Research on functioning of high-pressure water separation loop scheme during humid air processing

Аuthors

Starostin K. I.^*, Shustrov Y. M.^**

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

*e-mail: ki-star1969@yandex.ru
**e-mail: portartur34@mail.ru

Abstract

Purpose

Environmental Control Systems (ECS) of most modern aircraft include loop schemes of high-pressure water separation (Fig. 1).

Fig. 1. Loop scheme of high-pressure water separation: 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); 08 sections of air parameters calculations

The functioning of this loop during humid air processing is characterized by a number of specific features, which significantly influence the workability of separate units and the system as a whole. It is necessary to identify and analyze these features at the preliminary stage of ECS design. Thus the purpose of this paper is to compose the appropriate mathematical model of water separation loop scheme and create a program on its basis.
Final goal is to perform calculations (numerical studies) to identify the specific features of system functioning during humid air processing for various combinations of initial data.

Design/methodology/approach

Mathematical model of the considered system includes the general equation of humid air enthalpy and a system of equations for calculation of air parameters at each unit outlet in the loop scheme duct.
Enthalpy equation was solved with the usage of modified chord method developed with taking into account the specific features of the enthalpy function, which are connected with the physics of liquid evaporation and freezing. The numerical method for the equation system solution consists in successive iterations of enthalpy values in the loop scheme cross-sections towards their final values. This algorithm is similar to a checking calculation, which allows it to obtain the results on the particular modes of loop scheme functioning and model the transition modes. If it is taken into account that each of the iterations takes some time, it is possible to say that the calculation actually simulates the functioning of the system from the starting mode to the final.
A method of evaluation of workability of separate system units, which includes tests and criteria for each unit, was developed and applied to analyze system functioning during numerical experiment.
A series of numerical experiments was carried out with the help of the developed program. In this series the parameters of one or several units were changed with certain steps.

Findings

The distributions of air temperature along the duct for various initial data combinations were obtained. Comparative analysis of program calculation results with the results from similar programs and full-scale tests has shown that the model is highly reliable and the air temperature values along the duct differ by no more than 5 degrees from the reference results.
It was also discovered that certain combination of unit characteristics leads to unstable modes of system operation. Failures due to condenser and reheater icing are the most critical for the ECS system with three-stage cooling turbo-machine (one-stage turbine). Usage of four-stage cooling turbo-machine (two-stage turbine) within the loop scheme allows to significantly reduce the probability of icing of the heat exchanger components.
However, it also creates additional problems with moisture in the output section of the system. The research has shown that workability of the systems with high-pressure water separation scheme can only be achieved via precise and proper combination of unit characteristics and air parameters in the inlet section. Unsteady operating modes and icing possibility significantly affect workability and efficiency of modern aircraft ECS. This results in more stiff requirements to the ECS subsystem of preliminary thermal air processing and subsystem of automatic adjustment of ECS operational parameters.

Research limitations/implications

An assumption was made that air-flow rate does not change within the allocated ECS section and system functioning mode is considered to be stationary.

Originality/value

Application of the proposed chord method reduces the required iteration number by 1.5...2 times compared to a «normal» chord method and provides two-sided convergence. This method also helps to absolutely exclude the emergence of iterates out of the allocated interval bounds, which in turn increases the convergence robustness.
Application of 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, the mathematical model, calculations, high-pressure water separation loop sheme, environment control system, temperature, enthalpy

References

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