Description_of_the_Experiment6
3.4.3.1 Description of the Experiment
To detect the influence of free convection on the flow along a curved wall the equipment whose schematic configuration is displayed in Fig. 3–70 was manufactured. There two versions of the model equipment operation are possible (Fig. 3–69).
Version A represents the flow when the buoyancy force acts against the Coanda effect direction. Version B represents the flow where the buoyancy force acts in the Coanda effect direction.
The experimental equipment was designed and manufactured to enable simulation of different modes of air circulation with possibility to heat the circumfluenced cylinder wall and to enable visualisation of the flow along this cylinder wall through holographic interferometry. The chart of the equipment is shown in Fig. 3–70.
a) version A, b) version B
1 – ventilator, 2 – diffuser, 3 – canal, 4 – sensing head of thermo-anemometer, 5 – jet, 6 – air inlet, 7 – speed regulation, 8 – framework, 9 – thermo-anemometer, 10 – temperature tension converter, 11 – millivoltmeter, 12 – thermostat, 13 – cylinder, b – slit width
The main component of the equipment is a circular cylinder of aluminium alloy. Inside the cylinder 6 holes are longitudinally drilled through which the heated fluid (water) flows. The arrangement of the holes ensures uniform heating of the external cylinder surface. There is also an aperture on the cylinder for a sensing head to measure temperatures. The temperature is scanned by a diode thermometer.
Both the top and the bottom part of the jet are screwed to the side plates of acrylic material. The top part is adjustable to desired width of the slit.
To carry out the experiment the Mach-Zehnder interferometer adjusted to infinite fringe width was used.
The experimental equipment was properly located into the interferometer set-up so that the beams could propagate through the measured space. The desired water temperature (30–70 °C) was set on the thermostat of the boiler. When the cylinder temperature was fixed the holographic interferograms of the temperature field started to be scanned by the CCD camera. The recording started by scanning of the temperature field with no forced air circulation (free convection), then the ventilator was switched on and the air flow rate gradually increased. The interferograms were scanned at different boundary conditions (different cylinder temperatures, different flow velocity).
With the change of the air flow velocity in the slit also the Reynolds number changed and that was calculated by relation (Vít, 2004):
(3.73)
where ub – air flow velocity,
ν∞ – kinematical viscosity of the air,
R – radius of the cylinder,
b – width of the slit.
