The effect of restriction shape on laminar natural convection heat transfer in a vertical circular tube

Abstract EN

Natural convection heat transfer is experimentally investigated for laminar air flow in a vertical circular tube by using the boundary condition of constant wall heat flux in the ranges of (RaL) from (1.1*109) to (4.7*109).

The experimental set-up was designed for determining the effect of different types of restrictions placed at entry of heated tube in bottom position, on the surface temperature distribution and on the local and average heat transfer coefficients.

The apparatus was made with an electrically heated cylinder of a length (900 mm) and diameter (30 mm).

The entry restrictions were included a circular tube of same diameter as the heated cylinder but with lengths of (60 cm, 120 cm), sharp-edge and bell-mouth.

The surface temperature along the cylinder surface for same heat flux would be higher values for circular restriction with length of (120 cm) and would be smaller values for bell-mouth restriction.

The results show that the local Nussle number (Nux) and average Nussle number are higher for bell-mouth restriction and smaller values for (120 cm) restriction.

For all entry shape restrictions, the results show that the Nussle number values increases as the heat flux increases.

From the present work an empirical correlations were obtained in a form of (Log Nu-L) versus (Log Ra-L) for each case investigated and obtained a general correlation for all cases which reveals the effect of restriction existence on the natural convection heat transfer process in a vertical circular tube.