Heat transfer
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The document explains the concepts of heat transfer and thermodynamics, emphasizing that energy is the capacity to do work and heat energy is the transfer of energy due to temperature differences. It outlines the methods of heat transfer, primarily conduction, and distinguishes between thermal conductors and insulators. The document also illustrates practical applications of heat transfer principles in heating systems and various materials.
Heat transfer
- 5. Heat Energy ? What is Energy Energy is the capacity to do work. Ex - Mechanical energy, chemical energy, electrical energy, Nuclear energy and so on What is Heat Energy It is a form of energy in transit, the driving force for which is the temp difference
- 6. TD vs. HT One real example. Suppose you want to heat 5 kg of water from 30 to 100 C. By thermodynamics, you can know the amount of heat required. Lets say Q = 1463 kJ is required. Now, can you wait for a day Or can you wait for 5 hours to heat that quantity of water to 100 C. Or If you don’t have any work , you can do that But if you are a busy person and you want to heat within few minutes, so you have to take help of heat transfer here in order to fix time. Rate is fixed by knowledge of heat transfer. Only amount is fixed by knowledge of thermodynamics. If you want to heat within 15 mins, if you have to supply heat to the water at a rate of 1.63 kW. If you want to heat within 30 mins , you can supply heat at a rate of 0.81 kW. This is where Heat transfer makes a difference to thermodynamics
- 7. Heat Transfer We are concerned, as to How and What rate heat transfer takes place. HT is based on the Law of Thermodynamics, which states that heat flows from a body at higher temp to a body at lower temp. Conversely, heat can not flow from a body at lower temp to a body at higher temp, unless & until some external device, like heat pump, is employed It means, whenever there is temp difference between the two bodies, heat energy will flow.
- 8. Modes of Heat Transfer
- 10. Conduction
- 11. Conduction - 1 transfer of heat from one part of a substance to another part in the same substance or one substance to another substance by direct contact Different solids conduct different amounts of heat in a specific time. (copper vs. glass) Those materials that conduct heat well, are called thermal conductors while those that conduct heat poorly, are known as thermal insulators Most metals are excellent thermal conductors, while wood, glass, and most plastics are common thermal insulators.
- 12. Conduction mechanisms By Free flow of e- By Lattice Vibrations
- 13. Ty
- 15. Assumptions of Fourier’s Law 1. Unidirectional heat flow (only one direction) 2. Steady state heat flow 3. Constant temp gradient 4. Constant thermal conductivity (k) 5. Both faces isothermal 6. No Heat generation (Qg) 7. Material is Isotropic & Homogeneous
- 16. Thermal Conductivity (k) Those materials that conduct heat well, are called thermal conductors while those that conduct heat poorly, are known as thermal insulators
- 17. What if – required other side of the temp is less? Solution – Composite Walls
- 20. Examples of Composite walls Walls of buildings Walls of home refrigerators Insulated pipe carrying steam Walls of a furnace Walls of a cold storage Hot case for food
- 22. Heat conduction through composite cylinder
- 23. Fluid to Fluid HT across a Metal body
- 32. Fins
- 57. Planck’s Law
- 59. Wein’s Law