JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year I Semester UTILIZATION OF ELECTRICAL ENERGY Previous Question Paper (December – 2023/January - 2024 - R18)

Heat transfer occurs through three fundamental physical mechanisms, which describe how thermal energy moves between systems or within a medium due to a temperature gradient:

• Conduction: The transfer of heat through stationary matter by physical contact. It occurs due to the vibration and collision of atoms and molecules within a solid or fluid. The rate of heat flow is governed by Fourier's Law, which states that heat flux is proportional to the negative gradient of temperature and the thermal conductivity of the material.
• Convection: The transfer of heat by the actual physical movement of a fluid (liquid or gas). It involves the combined effects of conduction and fluid motion. It is categorized into natural convection, where fluid motion is caused by buoyancy forces due to density differences, and forced convection, where fluid is moved by external means such as a pump or fan.
• Radiation: The transfer of heat in the form of electromagnetic waves (specifically infrared radiation) emitted by matter as a result of its temperature. Unlike conduction and convection, radiation does not require a material medium and can occur through a vacuum. The rate of emission is determined by the Stefan-Boltzmann Law, which relates the power radiated to the fourth power of the absolute temperature of the surface.

• Uniform Heating: Unlike conduction or convection, which rely on heat transfer from the surface inwards, dielectric heating generates heat throughout the entire volume of the material simultaneously. This ensures uniform temperature distribution, which is critical for thick or non-homogeneous materials.
• High Speed: Because the heat is generated internally within the dielectric material itself, the process is extremely rapid. There is no time lag associated with thermal conductivity, significantly reducing processing times compared to conventional methods.
• Selective Heating: Dielectric heating can be frequency-tuned to target specific materials. In a mixture, the component with the higher loss factor will heat up faster, allowing for selective drying or curing of specific materials without affecting others.
• Cleanliness and Control: The process is inherently clean as it involves no combustion products or open flames. Furthermore, the power input can be controlled precisely and instantaneously by adjusting the voltage or frequency, allowing for high-precision automation.
• Efficiency: Since heat is generated directly in the workpiece, there are minimal heat losses to the surrounding environment or the heating equipment itself, leading to high overall thermal efficiency.