Maria A. Founti

Scrutinizing Gypsum Board Thermal Performance at Dehydration Temperatures

This article investigates the thermal performance of gypsum boards in the dehydration temperature region, where pronounced chemical reactivity is anticipated. Gypsum board samples were gradually heated up to 300°C, using a low heating rate. Temperature measurements at pre-selected board locations indicated three distinct stages of gypsum dehydration; free moisture evaporation, transformation of calcium sulfate dihydrate to calcium sulfate hemi-hydrate and of calcium sulfate hemi-hydrate to calcium sulfate anhydrate. Physical properties of the gypsum board sample, that is density, specific heat, and thermal conductivity, were measured as a function of the dehydration temperature range. Results suggested that the thermal conductivity of the calcium anhydrate sulfate is higher than that of the hemi-hydrate, despite the fact that gypsum porosity increases during the dehydration process. Based on the experimental results, correlations for the thermal conductivity and the dehydration energy as a function of the mass loss during the dehydration process and the initial water content of the gypsum board, respectively, were obtained.

Assessment of Chemical Markers for Heat-Release Rate Correlations in Laminar Premixed Flames

There is considerable interest in formulating practical correlations for the quantification of heat-release rate in flames via appropriate chemical markers, mainly due to difficulties in obtaining a direct estimation of the former. Experimental and numerical studies performed in methane-air flames have identified corrections between suitable kinetic information and heat-release rate. The present work investigates the validity of corrections developed for methane to the combustion of other fuel classes (e.g., alcohols, unsaturated hydrocarbons, and aromatics), and proposes a methodological approach for the development of generic correlations. Numerical assessment of such correlations and their underlying methodology is performed on the basis of experimental data from 24 laminar premixed flames, utilizing a single, in-house, recently published C1–C6 detailed chemical kinetic model. It is shown that the proposed correlations are valid for methane-based fuels but they are largely inadequate for unsaturated fuels under rich conditions and for oxygenated fuels under most conditions. It is shown that heat-release rate is correlated with the dominant carbon path and that the oxidation chemistry of the parent alkyl radical is of pronounced importance. Alternatives to the proposed heat-release rate correlations are also proposed and assessed.

Modeling of the gas–particle flow in industrial classification chambers for design optimization

Abstract A computational approach has been developed for the prediction of gas–particle flow and the simulation of the classification-settling characteristics of a particulate material flow in a gravitational classification chamber (GCC). By taking into account the classification chamber geometry and operating parameters, as well as the particulate material physical properties, the developed computational code allows the simulation, improvement and control of the operational characteristics of industrial settling, classification and/or separation chambers commonly used in process industries. The work assesses the performance of the computational code and demonstrates the capability of dedicated computational tools to model with accuracy complex industrial multiphase flows in order to support design criteria.

Turbulent Sprays Evaporating Under “Stabilized Cool Flame” Conditions: Assessment of two CFD Approaches

An “in-house” computational fluid dynamics code implementing a Euler-Lagrange approach is extended by incorporating the Euler-Euler (two-fluid model) approach, to improve prediction capabilities of flow and thermal characteristics of turbulent evaporating sprays. The performance of both approaches is assessed by comparing predictions with experimental data for a variety of evaporating-spray test cases. The applicability of the Euler-Lagrange and Euler-Euler approach is established in an isopropyl alcohol–air turbulent flow, in which characteristic droplet quantity predictions are in satisfactory overall agreement with measurements. The evaporating spray characteristics are then predicted under “stabilized cool flame” conditions and the computational results are compared to experimental data for a nonreactive case and a reactive case. In both cases, the velocity and thermal fields are successfully captured by both approaches. Overall, the article demonstrates an approach toward the development of performance-based computational tools.

Computational assessment of a full-scale Mediterranean building incorporating wallboards with phase change materials:

In this study, a lightweight residential building in Greece was investigated, focusing on the summer comfort when wallboards with phase change materials (PCM) were installed in the external and internal walls. The effectiveness of the PCM wallboards installed was numerically assessed, while the energy performance of the building was examined, in order to quantify the effect of PCM in the annual cooling load needs, as a way of saving energy. Potential bigger energy savings were evaluated by defining the appropriate PCM melting temperature range and the ‘energy-conscious’ occupant behaviour (passive vs. active). Results were expressed in terms of percentage savings of cooling loads and with comparison to wall elements incorporated with plain gypsumboards instead of the PCM wallboards. The optimum phase change temperature change for the specific location was investigated by examining two-phase change transition temperatures of the PCM wallboards (PCM24 and PCM26 respectively). The use of PCM24 produced a 29% reduction of annual cooling loads, compared to 16% reduction produced by PCM26. Five scenarios were also examined, showing the behaviour of the PCM which was enhanced when a cooling system was installed. The cooling needs were lowered by an average of 25.7%, compared to the respective no-PCM scenarios.

Experimental and Computational Investigation of CO Production and Dispersion in an Automotive Repair Shop

Carbon monoxide (CO), a highly toxic gas, is produced during the incomplete combustion of carbon-based fuels. In indoor environments, high CO concentrations constitute a serious occupational health hazard; this is especially true in the case of automotive repair shop (ARS) employees who are exposed on a daily basis to vehicle exhaust streams. The present study focuses on the experimental investigation and numerical simulation of CO production and dispersion inside an ARS facility. Detailed measurements of CO concentration, vehicle traffic and ventilation system velocities are performed; the obtained data are appropriately formulated to provide quantitative information for modelling purposes. A detailed Computational Fluid Dynamics simulation of the developing transient flow-field is performed. The numerical results are validated using the experimental data; an overall good qualitative and quantitative agreement is achieved. Aiming to improve the energy efficiency of the mechanical ventilation system, three alternative scenarios are investigated; it is shown that the utilization of a dynamic ventilation system may result in significant energy consumption benefits, while, at the same time, CO concentrations remain below the values suggested by current occupational health legislation. The obtained results may be utilized to assist the design of mechanical ventilation systems for ARS facilities.

NUMERICAL MODELLING OF TRANSPORT PHENOMENA IN A DIESEL SPRAY “STABILIZED COOL FLAME” REACTOR

The paper focuses on the numerical simulation of diesel oil droplet evaporation in a “stabilized cool flame” environment. For this purpose, a dedicated model is formulated, correlating cool flame induced heat release with local temperature and fuel concentration values. The developed model is capable of adequately describing the main physicochemical phenomena involved in the process and is deduced using both experimental data and chemical kinetic simulations. The model is implemented in an in-house developed two-phase CFD code and predictions are compared with available experimental data, achieving very good levels of agreement. Parametric studies examining the effects of temperature and fuel concentration variations on the thermal behaviour of the system are also conducted.

A NOVEL DRY PULVERIZER FOR LOW COST PRODUCTION OF POWDERS

ABSTRACT The paper presents the principles of a prototype dry pulverizer particularly suited for industrial applications together with performance tests demonstrating its advantages. The pulverizer offers considerable advantages when compared to commonly used comminution processes, especially in the fine and superfine milling of minerals. The originality of the pulverizer lies mainly in that it allows the variation of the ratio of the shear to the compression forces acting on a particle during its pulverization. The ratio is adjusted to the mechanical requirements of the material to be pulverized. Instead of a single roller or balls, the pulverizer employs several annular rings, thus allowing better grinding of the finer fractions of the original material. The adjustment of the operational conditions to the mechanical properties of the material to be pulverized and the increase of the “active” surface of the pulverizer are examined in terms of ratio of the work done by normal forces to the total work (due...

Solar wall enhanced with phase-change materials: a detailed numerical simulation study

ABSTRACT The use of phase-change materials (PCM) to improve the performance of conventional solar walls (SWs) has been recently proposed. A holistic analysis of the thermal and energy performance of a PCM-enhanced SW is performed by employing numerical simulations on two different physical scales. Initially, a computational fluid dynamics tool is used to investigate the thermal behaviour of a SW, by varying a range of operational parameters such as height, width, inlet air temperature, inlet air velocity and incident solar radiation. Predictions of air velocity and temperature distributions in the air cavity are used to determine the impact of each operational parameter. Furthermore, a building energy performance simulation tool is employed to provide a realistic estimation of the energy savings when a PCM-enhanced SW is used. A reference apartment is used to compare the energy demand and energy consumption ‘before’ and ‘after’ the installation of a conventional and a PCM-enhanced SW. A range of parameters, such as climatic conditions or U-values of external walls and windows, is varied; the selected values correspond to five typical European cities. It is shown that a PCM-enhanced SW results in higher savings in total energy consumption compared to a conventional SW.

Development and Parametric Evaluation of a Tabulated Chemistry Tool for the Simulation of n-Heptane Low-Temperature Oxidation and Autoignition Phenomena

Accurate modelling of preignition chemical phenomena requires a detailed description of the respective low-temperature oxidative reactions. Motivated by the need to simulate a diesel oil spray evaporation device operating in the “stabilized” cool flame regime, a “tabulated chemistry” tool is formulated and evaluated. The tool is constructed by performing a large number of kinetic simulations, using the perfectly stirred reactor assumption. n-Heptane is used as a surrogate fuel for diesel oil and a detailed n-heptane mechanism is utilized. Three independent parameters (temperature, fuel concentration, and residence time) are used, spanning both the low-temperature oxidation and the autoignition regimes. Simulation results for heat release rates, fuel consumption and stable or intermediate species production are used to assess the impact of the independent parameters on the system’s thermochemical behaviour. Results provide the physical and chemical insight needed to evaluate the performance of the tool when incorporated in a CFD code. Multidimensional thermochemical behaviour “maps” are created, demonstrating that cool flame activity is favoured under fuel-rich conditions and that cool flame temperature boundaries are extended with increasing fuel concentration or residence time.