Transient radiative heat flux characteristics in capillary discharge plasma jets

The capillary discharge plasma ignition device features a simple and reliable structure with a high ignition efficiency, and has become a research focus in both industrial applications and academic studies. The transient radiative heat flux characteristics of the plasma jet is a critical indicator for characterizing its ignition capability. In this work, a transient radiative heat flux measurement system based on a thin-film heatflux gauge is established. Design and optimization methods are proposed to address the measurement range, response time, and sensitivity of the thin-film probe. The results indicate that reducing the thickness of the film can enhance measurement sensitivity effectively, whereas changing the film material yields relatively limited improvement. Additionally, the effects of energy storage capacitor voltage and capillary diameter on the output radiative heat flux characteristics are investigated using polyethylene and polytetrafluoroethylene as capillary propellant. The results indicate that the radiative heat flux of capillary discharge exhibits a temporal delay compared with the main discharge current. Increasing the voltage of the energy storage capacitor enhances the energy deposition efficiency of the main discharge and the plasma temperature, thereby improving both the output radiative heat flux and the duration of the heat flux. Moreover, the growth rate of the heat flux exceeds that of the stored energy. Enlarging the capillary diameter reduces the discharge time constant, thereby shortening the heat flux duration. At the same time, the ablation of the propellant becomes more sufficient, resulting in fewer jet deposits and a weaker absorption of the heat flux. When the capillary diameter increases from 1.5 mm to 3 mm, the jet expansion velocity and the energy deposition efficiency are significantly enhanced, leading to a remarkable increase in the radiative heat flux density. However, when the diameter further increases from 3 mm to 6 mm, the jet expansion velocity changes marginally, while the decrease of energy deposition efficiencycan result in a reduction in radiative heat flux. The capillary discharge with polyethylene propellant exhibits a higher peak radiative heat flux, an earlier peak time, and a shorter duration than that with the polytetrafluoroethylene propellant.