Heat transfer and resistance characteristics of supercritical cyclohexane in a vertical mini-tube

With an increasing of March number of the near space aircraft, the effect cooling method is becoming a main factor for the development of engine. A regenerative active cooling, using hydrocarbon fuel as coolant flowing through the mini-tube, is proposed and verified prospectively. To overcome the uneven thermal load as the flight condition change, the flow and heat transfer process under the design condition should be studied. The conclusion affects firsthand on the heat exchange process and ultimately act on the engine design. Thus, the cyclohexane as a typical hydrocarbon was test at supercritical pressure in a vertical tube and its heat transfer and pressure characteristics were obtained in an upflow and downflow respectively. The experiment condition covered a range of heat flux from 158 to 450kw/m², pressure from 4.5 to 5.5MPa, inlet temperature from 30 to 70°C and the test stainless tube owned length of 0.36 to 0.79m and hydraulic diameter 1.0 to 2.0mm. The results shown that buoyancy and viscous force has a significant effect on the heat transfer characteristics in the vertical tube, leading to the effect of heat transfer of upflow is better than that of downflow. Meanwhile with the increase of pipe inner diameter the heat transfer was weakened. Conversely, with the increase of pipe length, the heat transfer coefficient is further enhanced. In experiments, hydrodynamic characteristics was also conducted and found that the onset of the static instability is significant to the heat transfer characteristics. In the outlet of the tube, when the static instability occurred, the heat transfer weakening also occurred at the same time.