Principles of free jets

Conference · Wed Mar 01 04:00:00 EST 1995

OSTI ID:110827

Miller, D R ^[1]

---

1. Univ. of California, San Diego, La Jolla, CA (United States)

The principles of the free jet expansion are well established and only a brief tutorial overview of selected concepts is presented below. Almost every important theoretical and/or experimental aspect of the free jet related to molecular beam sampling has appeared in one of the 19 International Symposium on Rarefied Gas Dynamics and many detailed reviews exist. The first stage of pressure reduction in a mass spectrometer sampling system is often simply an aperture or short capillary tube across which a significant pressure ratio is maintained. When this pressure ratio between the system being sampled and the first vacuum stage exceeds about two then the gas flow reaches sonic conditions at the exit, Mach number M = 1, and the exiting flow is known as {open_quotes}choked{close_quotes} flow. It is called choked because for fixed source conditions the mass flux out of the aperture will not exceed this M = 1 condition regardless of how low the exit chamber pressure is taken, ie how much pumping is added. Beyond the exit the gas expands in a supersonic free jet expansion as it adjusts to the low pressure in the exit chamber. The expansion is called a free jet because there are no diverging nozzle walls to constrain the flow, which would lead to viscous boundary layers and a much smaller rate of expansion. Diverging nozzle walls are used in rockets to direct the exiting flows along the axis for better forward thrust, and researchers use them to slow the rate of expansion to, for example, grow clusters or study fast kinetics. Only the free jet is dealt with here. The free jet nozzle only consists of the subsonic nozzle geometry. The nozzle exit to be referred to here is then just the aperture or capillary exit, and the free jet expansion is the unconstrained subsequent supersonic flow downstream of this nozzle exit.

• Research Organization: National Renewable Energy Lab., Golden, CO (United States)
• OSTI ID: 110827
• Report Number(s): NREL/CP--433-7748; CONF-9410343--; ON: DE95004052
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
JETS
MACH NUMBER
MOLECULAR BEAMS
NOZZLES
OPERATION
SUPERSONIC FLOW