Introduction Heat Transfer/ What is Heat Transfer ?
Heat will always be transferred from higher temperature to lower temperature independent of the mode. The energy transferred is measured in Joules (kcal or Btu). The rate of energy transfer, more commonly called heat transfer, is measured in Joules/second (kcal/hr or Btu/hr). Heat is transferred by three primary modes:
- Conduction (Energy transfer in a solid)
- Convection (Energy transfer in a fluid)
- Radiation (Does not need a material to travel through)
|Three Modes Of Heat Transfer
The conduction of heat takes place, when two bodies are in contact with one another. If one body is at a higher temperature than the other, the motion of the molecules in the hotter body will vibrate the molecules at the point of contact in the cooler body and consequently result in increase in temperature. The amount of heat transferred by conduction depends upon the temperature difference, the properties of the material involved, the thickness of the material, the surface contact area, and the duration of the transfer. Good conductors of heat are typically substances that are dense as they have molecules close together. This allows the molecular agitation process to permeate the substance easily.
So,metals are good conductors of heat, while gaseous substance, having low densities or widely spaced molecules, are poor conductors of heat. Poor conductors of heat are usually called insulators. The measure of the ability of a substance to insulate is its thermal resistance. This is commonly referred to as the R-value (RSI in metric). The R-value is generally the inverse of the thermal conductivity, the ability to conduct heat.
Typical units of measure for conductive heat transfer are:
Per unit area (for a given thickness)
Metric (SI) : Watt per square meter (W/m)
Metric (SI) : Watt (W) or kilowatts (kW)
The transfer of heat by convection involves the movement of a fluid such as a gas or liquid from the hot to the cold portion. There are two types of convection: natural and forced. In case of natural convection, the fluid in contact with or adjacent to a high temperature body is heated by conduction. As it is heated, it expands, becomes less dense and consequently rises. This begins a fluid motion process in which a circulating current of fluid moves past the heated body, continuously transferring heat away from it. In the case of forced convection, the movement of the fluid is forced by a fan, pump or other external means. A centralized hot air heating system is a good example of forced convection.
Convection depends on the thermal properties of the fluid as well as surface conditions at the body and other factors that affect the ability of the fluid to flow. With a low conductivity fluid such as air, a rough surface can trap air against the surface reducing the conductive heat transfer and consequently reducing the convective currents.
Units of measure for rate of convective heat transfer are:
Metric (SI) : Watt (W) or kilowatts (kW)
|Fig. Example Of Heat Transfer
Thermal radiation is a process in which energy is transferred by electromagnetic waves similar to light waves. These waves may be both visible (light) and invisible. A very common example of thermal radiation is a heating element on a heater. When the heater element is first switched on, the radiation is invisible, but you can feel the warmth it radiates. As the element heats, it will glow orange and some of the radiation is now visible. The hotter the element, the brighter it glows and the more radiant energy it emits.
The key processes in the interaction of a substance with thermal radiation are:
- Absorption- the process by which radiation enters a body and becomes heat.
- Transmission -the process by which radiation passes through a body
- Reflection -the process by which radiation is neither absorbed or transmitted through the body; rather it bounces off
Objects receive thermal radiation when they are struck by electromagnetic waves, thereby agitating the molecules and atoms. More agitation means more energy and a higher temperature.Energy is transferred to one body from another without contact or transporting medium such as air or water.In fact, thermal radiation heat transfer is the only form of heat transfer possible in a vacuum.
All bodies emit a certain amount of radiation. The amount depends upon the body’s temperature and nature of its surface. Some bodies only emit a small amount of radiant energy for their temperature, commonly called low emissivity materials (abbreviated low-E). Low-E windows are used to control the heat radiation in and out of buildings. Windows can be designed to reflect, absorb and transmit different parts of the sun’s radiant energy.
The condition of a body’s surface will determine the amount of thermal radiation that is absorbed, reflected or re-emitted. Surfaces that are black and rough, such as black iron, will absorb and re-emit almost all the energy that strikes them. Polished and smooth surfaces will not absorb, but reflect, a large part of the incoming radiant energy.
Typical units of measure for rate of radiant heat transfer
Metric (SI) ——Watt per square meter (W/m