Experiment Study On Transcritical Flow Visualization Of Endothermic Hydrocarbon Fuel In The Cooling Channel

Regenerative cooling using hydrocarbon fuel in the cooling channels of scramjet is an effective way to lower wall temperature, so it’s important to study the flow and heat transfer of the fuel in the cooling channels. As an effective way of flow field measurement, flow field visualization can be used to get intuitive information of flow field section.In the paper, we study the shaded images of flow field under subcritical and supercritical pressures with the high-speed camera. Heat sink and heat transfer coefficient can be calculated from experiment datas to compare with the flow field visualization pictures, which can be a effitive way to find the reason of changes of flow and heat transfer character.Firstly, flow visualization laboratory bench was build to display the flow details. Macro lens were used to increase the magnification of the image and background plate was used to get shaded images. What’s more, the light path of the visualization section was analyzed, which told us that black areas due to the lack illumination may occur near the wall. By using image process, we can make the character of image more clearly.Secondly, we studied on flow field development process pictures under subcritical and supercritical pressures. And we found that Boiling and Boiling flow pattern is the material manifestation of the discontinuity point. Under the relatively low pressure, boiling flow patterns are similar to traditional flow patterns. When the pressure gradually approaches to critical pressure, the bubbles are small and dense, and the gas-liquid collision and crushing intensifies compared to the low pressure boiling. Under the supercritical pressure, we can know that the the physical properties are continuous as no bubble appears. Wrinkles emerge in the flow field when the temperature closes to the critical point because of the dramatic physical properties changes. The flow field is pulsating and different density regions is mixing. These features can be clearly seen when we do some image processing.Finally, we studied on the character of heat sink, heat transfer and flow instability. The subcritical Boiling can cause the sudden increase of the heat sink, but the heat sink don’t increase suddenly at pseudo-critical point of supercritical pressure. At the same oil temperature, heat sink of supercritical pressure is much higher than subcritical pressure, which tells us that conversion rate of the fuel pyrolysis reaction is higher under supercritical pressure. Under the boiling region, Enhancement of heat transfer is more obvious than the critical region and the pyrolysis region. From the visualization picture, enhancement of heat transfer is closely related to gas-liquid collision and crushing and mixture of different density regions. Mechanism of flow instability is relatively complex. The flow instability occurs at a certain trigger mechanism under the boiling region, the pseudo-critical region and the the pyrolysis region. Periodic oscillation of flow parameters caused by flow instability can be divided into two categories, long cycle(12s) constant amplitude oscillation and short cycle(3s) irregular sawtooth oscillation. It can be infered that long cycle regular oscillation is caused by the alternate emergence of different density fuels.