A novel method, the Gaussian Integral Method (GIM), is presented for calculating void fractions in Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations. GIM is versatile and applicable to various grid types, including structured and unstructured polyhedral meshes, without requiring special boundary treatments. An optimization technique is introduced to make GIM independent of grid resolution and type. The method is validated against experimental data from a fluidized bed, demonstrating that GIM produces realistic simulations closely resembling experimental observations. Additionally, unstructured polyhedral grids using GIM outperform structured grids of equivalent resolution, yielding results more aligned with experimental data. The gradient of the void fraction is computed in the CFD solver and utilized in the DEM solver for precise estimation at particle locations. Overall, GIM provides an effective solution for void fraction calculations in particulate media simulations with complex geometries, enhancing the accuracy and applicability of CFD-DEM simulations for industrial processes.

Continuous fiber reinforced thermoplastics (CFRTPs) are valued for their high strength-to-weight ratios, corrosion resistance, and recyclability over traditional structural materials. However, CFRTPs are difficult to fabricate compared to thermoset composites, a significant barrier to their adoption in construction and infrastructure. The University of Maine’s Advanced Structures and Composites Center has developed several innovative manufacturing and post-processing techniques to address fabrication challenges. The Continuous Forming Machine (CFM) is a processing device for non-reactive thermoplastic pultrusion, efficiently producing structural profiles like rods, plates, and channels. The CFM platform can be expanded with co-processes to create parts such as field-bendable concrete reinforcing bar. Further, effective post-forming processes will be crucial for adapting CFRTP components to on-site infrastructure applications. Localized thermoforming targets specific bending regions within a CFRTP component, offering a field-friendly approach over stamp thermoforming. Using localized heating methods, such as embedded heating elements, CFRTP panels can be locally thermoformed into three-dimensional components. Additional work has been performed to mitigate wrinkle formation in the bending of thick-gauge CFRTP laminates by implementing modifications to component architecture. This research presents significant advancements towards the widespread adoption of CFRTPs for infrastructure, demonstrating the technology's adaptability for onsite manufacturing and its future impact on sustainable construction practices.

A novel graphical processing unit-based discrete element method solver is introduced to improve stability, performance, and provide seamless integration into commercial or open-source computational fluid dynamics software. A key innovation is eliminating a need for network communication between solvers, which was previously required for cross-platform coupling. This is accomplished by a direct coupling method that employs dynamic-linked libraries. Furthermore, the solver optimizes memory usage by streamlining the particle-cell search algorithm by eliminating the cells' searching grid. This ensures the solver is compatible with a wide range of mesh types, providing high geometric flexibility. The approach simplifies the simulation process by directly incorporating computational fluid dynamics mesh information into the discrete element method solver. The performance analysis indicates about sixteen times boost in computational speed compared to benchmark central processing unit-based solvers. The solver's compatibility with polyhedral meshes, a vital advantage for complex geometries, is tested against a referenced study regarding the simulation of an immersed-tube fluidized bed.

The capstone experience in engineering curriculums is a critical component focused on the unification of several years of student learning, but its unique nature can present challenges to engineering departments in faculty assignment and ensuring consistent, high-quality instruction. During a seven-year period, five instructional modalities, defining the interaction between students and faculty, were explored in capstone courses in the University of Maine Mechanical Engineering Department. By sharing the lessons learned from this case study, other engineering departments can make more informed decisions about how to operate capstone courses. We present the benefits and challenges of each modality and evaluate them for scalability, student satisfaction, project diversity, cost, and instructor workload. Annual data were collected on student, instructor, and project counts. Data from student evaluations and department budgets were used to evaluate student satisfaction and costs, respectively. Insights from the three authors, who were the primary capstone instructors during this study, are shared as part of the evaluations and lessons learned. Key results are that cost, student satisfaction, and project diversity did not depend strongly on the teaching modality. However, scalability and instructor workload were highly dependent on the teaching modality. The University of Maine Mechanical Engineering Department sees the most promise in a modality with multiple lead instructors who each oversee a portion of the teams, which provides scalability to add or remove instructors, and the ability for high-quality instruction through close coordination of a small instructor group.

A method is proposed to locally heat areas of thick (>25.4 mm) continuous glass fiber reinforced thermoplastic (C-FRTP) composites with embedded wire mesh resistive heating elements. Four test specimens including 10.6 cm wide nichrome mesh heating elements embedded in C-FRTP laminates were fabricated and used for heating trials, showing that the composite can be locally heated to near the thermoplastic forming temperature over its entire thickness in less than 1 hour. The heating trials were simulated using a purpose-built finite difference model to investigate detailed temperature distributions. The simulations show that the internal resistive heating elements are capable of locally increasing the temperature of the composite with negligible effect on the adjacent material. Heating efficiency is between 73% and 77%, with temperature differences in the z-direction at peak temperatures less than 35°C. Temperature uniformity can be improved by longer heating times and more heating elements. The local heating method does not cause deconsolidation of the part. The heating method was also experimentally assessed on a more complex cross-section laminate with varying thickness and a foam core using stainless steel mesh heating elements. Results from the first heating tests were applied to the experimental assessment of the more complex laminate to reduce z-direction temperature differences. Numerical simulation of the heating of the foam core laminate showed a z-direction temperature range of 15.5°C. The results of this study show that embedded resistive heating is a cost-effective and simple method for heating a local portion of a thick fiber reinforced thermoplastic composite.

This study presents three novel methods for the computational acceleration of simulations of high temperature radiative heat transfer in particulate media. The paper presents a novel algorithm for neighbor searching that can locate and sort multi-sized particles, based on their distance, with minimal computation. The traditional use of regression recurrent neural networks for time series data is modified to a regression function for distance-ordered data. LSTM and GRU cells are used in a deep recurrent neural network to predict radiation view factors of particle and face neighbors through the distance-ordered sequential data. The preprocessing of sequential data is addressed with two new transformation methods. Through the use of the transformation methods, both the particle and face neighbors are regarded as general objects, allowing one regression neural network to predict the view factor of any number of particles and faces sequentially, which represents a unique feature. With neural networks and preprocessing methods, it is possible to predict the view factor of different-sized objects (particles and faces), representing a significant development in the field. The models demonstrate high precision for objects located within the short-range radiation region. Objects located a long distance from the emitting particle have an over-prediction of view factor, but due to their negligible contribution to overall radiation heat transfer, a high coefficient of determination is obtained for particle and face neighbors, with several orders of magnitude improvement in computational speed compared to typical Monte Carlo ray tracing methods.(c) 2022 Elsevier Ltd. All rights reserved.

Construction material is one important need for long-term habitation on the moon. Solar radiation, when concentrated for high heat flux, can heat lunar soil, or regolith, until it sinters at temperatures above 900°C. The solid, sintered regolith can be used for construction material. This work explores the conditions which lead to effective sintering of lunar soil for both directly irradiated sintering and indirect sintering. Lunar soil simulants were sintered using concentrated light from a xenon-arc lamp with varying heat flux intensity. The resulting depth of sintering, amount of material sintered, and the compressive strength of the sintered material were recorded. A sintering range of 860°C–1140°C was identified. Limited compressive strength data showed higher strength for indirect sintering.

The achievement of low oxygen partial pressures is beneficial in a number of applications, including the production of nitrogen for the Habor-Bosch process, one of the largest industrial chemical processes. It has been suggested in the literature that a redox cycle can be utilized for selective oxygen adsorption, with the potential for application in oxygen separation and compression processes. In this work we demonstrate this concept and show the feasibility of using a temperature and pressure swing redox cycle to chemically absorb oxygen gas at a low partial pressure of 10(-6) bar. The redox material used was the perovskite oxide SrFeO3, chosen for its relatively low reduction temperature and strong oxygen affinity. The material's intrinsic thermodynamics and low temperature kinetics were both investigated using thermogravimetric analysis. Multiple reduction and oxidation experiments were conducted, with oxidation performed under both synthetic air and inert gas with 1% O-2 concentration. The results show successful trials of oxygen removal resulting in inert gasses with ppm level oxygen impurities and no degradation of the material over multiple cycles. Thermodynamic considerations suggest that with further development this technology could offer a very attractive, reversible cycle for the removal of oxygen impurities from gas streams, particularly in combination with conventional pressure swing adsorption. (C) 2019 Published by Elsevier Ltd.

The "holy grail" of solar chemistry and solar engineering, is the technical storage of solar energy into a more easily transformable and transportable form, namely an energy carrier such as H-2. The two-step redox-based solar thermochemical H2O splitting cycle is considered to be among the most promising approaches for the production of H-2 from entirely renewable sources (solar energy and water). In this redox cycle an active material is initially reduced thermally under inert atmosphere and at the next step it is oxidized from H2O producing H-2. The materials that have been in the core of solar chemistry research are metal oxides such as ferrites, cerium oxides, perovskites, etc. The reactor types that are being investigated for the redox thermochemical splitting of H2O are either powder-particle reactors or structured reactors. In the current work Ni-ferrite and Ce-oxide structured into different monolithic bodies (honeycombs, foams) were evaluated w.r.t. their redox activity. Based on this investigation, the most promising structure was further scaled-up for the construction of the full-scale reactors of the HYDROSOL-PLANT solar plant installation.

In the search for new and improved materials for hydrogen and syngas production by solar thermochemical looping, test-reactors are employed which include a temperature controlled sample chamber and adjustable gas flows through or past the sample. The experiments performed in these devices enable researchers to find limiting factors like mass transfer, heat transfer, kinetics, and material durability in a time and cost efficient manner. The devices have proven their utility by their near universal employment by groups seeking and studying new materials. A review of past studies has revealed that the measurement of oxygen partial pressure during the reduction state is key to the evaluation of material productivity, yet the methods for this measurement are varied across different publications and are often given little focus. The majority of O2 sensing is achieved using a mass spectrometer or gas chromatograph, inferring behavior at the sample from measurements of gas that has traveled for some distance and time. In this paper, we investigate the potential errors which may be introduced by taking a single measurement of oxygen production at the system outlet to infer O2 production curves, and demonstrate some methods to correct this. We also investigate some of the issues related to including an oxygen sensor near the sample. Issues discussed include temporal delays between sensors, oxygen leakage, sensing an incompletely mixed flow, diffusion, and mixing downstream from the sample. Oxygen entering the system through inlet gas or leakage accounted for the largest source of error, but these errors can be corrected by straightforward methods. Numerical simulations are employed to investigate the mixing of the flow, while diffusion is estimated with an analytical model. During an example experiment, the applied correction methods reduced differences between two sensors' data from 20% to 7%, while the corrections led to a 36% change in calculated total oxygen production from raw to corrected data.

Recently a novel design concept of a reactor-the cascading pressure reactor-for the thermochemical fuel production, using a solar-driven redox cycle, was proposed. In this concept, thermal reduction of metal oxide particles is completed in multiple stages, at successively lower pressures. This leads to an order of magnitude decrease in the pumping power demand as compared to a single stage, which in turn increases the solar to fuel efficiency. An important step in the process is the transfer of heat in the form of concentrated solar radiation to the particles, while providing reducing conditions in the space surrounding the particles. In this context, a novel system for heating and reducing particles, with a focus on operating at the small prototype scale (below 20 kW), is investigated. The key goals of the system are continuous operation, uniform heating of the reactive material, the ability to heat reactive material to 1723 K or higher, and flexibility of control. These criteria have led to the conceptual design of a continuous thin-layer particle conveyor, contained in an apertured, windowed cavity and enclosed in a vacuum chamber. This chamber, in combination with a water-splitting chamber and other system components, allows the possibility of testing multiple redox materials without any significant change in the reactor design. The present work shows a potential design for the proposed component, feasibility tests of the physics of moving particles with relevant materials, and series of interconnected numerical models and calculations that can be used to size such a system for the appropriate scales of power and mass flow rates. The use of a unified design strategy has led to efficient development of the system. Experimental investigations of the horizontal motion plate allowed effective determination of motion profiles and bed uniformity. The most important factors determined through the modeling effort were the aperture diameter, which serves as the coupling point between the solar simulator lamp array and the cavity particle heating, and the particle bed thickness, which has a strong effect on the outlet temperature of the particles.

The CONTISOL concept is a new vision of an integrated solar receiver/reactor for a variety of thermochemical processes. The concept includes a single monolithic solar absorber with two inter-mixed, but non intersecting sets of gas channels. One set of channels is always used for a chemical process. During daytime operation, the other set of channels is used to heat air which is sent to thermal storage. During nighttime operation, the air flow is reversed, transferring heat from thermal storage to the monolith through the same set of channels, thus providing energy to continue chemical processing continuously through day and night. In this paper we introduce the general operation of the system and discuss its benefits applied to solar methane reforming as an example process. Past solar reactors which influenced the development of CONTISOL are discussed. A 5 kW scale demonstration prototype has been constructed at DLR and thermal experiments have been conducted using the DLR high flux solar simulator. A statistical design-of-experiments procedure has been applied to evaluate the influence of absorber temperature, gas flow rates, and gas inlet temperatures on heat transfer rates to gas streams, and to construct a thermal performance map of the device. The target gas outlet temperatures of over 850 degrees C were reached during these tests. Limitations on the initial design of the monolith are discussed including recommendations for future improvements. (C) 2017 Elsevier Ltd. All rights reserved.

A volumetric solar receiver for superheating evaporated sulfuric acid is developed as part of a 100 kW pilot plant for the hybrid sulfur (HyS) cycle. The receiver, which uses silicon carbide foam as a heat transfer medium, heats evaporated sulfuric acid using concentrated solar energy to temperatures of 1000 °C or greater, which are required for the downstream catalytic reaction to split sulfur trioxide into oxygen and sulfur dioxide. Multiple parallel approaches for modeling and analysis of the receiver are used to design the prototype. Focused numerical modeling and thermodynamic analysis are applied to answer individual design and performance questions. Numerical simulations focused on fluid flow are used to determine the best arrangement of inlets, while thermodynamic analysis is used to evaluate the optimal dimensions and operating parameters. Finally, a numerical fluid mechanics and heat transfer model is used to predict the temperature field within the receiver. Important lessons from the modeling efforts are given, and their impacts on the design of a prototype are discussed.

In this paper we describe a novel receiver-reactor concept for continuous thermochemical fuel production powered by concentrated solar energy. The novelty of the receiver-reactor is a dual use of solar power. While half of the receiver-reactor is used to conduct an endothermic reaction producing solar fuels, the other half of the reactor has a heat exchanger characteristic. Through this heat exchanger part, energy can be transferred to a heat storage during daytime operation. During nighttime operation, the energy from the storage can be brought back into the reactor. We developed a prototype reactor and set up an experiment to test this new concept under solar-like as well as nighttime conditions.

Within the European research project SOL2HY2, key components for a solar hybrid sulfur cycle are being developed and demonstrated at pilot scale in a real environment. Regarding the thermal portion, a plant for solar sulfuric acid decomposition is set up and initially operated at the research platform of the DLR Solar Tower in Jüulich, Germany. One major component is the directly irradiated volumetric receiver, superheating steam and SO3 coming from a tube-type evaporator to above 1000 °C. At the design flow rate of sulfuric acid (50%-wt.) of 1 l/min, a nominal solar power of 57 kW is required at the receiver. With a flat ceramic absorber made from SiC and a flat quartz glass window, the design is based on lab scale reactors successfully demonstrated at the solar furnace of the German Aerospace Centre (DLR) in Cologne, Germany. A flexible lumped thermodynamic tool representing the receiver, compiled to assess different configurations, is presented in detail. An additional raytracing model has been established to provide the irradiation boundaries and support the design of a conical secondary concentrator with an aperture diameter of 0.6 m. A comparison with first experimental data (up to 65% nominal power), obtained during initial operation, indicates the models to be viable tools for design and operational forecast of such systems. With a provisional method to account for the efficiency of the secondary concentrator, measured fluid outlet temperatures (up to 1000 °C) are predicted with deviations of ±60 °C. Respective absorber front temperatures (up to 1200 °C) are under-predicted by 100–200 °C, with lower deviations at higher mass flows. The measured window temperature (up to 700 °C) mainly depends on the absorber front temperature level, which is well predicted by the model.

Sulfuric acid splitting is a key step of the hybrid sulfur cycle (HyS) for solar thermochemical hydrogen production. This exothermal reaction can be divided into two steps: firstly, the evaporation of liquid sulfuric acid (H2SO4) at about 400 °C forming sulfur trioxide (SO3), and secondly, the decomposition of SO3 to sulfur dioxide (SO2) and oxygen (O2) at 800 – 1000 °C. While the first sub-reaction has fast kinetics, the second one is rather slow and requires the introduction of catalysts to achieve sufficient conversion. Since 2004 the concept of a solar receiver-reactor for sulfuric acid splitting has been developed by DLR and operated in its solar furnace during the European projects HYTHEC and HycycleS. In the follow-up European project SOL2HY2, a scale-up of this concept has been designed, developing a solar tower demonstration plant. This demonstrator has a design flow rate of 1 l/min of sulfuric acid (50 w%) and consist of four main components arranged in series and connected by Joule heated piping: a 60 kW electrical evaporator for vaporization of the liquid acid, a solar receiver for superheating the SO3 to about 1000 °C, an adiabatic reactor with a fixed bed of an iron(III) oxide catalyst and a scrubber. The Joule heated evaporator consists of six vertical steel tubes with a siliconized silicon carbide (SiSiC) inner tubes filled with porous SiSiC foam structures for enhanced heat transfer. Liquid acid is injected at the bottom and vaporizes while passing upwards through the pipes. The vapors are collected in a steel manifold and, subsequently, conveyed to the solar receiver with an outer shell also made of steel. Concentrated radiation from the solar field passes through a quartz glass window closing the receiver and heats up a sectioned absorber composed of porous SiSiC foam structures. After superheating in the receiver, the process gas passed through the catalyst bed for an adiabatic reaction forming SO2. Before neutralization with sodium hydroxide solution in the scrubber, the SO2 concentration is measured by a customized gas analysis system via UV/Vis spectroscopy. The pilot plant was constructed and assembled on the research platform of the DLR concentrating solar power tower facility in Juelich, Germany. The layout of the plant was accompanied and supported by thermo-mechanical modelling of the most important components like the evaporator and solar receiver. Initial operation of the demonstrator was performed with air and water as process fluids. During water operation, the solar receiver reached the predicted design parameters achieving a gas outlet temperature of 1000 °C at an absorber front temperature of 1200 °C and a solar power on aperture of 50 kW. In the adiabatic reactor, however, temperatures of only about 400 °C were measured which are too low for SO3 decomposition. Therefore, the system was modified placing the catalytic fixed bed directly behind the solar absorber in the outlet section of the receiver. In the following test runs, the temperatures of this adiabatic reaction zone were sufficiently high with a minimum temperature in excess of 750 °C below which the catalyst would be deactivated due to sulfate formation. As a result, testing could proceed with sulfuric acid as the feed successfully demonstrating decomposition of SO3. A detailed analysis of all results of the systematic on-sun test series is given in the present paper.

Recent work regarding the efficiency maximization for solar thermochemical fuel production in two step cycles has led to the design of a new type of reactor—the cascading pressure reactor—in which the thermal reduction step of the cycle is completed in multiple stages, at successively lower pressures. This approach enables lower thermal reduction pressures than in single-staged reactors, and decreases required pump work, leading to increased solar to fuel efficiencies. Here we report on the design and construction of a prototype cascading pressure reactor and testing of some of the key components. We especially focus on the technical challenges particular to the design, and their solutions.