Clearance Seal Leakage

For reciprocating pistons of a Stirling engine, a clearance seal is a good alternative to conventional ring type seals. A clearance seal obviates rubbing surfaces and lubrication requirements. Obviously, due to the inherent clearance, this type of seal will leak to some degree. Therefore, thermodynamic effects of this leakage are important to the design. Clearance seals are usually placed on the cold side of the Stirling engine to avoid undesirable temperature-related effects. Consider a piston that is surrounded by a seal with clearance of g. If the instantaneous pressures on either side of the piston are pa and pb, the instantaneous mass flow rate, ml, is calculated as [33],

NPa + Pb { . g3 Pb — Pa to otA

Mi = nD~mT — {x P g — (338)

Where D is the mean clearance diameter, Tg is the gas temperature which would be near constant at the wall temperature, L is the seal length, and xP is the instantaneous piston position. Note that Eq. (3.38) incorporates both pressure-driven and shear-driven leakages. In addition to leakage, Eq. (3.38) can be utilized to predict instantaneous or mean enthalpy transfer through the seal using,

Hi = mi CpT (3.39)

Where T is the conditional nodal temperature at the point of application, depending on the gas flow direction.

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