Effects of pulse power on evaporation characteristics of an n-heptane droplet in a microtube

When combustion instability arises in liquid rocket engines, heat release fluctuations cause local temperature oscillations and affect the evaporation of fuel droplets. In this study, to investigate the responses of droplet evaporation characteristics to temperature oscillations, an n-heptane droplet is formed by continuous fuel injection, and a pulse power is applied to generate temperature oscillations. The pulse power outputs a square wave with a frequency of 0.1–10 Hz and a power of 16.6–30.0 W. A transient heat transfer and droplet evaporation model is established, and measured droplet diameters serve to validate the model. The effects of heating frequency, heating power, and waveforms on droplet vaporization are studied. The results show that pulse power induces oscillations in air temperature, droplet diameter, and evaporation rate, which correspond to the applied heating frequency. Due to heat transfer within the tube, a phase lag is observed in the air temperature relative to the applied power. The dynamic equilibrium between the evaporation rate and fuel flow rate causes the evaporation rate phase to lag behind the droplet diameter phase by a constant of 0.5π, and to lead slightly ahead of the air temperature phase. Increases in heating frequency reduce the amplitudes of air temperature, droplet diameter, and evaporation rate while maintaining their averages constant. Decreases in heating power lower the air temperature, consequently increasing the evaporation rate amplitude and average droplet diameter. At 16.6 W, the droplet descends due to exceeding the critical diameter of 2.02 mm. Replacing the square wave of pulse power with a sinusoidal or triangular wave only results in a decline in air temperature amplitude, consequently reducing the evaporation rate amplitude by an equivalent degree. The amplitude percentage ratio between them remains unchanged at 4.77 but is subject to interference from their phase difference. As frequency increases from 0.1 to 1 Hz, the phase difference rises from −0.08π and converges to 0, while the amplitude percentage ratio increases from 4.77, converging to 4.87.