Heat Transfer Lecture 3 PDF

Heat Lecture 3 Heat and Heat Transfer  Heat transfer Any matter which is made up of atoms and molecules has the ability to transfer heat. The motion of molecules and atoms is responsible for heat or thermal energy and every matter has this thermal energy.  The more...

Heat Lecture 3 Heat and Heat Transfer  Heat transfer Any matter which is made up of atoms and molecules has the ability to transfer heat. The motion of molecules and atoms is responsible for heat or thermal energy and every matter has this thermal energy.  The more the motion of molecules, more will be the heat energy. However, talking about heat transfer, it is nothing but the process of transfer of heat from a high-temperature body to a low temperature one.  Conduction Types of Heat Convection Transfer Radiation  Conduction is the transfer of energy from one molecule to another by direct contact. The rate of heat transfer by conduction is given by: 𝑸 ∆𝑻 𝑯= ∝ 𝑨 𝒕 𝑳 𝑸 𝑻𝟐 − 𝑻𝟏 A 𝑯 = = 𝑲𝑨 𝒕 𝑳 T2 Q T1 Where Q : The amount of heat transfer t : The time of heat transfer L : The length or thickness of the material A : The cross sectional area of the material K : The thermal conductivity of the material Unit : ( W/m. oC) L When heat conduction takes place through two or more solid materials of different thermal conductivities: (𝑻𝟐 −𝑻𝟏 ) 𝑯=𝑨 (1) 𝑳/𝒌 (𝑻𝟐 −𝑻𝟏 ) 𝑯 =A (2) 𝑹 - where R = L/k is called the thermal resistance of a material.  The heat flow through a wall composed of two different materials: 𝑺𝒆𝒓𝒊𝒆𝒔 𝑷𝒂𝒓𝒂𝒍𝒍𝒆𝒍 T k2 A2 k1 k2 T2 T1 T2 T1 k1 A1 L1 L2 L 𝑯 = 𝑯𝟏 = 𝑯 𝟐 𝑯 = 𝑯𝟏 + 𝑯𝟐 𝑨 (𝑻𝟐 −𝑻𝟏 ) 𝑨 (𝑻𝟐 −𝑻 ) 𝑨 (𝑻 −𝑻𝟏 ) 𝑨𝟏 (𝑻𝟐 −𝑻𝟏 ) 𝑨𝟐 (𝑻𝟐 −𝑻𝟏 ) 𝑯= = = 𝑯 = 𝑳 𝒌 + 𝑳 𝒌 𝑳𝟏 𝒌 + 𝑳𝟐 𝒌 𝑳𝟏 𝒌 𝑳𝟐 𝒌 𝟏 𝟐 𝟏 𝟐 𝟏 𝟐  Two bars of thermal conductivities k and 3k and lengths 1 cm and 2 cm, respectively have equal cross-sectional area. They are joined together as shown in the figure. If temperature at the end of this composite bar is 100 oC and 0 oC , then the temperature at the interface is …… a. 50 oC b. 100/3 oC T1 = 0 oC T T2 = 100 oC c. 60 oC k 3k d. 200/3 oC L1 = 1 cm L2 = 2 cm  Convection is the movement of heat by a fluid such as water or air. The fluid (liquid or gas) moves from one location to another, transferring heat along with it. This movement of a mass of heated water or air is called a current.  Radiation is the transfer of heat by electromagnetic waves. When you stand in the sun, you are warmed by the electromagnetic waves that travels from the sun to Earth. The rate of heat transfer by radiation can be calculated by Stefan- Boltzmann law: 𝑸 𝑷 = = 𝜎 𝒆 𝑨 𝑻𝟒 𝒕 where P is the radiant power in Watts 𝝈 is constant = 5.67× 10-8 (W/m2. K4), e is the emissivity – for black body (e = 1) T is the absolute temperature.  If the temperature of the surroundings Ts , the net rate of energy emitted by the object is: T Ts 𝑸 𝑷 = = 𝜎 𝒆 𝑨 (𝑻𝟒 −𝑻𝟒𝒔 ) 𝒕 where Ts is the temperature of surrounding. 1- Which one of the following is the unit of the thermal conductivity? a) N/ m. K b) W/ m. K c) M. K /W d) M. K /N 2- Which of the following is the correct relationship between thermal resistance and thermal conductivity? a) R = k/L b) R = k.L c) R = L/k d) R = L2/k 3- Which mode of heat transfer does not require any medium? a) Conduction b) Convection c) Radiation d) Both (a) and (b) 4- Which is the fastest mode of transfer of heat? a) Conduction b) Convection c) Radiation d) Both (a) and (b)