Microwave curing at an operating frequency of 2.45 GHz of Portland cement paste at early-stage using a multi-mode cavity: Experimental and numerical analysis on heat transfer characteristics

In this study, microwave curing at an operating frequency of 2.45 GHz of Portland cement paste at an early-stage using a multi-mode cavity is presented. First, the dielectric evolution of the cement paste at a water-to-cement mass ratio of 0.38 during a 24-h first-hydration period was measured. Secondly, the microstructural characteristics of the hardened cement paste after heating for 45 min in microwaves at a power of 390 W, with specific attention to the temperature rise were investigated experimentally and theoretically. The obtained results show that dielectric properties decrease rapidly during the hydration reaction and formation resuming and then proceeding with a high rate. During microwave heating, the temperature increased monotonically. The micrographs of the microwave-heat paste clearly indicate that the samples consisted of hydrated phases and pores, as well as cores of Ca(OH)₂ dendrite crystals, calcium silicate hydrate (C–S–H), and granular structure. The results of the temperature rise in our experiment without loss of moisture and steady heat transfer conduction, consistently agreed with the mathematical model developed for this study.     

Disruption of sugarcane bagasse lignocellulosic structure by means of dilute sulfuric acid pretreatment with microwave-assisted heating

Disruption of lignocellulosic structure of biomass plays a key role in producing bioethanol from lignocelluloses. This study investigated the impact of dilute sulfuric acid pretreatment on bagasse structure using microwave heating. Three reaction temperatures of 130, 160 and 190 °C with two heating times of 5 and 10 min were considered and a number of instruments were employed to analyze the properties of the bagasse particles. On account of microwave irradiation into the solution with dielectric heating, the experiments indicated that an increase in reaction temperature destroyed the lignocellulosic structure of bagasse in a significant way. The pretreated bagasse particles were simultaneously characterized by fragmentation and swelling. When the reaction temperature was as high as 190 °C, the fragmentation of particles became fairly pronounced so that the specific surface area of the pretreated material grew substantially. Meanwhile, almost all hemicellulose was removed from bagasse and the crystalline structure of cellulose disappeared. In contrast, the feature of lignin was remained clearly. However, a comparison between the heating times of 5 and 10 min revealed that the influence of the heating time on the lignocellulosic structure was not significant, indicating that the pretreatment with 5 min was sufficiently long.     

Mathematical modeling of continuous flow microwave heating of liquids (effects of dielectric properties and design parameters)

A detailed numerical model is presented to study heat transfer in liquids as they flow continuously in a circular duct that is subjected to microwave heating. Three types of food liquids are investigated: apple sauce, skim milk, and tomato sauce. The transient Maxwell's equations are solved by the finite difference time domain (FDTD) method to describe the electromagnetic field in the microwave cavity and the waveguide. The temperature field inside the applicator duct is determined by the solution of the momentum, energy, and Maxwell's equations. Simulations aid in understanding the effects of dielectric properties of the fluid, the applicator diameter and its location, as well as the geometry of the microwave cavity on the heating process. Numerical results show that the heating pattern strongly depends on the dielectric properties of the fluid in the duct and the geometry of the microwave heating system.     

Specific energy consumption in microwave drying of garlic cloves

The convective and microwave-convective drying of garlic cloves was carried out in a laboratory scale microwave dryer, which was developed for this purpose. The specific energy consumption involved in the two drying processes was estimated from the energy supplied to the various components of the dryer during the drying period. The specific energy consumption was computed by dividing the total energy supplied by amount of water removed during the drying process. The specific energy consumption in convective drying of garlic cloves at 70 °C temperature and 1.0 m/s air velocity was estimated as 85.45 MJ/kg of water evaporated. The increase in air velocity increased the energy consumption. The specific energy consumption at 40 W of microwave power output, 70 °C air temperature and 1.0 m/s air velocity was 26.32 MJ/kg of water removed, resulting in about a 70% energy saving as compared to convective drying processes. The drying time increased with increase in air velocity in microwave-convective drying process; a trend reverse to what was observed in convective drying process of garlic cloves.     

Microwave pre-curing of natural rubber-compounding using a rectangular wave guide

This article presents a new method to pre-cure natural rubber-compounding (NRc) by using microwave energy at a frequency of 2.45 GHz with a rectangular wave guide. The influences of microwave power input, specimen thicknesses, and vulcanized sulfur contents on the dielectric and thermal characteristics and cross-linked contents of microwave-cured NRc are studied. Furthermore, a generalized mathematical model for predicting temperature distribution inside the specimen during pre-heating is proposed. Significant results show that microwave energy can produce partial cross-linking at temperatures below the actual vulcanizing process. The numerical results from the model agree well with the results from the experiments.     

Effects of temperature dependent properties in electromagnetic heating

Electromagnetic heating such as microwave processing and radio frequency heating becomes very popular because of its non-contact, pollution free and fast distribution of thermal energy within the object of interest. In electromagnetic heating, the temperature distribution within a sample greatly depends on the dielectric properties which are functions of electromagnetic frequency, temperature and the composition of the object. There are many experimental and numerical investigations on electromagnetic heating because of its widespread use in food and other industries, but only few researchers have looked at this problem from analytic point of view specifically for the temperature dependent properties. In this paper, we developed an analytic expression for temperature distribution in a three dimensional rectangular object under electromagnetic heating and presented a method to incorporate temperature dependent properties of the object in determining the temperature distribution at different times. A simplified Maxwell’s equation is solved for plane wave to obtain electric field distribution in the body, and the electromagnetic power absorption is computed from the electric field distribution which was then used as a source term in the energy equation. Next an unsteady, three dimensional, non-homogenous energy equation is solved by integral transform technique to obtain temperature distribution. Finally, this closed form analytical solution is used to study the effects of electromagnetic frequency, dielectric properties, and heat transfer coefficient on temperature distribution in a rectangular salmon fillet. It is found that incident frequency, sample thickness and processing time have significant influence on the heating pattern. For radio frequency heating, the temperature dependent dielectric properties influence the temperature distribution significantly, but the effect of temperature dependent dielectric properties is less dominant for the microwave frequency used in the household microwave oven. Our results also show that microwave heating provides heterogeneous temperature distribution with alternate hot and cold spots. On the other hand, the radio frequency heating allows almost uniform temperature distribution within the body, which makes it a better choice for quick and convenient heating process especially for commercial and industrial heating.     

Cooling strategies,summer comfort and energy performance of a rehabilitated passive standard office building

One of the first rehabilitated passive energy standard office buildings in Europe was extensively monitored over two years to analyse the cooling performance of a ground heat exchanger and mechanical night ventilation together with the summer comfort in the building. To increase the storage mass in the light weight top floor, phase change materials (PCM) were used in the ceiling and wall construction. The earth heat exchanger installed at a low depth of 1.2 m has an excellent electrical cooling coefficient of performance of 18, but with an average cooling power of about 1.5 kW does not contribute significantly to cooling load removal. Mechanical night ventilation with 2 air changes also delivered cold at a good coefficient of performance of 6 with 14 kW maximum power. However, the night air exchange was too low to completely discharge the ceilings, so that the PCM material was not effective in a warm period of several days. In the ground floor offices the heat removal through the floor to ground of 2–3 W m⁻² K⁻¹ was in the same order of magnitude than the charging heat flux of the ceilings. The number of hours above 26 °C was about 10% of all office hours. The energy performance of the building is excellent with a total primary energy consumption for heating and electricity of 107–115 kW h m⁻² a⁻¹, without computing equipment only 40–45 kW h m⁻² a⁻¹.     

A theoretical analysis on the effect of containers on the microwave heating of materials

Microwave heating is generally performed by positioning the sample within a container. The container can reflect, absorb or transmit microwaves based on the dielectric properties and that can influence the microwave heating characteristics of the sample. This work is an attempt to theoretically analyze the alteration of the microwave heating characteristics of materials due to the use of either a low-lossy alumina container or a high lossy SiC container. The heating characteristics have been simulated for the high-lossy beef and low-lossy bread samples of a fixed dimension by solving the coupled energy balance equation and detailed Helmholtz wave propagation equations within the sample-container assembly. It has been shown that the microwave heating characteristics can be significantly altered in the presence of the container based on the relative dielectric properties of the materials. The alumina container has been found to be efficient to enhance the microwave heating efficiency of the high lossy material such as beef, while the rapid microwave heating of the SiC container has been found to be beneficial to enhance the heating of the low lossy material such as bread in some cases.     

The viscous dissipation effect on heat transfer and fluid flow in micro-tubes

The numerical modeling of the conjugate heat transfer and fluid flow through the micro-tube was presented in the paper, considering the viscous dissipation effect. Three different fluids with temperature dependent fluid properties are considered: water and two dielectric fluids, HFE-7600 and FC-70. The diameter ratio of the micro-tube was D_i/D_o = 0.1/0.3 mm with a tube length L = 100 mm. The laminar fluid flow regime is analyzed. Two different heat transfer conditions are considered: heating and cooling and three different Br = 0.01, 0.1 and 0.5. The influence of the viscous heating on Nu and Po is analyzed and compared with Br = 0.     

Frequency response and activation energy of palm-fatty acids at microwave region

Microwave technology has the potential to significantly enhance chemical reaction and knowledge of dielectric properties of materials plays a major role in microwave design for any process. In this paper, the dielectric properties (permittivity and conductivity) of palm-fatty acids were investigated. Palm-fatty acids were chosen being the most versatile and sustainable source of raw materials, also considered as the “gate-way” to other oleochemicals derivatives. Thus, the dielectric properties of commercial palm-fatty acids from various sources in flakes and liquid states were determined within the frequency range of 0.2 GHz to 4.5 GHz at temperature range of 303 K and 443 K (stabilized within 1 °C). The effects of temperature, moisture and unsaturation structure of palm-fatty acids on dielectric properties were also investigated. Real part of dielectric permittivity (ε_r^′) of all the palm-fatty acids increased with increasing temperature from 303 K to 443 K with correlation coefficient, R² > 0.95 to linear function; depending on temperature and moisture content of the palm-fatty acids. The conductivity increased with increase in temperature and following the model-fitting approach of Arrhenius equation with correlation coefficient (R² > 0.85) for all the measured samples. The calculated activation energy (E_a) for all measured samples was in the range between 0.06207 eV and 0.16442 eV. Hence, the data calculated would be useful to determine the quality of the palm-fatty acids and to specify their requirement for transportation, storage and process. Moreover, the information will also be useful in the design of a microwave (MW) reactor system for oleochemical downstream process as it is dependent on the dielectric properties of the materials being processed.     

Hydrogen and syngas yield from residual branches of oil palm tree using steam gasification

Wastes produced during oil palm production from agro-industries have great potential as a source of renewable energy in agriculturally rich countries, such as Thailand and Malaysia. Clean chemical energy recovery from oil palm residual branches via steam gasification is investigated here. A semi-batch reactor was used to investigate the gasification of palm trunk wastes at different reactor temperatures in the range of 600 to 1000 °C. The steam flow rate was fixed at 3.10 g/min. Characteristics and overall yield of syngas properties are presented and discussed. Results show that gasification temperature slightly affects the overall syngas yield. However, the chemical composition of the syngas varied tremendously with the reactor temperature. Consequently, the syngas heating value and ratio of energy yield to energy consumed were found to be strongly dependent on the reactor temperature. Both the heating value and energy yield ratio increased with increase in reactor temperature. Gasification duration and the steam to solid fuel ratio indicate that reaction rate becomes progressively slower at reactor temperatures of less than 700 °C. The results reveal that steam gasification of oil palm residues should not be carried out at reactor temperatures lower than 700 °C, since a large amount of steam is consumed per unit mass of the sample in order to gasify the residual char.     

Greenhouse heating and cooling using aquifer water

An aquifer coupled cavity flow heat exchanger system (ACCFHES) was designed using underground aquifer water for the heating as well as cooling of a composite climatic greenhouse. The performance of ACCFHES was experimentally evaluated for a full winter and a summer season. The ACCFHES makes use of constant temperature aquifer water (24 °C) available at an agricultural field through an irrigation tube well for heating in winter nights and cooling in summer days. The results showed that the average greenhouse room air temperature was maintained 7–9 °C above the outside air during extreme winter nights and 6–7 °C below the outside air in extreme summer days, and temperature fluctuations inside the greenhouse also decreased daily. The average relative humidity (RH) inside the greenhouse also decreased by 10–12% in the winter and increased by more than double in the extreme summer conditions as compared to the outside conditions. A comparison of economic feasibility of the ACCFHES coupled greenhouse was also conducted with conventional greenhouse and open field condition based on the yield of Capsicum annum. The ACCFHES was also compared economically with other existing heating/cooling technologies such as earth-to-air heat exchanger system (EAHES), ground air collector, evaporative cooling using foggers and fan & pad system in terms of net present worth (NPW) and pay back period. It was observed that the NPW of the ACCFHES coupled greenhouse was much higher as compared to the conventional greenhouse and open field condition. The payback period of the ACCFHES coupled greenhouse was the lowest among all other existing heating/cooling systems.     

Theoretical and experimental study on the heat transport in metallic nanofilms heated by ultra-short pulsed laser

In this paper the mechanism of heat transport in metallic nanofilms under ultra-short pulsed laser heating is examined theoretically and experimentally. In order to easily understand the non-equilibrium heat transport in metallic nanofilms the study of heat transport behavior is first carried out in dielectrics. The analyses indicate that there may be two kinds of wave phenomena in dielectrics subjected to a periodic surface temperature. One is the thermal wave governed by the C-V model based hyperbolic equation and the other is the diffusive wave governed by the Fourier model based parabolic equation. According to the hyperbolic two step model for non-equilibrium heat transport, such two kinds of wave phenomena can also occur simultaneously in the metallic nanofilms under pulsed laser heating, where the diffusive wave is induced by the electron temperature oscillation at the surface due to the non-equilibrium between electrons and lattices. Unlike the propagation speed of the thermal wave, the propagation speed of the diffusive wave depends not only on the medium properties but also the period of the temperature oscillation at the boundary. Hence, the propagation speed of the diffusive wave in the electron gas may be of as high as 10⁶ m s⁻¹, when the laser pulse duration is less than 1 ps. A transient thermoreflectance (TTR) system has been built to measure the transient electron temperature responses caused by the femtosecond laser heating and a pump-probe technique is used to ensure the femtosecond temporal resolution in the experiments. Different from the commonly used front heating-front detecting (FF) method for measuring the material properties, a rear heating-front detecting (RF) method is applied, so that measuring the propagation speed of heat becomes available. The non-equilibrium heat diffusion model is used to fit the measured normalized electron temperature profiles of 27.2 nm, 39.9 nm and 55.5 nm Au films. The best-fitted coupling factor G basically agrees with the theoretical value 2.3 × 10¹⁶ W m⁻³ K⁻¹. The propagation speed of the diffusive wave in the electron gas can be obtained by comparing the measured delay time of peak electron temperatures of Au films with different thicknesses. The average propagation speed of the temperature oscillation or diffusive wave in Au films for the range of thickness from 27.2 nm to 55.5 nm is equal to 8.1 × 10⁵ m s⁻¹, which is close to the value predicted by the non-equilibrium heat diffusion model.     

Rapid microwave hydrogen release from MgH2 and other hydrides

A honeycomb ceramic monolith (HCM) that was coated with 0.54 wt% Ni (corresponding to 0.2 micron Ni thin layer) could be heated rapidly from room temperature to white glowing within 7 s under 1000 W microwaves. Energy efficiency to convert microwaves to heat via this Ni-coated HCM was estimated to be more than 90%. By loading metal hydride powder samples into the monolith channels, Ni-HCMs were demonstrated to release 100% hydrogen from MgH₂ and other hydrides (NaBH₄, NaAlH₄, and LiH) under 200 W microwaves within 2-3 min. The new microwave/Ni-HCM method offers many advantages over the conventional electrical resistant or microwave heating methods on hydrides, including 1) very fast kinetics, 2) wide application range for various hydrides, 3) simple process with no ball-milling or catalyst addition, and 4) very high energy efficiency.     

Regeneration characteristics of desiccant rotor with microwave and hot-air heating

Microwave heating, because of its advantages of direct and rapid heating of materials, has the potential to be employed as a novel regeneration method of desiccant rotors in humidity conditioners. We proposed a combined regeneration process, which combines microwave heating and conventional hot-air heating. The system is expected to achieve high heating rate during an initial regeneration period by assisting water desorption using the additional energy of the microwave. In this study, the regeneration characteristics of a desiccant rotor were experimentally investigated under conditions of microwave heating, hot-air heating, and combined heating at various microwave powers and hot-air temperatures. The effectiveness of the combined regeneration was evaluated in terms of the regeneration ratio, the initial regeneration rate, the temperature distribution in the rotor, and finally in terms of the energy consumption. It was demonstrated that combined heating was effective at leveling non-uniform temperature distribution in the rotor. Combined heating achieved higher ratios and initial rates in regeneration compared to just microwave and hot-air heating. This result was obviously attributed to the additional input of microwave energy, resulting that average rotor temperature increased by microwave absorption of rotor. Moreover, it was also effective for enhancement of regeneration to level the temperature distribution in the rotor by combination of two heating methods with different heating mechanisms. Both the initial regeneration rate and the equilibrium regeneration ratio for combined heating were found to increase as the microwave power increased. A linear relationship was observed with respect to microwave power. From the viewpoint of energy consumption, it may be possible to apply combined and microwave heating to humidity control systems that switch between adsorption and regeneration in short cycle times, if the conversion and absorption efficiencies of the microwave are significantly improved.     

Experimental and simulation studies of primary vacuum freeze-drying process of random solids at microwave heating

This paper concerns experimental and theoretical studies of freeze-drying process at microwave heating. Two kinds of random solids were dried: material which are assumed to have no internal porosity (ground glass), as well as one containing internal porosity (Sorbonorit 4 activated carbon). Formulated one-dimensional two-region model of freeze-drying process at microwave heating takes into account unknown a priori sublimation temperature T_s(t) and mass concentration of water vapor C_s(t) at moving ice front. Steady capacity of internal heat source is correlated with electric field strength E and dissipation coefficient K(T) in both regions of the material to be dried. Linear temperature dependency of dissipation coefficient is assumed and described by two regression parameters: μ_1i and μ_2i for dry (i = I) and frozen (i = II) bed, respectively. A correlation between both measured and calculated temperatures of the sample and actual electric field strength was observed. Fairly good agreement between experimental and simulated results was stated.     

Study on the electrical and mechanical properties of phenol formaldehyde resin/graphite composite for bipolar plate

With phenol formaldehyde resin (PF) powder and graphite powder as raw materials, a kind of conductive composite for bipolar plate is obtained by hot-pressure molding. The effects of PF resin content, molding temperature and time on conductivity and bending strength of the composite were investigated in this paper; and the optimum PF resin content, molding temperature and time were obtained. The results show that: the conductivity decreases and bending strength increases with the increasing of PF resin content; the conductivity varies wave-like and bending strength increases firstly and then decreases with the increasing of molding temperature; the effects of molding time on properties of the composite are similar to that of molding temperature; and the best conductivity and bending strength of the composite are 142 s cm⁻¹ and 61.6 MPa, respectively, when its PF resin content is 15% molded at 240 °C for 60 min.     

Very low temperature radiant heating/cooling indoor end system for efficient use of renewable energies

Solar or solar-assisted space heating systems are becoming more and more popular. The solar energy utilization efficiency is high when the collector is coupled with indoor radiant heating suppliers, since in principle, lower supply temperature means lower demand temperature and then the system heat loss is less. A new type radiant end system is put forward for even lower supply temperature compared to the conventional radiant floor heating systems. A three dimensional model was established to investigate its energy supply capacities. Simulation results show that 50 W per meter length tube can be achieved with the medium temperature of 30 °C for heating and 15 °C for cooling. The predicted results agree well with the actual data from a demonstration building. Furthermore, it is demonstrated that a supply temperature of 22 °C in winter and of 17 °C in summer already met the indoor requirements. The new end system has good prospects for effective use of local renewable resources.     

Investigation of a novel porous anodic alumina plate for methane steam reforming: Hydrothermal stability,electrical heating possibility and reforming reactivity

A plate-type alumina support was synthesized through a novel anodization technology followed by a hot water treatment, which resulted in the drastically enlargement of support BET surface area from 16.5 to 204.6 g/m², and such BET value is even comparable to some commercial alumina supports. A high thermal stability of this kind of porous anodic alumina support was shown because as much as 63% of surface area remained after the support subjected to 700 °C air calcination for 50 h. Innovatively, an electrical heating pattern was allowed over this plate support due to the existence of Fe–Cr–Ni alloy interlayer among the support. Our work showed that the utilization of electrical heating pattern would shorten the reformer start-up time from 1 to 2 h to just a few minutes. With the porous anodic alumina support, a 17.9-wt% Ni catalyst with nickel aluminate layer was synthesized and its reforming reactivity was investigated during stationary and DSS SRM at 700 °C, under usual and electrical heating pattern. It showed excellent SRM reactivity and no deactivation was evidenced during 500 h stationary test and 100 times start–stop cycles DSS SRM test. Nevertheless, for the industrialization, some efforts should be made to alleviate the sintering of anodic supports, because after subjected to a hydrothermal treatment at 700 °C for 50 h, only 36% of surface area was kept.