Optimal choice of the hight pressure water separation structure and para-meters

Аuthors

Starostin K. I.

Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

e-mail: ki-star1969@yandex.ru

Abstract

The Environmental Control Systems (ECS) of most modern aircraft include High-Pressure Water Separation (HPWS) loop schemes. Fig. 1 shows a typical layout of such HPWS loop scheme.

Fig. 1. Layout of a loop scheme of high-pressure water separation.

where C, R are the heat exchangers (C is the condenser, R is the reheater); Т1, Т2 are the first and second cooling stages of multi-stage turbo-machine; MS-1, MS-2 are the moisture separators (dryer dehumidifiers).
The functioning of HPWS loop scheme during humid air processing influences the dependence of its workability on the altitude and speed of the airplane flight as well as the operability of separate ECS units. Thus the purpose of this paper is to calculate ECS workability areas for various values of altitude and speed of flight. The calculation is performed by using the available mathematical model of HPWS loop scheme and the computing program that is based on this model. The paper also aims at considering possibilities of the system optimization in terms of starting and equivalent weight criteria.
The mathematical model of the considered system includes the general equation of humid air enthalpy as well as a system of the equations, which are used for calculation of air parameters at the each unit outlet.
The equations were solved by using the modified method of chords, which was developed with taking into account the function features connected with the processes of moisture evaporation and freezing.
The developed program was used to obtain the workability areas depending on the altitude and speed of flight.
A method of evaluation of operability of separate ECS units was developed and applied to analyze system functioning in the course of the numerical experiment. This method includes tests and criteria for each unit.
Various ECS were compared according to the criteria of starting and equivalent weight with the help of the developed program. This calculation was performed for several versions of the system structure.
The conducted research showed that the workability area of ECS with the HPWS loop scheme is narrow when humidity has no profound effect on system units functioning (Fig. 2),

Fig. 2. ECS workability areas depending on altitude and speed of flight.
where number “1” is the area for the air cycle ECS with one-stage cooling turbine without the HPWS loop scheme; number “2” is the area for the ECS with the HPWS loop scheme, which includes a condenser, a reheater and a one-stage cooling turbine; number “3” is the area for the ECS with the HPWS loop scheme, which includes a two-stage cooling turbine.
However the HPWS loop scheme starts to gain an advantage upon the transition of the system outlet air temperature to negative values at low flight altitudes. This advantage emerges due to the reduction of the required conditioning air consumption for the HPWS loop scheme. The systems without the HPWS loop scheme are forcedly limited to positive values of the output temperature for all flight modes.
The results of the performed calculations are adduced in Fig. 3. Fig. 3 shows that there is a strong correlation between the adjusting weight of the system and its starting weight.

Fig. 3. The starting and equivalent weight of the considered ECS types.
where type #1 is the air cycle ECS with one-stage cooling turbine without the HPWS loop scheme; type #2 is the ECS with the HPWS loop scheme, which includes only a condenser; type #3 is the ECS with the HPWS loop scheme, which includes a condenser and a reheater; type #4 is the ECS with the HPWS loop scheme, which includes a two-stage cooling turbine.
Fig. 4 presents the calculated areas of system operability as well as the dependence of the total weight of the condenser and reheater on their efficiencies (h_c and h_R).

Fig. 4. Dependence of the total weight of the condenser and reheater on their efficiencies h_c and h_R.
The analysis allowed to establish the following: to optimize ECS with HPWS loop scheme in terms of the weight criteria it is desirable to have the values of condenser and reheater efficiencies close to each other.
During the research an assumption was made that the air-flow rate does not change within the allocated ECS section and system functioning mode is considered to be stationary.
The usage of the HPWS loop scheme as a part of the ECS narrows its workability area. Therefore, it is expedient to provide the ECS with bypass lines. These lines would allow the system to go around some of the units of the loop scheme during the flight at the altitudes, at which the amount of moisture in the atmospheric air is insignificant.
The comparative calculations showed that the system with HPWS loop scheme, which includes a condenser and a reheater, is optimum with respect to the criteria of starting and equivalent weight. Usage of the loop scheme with the two-stage cooling turbine can be justified by the increase of operation reliability of the heat exchangers.
Application of the proposed numerical methods for modeling of aviation ECS functioning at the early design stage can undoubtedly be useful and serve a practical purpose for organizations connected with manufacturing and operation of these systems.

Keywords:

humid air, high-pressure water separation loop sheme, heat exchanger, environment control system, workability area, installed mass, starting and equivalent weight criteria

References

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