Abstract
The heat transfer enhancement achieved by the additional electric field in Rayleigh–Bénard convection (RBC) of a dielectric fluid is numerically studied beyond the Rayleigh number Ra = 105. We carried out direct numerical simulations of RBC in a rectangular enclosure under the strong injection condition with a fixed non-dimensional injection parameter C = 10, a fixed mobility number M = 10, two Rayleigh numbers Ra = 105 and Ra = 106, and two Prandtl numbers Pr = 1 and Pr = 10 to investigate the characteristics of flow structure and heat transfer and evaluate the dependence on these parameters. It is observed that the flow structure exhibits multiple states with various steady or unsteady flow patterns such as four cells, three cells, and two cells (up/down). It is found that the introduction of an electric field is an effective way to achieve heat transfer enhancement. The heat flux is augmented more efficiently for a large Prandtl number and a low Rayleigh number, where the electric field has a strong effect relative to buoyancy. It is also found that heat transfer is most efficient when the flow pattern is in a three cells flow state.
Highlights
Rayleigh–Bénard convection (RBC), as a paradigm of thermal convection frequently encountered in nature and industrial applications, has been experimentally, theoretically, and numerically studied.1–4 Many approaches have been proposed to enhance or suppress the heat transfer efficiency of RBC, such as applying mechanical vibration,5,6 through roughness,7–9 using the rotation,10 by temporal modulation,11 using biphasic active particles,12 and through sidewall controlling.13–15 Among these methods, the introduction of an electric field in RBC is known as a possible way to control heat transfer, and such a system is usually termed an electrothermohydrodynamics (ETHD) system
From the result of Pr = 10 and Ra = 105, we notice in Fig. 5 that the mean Nu number of two unsteady flow states is increased by the electric field effectively—for the three cells flow state, the rate of heat transfer enhancement is 69.1% (T = 700), and for the four cells flow state, the rate of heat transfer enhancement is about 71.9% (T = 800)—demonstrating that the heat transfer was strongly augmented by the electric field for large Pr
The enhancement of RB convection heat transfer in a dielectric fluid layer due to the imposed electric field is studied numerically. In this ETHD system, the Coulomb force serves as the driving electric force, and the buoyancy comes from the thermal gradient
Summary
Rayleigh–Bénard convection (RBC), as a paradigm of thermal convection frequently encountered in nature and industrial applications, has been experimentally, theoretically, and numerically studied. Many approaches have been proposed to enhance or suppress the heat transfer efficiency of RBC, such as applying mechanical vibration, through roughness, using the rotation, by temporal modulation, using biphasic active particles, and through sidewall controlling. Among these methods, the introduction of an electric field in RBC is known as a possible way to control heat transfer, and such a system is usually termed an electrothermohydrodynamics (ETHD) system. Su et al. further extended the relevant study to non-Newtonian power-law fluids; their comprehensive analysis demonstrated that the power-law index has a marginal impact on the fluid flow and heat transfer considering two Rayleigh numbers 103 and 104 He et al. introduced a non-Ohmic solid of finite thickness into RBC and analyzed the effects of the solid–liquid interface on ETHD at two Rayleigh numbers 103 and 105. Various studies of ETHD have been conducted during the past few years, the nonlinear feature such as the multiple flow states in a wide cavity at large Rayleigh numbers has not been reported, and the corresponding flow structure and heat transfer of RBC under unipolar injection are still unclear.
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