Impact of relative humidity and PIV seeding on the electro-aerodynamic performance of needle-to-ring ionic fans

Abstract Electrohydrodynamic (EHD) fans represent a viable solid-state alternative to conventional thermal management, addressing the demand for miniaturized, silent, and vibration-free cooling in modern electronics. While Particle Image Velocimetry (PIV) is widely utilized for aerodynamic characterization, standard PIV assumptions break down under EHD flows. Classical tracer fluid coupling assumptions are invalidated due to the complex interactions between tracer particles, the induced electric field, and ambient conditions. This study experimentally quantifies the coupled effects of relative humidity (36%, 50%, and 80%) and PIV seeding materials (DEHS droplets and incense smoke) on the operational characteristics of a needle-to-ring EHD fan (22 mm diameter, 4–14 kV operating range). Experiments were conducted in a controlled environmental chamber to evaluate discharge current stability, current–voltage relationships, and the resulting flow fields. The findings confirm that while the linear corona discharge regime (Townsend relationship) is preserved across all humidity levels, increasing humidity suppresses the discharge current. Notably, higher humidity conditions yield higher outlet velocities, a trend attributed to condensation-enhanced local field effects near breakdown. More importantly, the choice of seeding material emerges as a primary source of measurement bias. Dielectric DEHS droplets suppress ionization and distort the flow topology, inducing a 36% reduction in peak axial velocity and a 17% underestimation of the volumetric flow rate at 14 kV. In contrast, electrically compatible incense smoke preserves the high-velocity ionic jet core. These findings establish that seeding selection and environmental conditions are active variables governing EHD flow behavior, with direct implications for diagnostic methodology and the deployment of ionic wind devices.