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The enhancement of silica gel regeneration by power ultrasound has been validated by a series of experiments in our previous studies, but there still lacks theoretical basis for illustrating its mechanism. In this paper, the mechanism of ultrasound-enhanced regeneration has been explored. Firstly, the benefit of ultrasonic mechanical effect to the enhancement of regeneration has been illustrated by the field synergy principle. The average synergy degree ( κ ¯ , only considering intersection angle between the velocity and the temperature gradient) and the average overall synergy degree ( κ ¯ overall , considering local values of velocity and temperature gradient based on κ ¯ ) in terms of the near wall region are suggested for analysis, and they are obtained based on the k –omega model which is suitable for the near wall free-shear flow velocity predicting. Results manifest that the ultrasonic mechanical effect can significantly enlarge the synergy degrees between the temperature and the velocity field around the particle, and this can be used to explain the enhancement of convective heat and mass transfer on the gas side due to the mechanical effect of ultrasound. Afterwards, a moisture diffusion model is developed to investigate spatial distributions of moisture ratio and temperature in a silica gel particle as well as its surface equilibrium humidity during the regeneration with and without ultrasound. Results show that ultrasonic heating effect can lead to an increase in the average temperature and moisture diffusivity in the silica gel particle, and this confirms the contribution of ultrasonic heating effect to the enhancement of silica gel regeneration. [ABSTRACT FROM AUTHOR]

Abstract: Adequately representing the dynamic performance of a water-to-air heat exchanger under transient conditions would help design optimized control strategies for a high-energy-efficiency HVAC system. In this paper, the water-to-air heat exchanger is considered as a multi-input-and-multi-output system (MIMO), and the corresponding dynamic model is developed with the mass and energy balance equations based on which the state-space model for transient behavior of the heat exchanger is derived. The model has been validated by a series of dynamic response experiments including the transient responses of exit air temperature and humidity as well as exit water temperature subjected to the disturbances of inlet water temperatures (Case I: Start up the chiller; and Case III: Stop the chiller) and a sudden change in the water flow rate (Case II: The water flow rate increases by 0.058kg/s). The time of experimental validation on the transient response simulation is set as 1200s in all the cases. The simulated results obtained by the state-space model have been compared with the experimental data, which shows that the state-space model developed in this study is capable of predicting the transient performance of the water-to-air heat exchangers satisfactorily (the average error is less than 15.0%). Meanwhile, the influence of input disturbances and initial conditions on the transient behaviors of the heat exchanger is discussed through the model simulation. The results show that the input disturbances and the initial conditions will produce significant influence on the proportionality coefficient of the response variables, but little influence on the time constant of the response ones. [Copyright &y& Elsevier]

Abstract: In the present work, a theoretical model is reported on the heat and mass transfer in silica gel packed bed during the regeneration process by using hot air combined with high-intensity ultrasound. The model consists of two parts: one is about the sound propagation in porous media; the other is about the fundamental heat and mass transfer process in the silica gel (particle) packed bed. The theoretical model is then validated by experiments in terms of the exit air temperature and humidity (kg/(kg dryair)) under different conditions of regeneration. The experimental error due to the measuring instruments is estimated to be within 0.4% and 3.5%, respectively, for the exit air temperature and humidity. The comparison between theoretical and experimental data shows that the mean relative errors (MREs) of the calculated exit air temperature and humidity compared with the experimental ones are mostly within 2.0%, which manifests the model developed in this study has a favorite agreement with the experiments. The theoretical model will help us conduct a further parametric analysis on the regeneration process in the presence of an ultrasonic field, and have a better understanding of the mechanism of enhancement of silica gel regeneration brought by the high-intensity ultrasound. [Copyright &y& Elsevier]

• Mathematical model for IMLDD was developed and validated experimentally. • Dehumidification performances for IMLDD of different flow types were compared. • Exergy destruction and entransy disspation were adopted to analyze IMLDD. • Unmatched coefficients were used to represent the loss of mass transfer. • Flow type of IMLDD with the best performance has been found. Internally-cooled membrane-based liquid desiccant dehumidifier (IMLDD) can well alleviate the dehumidification deterioration due to the increase of desiccant solution temperature. In the IMLDD, air channels and desiccant solution channels are separated on both sides of semi-permeable membrane. Cooling media flows through tubes inside solution channels to reduce desiccant solution temperature. There exist many flow patterns of fluids (air, desiccant solution and cooling water), which have been proved to have significant effects on the performance of IMLDD. Therefore, to find the optimal flow type of IMLDD and operating conditions, the mathematical model for IMLDD was developed and validated experimentally, and the exergy and entransy theories were first adopted to analyze the effects of inlet parameters on the performance of IMLDD of various flow types. The results showed that counter flow arrangement between air and solution and parallel flow arrangement between solution and cooling water had the best dehumidification performance. The trend of dehumidification efficiency was opposite to exergy efficiency with the increase of inlet air humidity ratio and solution concentration. The increase of inlet cooling water temperature improved exergy efficiency but decreased dehumidification efficiency accordingly. The research of this paper is helpful for the design and optimization of IMLDD. [ABSTRACT FROM AUTHOR]

• A state-space model for a membrane-based liquid desiccant dehumidifier was developed. • The dynamic model was validated experimentally. • The transient responses to sudden changes of inlet parameters were investigated. • The state-space model is helpful for control design and optimization. In this paper, a state-space model for a membrane-based liquid desiccant dehumidifier with counter-flow configuration was developed and validated experimentally. Compared with experimental data, the maximum relative error of model results is less than 2%, and the mean relative error is about 1%, which indicates that the state-space model can well predict the transient characteristics of the membrane-based liquid desiccant dehumidifier. Further, the transient responses of the dehumidifier to the step changes of inlet parameters (air temperature and humidity, air flow rate, solution temperature and flow rate) were investigated in detail. This dynamic model can clearly reflect the transient relationships among input, state and output variables. It was found that the effect of perturbation of the inlet air temperature on the moisture transfer in the membrane-based dehumidifier was too small to be neglected, and the variation of the inlet air humidity had little effect on the heat transfer. The response time constant of the outlet solution temperature to the inlet perturbations was obviously larger than that of the outlet air temperature and humidity. The air and the solution flow rate can be taken as the controllable parameters for the outlet air humidity control of membrane-based liquid desiccant dehumidifier. With the advantages of short calculation time and simple solving process, the state-space model is helpful for the control design and the optimization of the membrane-based liquid desiccant dehumidifier. [ABSTRACT FROM AUTHOR]