08-11-2012, 06:20 PM
Sealing system for a rotary expander
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Introduction
The expander to which this paper refers is shown schematically in Fig. 1. In this machine, an expansion chamber is bound by
four identical rotors and two side plates which are not shown. The rotors turn at the same speed and in the same direction causing
the chamber volume to vary from zero to a maximum value and back to zero twice in each revolution. The zero volume and
maximum volume conditions will be referred to as “top dead center” and “bottom dead center” respectively, by analogy to a piston
engine. Synchronization of the mechanism is achieved by having a gear in each rotor shaft mesh with a gear in a centrally located
output shaft. High pressure steam, or other working fluid, is admitted through a centrally located port in one side plate. A valve
mechanism, not shown, controls the beginning and cut-off of the admission period. The expanded fluid is exhausted through radial
passages in the rotors when the chamber volume decreases.
A number of patents for internal combustion engines based on variations of this concept have been issued at various times [1–4]. Of
these, Campbell's patent [4] is the only one claiming an enginewith rotors equipped with apex seals, although these are not described
in any detail.
Work cycle
The work cycle of the engine is illustrated in the sequence of Fig. 2. In 2(a), the rotors are shown in a position just past top
dead center, coinciding with the start of admission of high pressure steam through a port in the center of one side plate. Steam
cut-off occurs after further rotation of the rotors followed by an expansion phase terminating at 2(b), which corresponds to
the moment in which exhaust ports in the rotors pass into the expansion chamber allowing the expanded steam to flow out
through passages in the rotors themselves. 2© depicts the engine configuration at maximum chamber volume. After a
further 90° rotation of the rotors, the exhaust phase is completed as the chamber volume becomes zero, as shown in 2(d). A
few degrees of rotation later the position depicted in 2(a) is reached once more, ending one work cycle in one half revolution
of the rotors.
Seals
The basic considerations in designing seals for the UNAM engine were: to provide effective and reliable sealing of the working
chamber with minimum frictional power loss and minimum seal wear. Each rotor requires apex seals and side seals. Fig. 3 is a
simplified, exploded view of a rotor and one seal of each kind. Following is a discussion of both of these.
Side seals
In the UNAMengine, the seal between a rotor face and a side plate should ideally run along the edge of the rotor, rather than
some distance in, as is the case in Wankel engines. This consideration led to the design depicted in Fig. 3. The side seal is a plate
with a beveled rim that fits in a beveled depression in the rotor face. The top surface of the seal contacts the side plate only along
the raised narrow peripheral band, 4. The seal also acts as a spring by virtue of the grooves 5–8, which allow it to contract
transversely if pushed into the rotor depression against the resistance posed by the beveled seat. This action is illustrated in
Fig. 6, which shows a cross-sectional transverse view of a rotor and its side seals. The forces, F, developed at the contacts with
the side plates, result in displacements, δ, of the seals' rims. As may be appreciated, the side seals are effective all along the
rotors' edges during expansion of the work chamber, so that their function does not need to be complemented by the apex seals
at the rotor tips, as asserted in the previous section.