Testing of twisted-tube exchangers in transition flow regime

In most cases the main barrier to a heat flow in shell-and-tube heat exchangers is the convective resistance on the inner and outer tube surface. To increase thermal effectiveness, a number of augmentative techniques are available: twisted tapes, wire coil inserts, internal or external fins are frequently used. Following those efforts, it is proposed to use twisted tubes, instead of cylindrical tubes. This paper describes testing of an advanced shell-and-tube design with twisted tubes. Its performance has been tested under single-phase, fully developed, transition flow conditions. Subject to design constraints and based on specific optimization requirements, several configurations were tested, each test examining heat transfer and pressure drop for the particular heat exchanger. It was shown that overall heat transfer and pressure drop increase with a smaller tube twist pitch to diameter ratio, and that these exchangers have specific advantages and characteristics, previously open only to plate exchanger users. Twisted tubes increase the level of mixing and promote turbulence in the low Reynolds number range, on both, tube-side and shell-side of the tubes. Testing over a wide range of geometric parameters and process conditions confirmed that heat transfer results for twisted-tube exchangers could be satisfactorily correlated in the Nusselt, Reynolds, Prandtl equation form. Total friction factor through a twisted-tube bundle was found to be the sum of axial friction loss component and a drag contribution from the swirl flow.