Tube in tube helical coil heat exchanger heat transfer rate analysis

Recently, a study has been done to analysis the heat transfer rate to the tube in tube helical coil heat exchanger.

The tube in tube type heat exchanger consists of one pipe placed concentrically inside another pipe having a greater diameter. The flow in this configuration can be of two types: parallel flow and counter-flow. It can be arranged in a lot of series and parallel configurations to meet the different heat transfer requirements.

Numerical simulation has been carried out for tube in tube helical coil heat exchanger subjected to different boundary conditions. Nusselt number, Darcy friction factor, Log means temperature difference pressure drop variation with respect to Reynolds number for different D/d ratio is plotted. In practical application different boundary conditions imposed on the outer wall of exchangers are constant heat flux conditions in power plant boiler, condenser and evaporator etc. insulated outer wall condition in general case of exchanger used in laboratory and educational institutions, and convective heat transfer condition in food, automobile and process industries. Following are the outcome of analysis:

• With increase in the Reynolds number, the Nusselt number for the inner tube increase. However, with increases in flow rate turbulence between the fluid element increases which will enhance the mixing of the fluid and ultimately the Nusselt number or the heat transfer rate increases. • With increases in D/d ratio (inverse of curvature ratio) the Nusselt number will decreases; for a particular value of Reynolds number. Nusselt number has maximum value for D/d=25.

• The outer wall boundary condition does not have any significant effect on the inner Nusselt number, which can be confirmed from the results.

• Friction factor decreases with increase in Reynolds number due to relative roughness of surface, and velocity of flowing fluid.

• Log mean temperature difference increases at a steady rate with increase in Reynolds number.

• As long as the heat transfer is concerned from the hot fluid any boundary condition can be assumed for outer wall of external tube because it does not affect significantly the heat transfer rate.

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