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Acoustic streaming induced by longitudinal vibration at 30 kHz is visualized with particle imaging velocimetry (PIV). To gauge an increase in the velocity of air flow due to acoustic streaming, the velocity of air flow in a gap between the heat source and ultrasonic vibrator is measured using PIV. The ultrasonic wave propagating into air in the gap creates steady-state secondary vortex called acoustic streaming which enhances heat transfer from the heat source to neighboring air. Heat transfer through air in the gap is represented by experimental correlation involving Peclet number and Nusselt number. Theoretical analysis reveals that gaps for maximum heat transfer are the multiple of half wavelength. The acoustic streaming velocity of air in the gap is at maximum when the gap agrees with the multiples of half wavelength of the ultrasonic wave, which are specifically 6 mm and 12 mm. Considering the propagation loss of the ultrasonic wave and acoustic streaming velocity of air in the gap, gaps at which maximum heat transfer occurs are experimentally found to be half wavelength and one wavelength.