Abstract The time-averaged mass flux through a free-piston clearance seal can cause shifts in the equilibrium piston position, which will reach an ultimate value for a given set of operating conditions if the piston is restrained by a mechanical or magnetic stiffness. The mass flux and resulting piston offsets can be particularly significant in systems where the reactive component of dynamic pressure on the pistons is large (e.g. where gas-spring stiffness is important). Here we derive simple expressions for the time-averaged mass flux through a clearance seal, assuming that the piston is acting against enclosed volumes on either side (i.e., is a double gas-spring) and predict the resulting piston offset. We consider the case of solid pistons, which have a clearance gap that is constant in time, and the case of hollow pistons, which can flex in response to the pressure waves and therefore have a time-varying clearance. We compare these predicted offsets to data on several pressure-wave generators, taken with the compression volumes capped (so that they resemble double-gas springs). Agreement with theory is excellent for most cases of interest. These results suggest that a properly designed piston can harness the pressure-induced flexing effect to reduce or eliminate piston drift. Such an “anti-drift” piston is built and tested, with promising results.
A Novel Method for Controlling Piston Drift in Devices with Clearance Seals
Cryocoolers 13 ; 215-223
2005-01-01
9 pages
Aufsatz/Kapitel (Buch)
Elektronische Ressource
Englisch
A Novel Method for Controlling Piston Drift in Devices with Clearance Seals
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