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Coupled Problems
Published in S. Ratnajeevan H. Hoole, Yovahn Yesuraiyan R. Hoole, Finite Elements-based Optimization, 2019
S. Ratnajeevan H. Hoole, Yovahn Yesuraiyan R. Hoole
Heavy currents always lead to heating through the joule effect. This heat is often undesirable as in electrical machinery like alternators where the heat not only diminishes the efficiency of the generator but also can damage the insulation. In other cases this heat can be beneficial as in a) the metallurgical industry where the heat is used to melt the ore and mix it through electromechanical forces or b) hyperthermia treatment in oncology where cancerous tissue is burned off albeit with lower currents, achieving the heating by stronger eddy currents through a higher 1 kHz frequency (Vanderplaats, 1984; Brauer et al., 2001; Hoole et al., 1990). Whatever the situation, it is often desirable to accomplish a particular thermal distribution – whether to save an alternator from overheating or to accomplish the necessary melting of the ore or to burn cancerous tissue without hurting healthy tissue.
Nonthermal and Alternative Food Processing Technologies
Published in C. Anandharamakrishnan, S. Padma Ishwarya, Essentials and Applications of Food Engineering, 2019
C. Anandharamakrishnan, S. Padma Ishwarya
Ohmic heating or Joule heating is a process in which heat is generated inside the food by the passage of alternating current (AC) through it. Earlier in the twentieth century, ohmic heating was used for the pasteurization of milk. However, due to the high electricity cost, it was discontinued. Later in the 1980s, the technique was revived for the sterilization of liquids containing large particles. The underlying principle of ohmic heating is based on the resistance offered by the foods to the applied electrical current. Therefore, it is also known as resistive heating or electroconductive heating. Ohmic heating is based on the Joule effect, according to which the electrical energy supplied by the electrical conductor to the food material dissipates in the form of heat. Heat generated depends on the electrical conductivity of food material and square of the electrical field applied as given in Eq. (14.11). P=V2σAL
Theoretical Background
Published in Valery Rudnev, Don Loveless, Raymond L. Cook, Handbook of Induction Heating, 2017
Valery Rudnev, Don Loveless, Raymond L. Cook
An alternating coil current produces in its surroundings a time-variable magnetic field that has the same frequency as the coil current. This magnetic field induces eddy currents in the workpiece located inside the coil. Eddy currents will also be induced in other electrically conductive objects that are located near the coil. These induced currents have the same frequency as the coil current; however, their direction is opposite to the coil current. These currents produce heat by the Joule effect (I2R). Figure 3.2 shows a sample of a variety of inductor geometries used in IH. Recognizing that there is almost an endless variety of inductor types, it is convenient to review basic principles of IH considering a conventional solenoid-type coil that surrounds a cylindrical workpiece. This approach will be used in this chapter.
Improving microwave heating efficiency of asphalt concrete by increasing surface magnetic loss of aggregates
Published in Road Materials and Pavement Design, 2020
Wei Liu, Shengyue Wang, Xingyu Gu
The properties of asphalt pavements degrade over time due to micro cracks. These cracks can grow and lead to full scale failure (Liu, 2012). Researchers are now trying to introduce self-healing components to improve the service life of pavements. As a kind of polymer material, asphalt has the thermodynamic reversibility of dissolution and precipitation (Little & Bhasin, 2007). Healing of an asphalt concrete is usually related to the colloidal system of the bitumen. Once its temperature reaches a certain value, asphalt repossesses the flow plasticity. For this reason, a perfect timing and an appropriate heating method are the keys to the healing of asphalt pavements. Infrared heating is commonly used to heat the asphalt pavement but the low heating efficiency and surface coking (Terrel, Epps, & Sorenson, 1997) lead to some environmental problems. Electromagnetic heating is also successfully applied, including induction heating (Garcia, Schlangen, & Martin, 2009; Liu, Schlangen, García, & Ven, 2010a) and microwave heating (Agzenai, Pozuelo, Sanz, Perez, & Baselga, 2015; Mitchell, Link, Wang, Hao, & Xue, 2011; Norambuena-Contreras & Garcia, 2016). In induction heating, the conductive asphalt concretes are placed under a coil. When the power supply sends alternating current through the coil, the interaction between the alternating electric and magnetic field would induce currents flowing along the conductive loops. Then the induced current dissipates heat by Joule effect (Liu, Schlangen, Ven, & Poot, 2010b). The surface temperatures of the specimens are much higher than their internal temperatures because of skin effect (Liu, Miao, & Wang, 2017). And the aggregation of the conductive fibres is also a problem during the mixing process (Liu, Schlangen, & Ven, 2013). In microwave heating, dielectric polarisation is the process whereby heat is generated in polar molecules by means of rapid oscillations. The oscillation enhances the kinetic energy and collision of molecules and then the temperatures of the material increase (Nieftagodien, 2013). As microwave heating has the advantage of volumetric and uniform heating, the authors choose microwave technology to apply in the pavement healing.