The Investigation On Performance Of Ammonia Water Absorption Refrigeration System With Nanoparticles,Thermal Properties And Dispersion Stability Of Nanofluid

With the increasing attention to natural refrigerants,ammonia absorption refrigeration systems?AARS?have also received a lot of attention,especially in the context of the current global energy shortage.At the same time,as a new generation of heat transfer working fluid in the 21st century,nanofluids also have great potential in enhancing heat and mass transfer,and related research on ammonia-water nanofluids is gradually enriched.Therefore,in order to further improve the performance of the AARS,the ammonia-water nanofluid is applied to the solution circulation in the AARS,and the absorption process,the generation process,the heat transfer process and the coefficient of performance?COP?of the system are studied by the experimental method.The purpose of this paper is to reveal the influence of nanoparticles on various aspects of the system,thus providing new ideas for improving the COP of AARS and promoting system miniaturization.The work done in this paper mainly includes the following aspects:?1?Construction of a small-scale test bench system for AARS.The research of nanofluids applied in AARS mostly stays in theoretical calculation or local generation process,absorption process and heat transfer process.Based on the previous research work and theoretical calculations of the research group,this paper proposes the feasibility and necessity of building a small-scale test bench system for AARS.The test bench system includes a single-effect AARS test bench,a cooling water circulation system,a nanofluid preparation and dispersion system,a data monitoring and recording system,and so on.Taking into account the economics of the test bench construction and the later maintenance and the convenience of handling during the test,the test system adopts the single-effect form and the overall size is all miniaturized,and the actual footprint of the refrigeration system main body does not exceed 6m².At the beginning of the design of the test bench,the cooling capacity was set to 3kW.Other main design parameters included the heat source temperature of the generator was set to 150°C,the cooling water inlet temperature was set to 32°C,and the evaporation temperature in the evaporator was set to-15°C.In order to prevent the test system from rusting after a long period of operation or shutdown,which affects the test results,the components of the absorption refrigeration system are made of stainless steel.?2?Preparation and addition scheme of ammonia-water nanofluid.In this paper,the preparation of the ammonia-water nanofluid can be realized by the two-step method using the solution pump of the system,the solution in the absorber and the external dosing tank.During the preparation process,the ammonia-water nanofluid in the dosing tank can also be performed by using a high-power electromagnetic stirrer.Therefore,during the preparation of the ammonia-water nanofluid,the nanofluid is circulated to the dosing tank and the absorber under the action of the solution pump and is also stirred by the electromagnetic stirrer,thereby achieving the purpose of improving the stable dispersion of the nanoparticles in the base fluid.After the preparation of the ammonia-water nanofluid is completed,the nanofluid in the dosing tank is filled into the absorber by using the high-pressure ammonia gas in the external ammonia tank,and the circulation of the prepared nanofluid in the solution loop can be realized by opening the corresponding valve in the system solution loop.?3?Selection of nanofluids applied in test benches for AARS.By reviewing and comparing a large number of literature on the physical properties of ammonia-water nanofluid,and considering the physical properties changes of nanofluids under dynamic flow,this paper finally chose the TiO₂nanoparticles as the particle type for the preparation of nanofluids by the two-step method.After the investigation of the physical properties of dynamic TiO₂ ammonia-water nanofluid,it was found that excessive amounts of surfactant?SDBS?added can cause the nanofluid to produce a large amount of foam,thereby rendering the nanofluid unusable,which was not discovered when the same amount of surfactant was added under static conditions.Therefore,this paper finally determined the type of nanofluid and its corresponding surfactant content applied to the test bench of the AARS.?4?Study on the influence of nanoparticles on system'performance and other parameters.The main purpose of this paper is to study the effects of nanoparticles on various aspects of the system.Therefore,specific parameter indicators are needed to reflect the changes brought by the nanoparticles into the solution cycle under the same operating conditions.Among them,the most direct and comprehensive index is the system's coefficient of performance?COP?.The optimal amount of nanoparticles added was determined by directly comparing the effects of different amounts of TiO₂ nanoparticles added to the system on the COP of the system.Also,for the AARS,many parameters are affecting the system'COP,such as the outgassing range,the circulation factor,etc.,which are deeply affected by such processes as absorption and generation.Therefore,in order to further analyze the influence of the nanoparticles on the system after adding to the system,the paper has carried out a more detailed analysis of the absorption process,the generation and rectification process,and also the heat exchange process in the solution heat exchanger.?5?Study on the physical properties and stability of the ammonia-water nanofluid circulated in the solution circle.The ammonia-water nanofluid in the solution circle experienced the“Boost-heating-rectification-generation-cooling-decompression-absorption”process during the operation of the system.However,most of the existing literature focus on the study under the conditions of room temperature and atmospheric pressure,and the nanofluids applied to the AARS are subject to various abnormal conditions.Therefore,it is necessary to study the physical properties and stability changes of the ammonia-water nanofluid after undergoing the alternating cycle process.In this paper,by adding an appropriate amount of nanoparticles to the solution circle of the AARS,and the sample is taken from the solution circulation loop periodically.The physical properties and stability of the nanofluid were tested to study the effect of system operation on nanofluids.?6?Analysis of the thermodynamic effects of nanoparticles on solution circulation.In this paper,the combination of entropy analysis and enthalpy analysis is used to determine the distribution of irreversible losses in the solution circulation loop,which provides a theoretical basis for further optimization of the system.The results show that the addition of nanoparticles does not reduce the entropy production and damage of either process.On the contrary,due to the strengthening of the heat and mass transfer process,the absorption process,the process of generation and the heat exchange process are more irreversible.Thanks to the enhancement of the heat and mass transfer process brought by the nanoparticles,the coefficient of performance of the system can be improved under the same working conditions so that the overall efficiency of the system can be improved,and the optimal operating range of the system can be improved to some extent.?7?Study on the influence of ammonia purity on the system using a flooded evaporator.In a flooded evaporator,since water is a hardly volatile component,it is difficult to evaporate and absorbed by the absorber,thereby it is easy to accumulate in the flooded evaporator.As the operating time of the system increases,the content of liquid water in the evaporator gradually increases.Once the concentration of ammonia decreases,the vapor pressure will decrease at the same evaporation temperature and the other operation conditions,and it will cause decreasing absorption pressure,so that the absorption process is weakened,which results in the lowered outgassing range,the increased circulation factor,and the lowered COP of the system.So far,most researchers have only qualitatively analyzed the negative effects of impure ammonia after rectification and stripping on systems equipped with flooded evaporators.This paper quantitative analyzes the effects of impure ammonia in the flooded evaporator on the system.