Influence of thermodynamic disfigurement on the convective heat transfer of solar greenhouse

Abstract

Solar greenhouse is a typical greenhouse without any additional heating system, which has developed rapidly in Northern China. However, due to the construction quality, management methods, especially the long-term use and other factors, there are usually different degrees of thermodynamic disfigurements in the envelop enclosure of solar greenhouse. The purpose of this study was to investigate the influences of thermodynamic disfigurement on the temperature distribution and convective heat transfer of solar greenhouse. In this study, the east and west compartments of a typical solar greenhouse which is located in Yangling, China (108°4′E, 34°16′N) were tested. The air temperature of each compartment was collected using temperature recorders, and the thermal infrared images of different compartment envelopes were obtained by a thermal infrared imager on a typical cloudy day. Convective heat transfer coefficients and heat flux densities of different compartment envelopes in the solar greenhouse were calculated. The results showed that the temperature difference can be displayed in the thermal infrared images of compartment envelopes, the surface temperature of the front roof was the lowest, followed by the back roof, the wall surface temperature was the highest. The minimum average surface temperature of the front roof in the eastern compartment was only 3.8°C, which was 6.8°C and 9.2°C lower than the average surface temperature of the back roof and back wall, respectively. The surface average temperature of thermodynamic disfigurements located at the bottom of the south side in the front roof of the eastern compartment, whose area accounted for 16.5% of the total front roof in the eastern compartment, was only 5.4°C. Compared with non-thermodynamic disfigurement, the average convective heat transfer coefficient and heat flux density of thermodynamic disfigurements in the front roof of the eastern compartment were increased by 20.3% and 110.3%, respectively. The average air temperature in the eastern compartment was 3.5°C lower than the average air temperature in the western compartment of the solar greenhouse. Construction of brick wall at the bottom of the south side of the front roof in the solar greenhouse helped to increase the inner surface temperature of the front roof, with an average temperature rise of 6.2°C, and reduce the area of thermodynamic disfigurement, which only accounted for 2.6% of the total front roof in the western compartment. The average surface temperature of thermodynamic disfigurements mainly caused by the entry and exit door in the wall of the eastern compartment was only 9.8°C, which was lower 3.2°C than the average temperature of non-thermodynamic disfigurement of the wall. Thermodynamic disfigurement helped to increase heat loss. The weighted average proportion of thermodynamic disfigurement in the western compartment was 2.1%, while that of thermodynamic disfigurement in the eastern compartment was 10.7%. The thermal insulation performance of the western compartment envelope in the s...

Sensitivity of solar greenhouse envelopes to the thermal disfigurements and optimal selection of the thermal insulation quilt

“…Data was collected on a cloudy day (Dec. 10, 2018) at 17:30-18:30, and the greenhouse was covered with thermal insulation quilt. Details of the experimental process are described in the previous work [1] . 1 lists the test results of thermal disfigurements area fraction, average surface temperature, surface convective heat transfer coefficient, heat flux and calculated relative changes of them of different envelopes of the solar greenhouse.…”

“…The existence of thermal disfigurements in envelopes will deteriorate the thermal insulation performance of solar greenhouse envelopes and thus be detrimental to keeping indoor temperature high enough at night. The previous work [1] showed that the temperature difference can be displayed in the thermal infrared images of compartment envelopes, the surface temperature of the front roof was the lowest, followed by the back roof, the wall surface temperature was the highest. And the existence of thermal disfigurements will lead to the increase of convective heat transfer coefficient and heat flux in the disfigurements area, while the temperature will decrease.…”