(1) Test section: The test section consists of an upper horizontal circular disk and a lower parallel circular heated disk. The upper circular disk is made of 9-mm thick circular glass plate to allow for the observation of the vortex pattern and is supported by several stainless steel bars to keep it at a required jet-disk separation distance. The stainless steel bars have skews so that we can adjust the vertical distance between the exits of the injection pipes and the heated disk. The structure of the lower circular heated disk will be described in the next subsection. Besides, the experimental system is very sensitive to disturbances from the surroundings. Therefore, the system is
confined in a small room formed by three wood planes and one black sheet of lint to reduce the effects of the disturbances from discharging the flow to the ambient surrounding.
(2) Heating unit: The heating unit is designed to maintain the lower circular disk at the preset uniform temperature during the experiment. It mainly includes a 15-mm thick high purity copper plate of 16-inch in diameter, acting as the impinging disk, which is directly placed on the heating elements. The edge of the heated disk is connected with a concentric insulated portion with an outer diameter of 25 inches.
Besides, care is taken to insure that the upper surfaces of the heated disk and the insulated portion are kept at the same horizontal level so that the wall jet dose not experience a step change as it crosses the edge of the heated disk. After the jet impinges the heated disk, it moves in the radial direction over the heated plate and the insulated portion. Then the air is discharged into the ambient. More specifically, 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 the heated copper disk. The heater attached onto the back side of the copper plate is divided into 4 concentric zones (Fig. 2.3). 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 and its resistance is about 166 ohm. 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 20.0-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 at the
detection points through the small holes drilled from the backside of the plate (Fig.
2.4).
(3) Gas injection unit: 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 intends 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, regulated by the pressure regulator and later injected into the test section through a straight coaxial circular injection pipe.
The downward vertical air jet issuing from the pipe outlet impinges directly onto the heated plate. In the present study, an injection pipe with a diameter of 20.0 mm is chosen and the straight portion of the pipe is 600-mm long. The length of the injection pipe is selected to ensure that they are long enough to allow us to have a fully developed air flow at the exit of the injection pipe. The distance between the exit of the injection pipe and the heated disk can be varied from 12.5 to 25.0 mm. The air temperature at 600-mm upstream of the exit of the injection pipe is measured by a corrected and calibrated T-type thermocouple. The measured value is considered as the temperature of the air injected into the test section since the whole injection pipe is thermally well insulated by a superlon insulation layer of 16-mm thick.
(4)Visualization unit: A smoke-tracer flow visualization technique is employed to observe the flow patterns resulting from the jet impinging onto the heated disk in the test section. The air flow pattern in the test section is illuminated by 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-PC330. The recorded images are later examined carefully in a personal computer.
(5) Temperature measurement: To understand thermal characteristics of the steady and unsteady vortex flows, the temperature of the air flow in the test section is measured by inserting a thermocouple probe laterally into the test section from the outside of the insulated portion. In the experiment, the thermocouple tip is positioned at selected vertical distances from the upper surface of the heated disk. More specifically, the thermocouple probe is an OMEGA (model HYPO) mini hypodermic extremely small T-type thermocouple implanted in a 1-inch long stainless steel hypodermic needle. The transient air temperature at selected locations in the test section is recorded by a recorder (YOKOGAWA MV200) after the vortex flow reaches a statistical state in which the initial transients in the flow have died out. The sampling rate of the recorder is 8 Hz for each data channel.