Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.
Due to tube enhancements being used to improve process efficiencies, heat exchangers are starting to operate in the
transitional flow regime. Unfortunately, heat transfer and
pressure drop performances in this flow regime is un-explored for many heat transfer geometries. In this preliminary study, experiments were conducted on the annular passage of a horizontal concentric counter-flow tube-in-tube heat exchanger operated with water for non-fully developed flow associated with a standard inlet geometry type. The annular diameter ratio, defined as the inner wall diameter over the outer wall diameter, was 0.386. An approximate uniform wall temperature on the inner annular wall surface was considered for varying annular mass flow rates that covered all flow regimes. Both
heated and cooled cases of the annulus were examined. It was
found that the heat transfer and pressure drop characteristics,
based on the hydraulic diameter are different from those in
circular tubes. The transition from laminar to turbulent flow,
based on the Nusselt number, appeared to occur earlier than
based on the friction factor. Nusselt numbers for heated annulus case, based on Reynolds number, were high than that
for the cooled case. Conversely, the friction factors were higher
for the cooled annulus case than for the heated case, while the
adiabatic friction factors were the lowest.