A sketch of the experimental apparatus established here for the mixed convective air flow over a heated circular disk embedded in the horizontal duct with two curved blocks inserted in the upstream section 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 system 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 developing section of the duct two curved blocks also of 22.5 mm in height are installed in the region near the duct sides just upstream of the heated plate so that the cross section of the duct is narrowed and the forced flow is
accelerated in this region. The chosen geometry of the blocks symmetric and their positions in the duct are schematically illustrated in Figs. 2.2and 2.3. The blocks are both 600 mm long and the curved surface of the left block (Fig. 2.2(a)) can be represented by the equation
f
(z
′)=−3.68z
′4 +1.65z
′3 −0.26z
′2 +0.35z
′, here z′is measured from the upstream tip of the block indicated in Fig. 2.2(b) and has a unit of meter. Moreover, the position of the blocks is characterized by the longitudinal distance between the throat of the blocked section and the most upstream point of the heated circular plate, as illustrated in Fig. 2.3. Not that the blocks are placed at three different positions. The side and top walls of the duct are constructed of 10-mm thick transparent acrylic plates to allow for the visualization of vortex 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 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 (Figs. 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
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 section containing the blocks 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 vortex 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 the 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 test section, 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 temperature of the flow. The temperature data are recorded when the flow reaches steady or statistically stable state, usually 5-6 hours after starting the test.
It was noted that in all tests the maximum temperature differences between any two locations in the copper plate were below 0.1 . The error in the temperature ℃ difference between the copper surface and the air at the duct inlet is estimated to be within ±0.1 .℃