Instant hot water requirement is more in tropical countries during the winter season. The conventional flat plate collector and evacuated tube collector based solar water heaters are unable to delivesr instant hot water in the presence of low solar radiation. Passive heating of water in an evacuated tube with heat transfer fluids having low specific heat can deliver a better solution to the above problem. Therminol D-12 was identified as one such fluid and its performance was studied under

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... ng flow rates and radiation heat fluxes. The analysis was carried out with the CFD solver FLUENT 6.2. P1 radiation model was used to predict the static temperature and velocity magnitude of therminol D-12 at the exit of the evacuated tube. The result obtained from the CFD analysis was validated with the experimental result. The deviation of experimental result from the predicted result was found to be less than 5%. The error percentage increases with increasing radiative heat fluxes due to convection boundary conditions. The performance of P1 model was found to be good for solar radiation experiments less than 600 W/m2 .