Heat transfer

What is Heat transfer??

Talk about heat transfer, there are two words heat and transfer. The definition of heat is the form of energy that can be transferred from higher temperature to lower temperature. So, Heat transfer is heat energy transfer from one system to another system due to temperature difference.

Heat can be transferred by three ways, conduction, convection, and radiation. What is the difference?

Conduction

Conduction is kind of heat transfer without any movement of molecules. When we burn rod of metal at one of its edge, after a while we will feel the other edge is also hot. This is conduction phenomena. Heat transfer by this phenomena is affected by several parameters, those are surface area, thermal conductivity coefficient, temperature difference and length. As increasing surface area, thermal conductivity coefficient and temperature difference, heat transfer will also increase. As increase of length, heat transfer will decrease. We can say heat transfer by conduction is directly proportional to surface area, thermal conductivity, and temperature difference, But inversely to the length. Mathematically we can write

Q=(k . A. ∆T) / L

Convection

Convection is kind of heat transfer that involves movement of molecule. For example, when we heat water on a pan by stove, the highest temperature is on the bottom of pan. Water temperature at bottom is also higher than at the top. Density of matter is lower if the temperature is high. So, density of water at bottom lower than at the top, thus water from the bottom moves up and water from top moves down.

Convection heat transfer is affected by surface area, convection coefficient, and temperature difference. Mathematically we can write:

Q = h . A .   ∆T

Talk about convection heat transfer, the “game” of this phenomena is at convection coefficient. To determine this coefficient need complicated correlation between fluid mechanics, friction factor, geometry, pressure, etc.

Radiation

Radiation heat transfer is heat transfer without medium. Heat from sun to the earth is transferred by radiation because no atmosphere in outer space. This heat transfer can be written mathematically:

Q = ε . σ . A . T⁴

Where

ε : emissivity coefficient

σ : Stefan-Boltzmann constant

Heat Sink in Electronics cooling

Electronics technology has been developing rapidly since the first transistor was invented. Semiconductor technology is the most significant part for electronics technology development. At the first time one transistor with very small size could replace cathode tube. Now, thousand or even million transistors can be operated in one single chip. However, electronics have maximum temperature to be operated properly. A survey showed that the most cause of electronic failure is temperature. Thus electronics cooling cannot be separated in electronics technology.

Many types of cooling system for electronics have been introduced. The most common electronics cooling system is heat sink. Heat sink system can be divided into two parts, forced convection and natural convection.

Natural convection is convection heat transfer without any force applied to heat sink. This convection occurs because of buoyancy force naturally. Fluid (e.g air) has lower density if its temperature is high, this causes air moves up.  In electronics, heat dissipated from chip causes increasing temperature surrounding then air density becomes low, thus air moves up. Since natural convection only uses buoyancy force, usually natural convection heat sink is attached vertically.

Forced convection need additional force to move air flows on heat sink. Fan is usually used to support air flow on heat sink. Heat transfer rate at this heat sink is bigger than natural convection. Many electronic equipments use this type.

Heat sink is attached to heat source to enhance heat transfer rate. When heat dissipated from electronic device cannot be overcome by heat sink, it needs additional system or even different cooling system. Heat pipes or thermoelectric cooler may be an option for additional system in heat sink.