Modes Of Heat Transfer

 

MODES OF HEAT TRANSFER

Heat transfer occurs in different ways: Conduction, Convection, and Radiation. In real-time engineering problems, it can occur through 2 or 3 modes combined.

1. Conduction is the method through which heat is transferred in solids. It occurs due to the vibrational energy transfer between the molecules of the solid. Though conduction is majorly confined to solids, it also extends to some liquids and gases. Fourier's Law governs conduction.

    Q = kA(-dT/dX)

Which states that the rate of heat transferred (Q) is directly proportional to the heat transfer area (A) and the temperature (T) distribution across the length (X) of the solid, otherwise termed as the temperature gradient (dT/dX). The negative sign indicates the flow of heat from a region of higher temperature to a region of lower temperature. The proportionality constant (K) is called Thermal Conductivity. Thermal Conductivity indicates the ability of a solid to conduct heat. It decides if the solid is a conductor or an insulator.

2. Convection is the mode of heat transfer in fluid with a relative difference in the temperature at different portions. There are 2 types of convection: Free Convection and Forced Convection. Convection currents generated by the rise and fall of hot and cold molecules cause Free Convection. Example hot air rises while cold air sinks. Density differences result in convection currents and natural convection. By the application of an external force like agitation to speed up the circulation of the currents is what is termed Forced Convection. Newton's Law of cooling governs convection.

Q = hAdT

Here h indicates the heat transfer coefficient. It depends on the nature of the fluid and the nature of the agitation provided.

3. Radiation is the transfer from a source to a receiver. Much of the energy is absorbed while the rest is deflected. The Fourth-Power law governs radiation.

Q = AσεT4

A is the area of the body emitting the radiation. σ is Boltzmann's constant. ε is the emissivity of the body. Emissivity is the ability of a body to effectively transfer thermal energy in the form of radiation. It ranges from 0 to 1, with 1 being a perfect black body. Emissivity also depends on the smoothness of the surface. T is the absolute temperature at which the radiations are emitted. At absolute zero kelvin, a body can emit radiation.

Consider a pan of water over a stove. The heat spreads through the pan from the flame through internal molecular vibrations. These vibrations transfer the heat from one molecule to the other like a chain reaction. Here conduction is the mode. The liquid at the bottom of the pan is hotter than the portion at the top. The heat from the pan is transferred to the liquid and as the liquid molecules heat, the hotter molecules rise to the surface due to low density, thereby causing convection currents. These currents distribute the heat as they move. Here, convection is the mode. The heat from the hot pan is transmitted to the air surrounding it by radiation as it reaches saturation.

 

 

References:

Process Heat Transfer by D.Q. Kern

Heat and Mass Transfer, Fundamentals and Applications by Yunus A Cengel, and Afshin J Ghajar.

 

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