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Analytical and numerical study on the fast refill of compressed natural gas with heat removal

University of British Columbia

Heavy-duty vehicles powered by compressed natural gas (CNG) store the fuel in large onboard cylinders. Fast fill of depleted CNG cylinders is a common refueling method because it achieves refueling time comparable to liquid fuels. However, under-filling of CNG cylinders occurs due to the recompression work of CNG in the cylinder that heats the fuel. The heated fuel has a lower density at the rated pressure, and thus less mass of CNG can be stored than if it was at ambient temperature. While there are many previous works that have modeled or measured the pressure, temperature rise and the mass filled of the gas during filling, very few of them have investigated potential solutions to improve fill efficiency. Since heat generation during fast fill is inevitable, concepts that can actively or passively remove the generated heat during the fill process are required to achieve the needed improvements in fill efficiency. In this study, analytical and numerical models of the filling process of CNG into a Type-III cylinder with and without heat removal are developed. In the analytical study, 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 into the cylinder on the dispensed mass and fill time is quantified. The analytical study is compared to numerical simulations employing a two-dimensional axisymmetric computational fluid dynamics (CFD) model for unsteady, compressible turbulent flow without cooling, with active cooling and with pre-chilling. Dynamic average temperature, pressure and mass curves as well as the local temperature distribution in the cylinder are obtained at different time instances during the fill. The effect of the location of the heat removal coils is also investigated. The results illustrate the benefit of heat removal from the cylinder as a means of improving the fast fill efficiency, which can help increase the penetration of CNG into the Canadian transportation sector and reduce greenhouse gas emissions.

Text

2017-09-22

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/63113

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/