Introduction to Heat Transfer Concepts

Study Notes

Heat transfer is the science of determining the rate of energy transfer between systems due to temperature differences.
Heat flows from regions of higher temperature to regions of lower temperature.

Thermodynamics focuses on the total amount of heat transferred during a process, but doesn't address the rate of transfer. It ensures energy conservation.
Thermodynamics deals with equilibrium states and transitions between them. Heat transfer, on the other hand, studies systems that are not in thermal equilibrium, making it a non-equilibrium phenomenon.
Heat transfer problems can be categorized into rating and sizing problems:
- Rating problems determine the heat transfer rate for a given system at a specific temperature difference.
- Sizing problems determine the size of a system to achieve a desired heat transfer rate at a specific temperature difference.

Heat can be transferred through three primary modes: conduction, convection, and radiation.

Conduction is the transfer of energy through the direct interaction of molecules within a substance.
It occurs in solids, liquids, and gases.
In gases and liquids, conduction happens due to collisions and diffusion of molecules.
In solids, it involves vibrations of molecules in a lattice and energy transport by free electrons.
The rate of heat conduction through a plane layer is proportional to the temperature difference across the layer, the heat transfer area, and inversely proportional to the thickness of the layer.
Fourier's law of heat conduction describes the rate of heat transfer through a material as being proportional to the temperature gradient.
The thermal conductivity of a material quantifies its ability to conduct heat. It's defined as the rate of heat transfer per unit area, per unit temperature difference, through a unit thickness of the material.

Convection involves the transfer of heat between a solid surface and a moving fluid (liquid or gas).
It combines the effects of conduction and fluid motion.
Faster fluid motion leads to greater convection heat transfer.
Forced convection occurs when the fluid flow is driven by external means like fans, pumps, or wind.
Natural (or free) convection arises when the fluid motion is driven by buoyancy forces due to temperature variations within the fluid.
Newton's law of cooling expresses the convection heat transfer rate as proportional to the temperature difference.
The convection heat transfer coefficient (h) is not a material property but is determined experimentally, influenced by factors like surface geometry, fluid motion, fluid properties, and bulk fluid velocity.

Radiation involves the transfer of energy through electromagnetic waves.
It doesn't require a medium and can occur in a vacuum.
All objects emit radiation, with the intensity depending on the object's temperature and surface properties.
The rate of radiation heat transfer is governed by the Stefan-Boltzmann law, stating that the rate of energy emission per unit area is proportional to the fourth power of the absolute temperature of the object.