Experimental Research On The Flow Boiling Heat Transfer Characteristics Of NH₃-NaSCN Solution In Smooth Horizontal Tubes

In recent years,with the development of refrigeration technology,the absorption refrigeration technique gradually came to people's visions.Its advantages such as energy saving made it a research focus by many scholars.Most importantly,the working pairs utilized by the absorption refrigeration system are very environmentally friendly,it does no harm to the ozone.The conventional working pairs are ammonia-water and water-lithium bromide.In recent times,ammonia-salt solution is drawing more attention for its unique properties.As the working solution,ammonia is a natural refrigerant and very friendly to the environment.Besides,the ammonia-salt absorption refrigeration system can get a very low evaporating temperature.The system is also very simple with a low expense.In a word,it is very essential to research the properties of ammonia salt to determine the feasibility to put it into real refrigeration system.In this work,the key parts of the absorption refrigeration system,namely,generators and absorbers,were separated from the whole system to research the flow boiling heat transfer characteristics of ammonia-sodium thiocyanate solution in smooth horizontal tubes.During the research,a new experimental system was built to perform the measurement.The impact of the heat flux,mass flux and exit vapor quality as well as the inner diameter of the test tube on the flow boiling heat transfer coefficients was analyzed.Based on the properties of the ammonia salt solution,the concentration of the ammonia was around 45.5%.The heat flux was in the range of 10.27⁴2.90kW/m².The experiments were carried out in five different mass flux,namely,100,155,200,243 and 304kg/?m²*s?.The generating temperature was controlled from 78⁹2?.Besides,a comparison of the flow boiling heat transfer coefficients between the NH₃-LiNO₃ and NH₃-NaSCN working pairs was also carried out in a nearly same working condition to judge their differences.The experimental results indicate that the flow boiling heat transfer coefficients of the NH₃-NaSCN solution increase with the increase of heat flux or the mass flux.However,the heat flux seems to be in the dominant role.So,the flow boiling process of the solution in a smooth horizontal tube is decided by the nuclear boiling mechanism.It can be also observed that the flow boiling heat transfer coefficient in the larger tube is bigger than that in a smaller tube.When the solution is put into a real absorption refrigeration system,a balance point between the system performance and the size of the system needs to be found to get an optimum design.Besides,in the same working condition,the flow boiling heat transfer coefficient of NH₃-NaSCN solution is higher than the coefficient of the NH₃-LiNO₃ solution.The lower viscosity and higher thermal conductivity of the former solution is the main reason.The research conclusion in this work is very beneficial to the application of ammonia salt solution and the design as well as the optimization of the generator in the absorption refrigeration system.