A study on flow distribution characteristics of heat exchanger module integration scheme for aircraft environmental control system based on CFD

With the rapid development of the civil aviation industry, the performance requirements of the environmental control system for domestic commercial large aircraft are gradually improved, and the ram air inlet heat exchanger system is its main component. Generally speaking, a ram air heat exchanger system mainly includes an air conditioning system heat exchanger, air preparation system heat exchanger, and auxiliary cooling system heat exchanger. The integration mode of the heat exchanger will directly affect the flow and flow distribution characteristics of the ram air heat exchanger system. This paper develops three integration schemes referring to the integration mode of heat exchangers in Boeing 787 and Airbus 350 aircraft. Based on Fluent software, the calculation and simulation of three integrated scheme models under the seven working conditions of taking off, climbing, cruising, descent, hovering, emergency, and landing during flight were carried out, and the flow distribution and flow resistance values of different schemes were obtained, which were compared to meet the heat transfer requirements.


Introduction
With the rapid development of the civil aviation industry, the performance requirements for domestic commercial large aircraft's environmental control systems have gradually increased.Ram air is provided as a source of cooling and air supply, and is an important component of the environmental control system.The heat exchanger is an important refrigeration component, and its working efficiency will directly affect the compressor and turbine.Generally speaking, the ram air heat exchanger system mainly includes air conditioning system heat exchanger, air preparation system heat exchanger, and auxiliary cooling system heat exchanger, and the integration method of the heat exchanger will directly affect the flow and flow distribution characteristics of the ram air heat exchanger system [1].This paper conducted relevant research on the heat exchanger system and conducted CFD simulation calculations for the heat exchange requirements of the heat exchanger system under different flight conditions.CFD technology uses various numerical experiments by setting different flow parameters for comparison.The process is not limited by physical and experimental models and has low cost and high flexibility.It can provide detailed and complete information in the design phase to guide the construction of the system model.

Typical thermal management of commercial airplanes based on the thermal design method
In the entire ram-air heat exchanger system of the aircraft, there are generally five heat exchangers, which are the secondary heat exchanger of the air conditioning system, the primary heat exchanger of the air conditioning system, the auxiliary cooling system heat exchanger, the secondary heat exchanger of the air preparation system, and the primary heat exchanger of the air preparation system.The function of the secondary and primary heat exchangers of the air conditioning system is to regulate the temperature in the aircraft cabin [2]; The function of the auxiliary cooling system heat exchanger is to solve the cooling and heat dissipation problem of electronic equipment in the aircraft cabin; The function of the secondary and primary heat exchangers of the air preparation system is to control and regulate the temperature of the nitrogen generated in the nitrogen generation system.The schematic diagram of the integration scheme between the classic civil aircraft Boeing 737 and Airbus 320 is shown in Figure 1

Modeling of modular heat exchanger
After understanding the five heat exchangers of the ram air heat exchanger system, an integrated solution for the ram air heat exchanger system needs to be designed.After researching the ram air heat exchanger systems of Boeing 787 and Airbus 350 aircraft, we designed the integrated heat exchanger system concerning their integrated solutions.According to the refrigeration demand and the integrated heat exchanger design idea for the cold-side pressurized airflow channel, the three modular integrated schemes and characteristics of the three heat exchangers for the three systems are determined, as shown in Table 1 [4].Table 1.Design schematics of different integration schemes.

Study on flow distribution characteristics
Due to the complexity of high-altitude aircraft flight conditions, to conduct in-depth research on the integrated scheme of modular heat exchangers, simulation calculations were carried out on the flow distribution characteristics to determine whether the heat exchange requirements were met.The following seven operating conditions were selected for study: taking off, climbing, cruising, descent, landing, hovering, and emergency [5].
With given inlet and outlet conditions, a pressure-based solution method was selected, and the standard k-e turbulence model was used.The inlet and outlet pressure and temperature boundary conditions were changed to simulate the working conditions of the heat exchanger under different operating conditions.Finally, the mass flow rate of the heat exchanger was obtained as a function of altitude, and the pressure drop and velocity changes of the gas flowing through the heat exchanger as well as the total flow rate and total heat transfer power were simulated as the functions of altitude [6][7][8][9].

Scheme A
We select the corresponding section and calculate the gas flow through each heat exchanger in Fluent.The results are shown in Figure 2.    In summary, it can be seen that the pressure change of the gas before and after flowing through the heat exchanger is small and uniform, and the gas flow level within the heat exchanger component is relatively clear.Although there is a swirling phenomenon in the ram chamber, the gas flow at the outlet is relatively stable.Scheme A has the highest air mass flow rate through the auxiliary cooling system and primary heat exchanger of the air conditioning system and has a relatively high heat transfer power during taking off, cruising, hovering, and landing phases with a relatively low heat transfer power during emergency phases.

Scheme B
We select the corresponding section and calculate the gas flow through each heat exchanger in Fluent.The results are shown in Figure 5.  Taking the cruise as an example, the pressure cloud chart and trace chart of Scheme B are shown in Figure 7.Under various working conditions, there is a noticeable pressure change in the integrated area of the modular heat exchanger.There is a slight difference at the inlet due to the shunting effect.The pressure change from the inlet to the integrated area of the heat exchanger is slight, with a uniform distribution of pressure in the ram chamber and branch pressure.There is a sudden change at the junction, and the gas flows out more smoothly after passing through the ram chamber.The pressure variation of gas flow through the heat exchanger is small and uniform, and the gas flow level in the heat exchanger assembly is clear.There is a swirling phenomenon in the ram chamber, but the gas flow at the outlet is relatively stable.Scheme B maximizes the mass passing through the primary heat exchanger of the air conditioning system, with a maximum heat transfer power of 225. 9 kW during the hover phase and a minimum heat transfer power of 14. 0 kW during the emergency phase.

Scheme C
We select the corresponding section and calculate the gas flow through each heat exchanger in Fluent.The results are shown in Figure 8.The pressure change of the gas before and after flowing through the heat exchanger is small and uniform, and the gas flow level in the heat exchanger component is relatively clear.There is a swirling phenomenon in the stamping chamber, but the gas flow at the outlet is relatively stable.During the hovering and climbing phases, the heat transfer power is high, reaching 203 kW during the climbing phase, and the minimum heat transfer power during the emergency phase is 12.6 kW.

Conclusion
Firstly, the flow rates of the three schemes under different flight conditions are compared and analyzed.Due to the same inlet and outlet temperatures, it is easy to see that the integrated solution with the highest flow rate has the highest heat exchange, as shown in Figure 11.Under the hovering condition, the total flow rate of Scenario A is slightly lower than Scenario B and Scenario C.However, Scenario A has significant advantages over Scenarios B and C for other operating conditions.When analyzing and comparing the secondary and primary heat exchangers in the air preparation system of A, B, and C ramair heat exchanger systems, the cold side inlet flow rates of the corresponding systems are the same under corresponding working conditions because the two are designed in tandem.Due to the temperature increase after heat exchange between the cold and hot sides of the secondary heat exchanger, the warmed gas enters the primary heat exchanger, reducing the cold flow of the gas.In the primary heat exchanger, the heat transfer efficiency is reduced due to the reduced temperature difference in the heat exchange, leading to a reduction in the overall heat exchange amount.The cruise phase is the longest phase of the flight process.We select this phase and compare the flow resistance differences of the three different heat exchangers, as shown in Table 2  It can be concluded from the calculation results that the heat exchanger in the secondary cooling system of Scheme A has a significantly higher flow resistance than that of Schemes B and C, but the other heat exchangers have significant advantages over Schemes B and C in terms of flow resistance performance.It can be concluded from the previous paragraphs that the secondary cooling system with the maximum flow in the heat exchanger of Scheme A is much more suitable for the cooling system, compared to Scheme B and Scheme C under the same conditions.In summary, under the same conditions, Scheme A is the most suitable heat exchanger system.

Figure 1 .
Figure 1.The schematic diagram of the integration scheme.

Figure 2 .
Figure 2. Flow distribution of Scheme A. The heat transfer power change is shown in Figure 3.

Figure 3 .
Figure 3. Heat transfer power of Scheme A.Taking the cruise as an example, the pressure cloud chart and trace chart of Scheme A are shown in Figure4.

Figure 4 .
Figure 4. Cloud map distribution of scheme A.

Figure 5 .
Figure 5. Flow distribution of Scheme B.The heat transfer power change is shown in Figure6.

Figure 6 .
Figure 6.Heat transfer power of Scheme B.

Figure 7 .
Figure 7. Cloud map distribution of scheme B.

Figure 8 .
Figure 8. Flow distribution of Scheme C. The heat transfer power change is shown in Figure 9.

Figure 9 .
Figure 9. Heat transfer power of Scheme C.Taking the cruise as an example, the pressure cloud chart and trace chart of Scheme C are shown in Figure10.

Figure 10 .
Figure 10.Cloud map distribution of scheme C.

Table 2 .
below: Comparison of scheme flow resistance.