The processing chamber, which is made of 6.0-mm thick quartz glass to allow for the observation of the vortex flow in it, is cylindrical and has a diameter of 291.0 mm. The vertical distance between the jet inlet and exhaust ports of the processing chamber is 200.0 mm. The chamber can be installed with a horizontal top or an inclined top. For the horizontal top the distance between the injection pipe and the circular disk are, respectively, 10.0, 15.0, 20.0 and 30.0 mm (Fig. 2.3(a)). For the case with the tapering top the top of the processing chamber is inclined linearly downwards in the radial direction. More specifically, the top-to-disk separation
distance is reduced from 20.0 mm at the exit of the injection pipe to 10.0 mm at the chamber side (Fig. 2.3(b)). To facilitate the flow visualization, the two chamber tops are both made of acrylic plates. Air is injected vertically downwards from a long straight circular pipe into the cylindrical chamber along the axis of the chamber and impinges directly onto the heated disk. The air flows first over the heated disk, then moves through the annular section of the chamber, and finally leaves the chamber via twenty circular outlets of 12.7 mm in diameter opened at the bottom of the chamber (Fig. 2.2(a)). The chamber is sealed to prevent any gas leakage. The top, bottom and side walls of the chamber are thermally well insulated to reduce the heat loss from the processing chamber to the ambient by covering the entire chamber with a superlon insulator of 100.0-mm thick. The insulator can be opened during the flow visualization.
The heating unit is designed to maintain the circular disk at the preset uniform temperature during the experiment. It is composed of a 10.0 mm thick high purity circular copper plate of eight-inch in diameter, acting as the disk, placed on 15-mm thick stationary copper disk. The 10.0 mm thick circular disk is rotated by A.C. motor driver. The heater attached onto the back side of the stationary copper plate is divided into 3 concentric zones. The heating unit consists of a resistance heating element, a holder and an insulator. The holder which is made of stainless steel can support the resistance heating element and copper disk at the high temperature. The heater attached onto the back side of the copper plate is divided into 3 concentric zones (Fig.
2.4). Each zone is independently heated by a power supply with the D.C. current passing through the nickel coil placed on the stainless steel holder. The entire heating unit is then placed on a Teflon plate. Additionally, to reduce the heat loss from the sidewall of the copper plate and Teflon plate, the lateral surface of the entire heating unit is wrapped with a 16.0 mm thick thermal insulation layer of superlon. A proper
control of the voltage from each power supply allows us to maintain the copper plate at a nearly uniform temperature. Moreover, the copper plate temperature is measured by several corrected and calibrated T-type thermocouples at selected detection points located at 1-mm below the upper surface of the copper plate, which are fixed the detection points through the small holes drilled from the backside of the plate.
The gas injection unit consists of a 2HP air compressor, a flow meter, a smoke generator, filters, pressure regulator, connection pipes and injector. In the experiment, air is drawn from the ambient by the compressor and sent into a 300-liter and 100-psi high-pressure air tank and is filtered to remove moisture and tiny particles. The installation of the high-pressure air tank is to suppress the fluctuation of the air flow and extends the life of the compressor. Then, the air is mixed with smoke tracers in the smoke generator, and regulated by the pressure regulator and is later injected into the processing chamber through the straight circular injection pipe which is coaxial with the processing chamber. The downward vertical air jet issuing from the pipe outlet impinges directly onto the heated plate. In the present study, two injection pipes with diameters, 10.0 and 22.1 mm, are tested and the straight portions of the pipes are both 600.0 mm long. The length of the injection pipes is selected to ensure that they are long enough to allow us to have a fully developed air flow at the exits of the injection pipes. The air temperature at 600.0 mm upstream of the exits of the injection pipes is measured by a corrected and calibrated T-type thermocouple. The measured value is considered as the temperature of the air injected into the processing chamber since the whole injection pipe is thermally well insulated by a superlon insulation layer of 16.0-mm thick.
A smoke-tracer flow visualization technique is employed to observe the flow patterns resulting from the jet impinging onto the heated disk in the cylindrical chamber. The air flow pattern in the chamber is illuminated by the vertical and
horizontal plane light sheets produced by passing parallel lights from an overhead projector through two adjustable knife edges. The experimental system is located in a darkroom to improve the contrast of the flow photos. The time variations of the flow pattern during the entire transient stage from the top and side views are recorded by the Sony digital video camera DCR-PC100. The recorded images are later examined carefully in a personal computer.