﻿ Heat sink - Wikipedia apply for housing benefit

## Heat sink – Wikipedia apply for housing benefit

Consider a heat sink in a duct, where air flows through the duct. 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 the diagram gives the following set of equations: Q ˙ = m ˙ c p , i n ( T a i r , o u t − T a i r , i n ) {\displaystyle {\dot {Q}}={\dot {m}}c {p,in}(T {air,out}-T {air,in})} (1) Q ˙ = T h s − T a i r , a v R h s {\displaystyle {\dot {Q}}={\frac {T {hs}-T {air,av}}{R {hs}}}} (2)

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. M ˙ {\displaystyle {\dot {m}}} is the air mass flow rate in kg/s.

• when the air flow through the heat sink decreases, this results in an increase in the average air temperature.How can i apply for housing benefit 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 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.

Natural convection requires free flow of air over the heat sink. If fins are not aligned vertically, or if fins are too close together to allow sufficient air flow between them, the efficiency of the heat sink will decline. Design factors [ edit ] thermal resistance [ edit ]

For semiconductor devices used in a variety of consumer and industrial electronics, the idea of thermal resistance simplifies the selection of heat sinks.How can i apply for housing benefit the heat flow between the semiconductor die and ambient air is modeled as a series of resistances to heat flow; there is a resistance from the die to the device case, from the case to the heat sink, and from the heat sink to the ambient air. The sum of these resistances is the total thermal resistance from the die to the ambient air. Thermal resistance is defined as temperature rise per unit of power, analogous to electrical resistance, and is expressed in units of degrees celsius per watt (°C/W). If the device dissipation in watts is known, and the total thermal resistance is calculated, the temperature rise of the die over the ambient air can be calculated.

The idea of thermal resistance of a semiconductor heat sink is an approximation. It does not take into account non-uniform distribution of heat over a device or heat sink.How can i apply for housing benefit it only models a system in thermal equilibrium, and does not take into account the change in temperatures with time. Nor does it reflect the non-linearity of radiation and convection with respect to temperature rise. However, manufacturers tabulate typical values of thermal resistance for heat sinks and semiconductor devices, which allows selection of commercially manufactured heat sinks to be simplified. 

Commercial extruded aluminium heat sinks have a thermal resistance (heat sink to ambient air) ranging from 0.4 °C/W for a large sink meant for TO3 devices, up to as high as 85 °C/W for a clip-on heat sink for a TO92 small plastic case.  the popular 2N3055 power transistor in a TO3 case has an internal thermal resistance from junction to case of 1.52 °C/W.  the contact between the device case and heat sink may have a thermal resistance of between 0.5 up to 1.7 °C/W, depending on the case size, and use of grease or insulating mica washer.  material [ edit ]

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The most common heat sink materials are aluminium alloys.  aluminium alloy 1050 has one of the higher thermal conductivity values at 229 W/m•K  but is mechanically soft. Aluminium alloys 6060 (low stress), 6061 and 6063 are commonly used, with thermal conductivity values of 166 and 201 W/m•K, respectively. The values depend on the temper of the alloy.

Copper has excellent heat sink properties in terms of its thermal conductivity, corrosion resistance, biofouling resistance, and antimicrobial resistance (see main article: copper in heat exchangers). Copper has around twice the thermal conductivity of aluminium and faster, more efficient heat absorption. Its main applications are in industrial facilities, power plants, solar thermal water systems, HVAC systems, gas water heaters, forced air heating and cooling systems, geothermal heating and cooling, and electronic systems.How can i apply for housing benefit

Copper is three times as dense  and more expensive than aluminium.  copper heat sinks are machined and skived. Another method of manufacture is to solder the fins into the heat sink base. Aluminium heat sinks can be extruded, but the less ductile copper cannot.  

Diamond is another heat sink material, and its thermal conductivity of 2000 W/m•K exceeds copper five-fold.  [ unreliable source?] in contrast to metals, where heat is conducted by delocalized electrons, lattice vibrations are responsible for diamond’s very high thermal conductivity. For thermal management applications, the outstanding thermal conductivity and diffusivity of diamond is an essential. Nowadays synthetic diamond is used as submounts for high-power integrated circuits and laser diodes.

Composite materials can be used. Examples are a copper-tungsten pseudoalloy, alsic ( silicon carbide in aluminium matrix), dymalloy (diamond in copper-silver alloy matrix), and E-material ( beryllium oxide in beryllium matrix).How can i apply for housing benefit such materials are often used as substrates for chips, as their thermal expansion coefficient can be matched to ceramics and semiconductors. Fin efficiency [ edit ]

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.  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. Fin efficiency 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).How can i apply for housing benefit equations 6 and 7 are applicable for straight fins. Η f = tanh ⁡ ( m L c ) m L c {\displaystyle \eta {f}={\frac {\tanh(ml {c})}{ml {c}}}}  (6) m L c = 2 h f k t f L f {\displaystyle ml {c}={\sqrt {\frac {2h {f}}{kt {f}}}}L {f}}  (7)

Fin efficiency is increased by decreasing the fin aspect ratio (making them thicker or shorter), or by using more conductive material (copper instead of aluminium, for example). Spreading resistance [ edit ]

In industry, thermal analyses are often ignored in the design process or performed too late — when design changes are limited and become too costly.  of the three methods mentioned in this article, theoretical and numerical methods can be used to determine an estimate of the heat sink or component temperatures of products before a physical model has been made. A theoretical model is normally used as a first order estimate.How can i apply for housing benefit online heat sink calculators  can provide a reasonable estimate of forced and natural convection heat sink performance based on a combination of theoretical and empirically derived correlations. Numerical methods or computational fluid dynamics (CFD) provide a qualitative (and sometimes even quantitative) prediction of fluid flows.   what this means is that it will give a visual or post-processed result of a simulation, like the images in figures 16 and 17, and the CFD animations in figure 18 and 19, but the quantitative or absolute accuracy of the result is sensitive to the inclusion and accuracy of the appropriate parameters.

CFD can give an insight into flow patterns that are difficult, expensive or impossible to study using experimental methods.  experiments can give a quantitative description of flow phenomena using measurements for one quantity at a time, at a limited number of points and time instances.How can i apply for housing benefit if a full-scale model is not available or not practical, scale models or dummy models can be used. The experiments can have a limited range of problems and operating conditions. Simulations can give a prediction of flow phenomena using CFD software for all desired quantities, with high resolution in space and time and virtually any problem and realistic operating conditions. However, if critical, the results may need to be validated.