Brazilian Journal of Chemical Engineering THE COMPLETE MODELLING OF THE FILLING PROCESS OF HYDROGEN ONBOARD VEHICLE CYLINDERS

Complete modelling of the filling process occurring in a hydrogen-fueled vehicle storage cylinder is examined. A simultaneous modelling of the flow and heat transfer within the cylinder and cylinder wall has not been considered in previous studies. Rapid filling may result to an unexpected temperature rise and breaching of the safety standards. In the present study, initially a correlation was developed based on a numerical simulation for predicting the heat transfer rate between in-cylinder flow and the cylinder inside wall. Then, a thermodynamic model was developed for predicting transient variations of temperature and pressure inside the cylinder and wall temperature during the filling. The model was applied to a type III onboard storage cylinder filling process. The numerical results are compared with previously measured values and showed good agreement. The results also show that a great portion of heat dissipation from the incylinder flow is stored in the cylinder wall. It is also found that ambient temperature during the refueling process has considerable effects on filling behavior in general and in particular on the final in-cylinder temperature and filled mass.


INTRODUCTION
Utilization of hydrogen as a clean alternative fuel has a favourable impact on the environment (Farzaneh-Gord et al., 2013a).Being cleaner and more effective than petrol, hydrogen has been recognized as the primary choice for future fuels in automobiles (Maus et al., 2008;Zhao et al., 2012).Because of the recent developments in hydrogen fuel technology, the spread of hydrogen fuelling stations has gained more attention in the world (Rigas and Sklavounos, 2005;Schoenung et al., 2006).Studies indicates that 80 to 90% of hydrogen is stored using high-pressure compression in hydrogen fuelling stations and vehicle cylinders (Tzimas and Filiou, 2003), due to the advantages of being more practical, dependable, durable and admissible (Zheng et al., 2008;Maus et al., 2008;Zhang et al., 2006) as compared to other methods.
In this system, fuel is delivered from high-pressure hydrogen reservoirs into the onboard vehicle cylinder.The station dispensers control the rate of hydrogen passing into the cylinder and therefore the rate of temperature/pressure rise inside the cylinder.Clearly, reducing filling time has a favourable impact on commercialization of hydrogen vehicles, yet it Brazilian Journal of Chemical Engineering may result in unexpected temperature rise and breaching of the safety standards (Zhao et al., 2012;ISO15869 2005;ISO11439 2005).Due to the importance of temperature rise during the filling process, several experimental, numerical and theoretical studies have been performed on the issue.Yang (2009) developed a thermodynamic and heat transfer analysis of an onboard cylinder hydrogen refuelling process.The cylinder was assumed to be adiabatic, isothermal, or diathermal, while hydrogen was considered to be both an ideal and a real gas with a constant inlet flow rate.For an ideal gas, simple analytical expressions are derived for the tank temperature and pressure during adiabatic, isothermal, and diathermal refuelling conditions.Non-ideality is treated using the newly developed equation of state for normal hydrogen based on the reduced Helmholtz free energy formulation.Lower tank temperatures and pressures and longer filling times are always predicted when the real gas assumption is applied.
In another study, Mohamed and Paraschivoiu (2005) modelled hydrogen release from a high-pressure chamber based on the real gas assumption.Zheng et al. (2010) simulated an optimizing control method for a high utilization ratio and fast filling speed in hydrogen fuelling stations.It was shown that the optimizing control method can meaningfully improve the utilization ratio, while allowing for acceptable refuelling time.Farzaneh-Gord et al. (2012a) have also carried out a theoretical analysis to study the effects of storage types and conditions on the performance of hydrogen filling stations and the filling process for an adiabatic cylinder.Liss and Richards (2002), Liss et al. (2003), Newhouse and Liss (1999), Chan Kim et al. (2010) and Liu et al. (2010) have examined the fast filling of hydrogen cylinder experimentally.They all reported a high temperature rise in the cylinder during the fast filling process.Chan Kim et al. (2010) have also studied the thermal characteristics of a type IV cylinder filling process using computational fluid dynamics (CFD) analysis.The results show good agreement with the experiments, specifically as the initial in-cylinder pressure increases.Similar CFD analysis has been carried out by Heitsch et al. (2011), where the fast filling process of hydrogen tanks is simulated.It was found that the local temperature distribution in the tank depends on the materials of the liner and the outer thermal insulation.Different material combinations (type III and IV) were investigated.Dicken and Merida (2008) have also modelled the filling process of a hydrogen cylinder using CFD tools and experiments.They computed the heat transfer rate from in-cylinder gas to the ambient numerically but failed to present a general correlation.
Since the hydrogen and Compressed Natural Gas (CNG) infrastructures are similar, it is also constructive to consider the comparable studies on CNG (Thomas et al., 2002;Shipley, 2002).These studies also reported a temperature rise of about 40K during the filling process of storage gas cylinders.The temperature rise reduces the density of filled gas, resulting in an under-filled cylinder relative to its rated specification.It was also found that ambient temperature influences the filling process and storage capacity (Shipley, 2002).Farzaneh-Gord et al. (2007, 2008a, 2008b, 2011, 2012b, 2012c, 2013b) have also simulated the fast filling process of CNG in several studies.The results indicated that ambient temperature has considerable effects on the filling process and final cylinder conditions.They also employed a theoretical analysis to study the effects of buffer and cascade storage tanks on the performance of a CNG fuelling station (Farzaneh-Gord et al., 2011).The effects of natural gas compositions on the fast filling process for buffer and cascade storage tanks have also been examined (Farzaneh-Gord et al., 2012b, 2012c).The storage tanks are treated as adiabatic in all of their studies.
Nooralipour and Farzaneh-Gord (2013) studied the CNG fast filling process using commercial CFD software.Although their numerical findings compare well with the available experimental data, the computations were so time demanding that some improvements in the modelling are necessary.Also in another study, Deymi-Dashtebayaz et al. (2014) presented the full simulation of rapid refuelling of a CNG Vehicle on-board cylinder.
The above review of related literature indicates a shortage of information regarding the thermal aspect of the hydrogen cylinder fast filling process.In the present study, initially the filling process is numerically modelled to evaluate the heat transfer rate between in-cylinder gas and the cylinder wall by developing a correlation for predicting the inner convective heat transfer coefficient.Then, with the help of the developed correlation, a simultaneous thermodynamic modelling of the in-cylinder gas and cylinder wall was performed for predicting the thermal characteristics of the filling process with reasonable accuracy and minimal computational cost.As mentio ylinder is m Fluent), whi Later, the com illing process  density and i  can be calculated from the known values of the inlet temperature and pressure.In the present study, these properties are chosen to be identical to the values reported by Dicken andMerida (2007, 2008).Also the Reynolds and Mach numbers in the inlet could be obtained as:

THE METHOD AND MODELLING
where , , and Re Ma d  are the Reynolds and Mach numbers, diameter and viscosity of the inlet cylinder.
The heat released from the in-cylinder gas to the wall is calculated according to: where , , , and

A T T
 are the internal convection heat transfer coefficient, cylinder surface area, incylinder gas temperature and inside-wall temperature, respectively.i  can be calculated by employ- ing the definition of the Nusselt (Nu) number: where K and d are the thermal conductivity coefficient for in-cylinder hydrogen gas at its temperature and the inside diameter of the inlet tube, respectively.To our best knowledge there is no correlation available for variations of the heat transfer rate during the filling process of a compressed hydrogen onboard cylinder.In the present study, the following correlation was developed based on CFD results for the Nusselt number (Deymi-Dashtebayaz et al where RA m  is the ratio of inlet mass flow rate, m  , to the maximum mass flow rate, max m  : Maximum mass flow rate occurs in the choking condition and can be calculated according to Costhuizen and Carscallen (1997): where 0 0 , and P

 
are the isentropic exponent, density, and pressure of the high pressure hydrogen reservoir, respectively.
Combining Equations ( 1), ( 2) and ( 6), the following equation is obtained: which can be rearranged to: If the inside wall temperature, T iw , and the internal heat transfer coefficient are known (or the cylinder is assumed to be adiabatic), then from equations 10 and 1 the in-cylinder gas density and internal energy can be determined.Other thermodynamic properties are then obtained from thermodynamic tables.Farzaneh-Gord et al. (2007, 2008a, 2008b, 2011, 2012a, 2012b, 2013a) have used the same method to analyse the filling process of an adiabatic cylinder.Here, the energy equation for the cylinder wall was also solved simultaneously to determine the inside wall temperature.

The Energy Equation for the Cylinder Wall
Heat transfer between hydrogen and the ambient includes convective heat transfer between in-cylinder hydrogen and the inside wall, heat conduction within the wall and finally free convection heat transfer between the exterior wall and ambient.The cylinder wall is made of a highly conductive aluminium liner and a comparatively insulating carbon fibre and epoxy wrapping.For simplicity, the thermal properties of both liner and laminate are assumed to be constant.The in-cylinder temperature rise is affected by the material properties of the liner and laminate.
The cylinder wall is divided into two control volumes (liner and laminate) and the energy equation was applied to both systems according to: In above ensity, speci nd thermal ayers, respec eat transfer utside wall, qual to 10W/ Clearly, th t the interfac

By solving eously, the t erature can b
The Numeric Calculatio ynamic prop irst order Eu mass within th ng the volum f hydrogen c 12) and ( 13) nside wall t pecific intern nowing two here specific ther thermod ure) can be e ables.

Boundary Co
The   The numeri e measured addition to ut a numeric mpressed hy e filling proc e), they did n ansfer coeffic

Figure 1
Figure1shows an on-board storage cylinder which receives the compressed gas from high-pressure Figure 1: A s nvestigation.CFD Modelli A schemat CFD calculati s type III w xternal diam espectively.T um liner, wh arbon-fibre nlet conditio 2007, 2008), ata needed fo Fig atu (20

Figure
Figure 6: Co alculated in NIST values.
Figure 7 p nd Mach nu rocess.As s lds number ordingly, it c urbulent.Als he figure, du ions remain i

Table The C
Brazi