Figure 1 Flow chart for the research methodology of this study
The whole methodology can be simplified into following steps,
  1. APS – Average particle size has to be determined for particle aggregation
  2. Stability – The investigation on stability is a key issue that influences the properties of nanofluids for application, and it is necessary to study and analyze influencing factors to the dispersion stability of nanofluids [45].
  3. pH Measurement – It determines the nano particle aggregation and stability of the nanofluid.
  4. Experimental thermal conductivity and viscosity – Initially thermal conductivity values are used for degerming the stability of the nanofluid over a period and once the nanofluid is stable, further values are taken over various operating conditions.
  5. Regression analysis – The thermo physical properties values collected from experimental work and literature work have analyzed and correlations were formulated.
  6. Comparison - Experimental results were compared with literature and regression analysis to validate the correlations formulated.

2.1. Average Particle Size (APS) of Nanoparticles

One of the considerations for determining particle aggregation, upon suspension in the base fluid, involves nano particle size. To evaluate the solid nano particles’ APS state, various techniques are employed. Given solid nano particles, techniques that could be employed include transmission electron microscopy and scanning electron microscopy. Indeed, the electron microscope does not rely on light as its source of radiation. Instead, it relies on the electron beam. Figure 2 illustrates the scanning electron microscopy image for SiO2 nano particles. To ensure that the nano particles’ APS is assessed, an appropriate approach becomes the Dynamic Light Scattering technique [46]. In this technique, the speed of the particle is correlated with its size, a trend informed by the state of Brownian motion. The Stokes-Einstein equation illustrating this relation is shown below:
\(D=\ \frac{\text{kT}}{6\pi\mu r}\) (2)