CHAPTER 2
EXPERIMENTAL APPARATUS AND PROCEDURES
In order to conduct the experiment at reasonably low cost, we use air as the working fluid to replace the inert gases normally employed in real MOCVD processes.
In view of the similar thermodynamic and thermophysical properties for various gases, the results obtained here are still applicable to the MOCVD systems. The experimental apparatus and procedures used in the present study to examine how the installation of a showerhead at the injection nozzle affects the characteristics of the mixed convective vortex flow resulting from a gas jet impinging onto a horizontal heated disk confined in a vertical cylindrical chamber are modified slightly from that established in our previous study [15].
2.1 Experimental Apparatus
A schematic of the experimental system is shown in Fig. 2.1. The test section includes a circular disk held horizontally in a vertical cylindrical chamber with the gas injected vertically downward through a showerhead into the chamber. Note that for clear illustration the plots in Fig. 2.1 are not directly proportional to the actual dimensions of the apparatus. The present experimental system consists of four major parts: (1) gas injection unit, (2) processing chamber, (3) heating unit, and (4) flow visualization unit. The major parts are briefly described in the following.
Processing chamber: The processing chamber schematically shown by the top view and side view in Fig. 2.2, which is made of 6.0-mm thick quartz glass to allow
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for the observation of the flow pattern in the chamber, is cylindrical and has a diameter of 291.0 mm. The distance between the chamber top and bottom is 200.0 mm. The chamber top is made of an 15-mm thick acrylic plate and the showerhead contains 490 circular holes all having the same diameter dj of 0.5 mm and a pitch of 3.0 mm (Fig. 2.3). These holes form concentric rings and the showerhead has a diameter of 3-inch. To facilitate the flow visualization, air is injected vertically downward through the showerhead into the cylindrical 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. The outside surface of the chamber is thermally well insulated to reduce the heat loss from the processing chamber during the experiment. More specifically, the entire chamber is insulated with a superlon insulator of 10.0-mm thick. The insulator can be opened during the flow visualization experiment.
Heating unit: The heating unit schematically shown in Fig. 2.4 is designed to maintain the circular disk at the preset uniform temperature during the experiment. It is composed of 8-inch circular bakelite plate of 10.0-mm thick with a copper disk of 2-inch in diameter imbedded in its central portion. The copper disk is 11.0-mm thick and is placed on a thin heater of the same diameter which is heated by D.C. power supplies. The bakelite-copper plate and the heater are then placed on another bakelite disk. Note that the upper bakelite and copper disk surfaces must be at the exactly same horizontal level to avoid the wall jet flow to experience any step change between the surfaces. A proper control of the currents transferred from the power supplies to the heating coils leads to a nearly uniform copper disk temperature with a maximum deviation of 0.1℃ across the disk. The temperature of the copper disk at
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selected detection points is measured by four T-type thermocouples inserted into the disk by the small holes drilled on the backside of the disk. The locations of the detection points in the disk are 1-mm below the upper surface of the disk and are indicated in Fig. 2.5.
Gas injection unit: The gas injection unit consists of a 2 HP air compressor, a flow meter, a smoke generator, filters, a pressure regulator, connection pipes, a diffuser of 357-mm long, and a 3-inch showerhead. In the experiments, the 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 intends to suppress the fluctuation of the air flow and to extend the life of the compressor. The gas flow rate from the air tank is controlled by the mass flow meter and then, the air is mixed with smoke tracers in the smoke generator. Before injected into the processing chamber, the gas moves vertically downward into the diffuser and crosses the showerhead which is coaxial with the processing chamber. Finally, the small-diameter multiple air jets from the showerhead impinges directly onto the horizontal heated copper disk. In the present study, the diffuser is thermally well insulated by a superlon insulation layer of 16.0-mm thick to prevent heat loss from the air flow. The distance between the lower surface of the showerhead and the upper surface of the heated disk is varied from 20.0 to 40.0 mm. The air temperature at the inlet of the diffuser is measured by a T-type thermocouple. The measured value is considered as the temperature of the air injected into the processing chamber in view of the good thermal insulation over the diffuser.
Visualization unit: A smoke-tracer flow visualization technique is employed to observe the flow patterns resulting from the jet impinging onto a heated disk in the
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cylindrical chamber. The smoke is produced from burning incense prepared from sandalwood. The smoke is mixed uniformly in the smoke generator and is carried out by the inlet air and is sent into the cylindrical chamber. The gas 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 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 from the side views are recorded by the Sony digital video camera DCR-PC330.