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초임계 유체 유동에서의 관내 대류 열전달 특성에 관한 실험적 연구

초임계 유체 유동에서의 관내 대류 열전달 특성에 관한 실험적 연구
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An experimental investigation of turbulent heat transfer in supercritical vertical upward, downward and horizontal CO2 flow was conducted in a tube with an inner diameter of 4.5 mm. In vertical upward flow, the wall-temperature distribution was significantly influenced by the wall heat and mass flux. In upward flow, the wall temperature had a noticeable peak value when the wall heat flux and was moderate and the mass flux was low
in downward flow, the wall temperature increased monotonically along the flow direction for all cases. The difference between the wall-temperature distributions of the upward and downward flows was due to the relationship between the flow and buoyancy force directions near the tube wall.To determine the effect of buoyancy on the heat-transfer characteristics, the ratio of the Nusselt number from the experimental data to that of a reference correlation was examined with Bo* along the test section. The ratio was compared to q+ to determine the effect of flow acceleration on heat transfer. In our experimental conditions, the heat transfer characteristics were influenced by both effects of the buoyancy and flow acceleration. However, the analysis results indicated that in the experimental cases, the buoyancy effect was relatively smaller, and q+ representing the flow acceleration effect was a more appropriate parameter to represent heat transfer in the flow of fluids with severe property variations.Based on our analysis of the shear-stress distribution in the turbulent boundary layer and the significant variation of the specific heat across the turbulent boundary layer, a new heat-transfer correlation was proposed that reliably predicted heat-transfer phenomena in vertical upward and downward flows of a supercritical fluid. The new correlation was tested using various experimental data. The results demonstrated that the heat-transfer predictions of the new correlation were more accurate than those of other correlations for supercritical fluid flow. The heat transfer data for the horizontal flow were analyzed by using q+ and Gr/Reb2. In the horizontal experimental data, the ratios of Nusselt numbers from experiments and the reference correlation was closely related with the variations of q+ more than Gr/Reb2. The values of Gr/Reb2 decreased monotonically along the test section independently on the values of the Nusselt number ratio. Thus, in this study, the Nusselt number for horizontal flow could be considered to be identical with vertical upward and downward flow. Thus, the new correlation was compared with our experimental Nusselt number data in the horizontal flow. This new correlation predicted very well our experimental Nusselt number data in horizontal flowFinally, a two layer heat transfer model that could sufficiently reflect the effects of both flow acceleration and specific-heat variation was proposed to quantify the heat transfer characteristics of supercritical fluids. This model was based on the thermal resistance behavior in the viscous sub-layer and buffer layer. In our assessment of this model, the Nusselt number calculated from the model agreed with the selected experimental data within a margin of error of ±30%. Using the proposed model, we calculated the thermal resistance in the viscous sub-layer and buffer layer for the present experimental cases. The thermal resistance in the buffer layer had a maximum value near the pseudo-critical temperature line, and these results were closely related to the specific heat ratio. The thermal resistance in the viscous sub-layer was influenced considerably by the flow acceleration. Additionally, the location of the inner-wall temperature peak of the experimental data almost coincided with the maximum thermal resistance in the viscous sub-layer. Thus, the proposed model accurately predicts heat transfer characteristics due to the boundary-layer behavior caused by flow acceleration and specific-heat variation in the boundary layer.
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