Thrust imparting forward motion to an object, as a reaction to the rearward expulsion of a high-velocity liquid or gaseous stream can be effectively employed in jet propulsion. Air rushing toward the inlet of an engine flying at high speeds is partially compressed by the so-called ram effect. Ramjets that operate at supersonic speeds are often called scramjets. Scramjets are considered to be among the most promising power systems for hypersonic vehicles since their specific thrust is larger than that of any other propulsion system in the hypersonic flight regime when the Mach number is higher than 6. However, these systems are faced with a critical drawback in that the heat sink for the fuel consumed by hypersonic vehicle does not provide sufficient cooling at higher Mach numbers, which leads to a rapid increase in the heat loads. Despite the numerous studies regarding this issue, a concrete solution is yet to be arrived at and it is therefore imperative that an efficient cooling technique that will offer protection to the struts used when the hypersonic vehicles are operated at higher Mach numbers be developed.
Recently, a team of researchers (Dr. Gan Huang, Dr. Yinhai Zhu, Dr. Zhiyuan Liao, Dr. Taojie Lu) led by professor Pei-Xue Jiang from the Key Laboratory for Thermal Science and Power Engineering of Ministry of Education at Tsinghua University in collaboration with Zheng Huang from China State Shipbuilding Corporation in China investigated a combined transpiration and opposing jet cooling technique for protecting porous struts with micro-slits in the leading edge. They hoped to provide a breakthrough by developing an improved strut structure for better cooling efficiency. Their work is currently published in the research journal, Journal of Heat Transfer.
The researchers began their studies by identifying and adjusting a supersonic wind tunnel test facility to match their desired standards. Next, they prepared a physical model of the porous strut structures to be examined in the study. They then connected the test sections with the Laval nozzle section and used it to investigate the heat transfer of the struts. Eventually, the temperature distribution on the porous strut surfaces was measured using an infrared thermal imaging instrument.
From the Schlieren images taken, the researchers observed that their cooling technique considerably affected the stability of the flow field and the profile of the detached shock wave. Additionally, they recorded three different states of flow fields when increasing the coolant injection pressure of a strut having a 0.20 mm wide micro-slit. More so, it was also seen that the detached bow shock was pushed away by the opposing jet; after which it then became unstable and even disappeared when the coolant injection pressure was increased.
In their work, Pei-Xue Jiang and colleagues presented an empirical investigation of the combined transpiration and opposing jet cooling for sintered stainless steel porous struts. The results presented indicated that the combined opposing jet cooling and transpiration cooling had a significant influence on the flow filed. In addition, their work highlighted that the combined opposing jet and transpiration cooling was more effective than pure transpiration cooling for cooling the strut. All in all, the combined transpiration and opposing jet cooling with nonuniform injection made the temperature distribution uniform and caused the maximum temperature to decrease, which is a novel and effective technology to cool leading edge structures in the supersonic flow and reduce the thermal stress. It is of great significance to thermal protection system for spacecraft.
Figure1. Combined transpiration and opposing jet cooling structure
Figure2. Shock wave distributions
Gan Huang, Yinhai Zhu, Zhiyuan Liao, Taojie Lu, Pei-Xue Jiang, Zheng Huang. Experimental Study on Combined Cooling Method for Porous Struts in Supersonic Flow. Journal of Heat Transfer 2018, Volume 140 / 022201-1Go To Journal of Heat Transfer