Graduate Thesis Or Dissertation
 

Effect of vibration on forced convection to water from a cylinder at Reynolds numbers in the range of stable vortex shedding

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  • An experimental investigation has been made of the effect of vibration on heat transfer from a cylinder in crossflow. The cylinder used was 3/16-inch in diameter and was sinusoidally oscillated in the vertical direction by a mechanical drive mechanism while immersed in an open water channel. Nominal amplitude ratios, a/d, varied from 0.125 to 1.43 while frequency varied from 0 to 6.5 cycles per second. Data was taken at three flow Reynolds numbers: 64; 103 and 144. Temperature difference between the cylinder and the water was held at approximately 10°F. A hydrogen bubble technique was used in conjunction with dye studies to observe the flow near the cylinder. It was found that the heat transfer could be increased or decreased by vibration, depending on the amplitude ratio and the vibrational Reynolds number, (N[subscript Re])[subscript v] = awρ d/√2μ. Below a critical vibrational Reynolds number, which increased with the flow Reynolds number, oscillation had no effect on the heat transfer. After this critical point was passed, the data for the amplitude ratios 0.125, 0.250, and 0.500 showed a decrease in the heat transfer while the data for higher amplitude ratios (a/d = 1.00; 1.43) gave a steady increase. After reaching a minimum, the data for a/d = 0.500 increased to converge with the data of higher amplitudes; this trend was also suggested by the lower amplitude ratios but was not completely substantiated because of the frequency limitation of the drive mechanism. The maximum decrease in the heat transfer below stationary conditions was about 20 percent; the maximum increase in the heat transfer was 180, 125, and 90 percent for flow Reynolds numbers of 64; 103 and 144 respectively. The vortex shedding frequencies were in the range of frequency used for mechanical oscillation; noting this, an attempt was made to oscillate the cylinder through the immediate frequency range of vortex shedding to see if any special heat transfer effects would result. No change occurred other than the general increase due to increasing the vibrational intensity. The increases in heat transfer are attributed to vibrationally induced turbulence. An adequate explanation of the decreases in heat transfer was not provided by this investigation; however, it is thought that the phenomenon occurring may be similar to the observations of Kubanskii (11) who imposed acoustical vibrations on a cylinder in crossflow. By appropriately locating the cylinder with respect to the nodes of the sound field he found that the point of flow separation shifted toward the upstream side of the cylinder resulting in lower heat transfer rates. A critical (N[subscript Re])[subscript v]/(N[subscript Re])[subscript f] required to alter the heat transfer was established for this investigation and although it had a weak dependence on the amplitude ratio, a value of 0.35 serves as a good approximation for the data of this experiment.
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