External Heating of a Circular Annulus Cavity filled with Nano Fluid.

Nowadays, progress in direct absorption solar collectors requires further investigations to realize heat transfer occurring in a horizontal annulus cavity exposed to external heat from different angles. An annulus cavity filled with nano-fluid is numerically simulated by using Ansys-Fluent for Rayleigh number equals 105. The external wall of two-dimensional model for the cavity is equally divided into six parts to simulate heating from isothermal walls placed in different angles. The internal wall has constant low temperature simulating a pipe conveys cold fluid and acting as heat sink. Hot isothermal walls set in different angles to simulate several heating cases according to hot element position. “Upper”, “lower”, “upper-right”, “lower-right”, “upper & upper-right”, “lower & lower-right”, “lower-right & upper-right”, “upper-half”, and “lower-half” are the cases investigated to assess heating effectiveness for all possible directions of heating. It is found that heating in “lower” and “lower-right” cases is about six time that of “upper” case. In addition, heating improves slightly in the case of “lower-right” compared with that of “lower” heating. Heating effectiveness from two or three parts in the upper side appears less than heating of two or three parts from the lower side. Enlarging heating area not always reflects multiplication of heat transfer amount, natural convection on the cold wall decreases significantly due to restricted or weakened stream of fluid motion near to cold wall in poor heating cases. In conclusion, choosing correct direction of heating in annulus cavity demonstrates effective heating compared to that earned by enlarging heating area.

Keywords:

Annulus cavity, Natural convection, Thermal solar collector, Nano-fluid Rayleigh number,

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