Investigation and characterization of pressure drop in evenly spaced twisted tapes

Abstract

In his MIT Doctoral thesis on an inverted Hydride-fueled reactor concept, Paolo Ferroni (2010) suggested using short-length twisted tape inserts in order to increase the critical heat flux (CHF) at which departure from nucleate boiling occurs. Shortlength twisted tapes were proposed in his study because they offer the benefits of increased heat transfer and CHF, but also introduce a smaller pressure drop than a full-length twisted tape insert. This thesis project investigated the effects of short-length twisted tape inserts on the pressure drop in round tubes. The objectives of the project were two-fold. The first objective was to characterize the development of swirl flow in sequential twisted tapes. The second objective was to create a correlation for the Darcy's friction factor as a function of the Reynolds number (Re), tape twist ratio (y), and tape spacing (s) which could be used for both Ferroni's work and other future work. To characterize developing flow, 6 test sections were constructed. Measurements were collected for the pressure drop at several sequential twisted tapes, and the resulting friction factors were compared for each tape module. The results showed at most ±10% difference in friction factor between tapes and no significant trends between friction factor and the axial sequence location (or index) of the tape. To develop a correlation, 21 test sections were constructed by P. Ferroni and the author. Pressure drop measurements were collected for conditions spanning 10000 < Re < 90000, 1.5 < y < 6, and s = 30, 40, 50. A correlation for the Darcy's friction factor (f) was developed through the separation of the three variables to find f oc Re-- , y 1 87 and s- 67. Tap water at room temperature and nearly atmospheric pressure was used throughout the experiments, and for all short-length twisted tapes the number of revolutions. Nev, was held constant at 1.5.