Heat energy is the driving force of many day-to-day industrial as well as household activities and needs to be transferred from one place to other. Heat is not always generated in optimum quantity required for its desired use. Either it has to be taken away from the source or has to be driven-in into the location where it is being used.
This motion of heat whether on its own or controlled one for desired location, is nothing but a branch known as heat transfer. Heat can flow from higher temperature to lower on its own but can also be made to flow in reverse by using some driving force.
Heat transfer, an area of engineering based on the physics fundamentals is concerned with the transfer of thermal energy (heat) using a device called heat exchanger, built for heat transfer from one medium to another.
Heat transfer is the exchange of thermal energy between physical systems. Some of the examples of heat transfer are as:
• Air conditioner: a device that cools interior air, such as that of a building or vehicle
• Heat sink: a device used to absorb energy, typically by using its large mass to raise its temperature slightly or by changing phase
• Radiator: a device used to either move heat away from an object or heat an interior space by circulating a fluid through thin metal tubes
• Refrigerator: a device used to cool objects or interior spaces
• Space heater: a device used to heat spaces
• Hydraulic Oil Cooler or example will remove heat from hot oil by using cold water or air
• Swimming Pool Heat Exchanger uses hot water from a boiler or solar heated water circuit to heat the pool water.
The rate of heat transfer is dependent on the temperatures of the systems and the properties of the intervening medium through which the heat is transferred. The three fundamental modes of heat transfer are conduction, convection and radiation. Heat transfer, the flow of energy in the form of heat, is a process by which a system changes its internal energy, hence is of vital use in applications of the First Law of Thermodynamics. Conduction is also known as diffusion, not to be confused with diffusion related to the mixing of constituents of a fluid. The direction of heat transfer is from a region of high temperature to another region of lower temperature, and is governed by the Second Law of Thermodynamics. Heat transfer changes the internal energy of the systems from which and to which the energy is transferred. Heat transfer will occur in a direction that increases the entropy of the collection of systems.
Heat exchanger is a device which transfers heat from one medium to another. Heat is transferred by conduction through the exchanger materials which separate the mediums being used. A shell and tube heat exchanger passes fluids through and over tubes, where as an air cooled heat exchanger passes cool air through a core of fins to cool a liquid. There are many different types of heat exchanger available, the three main types are:
Shell and Tube Heat Exchangers: consist of a large number of small tubes which are located within a cylindrical shell. The tubes are positioned into the cylinder using a tube bundle or 'tube stack,' which can either have fixed tube plates. A floating tube stack which allows the tube bundle to expand and contract with varying heat conditions as well as allowing the tube bundle to be easily removed for servicing and maintenance.
Plate Heat Exchangers: operate in very much the same way as a shell and tube heat exchanger, using a series of stacked plates rather than tubes. Plate heat exchangers are usually brazed or gasketed depending on the application and fluids being used. Their compact stainless steel construction makes them an ideal choice for use with refrigerants or in food and beverage processing.
Air Cooled Heat Exchangers: are commonly used in vehicles or other mobile applications where no permanent cool water source is available.
Nanotechnology in Heat Transfer
Heat exchange has been a significant issue in many mechanical devices since the Industrial Revolution. There’s enough inefficiency in heat transfer, for instance, that for water to reach its boiling point of 100 degrees centigrade, the temperature of adjacent plates often has to be about 140 degrees centigrade. With the growing need and demand for all type of heat exchange, heat exchange should be economical and efficient; therefore, efforts are being directed to make efficient and economical heat exchangers. Nanotechnology can no longer be considered an emerging science. It has developed past the point of having a few applications. Everything from medical science to futuristic hologram projections is being developed using various forms of nanotechnology. Every aspect of our lives will improve in one way or the other thanks to this unique technology.
The other factor, surface wettability, has also been verified that hydrophilic characteristics of the surfaces result in the enhancement of surface re-wetting properties – and then it helps to increase Critical Heat Flux (CHF) to extend maximum heat dissipation capacity.
The problems of heat and energy consumption are interrelated. Energy consumption is lower when the hardware produces less heat. Likewise, the equipment needs less power when it runs cooler. Researchers have discovered a new way to apply nanostructure coatings to make heat transfer far more efficient, with important potential applications to high tech devices as well as the conventional heating and cooling industry. These coatings can remove heat four times faster than the same materials before they are coated, using inexpensive materials and application procedures. The discovery has the potential to revolutionise cooling technology. For the configurations investigated, this approach achieves heat transfer approaching theoretical maximums which is quite significant. The improvement in heat transfer achieved by modifying surfaces at the nanoscale has possible applications in both micro- and macro-scale industrial systems. Heat exchangers are what make modern air conditioners or refrigerators function, and inadequate cooling is a limiting factor for many advanced technology applications, ranging from laptop computers to advanced radar systems.
The problems of heat and energy consumption are interrelated. Energy consumption is lower when the hardware produces less heat. Likewise, the equipment needs less power when it runs cooler. Researchers have discovered a new way to apply nanostructure coatings to make heat transfer far more efficient, with important potential applications to high tech devices as well as the conventional heating and cooling industry. These coatings can remove heat four times faster than the same materials before they are coated, using inexpensive materials and application procedures. The discovery has the potential to revolutionise cooling technology. For the configurations investigated, this approach achieves heat transfer approaching theoretical maximums which is quite significant. The improvement in heat transfer achieved by modifying surfaces at the nanoscale has possible applications in both micro- and macro-scale industrial systems. Heat exchangers are what make modern air conditioners or refrigerators function, and inadequate cooling is a limiting factor for many advanced technology applications, ranging from laptop computers to advanced radar systems.
The new approach, through both their temperature and a nanostructure that literally encourages bubble development, water will boil when similar plates are only about 120 degrees centigrade.
To do this, heat transfer surfaces are coated with a nanostructured application of zinc oxide, which in this usage develops a multi-textured surface that looks almost like flowers, and has extra shapes and capillary forces that encourage bubble formation and rapid, efficient replenishment of active boiling sites. Many electronic devices need to remove a lot of heat quickly, and that’s always been difficult to do. This combination of a nanostructure on top of a microstructure has the potential for heat transfer that’s much more efficient than anything we’ve had before.
Researchers have shown that an advanced cooling technology being developed for high-power electronics in military and automotive systems is capable of handling roughly 10 times the heat generated by conventional computer chips. The miniature, lightweight device uses tiny copper spheres and carbon nanotubes to passively wick a coolant toward hot electronics. This wicking technology represents the heart of a new ultrathin "thermal ground plane," a flat, hollow plate containing water. Similar "heat pipes" have been in use for more than two decades and are found in laptop computers. However, they are limited to cooling about 50 watts per square centimeter, which is good enough for standard computer chips but not for "power electronics" in military weapons systems and hybrid and electric vehicles.
Cooling technology, which makes possible to dissipate generated or transmissive heat from hot spots to atmosphere, based on heat transfer is an essential ingredients for practical modern industry fields. As the quantity of heat generation rapidly increases according to the increase of the integration density of electric circuits, advanced cooling technologies are required as pre-requisite criteria for thermal designing of devices.
Bottlenecks
Heat transfer needs to be controlled as it is a critical aspect of many different technologies. Interfaces between different materials are often heat-flow bottlenecks due to stifled phonon transport. Inserting a third material usually only makes things worse because of an additional interface created. However, introducing an ultrathin nanolayer of organic molecules that strongly bond with both the materials at the interface gives rise to multi-fold increases in interfacial thermal conductance, contrary to poor heat conduction seen at inorganic-organic interfaces. This method to tune thermal conductance by controlling adhesion using an organic nanolayer works for multiple materials systems, and offers a new means for atomic- and molecular-level manipulation of multiple properties at different types of materials interfaces. Radiative heat transfer at nanoscale distances, while theorized, has been especially challenging to achieve because of the difficulty of maintaining large thermal gradients over nanometer-scale distances while avoiding other heat transfer mechanisms like conduction. All objects in our environment exchange heat with their surroundings using light. This includes the light coming at us from the sun, the glowing red color of the heating element inside our toaster ovens, or the "night vision" cameras that enable image recording even in complete darkness. But heat exchange using light is usually very weak compared to what can be achieved by conduction (i.e., by simply putting two objects in contact with each other) or by convection (i.e., using hot air).
Conclusion
To date, the benefits from nanomaterial-enhanced industrial heat transfer fluids have not been realized, due to some significant technical issues. There is a need to develop nanoparticle-containing heat transfer fluids by manipulating the local environment at the fluid-nanoparticle interface through both physical and chemical means. The resulting nanoparticle-containing heat transfer fluid will have flow properties close to that of the base fluid, providing a substantial improvement in thermal conductivity and heat transfer coefficient. The nanoparticle-enabled heat transfer fluid should improve the energy efficiency of existing industrial waste heat recovery systems that utilise fluid flow in a closed-loop. The new fluids should find applications in large industrial operations such as refineries, chemical plants, and paper mills. The principles developed also should be applicable to engine coolants, used in both off-road and consumer automotive applications, where weight and cost savings from smaller heat exchangers are important.
AUTHORS CREDIT & PHOTOGRAPH
Dr S S Verma
Department of
Physics S.L.I.E.T.
Department of
Physics S.L.I.E.T.
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