Experimental systems 47

There are two major experimental systems in this study: fluidization and visualization systems. Both have their own specific design and characteristics, which will be introduced separately in the following sections.

3.1.1 Fluidization system

All the experiments were performed 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 high downcomer of 38 mm×203 mm 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 3.1). A flip valve is used to control the solids flow rate. Air enters from 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, where they are fed into the riser again. Solids escaped from the first cyclone pass into the secondary cyclone for further 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. High pressure steam is supplied to the windbox of the riser with the primary air to eliminate the static electricity so as to avoid the misleading effects inside the riser.

Figure 3.1 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

3.1.2 Visualization system

The visualization system developed in the present study consists of three major parts: light source, high speed video camera and image processing and analyzing system (see Figure 3.2).

Figure 3.2 Visualization system

Light source

A 500Watt quartz halogen bulb (4-5/8’’ T-3 lamp, L-16, The Designers Edge, USA) with a lifetime of 1500 hours is selected as the light source (as shown in Figure 3.3). The reasons for the selection are that halogen bulb have higher luminance (Ellenberger and Young, 2000) and possess constant brightness on constant voltage (Zubler and Mosby, 1959). Moreover, the illuminance of the bulb is about 95000 lx, which eliminates the hotspot appearing in the images. 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 wall of the CFB riser from the radiation of the lamp. A digital illuminance meter or lux meter (LX-1330B, Easy Life Product, Hong Kong) is used to measure the illuminance before the experiment to make sure that every image is shot under the same luminance flux (see Figure 3.4).

CFB Riser Video Camera Platform Black box Images Monitor & Storage IEEE 1394 Cable Images Analysis Halogen Lamp Diffusion Panel 19 mm

Figure 3.3 Quartz halogen bulb

Figure 3.4 Digital illuminance meter

Figure 3.5 High-speed video camera

High speed video camera

The high speed video camera is the MotionScope M2 from Redlake (as shown in Figure 3.5). The camera allows frame rates up to 16000 fps (frame per second) 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 (see Figure 3.6), contains special self-calibrating circuitry that enables it to reduce its own column-wise fixed-pattern noise. It also has an

IR cut-off optical filter to block infrared light from entering the optical path so as to avoid its interference on the actual image. A Pentax C21228TH 12.5mm F1.8 manual lens is chosen to capture images of solids flow in the riser. During the whole experimental time span, the camera with the lens and the section of videoed column are covered by a black box to avoid the disturbance of external lightings.

Figure 3.6 1.3-megapixel CMOS active-pixel digital image sensor

From the video camera settings, it is known that for the resolution of the camera’s CMOS sensor, the higher the resolution, the lower the frame rate. Therefore, considering the frame size, speed and the illuminance of backup light, the frame speed of 2000 fps with a resolution of 1280×256 pixels and 500 μs shutter time were 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. With these settings, the valuable information, like flow patterns, shape and behavior of clusters, can be provided without any intrusion from the original recorded images. At least 100 images were taken under every operating condition at each shooting height to guarantee the statistical credibility of the observed results.

Image processing and analyzing

A desktop is used for real-time video monitoring and image storage and a laptop for digital image programming analysis. Through a standard Firewire (IEEE-1394) interface, the software from Redlake, Motionscope 2.0.3 allows the shooting video to be monitored in real-time and image sequences captured by the camera can be transferred to the

desktop and stored as well. Self-developed MATLAB programs enable the original images to be transformed into binary images so as to obtain the cluster equivalent diameter. The image arithmetic and logical operations enable the solids holdup inside the cluster and cluster velocity to be determined.