Update: Feb 2003
The cabin air outlet system enables a very equally distributed low speed flow with excellent air exchange rates. Large amounts of air are blown noiseless and without annoying jets into the cabin. Aircraft specifications for the Airbus A330/A340 request maximum flow velocities of 0.25 m/s in the seat area. For providing a good separation between smoking and non smoking cabin sections, longitudinal flow must be avoided. For all flow settings and service conditions like heating or cooling, the cabin air exchange rate has to be large and the temperature layer differences should be smaller than 3.5oC. To match this purpose perfectly for all areas of the passenger cabin with a compact minimum weight system suitable for the limited available space of modern aircraft, the air outlet geometry must be designed carefully. Computational Fluid Dynamics codes (CFD) are used for calculating optimized cabin air outlet geometry, which includes outlet size and area, location and blowing angle, as well as air flow velocity and temperature. For development and validation of those CFD codes experimental data must be provided.
The Flow Visualization Technique
While there are many flow measurement and flow visualization techniques in use, just a few of them suit the needs of the very low speed cabin A/C flow. On the point based method's side there are laser velocimetry or probe measurements. On the other hand, there are various particle image velocimetry techniques (PIV/PTV) using flow visualization. To get a bunch of quantitative results for the large and complex shaped cabin, the Helium Bubble technique was chosen. Tiny little soap bubbles filled with a mixture of air and helium to balance their weight exactly to the weight of the surrounding air can easily be tracked and measured by means of digital image analysis methods. A/C flow velocities are in the range of a few centimeters per second up to a maximum of approximately 1.0 m/s at the air outlets. As these are well below the normal 1 to 10 m/s visualized by the Helium Bubble technique, special care has to be taken when producing the bubbles. Even the smallest buoyancy of the bubbles, due to a wrong soap/air/helium mixture, results in large up/down speed of the bubbles compared to the actual flow speed. It proved to be a very delicate task to adjust the gas mixture correctly and takes a lot of experience. Once the setting is correct and the bubbles are hovering in still air, the complete measurement can be performed in very short time. The regular procedure is filling the complete room with bubbles, then take away the helium bubble nozzle to let the disturbances decay which were introduced by producing the bubbles, and then grab a live video sequence of several seconds duration directly into the imaging computer memory (Figure 1). A video tape recorder is of no use because of its limited geometric resolution.
The experiments have been performed at a radial cut of the rear part of the cylindrical fuselage section of an Airbus A330/A340 mock-up. Using several spot lamps, an approximately 0.7 m wide light sheet was illuminated, covering the passenger overhead area from the windows to the middle seats and to the head section of the aisle. It is assumed that the flow is mainly 2-dimensional, i.e. the flow in longitudinal direction of the cabin can be neglected. The "picCOLOR" image analysis system, developed at F.I.B.U.S., has been used to perform all the video sequence grabbing and the post processing. A Hitachi KPM1 CCD camera yielded an image resolution of 768*512 picture elements (pixel). The 64 MBytes of memory of the system allowed grabbing of 7 seconds real time sequences at 25 Hz or 14 seconds of half speed sequences at 12.5 Hz which was a sufficiently high frequency for the low speed flow experiments. Several image sequences have been stored on hard disk for later processing. For covering the relatively large interesting part of the cabin at a fine geometrical resolution, the viewing area had been separated into 3 regions, the head room over the window seats, the head room over the middle seats, and finally the aisle. Three camera positions have been selected to obtain as little geometrical image distortion as possible. With a little overlap of these regions, this yields a resolution of about 2mm/pixel. Using subpixel algorithms, the final resolution has been enhanced to better than 0.2mm which results in a velocity resolution of better than 10mm/s. This is sufficient for the actual velocity range of 50 to 500mm/s.
Image Analysis Procedure
As the number of bubbles was not large enough to use frequency domain based correlation techniques, a simple and fast particle tracking velocimetry (PTV) algorithm has been developed. After performing some image enhancement and processing like background removing and binarization, the location of the bubbles has been determined with subpixel accuracy and the bubble velocity and therefore the local flow velocity has been determined from the displacement of the bubbles in subsequent images. The randomly scattered velocity vector fields are interpolated to produce equally spaced grid vector plots and vorticity contour plots. Using the complete image sequences of several hundred images, an interpolation over time was calculated (Figure 2).
The vector plot (Figure 2) shows the air jet at the cabin air outlet, in the head room of the aisle, blowing toward the edge of the center bin and producing a well defined flow over the middle seats with some tendency to rotate slowly downward onto the seats. Two rotating flow regions can be found in the upper aisle area and in the area between the middle and window seat rows. Velocities of approximately 0.05 m/s are measured over the window seats. All other areas show air velocities in the range of 0.1 to 0.25 m/s. Aircraft A/C specifications are met in the complete cabin. For flow calculation the "Star-CD" software of Computational Dynamics Ltd. was used. The results have been compared to the experimental data and show excellent agreement.
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