(676e) Development and Feasibility of a Variable-Area, Sonic Nozzle Mass Flow Controller

Accurately controlling the mass flow rate of a compressible
fluid over a large dynamic range is of practical importance for a variety of
industrial and engineering applications.  The sonic nozzle is a potential
control device applicable to such a broad range of flow rates that is finding
wide spread acceptance for gases.  To determine the feasibility of such a
control device, two-dimensional, axisymmetric computational fluid dynamics
simulations are coupled with one-dimensional isentropic, compressible flow
models to develop a new, variable-area, sonic nozzle capable of dynamically
controlling the volumetric flow rate of a gas through almost five orders of
magnitude, from 0.5 sccm to 30 slm.  This achievement is in sharp contrast to
adjustable sonic nozzles currently available having dynamic flow rate ranges on
the order of 10:1 to 20:1, depending on the orifice diameter.  The volumetric
flow rate of air through the theoretical nozzle is calculated by varying two
independent variables, the inlet pressure and the cross-sectional area of the
nozzle throat.  The energy losses predicted by the computational fluid dynamics
simulations, which will be discussed in detail, are also taken into
consideration and incorporated into the isentropic flow model in terms of a
discharge coefficient, C_D.  The cross-sectional area of the
throat is varied by incremental motion of a rod centered in the nozzle.  The
distance from the surface of the rod to the orifice wall is calculated and
compared to typical fabrication tolerances.  It has been determined that, for
higher flow rates, machining tolerances do not preclude device fabrication.  At
low flow rates, however, extremely stringent control of the manufacturing
process is required to ensure that the rod surface does not contact the inner
wall of the nozzle.