The Study On The Flow Boiling Heat Transfer Characteristic Of Ammonia/salt Mixture In Horizontal Tubes And A Newly Online Concentration Measurement Method

Ammonia/salt solution attracts increasing attention in the field of refrigeration in recent years,becoming the most potential working pairs for small capacity units.Compared with the conventional and widely used H₂O-LiBr and NH₃-H₂O cycles,ammonia-salt absorption refrigeration systems are able to provide cooling capacity below 0°C without vacuum problems,and the need of a rectifier is eliminated.These advantages help to simplify the structure of the system and improve the reliability.Hence,in order to explore the application occasions of ammonia-salt absorption refrigeration system and its overall performance,this thesis starts to research on this field from two promising aspects.The most vital components of the absorption refrigeration system are the generator and the absorber.The generator is to obtain and utilize heat from the heat source,and the optimization of the structure of the generator will help to reduce energy consumption and enhance the overall efficiency of the whole system.In order to make the ammonia-salt absorption refrigeration systems to be applied in mobile transportation facilities,the first part on this thesis mainly research on the flow boiling heat transfer characteristic of ammonia/lithium nitrate solution in a smooth horizontal tube.The test conditions are selected based on practical working conditions of absorption refrigeration system with ammonia/lithium nitrate solution.The ammonia concentration of the solution at the inlet of test section is around 45.5 wt%.The mass flow rate and boiling temperature of the solution are in the ranges of 20–55 kg/h and 76.7–93.2?,respectively.The effects of the heat flux,mass flux and exit vapor quality has been investigated.Results show that heat fluxes have significant influence on boiling heat transfer coefficients.Nucleate boiling is found to be predominant during the tests.The experimental results are also compared with the available flow boiling heat transfer coefficients of NH₃/LiNO₃ solution and predicting correlations in literature.The existing correlations are all inappropriate to characterize the experimental boiling heat transfer coefficients,thus a new correlation especially for ammonia/lithium nitrate solution is proposed based on the experimental data,with a mean deviation less than10%.Pressure drops in the solution side are also measured under different working conditions.The results of the present study will be of great significance to the design and optimization of generators in the absorption refrigeration units using ammonia/lithium nitrate solution.Additionally,the absorption process in the absorber is the key of the whole running system,and the better absorption performance will lead to the improvement of the overall performance of the whole system.In order to evaluate the absorption efficiency of the absorber,the second part of the thesis proposes a new method for online measuring the concentration of working fluids in absorption refrigeration systems:electrical conductivity is measured to determine the concentration of the solution.Compared with the common density-concentration method,electrical conductivity-concentration method has the similar accuracy but helps to save the cost when applied in absorption systems with ammonia-salt solutions.Electrical conductivities of ammonia-lithium nitrate and ammonia-sodium thiocyanate solutions were measured between?293.15 and 333.15?K,using an Industrial Conductivity Meter.The ammonia mass fraction varied from 0.35 to 0.48 for ammonia-lithium nitrate solution and from 0.35 to 0.50 for ammonia-sodium thiocyanate solution.The experimental data were correlated as a function of temperature and composition using the extended Casteel-Amis equation,and the explicit format of this equation was also given.The experimental results and the obtained correlation will be of great help to the evaluation of the absorption efficiency of the absorber in absorption refrigeration systems.The content of this thesis is meaning for the design,optimization and evaluation of the components of the ammonia-salt absorption refrigeration system with small cooling capacity,which will also help to exploring its application situations.