Dionysios I. Kolaitis

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.

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.

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.

A 3D CFD Modelling Study of a Diesel Oil Evaporation Device Operating in the Stabilized Cool Flame Regime

Diesel fuel is used in a variety of technological applications due to its high energy density and ease of distribution and storage. Motivated by the need to use novel fuel utilization techniques, such as porous burners and fuel cells, which have to be fed with a gaseous fuel, a Diesel fuel evaporation device, operating in the “Stabilized Cool Flame” (SCF) regime, is numerically investigated. In this device, a thermo-chemically stable lowtemperature oxidative environment is developed, which produces a well-mixed, heated air-fuel vapour gaseous mixture that can be subsequently fed either to premixed combustion systems or fuel reformer devices for fuel cell applications. In this work, the ANSYS CFX 11.0 CFD code is used to simulate the three-dimensional, turbulent, twophase, multi-component and reacting flow-field, developed in a SCF evaporation device. An innovative modelling approach, based on the fitting parameter concept, has been developed in order to simulate cool flame reactions. The model, based on p...