Non-uniformity Of Heating Research Articles

Mathematical modeling of slab heating in a furnace with walking beams due to their curvature

Slab heating before hot rolling process is necessary for obtaining required metal ductility. The most effective for this purpose are furnaces with walking beams that provide heat supply to all sides of the slab. However, the places of slabs lower surfaces, contacting with water-cooled beams, are shielded from the radiation of the furnace lower heating zones and take the heat from the beams. Previously, the authors developed and programmatically implemented a mathematical model of slab heating in a furnace with walking beams, based on the numerical solution by finite difference method of the three-dimensional heat conduction problem with piecewise defined boundary conditions on the slab bottom surface. In this model, for the open zones of the slab bottom surface, boundary conditions were similar to those on the top surface, and for the zones of contact with the beams were set effective boundary conditions assuming duration of this contact. In this paper, the model was modified to take into account the curvature of the beams and to recalculate the configuration of zones with different boundary conditions on the slab bottom surface for each position of the slab along the furnace. By variant calculations at different values of heat transfer intensity from the slab bottom surfaces to the beams it was determined that curvature of a single beam can significantly change the characteristic of the corresponding “cold” spot, but it practically does not affect the general characteristic of the slab heating non-uniformity. If all fixed beams are subjected to curvature, the final drop across the slab is significantly reduced due to an increase in its minimum temperature. It was found that the influence of beam curvature on the temperature field at the end of heating process is higher the more intensive the heat transfer to the beams is.

On the defect detection limits of flash thermography in reflection mode: A comprehensive parametric 3D FE study

Optical flash thermography is a promising non-destructive testing technique which offers a fast, full-field and non-contact inspection. This study provides a comprehensive study into the influence of different factors on the physical defect detection limits of hidden defects with flash thermography, which is a missing key piece in the present literature.Therefore, a large-scale parametric 3D finite element study is performed in order to explore the actual defect detection limits for a substantial range of inspection conditions:i.Defect parameters: type, size and depth.ii.Material parameters: anisotropic diffusivity and stacking sequence.iii.Excitation parameters: heating non-uniformity, excitation energy and temporal excitation profile.iv.Acquisition parameters: sampling frequency and measurement noise.The finite element model is carefully designed using the commercial Abaqus software in order to ensure both accuracy and computational efficiency, and its reliability in quantifying defect detectability is experimentally validated. Decisive defect detection thresholds are substantiated, through which the detection limits in several (fiber reinforced) materials are concisely visualized and critically compared for the raw thermographic sequence and well-known processing techniques for flash thermography.

Effects of Conveyor Movement, Layer Rearrangement and Sample Mixing on Pest Mortality in Milled Rice Subjected to Radio Frequency Heating

Highlights Hot air-assisted RF heating accelerated temperature rise of samples. Thermal holding after RF heating improved heating uniformity and pest mortality. Layer rearrangement, conveyor movement, and mixing enhanced RF heating efficiency. Abstract. Application ofradio frequency (RF) treatment has been studied for disinfesting agricultural products, and RF heating non-uniformity has been demonstrated to directly influence pest mortality in treated products. The purpose of this study was to investigate the combined effects of holding time after conveyor movement, layer rearrangement, and sample mixing on the mortality of Rhyzopertha dominica (R. dominica) in milled rice treated with hot air-assisted RF heating. Results showed that hot-air assistance during the process of RF heating can lead to a rapid temperature rise and improve the temperature distribution. RF heating followed by holding samples in hot air for specific time periods effectively improved the heating uniformity of samples and raised the mortality of R. dominica in milled rice. Compared with the stationary conditions, conveyor movement, layer rearrangement, and sample mixing improved the hot air-assisted RF heating uniformity of the sample, raised pest mortality and reduced the thermal holding time after RF treatment required to achieve 100% pest mortality in milled rice. The findings of this study provide a valuable proof of concept for improving RF heating uniformity and optimizing parameters in RF disinfestation protocols for milled rice. Keywords: Heating uniformity, Milled rice, Mixing, Pest mortality, Radio frequency heating.

Analysis of microwave heating uniformity in berry puree: From electromagnetic-wave dissipation to heat and mass transfer

To understand the non-uniformity of microwave heating, berry puree selected as representative material with high moisture content, movement simulation and beach experiments were employed to analyze the non-uniformity formation and evolution in the blueberry puree loaded in a rectangular container (BLRC) under microwave heating. The results showed that the coupling process via microwave dissipation and heat & mass transfer dominates the uniformity of microwave heating BLRC. The decisively heating modes of microwave dissipation are transverse electric wave and transverse magnetic wave, whose features of microwave dissipation dominate the heat and mass transfer process in BLRC as three distinct stages of temperature changes, including slow elevation then stabilization followed by quick increase. Thermal accumulation, thermal conduction, and latent heat of water vaporization at varying degrees consume microwave dissipation in BLRC. The results provide guidance for understanding microwave heating uniformity and processing technology for agricultural products with similar properties as berry fruit.

Dependence of degradation of anthocyanins on non-uniformity of microwave heating in blueberry puree

To clarify the dependence of anthocyanins degradation on the non-uniformity of microwave heating in blueberry puree, the effects of temperature and moisture content distribution on the content of anthocyanins in blueberry were investigated. The temperature dispersion value (VT), moisture dispersion value (VM), temperature contrast value (CON), the highest temperature (MAX) and hot zone distribution (HTD) were introduced to evaluate the heating uniformity of blueberry puree during microwave heating. The Weibull model can characterize the distribution of anthocyanins content in blueberry puree as the function of the temperature and moisture under microwave heating. The degradation of anthocyanins in blueberry puree has the high dependency on non-uniformity in HTD, MAX, and VT inside blueberry puree under microwave heating. These research results may provide guidance for the quality control of berry products under microwave processing.

Developing cold air assisted radio frequency tempering protocol based on heating rate, uniformity, and quality of frozen chicken breast

Radio frequency (RF) heating as an emerging technology has been widely used for tempering and thawing of frozen meat products, while the RF heating non-uniformity is still the main problem before industrial applications. The objective of this study was to provide a new method for improving the RF heating uniformity by adding cold air (−5 °C, 1.5 m/s) around the frozen chicken breast in a rectangular polypropylene container. The effects of cold air assisted RF tempering (CARFT), RF tempering (RFT), air tempering (AT), water tempering (WT), and refrigerator tempering (RT) on heating behaviors and quality of chicken breast were also compared. The results showed that the cold air slightly reduced the RF heating rate but significantly ( P < 0.05) improved the heating uniformity of frozen chicken breast during RFT. Meanwhile, the optimal tempering method was CARFT at the electrode gap of 110 mm, which could provide a larger heating rate than AT, WT and RT and a better heating uniformity than AT, WT and RFT. Additionally, the quality, such as color, drip loss (1.42%), pH (6.38), and total volatile base nitrogen (10.37 mg/100 g), of CARFT treated chicken breast were better than those with other methods. Therefore, CARFT can further be expanded to optimize industrial-scale RF tempering protocols in frozen chicken breast. • RF heating non-uniformity is still the main problem of industrial tempering applications. • Added cold air improved the RF heating uniformity of frozen chicken breast. • Optimal tempering method was cold air assisted RF tempering with electrode gap of 110 mm. • Quality of CARFT treated chicken breast was better than that with other methods.

An Improved Model of the Heat Pipe Based on the Network Method Applied on a Heat Pipe Cooled Reactor

As the unique piece of heat transport equipment in a heat pipe cooled reactor, an accurate simulation of the heat pipe is helpful for understanding the real behavior of the reactor core and the reactor system. Based on the network method, an improved model for the heat pipe which considers the heat conductance in the wall, the vapor flow in the vapor space, and the liquid flow in the wick is proposed. Meanwhile, the gravity term is also added to the flow equation. Compared with the experimental results of a copper-water heat pipe, the validity of this model is verified. Then, a high-temperature sodium heat pipe of 1.0 m length is selected as the study object. Based on the analysis, it can be found that the total temperature difference of the heat pipe is 31.7 K, and the temperature drop caused by the vapor flow is only 2.6 K. As for the flow pressure drop in the heat pipe, the pressure drop is mainly concentrated in the wick region, which is 8,422.47 Pa, and the pressure drop in the vapor space is only 896.68 Pa. In cases of non-uniform heating and cooling, high heat leakage, and inclined operation, results indicate that the greater the non-uniformity of heating or cooling, the greater will be the temperature drop of the heat pipe. With the increase of heat leakage, the operating temperature of the heat pipe decreases significantly, and the total temperature drop increases. The heat pipe can operate at all positive inclination angles, but when the inclination angle exceeds−30°, the heat pipe will reach the capillary limit, and it may be damaged. All of these results can provide support for the design and simulation of a heat pipe cooled reactor.

Theoretical simulation of the closed currents near non-uniformly strongly heated surface of tungsten due to thermo-emf

A problem of surface melting under the impact of plasmas is one of the most important in the development of future magnetic confinement fusion reactors. The expected high heat loads can lead to melting of tungsten chosen as the material of a divertor and the first wall of ITER. The tungsten melt can move under the action of forces and quickly deform a surface. This article shows that at high temperatures, a tungsten vapor can be considered as a conductive material with conductivity high enough to be taken into account during simulation of current through a sample. We describe the mechanism of current generation by the thermo-emf due to non-uniform heating of the vapor/condensed substance interface without external sources of charges. Even without direct contact with the external plasma, the density of this current is high enough to cause noticeable movement of the melt in an external magnetic field. Simulation shows that the melt moving observed at BETA may be caused not by a current of an electron beam but by non-uniformity of heating and can be reproduced by any other similar heat source in the external magnetic field.

반도체 공정용 RF 용량성 결합 플라즈마 장비에서 발생하는 Striation 현상

This study provides the conditions and characteristics of a radial striation in a capacitively coupled plasmas, which is caused by self-organized pattern formation. With the variation of the gas pressure and the RF applied voltage, we found that the striation occurs in the parameter space near the breakdown boundary of the glow discharge. By observing the time-averaged plasma parameters, it was found that the striation is triggered by the local non-uniformity of electron heating. The radial electron flux distribution shows a strong shear with the evolving striations.

Improvement of radio frequency heating uniformity for millets by changing shape and adding polypropylene blocks

Radio frequency (RF) treatment is a burgeoning method in food processes. RF heating non-uniformity is a main obstacle for industrial implementations and influenced by the geometry of treated materials. In this study, the effect of convex-concavo shapes of the sample top surface and step shapes at the bottom of the rectangular container by adding polypropylene (PP) blocks on the heating uniformity was investigated using a 6 kW, 27.12 MHz free-running oscillator RF system. The results showed that the RF heating uniformity index values of millets decreased with increasing convex degree and decreasing concave degree of the sample top shape. Moreover, adding PP blocks at the bottom of the sample container was a better choice of improving RF heating uniformity owing to the wider adjustable range of sample sizes and better temperature distribution. This study provided a new method to improve the RF heating uniformity by simply changing the shapes of samples in either the top or bottom surface.

Vlasov–Fokker–Planck simulations of pre-magnetized ablating planar targets

Magnetic fields, spontaneously generated around laser heating nonuniformities, have been found to invert and enhance electron pressure perturbations in the conduction zones of laser-produced plasmas without applied magnetic fields. The application of a sufficiently strong magnetic field is predicted to damp this phenomenon, but may instead result in magneto-thermal instability. Two-dimensional Vlasov–Fokker–Planck simulations of the conduction zone of laser-produced plasmas, subject to externally applied magnetic fields of different field strengths, are performed. The effects of non-locality upon extended collisional transport terms and instability thresholds are investigated. It is found that magnetized transport terms Righi–Leduc heat flow and thermoelectric heat flow are strongly enhanced by non-locality (up to twofold), even at the top of the temperature gradient, due to larger magnetization of the mediating hot electrons (relative to their thermal counterparts). Meanwhile, the Nernst effect efficiently advects magnetic field out of the conduction zone before instability (which requires Hall parameters, χ&amp;gt;0.1) can take hold.

Results of experimental studies of changes in the level and heating of the main condensate in mixing-type LPH

In the thermal circuits of domestic steam turbines, mixing-type low-pressure heaters (LPH) with free-flow jet water distribution and counter-flow of water and steam are widely used. The choice of the counterflow variant of the media movement ensures the most efficient heat transfer. However, the technical problem of ensuring reliable operation of LPH in the entire range of design loads of TPP and NPP power units is still relevant.During the commissioning and operation of mixing-type LPH in 800÷1200 MW turbines of TPP and NPP, the presence of metal knocks in the zone of the check valve, hydraulic shocks in the heating section were revealed. A priori, these phenomena indicated design flaws in LPH or manufacturing defects in their production. Research carried out by NPO CKTI specialists showed that periodic hydraulic shocks in the heating section and metal knocks occur as a result of uneven distribution around the circumference of the main condensate and steam supply. This leads to a breakdown of the check valve and the destruction of perforated plates and off-design heating of water in the volume of the annular LPH water chamber. To clarify the causes of the damage, develop recommendations for the reconstruction of the apparatus and further account for the design, two series of experimental studies were carried out on mixing-type heaters of 800 MW turbine units PNSV-2000-1 and PNSV-2000-2 manufactured at PJSC Krasny Kotelshchik. The purpose of the experimental studies was to determine the change in the water level in the water chamber and the heating of the main condensate in the elements of the heating compartment during normal operation of the power unit at loads of 400÷850 MW. Based on the results of the research, the method for calculating the mixing-type LPH has been refined, taking into account the revealed non-uniformity of water heating in the water chamber, recommendations for their reconstruction have been developed and implemented.

Texture properties of microwave post-processed 3D printed potato snack with different ingredients and infill structure

3D printing has the advantage to modify food texture by regulating internal structure. In this study, 3D printing was used to design the infill structure and microwave vacuum drying (MVD) was used to post-process the potato-based snack. Results indicated that MVD was a promising post-processing method with a high drying efficiency and good maintenance of structure information of 3D printed samples. Increased microwave power reduced the drying time, but increased the heating non-uniformity. NaCl addition increased the dielectric loss factor (ε′′), reduced the drying time and hardness of MVD products. Sucrose addition did not affect the dying time, but increased the hardness of products. Infill percentage and infill pattern closely related to the hardness of samples. Increased infill percentage increased the hardness of samples. Cast-made samples and 100% infill 3D printed samples were not suitable for MVD post-processing. Finally, product with various texture within one sample was produced by adjusting the infill structure of each part. This study provides a new aspect to modify food texture by changing infill structure, with MVD as a post-processing method.

Comparison of heating performance between inverter and cycled microwave heating of foods using a coupled multiphysics-kinetic model

Non-uniform heating has been a challenging problem in microwave heating due to ‘standing wave pattern’ and ‘thermal runaway’ problems. Cycled microwave heating was used to improve microwave heating uniformity of frozen products, while a relatively new approach of inverter heating was claimed to help preserve food quality during thawing/heating. In this study, finite-element-method-based coupled multiphysics-kinetic models were developed, validated, and used to investigate these controversial results by comparing the heating performances of average temperature, heating non-uniformity, and nutritional value change between the inverter and cycled microwave heating of foods. The effect of food sizes/shapes and material properties (dielectric loss) was evaluated. Results showed that there was little difference in average temperature, heating uniformity, and Vitamin C content between cycled and inverter heating for different food products. A relatively more uniform heating and lower nutritional value degradation were observed for small size low loss material; however, the improvement was not significant enough to claim the benefit of inverter heating.

Analysis of the Heat Transfer of Ginkgo Biloba Seeds during Radio-Frequency Heating

AbstractFresh ginkgo biloba (GB) seeds are seasonal and susceptible to microbial spoilage, especially the growth of mold owing to GB seeds’ high-water content, which greatly limits their shelf life. As an efficient and special heating method, radio-frequency (RF) heating can be used to dehydrate ginkgo to preserve its nutritional value and reduce postharvest losses. However, the non-uniformity of RF heating restricts the development in industrialized application. In this study, the RF drying of GB seeds was performed to investigate the effect of RF heating on temperature. The distribution law of the entire temperature field was also observed. Using numerical simulation method, the coupling model of electromagnetic and heat transfer was established. The model was validated by the 6-min heating profile of GB seeds in a 12 kw and 27.12 MHz RF system. The model was also qualitatively validated by comparing the simulated temperature profiles on the three planes in the GB seeds with the corresponding thermal images. Quantitative validation was performed by comparing the simulated temperature of GB seeds on the three planes with experimental temperature acquired at places using thermocouples. Furthermore, the model can be effectively used to identify the distribution of electric fields in different positions and to achieve satisfactory heating uniformity.

Influence of the skin effect on the structure of relief–phase optical elements obtained by plasma–chemical etching

This paper discusses features of the phenomena that occur when optical glass is plasma–chemically etched on an apparatus with inductively coupled plasma during the fabrication of relief-phase optical elements, resulting in nonuniform damage of the metallic (chromium) mask. It is shown that, in fabricating such elements, which are characterized by very significant width variations of sections of the metallic mask, it is necessary to allow for the density nonuniformity of the eddy currents induced in the mask (a thin-film conductor) caused by the use of a high-frequency discharge. It is analytically shown that the nonuniformity of the damage of the mask–conductor is caused by the skin effect; as a consequence of heating nonuniformity, this causes the adsorption and desorption processes of chemically active particles to be dissimilar along the contour and in the middle of the mask. Diagrams of the amplitude distribution of the induced electric field transverse to the field vector are obtained that show that, although the consequences of the skin effect do not exceed 2% on relatively narrow sections of the mask (1–2 μm wide), the skin effect on relatively wide sections (10–20 μm wide) causes a difference in the conditions of the chemical reactions on the edge sections of the mask and in their middle by about a factor of 5.

Computer simulation of heat transfer for disinfestation of red flour beetle, Tribolium castaneum (Herbst) in stored canola seeds (Brassica napus L.) by radio frequency heating

To control insect pests in the stored grains effectively, radio frequency (RF) treatment is one of the novel alternative methods over the existing methods, such as pesticides, fumigations, and heat and cold treatments. The RF selective heating of red flour beetle, Tribolium castaneum (Herbst) in two different volumes of bulk canola seeds at 5%, 7%, 9%, and 11% moisture contents (MC) using a 1.5 kW, 27.12 MHz RF heating unit was simulated using a finite element method based commercial simulation package (COMSOL Multi-physics). The electric field formation, dielectric heat generation, non-isothermal fluid flow, and heat transfer including surface to surface radiation were coupled. The simulated and the measured temperatures of the seeds were compared with the different MCs and volumes of the seeds. Similar comparison was also conducted for the insects and the seeds. The differences between the simulated and the measured temperatures of the seeds were not more than 17.4%, 13.4%, 8.08%, and 19.0% for the small volume seeds and 14.8%, 15.6%, 19.5%, and 18.8% for the large volume seeds at 5%, 7%, 9%, and 11% MCs, respectively. Non-uniform RF heating of the seeds was observed regardless of the MC and the volume of the seeds. The RF selective heating of the insects was most effective for the small volume seeds at 11% MC. The temperature of the insects was 14.6 °C (maximum) higher than the temperature of the seeds. The RF selective heating of the T. castaneum in the canola seeds could be improved with a proper design of RF applicator that minimizes the non-uniformity of heating.

Metal loss reduction in the continuous furnace with a mechanized hearth due to the increase in uniformity of billet heating

The analysis of the negative influence of the uniform heating of billets along the length in the continuous furnaces with a mechanized hearth on the work of heating furnaces, rolling mill and the quality of metal products is given. The influence of the non-uniformity of heating on metal loss is investigated. Based on the experimental data analysis of temperatures measurements of billets surface heated in the walking-hearth furnace, a statistical analysis was carried out and the equation of regression of the temperature distribution along the length of the billet was obtained using the least squares method. It is established that an increase in the non-uniformity of surface billet heating leads to an increase in the metal loss, and this dependence has a nonlinear character.

Periodical metal cylinders for improving heating uniformity of small batch materials in microwave applicators with rotating turntables

Microwave applicators with rotating turntables have been widely applied in many fields, but the heating non-uniformity still limits the further applications. Specifically, when microwave applicators are used to process small batch materials, materials have to be away from the center of rotating turntables, where the problem of heating non-uniformity is more prominent. In order to overcome this difficulty, this paper presents a novel periodical metal cylinder structure to improve the heating uniformity for microwave applicators. With the proposed periodical metal cylinders being placed beneath rotating turntables, electromagnetic fields can be well adjusted, thus improving the heating uniformity. To demonstrate the heating uniformity with the proposed structure, the heating process of the arrays of two and three potato slices in a practical microwave applicator is simulated by the finite element method, respectively. The results show that both the heating uniformity of the two and three potato slices can be improved, which is validated by experiments. Furthermore, the results show that the temperature similarity among the processed potato slices with the proposed structure is better than that without the proposed structure.

Modeling radio frequency heating of granular foods: Individual particle vs. effective property approach

Radio frequency (RF) heat treatment is one of the most promising methods for disinfestations and pasteurizations of granular foods but the combined effects of geometry and dielectric properties of granular materials on the non-uniformity of heating in RF are poorly understood. A coupled thermal-electromagnetic model for RF heating (27.12 MHz, parallel plate) of a single layer of spherical particles (1–5 cm diameter) was developed using two different formulations of the particles − an approach that considers the geometry of individual particles and a simplified approach that considers the effective properties of an air-particle mixture. The spherical particles, prepared from soybean powder, represented dry granular foods. Model predictions were validated using point and spatial temperature measurements. Results show that while an effective properties approach can be sufficient for smaller (∼1 cm diameter) particles (with internal temperature range of 3 °C), consideration of the exact geometry (individual particles) would be needed for accurate prediction of heating non-uniformities (with internal temperature range of 39 °C) in larger diameters (diameter of 5 cm). Heating non-uniformities are present due to corner/edge heating, focusing effect, as well as contact heating between particles or between a particle and the tray. Results should greatly benefit computer-aided design and optimization of RF heating processes for granular products.