Heat transfer enhancement by localised time varying thermal perturbations at hot and cold walls in a rectangular differentially heated cavity

• Published in: International journal of thermal sciences 2020-05, Vol.151, p.106245, Article 106245
• Main Authors: Thiers, N., Gers, R., Skurtys, O.
• Format: Article
• Language: English
• Subjects: Heat transfer enhancement; Localised thermal perturbations; Natural convection; Rectangular differentially-heated cavity; Unsteady flows
• ISSN: 1290-0729; 1778-4166
• DOI: 10.1016/j.ijthermalsci.2019.106245

The objective of this work it is to determine the optimal properties of a thermal disturbance to enhance the heat transfer in a rectangular differentially heated cavity of aspect ratio 4. So, we study numerically the effect on the heat transfer of one or two small thermal actuators (5% of active plate surface) applied at the active walls of this kind of cavity. Thermal disturbances are sine or square waves. Influences of wave characteristics (amplitude, frequency, phase shift) and the vertical location of the disturbance area are investigated for various Rayleigh numbers below its first critical value (RaH=1.02×108). Only a fluid with Prandtl number equal to 0.71 (air) is considered. Flows are calculated solving the 2D unsteady Boussinesq–Navier–Stokes equations using the Spectral Element Method programmed in the Nek5000 opencode. A detailed description of the coupling between one or two thermal disturbances and cavity flow is presented. The main contribution of this work is that the best position to enhance the heat transfer is 70% of the hot plate height or 30% of the cold plate height, and not 0.1 for the hot plate (0.9 for the cold one) which is generally admitted. At this position this increase reaches 5.5% by disturbing at both active walls with local synchronised square waves of amplitude ε=1, of frequency f=0.403 and at a state slightly below the first critical Rayleigh number. •Heat exchange is improved due the presence of external thermal perturbation.•Optimal position for the disturbance is past the mid-height cavity.•Gain on the Nusselt number increases with the amplitude of the perturbation.•A perturbation that remains inside the boundary layer increase heat exchange.