J.J. Costa

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.

Indoor air quality audit implementation in a hotel building in Portugal

Abstract Hotels are designed to provide high levels of comfort for guests; however, frequent complaints related to uncomfortable thermal environment and inadequate indoor air quality (IAQ) appear. On the other hand, there is little research concerning IAQ audits of hotels up to now. This study is aimed to establish and demonstrate the comprehensive IAQ audit approach for hotel buildings, based on Portugal national laws. A 4-star hotel building in Portugal is used as a case study to demonstrate the IAQ audit application and evaluate its comprehensiveness and usefulness to the hotel or facility managers. The systematic approach involves the measurement of physical parameters – temperature (dry bulb), relative humidity and the concentration of the suspended particulate matter (PM10) – the monitoring of the concentrations of selected chemical indicators – carbon dioxide (CO2), carbon monoxide (CO), formaldehyde (HCHO) and total volatile organic compounds (TVOCs) – and the measurements of biological indicators (bacteria, fungi, Legionella). In the present case, air exchange rates are measured by the concentration-decay method using metabolic CO2 as the tracer gas. The comprehensive IAQ audit revealed four main problems in the hotel building: (1) insufficient ventilation rate; (2) too high particle concentration in some rooms; (3) contamination by Legionella of the sanitary hot-water circuit; (4) poor filtration effectiveness in all air handling units (AHUs).

Turbulent airflow in a room with a two-jet heating-ventilation system — a numerical parametric study

Abstract The two-dimensional turbulent airflow generated by two non-isothermal plane wall jets in a compartment is numerically investigated over a wide range of supply airflow rates. The low-Reynolds-number turbulence model of Nagano and Hishida, assembled in a finite-volume-based numerical code, is used after a previous validation study involving several other formulations of the k–e model. The results of an extensive parametric study are reported and discussed, and conclusions are drawn about the relative influences of the room aspect ratio, the jet inlet sections, the temperature of the walls and the ratio of ventilation-to-heating airflow rates on the flow structure and the maximum velocity in the return flow. Evidence is provided that the common requirements of ventilation rate in non-industrial buildings imply airflows that lie within a mixed convection regime, where deep changes of the flow pattern can occur. Attention is given to the parametric conditions associated with flow reversal, a situation that can seriously compromise thermal comfort. In fact, the maximum velocity in the occupied zone may then double for the same ventilation rate.

NEO-FFI: Psychometric properties of a short personality inventory in Portuguese context

The aim of this study was to assess the psychometric properties (validity and reliability) of the Portuguese adaptation of the NEO Five-Factor Inventory (NEO-FFI) and to compare the obtained structure to the original North American version. The methods of analysis used for cross-validation of the factorial structure were the Principal Axis Factoring (PAF) and the Confirmatory Factor Analysis (CFA). PAF reproduced the original structure of NEO-FFI and CFA revealed a satisfactory fit of single-factor models for the five dimensions. The reliability analysis showed high values of internal consistency which are congruent with previous international adaptations of the NEO-FFI. Multidimensional analysis showed significant main effects of gender and academic qualifications on personality using age as covariant. The findings suggest that the Portuguese version of the NEO-FFI is a reliable instrument to measure the five dimensions of personality.

Numerical evaluation of a phase change material–shutter using solar energy for winter nighttime indoor heating:

The incorporation of phase change materials in movable structural cells of shading elements associated with southward-facade windows is evaluated in this article. The proposed phase change material–shutter is a thermal energy storage system designed to take advantage of solar energy for winter nighttime indoor heating. A two-dimensional phase-change heat diffusion model based on the enthalpy formulation was considered. The numerical model follows the finite-volume method with a fully implicit formulation and allows the alternating melting and solidification of a phase change material submitted to cyclical thermal boundary conditions. Parametric investigations were carried out about the effects of thermophysical properties of the phase change material and temperature and convection heat transfer boundary conditions on the charge/discharge rates of energy. Due to the low thermal diffusivity of the phase change material, an aluminum fin arrangement was considered as a heat transfer enhancement technique. The distance between fins is directly proportional to the daily energy storage/release capacity of the system. The solar radiation flux has a strong effect on the charging/melting processes during the day. The indoor temperature and the interior convection heat transfer coefficient have a major influence on the discharging/freezing processes during the night. The design of the phase change material–shutter depends strongly on the thermophysical properties of the phase change material and on the interior and exterior boundary conditions considered.

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.

Experimental analysis of the use of wet porous media for thermal protection against high intensity heat fluxes

An experimental technique is implemented to analyse the behaviour of a wet porous medium under the direct impact of a flame, and the possibility of its use as a thermal protection device. The data obtained show that relatively thin and light thermal protections, with moderate values of water consumption, can be developed, presenting high levels of protection. The presentation of the data in a dimensionless form provides a useful tool to estimate the thickness and the water content of the protective device that are needed for expected values of incident heat flux and prescribed time of protection.

A systematic indoor air quality audit approach for public buildings

Good indoor air quality (IAQ) in buildings provides a comfortable and healthy environment for the occupants to work, learn, study, etc. Therefore, it is important to ascertain the IAQ status in the buildings. This study is aimed to establish and demonstrate the comprehensive IAQ audit approach for public buildings, based on Portugal national laws. Four public buildings in Portugal are used to demonstrate the IAQ audit application. The systematic approach involves the measurement of physical parameters (temperature, relative humidity, and concentration of the suspended particulate matter), monitoring of the concentrations of selected chemical indicators [carbon dioxide (CO2), carbon monoxide, formaldehyde, ozone, and total volatile organic compounds], and the measurements of biological indicators (bacteria and fungi). In addition, air exchange rates are measured by the concentration decay method using metabolic CO2 as the tracer gas. The comprehensive audits indicated some situations of common IAQ problems in buildings, namely: (1) insufficient ventilation rate, (2) too high particle concentration; and (3) poor filtration effectiveness and hygienic conditions in most of the air handling units. Accordingly, a set of recommendations for the improvement of IAQ conditions were advised to the building owner/managers.

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.

Modelling and performance analysis of an earth-to-air heat exchanger in a pilot installation:

Earth-to-air heat exchanger is an efficient complement to the space heating and cooling systems. An earth-to-air heat exchanger system consists of a set of subterranean pipes leading outdoor air to a ventilation room and by collecting geothermal energy saves some energy. This article presents testing of an earth-to-air heat exchanger installation in a residential building in Coimbra (Portugal) during 1 year. An analytical model is developed and verified with data from the system monitoring. Many variables affect the outcome of the model and they are included in the parametric study, namely, pipe length and diameter, air velocity, pipe wall material and type of soil. While the difference between outlet and inlet air temperatures is proportional to the pipe length and ground thermal conductivity and inversely proportional to pipe diameter and air velocity, the evaluation is made in terms of a coefficient of performance. For that, the most important factor is the air velocity while material of the pipe does not have a significant effect.