464831 Experimental Investigation of Relaminarization of Turbulence during Natural Circulation Cooling of a Very Hot Channel
Apoorva V. Rudra*a, Narbeh Artoun b, Sanjoy Banerjee a, c, Masahiro Kawaji b, c
aDepartment of Chemical Engineering, City College of the City University of New York, New York
bDepartment of Mechanical Engineering, City College of the City University of New York, New York
c The Energy Institute, City College of the City University of New York, New York
Gaseous coolants have attracted a lot of attention for usage in various industrial applications due to their high thermal efficiency, chemical inertness, and environmental acceptability. Gas coolants are also proposed for Very High Temperature Reactors (VHTR’s), which is one of six Generation IV reactors that have been proposed for DOE’s Next Generation Nuclear Plant . VHTRs also display benefits of passive safety systems including intra-core natural circulation for heat removal in accident scenarios . Our work addresses the issue of flow laminarization, in which a strongly heated turbulent gas flow, exhibits heat transfer characteristics of laminar flows in the downstream part . Under natural circulation conditions, reductions in the Reynolds number can trigger flow laminarization even if the incoming flow in the riser is above the critical Reynolds number . As the gas is heated, a reduction in the gas density causes the bulk flow to accelerate upward, and the viscosity also increases leading to reduced Reynolds numbers. This large increase in gas viscosity at strongly heated walls leads to relaminarization. In some cases, the buoyant forces can also lead to flow laminarization . On the other hand, the laminar flow can transition to turbulent flow in the downcomer channels as the gas is cooled .
In this study, we present the experimental measurements of the coolant flow and heat transfer behavior in a flow channel in a graphite block simulating a prismatic core of a Very High Temperature Gas-Cooled Reactor (VHTR). Natural circulation experiments are conducted using a helium-nitrogen mixture representing a helium-air mixture. A helium analyzer is used to investigate the nitrogen transport from the lower to upper plena as a function of time.
The natural circulation flow rate is measured and the flow rate data is related to the graphite temperatures, operating pressure, and overall concentration of nitrogen in helium. The amount of nitrogen gas injected and thus the overall nitrogen concentration in the gas mixture is systematically changed to determine its effect at different graphite temperature settings in the riser and downcomer. To what extent the local helium concentrations will differ between the lower and upper plena during both steady and transient natural circulation is determined in order to elucidate the air transport process following a postulated accident involving air ingress into the lower plenum. Under these conditions, the gas viscosity increases with temperature within the reactor cooling channels, causing a reduction in the Reynolds number which triggers flow laminarization. The natural circulation flow and heat transfer data along with the viscosity effects are analyzed to identify deterioration in heat transfer due to laminarization.
This experimental work will be supplemented by numerical simulations using CFD models  that will help to understand the physics behind the entire process better.
 Dan Gabriel Cacuci (ed.), 2010. Handbook of Nuclear Engineering, DOI 10.1007/978-0-387-98149-9_22, @Springer Science+Business Media LLC.
 Valentín, F.I., Artoun, N., Anderson, R. and Kawaji, M., 2015. “Study of Abnormal Heat transfer during forced and natural convection scenarios in a prismatic core of a VHTR: Numerical and Experimental Results,” Proc. of 16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (Nureth-16), Aug. 30-Sept. 4, 2015, Chicago, IL.
 Valentín, F.I., Artoun, N., Kawaji, M., and McEligot, D.M., 2015. “Investigation of helium flow laminarization at high temperatures and pressures in a graphite flow channel,” Proc. of the 1st Thermal and Fluids Engineering Summer Conference, New York, August 9-12, 2015.
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