Mathematical modeling of fast filling process at CNG refueling stations considering connecting pipes

• A new mathematical model developed to analyze the fast filling process.
• This new model is unique because connecting pipes between reservoir tanks and vehicle cylinder also taken into account.
• The results show that pressure loss in the fast filling process is very important and should be considered.
• Filling time for the buffer storage system is about 44% less than in the cascade.
• At the start of process, mass flow rates are constant and there is a temperature rise of about 85 K for the process.

One of the most important parameter in design of a compressed natural gas refueling station is detailed modeling of fast filling process. In this work, a new mathematical model was developed to analyze the fast filling process at compressed natural gas refueling stations. This new model is unique because connecting pipes between reservoir tanks and on-board vehicle cylinder also were taken into account. To model the process, the equation of state, continuity, momentum and energy equations have been solved for different parts of the system. The results have been validated against the experimental data to investigate effects of connecting pipes. The mass flow rate numerical values are in good agreement with the experimental values. The study carried out for both cascade and buffer storage systems. Variations of pressure and Mach number at different parts of the system, effects of the initial reservoir tank temperature on natural gas vehicle in-cylinder pressure and temperature are presented. The results show that pressure loss through pipes is very important and should be considered. Mass flow rate between reservoir tank and on-board cylinder is nearly constant (as the flow is chocked) during filling. Temperature growth rate at the beginning of the process is higher for the buffer system. It is also found that there is a temperature rise around 85 K for ideal gas model.