Performance Improvement And Testing Of The Natural Heat Exchange Air Conditioning With Parallel Pipes

The near zero energy consumption building has the characteristics of ultra-low energy consumption and high thermal comfort,which represents the development trend of the future building.In the near zero energy consumption building,the choice of indoor air conditioning terminal plays an important role in the system energy consumption and thermal comfort of personnel.In this paper,the natural heat exchange air conditioning with parallel pipes is a new type of indoor air conditioning.According to the different water supply temperature,it can provide cooling in summer and heating in winter.The natural heat exchange air conditioning with parallel pipes has the characteristics of silence and minimal interference to indoor air flow.One air conditioning terminal can meet the requirements of heating,cooling and dehumidification throughout the year.It is suitable for near zero energy consumption buildings.However,at present,the cooling capacity per unit projected area of the air conditioning is relatively low.In this paper,numerical simulation and experimental test are combined to simulate the air conditioning in different structural forms,propose the methods to improve its heat transfer performance and study the effect of improvement.Firstly,the influence of pipe diameter and pipe spacing on the heat transfer performance of the natural heat exchange air conditioning with parallel pipes is studied,and the whole structure is optimized.The equipment is composed of circular heat transfer tubes.The single circular heat transfer pipe and the whole model of the equipment are established in the CFD numerical simulation software.The heat transfer on the outside of the tube with the diameter of 20 ~100 mm and the distance of 30~110 mm between the centers of the tube is simulated and studied.The heat transfer performance of the equipment is measured by the heat transfer per unit projection area.The results show that the heat transfer per unit projected area of equipment increases with the increase of pipe diameter when the pipe spacing is constant.When the pipe diameter is constant,the heat transfer per unit projected area of equipment decreases with the increase of pipe spacing.In this paper,the concept of geometric projected area ratio is proposed.It is found that when the equipment surface temperature and indoor air temperature are constant,there is a linear correlation between the unit projected area heat transfer and the geometric projected area ratio at the natural heat exchange air conditioning with parallel pipes.The heat transfer per unit projection area of the equipment can be obtained by solving the geometric projection area ratio,which simplifies the calculation process.According to the heat transfer characteristics of the natural heat exchange air conditioning with parallel pipes in summer,a heat transfer model of the whole equipment consisting of elliptical finned heat exchanger tubes was established in CFD numerical simulation software.RNG k-? turbulence model and DO radiation model were selected in the simulation.In order to validate the simulation results,an experimental platform for cooling conditions was built to test the simulation conditions.The measured and simulated values of equipment cooling under two cooling and drying conditions were compared.The results show that the coincidence between the simulated and measured values is good,and the maximum relative error is less than 5%.This verifies the correctness of the selected model for this simulation calculation.In order to improve the heat transfer performance of the natural heat exchange air conditioning with parallel pipes,a three-dimensional fin with elliptical cross section outside the heat exchanger tube is proposed.The fin can not only greatly increase the heat transfer area between the equipment and the indoor air,but also form two vertical cavities between the outer wall of the circular heat exchanger tube and the elliptical fin,which can promote the natural convection heat transfer between the end of the air conditioner and the indoor air.The calculation model of three-dimensional elliptical fins with external long edge a=50mm and short edge b=20mm for D=10mm heat exchanger tube was established by using CFD software.The results show that the heat transfer capacity of single tube increases by 250.82%under the condition of heating in winter(water supply temperature is 50 C),when three-dimensional fins are installed outside the tube and natural convection is formed in the middle of the fins;when the distance between the tube walls is 10 mm,the overall heat supply capacity increases by about 40% compared with the equipment consisting of circular heat exchangers,and the heat supply per unit projected area increases by about 100 W.