A spatially
-
lumped
dynamic
model of a reciprocating
-
piston expander is presented in this paper. The model
accounts for the three main loss
mechanisms in realistic piston machines, namely: pressure losses through the
intake and exhaust valves, heat transfer between the gas and the surrounding cylinder walls, and the mass
leakage
between the compression/expansion chamber and the crankcase
throu
gh the piston rings.
The model
also accounts for real
-
gas effects with the fluid properties calculated from t
he NIST database using REFPROP.
The numerical calculations are first compared with experimental pressure
-
volume
-
temperature data obtained on
a cust
om reciprocating
-
piston gas spring over a r
ange of oscillation frequencies. The comparison between
numerical and experimental results shows good agreement. It also
allows the most accurate
heat transfer
correlation
to be selected
for calculating the gas
-
to
-
wall
in
-
cylinder
heat transfer. The semi
-
heuristic modelling
tool is then
used to design an expander for
specific pressure ratio
s
and mass flowrate, and to predict the
thermodynamic performance of the piston device over a
range of part
-
load conditions.