Heat transfer of twin-fin piezofan at various orientations and buoyancy levels

This experimental study explores the heat transfer characteristics over a horizontal heating plate cooled by the separated vortical flows downstream the trailing edges of two counteracting cantilever flapping fins. The cantilever fins are stimulated by piezoelectric ceramic discs (twin-fin piezofan) at five frequencies (f) of 135, 145, 150, 160 and 180 Hz with three oscillating amplitudes (λ) of 0.51, 0.62 and 0.85 mm for each f. Smoke detection method is used to visualize the vortical flow structures downstream the flapping fins; while the full-field Nusselt number (Nu) distributions over the horizontal plate are measured by the steady-state infrared thermography method with the flapping fins orientated at five attack angles (α) of 0°, 15°, 30°, 60° and 90°. At each α–f–λ test condition, heat transfer tests are performed with four ascending heat fluxes to vary the buoyancy level. As α increases from 0° to 90°, the near-wall flows above the horizontal plate are considerably modified, leading to the corresponding shrinkage of effective cooling region and the elevated local Nu  . Area-averaged Nusselt numbers (Nu‾) over the horizontal plate at all the α–f–λ test conditions are favorably correlative with modified Strouhal number (St) defined as 2πfλ  /gWP. The typical isolated/interdependent St   and buoyancy effects on Nu‾ are illustrated using a set of selective heat transfer results. While Nu‾ is increased by raising St at present test conditions, the increase of buoyancy level simultaneously extends the effective heat transfer region on the horizontal plate with local Nu   and Nu‾ elevated. Empirical Nu‾ correlation using α, St and buoyancy parameter (βΔT) as the determining variables is devised to permit the evaluation of isolated/interdependent α, f, λ and βΔT   effect on Nu‾ for electronic cooling applications.