Heat Transfer Concepts And Applications

Heat and Heat Transfer

Heat is the form of energy that is transferred between systems or objects with different temperatures (flowing from the high-temperature system to the low-temperature system). Also referred to as heat energy or thermal energy. It is typically measured in Btu, calories or joules. Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases. Heat transfer describes the flow of heat (thermal energy) due to temperature differences and the subsequent temperature distribution and changes.

The study of transport phenomena concerns the exchange of momentum, energy, and mass in the form of Conduction, Convection, and Radiation.

Conduction

Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules. Conduction occurs more readily in solids and liquids, where the particles are closer to together, than in gases, where particles are further apart. The rate of energy transfer by conduction is higher when there is a large temperature difference between the substances that are in contact. The process of heat conduction depends on the following factors: temperature gradient, cross-section of the material, length of the travel path, and physical material properties. The temperature gradient is the physical quantity that describes the direction and rate of heat travel.

The following equation calculates the rate of conduction:

Q = [k · A · (Thot — Tcold)]/d

where Q = heat transferred per unit time; k = thermal conductivity of the barrier; A = heat-transfer area; Thot = temperature of the hot region; Tcold = temperature of the cold region; and d = thickness of the barrier.

Applications of Conduction

Thermal Conduction:

1. You can warm your back muscles with a heating pad.

2. When you get sick and you use a mercury thermometer to measure your temperature, you are using the metal tip of the thermometer to transfer your body heat to the mercury.

Electrical Conduction:

1. The wires in your house conduct electricity and allow lights to come on when you flip the switch.

2. With a landline phone, sound waves are converted into electricity and conducted over wires to the other person’s phone where they are converted back to sound waves.

Convection

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection. The fluid above a hot surface expands, becomes less dense, and rises. At the molecular level, the molecules expand upon introduction of thermal energy. As temperature of the given fluid mass increases, the volume of the fluid must increase by same factor. This effect on the fluid causes displacement. As the immediate hot air rises, it pushes denser, colder air down. This series of events represents how convection currents are formed.

The equation for convection rates is calculated as follows:

Q = hc · A · (Ts — Tf)

where Q = heat transferred per unit time; hc = convective heat transfer coefficient; A = heat-transfer area of the surface; Ts = temperature of the surface; and Tf = temperature of the fluid.

Application of Convection

1. Car engines: Cooled by convection currents in the water pipes. Water is a very good substance to carry the unwanted heat away from the engine to the radiator.

2. Convection is used to cool down the laptop and super computer etc.

3. Air conditioners are installed near the ceiling of the room, to allow the setting up of convection currents. The air-conditioner releases cool dry air into the room. As cool air is denser, it sinks. The warm air, being less dense, will rise. The air circulated and the temperature of the air will eventually fall to the desired value.

4. Rising air over the land are convection currents and are used by glider pilots to keep their gliders in the sky.

Radiation

Radiation can be called as emission of energy which travels through space or a material medium in the form of waves/rays or high speed particles. Radiation can penetrate various materials.

Types of Radiation:-

1. Electromagnetic radiation:- Includes radio waves or RF, 3G, 4G waves, FM, Microwaves, Infrared , visible light, Ultraviolet, X-rays and Gamma radiation.

2. Particle Radiation:-include alpha radiation, beta radiation, proton radiation, neutron radiation.

3. Acoustic Radiation:-includes ultrasound and seismic waves. The sounds coming out of various musical instruments is an effect of this radiation.

4. Gravitational Radiation:-radiation in the form of gravitational waves or ripples in the curvature of space-time.

Radiation is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particle.

Thermal radiation is calculated by using the Stefan-Boltzmann law:

P = e · σ · A · (Tr4 — Tc4)

where P = net radiated power; A = radiating area; Tr = temperature of the radiator; Tc = temperature of surroundings; e = emissivity; and σ = Stefan’s constant.

Application of Radiation

Medical applications

1. Ionizing Radiation is used for Cancer treatment as it kill cancer cells or change genes so the cells cannot grow.

2. Ultrasonic rays are used in sonography to produce images of structures within your body.

Communication applications

1. Communication systems use forms of electromagnetic radiation.

2. Radio waves or RF such as FM, TV waves are all application of radiation.