27 June 2012

Basics of Thermodynamics And MIT News: Efficient Heat Dissipation


Thermodynamics is the branch of science that studies the relationships of heat, work, energy, and pressure.

In thermodynamics, it studies the interaction in relation to two kinds of systems: an isolated system and an open system. In an isolated system, material and energy of an object cannot exchange with its environment. In an open system, material and energy can exchange with its environment. Both this systems are precisely defined regions or spaces under study. Everything else outside the system is known as the surroundings.

A boundary separates the system and the surroundings. This is simply a surface surrounding the system or volume of interest. Anything that passes across the boundary that effects a change in the internal energy needs to be accounted for in the energy balance equation.

Laws of Thermodynamics

There are four laws that govern the behavior of energy and the passing of energy in thermodynamics. These are:

  • The Zeroth law: When 2 systems are in thermal equilibrium with a third system, these 2 systems are in thermal equilibrium with each other. Simply, if 2 objects have the same temperature with one other object, these two objects have the same temperature with each other.
    • There is no heat flow between objects of the same temperature.

  • The 1st Law of Thermodynamics: This is also known as the law of conservation of energy. It states that the total energy in an isolated system never changes.
    • Energy cannot be created or destroyed, it can only be transferred.

  • The 2nd Law of Thermodynamics: This law states that the entropy of an isolated system always increases or remains constant over time. Entropy as used in this law can be defined as disorder. An example of the 2nd law in action are ubiquitous in nature; ice melting, fires burning down, a battery running out of electrical charge, etc.
    • The 2nd law is the reason why heat always transfers from a relatively higher temperature to an object that is lower.

  • The 3rd Law of Thermodynamics: The 3rd law states that it is impossible to cool an object down to a temperature of absolute zero. It can also be stated in terms of heat; it is impossible to remove all the heat from a physical system.
    • The lowest possible limit on temperature is absolute zero. Temperature can never be lower than this in the whole universe. In quantitative terms, absolute zero can be measured as 0 Kelvin (K), -273.15 Celsius (C), or -459.67 Fahrenheit(F),
    • In 2003, MIT Researchers achieved the lowest temperature which is .45 nK (nano kelvin) or 4.5 x 10-10 K or 0.00000000045 K

  • These laws dictate how heat, energy, and pressure behave within a thermodynamic system.

    Heat Transfer

    As with the laws of thermodynamics, the transfer of heat between one object to another is defined by these. Heat is transferred from one object to another through one of four fundamental modes:
    • Conduction or diffusion: The transfer of energy between objects that are in physical contact
    • Convection: The transfer of energy between an object and its environment, due to fluid motion
    • Radiation: The transfer of energy to or from a body by means of the emission or absorption of electromagnetic radiation
    • Advection: The transfer of energy from one location to another as a side effect of physically moving an object containing that energy
    Better surfaces could help dissipate heat

    Cooling systems that use a liquid that changes phase — such as water boiling on a surface — can play an important part in many developing technologies, including advanced microchips and concentrated solar-power systems. But understanding exactly how such systems work, and what kinds of surfaces maximize the transfer of heat, has remained a challenging problem. Now, researchers at MIT have found that relatively simple, microscale roughening of a surface can dramatically enhance its transfer of heat. Such an approach could be far less complex and more durable than approaches that enhance heat transfer through smaller patterning in the nanometer (billionths of a meter) range.

    The new research also provides a theoretical framework for analyzing the behavior of such systems, pointing the way to even greater improvements. The work was published this month in the journal Applied Physics Letters, in a paper co-authored by graduate student Kuang-Han Chu, postdoc Ryan Enright and Evelyn Wang, an associate professor of mechanical engineering.

    Video: Thermodynamics

    “Heat dissipation is a major problem” in many fields, especially electronics, Wang says; the use of phase-change liquids such as boiling water to transfer heat away from a surface “has been an area of significant interest for many decades.” But until now, there has not been a good understanding of parameters that determine how different materials — and especially surface texturing — might affect heat-transfer performance. “Because of the complexities of the phase-change process, it’s only recently that we have an ability to manipulate” surfaces to optimize the process, Wang says, thanks to advances in micro- and nanotechnology.

    Chu says a major potential application is in server farms, where the need to keep many processors cool contributes significantly to energy costs. While this research analyzed the use of water for cooling, he adds that the team “believe[s] this research is generalizable, no matter what the fluid.”

    Scanning electron micrographs (SEMs) of the silicon microstructured surfaces for the boiling experiments. The dimensions of the micropillars are well-defined to allow systematic studies of surface roughness effects on critical heat flux (CHF).
    Image: Kuang-Han Chu et al, Applied Physics Letters
    The team concluded that the reason surface roughness greatly enhances heat transfer — more than doubling the maximum heat dissipation — is that it enhances capillary action at the surface, helping keep a line of vapor bubbles “pinned” to the heat transfer surface, delaying the formation of a vapor layer that greatly reduces cooling.

    To test the process, the researchers made a series of postage-stamp-sized silicon wafers with varying degrees of surface roughness, including some perfectly smooth samples for comparison. The degree of roughness is measured as the portion of the surface area that can come into contact with a liquid, as compared to a completely smooth surface. (For example, if you crumpled a piece of paper and then flattened it back out so that it covered an area half as large as the original sheet, that would represent a roughness of 2.)

    The researchers found that systematically increasing roughness led to a proportional increase in heat-dissipation capability, regardless of the dimensions of the surface-roughening features. The results showed that a simple roughening of the surface improved heat transfer as much as the best previous techniques studied, which used a much more complex process to produce nanoscale patterns on the surface. In addition to the experimental work, the team developed an analytical model that very precisely matches the observed results. Researchers can now use that model to optimize surfaces for particular applications.

    Time-lapse images of vapor bubble departure on the microstructured surfaces (a-d).
    Image: Kuang-Han Chu et al, Applied Physics Letters
    “There has been limited understanding of what kind of structures you need” for effective heat transfer, Wang says. This new research “serves as an important first step” toward such analysis. It turns out heat-transfer is almost entirely a function of a surface’s overall roughness, Wang says, and is based on the balance between various forces acting on the vapor bubbles that serve to dissipate heat: surface tension, momentum and buoyancy.

    While the most immediate applications would likely be in high-performance electronic devices, and perhaps in concentrated solar-power systems, the same principles could apply to larger systems such as powerplant boilers, desalination plants or nuclear reactors, the researchers say. The work was supported by the Battelle Memorial Institute and the Air Force Office of Scientific Research. The team received help in fabrication from the MIT Microsystems Technology Lab.


    MIT News
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    Written by David Chandler, MIT News Office