Suppression/Enhancement of Natural Convection by Use of Thermal Screen

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An experimental and numerical study has been made concerning the effects of intermediate cooling or heating by use of thermal screen on both buoyant and Marangoni convection. A porous screen, which is called as a thermal screen in this study and is constituted with a row of heat pipes placed in a fluid layer, is adopted for the intermediate cooling or heating. Visual observations are made by using liquid crystal particles migrated into test liquid.First, the effects of thermal screen on natural convection in a fluid heated from below are examined. It is found that the thermal screen introduces a rather strong uniformity in horizontal temperature distributions which can modify the buoyancy forces and can substantially suppress the buoyant convection. In addition to this thermal effect of the screen, the Darcy resistance retards flow through the porous screen. Thus, convective patterns were found to occur in the form of either the retarded large-scale convection through the whole layer or the local convection confined to a space bounded by the porous screen depending on the temperature level and the permeability of the screen. Further, the numerical and experimental results show that the local convection entrains fluid of the other side of the screen with different temperatures, which can yield time-dependent periodic motions.Secondly, Marangoni convection induced by heating a thin wire on the free surface is examined. It is also found that the thermal screen can reduce horizontal temperature gradients particularly near the heated wire and the Marangoni convection is weakened. Again, it is noted that the Darcy resistance of the thermal screen can cause an effect of attenuation of flow.Then, it is concluded that the intermediate cooling or heating by the thermal screen can provide a very efficient method of control of both buoyant and Marangoni convection.