Supersonic Micro-nozzles

Single-Phase Gaseous Hows in Microchannels Supersonic Micro-Nozzles... 

Micro- and Nanoscale Gas Dynamics Roughness Effect on Microscale Transport w Supersonic Micro-nozzles... 

In this entry, we provide an overview of the various engineering issues and challenges surrounding the modeling, design, and fabrication of micropropulsion systems and supersonic micro-nozzles in particular. From this, it is our hope and intent that the reader will develop an appreciation for the unique aspects of supersonic nozzle behavior on the microscale. 

A supersonic micro-nozzle consists of a micro-machined channel with a varying cross-sectional area A x), where the -tx-direction coincides with the flow direction within the channel. Under the quasi-one-dimensional (quasi-lD) flow approximation, all flow variables are assumed to vary only with the streamwise position x (and time if the flow field is unsteady). Variations across a given cross section are considered negligible. Such an approximation is valid provided that the rate of area variation dA/dx is sufficiently small. The quasi-lD model is, of course, only an approximation of an actual 3D flow however, it greatly simplifies analyses and often yields surprisingly good results in predicting basic nozzle behavior. An overview of the quasi-ID nozzle model and its limitations can be found in most gas dynamics references (e.g., Anderson [3]). 

A photograph of the NASA/GSFC MEMS-based microthruster etched into a silicon wafer and placed on top of a US penny for purposes of scale. (Bottom) A scanning electron microscopy (SEM) image of the NASA/GSFC microthruster with the converging-diverging supersonic micro-nozzle outlined for clarity (See Hitt et al. [2] for details)... 

Supersonic Micro-nozzles, Table 1 Design parameters for the NASA/GSFC MEMS-based monopropellant supersonic micro-nozzle... 

Supersonic Micro-nozzles, Fig. 2 Performance results for the NAS A/GSFC supersonic microthruster nozzle with decomposed H2O2 and decomposed N2H4 as the working monopropellants. Thrust production, as shown on the left axis, has been normalized by the theoretical quasi-lD inviscid value. The percentage of the micro-nozzle exit plane occupied by the viscous subsonic layer is also shown on the right axis for both monopropellants of interest. 

We close this entry with a brief discussion concerning future research opportunities and needs for supersonic micro-nozzle flow within the context of micropropulsion applications. In some instances preliminary work has been reported in various forms within the aerospace literature however, comprehensive and detailed studies have not yet appeared. 

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