31-08-2016, 02:57 PM
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ABSTRACT:
The increasing dependencies on a technology our standards of living requires more technologically trend workforce.
Heat and temperature are among the most misunderstood concepts in science.
Temperature is a physical state, based on the molecular activity of an object. If you cut an object in half, each half will have the same temperature.
Heat is a transfer of energy, which might change the state of temperature.
Heat transferis the exchange ofthermal energybetween physical systems, depending on thetemperatureandpressure, bydissipatingheat. The fundamental modes of heat transfer areconductionordiffusion,convectionandradiation.
Thermal conductivity plays an important role in heat transfer phenomenon. The conventional heat transfer fluids are having low thermal conductivity.
To increase thermal conductivity of these fluids and thereby the heat transfer rate ,
Nano fluids are the topic of research of most of the researchers.
Nanofluids are the nano sized particles of metals, oxides and carbides suspended in conventional heat transfer fluids.
In this paper, the performance of various systems using nano refrigerant, nano lubricant has been reviewed.
Nano fluids with different concentration of nano particles have been used to increase thermal conductivity.
From the literature reviews, it has been found that the heat transfer coefficient enhances and there is considerable saving in power consumption.
In the same way we can employ a nucleate boiling, CHF, Two phase flow billing to enhance heat transfer.
Keywords: 2phase Nucleate boiling , Nano fluids, critical heat flux ,Particle characterization Nano particles , Nano refrigerant, MWNT
INTRODUCTION
The way to improve heat transfer performance is referred to as heat transfer enhancement (or augmentation or intensification). Nowadays, a significant number of thermal engineering researchers are seeking for new enhancing heat transfer methods between surfaces and the surrounding fluid.
Heat-Transfer Challenges:
The heat rejection requirements are continually increasing due to trends toward smaller features (to <100 nm) for microelectronic devices, more power output for engines.
Cooling becomes one of the top technical challenges facing high-tech industries such as microelectronics, transportation, manufacturing, and metrology.
Conventional method to increase heat flux rates:
extended surfaces such as fins and micro-channels
increasing flow rates increases pumping power.
Nanofluids are promising to meet and enhance the challenges
These are the some advanced techniques by which we can enhance the heat transfer
1)NANOFLUIDS
With the rapid advancement in nanotechnology, it becomes possible to obtain nano sized particles (1 to 100 nm.) of metals,
oxides and carbides. These nanoparticles having high thermal conductivity are suspended in conventional heat transfer fluids which lead to emerging of new generation of heat transfer fluids called nanofluids.
Nanofluids have been introduced by Steve Choi of Argonne's Energy Technology Division and Jeff Eastman of the Materials Science Division on Argonne National Laboratory in 1995.
Thermal systems like refrigerator, air conditioners etc. consume large amount of electric power.
Many researches have been carried out in last decade to enhance the heat transfer rate, to improve performance of the systems.
To improve thermo physical and heat transfer capabilities of the system, nano particles are either suspended in the refrigerant or lubricating oil.
Nanofluids have the following characteristics as compared to normal solid-liquid suspensions.
A) Higher heat transfer between the particles and fluid due to high surface area of the particles.
B) Better dispersion stability.
C) Reduces particle clogging.
D) Reduces pumping power as compared to base fluid to obtain equivalent heat transfer.
Nano particles could be of metal
like copper, nickel, aluminum. Oxides like aluminum oxide, titanium oxide, copper oxide, silicon oxide. Base fluids are the thermo fluids like water, ethylene glycol, propylene glycol, engine oil and refrigerants.
Nanofluids are prepared by production of nano particles and then its dispersion in the base fluid.
There are two methods used for the preparation of nano fluids namely single step method and two step method. In single step method, nano particles are simultaneously produced and dispersed, while in two step method,
nanoparticles are first produced and then dispersed in the base fluid.
There should not be any agglomeration of particles and no chemical change in the base fluid. To achieve all this, there are three ways
A) To change pH value of suspension
B) To use surface activators/dispersants.
C) To use ultrasonic vibration.
Smart cooling nanofluids
Realizing the modest thermal conductivity enhancement in conventional nanofluids, a team of researchers at Indira Gandhi Centre for Atomic Research Centre, Kalpakkam developed a new class of magnetically polarizable nanofluids where the thermal conductivity enhancement up to 300% of basefluids is demonstrated. Fatty-acid-capped magnetite nanoparticlesof different sizes (3-10 nm) have been synthesized for this purpose. It has been shown that both the thermal and rheological properties of such magnetic nanofluids are tunable by varying the magnetic field strength and orientation with respect to the direction of heat flow. Such response stimuli fluids are reversiblyswitchable and have applications in miniature devices such as micro- and nano-electromechanical systems
All these techniques change the surface properties of the suspended particles and suppress clustering. It depends on the application how these techniques are used.
Nanofluids have vast application in different fields like engine cooling, engine transmission oil, cooling electronic circuits, refrigeration, drilling lubrication, thermal storage, solar water heating ,
Nuclear cooling system, space application, biomedical application, lubrication etc.
THERMAL CONDUCTIVITIES OF NANOFLUIDS
In electronic industry ,improvement of the thermal performance of cooling systems together with the reduction of their required surface area
has always been a great technical challenge research carried out on this subject can be
classified into three general approaches
finding the best geometry for cooling;
decreasing the characteristics length recently increasing he thermal performance of coolant;
the latest approach is based on the discovery of nanofluids.
Thermal conductivity is the ability of material to conduct or transmit heat.
Thermal conductivity plays an important role in construction of energy efficient thermal system.
nanofluids have a better thermal performance than conventional heat transfer fluids due to the high thermal conductivity of nano particles in recent years there are several investigations showing enhancement of thermal conductivity of nano fluids.
the cooling performance of water cooling kit with the addition of nano fluids is investigated
these fluids are engineered to enhance thermo physical, thermal diffusivity, viscosity and convective heat transfer coefficients compared to those base fluids like oil or water
nano particles used in nano fluids are typically made of metals, oxides, carbide, CNTs , MWNTs t (Multiwall nano tubes)
nano fluids have novel properties that makes them useful in many application in heat transfer, including microelectronics , fuel cells, hybrid powered engines, nuclear reactor coolant.
Nanocryosurgery
Cryosurgery is a procedure that uses freezing to destroy undesired tissues.
Introduction of nanoparticle enhanced freezing could also make conventional
cryosurgery more flexible in many aspects such as artificially interfering in the
size, shape, image and direction of iceball formation. The concepts of
nanocryosurgery may offer new opportunities for future tumour treatment.
2)Microchannels
With the advances in micro-machinining technology, the size of
Micro-Electro Mechanical Systems (MEMS) is reducing day by day
and power density of microdevices is increasing, posing a problem for
thermal control and heat dissipation from these devices. Microchannel
passages providing high surface area to volume ratio gives high heat
transfer rates from small areas has emerged as potential heat
dissipating and theremal control devices for MEMS.
Miniaturization of electronic gadgets has become necessity of today’s world. In the last
two decades researchers have devoted countless efforts in developing miniaturized
microdevices. The reduced size has increased the heat flux density which causes
overheating of devices and makes the overall well being and proper functioning of
these devices a big challenge for researchers. So there is a need to develop highly
efficient cooling technology and heat dissipation methods to meet the safety and stable
operation of MEMS. The simplest arrangements commonly used to this effect are
microchannels. In a microchannel a fluid is used to carry away heat from the small hot
surface by forcing it through passages having hydraulic diameters ranging from 10 µm
to 200 µm. As a microchannel has higher heat transfer surface area to fluid volume
ratio, so it provides high heat transfer coefficient for convective heat transfer. However
this small channel experiences a very high pressure drop.
3)
Particle characterization to enhance heat transfer
4)
CNT & MWNT advancement
Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1,[1] significantly larger than for any other material. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials.
All nanotubes are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis.
It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per Kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 watts per meter per K.
The temperature stability of carbon nanotubes is estimated to be up to 2800oC in vacuum and about 750oC in air.
ABSTRACT:
The increasing dependencies on a technology our standards of living requires more technologically trend workforce.
Heat and temperature are among the most misunderstood concepts in science.
Temperature is a physical state, based on the molecular activity of an object. If you cut an object in half, each half will have the same temperature.
Heat is a transfer of energy, which might change the state of temperature.
Heat transferis the exchange ofthermal energybetween physical systems, depending on thetemperatureandpressure, bydissipatingheat. The fundamental modes of heat transfer areconductionordiffusion,convectionandradiation.
Thermal conductivity plays an important role in heat transfer phenomenon. The conventional heat transfer fluids are having low thermal conductivity.
To increase thermal conductivity of these fluids and thereby the heat transfer rate ,
Nano fluids are the topic of research of most of the researchers.
Nanofluids are the nano sized particles of metals, oxides and carbides suspended in conventional heat transfer fluids.
In this paper, the performance of various systems using nano refrigerant, nano lubricant has been reviewed.
Nano fluids with different concentration of nano particles have been used to increase thermal conductivity.
From the literature reviews, it has been found that the heat transfer coefficient enhances and there is considerable saving in power consumption.
In the same way we can employ a nucleate boiling, CHF, Two phase flow billing to enhance heat transfer.
Keywords: 2phase Nucleate boiling , Nano fluids, critical heat flux ,Particle characterization Nano particles , Nano refrigerant, MWNT
INTRODUCTION
The way to improve heat transfer performance is referred to as heat transfer enhancement (or augmentation or intensification). Nowadays, a significant number of thermal engineering researchers are seeking for new enhancing heat transfer methods between surfaces and the surrounding fluid.
Heat-Transfer Challenges:
The heat rejection requirements are continually increasing due to trends toward smaller features (to <100 nm) for microelectronic devices, more power output for engines.
Cooling becomes one of the top technical challenges facing high-tech industries such as microelectronics, transportation, manufacturing, and metrology.
Conventional method to increase heat flux rates:
extended surfaces such as fins and micro-channels
increasing flow rates increases pumping power.
Nanofluids are promising to meet and enhance the challenges
These are the some advanced techniques by which we can enhance the heat transfer
1)NANOFLUIDS
With the rapid advancement in nanotechnology, it becomes possible to obtain nano sized particles (1 to 100 nm.) of metals,
oxides and carbides. These nanoparticles having high thermal conductivity are suspended in conventional heat transfer fluids which lead to emerging of new generation of heat transfer fluids called nanofluids.
Nanofluids have been introduced by Steve Choi of Argonne's Energy Technology Division and Jeff Eastman of the Materials Science Division on Argonne National Laboratory in 1995.
Thermal systems like refrigerator, air conditioners etc. consume large amount of electric power.
Many researches have been carried out in last decade to enhance the heat transfer rate, to improve performance of the systems.
To improve thermo physical and heat transfer capabilities of the system, nano particles are either suspended in the refrigerant or lubricating oil.
Nanofluids have the following characteristics as compared to normal solid-liquid suspensions.
A) Higher heat transfer between the particles and fluid due to high surface area of the particles.
B) Better dispersion stability.
C) Reduces particle clogging.
D) Reduces pumping power as compared to base fluid to obtain equivalent heat transfer.
Nano particles could be of metal
like copper, nickel, aluminum. Oxides like aluminum oxide, titanium oxide, copper oxide, silicon oxide. Base fluids are the thermo fluids like water, ethylene glycol, propylene glycol, engine oil and refrigerants.
Nanofluids are prepared by production of nano particles and then its dispersion in the base fluid.
There are two methods used for the preparation of nano fluids namely single step method and two step method. In single step method, nano particles are simultaneously produced and dispersed, while in two step method,
nanoparticles are first produced and then dispersed in the base fluid.
There should not be any agglomeration of particles and no chemical change in the base fluid. To achieve all this, there are three ways
A) To change pH value of suspension
B) To use surface activators/dispersants.
C) To use ultrasonic vibration.
Smart cooling nanofluids
Realizing the modest thermal conductivity enhancement in conventional nanofluids, a team of researchers at Indira Gandhi Centre for Atomic Research Centre, Kalpakkam developed a new class of magnetically polarizable nanofluids where the thermal conductivity enhancement up to 300% of basefluids is demonstrated. Fatty-acid-capped magnetite nanoparticlesof different sizes (3-10 nm) have been synthesized for this purpose. It has been shown that both the thermal and rheological properties of such magnetic nanofluids are tunable by varying the magnetic field strength and orientation with respect to the direction of heat flow. Such response stimuli fluids are reversiblyswitchable and have applications in miniature devices such as micro- and nano-electromechanical systems
All these techniques change the surface properties of the suspended particles and suppress clustering. It depends on the application how these techniques are used.
Nanofluids have vast application in different fields like engine cooling, engine transmission oil, cooling electronic circuits, refrigeration, drilling lubrication, thermal storage, solar water heating ,
Nuclear cooling system, space application, biomedical application, lubrication etc.
THERMAL CONDUCTIVITIES OF NANOFLUIDS
In electronic industry ,improvement of the thermal performance of cooling systems together with the reduction of their required surface area
has always been a great technical challenge research carried out on this subject can be
classified into three general approaches
finding the best geometry for cooling;
decreasing the characteristics length recently increasing he thermal performance of coolant;
the latest approach is based on the discovery of nanofluids.
Thermal conductivity is the ability of material to conduct or transmit heat.
Thermal conductivity plays an important role in construction of energy efficient thermal system.
nanofluids have a better thermal performance than conventional heat transfer fluids due to the high thermal conductivity of nano particles in recent years there are several investigations showing enhancement of thermal conductivity of nano fluids.
the cooling performance of water cooling kit with the addition of nano fluids is investigated
these fluids are engineered to enhance thermo physical, thermal diffusivity, viscosity and convective heat transfer coefficients compared to those base fluids like oil or water
nano particles used in nano fluids are typically made of metals, oxides, carbide, CNTs , MWNTs t (Multiwall nano tubes)
nano fluids have novel properties that makes them useful in many application in heat transfer, including microelectronics , fuel cells, hybrid powered engines, nuclear reactor coolant.
Nanocryosurgery
Cryosurgery is a procedure that uses freezing to destroy undesired tissues.
Introduction of nanoparticle enhanced freezing could also make conventional
cryosurgery more flexible in many aspects such as artificially interfering in the
size, shape, image and direction of iceball formation. The concepts of
nanocryosurgery may offer new opportunities for future tumour treatment.
2)Microchannels
With the advances in micro-machinining technology, the size of
Micro-Electro Mechanical Systems (MEMS) is reducing day by day
and power density of microdevices is increasing, posing a problem for
thermal control and heat dissipation from these devices. Microchannel
passages providing high surface area to volume ratio gives high heat
transfer rates from small areas has emerged as potential heat
dissipating and theremal control devices for MEMS.
Miniaturization of electronic gadgets has become necessity of today’s world. In the last
two decades researchers have devoted countless efforts in developing miniaturized
microdevices. The reduced size has increased the heat flux density which causes
overheating of devices and makes the overall well being and proper functioning of
these devices a big challenge for researchers. So there is a need to develop highly
efficient cooling technology and heat dissipation methods to meet the safety and stable
operation of MEMS. The simplest arrangements commonly used to this effect are
microchannels. In a microchannel a fluid is used to carry away heat from the small hot
surface by forcing it through passages having hydraulic diameters ranging from 10 µm
to 200 µm. As a microchannel has higher heat transfer surface area to fluid volume
ratio, so it provides high heat transfer coefficient for convective heat transfer. However
this small channel experiences a very high pressure drop.
3)
Particle characterization to enhance heat transfer
4)
CNT & MWNT advancement
Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1,[1] significantly larger than for any other material. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials.
All nanotubes are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis.
It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per Kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 watts per meter per K.
The temperature stability of carbon nanotubes is estimated to be up to 2800oC in vacuum and about 750oC in air.