Experimental Research On The Flow Boiling Heat Transfer Characteristics Of NH₃-LiNO₃-H₂O Solution In Smooth Horizontal Tubes

With the development of society and economy,the continuous deterioration of the global environment and the continuous consumption of energy have attracted more and more attention.The traditional vapor compression refrigeration mostly use Freon as the working fluids,which will aggravate the appearance of negative effects on the natural environment,such as the ozone layer cavity and the greenhouse effect,at the same time the total energy consumption has remained high level for long time.The absorption refrigeration system mostly uses environment-friendly natural working fluid pairs,which can make full use of low-grade energy such as industrial waste heat,ship exhaust heat,solar energy,etc.,which can well mitigate the impact of refrigeration working fluid on the environment and increase the diversity of energy utilization.Therefore,the development of absorption refrigeration systems with excellent performance is of great benefit to the sustainable development of society.The purpose of this article is to explore the experimental characteristics of the flow boiling heat transfer of an absorption refrigeration system using the NH₃-LiNO₃-H₂O ternary solution as the working fluid in smooth horizontal pipes,which can provide a certain reference for the structural design of the generator,which is the core component of the absorption refrigeration system,and the selection of the working fluid.At the beginning of the experiment,based on the existing physical property test data of NH₃-LiNO₃-H₂O ternary solution,this paper fitted the working pressure of this working fluid at different concentrations and different temperatures.At the same time,the calculation formulas of its physical parameters such as density,specific heat under constant pressure,dynamic viscosity,and thermal conductivity with solution concentration and temperature as variable are proposed.This article designs and sets up an experimental platform for the measure of boiling heat transfer in smooth horizontal tubes for ammonia/salt working fluid.By controlling variables,the qualitative study of the influence to boiling heat transfer coefficient due to the factors such as mass flow rate,heat flux density,tube diameter,exit vapor quality in the test section can get to begin.Based on the characteristics of the working fluid and the requirements of the experimental conditions,the mass concentration of each component in the selected ternary solution of NH₃-LiNO₃-H₂O was 50%,35%,and 15%,respectively.At the same time,five mass flows of 166,221,278,332,and 388kg/m²·s)were selected for testing during the experiment.The measured ternary solutions with boiling temperature of 59.0,62.4,and 65.5°C in different tubes.As for the outer diameter of the smooth horizontal tubes,it was 6mm,8mm and 10mm,and its heat flux density is recorded in the range of 4.6-78.5 kw/m².At the same time,the data of flow boiling heat transfer coefficients using the three solutions NH₃-LiNO₃-H₂O and NH₃-LiNO₃ and NH₃-NaSCN as working fluid under similar conditions are carried out for comparison.The experimental results show that both nuclear boiling and convective boiling play an indispensable role in the flow boiling heat transfer process.The convective boiling mechanism increases with the increasing of mass flow,while the nuclear boiling mechanism is the opposite.At the same time,higher heat flow density has a positive effect on the boiling heat transfer process,but there is a certain threshold.Excessively high heat flow density may cause complete suppression of nuclear boiling.The effect of the exit vapor quality on the boiling heat transfer coefficient is similar to the heat flux density.Under similar conditions,the flow boiling heat transfer coefficient in smooth horizontal tube with a smaller inner diameter is lower than that in smooth horizontal tube with a larger inner diameter.In addition,because of the presence of the H₂O component in the solution,its viscosity is low and its thermal conductivity is high.Under similar conditions,the boiling boiling heat transfer coefficient of the ternary solution should be larger than the binary solution of NH₃-LiNO₃ and NH₃-NaSCN.The content of this paper is of great significance to the structural design optimization of the ammonia/salt absorption refrigeration system in actual production practice and the selection of working fluid for the absorption refrigeration system.