The Investigation Of Heat And Mass Transfer In Ammonia-Water Falling Film Generation By Adding Nanoparticles

In recent years, ammonia-water absorption refrigeration system (AARS) have been gradually drawing renewed attention because of the advantages of utilizing ozone-friendly working fluids and the possibility of being energized by a waste heat or solar energy. At the same time, the tremendous potential of nanofluids in strengthening the convective and boiling heat transfer has been further discovered as the new generation of working fluid in the 21st century. This paper used nanoparticles into ammonia-water(AW) and through theoretical and experimental research methods to study the thermal mass transfer characteristics of nanoparticles in ammonia solution in the falling film generating process, which could clarify the heat and mass transfer process mechanism of the nanoparticles’ influence, and reveal the change rule among the vapor-liquid-solid heat and mass transfer. The research of this project provides with new ideas to improve the coefficient of performance of the AW absorption refrigeration system, miniaturize the generating equipment and develop novel efficient AW absorption refrigeration equipments. The specific work contents and conclusions are as follows:1) Preparation and investigation on dispersion stability of high temperature and high concentration AW nanofluids. By investigating the dispersion stability of 20 kinds of nanoparticles and 10 kinds of surfactants dissolved in high concentration AW at high temperature, the suitable nanofluids applied into falling film generating process were prepared. Ratio of absorbance (ROA) is defined to evaluate the dispersion stability of different kinds of nanofluids, and its reliability has been confirmed by sedimentation observation method and absorbance test method. According to the investigation on influence of boiling on dispersion stability of AW nanofluids, we find that the dispersion stabilities of boiled nanofluids are lower than unheated nanofluids to some extent. Besides this, Many factors have influence on dispersion stability of AW, including the type, structure, physical and chemical properties of dispersant and nanoparticles and the PH of AW. The criteria and optimal concentration are ascertained when adding nanoparticles and dispersants into AW to prepare the nanofluids by analyzing the action mechanism of surfactant and nanoparticles under different adsorption models. Meanwhile, this article explores the stability and surface tension properties of the anion and cation surfactant compound with AW nanofluids, the results showed that the anion and cation surfactant under the appropriate distribution proportion, can effectively improve the dispersion stability and lower solution surface tension of AW nanofluids.2) Investigation on Physical properties of AW nanofluids. Practical calculation equations on physical properties and (p, T, ξ) state equations of AW are concluded based on extensive related literatures. Six kinds of nanofluids with suitable amount of nanoparticles and surfactant and five kinds of nanofluids with suitable amount of surfactant only are prepared according to chapter 2, and all of them have well dispersion stability. Their thermal properties are measured include thermal conductivity, viscosity, and diffusion coefficient afterwards, and then analyze the theoretical calculation of above-mentioned physical properties in low concentrations nanofluids based on measured results. What’s more, the physical property calculation equations of AW nanofluids are derived which lay the foundation for the experimental study and theoretical modeling of AW falling film process. Explores the surface tension properties of the anion and cation surfactant compound with AW nanofluids, the results showed that the anion and cation surfactant under the appropriate distribution proportion, can effectively decrease the lower solution surface tension of AW nanofluids. Therefore, it provides a useful reference for the further exploration of low surface tension nanofluids.3) Experimental Research of falling film generation with AW nanofluids. The AW falling film test-bed was designed and established, and then well dispersed CB, ZnFe2O4, SiC, TiN and Fe2O3 AW nanofluids were prepared and applied to falling film experiments. At the same time, solutions without nanoparticles were applied to test-bed as a comparison. Results show that Fe2O3 and AW nanofluids can remarkably increase the generation rate, and the effective generation ratio can be improved to 1.70. TiN, CB and SiC nanofluids’strengthening effect are relatively lower than above mentioned 2 kinds of nanofluids in descending order. The results show that:Adding an appropriate mass fraction of nanoparticles and proper volume fraction of surfactant can increase the falling film generation rate of the AW. The mass fraction of the initial ammonia solution changes from 25% to 40%, setting ZnFe2O4 with mass fraction of 0.1% and the optional surfactant of SDBS with mass fraction 0.05%, the effective generation ratio can be increased by about 60%. Only adding surfactant of SDBS will produce certain inhibitory effect on the AW generation process. Select dispersant aspires to concern about both side of the disperse stability and the generation rate of the AW in order to achieve the optimal effect. Combined experimental results of AW falling film and previous research on enhancement of heat and mass transfer by adding nanoparticles. Finally, the mechanisms of nanofluids enhance the ammonia falling film generation are explained though the influences of micro motions of nanoparticles, field synergy theory, disperse phase particles theory, Marangoni effect and Rayleigh-Benard effect and the physical properties of nanofluids.4) Numerical simulation of falling film generation with AW nanofluids. This paper presents an equation suitable for calculating the AW nanofluids properties, establishes a more practical and convenient mathematical model to solve the heat and mass transfer in AW nanofluids falling film generation. By solving a discrete mathematical model, the influence of nanoparticle concentration and ammonia concentration to nanofluids physical parameters were obtained. Meanwhile the process parameters can not be directly measured in test, such as the velocity field, temperature field and concentration field, film thickness, interfacial mass and heat flow rate distribution. Simulation results show that:Viscosity, density and thermal conductivity of nanofluids increase with the increase of nanoparticles concentration, while heat capacity decreases with it; With the increase of ammonia concentration, heat capacity and density increase, viscosity and thermal conductivity decrease; Solution of radial velocity relative to the axial velocity is small, general need not consider fluctuations on the surface of the liquid film; The axial average speed and film thickness all along the length direction diminishing; Liquid film temperature distribution in the film thickness direction has a decreasing trend, while along the length direction, the film temperature increases gradually; The wall temperature, interface temperature and average temperature of liquid film all along the length direction increase gradually; Ammonia concentration distribution of the liquid film on the axial direction gradually decreases, and the film thickness direction also has a certain degree of downward trend; The wall concentration, interface concentration and the average concentration of liquid film all decrease along the length direction, and the interface concentration gradient is the largest; The gas-liquid interface of heat and mass transfer were mainly decided by the diffusion term. ③Thermal conductivity, viscosity and the diffusion coefficient of nanofluids have different effects on generation rate, heat and mass transfer coefficient in generation process because of the added nanoparticles. Results show that the thermal conductivity and diffusion coefficient have positive impact on the three above parameters, while the viscosity has a negative impact. Thermal conductivity has maximal effect on the heat transfer coefficient, followed by viscosity, and diffusion coefficient is the minimal property. However, diffusion coefficient has maximal effect on the generation rate and the mass transfer coefficient, followed by viscosity, and thermal conductivity is the minimal one.