ARTICLE:

• HEAT TRANSFER AND HYDRAULIC RESISTANCE OF SUPERCRITICAL-PRESSURE FLUID FLOWS  download article

J. H. Bae
Seoul National University, Seoul, Korea

J. Y. Yoo
Seoul National University, Seoul, Korea

H. Choi
Seoul National University, Seoul, Korea

Donald M. McEligot
Aero. Mech. Engr. Dept., Univ. Arizona, Tucson, Ariz. 85721; Idaho National Laboratory, Idaho Falls, Ida. 83415-3885 USA; and Inst. für Kernenergetik u. Energiesysteme (IKE), Uni. Stuttgart, D-70550 Stuttgart, Germany

ABSTRACT

Direct numerical simulations (DNSs) are performed to investigate turbulent heat transfer to CO₂ at supercritical pressure flowing in vertical pipes, at a Reynolds number of 5400 which is based on the inlet bulk velocity and tube diameter. Both upward and downward flows are considered in the vicinity of the pseudocritical temperature (T_pc) where very large property changes occur. The predicted wall temperature (T_w) and bulk Nusselt number (Nu_b) are generally in good agreement with some empirical correlations away from T_pc. Noticeable deviations are observed at some conditions where the bulk temperature (T_b) is in the close proximity to T_pc and where strong buoyancy effects are present in mixed convection. Investigation of the overall pressure drop shows that one-dimensional analysis based on the global force and energy balance equations is reasonably accurate at a low heat flux, but its accuracy is reduced when T_b approaches T_pc with increased heat flux. This difficulty arises because the one-dimensional analysis underestimates the pressure loss due to acceleration in the pseudocritical region. In mixed convection, the overall pressure drop is predominantly determined by the gravity term and the combined effects of acceleration and friction become negligibly small when the buoyancy effects are increased at a given heat flux. It is shown that these peculiarities in heat transfer to supercritical-pressure fluids occur mainly because the extreme variation of properties combined with strong buoyancy effects change the characteristics of turbulence significantly near the wall.

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