The objective of the present study is to analyze the heat transfer correlations of supercritical CO 2 cooled in horizontal circular tubes. In the paper, heat transfer correlations are first reviewed and compared with the experimental data at different heat fluxes. The results show that most of the previous correlations agree well with the experimental data under lower heat flux, but fail to predict the heat transfer coefficient well when the heat flux is as high as 33 kW/m 2 . The study of buoyancy effect on convective heat transfer shows that buoyancy effect significantly affects the heat transfer with the increase of heat flux, and both free and forced convections operate in the turbulence flow during supercritical CO 2 cooling process. The influencing factors on heat transfer coefficient are summarized and the new correlation can be developed with the four dimensionless numbers.
Cooling of supercritical CH4/N2 mixture is the most important heat transfer process during coalbed methane (CBM) liquefaction. In this paper, numerical studies of the turbulent convective heat transfer of supercritical CH4/N2 flowing inside a vertical circular tube have been conducted with Lam–Bremhorst low Reynolds turbulence model. The present numerical investigations focus on the effects of the nitrogen content, heat flux, and flow orientation. Results indicate that as nitrogen content increases, the maximum heat transfer coefficient gradually decreases and corresponds to lower temperature. Heat transfer coefficient is slightly affected by heat flux in the liquid-like region and increases with increasing heat flux in the gas-like region. Buoyancy effect gradually increases with decreasing bulk temperature, and reaches its maximum at the pseudo-critical point, and then drops as bulk temperature further decreases. It is significant in the liquid-like region and negligible in the gas-like region. At the same time, buoyancy effect enhances heat transfer in the upward flow and impairs it in the downward flow.
Coalbed methane (CBM) is a kind of mixed gas with the principal component of methane and nitrogen. Supercritical convective heat transfer of CH4/N2 cooled in horizontal circular tubes is one of the most important heat transfer processes during CBM liquefaction. In this paper, supercritical CH4/N2 cooling has been numerically investigated in a horizontal tube by using the low Reynolds number turbulence model proposed by Lam and Bremhorst. The study first focuses on the effect of nitrogen content on CBM heat transfer characteristics. The results indicate that supercritical convective heat transfer of CBM is mainly affected by the fact that the CBM properties change with nitrogen content. Then the study focuses on the buoyancy effect on heat transfer characteristics at different mass fluxes, heat fluxes and pressures. The results show that buoyancy effect increases with the decrease of mass flux or with the increase of heat flux, and the relationship Gr/Re2.7 predicts the buoyancy effect onset better than Gr/Re2. When the buoyancy effect is considerably strong, buoyancy effect on heat transfer in the top line of the horizontal circular tube is equivalent to buoyancy-opposed heat transfer, and buoyancy effect on heat transfer in the bottom line to buoyancy-aided heat transfer. The correlation of buoyancy-opposed heat transfer proposed by Bruch et al. predicts well for the supercritical heat transfer of methane. When the buoyancy effect is negligible, the calculated results agree well with the Gnielinski correlation.
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