ISSN Online: 2379-1748
ISBN Flash Drive: 978-1-56700-469-4
ISBN Online: 978-1-56700-470-0
Second Thermal and Fluids Engineering Conference
Increasing Fast Refill of Compressed Natural Gas by Active Heat Removal
Abstract
Heavy duty vehicles powered by compressed natural gas (CNG) have large CNG tanks onboard. The tanks need
to be fast refilled at CNG stations whenever the gas storage is low. The refill stops when the pressure inside the
tank exceeds the rated pressure, and at the same time the temperature inside the tank is higher than the ambient by
around 30K. The pressure goes down when the temperature reaches the equilibrium with the ambient. Defining
the ratio of the filled natural gas to the actual onboard storage capacity as the refill efficiency, the refill efficiency
is only about 80%. The driving range of the heavy duty CNG vehicles can be increased if the refill efficiency can
be improved. An active heat removal method based on a liquid cooling loop is proposed to improve the fast refill
of CNG vehicles. In this research, analytical and numerical models of the filling process with and without active
heat removal inside a type III cylinder is developed. In the analytical study, the mass and energy conservation
equations are coupled with an ideal-gas equation of state and orifice flow equations to predict the heat generation
rates during fast fill. The influence of heat removal via a cooling coil inserted in the cylinder on the fill efficiency
and fill time is quantified. The results show that although it takes more time to fill a cylinder with the cooling coil,
the cylinder can carry more mass of natural gas so that the vehicle's range can be increased. The analytical study
is compared against numerical simulations employing a two-dimensional axisymmetric computational fluid
dynamics (CFD) model for unsteady, compressible turbulent flow with and without active heat removal. Dynamic
average temperature, pressure and mass curves and local temperature distribution in the cylinder are obtained at
different time instances during the fill. The results of the analytical/numerical study achieve good agreement with
published experimental data and illustrate the benefit of heat removal from the cylinder as a means of improving
fast fill efficiency.