Microwave Heating Efficiency Research Articles

Effect of ultrasound-assisted osmotic dehydration pretreatments on drying and quality characteristics of pulsed fluidized bed microwave freeze-dried strawberries

In order to improve the shortcomings of low heat and mass transfer efficiency and uneven heating of freeze-drying technology, microwave heating and pulsed fluidized bed technology have been introduced, and ultrasonic-assisted osmotic dehydration (USOD) has been used for pretreatment of strawberries in this study. Using different ultrasonic power (180W, 240W, 300W) and different osmotic sugar solutes (sucrose, trehalose, sucrose and maltodextrin combination) to pretreat strawberries, and determine the dielectric properties, mass transfer, and water status and distribution of strawberries. The drying performance is evaluated by the drying rate, and the drying quality is determined by the color, texture, and total polyphenols and total flavones content. It was found that compared with freeze-drying, pulsed fluidized bed microwave freeze-drying (PFBMFD) can reduce the drying time by 45%, while USOD treatment helps to further increase the drying rate and reduce drying time by 10% because it increases water loss. In addition, dried strawberries pretreated with USOD may have better texture and nutritional quality, and sugar with low hygroscopicity and low sweetness value is more suitable for the USOD process of strawberries.

The Prospect of Microwave Heating: Towards a Faster and Deeper Crack Healing in Asphalt Pavement

Microwave heating has been shown to be an effective method of heating asphalt concrete and in turn healing the damage. As such, microwave heating holds great potential in rapid (1–3 min) and effective damage healing, resulting in improvement in the service life, safety, and sustainability of asphalt pavement. This study focused on the microwave healing effect on porous asphalt concrete. Steel wool fibres were incorporated into porous asphalt to improve the microwave heating efficiency, and the optimum microwave heating time was determined. Afterwards, the microwave healing efficiency was evaluated using a semi–circular bending and healing programme. The results show that the microwave healing effect is largely determined by the steel fibre content and the mix design of the porous asphalt concrete.. Besides, the uneven heating effect of microwave contributes to an unstable damage recovery in the asphalt mixture, which makes it less efficient than induction heating. However, microwaves exhibited the ability to penetrate further into the depth of the test specimen and heat beneath the surface, indicating deeper damage recovery prospects.

Investigating the microwave heating behaviour of lunar soil simulant JSC-1A at different input powers

For a sustainable human presence on the Moon, it is critical to develop technologies that could utilise the locally available resources (a.k.a. in situ resource utilisation or ISRU) for habitat construction. As the surface soil is one of the most widely available resources at the Moon, we have investigated the viability of microwave heating of a lunar soil simulant (JSC-1A). JSC-1A was thermally treated in a bespoke microwave apparatus using 2.45 GHz frequency, using five different microwave powers in the range of 250 W to 1000 W. The structural properties of the resulting products were analysed to determine whether their microstructures and mechanical strengths differ under different input powers; and whether input power plays a crucial role in triggering thermal runaway, for identifying the optimum power for developing a microwave-heating. Our key findings are: (i) the higher input powers (800 W and 1000 W) generate the highest yields and microstructures with the greatest mechanical strengths, at the shortest fabrication times, and (ii) thermal runaway improves the microwave heating efficiency despite the rapid increase in temperature, once it is triggered. Our findings are of key importance for developing a microwave-heating payload for future lunar ISRU demonstration missions, contributing towards 3D printing-based extra-terrestrial construction processes.

Improved microwave heating uniformity and self-healing properties of steel slag asphalt containing ferrite filler

As a preventive maintenance approach, microwave self-healing is an effective method to repair small pavement cracks at early stage and reduce maintenance costs. However, the difference in microwave absorption ability of asphalt pavement engenders the common problem of uneven heating. This study aims to evaluate the feasibility of applying ferrite to replace partial limestone filler in order to enhance the microwave heating uniformity and self-healing properties of steel slag asphalt mixtures. The morphological features and microwave absorption properties of ferrite were firstly characterized. The addition of ferrite content was estimated via rheological tests including Bending Beam Rheometer and Dynamic Shear Rheometer. Finally, the semi-circular bending and thermal analysis tests were performed to determine the optimal ferrite content. Results demonstrated that ferrite displayed excellent microwave absorption properties; nevertheless, excessive use of ferrite may weaken the rheological behavior of asphalt mortar at low temperatures. Moreover, the addition of ferrite can significantly promote the microwave heating uniformity and heating efficiency level of specimens, while slightly increase the self-healing ratio. Overall, replacing 20% limestone filler with ferrite in volume is a promising solution to properly enhance a uniform microwave heating of steel slag asphalt mixtures.

Glass-ceramic Foams Made of Very High Coal Fly Ash Weight Ratio by the Direct Microwave Heating Technique

A high mechanical strength glass-ceramic foam was produced by direct microwave heating at 853 ºC of a very high weight ratio of coal fly ash (82%), calcium carbonate (5%) as a foaming agent, sodium carbonate (13%) as a fluxing agent and water addition (10%). Due to the excellent energy efficiency of the direct microwave heating, the heating rate had a very high value (32 ºC/min), much higher than the heating rate of conventional processes and led to a very low value of the specific energy consumption (0.72 kWh/kg). The physical and mechanical characteristics of the optimal glass-ceramic foam sample were: apparent density of 1.44 g/cm3, porosity of 26.2%, thermal conductivity of 0.281 W/m·K, compressive strength of 41.3 MPa and water absorption of 0.5%. Given the features of the glass-ceramic foam (very high compressive strength, acceptable porosity and thermal conductivity, very low water permeability, fireproof, chemical stability, no-toxicity, etc., the application domain of this material type may include road and railway constructions, bridge abutments and retaining walls, foundations, drainages, sports grounds and other types of constructions that require high mechanical stress.

Preparation and Properties of Microwave-Absorbing Asphalt Mixtures Containing Graphite and Magnetite Powder

Abstract The concept of microwave-absorbing asphalt concrete has been proposed to create an asphalt mixture with high microwave heating efficiency. A mixture of 20 % graphite powder and 80 % magnetite powder was used as the microwave-absorbing composite to replace the common mineral filler in asphalt mixtures to improve its microwave absorption capacity. This study aimed to evaluate the effect of proposed absorbent composite on the properties and microwave heating efficiency of asphalt mixtures. The results indicate that the addition of graphite and magnetite powder exhibited an improvement in the microwave heating efficiency of asphalt mixtures. The heating rate of asphalt mastic was improved by more than 10 times, whereas the heating rate of asphalt mixtures containing the absorbent composite was improved by 47.8 %. The pavement performance test results show that the mechanical performances including the dynamic stability, bending strain, residual Marshall stability, and ratio of splitting tensile strength after and before the freeze-thaw cycle satisfy the relevant specification requirements.

Dielectric properties and electromagnetic wave transmission performance of aluminium silicate fibreboard at 915 MHz and 2450 MHz

Refractory lining is an important part of microwave heating equipment, and its wave-transparent effect significantly influences microwave heating efficiency. The complex dielectric constant of the material is the main factor determining its wave-transparent properties. In this study, with aluminium silicate fibreboard (ASF) as the study object, the complex dielectric constant of this material was investigated at temperatures ranging from 25 °C to 1000 °C at 915 MHz and 2450 MHz, respectively. Based on the theory of transmission lines, the wave-transparent properties of the material during the entire microwave heating process were calculated and analysed. It has been found that the dielectric constant and loss factor at both frequencies increase significantly when the temperature exceeds 700 °C; the wave-transparent properties of this material decrease with increasing temperature and present a fluctuating trend as the material thickness increases, with maximum and minimum values; in TM polarization mode, there is an optimal angle of incidence for total transmission, which contributes to a better overall wave-transparent effect of the material in TM polarization mode than that in TE polarization mode. Finally, this study provides the preferred thickness values at different working frequencies and temperatures under vertical incidence of electromagnetic waves for reference. This study is of certain theoretical significance to the research on the change mechanism of the material's dielectric and wave-transparent properties and has practical guidance on the optimization of microwave heating equipment and the selection of the technological parameters of furnace linings.

Energy recovery from sewage sludge: Product characteristics, heating value prediction and reaction kinetics

Energy recovery from sewage sludge was carried out by using microwave and conventional torrefaction. The microwave torrefaction was carried out by using a laboratory-scale microwave oven that provides single-mode microwave irradiation at 2.45 GHz, and the amount of sewage sludge for each experiment was approximately 20 g. The efficiency of microwave heating can be substantially promoted at higher power level, resulting in higher heating rate and maximum temperature. According to higher energy yield and heating value of torrefied sewage sludge, the optimum power level for bioenergy produced by microwave torrefaction of sewage sludge should be 200 W. Because of lower mass yield and temperature required to obtain the same yield, microwave heating can be more effective than conventional heating for sewage sludge torrefaction. The elemental composition of torrefied sewage sludge at 400 W was similar to that of anthracite, and its low hydrogen and oxygen contents could prevent excessive formation of smoke. Two correlations were obtained to predict the HHV of SS based on proximate and elemental compositions. With the recovery of liquid and gas products as bioenergy, the energy return on investment for microwave torrefaction of sewage sludge can be up to 16.4, much higher than the minimum value required for a sustainable society. Because of lower activation energy but higher pre-exponential factor, microwave heating can be approximately five times faster than conventional heating.

ИМИТАЦИОННОЕ МОДЕЛИРОВАНИЕ НАГРЕВА ЗЕРНА ЭНЕРГИЕЙ ЭЛЕКТРОМАГНИТНОГО СВЧ ПОЛЯ

High speed and efficiency of microwave heating of dielectrics makes it possible to use theelectromagnetic field to heat and dry almost any material containing moisture. One of the promisingareas for the use of microwave energy is the intensification of the grain drying process.To ensure rapid prototyping and reduce the possibility of losses caused by errors at various stagesof equipment development and implementation, model development is of particular importance.The purpose of this work is to perform simulation of a slotted waveguide grating for grain drying.The main task of the simulation is to ensure an even distribution of power to each slot. A specializedsoftware package FEKO is used as a modeling environment. To describe the model, an exactcalculation method is used - the method of moments. The distribution of the electric field and specificabsorbed power in a layer of grain exposed to microwave radiation is obtained. The modelswere verified by comparing the results obtained with different mesh values of surface elements.Modeling the distribution of the electric field strength showed that moving the waveguides awayfrom the cell with the grain leads to the leveling of the field irregularities, but in this case some ofthe radiated power is dissipated into space. In this case, an increase in the distance between thewaveguides and the cell with grain does not affect the antenna SWR. It should be noted that in areal situation, the wave nature of the field is smoothed out due to heat transfer, and this phenomenoncannot be taken into account in the process of electromagnetic simulation. In addition, theentire variety of possible changes in the dielectric parameters of grain should be investigated inorder to analyze the process of wave propagation in grain with high moisture. It is also advisableto consider other modifications of slotted waveguide antennas, which can provide more uniformheating in the container with grain.

Microwave heating improvement of asphalt mixtures through optimizing layer thickness of magnetite and limestone aggregates

The key to the development of microwave deicing technology in asphalt pavement engineering is to improve the heating efficiency using less magnetite aggregate with regards to the aggregate resource shortage. In this work, four asphalt mixtures with different magnetite layer thickness (MALT) by volume of the limestone aggregate were prepared. The microwave heating capacity test of magnetite aggregate was carried out firstly. Surface temperature, heat distribution, radiation depth, deicing time, and thermal diffusion for asphalt mixtures were also investigated. Results show that 4.75mm–9.5 mm is considered as the most suitable sieve size according to its higher microwave heating efficiency (MHE) and the highest proportion in aggregate gradation. The reduction of MALT in asphalt mixture can significantly improve the MHE ascribed to higher thermal conversion and utilization, related to the lower microwave reflectivity and shorter propagation path of internal heat to the surface obtained through adjusting MALT. The heat distribution of asphalt mixture with thinner MALT was more unevenly, which can be mainly attributed to the higher heat generating rate. Besides, the heat generated by the upper layer can be quickly transferred to limestone aggregate layer and thus the heat depth can not be used to represent the depth of radiation. In addition, both MALT and initial temperature have effect on the heat diffusion. The reason behind the higher deicing efficiency for asphalt mixture with thinner MALT was owing to both higher MHE and higher heat transfer efficiency.

Microwave heating based on two rotary waveguides to improve efficiency and uniformity by gradient descent method

This study aims to improve the efficiency and uniformity of microwave heating based on a rotary radiation structure. This study’s steps are summarized as follows. First, using the finite element method in a simulation model, two radiation ports revolve on their axes and heat the material. Second, the heating efficiency at different angles is analyzed to find efficient angles of radiation ports. Finally, to improve the uniformity of material with high efficiency, we use the gradient descent method in machine learning to optimize the heating time at selected angles. An experimental system is established for this research and the simulation results are verified by experiments, showing that this research has an obvious superiority and efficiency and uniformity of the material have improved compared to traditional heating results without angle selection and optimization of the heating time. What is more, influences of material with different shapes, positions and materials have also been discussed.

Microwave deicing efficiency and dielectric property of road concrete modified using different wave absorbing material

The efficiency of microwave de-icing is a factor in the popularisation and application of this method. In this study, iron black, silicon carbon, and graphite were added to road concrete to enhance the microwave de-icing efficiency. The microwave heating efficiency and temperature distribution on a concrete surface were also investigated. In addition, the de-icing area and dielectric property of wave absorbing concrete were also investigated and compared. The results indicate that the temperature time curve during microwave de-icing can be divided into three stages based on the different microwave heating efficiencies. An ice layer decreases the microwave heating efficiency of a concrete surface, and when the ice layer turns into water, the microwave heating efficiency of the concrete clearly increases. The addition of a wave absorbing material enhances the microwave de-icing efficiency of the concrete. When the addition of graphite reaches 15%, the microwave heating efficiency of the concrete increases by 0.98 °C/s, the relative dielectric constant increases by 3.5–4.7, and the de-icing area reaches 66.7 cm2, which is 5.1-times that of ordinary Portland cement (PC) concrete. Iron black also enhances the microwave de-icing efficiency of the concrete, and the microwave heating efficiency increases by 0.35 °C/s and the effective de-icing area increases to 33.6 cm2 with the addition of 15% iron black. The enhancements of the three absorbing materials from high to low are from graphite, iron black, and silicon carbon. Dielectric property tests of concrete show that the relative dielectric constant and dielectric loss tangent are closely related to the microwave de-icing efficiency.

The impact of pins on dual-port microwave heating uniformity and efficiency with dual frequency

This work aims to improve the uniformity and efficiency of microwave heating. A dual-port simulation model based on the finite element method was built, and two pins were applied to the heating cavity. The impact of dual frequency and pins on the coefficient of variation (COV) and heating efficiency were investigated. The results demonstrated that the uniformity of dual-frequency heating can be improved by 15.4%. Moreover dual-frequency with double pins can maximally increase heating efficiency to 166.7% and the COV value can be reduced to 60.2%, compared with single-frequency heating. The heating effect of other materials was also analyzed. The uniform and efficient microwave heating were realized at dual frequency with pins.

Microwave Healing Performance of Asphalt Mixture Containing Electric Arc Furnace (EAF) Slag and Graphene Nanoplatelets (GNPs)

Pavement preventive maintenance is an important tool for extending the service life of the road pavements. Microwave heating seems to be a promising technology for this application, as bituminous materials have the potential to self-repair above a certain temperature. As ordinary asphalt mixture has low microwave absorbing properties, some additives should be used to improve the heating efficiency. In this paper, the effect of adding Electric Arc Furnace (EAF) slag and Graphene Nanoplatelets (GNPs) on the microwave heating and healing efficiency of asphalt mixtures was evaluated. Microwave heating efficiency was assessed by heating the specimens using several heating times. In addition, the electrical resistivity of the mixtures was measured to understand its possible relationship with the microwave heating process. Furthermore, the healing rates of the asphalt mixtures were assessed by repeated Indirect Tensile Strength (ITS) tests. The results obtained indicate that the additions of graphene and EAF slag can allow important savings, up to 50%, on the energy required to perform a good healing process.

Dielectric properties and electromagnetic wave transmission performance of polycrystalline mullite fiberboard at 2.45 GHz

Refractory lining is an indispensable part of high temperature microwave heating equipment, and its wave transmission performance exerts an important impact on the mode and efficiency of microwave heating, while the complex dielectric constant (dielectric constant and dielectric loss) of the material is the decisive factor in determining the wave transmission performance of the material. In this work, we measured the complex dielectric constant of polycrystalline mullite fiber board (PMF) in the temperature range of 25–1000 °C, and the effect of temperature on dielectric constant and dielectric loss was analyzed; The wave-transmission properties of the material were calculated according to the theory of electromagnetic wave transmission line, and the effects of temperature, material thickness, polarization modes of electromagnetic wave and incident angle on the wave transmission performance were analyzed. The results reveal that the dielectric constant of PMF does not change much with the increase of temperature, which is about 1.6; The dielectric loss does not change much within 200 °C, but when the temperature is higher than 200 °C, the change presents approximately exponential increase with the rise of temperature. The wave transmission performance fluctuates with the increase of the thickness, and there are maximum value and minimum value, and the overall wave transmission performance decreases with the increase of the material thickness. In a transverse electric (TE) field, the overall wave transmission performance decreases with the increase of the incident angle, and better wave transmission performance can be obtained by priority selection of vertical incidence of electromagnetic wave. In a transverse magnetic (TM) field, with the increase in the incident angle, the wave transmission performance firstly climbs up then declines, and there is an optimal incident angle where total transmission can occur. Finally, this work selected the thickness corresponding to different temperature as the preferred thickness. This work is of important theoretical significance for understanding the mechanism of the dynamic change of the wave transmission performance of the thermal insulation materials in microwave heating, and provides important practical guidance for the design and optimization of microwave heating equipment.

Effect of dielectric properties on heat transfer characteristics of rubber materials via microwave heating

The effects of dielectric properties on temperature distribution, heat flux density distribution, heating rate, heating efficiency and heating uniformity of rubber sheets via microwave heating were studied. Experimental and numerical simulation studies were carried out, the reliability of the simulation method was verified. The results demonstrated that the ability of rubber to absorb microwave energy and convert it into heat is significantly affected by dielectric constant and dielectric loss tangent. Different dielectric properties led to different electric field strength distributions in rubber sheets during microwave heating process, resulting in different electrical energy densities in the rubber sheets, eventually, different amounts of electric field energy can be converted into heat, resulting in different temperature distributions. The temperature distributions of rubber sheets are uneven. The temperature difference of rubber sheet increases with increasing heating time, but the growth rate of temperature difference decreases with increasing heating time. The higher the dielectric constant or loss tangent of rubber, the higher the microwave heating efficiency and temperature rise rate, but the more uneven the temperature distribution. The longer the heating time, the higher the average body temperature of rubber sheet, but the growth rate of the average body temperature decreases with increasing heating time. The heat flux density of rubber sheet was also significantly influenced by the dielectric properties, the maximum temperature gradient value is in the central area of the rubber sheet rather than in the hot spot area.

Soil compaction enhances the impact of microwave heating on seedling emergence

An innovative microwave-based process to control invasive species in the field is currently under development. We investigated the effect of soil compaction levels under three microwave treatment pairings of power and duration (2kW4 min, 2kW8min and 4kW4 min), on seedling emergence of a diverse seed bank and three major invasive species in Europe (bohemian knotweed, giant goldenrod and jimsonweed). When soil was compacted, the number of seedling emergences of dicotyledons was significantly reduced by the 2kW4 min treatment. Furthermore, while some dicotyledon and giant goldenrod’s seedling emergences were observed respectively in plain soil in the 4kW4 min and 2kW4 min treatments, no seedling was recorded in compacted soil for the same treatments. Our results suggest that compacting soil increases the efficiency of microwave heating on seeds. Soil compaction level should therefore be considered in the design of the final microwave-based process.

Electromagnetic waves transmission performance of alumina refractory ceramics in 2.45 GHz microwave heating

Refractory materials are an important part of industrial microwave heating equipment, and their transmission performance has a significant influence on the efficiency and uniformity of microwave heating. In this study, electromagnetic wave power transmission coefficients (PTCs) of alumina refractory ceramics are calculated within the temperature range of 22–1379 °C and the thickness range of 0–0.1 m at 2.45 GHz, and the influences of the complex dielectric constant of the material, the thickness of the medium, the polarisation mode of the electromagnetic waves, and the incident angle of the electromagnetic waves on PTC are analysed. It was found that the transmission performance of alumina ceramics fluctuates with increasing thickness at different temperatures and that there are several transmission peaks. The transmission performance in a transverse electric (TE) field decreases monotonically with increasing incident angle, and the PTC is equal to zero when the incident angle is 90°. The transmission performance in a transverse magnetic (TM) field increases first and then decreases with increasing incident angle, and there is a transmission peak. Transmission peaks at different thicknesses form a ‘ridge’ at the Brewster's angle, at which the fluctuation amplitude of PTC becomes very small. As the temperature increases, the ‘ridge’ becomes steeper. Finally, the incident angle range with good transmission performance of alumina ceramics in the temperature range of 22–1379 °C is obtained, and the optimal thickness interval at the optimum incident angle (73.8°) is calculated for reference. This research has important guiding significance for the selection and optimisation of refractory ceramic materials for use in microwave high-temperature heating.

Importance of the Mixing and High-Temperature Heating Steps in the Controlled Thermal Coprecipitation Synthesis of Sub-5-nm Na(Gd–Yb)F4:Tm

In order to achieve a significant size reduction to get ultrasmall upconverting nanoparticles (UCNPs) following a thermal coprecipitation pathway, we identified two critical points: the UCNP precursor mixing and high-temperature heating steps. Significant differences could be observed according to the way the inorganic sodium and fluoride sources were mixed to the rare-earth oleate before the high-temperature heating step. More interestingly, accurate monitoring of the high-temperature heating step using microwave (MW) dielectric heating yielded major improvement toward ultrasmall UCNPs. Thus, hexagonal, Tm-doped sub-5-nm UCNPs with an unusual Na(Yb-Gd)F4 matrix with 53% Yb were produced, displaying satisfactory luminescence. Noticeably, MW heating was achieved in a weakly MW-absorbing oleic acid (OA)/octadecene mixture, and the influence of the OA content composition on the MW heating efficiency is discussed in this report.

Laboratory and Numerical Investigation of Microwave Heating Properties of Asphalt Mixture.

Microwave heating is an encouraging heating technology for the maintenance, recycling, and deicing of asphalt pavement. To investigate the microwave heating properties of asphalt mixture, laboratory tests and numerical simulations were done and compared. Two types of Stone Mastic Asphalt (SMA) mixture samples (with basalt aggregates and steel slag aggregates) were heated using a microwave oven for different times. Numerical simulation models of microwave heating of asphalt mixture were developed with finite element software COMSOL Multiphysics. The main thermal and electromagnetic properties of asphalt mixture, served as the model input parameters, were measured through a series of laboratory tests. Both laboratory-measured and numerical simulated surface temperatures were recorded and analyzed. Results show that the replacement of basalt aggregates with steel slag aggregates can significantly increase the microwave heating efficiency of asphalt mixture. Numerical simulation results have a good correlation with laboratory test results. It is feasible to use the developed model coupling electromagnetic waves with heat transfer to simulate the microwave heating process of asphalt mixture.