Abstract
A series of experiments were performed to investigate the thermodynamic instabilities that occur during heating of supercritical endothermic hydrocarbon fuel. A “power–temperature drop” characteristic curve is used to analyze the mechanism of thermodynamic instabilities. The results indicate that the heat-transfer process in a heated tube with increasing heating power can be divided into three periods: stable, developing, and instable; in which, the thermodynamic instabilities are found to occur. When the outlet fuel temperature reaches the pseudo-critical temperature, an acute decrease in fuel density and viscosity causes the flow to change from a transition flow to a turbulent flow, and the sharp increase of heat transfer in turbulent flow increases the thermodynamic instabilities. The intensity of the instability is related to the kinetic energy of the flow and the oscillatory extent. When the mass flow rate is increased from 1.0 to 1.5 g/s, the effect on the flow’s kinetic energy dominates the change in instability which causes the intensity of the instability to increase. While the intensity of the instability decreases with increasing inlet fuel temperature, which results from the decrease of the oscillatory extent. The effects of the operating pressure on the instability are not linear because of the properties of fuel change, obviously with pressure near the critical point.
Highlights
As one of the most potential options for the hypersonic air-breathing propulsion system, the scramjet has attracted an increasing attention worldwide,[1] but the aerodynamic heat attributed to high-speed flight is still a challenge for the current engine technology
The power–temperature drop characteristic curves in Figure 5 show that the heat-transfer process of all three different mass flow rates can be divided into three periods
The variations of outlet wall and fuel bulk temperature with time show that when instability occurs, the variation trend of fuel bulk temperature of the three different mass flow rates are the same, that is, the fuel bulk temperature first increases to the top sharply and remains still for about 0.3 s and decreases sharply to the bottom; the variation trends of wall temperature of 1.0 and 1.2 g/s are the same, that is, the wall temperature first decreases to the bottom sharply and increases gradually, while the wall temperature first increase to the top sharply and decreases gradually when the mass flow rate is 1.5 g/s
Summary
As one of the most potential options for the hypersonic air-breathing propulsion system, the scramjet has attracted an increasing attention worldwide,[1] but the aerodynamic heat attributed to high-speed flight is still a challenge for the current engine technology.
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