Theoretical and experimental study of absorption and absorption/diffusion refrigerating machines using ammonia as a refrigerant: Simulation under steady-state and dynamic regimes and experimental characterization of a pilot
- Autores: Rami Mansouri
- Directores de la Tesis: Alberto Coronas Salcedo (dir. tes.), Mahmoud Bourouis (codir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: Hatem Mhiri (presid.), Joan Manel Vallés Rasquera (secret.), Ahmed Hannachi (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Termodinámica de Fluidos por la Universidad de Burgos; la Universidad de Santiago de Compostela; la Universidad de Valladolid y la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Current research concerning absorption and diffusion-absorption refrigerating machines is carried out according to two main directions. Firstly, there are investigations aiming at deepening the understanding of functioning of these machines and at studying their performance at various operating conditions. Secondly, there are other investigations focused on improving the performance of the NH3/H2O and H2O/LiBr refrigerating machines and the search for new fluid mixtures susceptible to be used as working pairs in such machines. In this thesis, detailed models are developed to predict, evaluate and analyze the detailed behavior of functioning and the performance of ammonia-based absorption and diffusion-absorption refrigerating machines under steady-state and dynamic regimes. This thesis present also a detailed experimental analysis on the operation of a small capacity diffusion-absorption refrigerator, which will be useful to understand the optimal operation conditions of this machine.
Theoretical and experimental investigations on ammonia-based an absorption and a diffusion-absorption refrigerating machines were carried in this thesis. First, the steady-state operation of a commercial NH3/H2O gas-fired absorption chiller was theoretically investigated using the flow-sheeting software Aspen-Plus. Then, a small capacity diffusion-absorption refrigerator was experimentally and theoretically investigated under steady-state and dynamic regimes. The overall heat transfer coefficients and the refrigerator operation during the start-up period were established based on the experimental data. An Aspen-Plus model for the steady-state operation and a black-box model in the Matlab Simulink® environment for the dynamic operation were developed for this refrigerator. The methodology employed and most relevant results are summarized below.
In the case of the absorption chiller, the goal was to develop a reliable simulation model for the machine and to validate it basing on the published experimental data. The first step in this part of the work was to select the appropriate property model for the working NH3/H2O fluid mixture used. Nine different models implemented in Aspen-Plus were tested in two steps. Firstly, the vapor-liquid equilibrium (VLE) calculated by each one of the Aspen-Plus property models was compared with regressed VLE published data at different pressures. It was found that none of these models predicts the NH3/H2O vapor-liquid equilibrium with sufficient accuracy. Secondly, the Aspen-Plus data regression tool was used to deduce the binary interaction parameters of the Aspen-Plus property models by fitting the vapor-liquid equilibrium published data. It was found that the Peng-Robinson-Boston-Mathias equation of state (PR-BM) with regressed interaction parameters is the most suitable property model for the NH3/H2O fluid mixture in the temperature and pressure ranges encountered in absorption refrigerating machines. The sum of squared deviations for the selected model was 36.55.
Once the appropriate property package for the NH3/H2O mixture selected, a model of a commercial 3-ton NH3/H2O gas-fired absorption chiller was developed using the flow-sheeting software Aspen-Plus to simulate the steady-state operation of the machine. First, the overall heat transfer coefficients of the condenser, evaporator, absorber and refrigerant heat exchanger were deduced from experimental data at 35ºC cooling air temperature. The results were then compared with the experimental data and calculated data. This comparison showed a good agreement between the three sets of results. In a further step the model was modified to include the (UA) values of the heat exchangers calculated in the previous step as input parameters. The results were again compared with experimental and calculated data at cooling air temperatures of 26.7ºC and 38ºC. The model prediction showed a good agreement with the two sets of bibliographical data. The second part of this work was dedicated to the investigation of a small capacity (7.5W) commercial diffusion-absorption refrigerator, tested under different heat input conditions to the generator. The temperature at the inlet and outlet of every component was continuously recorded. Monitoring the temperature profiles allowed for the determination of the minimum power supply to the generator needed to ensure the functioning of the refrigerator and its stability. The results showed that a supply of 35 W ensured the functioning of the refrigerator but not its stability, while with 39 W a steady operation of the refrigerator was reached after two and a half hours. All the essential features of the refrigerator were determined experimentally, especially the heat exchange capacities of the cabin and the evaporator, respectively (UA)_cab=0.554 WK^(-1)and (UA)_int=0.3 WK^(-1). The best performance of the refrigerator was reached experimentally with an electric power supply of 46 W with a generator temperature of 167°C. The corresponding machine COP was found to be 0.159.
Here again an Aspen-Plus simulation model was then developed for the diffusion-absorption refrigerator for the Steady-state operating mode. To this propose, a set of assumptions based on the experimental tests were introduced as inputs. The refrigerator model developed was run, in a first step, using a sequential modular approach in which each block was calculated separately, with two “break points” constituting the convergence criterion. After convergence and synchronization, the Equation-Oriented (EO) approach was used, in a second step, to solve simultaneously in one block the governing equations of the diffusion-absorption refrigerator. The results obtained for the heating rates 46W, 56W and 67W were well in agreement with the measured temperatures. The deviations between predicted and measured COP and cooling capacity were less than 1%. This indicates that the model developed represents fairly well the functioning of the small capacity commercial diffusion-absorption refrigerator working under a steady-state regime.
Basing on the experimental data obtained for the diffusion-absorption refrigerator in nonstationary mode (set-up phase), a dynamic black-box model correlating the power input to the generator and the cooling capacity of the refrigerator was developed using the Matlab identification package. A first order transfer function with delay was found to describe quite accurately the time evolution of the cooling capacity for all considered heat rates supplied to the generator. In a further step, regressed analytical expressions of the three transfer function parameters of function of the generator heat supply were incorporated in the cooling capacity function. A generalized dynamic black-box model for the refrigerator was thus obtained which was then validated using the Matlab Simulink® environment. The model predictions were found to be in good agreement with the experimental data. In particular, the steady-state COP predictions of the model agree satisfactorily with experimental data with a maximum relative deviation of about 8%.
