| Author: | Ma, Xiaochen |
| Title: | Development of new water spray strategies for improving the energy performance of indirect evaporative cooling systems |
| Advisors: | Yang, Hongxing (BEEE) Lu, Lin (BEEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Air conditioning Evaporative cooling Energy conservation Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xxviii, 235 pages : color illustrations |
| Language: | English |
| Abstract: | Along with the increasing energy consumption of space cooling in buildings, the building industry is in urgent need of the environmental-friendly and proven cooling technologies. Indirect evaporative cooling is attracting wider attention as a sustainable technology based on the heat absorption properties of evaporation. It has been demonstrated that the plate surface covered by water membrane in the wet channels has a substantial effect on the operating performance in the practical use of indirect evaporative cooling technology, while the ideal state of the uniform water distribution assumed by existing models is usually difficult to be achieved. Even though several hydrophilic materials have been suggested for indirect evaporative cooler (IEC) to increase surface wettability, the non-uniform water film distribution brought on by improper nozzle settings remains a significant issue that worsens evaporation and leads to the consumption of large amounts of water. In order to enhance the evaporation process, it is crucial to tune the nozzle features in water supply systems and organize the nozzles suitably above the heat exchanger. Therefore, this thesis comprehensively analyzes the impact of the water spray system design on the evaporation efficiency of the IEC wetted channel passage by means of simulations and experiments, taking into account the actual wetting rate in the wetted channel. It also proposes the new water spraying strategies to optimize the evaporation efficiency of the secondary air channel of the IEC system from the perspectives of nozzle selection, installation spacing, air-water configuration, and operating parameters, so as to improve the working performance of the IEC system and to promote the further development of the system in terms of low energy consumption and high efficiency. First, a numerical model was developed and validated by experimental results to predict the spray water density distribution of the solid cone nozzles on the impact surface. The actual water spray density obtained from this model could be used to correct the wetting factor in the existing IEC numerical model. Besides, the control variable method was applied to compare the effect of inclined angle and distance between the nozzles, and the arrangement was optimized based on the uniformity coefficient as well as the coverage ratio. The results demonstrated that the optimized nozzle arrangement scheme led to improved air cooling and dehumidification, thereby enhancing the coefficient of performance (COP) of the IEC system. Second, based on the aforementioned investigations, it was discovered that the distribution of the water film within the wet channel of the IEC has a significant impact on its operational performance. To analyze the motion of the spray droplets and the state of water film coverage on the plate surface, a three-dimensions (3D) Computational Fluid Dynamics (CFD) model was proposed. For experimental validation of the CFD model, a test rig consisting the entire IEC system was constructed, and the simulation results were in good agreement with the experimental results. Using this model, the temperature distribution inside the IEC was demonstrated for variations in spray parameters. In addition, the characteristics of the primary air inlet were parametrically analyzed and the optimal spray design parameters were proposed. Third, to address the problem that the developed 3D CFD model requires high computer configuration and consumes large computational resources, this thesis developed a prediction model based on back-propagation artificial neural network (BP-ANN) focused on the influence of the six main operating parameters of the spraying system in order to predict the performance of the IEC system in a simpler way. The model was rigorously validated using relevant experimental data, and the importance of each influential parameter was assessed by gray relational analysis. In addition, the multi-objective optimization method based on Genetic Algorithm (GA) was introduced to efficiently determine the optimal operating parameters of the IEC system. Last, due to the high applicability of employing IEC as a heat recovery device in data centers (DCs), a 3D simulation model of the diamond-shaped IECs containing the water spray system was developed to compare and analyze the air-water arrangement options in IEC systems currently utilized in DCs, and the corresponding characteristics were explored from the perspective of CFD technology to determine the effects of nozzle configurations on the formation of a water film and evaporation in the wetted channel. Ultimately, it was determined that the top-configured nozzle performed best with the air-water counter-current form of the nozzles, and the effectiveness of the enhanced approaches with hydrophilic and fiber-coated simulations were further analyzed. The main academic contributions of this thesis are summarized as follows: 1) A spray volume flow distribution model incorporating spray inclination was developed and validated, and the existing IEC model was further modified. 2) A novel 3D CFD model was proposed and validated by the experimental results, which considered the actual coverage factor of the water film, including the cooling performance of the whole IEC system. 3) Based on the above two models, the nozzle arrangement scheme and setting parameters were optimized, and the improvement of thermodynamic performance as well as energy efficiency of the optimized IEC system was demonstrated in comparison with the original scheme. 4) A ML-based prediction model was developed to accurately estimate the performance of the IEC system, and GA was applied to optimize the IEC-ANN model to achieve efficient performance while maintaining low energy consumption. 5) A CFD model based on the diamond-shaped IEC was developed to explore the air-water flow, heat transfer, and mass transfer capabilities of the IEC as a heat recovery unit, and to identify the optimal nozzle configurations and performances, as well as the enhancement strategies for IEC in DCs. In conclusion, the analyzed and developed new water spray optimization strategies could effectively improve the performance of IEC systems and provide effective guidance for the application and further development of IEC systems. |
| Rights: | All rights reserved |
| Access: | open access |
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