수직 원형관내 혼합대류 유동영역지도 개발연구

Abstract

Hydrogen has drawn public attentions as the clean, secondary energy source. One of the promising production methods of hydrogen is to use the heat from the VHTGR (Very High Temperature Gas-cooled Reactor), a next generation nuclear reactor. Due to the high temperature requirement, the coolant for the VHTGR should be gas phase in order to prevent phase change during operation. One of the technical issues raised by the development of the VHTGR is the mixed convection, which occur when the driving forces of both forced and natural convection are of comparable magnitudes. It is classified into laminar and turbulent flows depending on the exchange mechanism and also into buoyancy-aided and buoyancy-opposed flows depending on the direction of forced flow with respect to the buoyancy forces. In laminar mixed convection, buoyancy-aided flow shows enhanced heat transfer compared to the pure forced convection and buoyancy-opposed flow shows impaired heat transfer due to the flow velocity affected by the buoyancy forces. However, in turbulent mixed convection, the situation is reversed. The buoyancy-opposed flows shows enhanced heat transfer due to increased turbulence production and buoyancy-aided flow shows impaired heat transfer at low buoyancy forces and as the buoyancy increases, the heat transfer restores and at further increases of the buoyancy forces, the heat transfer is enhanced. It is of prime interests for engineers to classify the convective flow regimes : forced, natural, and mixed convection. Metais and Eckert suggested a classical flow regime map. The map is based upon several experimental studies and the ranges of Reynolds number and Grashof number are far below the VHTGR ranges. The present work is to examine review the classical flow regime map and to develop the flow regime map applicable to the VHTGR ranges. Firstly, the literatures related to the classical flow regime map were reviewed. Based upon the reviews, the flow regime map was redrawn and compared with the classical flow regime map. Secondly, a series of mixed convection experiment was carried out for the wider ranges than the classical flow regime map. The achievement of large Grashof numbers with gas coolant requires very tall test facility as the Grashof number is strong function of facility height. In order to avoid tall and expensive test facility, this study adopted the analogy experiment method. Using the concept, the heat transfer system can be transformed into mass transfer system using the electroplating system. This system allows the large Grashof number to be achieved easily with short test facilities and accurate measurements of mass fluxes by electric means. The ranges of Grashof number are from 5.27×106 to 6.93×1010 and those of Reynolds number are from 4,000 to 14,000, which cover turbulent flows sufficiently higher than the classical flow regime map. The Prandtl numbers are varied from 2,014 to 2,129. Two kinds of the pipe with different diameters are used. The reviews of the existing flow pattern map revealed the limitations : limitation of cited studies, selective use of experimental results, lack of detailed description for the curved lines appeared in the flow pattern map etc. The experiments reproduced the typicals of the mixed convection heat transfer phenomena in a turbulent flow condition and agreed well with the existing studies performed by Y. Parlatan(1996) and Ko et al.(2008). The limits between the forced convection and the mixed convection and those of between the mixed convection and the natural convection were defined. A new flow regime map is suggested. Discussions are made regarding the effect of height, aspect ratio, problems of using characteristic lengths. It may be concluded that the flow regime map should be drawn into buoyancy-opposed flow and buoyancy-aided flow, respectively. The existing flow pattern map based upon the dimensionless numbers using the diameter as the characteristic length is not generalized enough to be widely used.