Non-uniform Flux Distribution Research Articles

A comprehensive investigation of filtration performance in submerged hollow fibre membrane modules with different fibre geometries

In this study, computational fluid dynamics simulation was performed to investigate the effects of fibre internal diameter (ID) and length on the filtration process and flux distribution during a dead-end, hollow fibre filtration. The model was combined with the Navier-Stokes and Darcy Brinkman equations to simulate a complete filtration run. A comprehensive membrane fouling mechanism, including reversible fouling resistance and irreversible fouling resistance, was considered. A moving fluid-solid boundary was adopted to evaluate sludge cake deposit along the fibre. Our results showed that non-uniform flux distribution was intensified with increase of fibre length, while an ID between 0.4 and 0.7 mm was optimal; it also demonstrated that an ID below 0.4 mm lead to drastic non-uniform flux distribution due to unfavourable hydrodynamic conditions in the internal channel. In actual filtration, the initial non-uniform flux distribution rapidly became uniform for a while, but over time the phenomenon of non-uniform flux distribution was re-established.

Numerical investigation of the heat transport in a very loose packed granular bed air receiver with a non-uniform energy flux distribution

This study analyzes the heat transfer of the solar energy absorbed by the porous media receiver to air where the porous media can reach temperatures up to 1000 °C. The heat transfer was studied numerically for a packed granular bed air receiver with a non-uniform energy flux distribution. The conduction is modeled as an effective thermal conductivity with the Ergun equation used to describe the pressure drop. The radiative heat transfer is modeled by a modified P-1 radiative heat transfer model. The local thermal non-equilibrium model is used to relate the air temperatures to the porous media temperatures. Comparisons with experimental data show that this model can accurately predict the heat transfer in the packed granular bed air receiver. The model is used to analyze the effects of flow direction, gas mass flow rate per unit area, particle emissivity, estimated fractional contact area, and particle size. The results illustrate that downward flows, larger gas mass flow rates, higher particle emissivities, larger estimated fractional contact areas, and smaller particle sizes all enhance the heat transport and that the packed bed receiver outlet air temperature does not vary much for relatively short term fluctuations of the solar flux.

Optical Design and Performance Analysis of Linear Focusing Solar Thermal Collectors

Linear Fresnel solar collector is a focusing concentrator suitable for direct steam generation, industrial process heat, and solar space cooling system and hot water generation for different uses. The optical design and performance simulations, experimental and numerical studies on linear Fresnel solar collector were reviewed. Studies on the optical designs and ray-tracing simulations results indicated non-uniform solar flux distributions on the receiver absorber surface. The optical quality of LFC is low due to its higher incidence angle and the cosine factor. Studies on optimizing the optical errors that affects the optical performance of the LFC are lacking in the literature. Ray tracing results at 0o incidence angle indicated three different optical losses-geometric configuration, material properties and focus errors losses. Studies on the lateral drift and uncertainty of the direction of the reflected rays, which adversely affect the concentration factor of the LFC are lacking in the literature. The optical performance of a LFC system can be improved through an optimized optical design - mirrors separation, shapes, width and their orientation.

Transient Core-Loss Simulation for Ferrites With Nonuniform Field in SPICE

A methodology to develop a subcircuit model for core loss simulation in LTspice is presented. The subcircuit implements the dynamic core loss model used in the transient solver of finite-element analysis (FEA) in order to provide equivalent results in the time domain for ferrite materials. The wipe-out rule is applied in the simulation so that the core loss during the transient behavior can be predicted. A field factor is derived from an effective flux density for a nonuniform flux distribution to simplify the relationship between the current and the field for arbitrary magnetic core shapes. The interface of the subcircuit is capable of being integrated to any power stage simulations. Simulation results of core loss equivalent to the FEA on an exemplary plate-core inductor are obtained from the subcircuit that significantly reduces computational cost.

Design and characterization of the non-uniform solar flux distribution measurement system

It is important to accurately measure the solar flux distribution on the heated surface for the utilization of solar energy. To this end, in this paper a new measurement system was proposed to measure the solar flux distribution of the absorber tube surface by taking advantage of the end loss effect. The new measurement system does not disturb the collected lights path and influence the operation of the solar collector. The solar flux distribution at different angles can be measured with a few number of heat flux gage by rotating the proposed solar flux measurement system. Compared with existing measurement techniques, the proposed solar flux measurement system has distinct advantages such as simple structure, flexible and reliable operation and easy application. An experimental platform was designed and constructed, and the solar flux distributions on the outer surface of the absorber under differentconditions are measured. The feasibility and effectiveness of the proposed measurement method are validated by comparing the experiment results and the numerical simulation results.

A new approach for evaluating flux uniformity for dense array concentrator photovoltaic cells

In this paper, we present two statistical methods to quantify the heterogeneity of the irradiance flux distribution, in a Concentrator Photovoltaic (CPV) dense-array, based on its operation and the optimization of current-matching. Preventing non-uniform flux distribution from design avoids the generation of hot spots, current mismatch and increases the overall efficiency of the system. This new approach considers the effects of the lowest irradiance values in the performance of the complete array, and its performance was corroborated by the simulations of a CPV array modelled in Matlab/Simulink; the irradiance distribution data as an input parameter was obtained from the images taken in a homogenization experiment, in the HoSIER, an 18,000 X solar furnace. The results are interpreted through the new concept of photovoltaic homogeneity, proven that the methodology successfully predicts the flux distributions, which enhances the efficiency of a series connected CPV array. Additionally, we found that the proposed methodology can also be used to optimize the electrical performance of dense-array CPV systems, working under the effects of non-uniformity illumination by rewiring the series connections into series-parallel configurations.

Non-uniform distribution of adsorptive fouling along hollow fiber membrane: Characterization and quantification

Abstract Non-uniform distribution of local flux along the hollow fiber (HF) membrane length poses a high risk of partially overloaded flux to exacerbate membrane fouling. In-depth analysis of its effect on fouling is critical to the design of membrane geometry (e.g. fiber length, inner and outer diameters). This study focused on HF membrane adsorptive fouling by soluble organic substances under the influence of local flux distribution. The non-uniform distribution of adsorbed foulant was experimentally observed via fluorescence and contact angle measurements. An HF-Thomas model was then developed to describe the dynamic adsorption process (free/adsorbed concentration as a function of time), by incorporating Langmuir and Linear adsorption kinetics into the hydrodynamics of the HF membrane lumen. The model fitting was found to be effective in determining the HF membrane adsorption parameters (equilibrium and rate constants) of typical polysaccharide, protein, and humic acid model compounds. Extended simulation was conducted using the HF-Thomas model to explore the impacts of local flux distribution on foulant mass transfer and adsorptive fouling distribution under different scenarios of membrane geometry. The non-uniformity of flux and adsorption was found to be markedly amplified by a comprehensive dimensionless factor, λl (which derives from fiber length, inner and outer diameters, and intrinsic filtration resistance). Therefore, λl may qualify as a critical parameter for HF membrane design. According to the results of this study, λl

Performance investigation of linear evacuated absorber of 2-stage solar Linear Fresnel Reflector module under non-uniform flux distribution

Performance and heat loss analyses have been carried out for a 2-stage parabolic receiver system of the solar Linear Fresnel Reflector module. Performance of the system with non-uniform distribution of the flux on the absorber and the influence of the wind on the receiver system are investigated. Solar flux distribution on the surface the absorber is applied by user-defined function to analyse the heat loss under the real-time scenario. The performance of the absorber tube when coated or uncoated with a selective material is studied for different flux conditions. This provides a perception to adopt appropriate coating material for different locations under varied DNI conditions. The investigation has been carried out to analyse the heat loss effects when the absorber is placed under evacuated and non-evacuated conditions. All the modes of heat transfer are considered in the study and the dominance of each mode of heat transfer is highlighted. Performance of the numerical data with the non-uniform flux distribution on the absorber with the measured velocity and emissivity values are validated with the experimental data. Deviation of ±10 % is observed between the experimental and the numerical values.

A model of wax deposition under oil-gas two-phase stratified flow in horizontal pipe

A model of wax deposition based on molecular diffusion mechanism, for oil-gas two-phase stratified pipe flow is developed. In the model, unidirectional fully developed flow analyses of momentum, heat and mass transfer are presented. And, a cube cage model is used to describe the wax deposit structure considering the effect of oil flow shear on the deposit. Calculation of wax deposit is compared well with a flow loop experiment. In particular, the model could give the wax deposit forming a crescent shape at the cross section of pipe, which is observed in different experiments. Furthermore, the cause of forming a crescent shape is revealed, which is indicated by the non-uniform circumferential distribution of mass flux for wax deposition along the pipe wall wetted by the oil. The mass flux from oil bulk flow to the oil-deposit interface is closely related to three parameters, diffusivity at oil-deposit interface, the temperature gradient at the oil-deposit interface at different time, and the slope of the wax solubility curve at oil-deposit interface temperature.

Heat loss investigation of 125kWth solar LFR pilot plant with parabolic secondary evacuated receiver for performance improvement

Numerical investigation is carried out on the secondary reflector system of the pilot solar linear Fresnel reflector module of 154 m2 in Vallipuram, Tamil Nadu. Initially, optical investigation on the design of the Linear Fresnel Collector module and the secondary reflector system is carried out. The optimized flux thus obtained is applied as a boundary condition to the thermal loss analysis. Investigation of the total heat losses (convective, radiative and conductive) with three different boundary conditions namely, the constant flux, variable flux and non-uniform flux distribution is carried out. The heat loss study has been carried out when the absorber is under non-evacuated and evacuated state. The study has been carried out with the DNI ranging between 250 W/m2 and 1000 W/m2. Uncoated and selectively coated absorbers with the emissivity of 0.01–1 are analyzed. Comprehensive analysis on the influence of wind flow on the receiver system is carried out. The variation of wind velocity from the ground level is incorporated in the forced convection. Investigation on the effect of the wind speed (0 m/s–10 m/s) and the wind direction (0°–90°) on this second stage reflector system is analyzed. The deviation between the different flux boundary conditions is comprehensively analyzed under different DNI and wind conditions. The variable flux distribution shows less deviation of about 15% from the non-uniform flux distribution for the DNI of 1000 W/m2 and is found to be less than 5% for 500 W/m2 under evacuated conditions. To avoid the computational complications in applying non-uniform flux boundary conditions to the absorber for DNI greater than 500 W/m2, variable boundary flux condition can be applied as an equivalent flux condition by augmenting 15% non-uniform error percentage to the final heat loss value.

In-situ prediction of focal flux distribution for concentrating photovoltaic (CPV) system using inverse heat transfer technique for effective design of receiver

The knowledge of heat flux distribution on the receiver area is very important to improve the overall performance of the solar Concentrating Photovoltaic (CPV) system. In a CPV system, non-uniform flux distribution is one of the common issues. Non-uniform illumination of heat flux is mainly due to the limitations in the design of concentrator optics, slope error in the concentrator profile, tracking system error, misalignment of concentrator, and the efficiency of refractive lens/reflecting mirrors. The prediction of the heat flux distribution will play a vital role in the CPV system such as designing the receiver area, selection of the materials, thermal management and to estimate the power output. In this paper, an experimental work for in-situ prediction of heat flux distribution profile on a flat plate receiver is presented. Inverse heat transfer technique is adopted to predict the heat flux distribution. A Gaussian distribution is assumed to model the distribution of the heat flux. The forward problem is a 3-D steady state heat conduction equation subjected to convection and radiation heat loss boundary conditions.The forward problem is solved using Finite Element Method in Ansys APDL. The unknown parameters of the assumed heat flux distribution are then estimated by minimizing the sum of squared error between measured and simulated temperature distribution. A deterministic search technique, Levenberg-Marquardt algorithm is used to solve the inverse problem. The simulated temperature distribution with the predicted heat flux are in good agreement with the measured temperature with a maximum residue of ±5 °C. Also, the deviation between theoretical and predicted total solar energy is found to be <6%.

Very High Frequency IVR for Small Portable Electronics With High-Current Multiphase 3-D Integrated Magnetics

As today's small portable electronics (smartphones, tablets, e-readers, etc.) becomes lighter, thinner, quicker, and smarter, the voltage regulator for the processor is expected to be efficient, miniaturized, integrated, and placed closer to the processor. In this paper, a concept of a very high frequency [tens of megahertz (MHz)] three-dimensional integrated voltage regulator (IVR) for small portable electronics is proposed. The magnetic characterization technique at tens of MHz is investigated, and the issues of and solutions for permeability and loss measurement are demonstrated. The LTCC and NEC flake materials are characterized and compared for the IVR inductor development. Both single-phase and five-phase integrated inductors are designed, fabricated, and experimentally tested at 20 MHz, featuring a simple single-via winding structure, small size, ultralow profile, ultralow DC resistance (DCR), high current-handling ability, air-gap-free magnetics, multiphase integration within one magnetic core, and lateral nonuniform flux distribution.

Multi-objective optimization of the aiming strategy for the solar power tower with a cavity receiver by using the non-dominated sorting genetic algorithm

The extremely non-uniform solar flux distribution in the solar power tower plant can badly cause some crucial problems for the solar receiver such as the local hot spot, the thermal stress, and the thermal deformation. Homogenization of the solar flux distribution is an effective method to avoid these problems, and has become an important research topic. The objective of the present study is to homogenize the solar flux distribution on the inner surfaces within the cavity receiver while keeping the optics loss as low as possible by replacing the conventional single-point aiming strategy with optimal multi-point aiming strategies. Multi-objective optimizations of the aiming strategy for the solar power tower with cavity receivers are performed by using the non-dominated sorting genetic algorithm. The distribution of the aiming points on the cavity aperture and the allocation of the aiming points for each heliostat are optimized simultaneously. The following conclusions can be made: (1) The uniformity of the solar flux distribution on the aperture does not always signify the uniformity of the solar flux distribution on the inner surfaces, where the later one is what we are truly concerned about. Therefore, the optimization of the aiming strategy should take charge of the solar flux distribution on the inner surfaces rather than on the aperture. (2) The multi-objective optimization can provide the trade-off between the non-uniformity of the solar flux distribution and the optics loss in the form of Pareto optimal fronts. (3) The optimal aiming strategies provided by the multi-objective optimization can significantly homogenize the solar flux distribution on the inner surfaces within the cavity at a minimum cost of optics loss. (4) For the optimal aiming strategies at all time except the noon, there exists a west-east asymmetry of the aiming point distribution on the aperture. Moreover, the asymmetry gets less obvious as the time gets closer to the noon.

Experimental efficiency of a low concentrating CPC PVT flat plate collector

PVT collectors aim to solar co-generation of electricity and heat. An new concept raises thermal efficiency by concentrating sunlight with CPC reflectors, in order to access a higher number of solar thermal applications. Losses in PV efficiency due to a higher operating temperature are accepted with regard to a higher overall collector efficiency. As a side effect, the concentration lowers the material usage of PV and enables a high efficient thermal coupling of the PV cell to the heat carrying fluid.The work focuses on the construction, as well as on the angle dependent electrical and thermal measurement of the real-size CPC PVT collector prototype (1460mm×600mm×150mm). In previous publications, calculations and experiments studied the influence of the CPC reflectors on the PV efficiency. Further, the thermal coupling between PV cell and heat carrier fluid has been measured in a lab experiment. These results, together with transient annual simulations, were considered in the design process of the CPC PVT prototype with an angular acceptance range of ±25°.The experiments were conducted on the outdoor solar test facility at ZAE Bayern in Garching, Germany. The thermal and PV efficiency has been measured with MPP tracking, as well as for open circuit voltage for fluid temperatures up to 107°C.It could be shown, that the thermal efficiency while MPP tracking is elevated to 34% compared to a glazed flat plate PVT with 17% for collector temperatures 60K over ambiance. At the same time, the electrical efficiency drops from 15% cell efficiency to an overall collector efficiency of 9%, due to the optical setup, temperature effects and a non-uniform flux distribution caused by the reflectors.

Non-uniform characteristics of solar flux distribution in the concentrating solar power systems and its corresponding solutions: A review

In this paper, the non-uniform characteristics of solar flux distribution and the consequent challenges in concentrating solar power (CSP) systems including parabolic trough system, linear Fresnel reflector system, solar power tower, and solar dish system are summarized. The corresponding solutions proposed to tackle the challenges caused by non-uniform solar flux distributions are particularly reviewed. Meanwhile, the authors’ previous work is also introduced. It is can be seen that the solar flux distributions in both tubular receiver and volumetric receiver are extremely non-uniform for the 4 types of CSP systems. The non-uniform solar flux leads to the non-uniform temperature distributions in the absorber. There exist high local temperature and large temperature gradient, which can causes great challenges for the safety and high-efficiency operation of CSP systems. The selective coating and the heat transfer fluid tend to degrade in high temperature environment; and the absorber also could be burned out in the hot-spot area. Thermal stress and thermal deformation resulted from large temperature gradient can cause damage of the key components of solar receivers. Then the authors pointed out the reason that the non-uniform solar flux distribution could cause the above challenges is attributed to the mismatching between the non-uniform solar flux distribution in the absorber and the uniform heat absorption capacity of the heat transfer fluid, which leads to the non-uniform temperature distribution. Accordingly, the existing solutions for tackling the challenges caused by non-uniform solar flux distribution can be classified into 2 groups: (1) Optimizing the heat absorption capacity to match the non-uniform solar flux distribution; (2) Homogenizing the solar flux distribution in absorber/receiver to match the uniform heat absorption capacity. Finally, the guiding principle of matching between the collecting process and the absorbing process is proposed for solving the problems caused by the non-uniform characteristics of solar flux distribution. The solar collectors and solar receivers in CSP systems should be optimized under the guidance of this principle to improve the safety and efficiency of CSP systems.

Integrated numerical study on the coupled photon-thermal conversion process in the central solar molten salt cavity receiver

In a solar power tower system, the concentrating and collecting subsystem, including the heliostat field and the receiver, contains some of the most costly and technically challenging components. Many types of solar receivers make up the solar power tower system. Among them, the cavity molten salt receiver has been widely applied due to its lower heat loss. The accurate simulation of the coupled heat transfer process in the cavity receiver and the prediction of receiver performance are of great importance for the receiver design and safe operation. The solar flux distribution on cavity interior surfaces is extremely non-uniform, which has great effect on the receiver thermal performance. Taking the non-uniform solar flux distribution into consideration, the photon-thermal conversion process is simulated integrally in this paper. A hybrid simulation approach, which couples Monte Carlo ray tracing and Gebhart methods, is applied to investigate the complete solar radiation transfer process in the solar power tower system with a cavity receiver and obtain the non-uniform solar flux distribution. On this basis, the coupled photon-thermal conversion process in the molten salt cavity receiver is simulated. The effects of the fluid flow layout on the receiver performance under the non-uniform solar flux distribution are particularly revealed. Also, the variation of the receiver performance with time is analyzed in detail. The results show that the solar flux distribution on the interior surfaces of the cavity receiver is extremely non-uniform, under which the molten salt flow layout has great effects on the receiver performance. In the case of flow layout that the cold molten salt flows into the receiver in the high-flux area, the temperatures of both the absorber wall and the molten salt increase rapidly along the flow path. The temperature of the whole absorber wall is relatively high, which leads to greater heat loss and less amount of hot molten salt, and even seriously, “heat transfer deterioration” near the outlet of the receiver. When the cold molten salt flows into the receiver in the low-flux area, the temperature of both the absorber wall and the molten salt increase slowly along the flow path. The temperatures of the whole absorber wall is relatively low, which results in less heat loss and larger amount of hot molten salt. Although the shape of the solar flux distribution on the interior surfaces varied greatly with time, the evolutions of the molten salt (absorber wall) temperature at different time display similar variation tendency along the flow path. Moreover, the reflective loss varies with time greatly, while the heat loss varies with time slightly.

Numerical investigation of the thermophysical characteristics of the mid-and-low temperature solar receiver/reactor for hydrogen production

With the considerations of the complex kinetic mechanisms of the methanol steam reforming using Cu/ZnO/Al2O3 catalyst, in this paper a multiphysics coupling model that integrates the mass, momentum and energy conservation governing equations is proposed to investigate the thermophysical performances of the mid-and-low temperature solar receiver/reactor for hydrogen production. The factors influencing the hydrogen production and temperature distributions of the catalyst bed, including the diameter of the receiver/reactor, the non-uniform distribution of the solar flux and the porosity of the catalyst bed, are numerically studied. The temperature distributions, mole fractions of the components and reaction rates are obtained. The influence rules of the diameter of the receiver/reactor tube on the performances of the receiver/reactor are revealed. The non-uniform distribution of the solar flux has a significant influence on the cross-sectional temperature difference of the receiver/reactor tube, the catalyst bed and the temperature rise of the catalyst bed, while has a slight impact on the methanol conversion and the collector efficiency. The effect mechanisms of the porosity on the performances of the receiver/reactor are revealed. The research findings provide a fundamental reference for the development of the mid-and-low temperature solar receiver/reactor.

Design and characteristics of a novel tapered tube bundle receiver for high-temperature solar dish system

Most of current receivers for high-temperature (>1000 K) solar thermal utilizations were confronted with non-uniform solar flux distribution on absorbers, which would result in cracking and melting as well as flow instability. This paper proposes a new design of tapered tube bundle receiver (TTBR) by investigating the geometrical principle of dish concentrators, which makes each passage of the absorber parallel with the concentrated sun rays to absorb uniform solar thermal. The performance of the new design was confirmed using the theory of energy balance, Ray-tracing simulation and CFX simulation. Calculations based on the theory of energy balance indicated that the thermal efficiency of TTBR achieves 80%. Ray-tracing simulation results showed that solar rays are able to irradiate the entire tubes and uniformly distribute on each tube. And according to the flow field simulation by CFX, attributed to the circumferential inflow structure, almost equivalent air mass rate was obtained in each tube, indicating the designed TTBR receiver can achieve uniform flow in the absorber passages, which laid a good foundation for the further stable and efficient heat transfer procedure.

RF discharge for in situ mirror surface recovery in ITER

Almost all optical diagnostic systems in ITER will require the implementation of mirror recovery and protection systems. Plasma cleaning is considered to be the most promising technique for the removal of metal deposits from optical surfaces. The engineering and physical aspects of RF discharge application for continuous or periodic plasma treatment are discussed with a focus on implementation under ITER conditions. The ion flux parameters obtained in capacitively coupled (CC) RF discharge were measured in the mock-up of a plasma cleaning system. The uniformity of sputtering in CC RF discharge with and without a magnetic field was studied experimentally for the cylindrical discharge reactor geometry and compared with numerical simulations. The sharp increase in the sputtering rate resulting from the non-uniform radial distribution of the ion flux was observed near the electrode edges. The longitudinal magnetic field improves sputtering uniformity. It was demonstrated that Al/Al2O3 deposits can be removed in the Ne and D2 plasma of CC RF discharge but long-term exposition results in the degradation of the polycrystalline molybdenum mirror surface. The efficiency of Al sputtering in the atmosphere containing O2 and N2 fractions was studied in the D2/O2 and D2/N2 plasma of glow discharge. The addition of 2% of oxygen or nitrogen increases the sputtering yield by 3–4 times as compared with that in a nominally pure D2 discharge. The impact of metal deposits on the performance of diagnostic mirrors is discussed. It was shown that an ultrathin metallic film with a thickness as low as a few nm may cause a significant degradation of diagnostic mirrors with a transparent coating.

Study on classical and excess eddy currents losses of Terfenol-D

In the present paper, classical and excess eddy currents losses of Terfenol-D are studied and effects of magnetic field frequency, peak of magnetic flux density and diameter of Terfenol-D on the eddy currents losses are investigated. To provide reliable data for the purpose of the paper, an experimental laboratory is fabricated and used to obtain major and minor hysteresis loops of Terfenol-D at different frequencies. In theoretical study, initially an analytical model based on uniform distribution of magnetic flux is developed which yields to calculation of classical eddy currents losses. Then, another eddy currents model based on non-uniform distribution of magnetic flux and nonlinear diffusion of electromagnetic fields is presented. The difference between output values of the two models is identified as excess eddy currents losses. Obtained results show that the values of excess losses are generally larger than classical losses and applying just classical model leads to wrong calculation of actual value of eddy currents losses. For the results obtained from two above models, empirical models with respect to the magnetic field frequency and the peak value of magnetic flux density are achieved which can predict the eddy currents losses precisely. To validate the empirical relations, experiments are repeated at a new frequency and values of power losses calculated from analytical equations are compared with the predicted values of the empirical models. The results point towards possibility to use the obtained empirical relations in order to calculate the classical and excess eddy currents losses of Terfenol-D at the frequencies below 200Hz and different values of magnetic flux density.