Flow characteristics and entrance length effect for MEMS nozzles

• The CBPR and discharge coefficient were measured for ISO and MEMS nozzles.
• For the same MEMS nozzle, the discharge coefficient with the longer entrance length was always higher than that for with shorter entrance length.
• The theoretical model for subsonic flow was used to analyze the influence of the entrance length on the discharge coefficient for supersonic flow, and the tendencies were the same.

The critical back pressure ratio (CBPR) and the discharge coefficients were measured for traditional ISO nozzles with circular throat sections and sonic MEMS nozzles with rectangular throat sections manufactured using Micro-Electro-Mechanical systems techniques. The measurements show that critical flow can be reached in the MEMS type nozzles, but that the CBPR is much smaller than for traditional ISO nozzles and is almost constant for the same MEMS nozzle with different Reynolds numbers. The CBPR for MEMS nozzles with similar shapes increased as the throat diameter increased with different CBPR values for different shapes. For MEMS nozzles with similar geometries, the discharge coefficients in the backward direction with the longer entrance length were always higher than in the forward direction with the shorter entrance length.The measurement results were used to analyze the effect of the entrance length effect on the discharge coefficient with comparisons to theoretical values. As with the Vena Contracta for subsonic flow, the longer entrance section resulted in larger effective throat diameters for the same MEMS nozzle, which resulted in larger discharge coefficients.