4 A novel method based on image processing to visualize clusters
4.2 Experimental 66
4.2.1 Circulating fluidized bed and operating conditions
The experiments were conducted in a cold-model rectangular CFB system that consists of a 7.6 m high riser with a rectangular cross section of 19 mm×114 mm and a 3.85 m height of 38 mm×203 mm downcomer with a 1.85 m cylindrical storage column of 203 mm i.d. on top, including two cyclones, a bag house filter, flapper valves on the cylindrical storage section for solids circulation rate measurement and a gas distributor at the bottom of the riser (Figure 4.1). A flip valve is used to control the solids flow rate. Air enters into the bottom of riser through the distributor, mixes with the particles fed from the downer and carries the particles up the riser into the first cyclone where solids are separated from the air. The separated particles flow down into the downcomer, from where they are fed into the riser again.
Figure 4.1 Schematic diagram of the rectangular circulating fluidized bed Bag House Filter
Secondary Cyclone Primary Cyclone Narrow Rectangular Downcomer Narrow Rectangular Riser Flip Valve Flapper Valves Air 114 mm 19 mm Cylindrical Storage Column 203 mm 38 mm Air Distributor
Solids escaped from the first cyclone pass into the secondary cyclone for another separation, with the last gas-solids separation being carried out by a bag filter. Fine particles collected at the bottom of the secondary cyclone and the large capacity bag filter are also fed back to the downcomer. The bed material was FCC particles of 67 μm (Sauter mean diameter) with particle density 1877 kg/m3. Solids circulating rate, Gs,
ranged from 50 to 150 kg/m2s, while the superficial gas velocity, Ug, ranged from 3.0 to
9.0 m/s.
Figure 4.2 Visualization system
4.2.2 Visualization system
The visualization system consists of a high speed video camera, a light source, real-time video monitoring and image storage and digital image programming analysis, as shown in Figure 4.2. In order to eliminate the entrance and exit effects, the shooting position is focused on the upper fully developed region at the 5.33 m height of the riser. The camera and the section of videoed column are covered by a black box to avoid the disturbance from external lightings. A 500 Watt quartz halogen bulb with irradiation of strong lights about 95000 lx is used as the experimental light source. A diffusion panel is applied to make the recorded area uniformly illuminated and eliminate undesirable shadows as well as intensity gradients. The panel also acts as an insulator to prevent overheating of the
CFB Riser Video Camera Platform Black box Images Monitor & Storage IEEE 1394 Cable Images Analysis Halogen Lamp Diffusion Panel 19 mm
wall of the CFB riser from the radiation of the lamp. Through a standard Firewire (IEEE- 1394) interface, the software from Redlake, Motionscope 2.0.3 allows the shooting video to be monitored real-time and image sequences captured by the camera can be transferred to a computer and stored as well. Self-developed MATLAB programs enable the image process and frame-by-frame analysis later.
4.2.3 Description of the apparatus
The high speed video camera is the MotionScope M2 from Redlake. It allows frame rates of up to 16000 fps and a maximum resolution of 1280×1024 pixels at 500 fps leading to a record time of 4s for the built in memory. Its equipped sensor, MI-MV13, contains special self-calibrating circuitry that enables it to reduce its own column-wise fixed- pattern noise. The camera also has an IR cut-off optical filter to block infrared light from entering in the optical path so as to avoid its interference on the actual image. A Pentax C21228TH 12.5mm F1.8 manual lens was chosen to capture images of solids flow in the riser. A 500Watt quartz halogen bulb (4-5/8’’ T-3 lamp, L-16, The Designers Edge, USA) with a lifetime of 1500 hours was selected as the light source due to its high luminance over the complete spectrum and constant light emission during its lifetime (Ellenberger and Young, 2000), and also their advantage of possessing constant brightness for constant voltage. A digital illuminance meter or lux meter (LX-1330B, Easy Life Product, Hong Kong) is used to measure the illuminance before experiment to make sure every image was shot under the same luminance flux.
Another point worth mentioning is that for the resolution of the camera’s CMOS sensor, the bigger the frame size, the lower the frame rate. Therefore, considering the frame size and the illuminance of backup light, the frame speed of 2000 fps (frame per second) and 500 μs shutter time are chosen during the whole experiment. Output images are uncompressed full frames without any loss of original information, which guarantees the precision of later image processing. Thus a prominent advantage of the visualization system is that some valuable information, like flow patterns, shape and behavior of clusters, can be provided without any intrusion from recorded images. At every operating condition, at least 100 images are taken in order to guarantee the statistical significance of the observed results.