Heat Transfer Power Point presentation 1.1.pptx

heat transfer

1. Heat Transfer By Mr. P. Solomon Durairaj , Assistant Professor, Department of Mechanical Engineering, CK College of Engineering & Technology, Cuddalore.

2. After this session the learner will be able to Understand the meaning of Heat Transfer. What are the Modes of Heat Transfer. Introduction about Conduction, Convection & Radiation Understand about the Fourier’s Law of Conduction What is Thermal Conductivity. What is Thermal Diffusivity Mechanism of conduction heat transfer SESSION OUTCOME:

3. In general, we defined heat as the form of energy that can be transferred from one system to another as a result of temperature difference. A thermodynamic analysis is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another. The science that deals with the determination of the rates of such energy transfers is called as heat transfer. The transfer of energy as heat is always from the higher-temperature medium to the lower-temperature, and heat transfer stops when the two mediums reach the same temperature. Heat Transfer

4. Define Heat Transfer In simple terms, Heat transfer is defined as the transfer of heat from one place to another place. Or one material to another material. This is done due to the temperature difference b/w one material to another material.

5. Modes of Heat Transfer There are three modes of heat transfer: Conduction Convection Radiation

6. Introduction about Conduction Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. Conduction can take place in solids, liquids, or gases. In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion. In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons.

7. Introduction about Convection: Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion , and it involves the combined effects of conduction and fluid motion. The faster the fluid motion , the greater the convection heat transfer . In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction. The presence of bulk motion of the fluid enhances the heat transfer between the solid surface and the fluid, but it also complicates the determination of heat transfer rates.

8. Convection: Forced convection if the fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. In contrast, natural (or free) convection if the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid. The rate of convection heat transfer is observed to be proportional to the temperature difference, and is expressed by Newton’s law of cooling as. ∆Q = h. A. ∆T Where, ∆Q = heat transferred per unit time ∆T = temperature difference between the surface and the bulk fluid, A = heat transfer surface area h = convective heat transfer coefficient

9. Introduction about Radiation: Radiation is the energy emitted by matter in the form of electromagnetic waves as a result of the changes in the electronic configurations of the atoms or molecules. Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an medium . In fact, energy transfer by radiation is fastest (at the speed of light ) and it suffers no affect in a vacuum . The maximum rate of radiation that can be emitted from a surface at an absolute temperature Ts (in K) is given by the Stefan–Boltzmann law as Q = σAT ⁴ Where, Q = heat transfer per unit time σ = stefan Boltzmann constant A = Area of the emitting body T = absolute temperature

10. Fourier’s Law of Heat Conduction: Now we consider a case of heat transfer through the wall by the process of conduction. It states that the time rate of heat transfer through the material is proportional to the negative gradient in the temperature and to the area, at the right angles to that gradient, through which the heat is flowing, then the rate of heat conduction is given by ∆Q = - kA.dT / dX Where, ∆Q = rate of heat transfer K = Thermal conductivity of the wall A = surface area perpendicular to the direction of heat flow dT / dX = Temperature gradient The negative sign indicates that heat always flows from hot to cold areas

11. Thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat . The thermal conductivity is denoted as k and unit is W/m °C. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For example, metals having high thermal conductivity are very efficient at conducting heat.

12. Thermal diffusivity The thermal diffusivity, which represents how fast heat diffuses through a material and is defined as = (Heat conducted / Heat stored) (k / ρ Cp) (m2 /s) The ratio of the heat conducted through the material to the heat stored per unit volume . A material that has a high thermal conductivity or a low heat capacity will obviously have a large thermal diffusivity . The larger the thermal diffusivity , the faster the propagation of heat into the medium . A small value of thermal diffusivity means that h eat is mostly absorbed by the material and a small amount of heat will be conducted further.

13. Mechanism of conduction heat transfer