Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.
For fast and effective simulation of filling processes of
pressure vessels with compressed gaseous media the governing
equations are derived from a mass balance equation for the gas
and from energy balance equations for the gas and the wall of
the vessel. For simplicity the gas is considered as a perfectly
mixed phase and two heat transfer coefficients are introduced.
The first one is the mean heat transfer coefficient between the
gas and the inner surface of the pressure vessel and the second
one is the heat transfer coefficient between outer surface of the
vessel and the surroundings. Although the process is transient,
steady-state heat transfer coefficients for free convection are
used between outer surface of the vessel and the surroundings.
The use of available correlations for steady-state heat transfer
coefficients to describe transient processes is common practice,
e.g. in the modelling of the transient behaviour of heat
exchangers [1]. But no correlations – neither steady-state nor
transient – are available for the heat transfer coefficient
between inflowing gas and inner surface of the vessel. To solve
this problem a CFD tool is used to determine the gas velocities
at the vicinity of the inner surface of the vessel for a number of
discrete surface elements. The results of a large amount of
numerical experiments show that there exists a unique
relationship between the tangential fluid velocities at the
vicinity of the inner surface of the vessel and the gas velocity at
the inlet. Once this unique relationship is known the complete
velocity distribution at the vicinity of the inner surface can be
easily calculated from the inlet velocity of the gas. The nearwall
velocities at the outer limit of the boundary layer are
substituted into the heat transfer correlation for external flow
over flat plates. The final heat transfer coefficient is the areaweighted
mean of all local heat transfer coefficients. The
method is applied to the special case of filling a 70 MPa
composite vessel for fuel cell vehicles with hydrogen. Because
of the heat capacity of the composite wall consisting of an inner
aluminium liner wrapped with carbon fibre, heat transfer from
the compressed gas to the vessel wall strongly influences the
temperature field of the gas which is predicted by the model
and confirmed by experiments.
