등온조건에서 수평관내 혼합대류 열전달 연구

Abstract

The mixed convection in horizontal pipes occurs in heat exchangers designed for viscous liquids, pipelines transporting oil, and gas flows. Nevertheless, it is difficult to interpret the phenomena that occur in the mixed convection. Therefore, it receives attention from the field of the heat transfer, but it was treated as a hard task. The mixed convection means that the forced convection coexist with the natural convection. When the forced convection happens, that is no longer the pure forced convection since little natural convection exists. Thus, it was combined with the forced and natural convection. The heat transfer phenomena of mixed convection was not combined with characteristics of forced and natural convection physically, new and complex phenomena occur as phenomena of forced and natural convection are mixed. The mixed convection has 3 types of flow, that is, buoyancy-aided flow, buoyancy-opposed flow and buoyancy-transverse flow. When the direction of forced and natural convection is same, it is called buoyancy-aided flow. And when direction of forced and natural convection is reverse, it is called buoyancy-opposed flows. Two flows occur in a vertical pipe. Buoyancy-transverse flow happens that the direction of forced and natural convection is transverse in horizontal pipe. A lot of experimental and numerical studies were performed in a vertical pipe. However, the studies of mixed convection on a horizontal pipe are lacking. Some results of experiments were the old data, on the other hand, numerical studies on recent studies were just. The result of researchers confirmed the mixed convection region as dimensionless length only. And existing author to use the buoyancy coefficient is not clear. In this study, using analogy concept: the heat transfer system can be transformed into the mass transfer system using the electroplating system. Firstly, we carried out experiments of natural convection. It measured the natural convection heat transfer inside horizontal pipe with varying the diameters of the pipes at various angles. The Prandtl number is 2,094 and the ranges of Rayleigh numbers are 6.8×108∼1.5×1012.The results are agreed well with the correlation of Sarac and Korkut. It is confirmed that heat transfer decreases as increased angle for piece wise electrodes. And then, we suggests a mass transfer correlation with laminar and turbulent region. Secondly, it measured the forced convection heat transfers inside horizontal pipe. The Prandtl number is 2,094 and the ranges of Rayleigh numbers are from 3.0×109 to 5.5×109 and those of Reynolds numbers are from 58 to 1,270. And length of pipes is fixed 0.03m. Then, we developed correlations with geometry on length of pipe is 0.03m and diameter of pipe is 0.026m in forced convection experiments. The heat transfer rates of mixed convection were measured with length increases, 0.03m∼0.50m, for two different diameters (0.026m and 0.032m). Thus, it is confirmed the region of mixed convection and compared experimental results and correlation of forced convection. It is defined as semi-empirical buoyancy coefficient derived from correlation of natural and forced convection. Because of existing authors to use the buoyancy coefficient is not clear and it cannot find a physical basis. Also shown in the experimental results, the impact of the L/D was added. Then, the buoyancy coefficients were compared with existing others buoyancy coefficient. As a result, newly defined the buoyancy coefficient well explains not only this result of experiments but also existed experiments. Consequentially, the phenomena of the experimental results can be summarized as a single line. Finally, the correlation equation using buoyancy coefficient was developed. It applied to other systems, it means that good description of the horizontal pipe in laminar mixed convection.