Abstract
When water vapour condenses at a sub-atmospheric pressure, the pressure drop may be a
significant fraction of the absolute pressure. Furthermore the pressure drop in a condenser
passage also reduces the absolute vapour temperature and therefore affects the heat transfer
capacity of a condenser.
For a tubular heat exchanger the pressure loss in the heat exchanger tubes can be minimized
by the use of contoured or rounded inlet sections at the inlets of the tubes instead of using a
sudden contracting inlet section or a protruding inlet section for the tubes.
The pressure loss characteristics of different inlet sections to the tubes were obtained through
a literature survey of the pressure loss coefficients. The pressure loss at the inlet sections were
also investigated with computational fluid dynamics, using the Star-CD software system. The
flow regimes for which the pressure loss was investigated were for the laminar incompressible
and turbulent incompressible flow regimes. The inlet sections investigated were a sudden
contraction and two rounded inlet sections with a rounding radius of 52% and 105% of the
tube diameter respectively.
The computational fluid dynamics results of the laminar flow simulations revealed that the
pressure loss coefficients of the sudden contraction and rounded inlet sections were very
similar. The pressure loss coefficient of the sudden contraction inlet sections only being 3 to
6% higher than the rounded inlet sections. This is due to the dominant effect of viscosity in
the laminar flow regime. The viscosity reduces the extent of flow contraction occurring since
transverse momentum is damped by the viscous dissipation. The dominant pressure loss
mechanism in the laminar flow regime is hydrodynamic flow development. With
hydrodynamic flow development the flow velocity profile changes from a uniform velocity
profile before the inlet section into a pointed parabolic profile downstream in the tube.
The turbulent flow simulation results revealed that the pressure loss coefficients of the
rounded inlet sections investigated in this study were very similar. The pressure loss
coefficient of the sudden contracting inlet section was higher than the rounded inlet sections’
pressure loss coefficient.
The results indicated that rounded tubular inlet sections would be of limited value in the
laminar flow regime; it would however be beneficial in the turbulent flow regime.
Prof.J.P. Meyer
Prof. L. Pretorius