How and Why Flow Resistances of Cross Flow over Inline Tube Bundles Vary with Reynolds Number

Abstract

Helical tube bundles are used for steam generators (SGs) and intermediate heat exchangers (IHXs) of High Temperature Gas-cooled Reactors (HTGRs) due to its compactness, high heat transfer coefficient and thermal expansion adaption performance. Accurate prediction of the pressure drop coefficient of cross flow over inline tube bundles is crucial for the design of such heat exchangers. According to the previous measurements from the wind tunnel experiments, the pressure drop coefficient of cross flow over tube bundles with pitch to diameter ratio of 1.875 (S/D=1.88) decreases as Reynolds number (Re) increases, which agrees well with the Zukauskas’ correlation. In current investigation, the cross flow over tube bundles with S/D=1.875, Re=18000 and 36000 are simulated using Large Eddy Simulation (LES). The dynamic Smagorinsky model is used to close the sub-grid stress term. The numerical method is validated using the measured surface pressure coefficient and the pressure drop coefficient. According to the analysis on surface pressure coefficient distributions, the reason for why pressure drop coefficient decreases with Re increasing is attributed to that the surface pressure coefficient at rear side of the tube increases as Re increases. By analyzing the streamwise force balance on the time averaged separation bubble boundary, the pressure coefficient at the separation streamline (wake bubble boundary) increases as Re increases, which leads to the increase of pressure coefficient at rear side of the tube as Re increases. Finally, by comparing flow field of cases with different Re, the wake bubble is found shrink as Re increases, which results in the increasing of pressure coefficient at the separation streamline (wake bubble boundary) as Re increases and ultimately leads to the decreasing of pressure drop coefficient.