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• This work focuses on interphase mass transfer mechanism of liquid desiccant dehumidification. • Mass transfer correlations are developed based on the interface instability of falling film. • Correlations are verified by test data from several literatures with a wide range of conditions. • Predictions show close trends to previous experiments, with overall errors of 20~30%. • Factors affecting the mass transfer by changing film instability and wetting factor are analyzed. The focus of this work is on the liquid-air mass transfer mechanism that is critical for liquid desiccant dehumidification and many other absorption processes. Most existing mass transfer correlations heavily rely on specific experiments and show poor universality. Therefore, we proposed a new set of mass transfer correlations theoretically based on the film instability during falling film dehumidification. The flow dynamic, Marangoni effect and liquid/air contact conditions that affecting the interface characteristics and wetting factors are considered. The correlations were verified by comparing with experimental data from several widely-cited literatures. The tests in these literatures were conducted under a wide range of operating conditions and dehumidifier types. The newly-developed correlations provide an acceptable prediction for liquid-air mass transfer, showing close trends to all previous experimental results. The overall error of the new predictions, 20–30%, is close to those of empirical equations built in the specific literature. The factors that affect the interphase mass transfer by changing the film instability and the wetting factor are also analyzed. The increase in liquid Reynolds number shows the most significant effect as it could effectively increase the film instability and liquid-air contact area. The liquid contact angle on solid surfaces, regarding the wettability, also affects the mass transfer considerably. By reducing the contact angle from 90° to 10°, although the increase in Sherwood number is slight due to the suppression of film instability, the wetting factor is almost doubled, resulting in a significant growth in mass transfer performance. This new correlation examines the falling film mass transfer process in more detail, and is based on fewer simplifying assumptions and attempts to take more realistic situations into account. Findings presented herein contribute to a more fully understanding on the falling film behaviors during liquid/gas contact such as liquid desiccant dehumidification. [ABSTRACT FROM AUTHOR]

Highlights • A model for surface waves during liquid desiccant dehumidification was developed. • The influences of interfacial mass transfer were considered. • The time-series performance was solved and experimentally verified. • Increasing liquid temperature or decreasing concentration enhances wave intensity. • The contact area increases by 10~40% as the surface tension dropped by 10~50%. Abstract The wavy surface of a falling film has a significant effect on the heat/mass transfer of liquid desiccant dehumidification systems. This paper developed and analytically solved a 2D theoretical model for predicting the time-series surface waves of falling film desiccant dehumidification process. The influences of operating conditions, physical properties of desiccant and air, vapor pressure and thermocapillary forces due to interfacial mass transfer, and the interaction between the desiccant and working plates were considered. The model was validated by experiments with an acceptable error of 10.9% for the frequency, 6.59% for the amplitude and 14% for the contact area changing rate. The actual changing rate of liquid/air contact area with time could be predicted with this model, which was 20–90% higher than the flat film assumptions at a solution mass flow rate from 0.0126 to 0.043 kg/s. Then, parameter analyses of fluctuating desiccant film were numerically performed. Although increasing the temperature or decreasing the concentration of a liquid desiccant increased the wave intensity, the mass transfer driving force was also reduced, which is not suitable for practical systems. However, if the surface tension dropped to the original 10–50% by surface modification or surfactant addition, then the contact area could increase by approximately 10–40% due to the surface wave enhancement. In addition, this method is cost efficient and has little impact on the system structure. Therefore, with this model, it is theoretically possible to evaluate the time-series characteristics of a wave-wise liquid/air interface with heat and mass transfer, which could improve the evaluation accuracy of falling film liquid desiccant dehumidification systems. Thus, this study effectively helps the prediction and optimization of practical dehumidification systems, and other applications related to gas/liquid flow. [ABSTRACT FROM AUTHOR]

The cooling system with liquid desiccant dehumidification is an energy-efficient and eco-friendly solution for the ever-increasing energy consumption and environmental impact of modern buildings. By significantly influencing the wetted area, the contact angle of the liquid–air interface is critical to the performance of desiccant dehumidification. In this study, to enhance heat and mass transfer performance, a new approach was developed with a titanium dioxide superhydrophilic self-cleaning coating on the dehumidifier surfaces that was made of a highly dispersed paste with nanoscale titanium dioxide particles and activated with ultraviolet light. We first tested the effect of the coatings on samples made of three commonly used materials, SUS304, SUS316, and SUS410, with a 30% LiBr solution and deionized water. Results show that the contact angle on the coated samples dramatically decreased to only 1/7 for water and 1/6 for the LiBr solution compared to uncoated samples. The durability of the coatings was studied experimentally by repeatedly immersing the samples in 30% LiBr, exposing them for 2 months, and reactivating them. It was found that the angle changed slightly for SUS304/316 samples with good anticorrosion performance, whereas the angle increased by 10°–20° for SUS410 samples due to the accelerated catalytic oxidization of the Titanium dioxide coating, which could possibly be solved by precoating an SiO 2 compact layer. Furthermore, we found numerically that the wetted area during desiccant dehumidification could be effectively increased by 5–7 times with the coating. Accordingly, the heat exchange rate and moisture removal rate between the air and the desiccant could be increased by 1.2 and 2 times, respectively. Therefore, by dramatically reducing the desiccant contact angle on dehumidifier surfaces and enlarging the wetted area accordingly, the titanium dioxide self-cleaning coating could significantly improve the heat and mass transfer performance of liquid desiccant dehumidification, especially the mass transfer performance. The coatings' manufacture is cheap and facile for industrialization. This measure can be considered an effective and economic way to improve the performance of a dehumidifier and also has other applications in heat exchangers, evaporators, and absorption towers. [ABSTRACT FROM AUTHOR]

Abstract: Currently, evaluating the performance of internally cooled/heated liquid desiccant dehumidification system (ICHLD) is complicated, requiring a large number of iterative calculations. This study developed a simplified numerical model, for obtaining the outlet parameters of all fluids (air, solution and heating/cooling fluid) and system performance of dehumidifiers and regenerators in ICHLD accurately and directly. The model was built by defining three kinds of effectiveness, i.e. enthalpy effectiveness, moisture effectiveness and temperature effectiveness. Based on the parameter study under different design and operation conditions, the correlations of effectiveness were obtained by multiple linear regressions, using heat and mass transfer gradients and other related parameters as variables. Comparisons between our results and those by existing mathematical models show that the composite errors are acceptable, with an average difference of 14.5% for dehumidifiers and 6.83% for regenerators. Similar trends were also observed in comparisons with previous experimental data, with average errors about 20%. The inlet solution temperature has a greatest impact on the enthalpy effectiveness and moisture effectiveness. The temperature effectiveness is mainly determined by the temperature difference between cooling/heating fluid and solution. This simplified model is very useful for researchers and engineers, especially those interested in the dynamic performance of ICHLD systems. [Copyright &y& Elsevier]

• A model coupling falling film rupture and liquid/gas absorption was developed. • Film break-up positions considering mass transfer and Marangoni effect can be calculated. • The model was validated by LiCl halide/air dehumidification experiments with errors less than 20%. • Force caused by Marangoni effect dominated, and its increase was most obvious with the flow distance. • Since the upward force on film edge increased with flow distance and mass transfer, where it was equal to gravity, the possibility of film rupture was very high. The falling film flow pattern and heat/mass transfer severely interact. Considering the unbalanced forces caused by liquid-gas mass transfer and Marangoni effect, this paper developed a theoretical model to predict the break-up characteristics of gas-liquid falling film convective absorption such as dehumidification. Analytical solutions were provided by calculating the uneven distribution of falling film parameters such as film thickness, interface temperature and concentration. Full consideration was also given to the influence of liquid film fluctuations and liquid-gas mass transfer coefficients that vary with operation and design conditions. Experiments of liquid desiccant dehumidification with LiCl saline aqueous were conducted for model validation. Though the model cannot reflect the volatility of film break-up positions, the trend was close and the error was less than 20% between the calculated and experimental results. Results showed that under absorbtion, except for the top of liquid film, the force caused by the surface tension difference due to liquid-gas mass transfer dominated, and its increase was most obvious as the flow distance increased. Liquid/solid friction was also significant, while gas/liquid shear force had little effect. Thus, since the upward force on film edge increased with the flow distance and mass transfer, where it was equal to gravity, the possibility of film rupture was very high (0.6–0.7 m in the conditions of this paper). All forces acting on the falling film increased, while the largest increment with liquid Re was liquid/solid friction and that with air Re was liquid/gas friction. Besides, reducing the liquid contact angle can efficiently relieve film rupture and enhance mass transfer, mainly by reducing the liquid/solid friction. This study provided a theoretical explanation for the coupling of falling film rupture and gas-liquid mass transfer, which helps to improve the performance of dehumidification and other wetted-wall absorptions. [ABSTRACT FROM AUTHOR]

• Manipulations from component, system to material of liquid desiccant dehumidification are reviewed. • Limitations in practical applications are introduced. • Corrosive desiccants with low heat capacity and insufficient wettabilityare a major obstacle. • Manipulations can partially solve the bottlenecks, while their feasibility needs further research. • Long-term stability, complexity and economics of need more attention. This paper provides a comprehensive literature review of the latest research on liquid desiccant air dehumidification, with an emphasis on manipulation methods from components, systems to materials. Energy comsuption for indoor humidity control is significant, especially in tropical and subtropical areas. Liquid desiccant dehumidification can independently remove moisture from the supply air. It has many advantages, including effective humidity control, utilization of low-grade thermal energy, higher supply air quality and energy storage potential. With the development in recent decades, this technology and its economic value are close to being viable in practice. However, the system still faces some limitations due to the use of corrosive desiccants with low heat capacity and insufficient wettability on packing columns. This paper firstly summarized the bottlenecks of liquid desiccant dehumidification in practical applications, including droplet carrying problems, low liquid/air contact area due to poor wettability, large temperature change during heat/mass transfer, and large heat requirement, etc.Then the optimization methods for improving the system feasibility and performance were reviewed, from these three aspects: 1) Component optimization: New type of dehumidifers/regenerators, including the hollow-fiber membrane dehumidifiers, electrodialysis and vacuum regenerators; 2) System optimization: Combined with solar collector, vapour compression, heat pump or waste heat recovery; 3) Material optimization: Modification of desiccants and membrane materials, and modifications of packing/plate surfaces. This review will help to identify research gaps and explore new pathways to further improve the practical feasibility of liquid desiccant air dehumidification. [ABSTRACT FROM AUTHOR]

Abstract Liquid desiccant air-conditioning systems are promising as they can achieve independent dehumidification. Non-metallic materials, such as plastics, are good internal working plate materials due to their natural anti-corrosion and easy-processing characteristics. However, poor wettability severely limits the application. In this study, the enhancement of surface wettability and mass transfer performance for liquid desiccant dehumidification with plastic internal plates was achieved, with a newly developed, durable, super-hydrophilic modified surface. The plastic plates were first etched into micro-rod structures to improve durability, and then the surfaces were prepared by adhering the super-hydrophilic SiO 2 particles to the microstructures. Results showed that the liquid desiccant could completely spread on the coated plastic surfaces within 2 s, and the contact angle could be as low as 0°. Durability tests showed that after immersing in 40 wt% LiCl solution for 15 days, the liquid desiccant could still completely spread in 3 s. Furthermore, after 48 h of high-speed scouring (more than 1000 times of common conditions), the contact angle of desiccant maintained 0° on the coated surface, and fully spread within 22 s. By comparing coated to uncoated surfaces through experiments with dehumidification rigs, the wetted area of desiccant could be effectively increased by 1.5–5 times when the novel plates were used. The average film thickness decreased by 1.4–2.4 times and the moisture removal rate increased by 1.3–1.8 times for super-hydrophilic surfaces relative to original surfaces. For a case study with a typical building in Hong Kong, the annual energy consumption of air-conditioning systems could be predicated to decrease about 1/3 by using coated plastic plates. Therefore, this durable super-hydrophilic surface could significantly enhance the wettability, and thus enhance the performance of plastic LDACS. The manufacturing process of the surfaces is cheap and facile for industrialization. This study helps the optimization of liquid desiccant dehumidifiers, and other plastic applications with liquid-gas flows. [ABSTRACT FROM AUTHOR]

The dynamic characteristics of solution film flow with counter-current air flow, especially the two phase contact area, are important for the optimal design and operation of LDD (liquid desiccant dehumidifier). However, the unsteady interfacial information, including the interfacial pressure, velocity and film thickness, is hardly to be examined accurately by theoretical prediction or experimental measurement due to its complexity. In this study, CFD (computational fluid dynamic) models for the counter-current flow of the LDD were established based on the VOF (volume of fraction) and RNG (Renormalization group) k-ε turbulence model. Experimental research had been conducted to validate the liquid film thickness and typical interfacial waves obtained in simulation. With the established model, the dynamic formation process of unsteady counter-current flow is evaluated. Moreover, the effects of various parameters on the liquid film waves, pressure drop, liquid film thickness and interfacial information were obtained and analyzed by the built models. With the increase of liquid flow rate, the roll waves appear and the wave amplitudes are increased significantly, which may obviously enhance the performance in the LDD absorption and regeneration process due to the increased contact area. [ABSTRACT FROM AUTHOR]