Study on non-monotonic pressure-drop of supercritical n-decane with pyrolysis in heated channels

Abstract

The non-monotonic relation between pressure-drop and flow-rate leading to the flow instability of fuel with pyrolysis was investigated. A characteristic pressure-drop model of n-decane coupled pyrolysis reactions under supercritical pressure was established. The model includes one-dimensional flow control equation, species equation, RK-PR equation of state, high-pressure viscosity model and a reaction mechanism consisting of 10 species and 15 reactions. The non-isothermal friction factor formula with wall correction coefficient fitted by experimental data was used in the model. To verify the model, the pressure-drops at different flow-rates were measured in a horizontal stainless steel tube with the inner diameter of 1.93 mm under different back pressures (3, 4 and 5 MPa). The results show that the model can accurately capture the non-monotonicity of characteristic pressure-drop. Under 3 MPa condition, the characteristic pressure-drop curve of n-decane has a pseudo-critical negative slope (PCNS) region and a reaction negative slope (RNS) region caused by the pyrolysis. With the increase of pressure, the PCNS region disappears but the RNS region still exists. The high pressure makes the onset of RNS region appear earlier due to the accelerated initial reaction rate. The total pressure-drop is separated and analyzed based on the model mathematically. In the PCNS region, the derivative of acceleration pressure-drop to mass flux is negative, whereas the derivative of friction pressure-drop to mass flux is positive. In the RNS region, the two derivatives are both negative because of the sharp decrease in density and the increase in friction factor caused by the pyrolysis reaction. The two parallel channels’ instability with constant total mass flow rate is analyzed based on the characteristic pressure-drop curves for reacting and non-reactive flow. The results indicate that the fuel pyrolysis reaction not only expands the range where the flow excursion may occur but also makes it difficult for the flow-rate to reach stable state again after the flow excursion caused by pseudo-critical instability.