  Hydraulic accumulators play a central role as energy storage in nearly all
fluid power systems. The accumulators serve as pulsation dampers or energy
storage devices in hydro-pneumatic suspensions. The energy carrying gas is
compressed and decompressed, often periodically. Heat transfer to the outside
significantly determines the transfer behaviour of the accumulator since heat
transfer changes the thermodynamic state of the enclosed gas. The accumulators
operating mode ranges from isothermal to adiabatic. Simulating fluid power
systems adequately requires knowledge of the transfer behaviour of the
accumulators and therefore of the heat transfer. The Engineer's approach to
model heat transfer in technical system is Newton's law. However, research
shows, that in harmonically oscillating gas volumes, heat flux and bulk
temperature difference change their phase. Newton's law is incapable of
representing this physical phenomenon. We performed measurements on two sizes
of commercial membrane accumulators. Experimental data confirm the failure of
Newton's approach. Instead the heat transfer can be modelled with an additional
rate dependent term and independently of the accumulator's size. Correlation
equations for the heat transfer and the correct accumulator transfer behaviour
are given.
