1,721,011 research outputs found
A new approach for the vitrification of municipal solid waste incinerator bottom ash by microwave irradiation
Full text linkEncouraging the transition to a circular economy, the valorization of municipal solid waste incinerator (MSWI) bottom ash (BA) has received considerable attention in many processes. In the present work, flash microwave vitrification was effectively realized in a single mode cavity operating at 2.45 GHz within 1.5 min. The closed-loop process was evaluated in terms of energy and power input, treatment time and vitrified bottom ash (VBA) yield rate. The required minimum energy consumption was ∼3300 kJ/kg. By conducting thermo-electromagnetic multiphysics simulations, the heating mechanism of BA by microwave irradiation was underpinned. This relied on the generation of microwave-induced hot spots inside the material and high power density, in the order of 3 × 107 W/m3, that triggered the onset of BA melting at high heating rates. The inherent cold environment of the microwave cavity, due to the absence of any insulation material, in conjunction with the high silica content of BA promoted the glass forming ability of the melt. This allowed a natural fast cooling of the melt and VBA production, avoiding the cost and environmental impact accompanying conventional quenching. Preliminary characterization of the highly amorphous VBA product was performed and its exothermal heat flow after alkali activation revealed the potential incorporation in the binder of novel building materials
Dynamic Superficial Dielectric Constant Models and Void Fraction Prediction by Microwave Resonant Cavity for Gas Liquid Flow
Full text linkIn the investigation of non-uniform flow measurement utilizing microwave technologies, both the dielectric properties and phase fraction of the flow are crucial factors. This study introduces a new concept of dynamic superficial dielectric constant (DSDC) to characterize the variations in dielectric constant of the fluids, thereby facilitating the development of a method to measure void fraction in air-water flows. A detection technique employing a microwave resonant cavity (MRC) was implemented, and experiments were conducted for various air-water flow void fractions using an MRC sensor under pressures ranging from 0.3 to 0.9 MPa and superficial gas velocities from 5 to 12 m/s. The experimental results demonstrated that the void fraction of the air-water flow significantly affects the resonant frequency, while the pressure and superficial gas velocity have no direct impact. Furthermore, DSDC models were constructed and assessed based on the MRC sensor data. The proposed methodology, when applied to void fraction measurement in stratified and annular flows, yielded an accuracy within ±5% for approximately 91% and 95% of tested samples, respectively. The introduced concept, method, and DSDC model offer a novel, feasible and precise measurement system capable of characterizing the dielectric properties and estimating phase fractions in non-uniform flow fields
Experiments and modelling of phase fraction in gas-liquid two-phase flow using a microwave resonant cavity sensor
No full textGas-liquid two-phase flow is prevalent in the natural gas industry, and accurate phase fraction measurement is crucial for enhancing productivity and energy efficiency in industrial processes. However, achieving high-precision, in-situ measurement remains challenging. To address this issue, this study proposes novel prediction models based on the microwave cylindrical resonant cavity (MCRC) sensor. Firstly, the MCRC sensor was implemented, and the experiments were conducted by incorporating a quick-closing valve calibration system into an existing gas-water reference system, capturing a multi-parameter dataset. The analysis indicated that a complex nonlinear relationship existed among phase fraction, relative frequency shift, pressure, and superficial gas velocity. Then, phase fraction prediction models, including void fraction and gas volume fraction (GVF) model, were developed using the empirical and machine learning modelling methods. The results revealed that empirical models without intermediate dielectric constant complex calculation achieved relative errors within ±5 %. Among the 5 machine learning models compared, the XGBoost model performed the best, with over 95 % of data points within ±2 %. Additionally, extended experiments were used to estimate the generalization ability of the GVF prediction models, demonstrating excellent performance. Finally, the comparative error analysis confirmed the superior accuracy of the proposed models. The findings suggest that the proposed models offer notable improvements in prediction accuracy and practical applicability, making them promising methods for phase fraction prediction in gas-liquid flow using the MCRC sensor in the natural gas industry
Experimental and computational investigation of heat transfer in a microwave-assisted flow system
Full text linkMicrowave technology is gaining popularity as a tool for chemical process intensification and an alternative to conventional heating. However, in flow systems non-uniform temperature profiles are commonly encountered and hence methods to characterise and improve them are required. In this work, we studied the effects of various operational parameters-microwave power, inlet flow rate, tube orientation and pressure-on the electric field and temperature profiles of water flowing in a PTFE tube (2.4 mm internal diameter), placed in a commercial single-mode microwave applicator. A finite element model was developed to estimate the longitudinal temperature profiles and the absorbed microwave power, while in situ temperature monitoring was performed by a fibre optic probe placed at multiple locations inside the tube. The water temperature inside the tube increased by increasing the microwave power input and temperature profiles stabilised beyond 20 W, while the percentage absorbed microwave power showed the inverse trend. When changing the tube orientation or decreasing the inlet flow rate, microwave absorption decreased significantly. When the pressure was increased to 2.3 bara, water temperature increased by ~ 20 o C. Results from this study provide valuable insights on achievable temperature profiles and energy efficiency of microwave-assisted flow synthesis systems.
Dielectric characterisation of solar salt for volumetric heating applications in Power-to-Heat-to-Power systems
Full text linkCarnot batteries, or Power-to-Heat-to-Power systems, rely on solar salt as a thermal energy storage medium and require efficient and controllable heating technologies. However, conventional resistive heating is constrained by the low thermal conductivity of solar salt, leading to temperature gradients, local overheating, and material degradation, which motivates the exploration of alternative volumetric heating approaches. In this context, this study evaluates the feasibility of microwave-based volumetric heating of solar salt by analysing its dielectric behaviour across both solid and molten states. Dielectric properties were measured using the cavity perturbation method at 912 MHz and 2.45 GHz with different sample volumes and electromagnetic field configurations. Under these conditions, the sharp increase in electrical conductivity in the molten state results in high effective dielectric losses that violate the small-perturbation assumption underlying this technique. Consequently, the microwave measurements were complemented by four-electrode electrochemical impedance spectroscopy from 100 Hz to 1 MHz up to 550 • C to confirm the dominance of ionic transport mechanisms. The results show activation energies of 0.810 eV in the solid state and 0.148 eV in the liquid state, while extrapolated conductivities of approximately 160-170 S m −1 correspond to microwave penetration depths of about 1.3 mm at 912 MHz and 0.8 mm at 2.45 GHz, providing an application-relevant measure of the interaction between molten solar salt and electromagnetic fields. These findings indicate that accurate dielectric characterisation of molten solar salt at microwave frequencies requires measurement systems specifically adapted to highly conductive liquids and suggest that effective microwave heating strategies may rely on solar salt-compatible ceramic materials combined with appropriately tailored electromagnetic field distributions
Predicting the Behaviour of Near-Critical and Supercritical Alcohols at Microwave Frequencies: Validation of Molecular Dynamic Simulations as a Tool that can Substitute for Measurements under Extreme Experimental Conditions
Full text linkEquilibrium and non-equilibrium molecular dynamic simulations, predicting the dielectric properties of near-critical and supercritical methanol and ethanol at microwave frequencies have been carried out. The autocorrelation functions of the dielectric relaxation, show dependency on the slow component at the near-critical region for both alcohols. At the supercritical region, two competing relaxation mechanisms are observed, related to the large breakdown of the hydrogen-bonding network and the degree of clustering between the molecules. This approach closely matches experimental data at microwave frequencies and identical temperature and pressure conditions, validating the predictions of how the molecular structure and dynamics manifest themselves into the complex permittivity and dielectric relaxation behaviour. Thus, introducing a modelling-based solution to deliver accurate dielectric property values for materials at supercritical conditions for “a priori” screening of solvents, whilst removing the need to overcome engineering and safety challenges associated with the development of experimental equipment to practically generate such data
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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