Basics of Heat Transfer

 

BASICS OF HEAT TRANSFER

What is Heat transfer?

We know through thermodynamics that energy can exist in many forms. One such form of energy is heat. The energy that is transferred between systems as a result of the temperature gradient is called heat. Heat transfer is the science that indicates the rate at which this heat is transferred between the systems. Imagine holding a bottle of water in a refrigerator. There is going to be energy transfer between the bottle and the cold air inside. The energy transfer takes place until the temperatures of the cold air and the water is the same. Therefore, we can conclude that heat transfer occurs until equilibrium is obtained. Heat is transferred from higher temperature to lower temperature.

Here comes thermodynamics that can tell us about the amount of heat to be transferred to effect the change in the state according to the law of conservation of energy. But the amount is not everything that we need. Consider a scenario where we purchase a flask or a casserole to keep the food warm. We tend to look for how long the food stays warm and the material to ensure heat doesn't escape quickly or easily. These conditions are not satisfied by thermodynamic analysis. So we bring in heat transfer that will talk about the rate at which heat is transferred. The rate of heat transfer is defined as the heat transferred per unit of time.

Thermodynamics moreover relies on state equilibrium, while heat transfer is the condition to approaching equilibrium. Thermodynamics is still the physics behind the framework of heat transfer.

The rules that govern this phenomenon are:

1. The rate of heat lost from one body should be equal to the rate of heat gained by the other body.

2. Heat transfer should occur in the direction of reducing temperature.

Heat transfer majorly depends on the thermal gradient across the system. The thermal gradient is the temperature difference over the unit length of the system. The greater the thermal gradient greater is the rate of heat transfer. We can see the real-time applications of heat transfer through Refrigeration, Air Conditioning, Thermostats, Water heaters, Cloth Irons, Hairstyling equipment, Computers, electronics, etc.

Heat transfer occurs through three modes in general: Conduction, Convection, and Radiation. Each of which will be discussed in further posts.

 

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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