How Heat Is Transferred From One Body To Another

Heat Transfer

According to Vedat S. (2000) heat transfer is a sub field of mechanical engineering in which the transfer of heat between different item and objects with or without any medium is studied. Heat transfer between two bodies take place as a result of difference in temperature between two bodies. Heat always transfers from region of high potential to the region low potential. In general the motion of heat in or out of the systems is mainly deal by the laws of thermodynamics. 

According to C.P. Kothandaraman (2006) there two thermodynamics laws which are used in heat transfer and they are first and second law of thermodynamics. First law of thermodynamics establish a relationship between the energy generated, heat flow and energy stored in any thermodynamics system. For any closed system this law say that the total energy generated inside a system is equal to the heat transfer through that system plus the internal energy of that system. Second law of heat transfer shows the direction of heat transfer and according to it heat transfer from body of high temperature to the body of lower temperature.

Modes of Heat transfer

According to Vedat S. (2000) there three different modes in which heat can be transfer from one body to another body. Those three modes of heat transfer are as follows

Conduction

Convection

Radiation

Conduction

Heat transfer with in a solid body or different solid bodies that physically attached to each other is called conduction (C.P. Kothandaraman 2006). According to J.P Holman (2010) heat transfer through conduction depends on the cross section area of body, thermal conductivity of the material and temperature difference between two sides of body. In the case of heat transfer between two different bodies, heat transfer depends on the overall heat transfer coefficient of both bodies (considered as one body) and temperature difference cross the bodies. 

Q=k*A*∆T

Where

Q is the heat transferred

k is the thermal conductance of material

A is cross section area of body

∆T is temperature difference between two sides of body 

Convection

Heat transfer between solid and fluid is called convection (heat transfer can be from solid to fluid or from fluid to solid). According to J.P Holman (2010) convection heat transfer from solid to fluid or from fluid to solid depends on the temperature difference between solid and fluid, surface area of solid exposed to fluid and convection heat transfer coefficient between fluid and solid. Convection heat transfer coefficient between solid and fluid is a complex thing; it depends on air velocity and temperature difference between solid and fluid. For a given temperature difference convection heat transfer coefficient will be different for still air and air moving with some specific speed. 

Q=h*A*∆T

Where

Q is convection heat transferred

h is the convection heat transfer coefficient

∆T is temperature difference between solid and fluid 

Radiation

Heat transfer in the case of conduction and convection was done in the presence of some medium; heat transfer without any medium in a perfect vacuum is called radiation. In this case the heat is transfer through electromagnetic radiation as a result of temperature difference called thermal radiation (C.P. Kothandaraman 2006). According to J.P Holman (2010) heat transfer through thermal radiation depends on the surface area of body, temperature difference and proportionality constant called Stefan Boltzmann constant whose value is 5.67*〖10〗^(-8)  W/m^2 K^4 

Q=σ*A*T^4

Where

σ is proportionality constant called stefan Boltzmann constant 

A is the surface area

T is temperature difference