Study Of The Thermal Insulation Of Natural Convection Of Double-skin Roof

In recent years, many scholars have made a lot of research on thermal performance of the double-skin roof as a new style of thermal insulation. However, there is still much faultiness in theories and application. Those studies almost paid lots of attention to forced-air convection, but paid not enough attention to natural convection in the roof cavity. Therefore, the effect of natural convection in the roof cavity on the thermal performance of the roof was the main research issue in this paper. This paper summarized the theory of the natural air convection in the cavity of roof, and made some numerical simulation using CFD software. It has the important theory significance and the project practical value, and also corresponds to the mainstream direction of promoting technology development of energy saving of building.First, Heat Circuit Diagram Methods was used to describe the superiority of the double-layer roof over traditional roof. Based on the thermal boundary layer theory and experience formula, this study summarized the analytic solutions of the natural air convection in the cavity of double-layer roof and proposed the optimal spacing of air layer. Then, a series of 2-D numerical simulations were implemented using CFD software to discuss the effects of different factors on the heat-insulating property. At last, according the new thought using profiled steel sheet to rebuild the roof of pre-existing building presented by a literature, 3-D numerical simulations was used to study the thermal performance of the new ventilated roof of profiled steel sheet. To verify the feasibility of numerical method, the results of 3-D numerical simulations of ventilated roof of profiled steel sheet was compared with the experimental results, and the results agreed well with experimental data. The study presented the effects of the several factors on the thermal performance of the rebuild ventilated roof. The results have instructive significance for application of the rebuild method.The conclusion showed the importance of the heat transfer coefficients for heat-insulating property of double-layer roof, indicated that the temperature difference between the plate and the air in the cavity, tilt angle, the thickness and the length of the air layer were the important factors of heat-insulating property, and proposed the optimal inter-plate spacing which was very close to the sum of both plate's thermal boundary layer thicknesses. The result of the numerical simulation shows that the roof would perform better at higher temperature, larger tilt angle and spacing near the optimal spacing under good ventilation. As using profiled steel sheet to rebuild the roof, thermal boundary thickness should be forecasted to select the appropriate type of profiled steel sheet.The study proposed the optimal spacing for natural convection layer and the effects of some factors on the thermal performance of the roof which can give the references for applications.