Heat is transferred in three different modes namely conduction, convection, and radiation. Electrical and electronic appliances, process equipments and automobiles all these objects run on different kinds of energy sources. When they operate heat energy is released or observed from a point or location to another. In case of a mobile phone charging, electricity pass from a power socket to the battery and during this operation some energy escapes in the form of heat. Heat generation happens due to the resistance of the materials. From the inner core of the battery and circuits of the charger, heat will be transferred by conduction.
Thermal conductivity is a property derived in cases where conduction mode of heat transfer is studied and conduction of heat occurs through the non-flow motion of molecules example as solids where the molecules are not free to move. Unbound electrons play the role of transferring heat in conduction but when in case of liquids and gases molecules themselves move towards the temperature gradient.
In case of a solid heat transfer rate in conduction explained by a formula for steady state condition given by Fourier’s Law
Thermal conductivity is a property derived in cases where conduction mode of heat transfer is studied and conduction of heat occurs through the non-flow motion of molecules example as solids where the molecules are not free to move. Unbound electrons play the role of transferring heat in conduction but when in case of liquids and gases molecules themselves move towards the temperature gradient.
In case of a solid heat transfer rate in conduction explained by a formula for steady state condition given by Fourier’s Law
This says the rate of heat flow per unit area is given by the product of thermal conductivity of the substance to the temperature gradient of length, n.
The above equation is written for ∆T of temperature difference (T1, T2) and B thickness of a solid where ∆T exist is q/A=k. ∆T/B
The same analogy is considered when heat is transferred to liquid which is in contact with solid surface, it is assumed that when hot solid surface comes in contact with a liquid, a film (a thin sub-layer at walls of the solid surface) is formed on liquid side in the region of contact and this film is the only thickness considered as solid thickness and temperature is homogenous outside of this film. The resistance of the heat flow lies in this film only.
A property for liquids has been introduced for calculation of heat flow from solid to liquid, which is derived from Fourier’s law of conduction where the term B thickness is incorporated into the term k and a term called heat-transfer coefficient is generalized the reason is fluid thickness cannot be measured accurately.
Thermal resistance is the inverse of heat transfer coefficient. In a heat exchanger heat is transferred from a hot liquid to solid tube and from tubes to cooled liquid. Thus this coefficient is specified as individual heat transfer coefficient with respect to hot and cool liquid streams and referred inside heat transfer coefficient and outside heat transfer coefficient.
The rate of heat transfer by convection under steady-state conditions: Q = h A ∆T
‘h’ is the heat transfer coefficient, W/m2K, h= k/B
For convective heat transfer both in laminar and turbulent flow Q = h A (Tw - Tavg)
Tw = temperature at the surface of the wall
Tavg = Free stream temperature (for external flow)
Tw = temperature at the surface of the wall
Tavg = Free stream temperature (for external flow)
Bulk temperature (for internal flow)
For fluid flowing inside the pipe, where heat transfer occurs from heated wall of the pipe to the fluid:
Q = h A (Tw – Ti)
Ti is the average temperature of the fluid
For fluid flowing outside a heated pipe,
Q = h A (Tw – To)
To is the temperature far from the surface.
Overall heat transfer coefficient for hydrocarbons against water coolant vary normally in the range of 50 to 100 Btu/ hr.ft2.oF
Few heat transfer thumb-rules:
For fluid flowing inside the pipe, where heat transfer occurs from heated wall of the pipe to the fluid:
Q = h A (Tw – Ti)
Ti is the average temperature of the fluid
For fluid flowing outside a heated pipe,
Q = h A (Tw – To)
To is the temperature far from the surface.
Overall heat transfer coefficient for hydrocarbons against water coolant vary normally in the range of 50 to 100 Btu/ hr.ft2.oF
Few heat transfer thumb-rules:
- The law that governs the conduction mode of heat transfer is Fourier’s law.
- Thermal Conductivity of water as the temperature increases goes through a Maximum.
- When different resistances are in series, the quantity that is constant throughout the length of steady state conduction is heat transfer.
- More slope in the steady state temperature profile denotes more thermal resistance.
- A pool of water is being heated by an immersed heated plate. Then the mode of heat transfer at the interface of water and the heated plate is conduction.
- For a particular material, thermal conductivity is the function of Temperature.
- The significant dimensionless number in unsteady state heat transfer is Fourier number
- The thermal boundary layer is thinner than hydrodynamic boundary layer when Prandtl number>1
- When warm end approach of the heat exchanger is approximately equal to the cold end approach them LMTD is equal to the arithmetic mean.
