Research results pertinent to heat exchangers are available in existing literature and heat transfer text books. However, it was short of sufficient flow visualizations in louver fins because of the difficulty in its flow visualization. Flow visualization by means of the dye injection method and the laser Doppler velocimetry (LDV) was performed to disclose flow characteristics in louver fins. Fin pitches P_f and louver angles θ in louver fins were varied. Flow patterns in each louver fin are visualized using the fluorescent dye illuminated a laser light sheet. The LDV data were used to calculate Reynolds stress distributions and power spectrum distributions. It is disclosed that vortices in cases of θ=15° and 20° for P_f=100 mm exhibit a distinctive periodic characteristics, and the Strouhal number is uniquely related to the Reynolds number.

View full abstract

The possible drag reduction and mechanism of flow management over a circular cylinder by internal acoustic excitation through a slit are studied by pressure measurements and flow visualizations. The drag and lift forces acting on the cylinder are evaluated by using the measured pressure distributions. The results indicate the drag reduction up to 30 percent by an optimum combination among the slit angle, the forcing Strouhal number and the velocity amplitude of the acoustic excitation. The mechanism of drag reduction was observed by visualizing the flow field using the smoke-wire method in air and the two-color dye injection method in water. Visualization results indicate that the correlation between vortices of shedding shear-layers is attenuated by the acoustic excitation. This is cause of modulation of the shedding frequency in the shear layers and the cancellation of the shedding vortices due to the vortices generated by the slit.

View full abstract