Research On The Effects Of Direct Sunlight On The Floor Radiant Heating System Of Low-temperature Hot Water

Railway transportation is the main mode of transportation in China,to meet the requirements of personnel flow,wide vision,and beautiful architectural appearance,railway station buildings generally have the characteristics of high space,large span,and large window-to-wall ratio in the building structure.This leads to a large amount of solar radiation directly irradiating the indoor floor through the transparent envelope structure,which affects the floor radiant heating system of the passenger station,especially for the railway passenger station along the Sichuan-Tibet Railway,the strong solar radiation in alpine regions intensifies the effects,resulting in a large temperature difference in areas with different solar radiation intensity.To solve the problem of“overheating”on the sunny side of the building,based on the floor radiant heating system of Shannan Station of Sichuan-Tibet Railway,this thesis researched the effects of direct sunlight on the heat transfer characteristics of floor radiant heating;and put forward the system optimization strategies.Firstly,the heat transfer process of floor radiant heating under direct sunlight and the transfer process of solar radiation from outdoor to the indoor ground were analyzed theoretically,and the calculation formulas of convection,radiation heat transfer coefficient,and heat transfer between the upper surface of the floor and indoor space were deduced,and the calculation method of solar radiation intensity absorbed by the floor surface under direct sunlight was clarified,which provides the calculation basis for the parameter selection range of the following experimental research and numerical calculation.Next,the experimental room with a low-temperature hot water floor radiant heating system was built,and the electric heating film was laid on the floor surface to simulate the direct sunlight on the floor.6 steady-state experiments and 4 unsteady-state experiments were carried out by changing the water supply temperature,hot water flow rate,and solar radiation intensity.The steady-state experimental results showed that the range of convective heat transfer coefficient between the upper surface of the floor and indoor space is 3.8～4.2 W/(m~2·℃),the range of radiant heat transfer coefficient is 5.0～5.4 W/(m~2·℃),and the radiant heat transfer accounts for 55.2%～56.6%of the total heat transfer;The unsteady-state experimental results show that the direct sunlight on the floor surface increases the floor temperature and inhibits the heat dissipation of hot water in the buried coil to the floor structure layer,resulting in increasing the water return temperature and decreasing the quantity of heat supplied by hot water.Then,the numerical calculation model of the radiant heating floor was established.For the two boundaries of“heat flow boundary of direct sunlight on the upper surface of the floor”and“convection boundary between the upper surface of the floor and indoor space”,the former was simplified as a“body heat source”in a very thin surface layer(0.01 mm)on the upper surface of the floor.Given the effects of hot water alone,solar radiation alone,and the coupling of hot water and solar radiation,the steady-state and unsteady-state numerical calculations were carried out by changing the water supply parameters(water supply temperature 30～50℃,hot water flow rate 0.2～0.6 m/s,water supply duration 0～12 h)and solar radiation conditions(solar radiation intensity 0～200W/m~2,direct solar area ratio 0～1/4,direct solar position,direct solar duration 0～9 h).The heat transfer characteristics of floor radiant heating under direct sunlight were analyzed and obtained,it is proposed that for the floor radiant heating system of large-space buildings in alpine regions,the“direct sunlight area”and“non-direct sunlight area”indoor should be“zoning control”.Finally,based on the Shannan station of Sichuan-Tibet Railway,the“sunlight zoning”scheme was proposed,and the floor radiation system was established for simulation.“Zoning control”reduced the maximum operating temperature of the“direct sunlight area”by about 3℃and the“overheating”time by about 5 h by shortening the water supply time of the“direct sunlight area”,which effectively improved the indoor comfort;Under“zoning control”,the“direct sunlight area”,only accounts for 32%of the total area,making full use of the solar radiation passively absorbed by the floor,and saved about 9.2%of the heating energy consumption using“active and passive collaborative heating”,which effectively improved the energy conservation of the system.