Heat transfer fluid (HTF) plays an important role in the efficiency and cost effectiveness of solar thermal collector. From several choices, Therminol VP-1 (eutectic mixture of diphenyl oxide and biphenyl) are successfully used as heat transfer fluid in the collector field of parabolic trough plants. This paper experimentally analyzes the buoyancy driven heat transfer performance of Therminol VP-1 in a square enclosure with uniformed heating from below and for a Rayleigh number up to 1.11×108. Thermophysical properties such as density, viscosity, and thermal conductivity of Therminol VP-1 were also measured experimentally. The results show that the density, viscosity, and thermal conductivity decrease with temperature within the measured temperature range 10–60°C. The obtained convective heat transfer coefficient was lower than the De-Ionized (DI) water, whereas the dimensionless Nusselt number was observed higher than DI water, which is because of the low thermal conductivity (approximately 25% of DI water) of Therminol VP-1.

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