FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
1. Title of the invention: NANOCOOLANTS FOR USE IN HEAT TRANSFER APPLICATIONS 2.Applicant(s)

NAME NATIONALITY ADDRESS
TATA CONSULTANCY Indian Nirmal Building, 9th Floor, Nariman Point,
SERVICES LIMITED Mumbai, Maharashtra 400021, India
3. Preamble to the description
COMPLETE SPECIFICATION
The following specification particularly describes the invention and the manner in which it
is to be performed.

TECHNICAL FIELD
[0001] The present subject matter relates to coolants and, particularly, but not exclusively, to nanocoolants for use in the heat transfer applications.
BACKGROUND
[0002] Generally, engines, such as internal combustion (IC) engines, generate mechanical power by extracting energy from fuel. The engines typically convert chemical energy, which is provided by way of combustion of the fuel, into heat energy, which in turn is converted into the mechanical power. However, engines are thermodynamically inefficient, and so only a portion of the heat energy is converted into the mechanical power, i.e., thermodynamic work. A portion of the heat energy not converted into the mechanical power may be referred to as waste heat, which must be removed. IC engines often remove the waste heat through intake of cool air, expulsion of hot exhaust gases, and other methods of direct engine cooling.
[0003] Further, as IC engines burn fuel, heat is generated due to which the temperature in the engine rises, and can increase to a temperature higher than the melting temperature of engine materials. In some cases, the temperature may be high enough to set fire to the lubricants. Therefore, cooling may be performed to keep temperatures low inside and around the IC engine, thereby preventing any damage to engine materials and lubricants. The IC engines may be fluid cooled and can use either a gaseous fluid, such as air; or a liquid coolant running through a heat exchanger, such as a radiator cooled by air. Nowadays, most of the IC engines are liquid cooled instead of air cooled owing to better cooling efficiency.
[0004] Most liquid-cooled engines use a mixture of water and chemicals, such as antifreeze and rust inhibitors, as the cooling liquid. A liquid-cooled engine usually dumps heat from the engine to a liquid coolant, thereby heating the liquid coolant to about 135°C, which is then cooled using air having a temperature of about 20°C. Since the liquid coolant may be heated to a temperature as high as 135°C, liquids having a lower boiling point, such as water having a boiling point of 100°C, may not be preferred for cooling purposes. Therefore, some liquid cooled engines use a higher boiling liquid, such as propylene glycol or a combination of propylene glycol and ethylene glycol, instead of using water. Further, stability of the liquid coolants may also govern the selection of the coolant. For example, when certain coolants having glycols are

subjected to high temperatures, they tend to dissociate to result in acids, thereby making the coolant corrosive and practically non usable.
[0005] Also, the properties of cooling liquids affect the size, i.e., area of heat transfer, of the engine considerably. As an example, as compared to water, one gram of oil can absorb about 55% of the heat for the same rise in temperature. Further, oil has about 90% of the density of water, and so a given volume of oil can absorb only about 50% of the energy absorbed by the same volume of water. On the other hand, thermal conductivity of water is about four times that of oil, which can aid heat transfer. Also, the viscosity of oil can be ten times greater than water, thereby increasing the energy required to pump oil for cooling and, in turn, reducing the net power output of the engine.
[0006] Thus, availability of efficient coolants would make it possible to have more efficient machines and better engines. Such machines and engines may be smaller and cheaper, and may have lower fuel demands, thereby making them environment friendly.
SUMMARY
[0007] This summary is provided to introduce concepts related to method of producing nanocoolants and these concepts are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter. [0008] In one embodiment of the present subject matter, method(s) for producing stable nanocoolants are described. The method includes mixing a coolant with a dispersant and a metal oxide powder to form a primary mixture. The primary mixture is ground to obtain a ground suspension of nanoparticles. To the ground suspension of nanoparticles, a pH buffer is added to obtain a pH adjusted nanocoolant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of the method(s) in accordance with the

present subject matter are described, by way of example only, and with reference to the
accompanying figures, in which:
[0010] Fig. 1 illustrates a system for producing stable nanocoolant, in accordance with an
embodiment of the present subject matter.
[0011] Fig. 2 illustrates particle size analysis graph for the suspension of nanoparticles,
according to an embodiment of the present subject matter.
[0012] Fig. 3 illustrates a test apparatus for testing the heat transfer capability of the
nanocoolants, according to an embodiment of the present matter.
[0013] It should be appreciated by those skilled in the art that any block diagrams herein
represent conceptual views of illustrative systems embodying the principles of the present
subject matter.
DETAILED DESCRIPTION
[0014] Method(s) for producing nanocoolants with increased heat transfer capability are described. In an example, a nanocoolant may be understood as a dispersion of nanoparticles in commercially available coolants. In an implementation, the nanocoolants may be used in internal combustion (IC) engines, thermal management of batteries, electronic equipment cooling, etc. Further, the nanocoolant produced using the present subject matter is stable. A stable nanocoolant may be understood as a suspension of the coolant in which nanoparticles either do not agglomerate for long durations, say, several months or years, or form weak agglomerates that may be disintegrated by imparting small amount of energy. The methods described herein can be implemented to produce nanocoolants based on a variety of coolants, such as commercially available engine coolants. Although certain coolants have been described for the purpose of nanocoolant production, it would be understood by those skilled in the art that other fluids may also be similarly used for the production of nanocoolants.
[0015] According to an embodiment of the present subject matter, methods for producing nanocoolants are described. The nanocoolants are prepared using a coolant, such as commercially available IC engine coolant. The coolant may include a blend of additives, including, for example, lubricants, buffers and corrosion inhibitors. Examples of additives

include, but are not limited to, antioxidants, anti-corrosive agents, anti-freezing agents. The additives may be added to enhance the efficiency of the coolant and thus, of the nanocoolant produced using such a coolant. For example, Disodium fluorescein dyes are added to help trace the source of leaks, and tolytriazole may be added as a corrosion inhibitor. [0016] To the coolant, a metal oxide powder and a dispersant are added to form a primary mixture. The metal oxide powder may include particles of one or more metal oxides and the metal oxides may include, but are not limited to, the oxides of metals like titanium, aluminum, iron, silicon, zirconium, zinc, and the like. Further, the metal oxide powder may include particles of metals, metal alloys, and combinations thereof. The average particle size in the metal oxide powder may be in the range of about 0.1-100 microns.
[0017] Further, a dispersant may be understood as a substance added to a medium to promote uniform suspension of particles and prevent agglomeration. Based on the coolant and the metal oxide powder used, a suitable dispersant may be selected. In one implementation, a dispersant is selected based on a measurement related to interaction of the dispersant with a given metal oxide powder and a coolant. The interaction may be measured as the interaction energy of a given dispersant with surface of the metal oxide particles in the presence of the coolant. The selection of a dispersant based on the interaction energy helps identify a dispersant that is strongly attracted to metal oxide particles surfaces, thereby allowing for strong dispersant adsorption. Consequently, the metal oxide particles repel each other, which in turn prevents agglomeration. Examples of the dispersants that may be used include, but are not limited to, carboxylic acids, which may be aliphatic, aromatic, or polymeric acids, esters, ethers, alcohols, cellulose, sugar derivatives, phosphates, amines, or any combination thereof.
[0018] The primary mixture is then ground to produce a suspension of nanoparticles. In an implementation, before grinding the primary mixture, the metal oxide powder and the dispersant are dispersed in the coolant, for example, using magnetic stirring, to form a primary mixture slurry. This helps to evenly distribute the particles of metal oxide in the primary mixture and avoids agglomeration of the particles. It would be understood by those skilled in the art that different methods of dispersion known in the art, such as ultrasonication may also be used to disperse the primary mixture and to form a slurry of the primary mixture.

[0019] Subsequent to dispersion, the primary mixture may be ground to form a ground suspension of nanoparticles. In an example, the ground suspension may be concentrated nanoparticle suspension. In one implementation, the primary mixture is ground using wet milling techniques. The primary mixture may be ground till average particle size becomes of the order of 100 nanometer (nm) or less. Further, while the primary mixture is ground, the dispersant may be added after predetermined time intervals to maintain the viscosity and state of dispersion. It will be understood that the dispersant and the coolant may be added separately or as a mixture. [0020] The suspension of the nanoparticles thus produced may contain a high concentration of nanoparticles and the concentration may vary between 10 to 40 wt% (weight percentage), where the concentration in weight percentage reflects the weight percentage of the metal oxide powder in the primary mixture. Since the nanoparticles in the obtained suspension were formed by wet milling in the presence of the dispersant and the coolant, the nanoparticles do not tend to agglomerate. Further, the addition of dispersants during grinding, after predetermined time intervals, also provides additional stability and prevents agglomeration. Moreover, to prepare a stable suspension, various process parameters, such as grinding time, size of grinding medium, grinding medium to powder ratio, primary mixture slurry concentration, and the amount of dispersants may also be controlled and monitored.
[0021] Further, pH of the dispersion produced subsequent to milling may be monitored and maintained in the range of about 7 to about 11 to obtain a pH adjusted nanocoolant. In an example, a pH buffer may be added to the ground suspension to obtain a pH adjusted nanocoolant. The addition of the pH buffer may help maintain the pH in above mentioned range. As mentioned before, the coolant may include certain additives, which may hinder with the stability of the produced nanocoolant. The adjustment of the pH post milling helps in maintaining the stability of the suspension. The adjustment of the pH in the range of about 7 to about 11 helps in providing stability, since pH has a critical relationship with the charge distribution and the electric double layer, which in turn dictates the stability of dispersion in such cases.
[0022] In one implementation, the pH adjusted suspension is diluted with the dispersant and the coolant to obtain a diluted suspension of the nanoparticles. Further, the diluted suspension may be dispersed using dispersion techniques known in the art, such as ultrasonication, and

magnetic dispersion. Furthermore, in certain cases, after the dilution with the dispersant and the coolant, the pH may change and in one example, nanocoolant may become more alkaline. Therefore, to bring the pH in the range of about 7 to about 11, pH buffers like organic acids, such as, citric acid may be added. The addition of the pH buffer maintains the pH in the above mentioned range, thereby reducing the corrosiveness of the fluid and making it usable in machines, such as IC engines. It will be understood that pH adjustment of the ground suspension may done post grinding, post dilution, or at both the stages.
[0023] Thus, a nanocoolant can be produced, the nanocoolant including a coolant having one or more additives selected from antioxidants, anti-corrosive agents, and anti-freezing agents, nanoparticles of a metal oxide powder, a dispersant to keep the nanoparticles dispersed in the coolant, and a pH buffer to maintain pH in a range of 7 to 11.
[0024] The described method thus provides for production of stable nanocoolants with increased heat transfer efficiency. Also, the nanocoolants produced are stable for long durations, such as weeks, months, and years. Thus, the described method produces nanocoolants where the nanoparticles of a metal oxide, do not agglomerate or coagulate and remain stable even in the presence of the various additives present in the coolant. Further, the pH of the nanocoolant is maintained for longer durations thereby making it suitable for engines and other such machines. Additionally, the long term stability of the nanocoolant may be ensured by modifying the pH of the nanocoolant. Since the nanocoolant includes solid nano-sized particles or fibers having relatively high surface area and high thermal conductivity, the heat transfer capabilities of the nanocoolant is considerably improved. The nanocoolants thus prepared also exhibit unchanged heat transfer efficiency for multiple heating and cooling cycles and are stable for several years. [0025] The present method also provides for scalable process for in situ production of stable dispersions of nano-particles in commercial coolants. Thus, the present process may be seamlessly integrated with existing coolant manufacturing line with little modifications to produce commercial nano-coolants in large quantities.
[0026] In many heat transfer applications, there is a critical relationship between size of a mechanical system and the cost associated with manufacturing and operation. In comparison to conventionally used coolants, the stable nanocoolant produced by methods described herein provides increased heat transfer efficiency and the use of such stable nanocoolant would result in

a lesser heat exchanger surface area, thereby reducing the space required to handle a specified amount of cooling load. The use of stable nanocoolant can also enable smaller heat transfer systems with lower capital costs and higher energy efficiencies.
[0027] It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended to be only for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art. Moreover, all statements herein reciting principles, aspects, and embodiments of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof.
[0028] While aspects of described methods for preparing stable nanocoolant of the metal oxide nanoparticles can be implemented in any number of different production environments, and/or configurations, the embodiments are described in the context of the following environment(s).
[0029] Fig. 1 illustrates a block diagram representation of a system 100 implementing a method to produce stable nanocoolants, according to an embodiment of the present subject matter. It will be understood that intermediate mixtures produced at different levels and stages of preparation of the suspension may be produced in different batches and in various quantities, as would be understood by those skilled in the art.
[0030] In one implementation, a coolant 102, a dispersant 104, and a metal oxide powder 106, which may include particles of at least one metal oxide, are mixed together to form a primary mixture 110. The metal oxide powder 106 may include oxides of metals, such as titanium, iron, silicon, aluminum, zirconium, and zinc. Further, it would be appreciated that in other implementations, the metal oxide powder 106 may include particles of metals, metal alloys, and combinations thereof. In an example, metal oxide powders used in crystalline form. In one implementation, average particle size of the metal oxide powder 106 is greater than 100 nm, for example, in the range of about 0.1 to 1.0 microns. In an example of said implementation, average particle size of the metal oxide powder 106 is in the range of about 0.5 microns.

[0031] Further, the coolant 102 may include a variety of heat transfer liquids which may include, but are not limited to, water, commercially available IC engine coolants, water based antifreezes, such as glycols, alcohols, and combination thereof; polyols, and the like. Furthermore, the coolant 102 includes a blend of additives, including, for example, lubricants, buffers, and corrosion inhibitors. In one implementation, the coolant 102 can be any commercially available coolant known in the art.
[0032] The dispersant 104 may be a suitable substance for the chosen coolant 102 and may include, but is not limited to, carboxylic acids, esters, ethers, alcohols, sugar and its derivatives or polymers, phosphates, amines, and the like or combinations thereof. In one implementation, the dispersant 104 for the given coolant 102 and the metal oxide powder 106 is selected based on interaction energy of coolant 102 and the metal oxide powder 106. In one implementation, the interaction energy may be computed using molecular modeling techniques. The molecular modeling techniques may be understood as theoretical or computational techniques used to quantify the interaction of dispersant molecules with the coolant 102 and the metal oxide in the metal oxide powder 106. For the purpose, any molecular modeling technique known in the art may be used. The interaction energy thus computed may be used to select a suitable dispersant 104 for the given coolant 102 and the metal oxide powder 106. In other implementations, the dispersant 104 may be selected by experimentation using trial and error, or any other technique known in the art.
[0033] In one example, the primary mixture 110 is prepared by mixing the coolant 102 and the dispersant 104 with about 30-40 wt % of the metal oxide powder 106. In one implementation, to prevent agglomeration and precipitation of metal oxide particles, the primary mixture 110 is dispersed using a first disperser 112-1. The first disperser 112-1 may use any physical dispersing technique known in the art, such as ultrasonication, and magnetic stirring, for dispersing the primary mixture 110.
[0034] The dispersed primary mixture may be ground using a grinder 114 to reduce the particle size of metal oxide powder 106 to provide a suspension of metal oxide nanoparticles. However, it will be understood that the un-dispersed primary mixture 110 may also be directly ground by the grinder 114. In one implementation, the grinder 114 is a milling device, for example, a planetary ball mill, stirred media mill, attrition mill, and a bead mill. In one

implementation, the primary mixture 110 may be ground using wet milling. Further, the grinder 114 may have multiple jars to receive and grind the primary mixture 110. Additionally, the grinder 114 includes a grinding medium, such as Yttrium stabilized Zirconium Dioxide. During milling, various process parameters, such as the grinding time, grinding speed, size of grinding media, percentage filling of the jars, grinding media to metal oxide powder ratio, primary mixture concentration, and the amount of dispersants, may be controlled and monitored.
[0035] In an implementation, the average particle size of the grinding medium is in the range of about range of 0.4 - 25 millimeter (mm); percentage filling of the jars is in the range of about 50 - 70%; the ratio of the grinding media and the metal oxide powder 106 in the grinder 114, in terms of weight, is in the range of about 10:1 to 40:1, grinding time is in the range of about 30 minutes to 24 hours. In an example of said implementation, the average particle size of the grinding medium is in the range of about 0.4-0.7 mm; percentage filling of the jars is in the range of about 55 - 65%; the ratio of the grinding media and the metal oxide powder 106 in the grinder 114, is in the range of about 25:1 - 40:1; and grinding time is in the range of about 2-10 hours.
[0036] Further, during grinding, particle size of the metal oxides may be analyzed after every pre-determined time interval using a particle size analyzer 116. It will be understood that as the milling process nears completion, the time period after which the particle size are analyzed may be smaller as compared to the initial stages of the milling process. The particle size analyzer 116 may utilize different techniques known in the art to analyze particle size, such as laser diffraction technique, acoustic spectroscopy technique, and ultrasound attenuation spectroscopy technique. In an example, a laser scattering particle size analyzer is used to analyze the size of particle in the grinder 114.
[0037] In one implementation, small amounts of the dispersant 104 are added to the mixture in the grinder 114 after every pre-determined time interval to modify the viscosity and keep the produced nanoparticles well dispersed, i.e., to ensure that the suspension is well dispersed and remains stable. As mentioned previously, the dispersant 104 coats the metal oxide particles so as to prevent agglomeration. In one example, the average size of the nanoparticles in the suspension is less than 100 nm.

[0038] In an implementation, upon grinding, a pH buffer 118 may be added in the ground suspension to maintain the pH in the range of about 7 to about 11 and to obtain a pH adjusted nanocoolant. In an example, slightly alkaline pH is chosen because the coolants usually include glycols, which when subjected to high temperatures tend to dissociate to result in acids, which make the coolant corrosive and practically non usable. By maintaining an alkaline pH initially, the decrease in pH due to disassociation of glycols can be controlled. Examples of the pH buffer include but are not limited to organic acids, Citric Acid/Sodium Citrate, Borax/Sodium Hydroxide, Sodium Hypophosphate/NaH2P04, Potassium Hypophosphate/KFkPO4 and N-Cyclohexyl-2-aminoethanesulfonicacid.
[0039] Further, in an implementation, the pH adjusted nanocoolant may be diluted using the coolant 102 and the dispersant 104. The diluted suspension thus prepared may be dispersed using a second disperser 112-2 to evenly mix the nanoparticles of the metal oxide powder 106. The dispersion may also provide stability to nanocoolant to be produced. Similar to the first disperser 112-1, the second disperser 112-2 may use any physical dispersing technique known in the art for dispersing the concentrated suspension to obtain a dispersed diluted suspension. In one implementation, the second disperser 112-2 may implement an ultrasonication technique. To the diluted suspension, the pH buffer 118 may be added again to maintain the pH in the above mentioned range and to obtain a nanocoolant product 120. In an example, the concentration of the nanoparticles in the nanocoolant product 120 may be varied from 0.1 to 5 wt%. [0040] The nanocoolants thus produced, such as the pH adjusted nanocoolant or the nanocoolant product 120, have a composition including the coolant (102), the dispersant (104), and nanoparticles of the metal oxide powder (106) produced by wet grinding in presence of the coolant (102) and the dispersant (104). The composition further includes the pH buffer (118) to maintain pH in a range of about 7 to 11. As mentioned earlier, the coolant (102) includes one or more additives selected from antioxidants, anti-corrosive agents, and anti-freezing agents. Further, in the nanocoolant, the nanoparticles of the metal oxide powder (106) have an average size that is less than 100 nm.
[0041] The nanocoolants prepared through the methods described herein are stable for longer time durations and have better heat transfer properties. Typically, the nanocoolants are stable for long durations and, if at all agglomeration takes place, it results in formation of weak

agglomerates, which may be broken down with small amounts of energy, for example, using magnetic stirring or ultrasonication. Further, the nanocoolants are stable under both, isothermal, static, and thermal shocks conditions. The nanocoolants also show good heat transfer capability with consistent stability even when used for multiple heating and cooling cycles under various flow conditions.
[0042] With such properties, the nanocoolants can be used in automobile industries as coolant in batteries, fuel cells, engines, etc. Also the nanocoolants can be utilized in electronic industry for cooling super computers and electronic equipments. Similarly, in the heating, ventilation and air conditioning (HVAC) industry the stable nanocoolants can be useful for cooling purposes. Further, the nanocoolants can also be prepared at large scale and for commercial purposes, in batches of more than 100 liters, without any change in their stability or heat transfer capability. Furthermore, since the present method does not involve high pressure or temperature, it provides for reduction in the cost of manufacture, and therefore may be used for commercial production of the nanocoolants.
VALIDATION AND RESULTS
[0043] The results of methods for preparing stable nanocoolants have been validated using following examples. It will be understood that the examples discussed herein are only for the purpose of explanation and not to limit the scope of the present subject matter. Further, the test results are shown for a specific example of nanocoolant and should in no way be construed as the only stable nanocoolant that can be formed through the described method.
[0044] In the present example, alumina powder is taken as the metal oxide powder 106, commercially available Castrol™ Heavy Duty Coolant, hereinafter interchangeably referred to as the coolant, is taken as the coolant 102, and sodium citrate as the dispersant 104. In said example, 10 grams (gm) of sodium citrate was dissolved in 100 milliliters (ml) of the coolant and, to this solution, 25 gm of alumina powder was added to obtain the primary mixture 110. The primary mixture 110 was dispersed using magnetic stirring. Further, to grind the primary mixture 110, a planetary ball mill having four well cleaned alumina jars having volume 500 ml each was used. The jars were charged with 1000 gm of 0.4 - 0.7 mm Zirconox grinding media, obtained from Jyothi Ceramics, Nasik, India, to make the effective volume of the jar correspond to about 2/3rd of the total volume.

[0045] The dispersed primary mixture was poured into the each jar and ground at 420 rpm. Further, 0.02 gm of sodium citrate and 2ml of water were added every hour of grinding to modify the viscosity and keep the produced nanoparticles well dispersed. The grinding was continued for nine hours and the particle size of the ground slurry was analyzed every hour till further grinding, i.e., reduction in particle size, was observed to be minimal. In the present example, the particle size analyzer was Horiba LA-910, Horiba™ Japan, As can be observed from Fig. 2S the nanoparticles of alumina in this suspension had d50 size of about 85 nm and about 93% particles were finer than 100 nm. The d5o particle size represents the median or the 50th percentile of the particle size distribution as measured by volume. The d50 particle size is a value on the particle size distribution such that 50% of the particles have a volume of this value or less.
[0046] The nanoparticle suspension obtained above was used to prepare the nanocoolants. A given volume of the nanoparticles suspension, for example, in the range 30 mI-100 ml, was diluted with Castrol Heavy Duty Coolant containing 0.1 wt% of sodium citrate. In an example, the diluted suspension was ultrasonicated for 45 minutes using an ultrasonic probe. In said example, for ultrasonication Branson D450 was used. In another example, the diluted suspension was ultrasonicated for 3 hrs in an ultrasonication bath. The nanocoolant thus prepared was characterized by particle size measurement. It was observed that the nanoparticles remain well dispersed even after dilution. Further, the nanocoolant was found to be stable for several months. Additionally, pH of the nanocoolant was adjusted using one of Citric acid and Sodium citrate post milling and post dilution.
J0047] The effect of pH on stability of the nanoparticle-coolant obtained is shown in Table 1.

Without pH modification With pH modification
Stability in terms of number of
days (Approximate value
under zero shear condition) 150-170 200-220
Table 1
[0048] Further, rheological characteristics, which are to be considered for flow based applications, have also been tested. It was found that the viscosity of the above prepared nanocoolant is less than or same as the viscosity of the coolant without particles. This has been

tested for various concentrations of particles in the coolant using a concentric cylinder viscometer. The absolute values are tabulated in Table 2.

Shear Rate (s1) Viscosity (cP) for Different Concentrations of Particles in Nanocoolant

0vol% 0.5vol% 1.5 vol%
80 5.1 4.2 5.1
132 5.0 4.2 5.0
Table 2
[0049] In the present case, it is found that, post-dilution, the viscosity of the dispersion does not increase beyond the viscosity of the bare coolant even for concentrations as high as 1.5 vol% and decreases for lower concentrations. The viscosity of the nanocoolant was measured in centipoises (cP). Thus, the present nanocoolant may be used in various heat transfer applications without higher pumping power, thereby making it suitable for such heat transfer applications. Further, the shear enhances the stability of the nanocoolant.
[0050] The nanocoolant prepared with the above described method was tested for heat transfer efficiency under convection using a test apparatus 300 as illustrated in Fig. 3. The test apparatus was a shell and tube type of heat exchanger. The test apparatus 300 included a customized double pipe heat exchanger 305, which was fabricated using materia], such as glass, steel, copper, and combination thereof. For the purpose of testing, the length of the pipe heat exchanger 305 was chosen to be in the range of 370 mm to 2700 mm with an inner diameter of the tube varying from 6 mm to 8 mm and wall thickness of 2mm. It would be understood by those skilled in the art that the inner tube can either be a straight tube, or can form a coil. In the present example, the inner tube 305- 1 used was in coil form having around 23 to 40 coils with a pitch between two coils of around 10 to 12 mm. Further, according to an embodiment of the present subject matter, the outer pipe 305-2 was a straight tube with a diameter in the range of 25 mm to 100 mm which houses the inner coiled tube.
[0051] To provide insulation from the atmosphere, the outer surface of the double pipe 305 was insulated with Polyurethane Foam (PUF) and glass wool. To test the heat transfer capability of the nanocoolant produced using the above described method, nanocoolant 310 was passed through the inner tube 305- land heated water from a hot water bath 315 was passed through the

outer tube 305-2 to transfer heat to the nanocoolant. In another implementation of the present subject matter, instead of heated water, steam was used for the purpose of heat transfer. The nanocoolant 310, heated water or steam may be provided using a pump 312-1 and 312-2, respectively. The temperature at the inlet and the outlet of the inner tube 305-1 was measured using thermocouples and recorded through a data acquisition system 320 as a function of time. Based on the recordings, a time-temperature relation, of both the water flowing through the outer tube 305-2 and the nanocoolant 310 flowing through the inner tube 305-1, was measured on a continuous basis. Although, it has been described that the cold nanocoolant was passed through the inner tube and the heated water was passed through the outer tube 305-2, it would be appreciated that the flow can be in the opposite manner where the nanocoolant 310 is passed through the outer tube 305-2 and the other fluid, such as water is passed through the inner tube 305-1. Further, the nanocoolant 310 after passing through double pipe heat exchanger 305 may be collected in container 325.
[0052] To analyze the data obtained in the form of inlet and outlet temperatures of the two fluids at steady state, Log Mean Temperature Difference (LMTD) and Overall Heat Transfer Coefficient (U) were calculated. As would be known to a person skilled in the art, "U* reflects the effectiveness of the heat exchanger. In other words, higher the value of *U', more effective is the heat exchanger. The flow rates of hot water bath or steam 315 were varied from 100- 3300 ml/min, preferably at 200 ml/min or 2800 ml/min; while that of nanocoolant 310 was varied from 40-850 ml/min., preferably at 830 ml/min. Several cycles of heating cooling, in an example, about 3000 cycles, were followed to measure the heat transfer efficiency of the nanocoolant 310. The nanocoolant 310 exhibited about 20 % about 100% enhancement in overall heat transfer efficiency over several heating- cooling cycles for concentrations of 0.5-1.5 vol %.
[0053] Additionally, since the coolants are often subjected to high temperatures during their operating cycle and a low boiling point may lead to higher loss of the coolant, and reduce life of a product having the coolant, therefore boiling point may be considered as one of the key characteristics of a coolant. Further, elevation in boiling point of the present nanocoolant was also measured.
[0054] Boiling point was measured by a standard distillation test that measures the temperature of the vapour above a boiling liquid. The test results for boiling point of the above

prepared nanocoolant is summarized in Table 3. It may be gathered from table 3 that an elevation of 2-4 degree Celsius in the boiling point is achieved on preparation of the nanocoolant.
Test Fluid Boiling Point (deg. C) depending on
Commercial Coolant 101-110
1.5 vol% Alumina in Commercial Coolant 103-114
EG-Water (1:1) 100-109
Table 3
[0055] Thus, the present subject matter provides for scalable process for in situ production of stable dispersions of nano-particles in commercial coolants. Based on the described method, commercially available coolants may be easily converted into stable nanocoolants in large quantities. Additionally, the nanocoolants thus produced have an elevated boiling point thereby making them suitable for heat transfer applications. The nanocoolants thus produced exhibit enhancement in heat transfer efficiency with little or no trade off in pumping power, since there is no change in the viscosity of coolant owing to addition of nanoparticles. Further, such enhancements in heat transfer efficiency may provide for reduction in the size of the engine, enhanced power output with existing engine designs, and increased mechanical work for the same heat input. Consequently, fuel efficiency of a machine, such as an engine having the nanocoolant may improve and carbon footprint may reduce.
[0056] Although implementations for preparation of stable nanocoolants has been described in language specific to structural features and/or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as examples and implementations for producing stable nanocoolants.

I/We claim:
1. A method of preparing a nanocoolant, the method comprising:
mixing a coolant (102) with a dispersant (104) and a metal oxide powder (106) to form a primary mixture (110), wherein the coolant (102) includes one or more additives;
grinding the primary mixture (110) to obtain a ground suspension of nanoparticles; and
adding a pH buffer (118) to the ground suspension to obtain a pH adjusted nanocoolant.
2. The method as claimed in claim 1, wherein the method further comprises:
mixing the dispersant (104) and the coolant (102) to the pH adjusted nanocoolant to obtain a diluted suspension; and
adding the pH buffer (118) to the diluted suspension to obtain a nanocoolant product (120).
3. The method as claimed in claim 1 or claim 2, wherein the pH buffer (118) is added to maintain pH in a range of about 7 to about 11.
4. The method as claimed in any one of the above claims, wherein the pH buffer (118) includes organic acids.
5. The method as claimed in claim 2, wherein the mixing the dispersant (104) and the coolant (102) to the pH adjusted nanocoolant further comprises dispersing the pH adjusted nanocoolant.
6. The method as claimed in claim 2, wherein metal oxide particles in the nanocoolant product (120) and the ground suspension have an average size less than 100 nm.
7. The method as claimed in claim 1, wherein the mixing further comprises dispersing the primary mixture (110) to form a slurry of the primary mixture (110).
8. The method as claimed in claim 1, wherein the metal oxide powder (106) includes particles of at least one metal oxide having an average particle size in the range of about 0.1 microns to 100 microns.
9. The method as claimed in claim 1, wherein the grinding further comprises adding the dispersant (104) to the primary mixture (110) at predetermined time intervals.

10. The method as claimed in claim 1, wherein the method further comprises, while grinding the primary mixture (110), periodically measuring particle size of metal oxide particles in the primary mixture (110), and grinding the primary mixture (110) till average particle size of metal oxide particles is less than about 100nm.
11. The method as claimed in claim 1, wherein the one or more additives include antioxidants, anti-corrosive agents, and anti-freezing agents.
12. The method as claimed in claim 1, wherein the dispersant (104) is at least one of a carboxylic acid, an ester, an ether, an alcohol, sugar, a sugar derivative, a phosphate, and an amine.
13. The method as claimed in claim 1, wherein the metal oxide powder (106) includes an oxide of at least one of aluminum, titanium, iron, silicon, zirconium, and zinc.
14. The method as claimed in claim 1, wherein the coolant (102) includes at least one of water, commercially available internal combustion (IC) engine coolants, and water based antifreezes.
15. The method as claimed in claim 1, wherein the dispersant (104) is selected based on an interaction energy of the dispersant (104) with respect to the metal oxide powder (106) in presence of the coolant (102).
16. A nanocoolant comprising:
a coolant (102), wherein the coolant (102) includes one or more additives selected from antioxidants, anti-corrosive agents, and anti-freezing agents;
nanoparticles of a metal oxide powder (106), wherein the nanoparticles have an average size less than 100 nm;
a dispersant (104) to keep the nanoparticles dispersed in the coolant (102); and
a pH buffer (118) to maintain pH in a range of about 7 to about 11.
17. The nanocoolant as claimed in claim 16, wherein the nanocoolant is produced by a method
comprising:
mixing the coolant (102) with the dispersant (104) and the metal oxide powder (106) to form a primary mixture (110);
grinding the primary mixture to obtain a ground suspension of the nanoparticles; and

adding the pH buffer (118) to the ground suspension to obtain a pH adjusted nanocoolant.
18. The nanocoolant as claimed in claim 17, wherein the method further comprises:
mixing the dispersant (104) and the coolant (102) to the pH adjusted nanocoolant to obtain a diluted suspension; and
adding the pH buffer (118) to the diluted suspension to obtain a nanocoolant product (120).
19. The nanocoolant as claimed in claim 16, wherein the nanocoolant has a boiling point
elevation of at least 2 degrees over the coolant (102).