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The present investigation can be divided into two parts: (a) experiments made to examine the mechanism of dropwise condensation of steam with particular reference to the stability of drop promoting surfaces as affected by the material of cooled surface, the drop promoter, the surface finish, the rate of heat transmission, and the presence of non-condensable gas. and (b) a theoretical analysis of the beat transmission through individual droplets, the transient heat transfer through exposed areas, the statistical study of drop size distribution, and the estimation of steam side coefficient. An apparatus was developed to examine qualitatively,the behaviour of drop promoting surfaces on a small scale. It is considered that sufficient evidence was found to show that steam in contact with a cooled surtace condenses as a thin liquid film which later breaks into droplets. surfaces treated to give dropwise condensation deteriorate into mixed condensation in due time, and the duration tor which a treated surface maintains dropwise condensation varies between a few hours to several days, depending on many factors among which· the presence of non-condensable gas must not be overlooked. An approximation to the heat transmission through individual droplets has been worked out with assumed heat flow lines. The result, checked by the relaxation method. is correct within . + 10%. An analysis Of the transient heat transfer through exposed areas was made neglecting the increasing resistance of any accumulating liquid. The drop size distribution was analyzed tor one drop promoting surface at three different heat transmission rates. Based on this drop size distribution, the heat transmission through the drops was estimated by assuming they were held at rest on a cooled surface conducting heat under a steady state. . The estimated coefficient comes within the range or experimental results of many investigators.

Colloidal solvents, containing dispersed nanometer (~1-100 nm) sized particles, are categorized as nanofluids. With the growing heat loads in engineering systems that exceed the current technological limits, nanofluids are considered as an attractive option for more efficient heat removal for thermal management applications. Recent results reported in the literature show that the thermo-physical properties of coolants are enhanced considerably when seeded with very minute concentrations of nanoparticles. Hence, nanofluids research has provoked interest in thermal management applications. The convective heat transfer characteristics of nanofluids are reported in this study. Exfoliated graphite nanoparticles were dispersed in poly-alpha-olefin (PAO) at concentrations of 0.3% and 0.6% (by weight). The heat flux into a convective cooling apparatus was monitored and the results for nanofluid and the base fluid are presented. Thermo-physical properties of the nanofluid were measured and compared with the base fluid. The thermo-physical properties of the fluid are observed to increase with the addition of the nanoparticles. The specific heat of nanofluid was increased by ~50% compared to PAO. The thermal diffusivity was enhanced by ~400% compared to PAO. The viscosity of the nanofluid was enhanced by 10-1000 times compared to PAO. The viscosity of the nanofluid was observed to increase with temperature while the viscosity of PAO decreases with temperature. The convective heat flux was enhanced by the nanofluids by up to ~8 % for experiments performed at different heat inputs. The experimental results show that the convective heat transfer enhancement potentially results from the precipitation of nanoparticles on the heated surface and results in enhanced heat transfer surfaces (“nano-fins”).