Experimental analyses of temperature and pressure oscillation frequencies of a flat plate pulsating heat pipe tested under various edge orientation angles and heat loads

A closed loop flat plate pulsating heat pipe, filled with Opteon^TM SF33 (with a filling ratio of 50%), was experimentally studied in different orientations: 0° (horizontal), 22.5°, 45°, 67.5°, and 90° ( "edge" : vertical with horizontal channels). The results confirm the interest of such configurations, rarely investigated in the literature, on the thermal behavior of the device and on the regularity of the temperatures and pressure signals: If dried-out occurred in horizontal orientation, increase of inclination angle (starting from 22.5°) led to regular oscillatory movement due to help of gravity pressure drop between channels. The thermal performance remains very similar for the device inclination angles from 45° to 90°. Both FFT and wavelet analyses of the pressure signal and temperatures of the external wall of the device (measured with IR camera) were done to characterize the dominant oscillatory frequencies. These orientations led to dominant frequencies, rarely detected in the literature for other classic configurations (with vertical/inclined channels). Similar internal pressure and temperature signals both showed that the dominant frequency increases with decreasing angle (from edge to horizontal orientation), but also with increasing applied heat power, and finally tends to spread and disappear for the highest heat loads.