Outside humidity treatment is a main contributor to aircraft ECS (environmental control system) power requirements. To reduce this power, it is important to know exactly how to not exceed outlet humidity at the pack outlet. Liquid water at Low Pressure ducting participates to cabin cooling but, if this quantity is too high, a fog can be seen.
The purpose of the “Humidity Optimization Tool” project was to develop a tool able to predict generation and growth of water fogs in the ECS, in order to avoid them. The project was separated into three sub-topics:
- Determination by tests of a visibility fog criteria based on physical properties (air speed, liquid water content, blowing temperature, droplet size, etc.);
- Development of a 0D tool to predict the size of the water droplets (liquid or ice) downstream a: (i) Heat exchanger, (ii) Pack outlet turbine and (iii) Mixmanifold;
- Validation of this tool.
The proper implementation of this research required two-phase flow expertise in two different fields: (i) numerical modelling and (ii) measurement techniques.
The energy consumption cost of Aircraft Environmental Control Systems (ECSs) reaches 1.000 million euro per year. These systems are responsible for the conditioning and quality of the air passengers are breathing. Besides this, their performance also has to take into account some primary safety issues. One example is the avoidance of condensation at landing windows in helicopters.
- The technology used at ECSs is well known and mature, regarding machinery and humid air behaviour. On the contrary, for the processes that include condensation, the prediction of some key aspects is at the edge of scientific knowledge. These include: (i) the condensation droplet number and size, (ii) the fog evolution and visibility, as well as (iii) the fraction of liquid water laden on the flow in respect to the fraction wetting the walls. This is the reason why this Humidity Optimization project for Aircraft ECSs had been commissioned within the Clean Sky Joint Undertaking structure by the EU-Commission, DG Research.
- The structure of the project was divided in three blocks that have achieved the following advancements in respect to the previous state of the art:
A) Fog visibility criteria and description of the fog plume generated by an axisymmetric nozzle:
- Development of a “Fog entity” parameter criterion;
- Capability and means to model the generation and evolution of an axisymmetric nozzle fog;
- Capability and means to predict the visual impact of the fog plume.
B) Generation of a tool for prediction of flow downstream of selected ECS elements:
- Module for two-phase flow prediction at the exit of a generic turbine;
- Module for two-phase flow prediction at the exit of a mix-manifold;
- Module for two-phase flow prediction at the exit of a post-turbine Heat exchanger.
C) Experimental validation of the tool:
- Test campaign for validation of turbine outlet and post-turbine heat exchanger outlet;
- Test campaign for validation of the axisymmetric nozzle fog model;
- Test campaign for validation of generic mix-manifold outlet.
The main result of the project was an experimentally validated tool able to provide the commented relevant two-phase flow characteristics. This is a key element for improved detail-design of ECSs. In addition, a database with the valuable experimental data from the test campaigns had been compiled. This data is useful for studies or validation of any further issue related to these two-phase flows. Also the state of the art regarding numerical calculation of two-phase flow had been improved along this project. These techniques can be applied to a large deal of technological fields where these flows play a relevant role, including environmental and health related issues.
The impact potential of this project on the society surpasses its cost by orders of magnitude. Just a saving of 1% in the energetic performance of aircraft ECSs would save 10 million euro per year (i.e. 36.000 tons/year of kerosene). This means a reduction of the emissions to the atmosphere of 100.000 tons/year of CO2 and a saving of 400 TWh/year of energy. To this primary impact, additional benefts from the application of this knowledge to any other two-phase flow technological field should be taken into account.