摘要

Natural convection within a square cavity filled with Cu-water or Al₂O₃-water nanofluids is investigated numerically. The temperature of the left vertical surface (cold temperature) is maintained constant, while that of the opposite surface (hot temperature) is varied sinusoidally in time. The horizontal walls are considered adiabatic. The parameters governing the problem are the amplitude (0 ≤ a ≤ 1) and the period (0.001 ≤ τ ≤ 1) of the variable temperature, the Rayleigh number (10³ ≤ Ra ≤ 10⁶), and the solid volume fraction (0 ≤ φ ≤ 0.1). By adding nanoparticles to the pure fluid, a substantial enhancement of heat transfer is observed. In comparison with the constant heating conditions, it is found that the variable heating temperature could lead to a drastic change in the flow structure and the corresponding heat transfer, especially at specific low periods of the hot variable temperature. This leads to a resonance phenomenon characterized by an important increase in heat transfer by about 25.5% with respect to the case of a pure fluid subject to constant hot temperature boundary conditions.