governing PDEs are reduced into ODEs by similarity variables. ODEs were tackled analytically using the homotopy analysis method (HAM). The analysis is carried out up to the 15th order of approximation. HAM contains an h-curve, allowing a flexible method of controlling the series solution convergence area. Graphs and tables are plotted to analyze the influence of different parameters on the fluid. Rates of heat and mass flux are depicted using tables. Our results indicate that the increasing viscoelastic flow enhances the boundary layer, and the thermal boundary layer reduces as radiation enhances. The numerical demonstrations of the wall drag coefficient, Nusselt number and Sherwood number have been tabulated. The growing parameters are taken as [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]

Innovative mini-channel design for a compound parabolic solar thermal collector serving intermediate temperature applications

The heat flux in the focal region of a compound parabolic collector (CPC) and the receiver's surface is varying and ununiform, and its effective absorption is one of the challenging tasks. It opens new opportunities for suitable and appropriate designs for the receiver. Thus, a new design of the mini-channel receiver is proposed based on fluid flow optimization to address the issue. The pressure drop analysis enables the decision of the dimensions of the proposed mini-channel receiver. MATLAB programming enables flow optimization through a mini-channel receiver under variable solar flux falling its surface to attain the highest possible temperature. The performances of the optimized and parallel fluid flow mini-channel receivers are compared by integrating them with CPC.The results revealed that the thermal gain of CPC-optimized fluid flow mini-channel increases by 23% with minimum pressure drop and shows the variation in the outlet fluid temperature from 82 to 155 °C under the fluid flow rates 1.5–4 l/h. The results show a reduction in the heat loss factor, which ultimately contributed to increasing CPC's efficiency. Thus, the proposed new design of an optimized fluid flow mini-channel receiver will help researchers design and develop efficient solar thermal systems.

Design and Analysis of Wide Speed Regulation of Variable Leakage Flux Reverse Salient-Pole Motor

In this article, a new variable leakage flux reverse salient-pole motor (VLF-RSPM) is raised to widen the speed range. The innovation is to realize both reverse salient-pole characteristics and variable leakage flux characteristics by using a method of adding magnetic bridges and magnetic barriers. Firstly, the evolution of the topological structure and working principle of the motor are introduced. Secondly, based on 2D Finite Element Analysis (FEA), the electromagnetic properties and noise of the motor are analyzed in detail, and the electromagnetic properties are contrasted with that of the conventional V-type synchronous motor (CVTSM). The results show that VLF-RSPM has the advantages of small torque ripple, strong magnetic weakening ability, low noise, high efficiency, and low risk of permanent magnet demagnetization under different conditions. In addition, it is verified that the proposed motor extends the speed range.

Variable Flux Direct Torque Control System of Switched Reluctance Motor

Direct Torque Control (DTC) strategy is very suitable for controlling Switched Reluctance Motors (SRM) due to its advantages of the simple control structure and fast system response, etc. This paper first analyzes the principle of DTC and establishes the DTC model of three-phase 6/4 SRM by Matlab/Simulink. In this paper, a three-closed-loop variable flux DTC system with velocity as the given quantity is constructed to solve the problem that the stator current of constant flux DTC is too large when the motor is in a steady state. The simulation results show that the variable flux DTC system has a better inhibition effect on the stator current and torque ripple of SRM than the conventional DTC system.

Novel Harmonic Current Controller for DC-Biased Vernier Reluctance Machines Based on Virtual Winding

Direct current-biased vernier reluctance machine (DC-biased VRM) adopts a doubly salient structure and sinusoidal phase currents. It has better performance than switched reluctance machines and variable flux reluctance machines. The back electromotive force (EMF) waveforms of DC-biased VRMs contain low-order harmonics. It leads to harmonic currents in phase windings. The current harmonics increase the losses and worsen controller performance. The suppression of current harmonics has become a hot research topic in literatures. However, the existing harmonic suppression methods have their inherent defects. In this paper, a novel harmonic current controller based on virtual winding is proposed. Harmonic currents are decoupled and transformed into different harmonic planes, and independent closed-loop control is realized. The characteristics of harmonic planes and the relationship between them are analyzed. According to the analysis, the method of choosing valid planes and harmonic distribution is introduced. The major advantage of the proposed method is that, harmonic current sampling and control can be realized without the use of filters, and it avoids complicated design of controller parameters. Experimental results based on a DC-biased VRM are presented. It verifies that the proposed current control strategy based on virtual winding shows excellent harmonic control performance.

Investigation of Delta-Type Variable Flux Memory Machines Considering Magnetizing Current Limits

A simple delta-type interior-permanent magnet (PM) variable flux memory (VFM) machine exhibiting series-parallel magnetic coupling is investigated, and the limits of magnetizing current capacity are considered. The series-parallel coupled high coercive force (HCF) PMs and low coercive force (LCF) PMs integrate the merits of stable PM working point in series-type VFM machines with the high torque output in parallel-connected cases. Moreover, it is revealed that the iron bridges between the two kinds of the PMs play an essential role on magnetic coupling, de-/re-magnetization and torque capability. The VFM machines with wider iron bridges normally require higher magnetizing currents. The effects of the iron bridges are investigated based on the theoretical analysis and finite element methods. It is found that the narrower iron bridges contribute to the more significant series coupling and wider flux variation range, while the wider iron bridges sacrifice the flux variation but facilitate the torque improvement. A prototype machine is manufactured and the experiments are carried out to verify the investigations.

Eruptive dynamics reflect crustal structure and mantle productivity beneath volcanoes

Abstract Volcanoes exhibit a wide range of eruptive and geochemical behavior, which has significant implications for their associated risk. The suggested first-order drivers of intervolcanic diversity invoke a combination of crustal and mantle processes. To better constrain mantle-crustal-volcanic coupling, we used the well-studied Lesser Antilles island arc. Here, we show that melt flux from the mantle, identified by proxy in the form of boron isotopes in melt inclusions, correlates with the long-term volcanic productivity, the volcanic edifice height, and the geophysically defined along-arc crustal structure. These features are the consequence of a variable melt flux modulating the pressure-temperature-composition structure of the crust, which we inverted from xenolith mineral chemistry. Mafic to intermediate melts reside at relatively constant temperature (981 ± 52 °C; 2σ) in the middle crust (3.5–7.1 kbar), whereas chemically evolved (rhyolitic) melts are stored predominantly in the upper crust (<3.5 kbar) at maximum depths that vary geophysically along the arc (6–15 km). Our findings are applicable worldwide, where we see similar correlations among average magma geochemistry, eruptive magnitude, and rate of magma input.

High Fault-Tolerance Dual-Rotor Synchronous Machine With Hybrid Excitation Field Generated by Halbach Permanent Magnets and High Temperature Superconducting Magnets

Superconductors can help electric machines achieve extremely high torque and power densities. However, operational instability is still a severe challenge for most superconducting electric machines. This paper proposes a novel dual-rotor hybrid excitation variable flux synchronous motor (DRHE-VFPM) suitable for applications with high safety requirements. The primary characteristic is using series-connected permanent magnet (PM) and High-temperature superconducting (HTS) magnet as the rotor's flux sources. These two magnets are on different rotors and steadily excited by each other through armature slots, improving several critical characteristics of the superconducting motor. First, the new electromagnetic topology can avoid severe quench accidents. At normal operation, the PM and HTS rotors can produce a large total electromagnetic torque. When the HTS magnets are damaged by the overtemperature or the mechanical shocks, the motor can still use the PM rotor to generate continuous torque instead of sudden interruption. Secondly, because an excessively strong rotor field will limit the speed, the variable flux density of the HTS magnet allows fuller use of voltage to extend the speed range and improve the power density. Moreover, the main magnetic circuit of the motor changes significantly under different operating states, which is analyzed theoretically in detail.

Two phase simulation of solar still in the presence of phase change materials in its bottom and aluminum nanoparticles in the water

This article performs a numerical study on a transient solar still (SOST) by utilizing a variable heat flux using COMSOL software. A phase change material (PCM) layer with a thickness of 10–50 mm is placed at the bottom of the desalination. Some aluminum nanoparticles are added to the water in the water desalination, and the two-phase method is used to simulate this part of the desalination water. Other variables include the angle of the glass that changes from 10 to 45⁰ and the heat transfer coefficient (HTFC) on the glass that varies from 5 to 300 W/m2. The effect of these variables on the average PCM temperature (T-PCM), PCM volume fraction (VOF-PCM), average moisture (AV-MO) temperature, and AV-MO concentration is examined in 12 h. The simulations are done using the finite element method (FEM). The results demonstrate that the average temperature and VOF-PCM, AV-MO concentration, and AV-MO temperature are enhanced from morning to noon and decreased from noon to evening. An increment in the thickness of PCM causes the VOF-PCM in desalination water to reduce by 35%. An enhancement in the glass angle reduces the average temperature of PCM, especially in the morning and afternoon. The change in PCM thickness, especially in the evening, significantly enhances the AV-MO temperature.

Numerical simulation for impact of implement of reflector and turbulator within the solar system in existence of nanomaterial

Turbulent flow of oil based hybrid nanofluid within an absorber tube of concentrated solar system has been evaluated in this article. To concentrate the solar irradiation, the parabolic plate has been located below the tube and variable heat flux was considered as the boundary condition of the tube. The presence of a turbulator within the circular tube causes secondary flow to increase. Both thermal (Sgen,th) and frictional (Sgen,f) components of irreversibility were reported in outputs. As Re increases, the residence time decreases and lower outlet temperature has been achieved. Sgen,th decreases about 57.36% with growth of Re while Sgen,f increases about 17.44 times. As the number of rows of tapes increases, the value of Sgen,f enhances about 69.23% while the value of Sgen,th decreases around 3.67%. Increase of pitch ratio causes Sgen,th to decrease about 11.25% while frictional component increases around 76.7%.

Numerical simulations and modeling of MHD boundary layer flow and heat transfer dynamics in Darcy-forchheimer media with distributed fractional-order derivatives

In this study, we extend the application of an existing Cattaneo heat flux model integrated with magnetohydrodynamic (MHD) Maxwell boundary layer flow using distributed fractional-order derivatives. This offers a novel approach for the use of disturbed order along with the midpoint quadrature rule, not previously considered in the literature for such phenomena. Moreover, the application of a variable heat flux with distributive order fractional constitutive equation, also an unexplored area in the literature, adds to the novelty of our study. We investigate the dynamics of the model over a flat surface in the presence of variable heat flux and Darcy-Forchheimer media. The governing equations, along with their respective initial and boundary conditions, are derived and solved using the finite difference method employing the L1 scheme. A benchmark 'exact solution' is obtained for a simplified version of the problem and a numerical solution is provided for a more complex, realistic version. The accuracy of our numerical scheme is validated by comparing results with the benchmark solution, and a mesh independence study further validates our code. Our study uncovers unique insights into the influence of various physical parameters on the velocity and temperature boundary layer thickness, as well as the flow and heat transfer mechanisms, elucidating the rheological characteristics of the system.

Production of pure water and dry salt from NaCl solutions via percrystallisation by supported carbon membranes

Membrane percrystallisation is an alternate crystallisation process where a membrane disperses aqueous salt solution into a thin film on the permeate side. This thin film can be instantaneously vaporised to form pure water vapor and dry, crystallised salt under vacuum conditions. This work demonstrates, for the first time, that supported carbon membranes are a viable configuration for the percrystallisation of saline solutions. Within the work, the carbon materials in powder and supported membrane form were produced from polyvinylpyrrolidone (PVP) by pyrolysis at temperatures ranging from 450 and 800 °C. Porosity analysis identified that the materials do not adsorb gas. This suggests that the materials are non-porous or the pores are not accessible at the analysis conditions. All produced membranes were of supported nature with approximate thickness of 8 μm. The flux measurement revealed that the membranes, despite being similar in thickness, resulted in variable flux exhibiting a parabolic profile, with a maximum flux obtained (18 kg m−2 h−1) when tested for percrystallisation at 50 °C. The membranes maintained structural integrity when operated with 3.5 to 10 wt% NaCl solutions at 50 °C. However, membranes began to degrade when operating with 20 wt% solutions at 70 °C. The solid product analysis showed that the NaCl produced by percrystallisation is crystalline NaCl, with various morphological properties. The analysis also revealed that the particle size and shape depends on the solution flux through the membrane with smaller crystals at higher fluxes. The results of this study are used to propose a mechanism of salt formation for the percrystallisation of aqueous salt solutions.

Thermal description and entropy evaluation of magnetized hybrid nanofluid with variable viscosity via Crank–Nicolson method

The dominant characteristics of hybrid nanofluids, such as low cost, improved heat transfer rates, and higher thermal and electrical conductivity, make them preferable fluids in thermal energy systems. In light of these incredible features, our goal in the present research is to analyse heat transfer and entropy generating in (Fe3O4–Cu)/water hybridity nano liquid flowing via a vertical cone with variable wall temperature inserted in a porous material. The effects of variable viscosity, magnetic force, and thermal radiative flux are additional aspects that contribute to the originality of the constructed model. The mathematical model is solved utilising the Crank-Nicolson technique, and the numerical findings are showed graphically and in a tabular manner. The heat transfer process is improved by hybrid nanoparticles and porosity while being hindered by magnetic interaction, viscosity, and thermal radiation. Energy loss in the form of increased entropy can be noted at the further vertical end of the cone than the base. The parametrical influence was reported to be nominal compared to the other physical aspects. Domination of heat transfer induced entropy generation and be observed for the porosity improvements.

Unsteady temperature distribution in a cylinder made of functionally graded materials under circumferentially-varying convective heat transfer boundary conditions

Abstract A novel model on 2D unsteady conductive heat transfer in an infinite hollow cylinder is proposed. The cylinder is made of functionally graded material (FGM) that has variable properties both in radial and angular directions. Volumetric heat capacity and thermal conductivity coefficient are changed according to the power function of the radius. In the presence of variable coefficients, the governing equations of unsteady heat transfer in FGMs have caused the complexity. The Laplace transform method is used to transfer the energy equation from time to frequency domain whereas the meromorphic function is used for the inverse Laplace transform to obtain the desired solutions. The closed form solutions have been well validated and the results have been presented for different values of functionally graded indices for thermal conductivity coefficients and volumetric heat capacity. Two different FGM cases with different complicated thermal boundary conditions have been investigated. The first case has a constant temperature in the inner radius and a variable heat flux along with the convection condition in the outer radius. In the second case, the inner radius has a specific harmonic temperature and the outer radius is exposed to the convective conditions. It was observed that in both cases, the temperature value in the cylinder decreases with the increase of the FG index for the conductivity coefficient. The presented analytical solution provides a good tool for validating unsteady numerical solutions presented in the field of heat transfer in FGMs.

Formation process of Pliocene cold seep carbonates from the southern Western Foothills, Southwestern Taiwan: A synthetical rare earth element and C–O–Sr–Nd isotope study

In this work, the Pliocene dolomitic carbonate concretions and pipes from Chiahsien of the Western Foothills, southwestern Taiwan were studied. We conducted a synthetical analysis of stable carbon and oxygen isotopes, rare earth element (REE) contents, and Sr and Nd isotopes, to trace the compositions and sources of pore fluids from which carbonates have precipitated and reconstruct the carbonate formation processes. The carbonates with low δ13C values (as low as −48.6‰) are promoted by the anaerobic oxidation of biogenic methane. The 87Sr/86Sr ratios of samples reflect the mixing of coeval bottom seawater and seep fluid enriched in 87Sr. The Nd isotopic signatures of studied carbonates indicate multiple Nd sources, including detrital components, Fe–Mn oxyhydroxides, organic matter, and seawater. The MREE-bulge patterns in samples are typical of anoxic conditions, which are supported by Ce anomalies. Our results demonstrate that the seepage intensity played a key role in the formation processes of concretions and pipes by controlling the depth of the sulfate-methane transition zone (SMTZ). The concretions characterized by the lowest δ13C values formed at a deeper depth in sediments under low methane flux, which suffered no/less influence of seawater. The representative pipe with obvious concentric lamination grew in a rim-to-core pattern. In the early stage, the variable flux of highly channelized seepage caused the fluctuation of carbon and oxygen isotopes. During the later stage, the modification of isotopic compositions in the core part were resulted from meteoric alteration. Our work provides insights into the formation process related to seepage evolution in ancient cold seeps on Taiwan Island.