3.3 CALCULATION OF THE AMOUNT OF MWCNT TO BE ADDED TO THE BASE FLUID

x = volume fraction

x = V_n/(V_n+V_b)

0.2/100 = V_n/(V_n+ 3 l)

(1 – 0.002)V_n = 0.002 x 3000 cc

V_n = 6.000/0.998

                                                                     =6.01202 cc

The weight of the nanoparticles to be added to the base fluid can be calculated if density of nanoparticles is known, which is equal to 3.80 g/cc

                                                          w = V_n x ρ

= 6.01202 x3.8

W_n   = 22.8456 grams

The Nanofluids can be prepared by two methods,

·        One-step method and

·        Two-step method.

     The one-step method 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. One-step method cannot synthesize nanofluids in large scale and the cost is also high.

      The two-step method is the most commonly used and it involves magnetic stirrer and ultrasonication process. A magnetic stirrer or magnetic mixer is a laboratory device that employs a rotating magnetic field to cause a stir bar immersed in the liquid to spin very quickly, thus stirring it. Fig 3.2 shows the magnetic stirrer in which the stir bar is made to rotate in the clockwise direction at a speed of 1200 rpm. The process is carried out for one hour.

    Preparation of a stabilized nanofluid is of utmost importance as poorly prepared nanofluids can greatly affect the performance of the system. For proper homogenization and prevent agglomeration compounds called surfactants are used. Surfactants tend to lower the surface tension of a liquid. In this study, calculated amount of Sodium diodical benzene sulphate (0.1%) is used as the surfactant and added at the start of the stirring process.  

                                     Fig 3.2 Magnetic Stirrer                                       

                                                                Fig3.3 Ultrasonication

        This is followed by the ultrasonication process. Ultrasonication generates alternating Low-pressure and high-pressure waves in liquids, leading to the formation and violent Collapse of small vacuum bubbles. Fig 3.3 shows the ultrasonication process which is done for an hour in sets of 15 minutes. These effects are used for the deagglomeration

of Nano Material fluid particle used is Al₂O_3. The resultant fluid is a homogenous.

       Mixture of base fluid and nanoparticles. A main concern in nanofluid preparation is the Proper homogenization of the nanoparticles suspension. Particle instability results in Particle fouling in reservoir, corrugated channel, pumps and other equipment of heat exchanger, as well as reduced pressure, all of which is highly undesirable. Addition of Suitable quantities of surfactants and the two methods of nanofluids preparation are Carried out for uniform dispersion and stabilization of the suspensions.

Table 3.1 Properties of Base fluid and nanofluid

Fluid	Density ρ (kg/ m3)	Specific  Heat Cp (kJ/kg K)	Dynamic Viscosity µ (Pa sec)	Thermal Conductivity k (Kw/m K)
Base Fluid	990	4.187	0.003121	0.0630
Nanofluid (0.2% vol.)	996	4.160	0.0011	0.0765

          The thermal conductivity was measured by using a KD2 thermal property meter. The Viscosity was measured using a rheometer.