Aircraft cabin flow pattern under unsteady air supply

Abstract

Maintaining a suitable cabin environment for an aircraft at cruise requires constant extraction of fresh air from the engine, resulting in a loss of thrust. An effective air delivery method for ventilating the cabin will minimize such loss and thus improve the fuel economy of an aircraft. This study investigates an unsteady air supply method and finds that the flow pattern becomes transitional under periodic air supplies. This transitional feature leads to lower average temperature and CO 2 concentration levels than the conventional steady supply condition with the same amount of fresh air. Unsteady air supply shows the promise in improving the fresh air delivery efficiency inside the cabin. I. Introduction safe and healthy cabin environment is essential for commercial airlines since modern turbofan planes are able to cruise at an altitude of over ten kilometers where the local air temperature and pressure could reach respectively -55°C and 30Kpa. Obviously passengers are not survivable under such air conditions and the ECS (environment control system) is employed to provide conditioned air for the cabin. One widely used type of ECS works by bleeding air from the compression stages of the engine and supplying, after cooling, filtering and ozone removing, to the cabin to control the internal temperature, pressure and contaminants. A higher air supply rate may achieve better cabin air quality, but at the price of a greater engine thrust loss due to the bleed and consequently an increase in fuel consumption. A more efficient and effective method of delivering conditioned air to the cabin will have the benefit of improved fuel economy and is worth some research efforts. The prevailing air delivery pattern at present is the mixed ventilation. Fresh air is introduced at top level and travels towards the floor along the side or through the center part of the cabin depends on the direction of the inlet diffuser. After reaching the floor, part of the air leaves the cabin through outlets and the rest climbs up and meets the newly supplied air to form the recirculation. During the travel, fresh air diffuses, exchanges heat and momentum with the surrounding air and dilutes the contaminant generated from passengers. A relatively uniform temperature and CO 2 distribution is arrived within the whole cabin 1 . Such pattern is justified by some detailed investigations on cabin flowfields 2-4 thanks to the advances in flow visualization techniques including particle image velocimetry and volumetric particle streak velocimetry. These studies also indicate that the actual flowfield is unsteady in nature and greatly affected by the incoming air location, flow rate, temperature and the occupancy of passengers. Particularly, the buoyancy effect, which results from the local temperature difference, is a significant drive of the cabin airflow and contributor to the unsteadiness and should not be ignored in the study of cabin flowfields. Novel concepts of air delivery method have been proposed recently including the displacement ventilation and personalized ventilation 1,5,6