Natural convection and radiation heat transfer of an externally-finned tube vertically placed in a chamber

A three-dimensional numerical study was made to investigate effects of fin angle, fin surface emissivity, and tube wall temperature on heat transfer enhancement for a longitudinal externally-finned tube placed vertically in a small chamber. The numerical model was first validated through comparison with experimental measurements and the appropriateness of general boundary conditions was examined. The numerical results show that the mean Nusselt number increases with Rayleigh number for all the fin angles investigated. The maximum heat transfer rate per mass occurs when the fin angle is about 60 for fin surface emissivity between 0.7 and 0.8 and 55 when the surface emissivity increases to 0.9. With increasing tube wall temperature, both the natural convection and radiation heat transfer are enhanced, but the fraction of radiation heat transfer decreases in the temperature range studied. Radiation fraction increases with increasing fin surface emissivity. Both convection and radiation heat transfer modes are important. List of symbols A Total surface area Cp Specific heat capacity (J kg -1 K) D Outer diameter of base pipe (mm) g Gravity acceleration vector H Height of base tube (mm) Nu Nusselt number L Fin length (mm) Qc Total convective heat rate (W) P Pressure (Pa) Ra Rayleigh number S1 Thickness of base tube (mm) S2 Fin thickness (mm) T Temperature (K) DT Temperature difference (K) V Velocity vector h Heat transfer coefficient (W m K) Greek symbols a Thermal diffusivity (m s) e Surface emissivity q Density (kg m) t Kinematic viscosity (m s) b Thermal expansion coefficient (K) k Thermal conductivity (W m K) Subscripts ? Infinity or at ambient environment w Tube wall