Investigation of combined free and forced convection in a 2 x 6 rod bundle during controlled flow transients

Abstract

An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients. A brief investigation of steady-state flows for the purely free-convection regime was also completed. The study was performed using an electrically heated 2 x 6 rod bundle contained in a flow housing. In addition a transient data base was obtained for evaluating the COBRA-We thermal-hydraulic computer program (a modified version of the COBRA-IV code).The objective of the study was to develop an understanding of the thermalhydraulic phenomena at low flows in a rod bundle subjected to lateral power skews. Test conditions included 500-, 300-, 150-and 45-second flow coastdown transients, a Reynolds number range of -90 to -1300, and power skews of 1:0, 2:1, 3:1, and 4:1. The dimensionless group Gr*/Re 2 , which represents the ratio of buoyancy to inertial forces, was varied in the range 0.0 to 2.4 at selected lateral power skews. Flow recirculation zones were detected in the bundle during transient conditions at large ratios of buoyancy to inertial forces as indicated by the measurements of downward flow in the colder subchannels and upward flow in the hotter subchannels. No flow recirculation was detected in the bundle at steady-state conditions except at extremely low Reynolds numbers where heat loss and/or hydraulic effects of the discharge geometry become important. The length of the recirculation zone in the axial direction was observed to be dependent on rod power level, the lateral power skew, and the total flow rate.The data were compared to predictions obtained with the COBRA-We computer code. COBRA-We satisfactorily predicted the measured bundle subchannel velocity and temperature histories. At Reynolds numbers below 450 an improvement in the turbulent mixing model in COBRA-We was required to satisfactorily predict experimental results. This improvement was necessary because as the magnitude of the buoyancy forces increased, thermal plumes grew laterally as they moved axially upward. When adjacent thermal plumes interacted, turbulence was v generated which appeared to promote lateral mlxlng between subchannels. A model incorporating the enhanced turbulent mixing due to the thermal plume interactions was used to improve the COBRA-We predictions at large ratios of Gr*/Re 2 . The enhanced turbulent mixing parameter was correlated as a function of Grashof and Reynolds numbers (Gr/Re 2 ) and the ratio of local heat generating rate to the bundle average heat generation rate.vi ACKNOWLEDGMENTS

CTF Validation and Verification: Version 4.4

“…The CTF model contained 12 rods and 21 subchannels arranged as shown in Figure 20. The numbering from the steady-state test reference [8] was used, in contrast to that in the transient test reference [9], where the subchannel numbering began in the middle row. The axial meshing of the test section is shown in Figure 21.…”

Development of a Single-Phase, Transient, Subchannel Code, within the MOOSE Multi-Physics Computational Framework

“…The PNNL 2 × 6 benchmark [41] was performed at Pacific Northwest National Laboratory in 1977 for investigating the buoyancy effect in the mixed (combined free and forced) convection regime for specific flow coast-down transients. The objective of the study was to develop an understanding of the thermal hydraulic phenomena at low flows in a rod bundle subjected to non-uniform lateral power distributions.…”

“…Subchannel cannot calculate negative mass flows in the axial direction. For this reason we cannot capture recirculation phenomena in cases where we have the formation of eddies due to high forced convection and low flow conditions, such as those seen in [41], or due to blockages in the flow path such as those seen in [47]. Possible accident scenarios that could produce these types of flow effects could be the loss of power in a large part of the core, or pin swelling and pin rupture in the middle of the core.…”

Development of a Subchannel Capability for Liquid-Metal Fast Reactors in Pronghorn

“…If the flow is mainly one dimensional, such as in standard LMFRs operation conditions, one may solve one axial level at a time before solving the axial level above. This gives rise to the commonly implemented marching scheme in subchannel codes [2,18] as one "marches" from the bottom to the top of the assembly while solving. The advantage of this sweeping-type discretization scheme is that one can achieve a high-speed solution of the subchannel system as only small systems are obtained per level.…”