Technical Seminar Presentation

Technical Seminar On

NANOFLUIDS

Darshan

CONTENTS:  1. INTRODUCTION   2. PREPARATION METHODS FOR NANOFLUIDS  3. THERMAL CONDUCTIVITY OF NANO FLUIDS  4. MATERIALS USED FOR NANOPARTICLES AND BASE FLUIDS  5. ADVANTAGES OF NANOFLIDS  6. LIMITATION  7. APPLICATION OF NANOFLUIDS 

 • Suspended nanoparticles in various base fluids can alter the fluid

flow and heat transfer characteristics of the base fluids. These suspensions of nano sized particles in the base fluids are called nanofluids.

• Nanofluids are suspensions of nanoparticles in a base fluid, typically water. The term nanoparticle comes from the Latin prefix ‘nano’. It prefix is used to denote the 10-9 part of a unit.

• Recent development of nanotechnology brings out a new heat transfer coolant called ‘nanofluids’. These fluids exhibit larger thermal properties than conventional coolants.

• The much larger relative surface area of nanoparticles, compared to those of conventional particles, not only significantly improves heat transfer capabilities, but also increases the stability of the suspension.

What are Nanofluids ?

Concept of Nanofluids

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Thermal conductivity of typical materials

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1-Engine Oil2-Ethylene Glycol3-Water4-Alumina5-Silicon6-Aluminum7-Copper8-Silver9-Carbon

Solids have thermal conductivitiesthat are orders of magnitude larger than those of conventional heat transfer fluids.

• Conventional heat transfer fluids have inherently poor thermal conductivity compared to solids.

• Conventional fluids that contain mm- or m-sized particles do not work with the emerging “miniaturized” technologies because they can clog the tiny channels of these devices.

• Nanofluids are a new class of advanced heat-transfer fluids engineered by dispersing nanoparticles smaller than 100 nm in diameter in conventional heat transfer fluids.

Nano-particles can be produced from several processes such as gas condensation, mechanical attrition or chemical precipitation techniques. Gas condensation processing has an advantage over other techniques.”

Materials used for nanoparticles and base fluids: Nanoparticle materials include: Oxide ceramics – Al2O3, CuO Metal carbides – Sic Nitrides – AlN, SiN Metals – Al, Cu Non-metals – Graphite, carbon nanotubes Layered – Al + Al2O3, Cu + C

Base fluids include: Water Ethylene- or tri-ethylene-glycols and other coolants Oil and other lubricants Bio-fluids Polymer solution

PREPARATION METHODS FOR NANOFLUIDS

 1. TWO-STEP METHOD

• Two-step method is the most widely used method for preparing nanofluids.

• Nanoparticles, Nanofibers, nanotubes or other nanomaterials used in this method are first produced as dry Powders by chemical or physical methods. Then the nanosized powder will be dispersed into a fluid in the second processing step with the help of intensive magnetic force agitation, Ultrasonic agitation, high-shear mixing, homogenizing and ball milling. Two-step method is the most economic method to produce nanofluids in large scale, because nanopowder synthesis techniques have already been scaled up to industrial production levels. Due to the high surface area and surface activity, nanoparticles have the tendency to aggregate.

• The important technique to enhance the stability of nanoparticles in fluids is the use of surfactants. However the functionality of the surfactants under high temperature is also a big concern, especially for high temperature applications.

2. SINGLE STEP METHOD

• To reduce the agglomeration of nanoparticles they developed a one-step physical vapor condensation method to prepare Cu/ethylene glycol nanofluids . The one-step process consists of simultaneously making and dispersing the particles in the fluid.

• In this method, the processes of drying, storage, transportation, and dispersion of nanoparticles are avoided, so the agglomeration of nanoparticles is minimized, and the stability of fluids is increased . The one-step processes can prepare uniformly dispersed nanoparticles, and the particles can be stably suspended in the base fluid.

• However, there are some disadvantages for one-step method. The most important one is that the residual reactants are left in the nanofluids due to incomplete reaction or stabilization. It is difficult to elucidate the nanoparticle effect without eliminating this impurity effect.

Structure of Nanofluids

Figure 1: ZrO2 in water that producedwith Two Step method

Figure 2: Cu nanoparticles in ethyleneglycol produced with One Step method

THERMAL CONDUCTIVITY OF NANO FLUIDS

The fluids that have been traditionally used for heat transfer applications have a rather low thermal conductivity. Taking into account the rising demands of modern technology, it has been recently proposed that dispersion of small amounts of nanometres-sized solids in the fluid called nanofluids can enhance the thermal conductivity of the fluids.

This increase in the thermal conductivity is predicted to be because of the following reasons:

1. Brownian motion2. Interfacial layer 3. Volume fraction of particles

 1. Brownian motion

It has been found that the Brownian motion of nanoparticles at the molecular and nanoscale level is a key mechanism governing the thermal behavior of nanoparticle–fluid suspensions ("nanofluids"). The enhancement in the effective thermal conductivity of nanofluids is due mainly to the localized convection caused by the Brownian movement of the nanoparticles.

• It is postulated that the enhanced thermal conductivity of a nanofluids when Compared to conventional predictions, is mainly due to• Brownian motion which produces micro-mixing.• This effect is additive to the thermal conductivity of a static dilute suspension.• Keff = kstatic + kbrownian• Since the speed of thermal wave propagation is much faster than the particle Brownian motion, the static part cannot be neglected

2. Interfacial layer

Fig: Schematic cross section of nanofluids structure consisting of nanoparticles, bulk liquid, and nanolayers at solid/liquid interface

Fig: Single spherical particle with interfacial layer in a fluid medium.

• Although liquid molecules close to a solid surface are known to form Layered structures, little is known about the connection between this Nanolayer and the thermal properties of solid/liquid suspensions. It is assumed that the solid-like nanolayer acts as a thermal bridge between a solid nanoparticle and a bulk liquid and so is key to Enhancing thermal conductivity.

• From this thermally bridging nanolayer idea, a structural model of nanofluids that consists of solid was suggested. Nanoparticles, bulk liquid and solid-like nanolayers.

• Conventional pictures of solid/liquid suspensions do not have this nanolayer. The thermal conductivity of the nanolayer on the surface of the nanoparticle is not known. However, because the layered molecules are in an intermediate

3. Volume fraction

Highly conductive nanoparticles of very low volume fractions distributed in a quiescent liquid (called ‘nanofluids’) may measurably increase the effective thermal conductivity of the suspension when compared to the pure liquid.

ADVANTAGES OF NANOFLUIDS High specific surface area and therefore more heat

transfer surface between particles and fluids.

High dispersion stability with predominant Brownian motion of particles.

Reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification.

Reduced particle clogging as compared to conventional slurries, thus promoting system miniaturization.

Adjustable properties, including thermal conductivity and surface wet ability, by varying particle concentrations to suit different applications.

LIMITATION• Lower specific heat From the literatures, it is found that specific heat of nanofluids is lower than base fluid . Namburu et al reported that CuO/ethylene glycol nanofluids, SiO2/ethylene glycol nanofluids and Al2O3/ethylene glycol nanofluids exhibit lower specific heat compared to base fluids. An ideal coolant should possess higher value of specific heat which enable the coolant to remove more heat.

• High cost of nanofluidsHigher production cost of nanofluids is among the Reasons that may hinder the application of nanofluids in industry. Nanofluids can be produced by either one step or two steps methods. However both methods require advanced and sophisticated equipment's.

• Difficulties in production process Another difficulty encountered in nanofluid manufacture is

nanoparticles’ tendency to agglomerate into larger particles, which limits the benefits of the high surface area nanoparticles. To counter this tendency, particle dispersion additives are often added to the base fluid with the nanoparticles.

  Unfortunately, this practice can change the surface

properties of the particles, and nanofluids prepared in this way may contain unacceptable levels of impurities.

Most studies to date have been limited to sample sizes less than a few hundred milliliters of nanofluids. This is problematic since larger samples are needed to test many properties of nanofluids and, in particular, to assess their potential for use in new applications

APPLICATIONS

Electronic applications Transportation Industrial cooling applications Nuclear systems cooling Space and Defence Medical application Cooling of Microchips

Studies of nanofluids reveals high thermal conductivities and heat transfer coefficients compared to those of conventional fluids.

These characteristic features of nanofluids make them suitable for the next generation of flow and heat-transfer fluids.

Pioneering nanofluids research has inspired physicists, chemists, and engineers around the world.

CONCLUSIONS

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