|Simulation and experimental studies on indirect evaporative cooling system with porous material
|Yang, Hongxing (BEEE)
Lu, Lin (BEEE)
Hong Kong Polytechnic University -- Dissertations
|Department of Building Environment and Energy Engineering
|xxviii, 178 pages : color illustrations
|Indirect evaporative cooling, an air-conditioning (AC) approach based on physical evaporation process, is one of the promising ways for energy saving of central air-conditioning systems, especially in dry-weather regions with low wet-bulb temperature. Researchers have recently established various simulation models with higher accuracy for different types of indirect evaporative coolers (IECs) and optimized the experimental schemes for both IECs and spraying systems. Existing studies show that reliable cooling performance can be guaranteed by stable and continuous water spraying on the secondary air channel surface. However, the water retention ability and wettability are usually unsatisfactory in the traditional IEC due to the effect of gravity and surface property. Because water can only transiently adhere to the vertical channel wall, the IEC has always been highly dependent on water spray systems, which consumes much energy, and thus limits further improvement of the system coefficient of performance (COP). It has been generally realized that more space needs to be provided for water storage, and porous material is promising for solving this problem because its small cavities are natural rooms for the liquid holdup. Accordingly, a novel plate-type cross-flow indirect evaporative cooler with porous material (PIEC) was newly proposed in this thesis. The developed porous structure in the IEC can effectively improve the water retention ability of the secondary air channel surface, which is beneficial to reducing the system's dependence on the water spraying system.
Firstly, the feasibility of using the porous material in the secondary air channel of the plate-type cross-flow IEC for liquid holdup was experimentally demonstrated, and the cooling performance of the PIEC was achieved under different spraying modes. The hybrid plate, comprising a porous nickel layer sintered on a stainless-steel sheet, was designed and manufactured. The porous nickel is to store water, while the smooth stainless-steel plate can prevent water from seeping into the primary air channel. A water retention test was conducted for the hybrid plate, which showed that the sprayed water can be collected in the porous zone to support evaporation during the non-spraying period. Then, a cross-flow PIEC prototype assembled by hybrid plates was tested in the laboratory. Experimental results confirmed that the PIEC could not only cool the air under consistent spraying, but also keep the cooling effect for a period of time at the cost of a slight temperature rise when water spraying was interrupted, which indicates the relief of dependence on the spraying water.
Secondly, a three-dimensional (3-D) PIEC simulation model was established based on the computational fluid dynamics (CFD) approach to forecasting the cooling performance of the PIEC with different inlet air conditions under consistent and periodic spraying conditions. Results predicted by this 3-D model were compared with the data obtained from the previous experimental study for validation, which are in good agreement. Using this model, the temperature and humidity distributions in the PIEC were presented over time when intermittent spraying was implemented. In addition, the effects of various parameters on the dynamic variation of the primary air outlet temperature, average wet-bulb efficiencies, and non-spraying intervals have been quantitatively investigated.
Thirdly, considering the high computational requirements and resource consumption of the established 3-D PIEC model, response surface methodology (RSM)-based regression models were developed for this novel heat exchanger. These models were developed to forecast the system performance more simply under the consistent spraying and periodic spraying modes. The analysis of variance (ANOVA) was carried out for each response, and the influence of the single factors and interactive terms of the controllable parameters on the response was both revealed. In addition, a multi-objective optimization of the operating parameters of the PIEC has been achieved based on the desirability function approach.
Finally, the PIEC system performance was investigated and compared from the energy, exergy, and environmental (3E) perspectives under different spraying modes to exhibit the advantages of using the period spraying resulting from the application of porous media. Results show that the periodic spraying scheme can substantially improve the COP among the studied cases, albeit with minor temperature fluctuations. Furthermore, the periodic spraying mode increased exergy efficiency and reduced exergy loss ratio due to the decreased temperature potential difference in the primary air side and humidity potential difference in the secondary air side. Regarding the environmental benefits, the greenhouse gas emission of the PIEC in the periodic spraying scenarios is less than that under the conventional consistent spraying mode.
The key academic contributions derived from this thesis are summarized as follows: 1) A cross-flow PIEC was proposed to improve the liquid holdup on the secondary air channel surface so as to alleviate the dependence on consistent water spraying operations. 2) The feasibility of using the porous material and implementing the periodic spraying to replace the traditional consistent spraying were fully verified by the designed experiments. 3) The established 3-D model lays the foundation for predicting the PIEC performance under consistent and intermittent spraying conditions, and the RSM-based regression models of the selected responses offer a more straightforward approach for IEC performance forecasting and optimization. 4) The advantages of the period spraying operation resulting from using the porous media in the IEC were presented based on the performance comparison under the two spraying modes from 3E perspectives. Overall, the novel PIEC proposed in this thesis exhibits enhanced water retention ability and reduced dependence on spraying systems compared to traditional IECs, providing valuable insights for the development of next-generation IECs.
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