A sketch of the experimental apparatus for the mixed convective air flow over a heated circular plate in a horizontal plane channel and the adopted coordinate system are schematically shown in Fig. 2.1. The apparatus begins with the air regulated from a 300-liter and 100-psi high-pressure tank. Then, the air passes through a settling chamber, a contraction nozzle and a developing channel before entering the test section.
After leaving the test section, the air is sent through an exhaust section and discharged into the ambient.
The test section of the experimental loop is a horizontal rectangular duct and has a
cross section of 22.5 mm in height and 450 mm in width, providing an aspect ratio of A=20, and has a total length of 450 mm. In the first part of this study the sidewalls of the test section are tilted symmetrically toward the duct core at the same convergent angle ψ, which is defined as the angle between the sidewalls and the vertical central plane at x = 0. Both the bottom and top walls of the test section are horizontal. Thus the test section, in fact, is a sidewall convergent flat duct and is schematically shown in Fig. 2.2. Therefore the duct aspect ratio is 20 at the inlet of the test section, which is reduced to 16 and 12 at the outlet of the test section in the present experiment corresponding to ψ= 5.7°& 11°. The side and top walls of the duct are constructed of 10-mm thick transparent acrylic plates to allow for the visualization of secondary flow patterns. The bottom of the test section is a thick flat bakelite plate embedded with a 15-mm thick, high purity circular copper plate of 300 mm in diameter to model a 12-inch semiconductor substrate. The upper surfaces of the bakelite and copper plates are kept at the same horizontal level so that the air flow does not experience any step when moving over the copper plate. To obtain the uniform copper plate temperature, the heating elements attached onto the lower surface of the copper plate are divided concentrically into seven semi-circular zones and the heater for each zone is independently controlled by a GW GPC 3030D laboratory power supply. Besides, a mica sheet is placed between the copper plate and heating elements to prevent the
electric current leaking to the copper plate (Fig. 2.3). While in the second part of the present study the rectangular duct is slight inclined from horizontal so that its exit end is above its inlet at a small angleψ, as schematically shown in Fig. 2.4.
A good control of the flow condition upstream of the test section is essential in the experiment. More specifically, at the inlet of the loop the working fluid (air) is driven by a 7.5-hp air compressor and sent through a dryer installed with water vapor and oil filters. This dry air then moves into the high-pressure tank. To proceed with the experiment, the air flow is further controlled by a pressure regulator and its volume flow rate is measured by Brooks 5850E and/or 5851E flow controllers both having an accuracy of ±1%. These two flow controllers individually operate in the ranges of 0 to 10 and 0 to 50 liter/min. Through a flexible tube, the air enters the settling chamber, in which four fine-mesh screens, a divergent buffer section, a honeycomb and another four fine-mesh screens are installed in sequence to reduce the turbulence in the air flow.
The air turbulence was further suppressed by passing the air through a contraction nozzle with a contraction ratio of 44:1, which provides a nearly uniform velocity at the inlet of the developing section.
The developing section is 1400 mm in length, approximately 62 times of the duct height. This insures the flow to be fully developed before it arrives at the test section inlet for Re≤100. An insulated outlet section of 450 mm long is added to the test
section to reduce the effects of the disturbances from discharging the air flow to the ambient. The developing section and outlet sections are both made of 10-mm thick acrylic plate, whereas the settling chamber and contraction nozzle are made of stainless steel SS-304 plates. The settling chamber, developing section, test section and outlet section are all thermally insulated with a 20-mm thick Superlon insulator and the entire loop is fixed on a rigid supporting frame.
Visualization of the buoyancy driven secondary flow in the test section is realized by injecting smoke at some distance ahead of the settling chamber. The smoke is produced by a smoke generator, which is a cubic space with incense burned in it. By keeping the smoke concentration at a suitable level, the incense particles can be illuminated by a plane light sheet from a 550 Watt overhead projector. With an adjustable knife edge a sharp contrast could be achieved between the duct walls and smoke. The flow photos from the top, side and end views of the test section can then be taken. The exposure time is about 1/125 second in taking the photos.
The temperature of the heated copper plate is measured by 17 calibrated and electrically insulated T-type thermocouples embedded at selected locations in the plate (Fig. 2.5). The thermocouple beads are fixed at about 1 mm from the upper surface of the copper plate through the small holes drilled from the back side of the plate. A T-type thermocouple is also used to measure the inlet air temperature at locations just
upstream of the test section. The signals from the thermocouples are recorded by the Hewlett-Packard 3852A data acquisition system with a resolution of ±0.05℃.
To measure the temperature distribution of the air flow in the horizontal duct, a thermocouple probe is inserted from the downstream end of the test section. The probe is supported by a three-way traversing device. More specifically, the thermocouple probe is an OMEGA (model HYP-O) mini hypodermic extremely small T-type thermocouple (33 gauge) implanted in a 1-inch long stainless steel hypodermic needle.
This movable thermocouple probe can measure the time-average and instantaneous