A component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid….

HEAT SINK DESIGN::::

A heat sink is physically designed to increase the surface area in contact with the cooling fluid surrounding it, such as the air. Approach air velocity, choice of material, fin (or other protrusion) design and surface treatment are some of the design factors which influence the thermal resistance, i.e. thermal performance, of a heat sink. One engineering application of heat sinks is in the thermal management of electronics, often computer CPU or graphics processors. For these, heat sink attachment methods and thermal interface materials also influence the eventual junction or die temperature of the processor(s). Thermal adhesive (also known as thermal grease) is added to the base of the heatsink to help its thermal performance. Theoretical, experimental and numerical methods can be used to determine a heat sink’s thermal performance.

Basic heat sink heat transfer principle

To understand the principle of a heat sink, consider Fourier’s law of heat conduction. Joseph Fourier was a French mathematician who made important contributions to the analytical treatment of heat conduction.[1] Fourier’s law of heat conduction, simplified to a one-dimensional form in the x-direction, shows that when there is a temperature gradient in a body, heat will be transferred from the higher temperature region to the lower temperature region. The rate at which heat is transferred by conduction, qk, is proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred.




Consider a heat sink in a duct, where air flows through the duct, as shown in Figure 2. It is assumed that the heat sink base is higher in temperature than the air. Applying the conservation of energy, for steady-state conditions, and Newton’s law of cooling to the temperature nodes shown in Figure 2 gives the following set of equations.

 

Using the mean air temperature is an assumption that is valid for relatively short heat sinks. When compact heat exchangers are calculated, the logarithmic mean air temperature is used.  is the air mass flow rate in kg/s.

The above equations show that

  • When the air flow through the heat sink decreases, this results in an increase in the average air temperature. This in turn increases the heat sink base temperature. And additionally, the thermal resistance of the heat sink will also increase. The net result is a higher heat sink base temperature.
    • The increase in heat sink thermal resistance with decrease in flow rate will be shown in later in this article.
  • The inlet air temperature relates strongly with the heat sink base temperature. For example, if there is recirculation of air in a product, the inlet air temperature is not the ambient air temperature. The inlet air temperature of the heat sink is therefore higher, which also results in a higher heat sink base temperature.
  • Therefore, if there is no air or fluid flow around the heat sink, the energy dissipated to the air can not be transferred to the ambient air. Therefore, the heat sink functions poorly.
  • Furthermore, a heat sink is not a device with the “magical ability to absorb heat like a sponge and send it off to a parallel universe”[2].

Design factors which influence the thermal performance of a heat sink::

  • Material

  • Fin Efficiency

Fin efficiency is one of the parameters which makes a higher thermal conductivity material important. A fin of a heat sink may be considered to be a flat plate with heat flowing in one end and being dissipated into the surrounding fluid as it travels to the other[7]. As heat flows through the fin, the combination of the thermal resistance of the heat sink impeding the flow and the heat lost due to convection, the temperature of the fin and, therefore, the heat transfer to the fluid, will decrease from the base to the end of the fin. This factor is called the fin efficiency and is defined as the actual heat transferred by the fin, divided by the heat transfer were the fin to be isothermal (hypothetically the fin having infinite thermal conductivity). Equations 6 and 7 are applicable for straight fins.

[8] (6)

[8] (7)

Where:

  • hf is the convection coefficient of the fin
    • § Air: 10 to 100 W/(m2K)
    • § Water: 500 to 10,000 W/(m2K)
  • k is the thermal conductivity of the fin material
    • § Aluminum: 120 to 240 W/(m·K)
  • Lf is the fin height (m)
  • tf is the fin thickness (m)

To increase the fin efficiency of fins:

  • Decrease the fin aspect ratio, by:
    • § Increasing the fin thickness, or
    • § Decreasing the fin length
  • Increase the thermal conductivity of the fins,

  • Spreading resistance:::

Another parameter that concerns the thermal conductivity of the heat sink material is spreading resistance. Spreading resistance occurs when thermal energy is transferred from a small area to a larger area in a substance with finite thermal conductivity. In a heat sink, this means that heat does not distribute uniformly through the heat sink base. The spreading resistance phenomenon is shown by how the heat travels from the heat source location and causes a large temperature gradient between the heat source and the edges of the heat sink. This means that some fins are at a lower temperature than if the heat source were uniform across the base of the heat sink. This nonuniformity increases the heat sink’s effective thermal resistance.

To decrease the spreading resistance in the base of a heat sink:

  • Increase the base thickness
  • Choose a different material with better thermal conductivity
  • Fin arrangements

Courtesy : http://en.Wikipedia.org

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