C.R. Ruivo

Analysis of Simplifying Assumptions for the Numerical Modeling of the Heat and Mass Transfer in a Porous Desiccant Medium

ABSTRACT The aim of this work is to assess the accuracy of different simplifying assumptions commonly adopted in the modeling of the thermodynamic behavior of porous desiccant media such as those composing the channel walls of compact heat and mass exchangers, such as desiccant wheels. The study is based on the one-dimensional numerical solution of the conservation equations for heat, water vapor, and adsorbed water inside the porous medium under the constraint of local equilibrium between the two phases, which is characterized by sorption isotherms without hysteresis. Systematic calculations are performed for both adsorption and desorption processes and particular air flow conditions. It is concluded that the surface diffusion is the most important mechanism of water transport within the porous medium and that the internal thermal resistance may be locally neglected, allowing a lumped heat capacitance model in the cross direction of the channel wall. Furthermore, it is also important to account for the local variation of some properties of the porous medium.

Numerical Study of the Cyclic Behavior of a Desiccant Layer of a Hygroscopic Rotor

Enthalpy and desiccant wheels are often used in commercial and industrial applications for energy recovery and air dehumidification operations, respectively. The modeling of these hygroscopic rotors is of great relevance either for manufacturer product optimization or for hourly simulations, when integrated in air treatment installations. Some simplified numerical methods have been proposed to predict the behavior of hygroscopic rotors, most of them assuming negligible internal resistances to heat and mass transfer and/or constant properties of the desiccant wall. In this article, a one-dimensional physical model is used to numerically investigate the behavior of an element of a porous desiccant wall that is assumed to belong to a hygroscopic wheel, and is submitted to an adsorption/desorption cyclic operation. The mathematical model is based on the solution of the differential equations of mass and energy conservation inside the porous medium. Moreover, the model was developed to treat the coupled heat and mass transfer phenomena in a detailed way, taking into account specified convective boundary conditions and considering the local and time changes of the medium properties during the sorption processes. The corresponding numerical model is used to perform simulations considering two distinct values of the wall thickness and different durations of the adsorption/desorption cycle. The results lead to a good understanding of the relationship between the characteristics of the sorption processes and the behavior of hygroscopic wheels, and provide guidelines for the wheel optimization, namely of the adsorption/desorption partition of the wheel frontal area.

Influence of Altitude on the Behavior of Solid Desiccant Dehumidification System

A study is described regarding the influence of the altitude, from 0 to 4,217 m (corresponding to atmospheric pressure from 101,325 to 60,000 Pa), on the behavior of a simple solid desiccant system used for air dehumidification purposes. The heating coil and the desiccant wheel are the main components investigated. The effectiveness method is used to evaluate the global behavior of the heating coil, and a detailed numerical model developed by the authors is used to predict the behavior of the desiccant wheel. Fixed-mass and fixed-volume airflow rate operations are considered in the comparison of the results at different altitudes. Two modes of specifying the inlet states of both airflows in the system are taken into account: (1) temperature and water vapor content and (2) temperature and relative humidity. As the atmospheric pressure decreases, the heat and mass transfer rates increase or decrease, depending on the mode of fixing the airflow rates and the inlet states of both airflows. Correction factors are determined for fixed-volume and fixed-mass airflow rate operations. The results show that these correction factors are also affected by the rotation speed of the desiccant wheel. Sea level data can be adopted for sizing the system without the need of correction when fixed-mass airflow rate and specifying the inlet states by the temperature and water vapor content.

An alternative approach to the inversion temperature

Abstract Above a given temperature, referred to as the inversion temperature, superheated steam is a more effective drying agent than humid air or even than dry air. However, no agreement has been reached in what concerns the definition of both the inversion temperature and its numerical value. Recent works attempted to clarify the different definitions of the inversion temperature, taking into account the obtained different numerical values. In this work, some of the ideas presented recently are developed and worked out in such a way that new graphical presentations of data are obtained, leading to a better understanding of the inversion temperature and of its value. The issues concerning the influence of the steam content of the drying agent on the evaporation rate, for different drying conditions and for a given inlet temperature of the drying agent, are clarified. The present results provide useful information on what concerns the influence of the convective drying conditions and parameters over the evaporation rate.

Heat and Mass Transfer in Matrices of Hygroscopic Wheels

In the present chapter the sorption and the mass diffusion phenomena in the porous desiccant layer are described aiming a detailed numerical modelling of hygroscopic wheels. A particular desiccant medium (silica gel Regular Density) is characterized as well as a representative cell of the corrugated matrix. The heat and mass transfers in the gas domain of the matrix channels are analysed addressing also the validity of the so-called low mass transfer rate theory. The mass transfer convective coefficient is evaluated through the use of the Chilton-Colburn analogy. The detailed numerical model proposed for the simulation of the behaviour of a representative channel of the matrix is briefly described. Results of the cyclic behaviour of an element of the channel wall are presented and a parametric study is performed concerning the influence of the rotation speed and of the thickness of the desiccant layer on the behaviour of hygroscopic wheels.

Heat and Mass Transfer in Desiccant Wheels

1.1 Background Nowadays the interest in heating, ventilation, air-conditioning and refrigerating systems (HVAC&R) based on desiccant wheels is increasing due to the possibility of using renewable energy sources, making them an attractive alternative or complement to conventional systems. The thermally driven desiccant systems can potentially reduce the peak electricity demand and associated electricity infrastructure costs. They generally incur in higher initial cost compared with equivalent conventional systems, but cost reduction can be achieved at the design stage through careful cycle selection, flow optimisation and size reduction. The performance of these systems can be evaluated by experimental or numerical approaches. To date there still exists a lack of data of real manufactured wheels enabling to perform a dynamic energy analysis of such alternative systems with reasonable accuracy at design stage. The data given by the manufacturers of desiccant wheels are usually restricted to particular sets of operating conditions. Besides, the available software for sizing is usually appropriate to run only stationary operating conditions. For these reasons, it is recognized the importance of the use of a simple predicting method to perform the dynamic simulation of air handling units equipped with desiccant wheels. In this chapter, the results of a detailed numerical model are used to determine the effectiveness parameters for the coupled heat and mass transfer processes in desiccant wheels, allowing the use of the effectiveness method as an easy prediction tool for designers.

Numerical Modelling of the Heat and Mass Transfer in a Channel with Hygroscopic Walls

In this paper the numerical modelling of the behaviour of a channel of a hygroscopic compact matrix is presented. The heat and mass transfer phenomena occurring in the porous medium and within the airflow are strongly coupled, and some properties of the airflow and of the desiccant medium exhibit important changes during the sorption/desorption processes. The adopted physical modelling takes into account the gas side and solid side resistances to heat and mass transfer, as well as the simultaneous heat and mass transfer together with the water adsorption/desorption process in the wall domain. Two phases co-exist in equilibrium inside the desiccant porous medium, the equilibrium being characterized by sorption isotherms. The airflow is treated as a bulk flow, the interaction with the wall being evaluated by using appropriated convective coefficients. The model is used to perform simulations considering two distinct values of the channel wall thickness and different lengths of the channel. The results of the modelling lead to a good understanding of the relationship between the characteristics of the sorption processes and the behaviour of hygroscopic matrices, and provide guidelines for the wheel optimization, namely of the duration of the adsorption and desorption periods occurring in each hygroscopic channel.

Characterising the Differences between the Adsorption and Desorption Processes in a Desiccant Layer by Detailed Numerical Modelling

In this paper, the performance of a channel element of a hygroscopic matrix is evaluated by detailed numerical modeling. The adopted physical model takes into account the gas-side and solid-side resistances to heat and mass transfer, as well as the simultaneous heat and mass transfer occurring simultaneously with the water adsorption/desorption process in the desiccant porous channel wall domain. The desiccant medium is silica gel RD, the equilibrium being characterized by sorption isotherms. Appropriate convective transfer coefficients are taken into account for the calculation of the heat and mass transfer phenomena between the airflow and the channel wall. The response of the channel element to a step change in the airflow states is simulated, the results enabling the investigation of some differences between the adsorption and desorption processes.

Effectiveness Parameters for the Heat and Mass Transfer in a Desiccant Wheel

The desiccant wheel is the key component in a solid-desiccant system for air dehumidification. The heat and mass transfer phenomena occurring within the porous channel walls of the wheel and with the airflow are strongly coupled, and some properties of the airflow and of the desiccant medium exhibit important changes during the sorption/desorption processes. The dynamic analysis of such devices integrated in non-conventional HVAC&R systems can be easily done by a project designer using the NTU-effectiveness method, provided that appropriate correlations for two independent effectiveness parameters are available. In this work, the performance of a desiccant wheel was evaluated by numerical modelling the cyclic behaviour of a representative channel of the hygroscopic matrix. The physical model adopted takes into account the gas-side and solid-side resistances, as well as the simultaneous heat and mass transfer coupled with the water adsorption/desorption process in the channel wall domain. Two phases co-exist in equilibrium inside the desiccant porous medium, the equilibrium being characterized by sorption isotherms. The desiccant medium considered is silica gel RD. In the numerical model, the airflow is treated as a bulk flow, and its interaction with the wall channel matrix is represented by appropriate convective heat and mass transfer coefficients. Two independent effectiveness parameters were defined. A set of cases was numerically simulated and the results were analysed to assess the dependence of those effectiveness parameters on the process and regeneration airflow rates and on the channel length. As a conclusion, novel empirical correlations are here purposed.