Increasing the efficiency of contact humidifiers in the operating conditions of thermal water desalination systems
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Дата
2025
Автори
Науковий керівник
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Видавець
Igor Sikorsky Kyiv Polytechnic Institute
Анотація
Liu Yang. Increasing the efficiency of contact humidifiers in the operating conditions of thermal water desalination systems.
Dissertation for a Philosophy Doctor degree in specialty 144 Thermal Power Engineering. National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” MES of Ukraine, Kyiv, 2025.
This dissertation is dedicated to the study of the air humidification process in a acrylic film-type contact heat exchanger under typical operating conditions of thermal water desalination systems. The introduction explains the importance of the research topic, defines the object and subject of the study, highlights its scientific novelty, and states the aim and objectives. It also justifies the scientific principles and conclusions presented in the dissertation. Additionally, it details the practical significance of the findings, the author’s individual contribution, the validation of the findings, related publications, and outlines the scope and structure of the work. Chapter 1 is devoted to the analysis of the current state of water desalination technologies and a review of contact humidifier designs used in thermal desalination systems. The chapter begins with an analysis of freshwater supply issues in China, identifying the regions most affected by water scarcity. It is shown that decentralized, small-scale seawater desalination is a promising solution for coastal areas and offshore islands. The chapter describes the main types of small-scale desalination units, including solar distillers, membrane systems, reverse osmosis, and humidificationdehumidification (HDH) systems. The advantages of HDH systems for use in remote and economically disadvantaged regions are discussed, along with factors that limit their implementation. It is noted that significant attention in modern research is paid to the integration of HDH systems with power units based on steam turbines. In such hybrid schemes, the steam turbine acts as an electricity generator, and its exhaust steam serves as a heat source for the desalination process. Various configurations are considered in which the steam turbine condenser is used as an air or water heater for the HDH cуcle. Analysis of thermodynamic parameters showed that the optimization of working fluid expansion modes in the steam turbine and effective recovery of condensation heat allow increasing the total efficiency of the unit up to 94%, ensuring stable production of fresh water and electricity. Particular attention is paid to the analysis of contact heat exchanger designs within HDH systems. It is established that the most common types of humidifiers are packedbed towers and bubbling columns. The advantages and disadvantages of existing humidifier designs are summarized. The literature review reveals a lack of sufficient data on air and water pressure losses, which hinders a comprehensive assessment of humidifier performance, their energy and exergy indicators. Special attention is required for the selection of materials, which must combine corrosion resistance to aggressive environments (hot brine) with the necessary mechanical strength of the structure. Furthermore, the influence of thermal state of humidifier elements on their operational life and the stability of heat and mass transfer processes remains insufficiently studied. The research presented in this dissertation is aimed at reducing the specific energy consumption of contact humidifiers under typical operating conditions of thermal desalination systems. Chapter 2 describes the design of the experimental setup developed and constructed to investigate the air humidification process. The system features an open-air circuit and a closed water circuit. Its main components include a humidifier, a water heater, a pump, a fan, and a control and measurement system. The humidifier is a transparent acrylic tube, two meters in length, and with an internal diameter of 26 mm. The choice of acrylic as a structural material is conditioned by its ability to ensure a stable stress state and a uniform thermal state of the wall due to low thermal conductivity. Unlike materials for bubble and spray humidifiers, acrylic demonstrates higher fracture toughness, which minimizes the risks of crack growth initiation and fatigue damage accumulation under conditions of frequent system startup and shutdown cycles. To monitor the air temperature and humidity in real time, the setup employs instrumentation from Regmik, which enables a detailed analysis of heat and mass transfer dynamics during the humidification process. The system allows for adjustments to the humidifier’s thermal load, regulation of water and air flow rates, and systematic collection of experimental data. This setup makes it possible to study the energy performance of the humidifier under typical operating conditions of HDH systems. An experimental procedure was developed, including data collection and processing methods, as well as calculation of the thermal balance between water and air, with a distinction between latent and sensible heat components. An error analysis was also conducted, accounting for uncertainties in both measurements and calculated parameters. Chapter 3 investigates the operating parameters and energy performance of a tube film humidifier. It presents a classification of water film and air flow regimes within a vertical tube, along with an explanation of the mechanisms leading to flooding. The lack of clearly defined transition boundaries between flow regimes and the absence of accurate methods for determining air pressure losses below the flooding point have been noted. The visual identification of flow regimes in the humidifier was carried out, and threshold values of water and air mass flow rates corresponding to regime transitions were established. It was found that at a superficial air velocity below 0.34 m/s, air pressure losses can be reliably determined using the Darcy-Weisbach equation. Based on this, it is recommended to limit the superficial air velocity in the humidifier to 0.34 m/s to minimize pressure losses and prevent contamination of the condensate with saltwater droplets. The minimum water mass flow rate required to maintain continuous film flow was determined experimentally. The feasibility of using a minimum mass flow rate ratio of water to air (MR = 2) was substantiated. Under these conditions, maximum heat and mass transfer efficiency was achieved, ensuring optimal performance of the humidifier. The influence of inlet water temperature (in the range up to 60 °C) on the intensity of heat and mass transfer processes was established. It is shown that increasing the temperature leads to an increase in humidifier productivity and outlet air temperature. Thermodynamic analysis revealed that with an increase in the temperature difference, the humidification efficiency decreases due to the increase in process irreversibility, while the exergy efficiency remains constant (ηhum=0.58). The obtained results confirm the energy feasibility of the proposed humidifier operating modes. An important result of the operational tests is the confirmation of the durability of the humidifier's structural material. In the studied range of operating parameters, no signs of thermal deformation, clouding of the acrylic, or crack growth initiation were detected. This indicates that the heating-cooling operating cycles do not lead to critical changes in the stress state of the tube wall or the accumulation of fatigue damage, guaranteeing the durability of the structure. Chapter 4 presents the developed thermodynamic model of a film humidifier based on heat and mass transfer equations. The model was validated using experimental data. New simplified empirical formulas were proposed to determine the heat and mass transfer coefficients in film humidifiers under the operating conditions of HDH systems. A calculation method was developed, which can be used for engineering design, technoeconomic analysis, and operational optimization of film humidifiers in thermal desalination systems. Modeling of the humidifier’s performance was conducted, and the optimal geometric dimensions of the tube film humidifier were determined: a diameter of d = 50 mm and a height of l = 1 m. These parameters maximized the efficient use of the humidifier’s volume, allowing high performance with minimal thermal and electrical energy consumption. In addition, the influence of ambient air parameters on the operational characteristics of the film humidifier was analyzed. It was established that the apparatus productivity depends weakly on the ambient air temperature but decreases significantly with an increase in its relative humidity due to a decrease in the mass transfer potential. At the same time, it is shown that the gain output ratio (GOR) demonstrates a slight increase with an increase in both climatic parameters. This effect is explained by the decrease in the density of humid air and, consequently, the decrease in air mass flow rate, which leads to a reduction in the total thermal load on the humidifier. Chapter 5 compares the energy performance of the investigated film humidifier with other common designs. A description of various types of packing materials widely used in HDH systems is provided. The operating parameters and energy characteristics of the humidifiers selected for comparison are presented. The influence of the water-air mass flow ratio (MR) on evaporation intensity, thermodynamic efficiency, and air aerodynamic resistance was analyzed. It was established that in the temperature range characteristic of solar thermal systems (30–60 °C), the evaporation rate (ER) values for the film humidifier reach 200–800 kg/(m³·h), which is an order of magnitude higher than the indicators of systems with cellulose pads, Raschig rings, or paddy grass. The hydraulic advantages of the developed humidifier are separately substantiated: the absence of nozzles for water spraying allows creating the operating head exclusively due to the geometric height of the tube. This ensures minimal hydraulic losses on the liquid side compared to packed bed analogs, where energy costs for overcoming the resistance of sprayers are significant. It is demonstrated that the film humidifier ensures the highest evaporation intensity among all humidifier types. At the same time, its aerodynamic and hydraulic resistance is minimal. It is established that the use of a film humidifier allows reducing electricity consumption in the HDH system without deteriorating its productivity. It was shown that the film humidifier achieves the highest evaporation intensity among all types analyzed, while maintaining minimal aerodynamic and hydraulic resistance. The results confirmed that using a film humidifier reduces the electricity consumption required for circulating water and air in the HDH system without compromising overall system performance. The dissertation materials and research findings have been implemented at China Hydrogen Energy Group Co., Ltd and Liaoning Hongsheng Environmental Solutions Technology Co., Ltd.
Опис
Ключові слова
cycling air, energy efficiency, environmental safety, heat exchanger, heat transfer, mathematical model, separation, pressure losses, renewable energy, temperature gradient, thermal state, turbine, two-phase flow, water desalination plant, wastewater, відновлювана енергетика, втрати тиску, двофазний потік, екологічна безпека, енергоефективність, математична модель, опріснювальна установка, розділення, стічні води, температурний градієнт, тепловий стан, теплообмінник, теплопередача, турбіна, циркуляційне повітря
Бібліографічний опис
Liu Yang. Increasing the efficiency of contact humidifiers in the operating conditions of thermal water desalination systems : dissertation submitted for the Doctor of Philosophy degree : 144 Thermal Power Engineering / Liu Yang. – Kyiv, 2025. – 137 p.