3D Numerical Thermal Study Of The Hollow Roofs

Energy used to construct and operate buildings accounts for 46% of total energy consumption, so building energy efficiency is related to the improvement of the environment and the sustainable development of the society. Roof insulation for building top floor room also has great effect on the thermal comfort performance. Cast-in-situ concrete hollow floor system, with advantages of light weight, energy saving and environmental protection, is widely used in present engineering. The convective and radiation within hollow building materials enhance the heat transfer significantly, so thermal optimization of the roofs constructed with cast-in-situ hollow concrete floor system has an important significance.The thermal performances of hollow roofs with different box filler are simulated numerically by 3D self-made code based on the finite volume method, which can deal with complicated heat transfer efficiently. In order to enhance the insulating performance, the partitions are designed in the box filler of hollow roofs to suppress the convection and radiation. At the same time, the effects of concrete materials, box filler materials, partition surface emissivity, indoor and outdoor temperature difference on the equivalent thermal conductivity was investigated. The numerical results indicate that:1. Inserting partitions vertical to heat flux can reduce the equivalent thermal conductivity of the hollow roofs efficiently. With the increase of the number of cavities in height direction, the equivalent thermal conductivity decreases nonlinearly. While, the partitions parallel to heat flux have few effects on the thermal behavior of the hollow roofs, but cause influence on temperature distribution of the hollow roofs.2. For hollow roofs with higher void fraction, partitions vertical to heat flux reduce the equivalent thermal conductivity more efficiently. For hollow roofs with 900×900×500 mm box fillers,7 and 14 horizontal partitions reduce the equivalent thermal conductivity 29,91% and 40.42% respectively,while for hollow roofs with 400 X 400 X 200 mm box fillers,7 horizontal partitions reduce the equivalent thermal conductivity 10.99%, whose equivalent thermal conductivity reaches the minimum.3. Selecting box filler materials and partition materials with higher thermal conductivity, the equivalent thermal conductivity of hollow roofs increase. The equivalent thermal conductivity of B552L1W1Hn increase 1.93% - 2.99%, when thermal conductivity of box filler materials and partition materials increase from 0.3 to 1.0 W/(m.K).4.Choosing different thermal conductivity of concrete, the difference of the equivalent thermal conductivity is obvious. When thermal conductivity of concrete decrease from 1.51 to 0.76 W/(m.K), for the roof B552L1W1Hn, equivalent thermal conductivity decreases by 8.99% - 16.39%, and for roof B555L1W1Hn, equivalent thermal conductivity decreases by 9.17 to 22.40%.5. Indoor and outdoor temperature difference cause few effects on equivalent thermal conductivity of hollow roofs. Comparing the case that Indoor and outdoor temperature difference is 40 ℃, when Indoor and outdoor temperature difference is 20 ℃ and 50 ℃, the equivalent thermal conductivity of B552L1W1Hn vary 0.30% - 0.97% and 0.08% - 0.34% respectively.6. Radiation within box fillers enhance the equivalent thermal conductivity of hollow roofs. For B552L1W1Hn and B555L1W1Hn hollow roofs, when surface emissivity of box fillers is 0, the equivalent thermal conductivity is lower 3.85%-7.38% and 7.07%-12.91% than the case of surface emissivity is 0.85.7. There are not much difference between the equivalent thermal conductivity of inverted hollow roofs and positive hollow roofs. For hollow roofs with same exterior size and interior structure, the equivalent thermal conductivity of inverted hollow roofs is slightly lower.The simulated results are aimed to provide references for engineers toward energy-efficient design and selection of hollow roofs for better heat preservation.