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
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:7322 |
| Date | 18 July 2008 |
| Creators | Bijkersma, Jan |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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