Determination of Nusselt number in Poiseuille-Rayleigh-Bénard convection using simultaneous temperature PLIF and PIV

• A simultaneous PLIF and PIV measurement system was developed for full-field temperature and velocity measurements in Poiseuille-Rayleigh-Bénard Convection (PRBC). • A fluid test rig was design and fabricated to generate PRBC flow. • Experiment were conducted within mixed convection regime from Richardson number in the range of 2.2 to ∞ to identify the velocity and temperature profile within the flow field. • The Nusselt number is defined within the mixed convection regime using both flow vertical convection and vertical temperature gradient to investigate the influence of each term in convection heat transfer of PRBC. • New physical finding based on the measurement of the velocity and temperature indicates that the influence of flow vertical motion is significant when Richardson number higher than 8.8. However, the influence of vertical flow motion is negligible for Richardson number of 8.8 to 2.2. Poiseuille-Rayleigh-Bénard convection (PRBC) is a mixed convection system where buoyancy-driven flow interacts with an imposed streamwise flow, leading to complex heat and momentum transport. Similar to every convective heat transfer phenomenon, determining the heat transfer coefficient, the Nusselt number, is critical in PRBC. However, it is more sophisticated since PRBC is in mixed convection regime and both vertical (buoyancy-driven) convection and temperature gradient may play an important role in heat transfer. Experimentally estimating both of these terms requires simultaneous measurement of the velocity and temperature. Heat transfer in PRBC is investigated here by determining the Nusselt number over a wide range of Richardson numbers, Ri = 0.5 to Ri = ∞ . An optical measurement system combining planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) was developed, to provide high spatio-temporal resolution measurement of the temperature and velocity fields. For PLIF, sodium fluorescein with positive temperature sensitivity was used. Rhodamine B coated seeding particles with a distinctive spectrum from sodium fluorescein were also used to apply PIV. Experiments were conducted in a custom-designed fluid test rig with a rectangular channel heated from below, resembling the PRBC flow. Results from the simultaneous measurement of the temperature and the velocity reveal that at high Richardson numbers, Ri > 2.2 both temperature gradient and vertical convection have a critical role in heat transfer. However, for lower Richardson numbers 2.2 ≥ R i ≥ 0.5 heat transfer is dominated by the temperature gradient while the influence of the vertical convection is still vivid.