Geometry effects on flow characteristics of micro-scale planar nozzles

Abstract

A numerical investigation of geometry effects on the flow characteristics associated with converging-diverging 2D planar micronozzles is reported. The classical N–S with linear slip model, DSMC method, and a hybrid N–S/DSMC based on the continuum breakdown concept are employed for the simulations. The various methods adopted are validated with available experimental data. The study addresses the issue of heterogeneous findings on optimized half divergence angle recommended by previous studies. The analysis is conducted for various throat dimensions (2–200 µm thoat height), divergence angles (5∘–30∘), and divergence length. The performance and flow characteristics are compared at two distinct operating conditions, i.e. when the flow is mostly subsonic and supersonic in the divergent section. Results show an optimum divergence angle and length, maximizing the performance for a specific operating condition and nozzle size. Literature data suggest various divergence angles for optimum performance. This is due to the differences in operating conditions and the size of the micronozzles used. The current study shows that differences in flow behavior are mainly due to the pressure difference across the nozzle. When the micronozzle is operating under low-pressure differences, the flow is primarily subsonic in the divergent section. However, it accelerates due to the effect created by the growing thick boundary layer. The flow at higher pressure differences is characterized by clear flow separation and the occupation of the separated flow to a considerable portion of the divergent section. The separated phenomenon becomes less pronounced as the pressure difference and nozzle size decreases. As the nozzle size decreases to the nanoscale, Re is very low, and the subsonic layer fully occupies the divergent nozzle section even at very high-pressure differences. The results show that the performance is significantly influenced by the wall thermal conditions. The study highlights the differences in flow behaviour between micro and nano-scale nozzles at various operating conditions. Based on numerical data obtained in this present work, a new correlation is proposed to predict thrust per unit width for micronozzles.