Performance of different material heat transfer pipes and CFD simulation of thermal storage soil temperature distribution

Abstract: Heat transfer pipe has been widely adopted in horticultural facilities, such as Chinese solar greenhouses, which serves as a media to transfer heat between hot air and soil or north wall, and has many advantages such as easier operation, lower cost and better effect. Many scholars have studied the pipe heat transfer performance from piping materials and air flow, and they found the piping material has little influence on heat transfer, but air flow rate has a big influence on heat transfer. However, whether the holes on the heat transfer pipe wall have positive roles in heat transfer is still elusive. Therefore, we compared the heat transfer ability between closed type pipe and breathable type pipe. To study the temperature distribution of thermal storage soil, a typical heat transfer pipe experimental platform with 3 different kinds of pipe material, including polyvinyl chloride (PVC), galvanized iron (GI), and steel mesh skeleton - geotextile composite (SFG), was established in this research. Combined with computational fluid dynamics (CFD) technique, the heat storage experiment on soil was performed. Results showed that there was no temperature and humidity difference at the inlet of heat transfer pipe among 3 kinds of different material, however, the temperature difference was significant at outlet: PVC > GI > SFG; in addition, the humidity level also showed a clear trend of GI > PVC > SFG. This can be attributed to the physical characteristics of SFG such as the high permeability for air and moisture, from which the soil had a chance to absorb both heat energy and moisture from the hot air. Additionally, SFG resulted in the most effective heat transfer activity with a heat exchange amount of 299.44 kJ during the whole test, which was at least 4 times that of GI, and 3 times that of PVC. The location with soil temperature fluctuation more than 1 ℃ is in effective heat range, and that with soil temperature fluctuation more than 5 ℃ is in efficient heat range. The effective heat storage ranges at inlet of SFG were much greater than 240 mm in all 4 directions, i.e. up, down, left, and right direction; the effective heat storage ranges at outlet of SFG were more than 240 mm; the efficient heat storage ranges at inlet of SFG were also more than 240 mm; and the efficient heat storage ranges at outlet of SFG were higher than 240, 180-240, 180-240, and 180-240 mm respectively in the 4 directions. Our results showed that the heat transfer ability of SFG was the best in our experiment, and PVC was slightly better than GI. Therefore, the heat transfer ability of breathable type pipe was better than closed type pipe. Moreover, each CFD simulation model was established individually based on 3 different pipes, with the maximum relative error between measurement and simulation value of 4.4%. Simulated results showed that the temperature was distributed unevenly at the end surface of the soil: a higher temperature at the upper layer, a lower temperature at the lower layer, and a symmetrical distribution between left and right sides. There was also a distinguishable decrement of section temperature between the air inlet and outlet. Therefore, we conclude that SFG performs significantly better than both PVC and GI, which can be potentially applied in Chinese solar greenhouses or other similar agricultural facilities, and/or popularized to the construction market.