Natural convection circulation characteristic between the air gap and indoor room of passive solar house with Trombe wall has been investigated. Experimental data show that the period of thermo-circulation occurring lags behind the period of Trombe wall receiving solar radiation due to the existence of natural circulation inside the air gap. A big temperature difference between the surface of massive wall and glazing results in intensive natural convection inside the air gap during night, and it is important that some methods should be implemented to weaken this natural convection in order to reduce the heat loss. By means of analogy analysis the natural convection heat transfer in the air gap is regarded as the superposition of two natural convection heat transfer processes occurring over isolated vertical flat plate. Two empirical formulas are found in this paper, giving Q and Nu as functions of temperature difference between the massive wall surface or glazing surface and inlet air. Also the formulas have been validated by the experimental data, and it is felt that they can accurately predict the natural convection heat transfer through thermo-circulation process of Trombe wall included in a full scale passive solar house.
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