Experimental investigation and analysis on the cross flow characteristics over inline tube bundles with S/D=1.875

Abstract

Helical tube bundles are usually used for steam generators (SGs) and intermediate heat exchangers (IHXs) of High Temperature Gas-cooled Reactors (HTGRs). The thermal hydraulic performance of SGs/IHXs is dominated by the cross flow convection over tube bundles in the primary side. Though cross flow over tube bundles were widely investigated as a basic flow pattern, few of them focused on tube bundles with 1.4-1.6<S/D<2-2.2 (S, tube pitch, D, tube diameter). However, the designs of HTGR SGs or IHXs prefer smaller tube pitches due to the relatively low heat transfer coefficient. In present investigation, velocity distribution, surface pressure distribution and drag/lift force of cross flow over tube bundle with S/D=1.875 are measured and investigated thoroughly in a wind tunnel. The velocity fields are measured using time resolved PIV and split hot film methods. The surface pressure distribution is measured using dynamic pressure transducers. The drag and lift forces are measured using high frequency wind tunnel balance. Due to the friction drag introduced by the bounded wall, the time averaged velocity in the flow passage next to the bounded wall is smaller than those in the middle flow passages. The surface pressure distributions between the impinging point and the separation point on tubes are characterized by the acceleration effects when fluid flowing into the region between neighboring tubes in the same row. The increasingly significant pressure recovery after separation results in the decreasing tendency of flow resistance coefficient with Reynolds number (Re) increment. A main frequency corresponding to Sr=0.114 (Sr, Strouhal number) is observed in both surface pressure and velocity spectrums, indicating that the vortex shedding induces the turbulent wake swinging. The turbulent wakes after tubes in the same row swing with no phase lag, while turbulent wakes of tubes in neighboring rows swing with a phase lag around 0.33 periods. By using an improved phase average method, phase averaged turbulent statistics and instantaneous coherent structures are obtained. The main flow region swings with vortex shedding. The region with larger 〈u′u′〉 (u′, fluctuating component of stream wise velocity) mainly locates at the shear layer, swinging with the vortex shedding. The region with larger 〈v′v′〉, (v′, fluctuating component of transverse velocity) mainly locates in front of the tubes, where transverse flow sweeps the upwind side of the tubes.