Air conditioning in aircrafts is an important issue for improving energy performances of this mode of transportation whether we consider the energy consumption for air conditioning or the weight reduction of the aircraft.
The project objective is to provide in a medium term view predictive tools suited to the challenges that must be faced by aircraft industry to improve the performances of air conditioning system loaded in planes. The project will enable improving our knowledge of physical phenomena involved in these air conditioning units used in aircrafts, and whom conception is carried out by specialized firms, as Liebherr Transportation Systems. For reducing the weight of these systems, the project emphasizes on the water extractor (condenser) that allows drying air conditioning. In this project, different studies are proposed on the following two scientific issues which are significant when considering the water extractor performances, namely droplet condensation in a channel, and primary atomization of a liquid by a high speed gas flow. <br />The first part of the project on the condensation of droplet and liquid film <br />will rely on the skills previously developed in IMFT on the Direct Numerical Simulation of two-phase flows with phase change. In the second part of the project, an experimental study on the primary atomization of a liquid film by an internal gas flow is proposed. These two studies will allow proposing simplified models on liquid condensation for various geometries (one droplet, several droplet, liquid film) and on primary atomization. These simplified models will be next integrated in 1D computational loops used by engineers in order to improve the reliability of the numerical predictions of these industrial solvers.
The direct numerical simulations of the liquid condensation are performed by using the home-made code DIVA (Dynamics of Interface with Vaporization and Atomization) which has been specifically developed for that kind of applications. In this part of the project, our objective is to carry out three dimensional direct numerical simulations with very refined grids in order to model accurately heat transfer and condensation for an arbitrary liquid interface shape (liquid film, single droplet on a wall, several droplets on a wall). A specific care has been devoted on the efficiency of the computational code in order to fully benefit of the computational power available for research works on numerical simulations, by using massively parallel supercomputers. Another important part of the project is an experimental study about the different liquid-gas flow regimes occurring in small millimeter-sized channels with rectangular section. This study will allow characterizing the size of the droplets which are atomized in the channel outlet depending on the flow regime. Droplets size is an important feature since the efficiency of the water extractor will depend strongly on this parameter. This work, based on high frequency imagery, will allow the characterization of the different atomization mechanisms and the measure of the droplets size and velocity by using image processing.
By considering a simplified configuration, in which a static liquid film interacts with a sub-cooled laminar flow of saturated vapor, a parametric study has been performed in order to analyze the impacts of the main dimensionless numbers (the Jakob number, the density ratio, the Reynolds number, the Prandtl number) on the Nusselt number (namely, the condensation rate). Thanks to this study a semi-empiric correlation defining the condensation rate at the liquid-gas interface depending on the problem parameters has been defined. Unsteady three-dimensional well-resolved numerical simulations, characterized by computational domains containing more than 1 billion points (1024x1024x1024 mesh), have been performed as well by using the DIVA code. In order to deal with this study, a coupling between different solvers was required: a solver to resolve the basic fluid dynamics equations (Navier-Stokes), a solver to deal with all the two-phase phenomena (interface captruring, the jump conditions), and a solver able to evaluate the heat transfer in all the computational domain as well as the condensation of the vapor phase at the liquid-gas interface. These solvers have been integrated in a code which allows to perform massively parallel numerical simulations. All these characteristics make DIVA, from a methodological point of view, one of the first code of this kind in the world.
An experiment able to reproduce the atomization process in the water extractor has also been realized.
Nown, the potential of the developed numerical methodologies must be fully exploited in order to improve our understanding of the condensation phenomena, which is one of the main objective for project follow-up, together with the possibility of deducing new correlations for more complex configurations by taking advantage of numerical simulations. In this way it will be possible to evaluate in a simple way the condensation rate, as well as the heat flux exchanged between a fluid medium and a solid wall, for a given device. For more complex configurations we mean configurations where a turbulent external flow can be considered and different kind of liquid-gas interfaces can be investigates (multi-drops, drop-film transition …). The already performed studies open to promising perspectives which could be exploited by means of other research projects on connected subjects.
To be submitted in the next week a letter on the condensation correlation
Invited speaker Gordon Research Conference – Micro and Nanoscale Phase Change Heat Transfer, Gavelston TX 2017
communications proposed to ICBCHT (International Conference on Boiling and Condensation Heat Transfer) 2018 at Nagasaki (condensation correlation)
communications proposed to ICBCHT (International Conference on Boiling and Condensation Heat Transfer) 2018 at Nagasaki (3D simulation on condensation)
In order to meet the requirements of economic and ecological performances from airline companies and civil society in general, aircraft manufacturers propose to resort to more efficient systems, leading to less fuel consumption. This impacts the aeronautical air conditioning, and can be applied also to railway sector. Thus, weight reduction, compactness improvement and better efficiency is a great challenge for air conditioning systems.
The air conditioning pack is equipped with a condenser and a water extractor that allows drying the air before its expansion in the turbine of the air-conditioning system in order to avoid the condensation of droplets, which would damage the blades of the turbine, and to keep the cockpit and cabin from fogging on ground operation and low altitudes. The water extraction loop is about 25% of the mass and volume of the air conditioning pack, and also contributes to its performances. A better understanding of the physical phenomena in this loop is therefore crucial for optimizing the system.
The aims of the COALA Project (COndensation And Liquid Atomization), is to improve the physical understanding and the prediction of the condensation and of the atomization of liquid films downstream the water extractor. This project focuses on two distinct phenomena:
. The condensation and liquid film formation in the mini channels of the condenser core
. The liquid film stability in the mini channels and atomization
These two distinct problems will be studied separately but in complementary ways by the Institut de Mécanique des Fluides de Toulouse. A numerical study is proposed in the project to quantify the condensation rate in the milimetric channels. An experimental study aims at studying the atomization into droplets of the liquid film by an airstream to quantify the velocities and the droplet size distributions at the outlet of the condenser. The modeling proposed by IMFT concerning the condensation rate in a mini channel and the droplet size distribution will be used by Liebherr to be implemented in a 0D model of the air-conditioning loop or in a 3D model of the water separator.
By joining academic skills (Institute of Fluid Mechanics of Toulouse) and industrial skills (Liebherr Aerospace Toulouse), this collaborative project targets the energetic optimization of air-conditioning system in aircrafts or trains.
Monsieur Sébastien Tanguy (Institut de Mécanique des Fluides de Toulouse)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IMFT Institut de Mécanique des Fluides de Toulouse
LTS LIEBHERR AEROSPACE TOULOUSE SAS
Help of the ANR 358,600 euros
Beginning and duration of the scientific project: January 2016 - 48 Months