Effects of low slope variable section double-skin roof on the heat-shielding performance of grain warehouse roof

Abstract: The double-skin roof has been applied in the design of modern granary. The inclined upper-plate has the better thermal performance to effectively improve the situation of "hot roof" of granary, compared with the ordinary parallel double-plate. In this study, an experimental and numerical investigation was carried out to determine the thermal performance of double-skin roof with the inclined upper-plate, and the effects of physical parameters on the convective heat transfer. According to the actual size of grain warehouse in application, an experiment setup was established using the similarity theory with a scale of 1:28 to the real one. The double-skin roof consisted of two plates: the lower one was used as the original surface of the warehouse, and another was suspended above. The two flat plates were arranged parallel or inclined for comparison. Changeable parameters included the flow rate through the air gap between the double plates, the thickness of the air gap, and the reducing ratio. Other parameters were the same, such as the air temperature. A systematic investigation was made to clarify the influence of the above factors on the performance of the double-skin roof. The results showed that both the flow rate and the thickness of the air gap dominate the thermal performance of the roof. Under the optimal working condition in the parallel mode (with an air gap of 0.07 m and a flow rate of 60 m3/h), the temperature increment at the surface of the grain bulk was 1.75 ℃, which was lower than the mode without suspended upper-plate. Moreover, the inclined design of the upper-plate increased the air velocity along the flowing direction. As such, the convection was strengthened at the outlet, and thus improved the average thermal performance of the opened roof. The inclined mode was used to control the temperature at the grain bulk surface within 24 ℃, and only dropped by 1 ℃, compared with the parallel mode under the same working conditions. In addition, a geometric model of the experiment setup was created, where the grid was divided using the software ANSYS ICEM. The convection term in the conservation equation was discretized with the QUICK scheme, and the solution for the flow field was obtained by the SIMPLE algorithm. The experimental data was used as the input profile for the model. The numerical results were compared with the experiment to verify the accuracy of the model. The average and the maximum error were within 3% and 8.9%, respectively. Simulation results demonstrated the effects of the reducing ratio on the roof thermal performance with the analysis of the Ra number and the Nu number. In the inclined mode, the smaller reducing ration was beneficial to the air convection. The outlet velocity of the roof was higher by 30.6% with a reducing ratio of about 0.500, resulting in a larger local Nu number and better thermal performance. More importantly, the Nu number was increasing, when the dimensionless length was in a range of 0.34-0.37. Furthermore, the Nu number increased with the Ra number given a dimensionless length of below 0.15 near the inlet of the double-skin roof. Therefore, the ventilated double-skin roof with an inclined upper plate was recommended for the roof reconstruction design, because of the advanced air convection.