Study On Heat Transfer Characteristics Of Floor Based Direct Expansion Capillary Heating Of Air Source Heat Pump

In In recent years,energy shortage and environmental pollution have been worsening.At the same time,people’s demand for a comfortable and healthy living environment is also increasing.In order to respond to the national call of "energy saving and emission reduction" and people’s growing health needs,the research of renewable energy technology and the promotion of new energy-saving heating methods have great potential in the field of heating in China.In the current "clean heating" work,the air source heat pump has become a renewable energy technology that can replace the traditional forms of heating such as coal burning,and has been widely used in the "coal to electricity" project in north China.Air source heat pump system based on refrigerant capillary floor heating is a new type of anhydrous floor radiant heating system,which eliminates the heat exchange link between refrigerant and hot water,has the advantages of simple overall system,low maintenance cost and greatly improved system performance.However,the current research on the new system is mainly carried out in the laboratory,and there is a lack of practical research on its heating effect and system operation characteristics.This study is based on a small natatorium project in Qingdao.Through field measurement and simulation method,the heat transfer characteristics of the floor based direct expansion capillary heating of Air source heat pump are studied.The main research conclusions are as follows:(1)Through the actual test of the project,the floor surface temperature distribution of the refrigerant capillary system is relatively uniform during the heating period,and the temperature of the condensing section is relatively constant in the heating stable stage.The average condensing temperature at 24℃ and 22℃ indoors is 38.8℃ and 33.1℃,respectively,and the average floor surface temperature is 32.6℃ and 30.9℃.Due to the heat storage characteristics of the floor itself,the temperature of the floor surface rises or falls slowly within 2～3h when the system is started/stopped.At the initial stage of the system operation,the surface heating heat flow increases rapidly,and then the growth rate slows down and tends to be stable.At 24℃,the heat supply in the stable stage is stable in the range of 97W/m2 ～ 119W/m2,and at 22℃,it is stable in the range of 89W/m2 ～101W/m2.Under the setting conditions of 24℃,22℃ in the daytime and 18℃ at night,the indoor air temperature all exceeded the set value.During the process of defrosting,the indoor temperature has no obvious change due to the unique heat storage performance at the end of the floor,which is conducive to maintaining a stable indoor thermal environment.(2)The unit operation characteristics of typical weeks and typical days during the test period were analyzed.During the typical test period,the average outdoor temperature was 2.6℃ and the average indoor temperature was 22.3℃,and the average COP of the unit during the period was 2.97.The average COP in the stable stage of unit operation decreases with the increase of indoor setting temperature,and also increases with the increase of outdoor temperature.The COP at 24℃ is between 2.07 and 2.91.It is between2.15 and 3.9 at 22℃.Under two typical daily working conditions,the average COP of the whole day is 3.38 and 3.5 respectively.Compared with the thermosiphon radiator and convection terminal air source heat pump under similar working conditions,the COP of the refrigerant capillary floor radiation system used in this project under typical daily working conditions of 22℃ increases by 8.6% and 16.6% respectively,and the system runs more efficiently.(3)Through the simulation of capillary heat transfer characteristics,the influence factors such as tube spacing,condensation temperature and ground laying materials were studied.The smaller the tube spacing,the larger the floor surface temperature and heat flux,the better the temperature distribution uniformity;Under the same condensation temperature and pipe spacing conditions,the highest surface temperature is obtained when the floor decoration material is stone,followed by ceramic tile and wood floor.The type of floor decoration material has no significant effect on the uniformity of temperature distribution of heating floor.The floor surface temperature and heat flux decrease with the decrease of condensation temperature.Condensation temperature has little effect on the uniformity of surface temperature distribution,but great effect on the average surface heat flux.Under the condition of constant indoor temperature,the ground average heat flux decreases by about 10 W/m2 for each 2℃ decrease of condensation temperature.Based on the simulation analysis,the average floor surface temperature and heat transfer data under different pipe spacing,surface material and condensation temperature are given under the commonly used setting conditions of 20℃ for office buildings in winter and 18 ℃ for residential heating,which can be used as a reference for practical engineering design.To sum up,this paper studied the indoor environment characteristics,unit operation characteristics and capillary end heat transfer characteristics of the refrigerant capillary floor heating system through the combination of experiment and simulation.The research results can provide certain theoretical basis and design reference for the promotion and application of the heating end of the new air source heat pump.