Abstract
The effects of gravity on the heat transfer performance of supercritical CO2 flowing within a vertical tube with a diameter of 4.75 mm are numerically studied in this paper. The main objectives are to comprehensively investigate the action of gravity and buoyancy on the supercritical heat transfer. An effective numerical method, which employs a modified Shear Stress Transfer k-ω model (SST k-ω), is applied at various gravity conditions. It is found that, for both upward and downward flows, the heat transfer of supercritical CO2 is improved with increased gravity magnitude. The effect of gravity on heat transfer are more pronounced under a low mass flux condition than that under a high mass flux condition and it is closely related to the variations of thermal properties. For the upward flow, the increased gravity magnitude accelerates the near wall fluid and creates a classic “M-shaped” radial velocity distribution. For the downward flow, the increased gravity magnitude decelerates the near wall fluid and creates a parabola-like radial velocity distribution. On one hand, the turbulent kinetic energies of both the upward and downward flows are enhanced as the gravity magnitude increases, which benefits heat transfer dominated by turbulent eddy diffusion. On the other hand, high-density fluid with high thermal conductivity occupies the near wall region as the gravity magnitude increases, which benefits heat transfer dominated by molecular diffusion. The results might provide some instructive advice to improve the design and operation safety of heat exchanger at various gravity conditions.
Highlights
With the development of aero-engine technology, the operation parameters, for example, pressure, heat flux and wall temperature, are becoming increasingly higher to meet the demand for a new generation of aero-engines
When the bulk temperature is higher than 330 K, no obvious differences in heat transfer coefficient (HTC) are found at different gravity magnitudes, indicating that gravity has much less effect on the heat transfer performance in the high bulk temperature region
Numerical simulations are conducted in this study to evaluate the effects of gravity on the heat transfer of supercritical CO2 flowing in a vertical round tube
Summary
With the development of aero-engine technology, the operation parameters, for example, pressure, heat flux and wall temperature, are becoming increasingly higher to meet the demand for a new generation of aero-engines. To protect aircraft engines and improve system efficiency, the high efficiency heat transfer technologies are essential. Supercritical fluids, such as hydrogen, nitrogen, carbon dioxide and other fluid materials are adopted or are under consideration as the working medium due to their high heat absorption capacity [1]. Many studies have been conducted to improve the industrial applications of supercritical fluid. Conducted a comprehensive review of the research on the supercritical fluid gaseous and liquid states. By reviewing the experimental evidence from both historic and modern literature, they concluded that the non-continuity description of liquid-gas criticality is the only plausible description that is consistent with the results of 150 years of experimental thermodynamic measurement research. Gkountas et al [3]
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