Abstract:

Effects of the endothermic fuel pyrolysis of n-decane on fluid flows, heat transfer and chemical component distributions in a helical tube at a supercritical pressure were examined. Results reveal that a large amount of heat is absorbed and converted into chemical energy during the endothermic pyrolytic reactions. Comparing with the test case without consideration of fuel pyrolysis, the bulk fluid and averaged wall temperature near the tube exit can be reduced by around 50 K and 70 K, respectively, with fuel pyrolysis, and the heat transfer coefficient is increased by around 10%. These phenomena are attributable to the strong flow acceleration, caused by the drastic decrease of fluid density, and the enhanced turbulent kinetic energy, resulted from the increased secondary flows in the tube cross sections. The pyrolytic rate of n-decane is higher in the high fluid temperature region on the inner side of the tube, which leads to the reduced wall temperature in the region and more uniform temperature distribution in the circumferential direction. The higher is the fuel conversion rate, the more uniform becomes the circumferential wall temperature. As the chemical concentrations of the coking precursors, such as the alkenes of C₂H₄, C₃H₆, are higher on the inner side of the tube at high fluid temperatures, carbon deposition is more easily accumulated in this region.

Key words: hydrocarbon fuel, pyrolysis, coking, secondary flow, convection, numerical simulation, supercritical fluid

摘要：

探讨了正癸烷的裂解吸热反应对流动、传热和化学组分分布的影响。结果表明，由于裂解吸热反应把加热热量转换成了燃料分子的化学能，与未考虑热裂解的计算结果相比，考虑裂解反应时出口处流体平均温度降低了约50 K，壁面温度降低了约70 K，对流传热系数提高了10%左右。这一方面是因为裂解引起的密度降低、轴向速度增加，另一方面是由于裂解反应提高了螺旋管截面的径向速度，加强了二次流动，增加了壁面湍动能。正癸烷在内侧温度较高的区域裂解度较高，因此螺旋管内侧温度降低，环向温度分布变均匀；裂解度越大，环向温度分布越均匀。与热结焦有关的烯烃类裂解产物C₂H₄，C₃H₆在温度较高的内侧质量分数较大，表明结焦更可能发生在螺旋管的内侧。

关键词: 碳氢燃料, 热解, 结焦, 二次流动, 对流, 数值模拟, 超临界流体