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Introduction:
Power dissipation, chip power consumption, heat flux in electronic devices & less heat transfer in radiators have been rapidally increasing over the past century, creating the need for improved methods of improving them. Choi et al. reported the limitations of used technologies:-
Liquid Coolants:- Traditional coolants such as water & oils have poor heat transfer properties.
Air as coolants:- Radiator designs used in last decade for increasing air side heat transfer have already adopted extended surface technologies to its limits.
So there is need of new technology & advanced fluids which having peculiar properties & should have great potential to improve the flow (Heat) & thermal characteristics, so that heat transfer rate can be enhanced.
Nanofluids are formed by suspending metallic or non-metallic oxide nanoparticles in traditional heat transfer fluids. These so called nanofluids have good thermal properties compared with fluid which were convectionally used for heat transfer.
Radiators are heat exchangers used to transfer thermal energy from one medium to another for purpose of cooling and heating.
Since from the middle of 1950s great efforts have been done on variation in geometry using different types of fins but with increasing power to weight or volume ratio, increasing demand to dissipate maximum amount of heat for any given space, only small change has been occurred to the design and manufacturing of Automobile Radiator.
Only a portion of total power generated is supplied to Automobile for motion, the rest is wasted in exhaust and heat. If that excess heat is not removed, the engine temperature becomes too high which causes viscosity breakdown of lubricating oil, stress among engine parts which results in wearing and tearing & product life is reduced.
So it’s need to design more compact cooling system with smaller and lighter radiator. Two options are available for greater cooling
1. Additions of fins is one of the approaches to increase the cooling rate of radiator as it provides greater heat transfer area & enhance the air convective heat exchanger coefficient.
2. The addition of nanofluids as additives in liquid coolant to improve specific properties.
Yu et al. reports about 15-40% of heat transfer enhancement can be achieved by using various types of naonofluids. For ex: glycol are added to water to depress its freezing point & to increase its boiling point. The heat transfer coefficient can be improved via the addition of solid particles to liquid coolant.
Eastman et al reported that the thermal conduvtivity of ethylene glycol nanofluids containing 0.3% volume fration of copper particles can be enhanced up to 40% compared to that of ethylene glycol basefluid. Hwang et al found that thermal conductivity of the nanofluid deoends on the volume fration of particles and thermal conductivity of base fluid and particles.
Mintsa et al investigated the effected of temperature, particle size and volume fraction on thermal conductivity of water based nanofluids of copper oxide and alumina. Authors suggested that thermal characteristics can be enhanced with increase of particle, volume fraction, temperature and particle size. Authors found that the smaller the particle size, the greater the effective thermal conductivity of the nanofluids at the same volume fraction. Contact surface area of partocles with fluid and Brownian motion can be increased when smaller particles are used in the same volume fraction. This consequently increased thermal conductivity of nanofluids.
Since the thermal conductivity of solids is greater than liquids, disparsion of solid particles in a given fluid is bound to increased thermmal conduvtivity.
By reducing the size and changing the location of the radiator, a reduction in weight and wind resistance could enable greater fuel efficiency and subsequently lower exhaust emmisions. Computer simulations from the US department of energy’s office of vehicle technology showed that nanofluid coolants coolant could reduce the size of truck radiator by 5%. This would result in a 2.5% fuel saving at highway speeds.
Thermal conductivity of copper at room temperature is about 700 times greater than that of water and about 3000 times greater than that of engine oil . the thermal conductivity of metallic liquids is much greater than that of non- metallic liquids.
The heat transfer in a heat exchanger involves convention on each side of fluid and conduction taking palace through the wall which is seprating the two fluids. In a heat exchanger, the temperature of fluid keeps on changing as it passes through the tubes and also the temperature of dividing wall located between the fluids varies along the length of heat exchanger.
The thermal resistance between coolant and environment is as follows:-
R = R1 + R2 where R1 = Resistance (Thermal) between coolant and inner walls of tubes of radiator.
R1 = 1/h where h = convective heat transfer co-efficient between coolant and inner walls of tube.
R2 = resistance between outer wall to fin to surroundings (Air). It is independent of what coolant is used either Ethylene glycol or Al2O3. So total thermal resistance is only the function of R1
Total heat transfer via coolant is:-
Q = T coolant – T environment/ R
The more heat transfer could only be possible when there is higher value of T coolant & smaller value of R (Thermal resistance).
That’s why liquid metal is adopted as heat transfer coolant. And nanofluids are called as “ Boiling point increasers”.
Challenges
Smaller the size greater the stability of colloidal dispersion, greater the stability of colloidal dispersion greater is the probability of interaction & collision among particles in fluid and with fluid so due this particle interact with each other then settle down due to gravity & tends to agglomerate. Due to agglomeration, blocking of fins as well as properties of nanofluids are decreased. In order to reduce agglomeration addition of surfactants is the only option.
But addition of surfactants causes widespread problem as they may produce foams in alternate cooling- heating process.
Cost
Nanofluids are prepared by following process
1. 2 step- in 2 step first nano powder is produced then secondly powder is dispersed into liquid coolant.
2. 1 step- in 1 step process both production of nano powder & dispersed into liquid are simultaneously done by one chemical process
Both these process have their own limitations & require advanced and sophisticated equipments
Panzali ey al stressed that high cost of nanofluids is among the drawback of nanofluids applications.
Specific heat
Nano coolants actually possess lower specific heat than base fluid but ideal coolant should possess highest specific heat value as function of coolant is to draw more heat from engine. But specific heat possess a great challenge to researchers. Namburu et al. revealed that Al2O3/ ethylene glycol nanofluids exhibit lower specific heat compared to base fluids.
Viscosity
Due to agglomeration at higher concentration of nano particles in coolants results in higher viscosity of nanofluids. As a result of this, heat transfer rate is decreased and clogging of fins is major problem. So p[article mass fraction has to be increased in limited manner. Lee reported that the viscosity increased so rapidally with increasing particle concentration that volume% of CNTS are limited to less than 0.2% in practical systems.
Pressure drop & pumping power
Increased flow rate increases pumping power and as pressure drop is closely associated with pumping power so there is effect on pressure drop also. The major properties of nanofluids that influences the coolant pressure drop are
1. Density
2. Viscosity
The coolants with higher density and viscosity experiences higher pressure drop. So this caused the real disadvantage of nanofluids as liquid coolant.
Lee and Mudawar revealed that single phase pressure drop of Al2O3 nanofluids in microchannel heat sink increases with nanoparticles concentration. Vasu et al studied the thermal design of compact heat exchanger using nanofluids. In this study, it is found that pressure drop of 4%. Al2O3 + H2O nanofluids is almost double of the basefluid.
Future work a. Study is required to identify the reason for and effects of particle deposition. b. Study in use of nanofluids as refrigerants. c. Exact mechanism of heat transfer for nano fluids is still unclear as reported by many researchers. d. Nanofluids stability & its production cost are major factors that hinders the commercialization of nanofluids. e. in contrast to traditional unilateral approach, researchers need to examine the following factors:- such as synthesis, characterization, thermo- physical properties, heat and mass transport, modeling and device as well as system level applications of nanofluids.