Performance optimization design of wide-body aircraft pressure refueling system

In order to reduce the wide body aircraft refueling process time-cost and improve the airport operation efficiency, pressure refueling system design strategy was innovated. The shutoff valves’ operation sequence in refueling process was optimized. The diameter of the orifices on the pressure refueling system fuel line was designed. Pressure refueling performance was simulated and analyzed at different scenarios. The results show that it can improve the pressure refueling system performance at different scenarios by optimizing the pressure refueling process strategy design including optimizing the fuel line orifice design and optimizing the wing tank inner side shut-off valve operation sequence.


Introduction
The pressure refueling process is an important part of the airliner ground service.In general, the refueling process occupies most of the ground operation time, especially for the long-range wide body aircraft.Shortening the refueling process time-cost is beneficial for airlines, airports, passengers, and others by improving civil aircraft operational efficiency and reducing airport working load [1][2].
Simultaneous refueling all the fuel tanks from empty or a certain value to full at the same time is usually chosen as the refueling system design condition.Besides. in order to meet the fuel line flow velocity limit [3], the fixed geometric orifices are designed and set on the refueling outlet line to limit the fuel flow rate, and the pressure refueling process is adjusted by the fuel gauging and controlling system.For the off-design condition, the refueling system performance will deviate from that of the design point.For example, when refueling only a portion of the tanks, i.e. not refueling all the tanks to the maximum quantity, it is not possible to refuel at the maximum refueling flow rate due to the fixed geometric orifices at the fuel line, and the time-cost of the refueling process can not be significantly reduced at this scenario.Actually, in most operation cases, the airliners refuel the aircraft fuel tanks according to the need of the flight task rather than refueling all the tanks to the maximum fuel quantity for economic consideration.
In order to increase the refueling flow rate, current technologies adopted in the wide-body aircraft are as follows.
(a) Using the refueling adapter with two nozzles, or setting two independent refueling adapters on one side of the aircraft wing and connecting them in parallel to the refueling line.It can increase the total refueling upstream flow rate of the refueling system.
(b) Installing the refueling adapters on both aircraft wings and adding some refueling pipes.It can increase the total refueling flow rate of both upstream and downstream of the refueling system.
As above technologies will increase the refueling system weight or cause some difficulties on the refueling system layout, design optimization of the refueling system is needed.
Many scholars have conducted relevant research on the aircraft pressure refueling system design optimization.Cao K Q optimized the aircraft ground pressure refueling system by installing variable geometric throttling devices in each refueling branch line to ensure the refueling time can be shortest for any refueling quantity [4].Cao K Q conducted flow balance design for aircraft pressure refueling systems aimed at different actual refueling needs and control strategies [5].Shen Y L used the Bayes method to optimize the design of the orifice geometry, which improved the efficiency of the aircraft pressure refueling system [6].Wan F Q applied an improved BP neural network algorithm to faulttolerant control of aircraft pressure refueling systems, which shortened refueling time and improved refueling control accuracy [7].Wang J conducted simulation analysis on the aircraft ground pressure refueling system and calculated the refueling time and maximum flow rate in fuel line [8].Pinheiro N conducted modeling and simulation of the fuel system based on AMESim [9].Tu Y conducted modeling and simulation of the A320 aircraft refueling system based on Flowmaster, and analyzed the pressure distribution in the fuel line network [10].Liu M developed a pressure refueling model based on Flowmaster, checked the impact pressure and ventilation capacity of the system [11].Gong H proposed an innovative pressure refueling system from the perspective of control strategy [12].
In addition, due to the differences in policies and requirements of different countries, the aircraft refueling adaptor may not be set on the same side with the airport stand refueling station.If the refueling adaptor is quipped in only one wing, it may cause some inconvenience to the airline operation in the different countries.
This article optimized the working sequence of the refueling shut-off valves and the geometry of the refueling line orifices for a three-tank layout wide-body aircraft application.Simulation and analysis have been conducted on the different refueling scenarios including using a single side refueling adapter and using both sides of the refueling adapters simultaneously.

System description
This article analyzed the pressure refueling system for a three-tank layout wide body aircraft application.The system scheme is shown in Figure 1 [12].The mainly features of system are as follows.
(a) To adapt to the layout of ground refueling devices at different airports, one refueling station is set on the aircraft left wing and the other one is set on the right wing.Each refueling station is equipped with two standard refueling connectors, i.e. 2.5-inch couplings conforming to ISO45.The onboard refueling line is connected with the external refueling hoses (not shown in Figure 1) through the pressure refueling couplings.
(b) The aircraft's fuel storage tanks are divided into three independent parts including left-wing tank, central tank and right-wing tank.In order to ensure the reliability of the pressure refueling system, two pressure refueling outlets are set on the pressure refueling line for each independent fuel tank, and a shut-off valve and a orifice are installed on the refueling outlet line.
(c) Independent control shown in Table 1 is carried out on the refueling shut-off valves which are labeled as 3A, 3B, 3C, 3D, 3E, 3F in Figure 1.It can maintain a high refueling flow rate in various situations to shorten refueling time and ensure that the refueling flow rate does not exceed the limit value.When the fuel quantity of a certain fuel tank reaches the target value, the refueling shut-off valves inside the tank will be closed.As traditional design, the refueling shut-off valves are opened synchronously, and the restriction of refueling flow rate is achieved through the orifices.The operational logic of refueling shut-off valves is shown in Table 1.The differences of the operational logic of refueling shut-off valves 3B and 3E between the proposed innovative solution and the traditional solution are indicated by * in Table 1, which reflects different design ideas.
(d) When setting each tank refueling quantity, the wing tanks shall be prior filled.When refueling to all fuel tanks, the wing tanks and central tank starts refueling simultaneously.Opened Opened Note, the content marked with * is only for proposed innovative design.For traditional design, it shall be "Opened".

The design process
Based on the conceptual scheme shown in Figure 1 and Table 1, the design process of the pressure refueling system is shown in Figure 2. The different operational scenarios have been taken into account.

The design objectives
The design objectives are as follows.
(a) It takes no more than 40 minutes to refuel the central tank and the wing tanks from zero to maximum quantity at the pressure of 50 psig.
(b) It takes no more than 40 minutes to refuel the wing tanks from zero to maximum quantity at the pressure of 50 psig.
(c) It is better to shorten the time cost of the refueling process as much as possible.

The design constraints
The design constraints are as follows.
(a) The fuel flow velocity at any section of the refueling line shall not exceed 7 m/s.(b) The fuel flow velocity at the outlet of the refueling line shall not exceed 1 m/s to avoid static electricity accumulation.
(c) At the end of refueling, the difference between the actual fuel quantity and the target refueling quantity shall be less than 100 kg for each fuel tank.A nominal fuel density of 0.8 kg/L is assumed.
(d) During the refueling process, the approved pressure range at the refueling adapter is between 35 psig to 55 psig.

The design scenarios
The design scenarios are as follows.
(a) To refuel all the fuel tanks using only one side refueling adapter, and prioritize ensuring that the wing tanks are filled to the maximum quantity.
(b) To refuel the wing tanks using a single side refueling adapter.
(c) To refuel all the fuel tanks using both sides of refueling adapters, and prioritize ensuring that the wing tanks are filled to the maximum quantity.
(d) The other scenarios in Table 1 are used to verify whether the design constraints are met.

The performance model
The refueling line network architecture of the innovative refueling system design is the same with that of the traditional refueling system design.The differences between them lies in the design ideas, which lead to the differences of system design parameters such as the orifices' geometry and the control logic of the shut-off valves.As shown in Figure 3, a 1-D performance model of the pressure refueling line network was developed based on AMESim software.The model can be used to analyze the performance of the traditional refueling system design and the innovative refueling system design as shown in Figure 1 and Table 1.
Under the guidance of the process shown in Figure 2, the main parameters of the model have been calculated and defined.The results are shown in Table 2.The detail of the calculation process is not displayed here.Due to the fact that the maximum fuel capacity of the central tank is much greater than that of the wing tank, the refueling time of the central tank plays a decisive role in the total refueling process.The size of the orifices of central tank refueling line is similar for the traditional refueling system design and the innovative refueling system design.However, there are significant differences in the size of the orifices of wing tank refueling line between the traditional system design and the innovative system design.

Simulation and discussion
Simulation on the pressure system performance at different refueling scenarios has been conducted based on the shut-off valve controlling scheme defined in Table 1.The simulation condition is to refuel Jet-A1 fuel to the target fuel tanks under 50 psig refueling pressure on a normal temperature day at sea level, and refueling process to all the target fuel tanks starts at the same time.Differences on pressure refueling performance between the innovative system design and the traditional system design are shown in Figure 4 to Figure 7.    Calculation results show that, for the traditional refueling system design, the refueling pattern of using single side of the refueling adapter is basically consistent with that using both sides of the refueling adapter, and the performance difference between them is relatively small.Therefore, for the traditional refueling system design, Figure 4 to Figure 7 only show the situation when using single side of refueling adapter.

Refueling process description for the innovative design
For the innovative design which has been shown in Figure 1 and Table 1, three commonly used automatic refueling scenarios were taken as examples to describe the refueling process.
(a) When refueling all the fuel tanks automatically by using the right side refueling adapter, and prioritize ensuring that the wing tanks are filled to the maximum quantity, the refueling process is shown in Figure 5 (a) and can be described as follows.Firstly, open the shut-off valves 3A, 3C, 3D and 3F on the refueling line, and keep the shut-off valves 3B and 3E closed.Secondly, start to refuel both wing tanks and central tank at the same time, and continue the refueling process until the central tank reaches the preset value.And then, close the shut-off valves 3C and 3D.Thirdly, continue to refuel the wing tanks until the fuel level of the wing tanks reaches the preset value.And then close the shut-off valves 3A, 3B, 3E and 3F.
(b) When refueling only the wing tanks automatically by using the right side refueling adapter, the refueling process is shown in Figure 6(a) and can be described as follows.Firstly, open the shut-off valves 3A, 3B, 3E and 3F on the refueling line, and keep the shut-off valves 3C and 3D closed.Secondly, start to refuel both wing tanks at the same time, and continue the refueling process until the wing tanks reach the preset value.And then, close the shut-off valves 3A, 3B, 3E and 3F.
(c) When refueling all the fuel tanks automatically by using the both side refueling adapters, and prioritize ensuring that the wing tanks are filled to the maximum quantity, the refueling process is shown in Figure 4(b) and Figure 5(b), and can be described as follows.Firstly, open the shut-off valves 3A, 3B, 3C, 3D, 3E and 3F on the refueling line.Secondly, start to refuel both wing tanks and central tank at the same time, and continue the refueling process until the wing tanks reach the preset value.And then, close the shut-off valves 3A, 3B, 3E and 3F.Thirdly, keep the shut-off valves 3C and 3D opened, and continue to refuel the central tank until the fuel level of the central tank reaches the preset value.And then close the shut-off valves 3C and 3D.

Refueling performance analysis
Based on the analysis to Figure 4 to Figure 7, it can be seen that the restrictions on refueling flow rate come from the following three aspects.
(a) The geometry of the orifice on the outlet of the branch refueling line.The throttling effect of the orifice can limit the fuel flow rate in the refueling line to a certain range, ensuring that the flow velocity in the refueling line does not exceed the limit under any possible operating condition.The optimization design for the orifices was carried out under the consideration that ensuring the flow velocity does not exceed the limit and increasing the total refueling flow rate as much as possible.In the scenario of maximum refueling quantity, the difference of the maximum refueling performance between the innovative design and the traditional design is relatively small, as shown in Figure 4 and Figure 7(a).
(b) The number of the refueling line outlets.On the premise that the size of the orifices remain unchanged and the number of refueling adapters has not yet reached the limit of further increasing the fuel flow rate, the fuel flow correspondingly increases when the number of refueling outlet increases.Taking Figure 6 and Figure 7(c) as an example, when only the wing tanks are refueled, it takes more than 30 minutes to complete the refueling process from zero fuel to maximum quantity for the traditional system.However, it can significantly shorten the refueling process time to within 20 minutes for the innovative system.
(c) Number of the refueling adapters.With the number of the refueling adapter increases, the refueling flow rate correspondingly increases, but the fuel flow rate is also constrained by the number of the refueling line outlets and the orifice 's geometry.If the number of refueling line outlet increases, the total refueling flow rate can be significantly increased for the innovative system design when using both wing sides of refueling adapters to refuel all the fuel tanks, as shown in Figure 5.However, for the innovative system design that only the wing tanks are refueled (as shown in Figure 6) and the traditional system design, there are limitations in improving the overall refueling flow rate if only the number of refueling adapter increases.That is due to the constant number of downstream refueling line outlets and the size of orifices on the refueling line.But it can reduce the fuel flow velocity in the refueling line section where the refueling adapters are located.In addition, setting the refueling adapters on both the left and right wings or setting a refueling adapter on one side as an aircraft optional item is beneficial for improving refueling operation flexibility.

Conclusion
This article conducts pressure refueling system design and analysis for a three-tank layout wide body aircraft.The main conclusions are as follows.
(a) The refueling flow rate can be maintained high in different refueling scenarios by optimizing the design of pressure refueling system, especially the control logic of the shut-off valves in the inner side of the wing tank.It can improve the operating performance of the refueling system.
(b) Optimizing the refueling strategy can reduce the refueling process time cost in the conditions that the central tank is not fully filled or only the wing tanks are refueled.It can improve the operational efficiency of airlines and airports.
(c) Setting the refueling adapters on both wings of aircraft or setting one side of refueling adapter as an aircraft optional item can improve the operational adaptability of the aircraft at different airports.

Figure 3 .
Figure 3. Pressure refueling 1-D fuel line performance model based on AMESim software.
fuel line in which refueling adapter is installed mm 101.6 101.6 Length of the fuel line in which refueling adapter is installed m (a) Innovative design with one side adapter (b) Innovative design with both side adapters (c) Traditional design with one side adapter

Figure 4 .
Figure 4. Variation of onboard fuel quantity over time when refueling from empty to 120 m 3 .

Figure 5 .Figure 6 .
Figure 5. Variation of onboard fuel quantity over time when refueling from empty to 100 m 3 .

Figure 7 .
Figure 7. Variation of onboard fuel quantity over time when refueling from empty.

Table 1 .
Shut-off valve controlling scheme at different scenarios.

Table 2 .
Pressure refueling system model parameter.