Papers by Dimos A Kontogeorgos
In this study, a new method for the determination of the thermal conductivity of the VIP'... more In this study, a new method for the determination of the thermal conductivity of the VIP's core material (i.e. fumed silica) at elevated temperatures, up to 900 o C, is proposed. An experimental set up, based on the heat flow meter apparatus method, is designed and realized. Two VIPs are joined together to form a specimen, which is subjected to fire conditions from one side, while the other side faces ambient conditions, simulating realistic fire conditions. The temperatures on both sides and the heat flux on the unexposed side are recorded and utilized for the determination of the thermal conductivity. The experimental data are coupled with a detailed numerical model, used for the determination of the VIP's core material thermal conductivity. An optimization technique is utilized in order to define the parameters of the model. The contribution of each heat transfer mechanism, i.e. gas conduction, solid conduction and radiation, of the detailed numerical model to the overall thermal conductivity is investigated.
Journal of Facade Design and Engineering, 2016
A numerical code for the simulation of the one-dimensional heat transfer through a commercially a... more A numerical code for the simulation of the one-dimensional heat transfer through a commercially available gypsum board exposed to fire is presented. A parametric study regarding the physical properties of the gypsum is carried out. The predictions obtained with the in-house developed code are in good agreement with experimental data, when temperature dependent physical properties are taken into account. The good performance of gypsum boards under fire conditions, from the fire safety point of view, due to the dehydration/calcination process and the occurring energy absorption is pointed out by the numerical results.
A numerical code for the simulation of the one-dimensional heat transfer through a commercially a... more A numerical code for the simulation of the one-dimensional heat transfer through a commercially available gypsum board exposed to fire is presented. A parametric study regarding the physical properties of the gypsum is carried out. The predictions obtained with the in-house developed code are in good agreement with experimental data, when temperature dependent physical properties are taken into account. The good performance of gypsum boards under fire conditions, from the fire safety point of view, due to the dehydration/calcination process and the occurring energy absorption is pointed out by the numerical results.
This paper addresses the thermal bridges issues of a two storey lightweight steel framed envelope... more This paper addresses the thermal bridges issues of a two storey lightweight steel framed envelope in which the VIPs are placed in an inner " protected " layer of the external walls. This configuration provides " protection " for the VIPs, allows flexibility in installation of facade elements and at the same time permits interventions and modifications (e.g. drilling, installation of appliances) on the internal side of the wall. The envelope is extensively analysed in terms of all the different types of thermal bridges utilizing commercial computational tools and standardized methodologies, and their effect on the overall thermal performance is evaluated. A total improvement of 33% on the heat transfer coefficient of the building is calculated. Results indicate the junctions between the external and internal walls, the external walls and the ceiling, the internal walls and the roof and the internal walls and the floor, respectively, as the most crucial thermal bridges. Different design modifications and solutions are assumed in order to further reduce the impact of the most crucial thermal bridges. The implementation of the modifications resulted in a further reduction of the overall thermal losses by 27.5%, leading to an overall thermal loss reduction by 60.5% as compared to the reference building.
Applied Thermal Engineering, 2010
This paper investigates the simultaneous heat and mass transfer mechanisms occurring in a gypsum ... more This paper investigates the simultaneous heat and mass transfer mechanisms occurring in a gypsum board exposed to fire conditions. An in-house developed code (HETRAN), simulating heat and mass transfer in porous building materials, has been used to predict the heat and mass transfer characteristics within gypsum boards. The code solves numerically a set of mass and energy equations appropriate for the heat and mass transfer in porous materials, assuming homogeneity, local thermodynamic equilibrium and mass transfer due to diffusion and pressure gradients. The predicted temperature evolution within the gypsum sample, with and without mass transfer, is compared to experimental data, demonstrating that vapor migration through the sample holds a significant role in the board behavior under elevated temperatures. The results demonstrate that vapor migrates towards both directions of the board (fire and ambient side), with diffusion mass transfer being the dominant mass transfer mechanism, whereas air moves towards the “fire side”. The dehydration front moves from the “fire side” to the ambient side, with a high velocity in the beginning, which reduces as the front moves through the gypsum sample to the ambient side.
Fire Safety Journal, 2016
This paper studies the fire resistance of innovative high thermally insulated multilayer drywall ... more This paper studies the fire resistance of innovative high thermally insulated multilayer drywall assemblies incorporating conventional insulation materials, Phase Change Materials (PCMs) and Vacuum Insulation Panels (VIPs). An experimental study was developed and implemented into two directions. In the first direction, four different multilayer drywall configurations were subjected to fire temperatures up to 900 °C from one side, while the other side was at ambient conditions. Each configuration consisted of a gypsum board with PCMs (PCM-GB), a standard gypsum board (S-GB), an Expanded Polystyrene (EPS) layer, a thermal insulation render containing EPS (TIR) and an insulation layer located between the PCM-GB and the S-GB. A different insulation layer was used for each configuration: cavity (no insulation), EPS, mineral wool (MW) (both conventional insulation materials) and Vacuum Insulation Panels (VIP) (super insulation material). In the second direction, Differential Scanning Calorimetry (DSC) measurements, at inert (nitrogen) and oxidized (air) environments, were performed for all the utilized materials.
Renewable Energy, 2015
This paper presents a new hybrid methodology for the determination of the effective heat capacity... more This paper presents a new hybrid methodology for the determination of the effective heat capacity (Ceff) of phase change materials (PCMs) for use in numerical models. The methodology focuses on PCM enhanced building panels utilizing a heat flow meter apparatus (HFMA) operating in dynamic mode and a numerical model based on the effective heat capacity method. It comprises of: a) experimental analysis of the panel by means of differential scanning calorimetry (DSC) and HFMA for the estimation of initial Ceff curves, b) optimization of the initial Ceff curves with an algorithm incorporating the numerical model and c) validation of the obtained results. Starting from a complete description of the concept and its main elements, the proposed approach has been successfully employed for the determination of Ceff curves of a lightweight building component combining insulation with thermal storage properties. The derived curves yielded more accurate results when incorporated in the numerical model than the respective curves measured by means of DSC. Simulations of the thermal performance of the building component in different conditions than those used for the determination of the curves validated the effectiveness of the methodology.
Fire and Materials, 2014
The current work proposes for the first time an integrated set of simplified correlations for the... more The current work proposes for the first time an integrated set of simplified correlations for the thermal properties, i.e. effective thermal conductivity, effective specific heat and effective density, of commercial gypsum boards as a function of temperature that can be easily incorporated in dedicated computational tools in order to simulate the fire behavior of a gypsum board. The proposed correlations are based on experimental data purposely performed in the frame of this work, as well as on literature experimental data and theoretical approximations. The applicability and the accuracy of the correlations are established by simulating the fire behavior of various types of gypsum boards exposed to different fire conditions. For the validation of the developed correlations, an in-house developed code is utilized, taking into account thermal properties produced by the proposed correlations. The predictions are compared with two published sets of experimental data, as well as with one experimental data set performed in the current work. The results indicate that the proposed correlations can be reliably utilized in computational tools in order to accurately predict the fire behavior of commercial gypsum boards.
Fire and Materials, 2014
This paper investigates the fire behavior of a regular and an energy storage gypsum board with la... more This paper investigates the fire behavior of a regular and an energy storage gypsum board with latent heat storage characteristics when exposed to fire temperatures. Gypsum board samples, with and without a microencapsulated paraffin mixture phase change material, are studied at material and board level. At the material level, measurements of the physical properties, that is, mass and effective thermal conductivity, as a function of temperature, as well as differential scanning calorimetry experiments, in inert and oxidized environments, are performed. At the board level, specimens are inserted into a preheated oven, and the temperature evolution at preselected board locations is recorded. Both experimental procedures reveal significant information concerning the evolution of the various thermochemical processes taking place inside the gypsum boards during their heating. Results indicated the different fire behavior of the samples at different temperature ranges. At temperatures up to 300°C, the materials act as a fire retardant because of the dehydration of the free and chemically bound water contained in the gypsum boards. On the other hand, at temperatures higher than 300°C, the temperature rise within the samples is enhanced and accelerated because of the oxidation of the phase change material and their external finishing.
Energy and Buildings, 2013
ABSTRACT A comparative assessment of internal versus external thermal insulation systems for ener... more ABSTRACT A comparative assessment of internal versus external thermal insulation systems for energy efficient retrofitting of residential buildings is performed by means of detailed numerical simulations. A 99.6 m2 one-storey apartment located at a mid-level of a multi-storey building is utilized as a “benchmark” case; the external walls of the building are considered to be non-insulated, a typical condition for the majority of the existing Greek building stock, which has been constructed before 1980. The annual thermal and cooling energy requirements are estimated by performing simulations using the TRNSYS software; the effect of insulation layer location (external, internal), meteorological conditions (warm Mediterranean and temperate Oceanic climate regions) and “energy conscious” occupant behaviour (passive, active) is examined by means of a parametric study. Both external and internal thermal insulation configurations are found to significantly reduce the total energy requirements; on average, external insulation outperforms the internal insulation configuration by 8%. Meso-scale hygro-thermal simulations are also performed using the in-house developed HETRAN code. A significant risk for water vapour condensation emerges only in the case of internal insulation installed in the temperate Oceanic climate region. Internal insulation requires approximately 50% less investment cost than the external insulation, thus resulting in a lower payback period.
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Papers by Dimos A Kontogeorgos
Passive fire protection measures compose not only the modern, but also the necessary way for the fire protection of a costruction. On the other hand, Light Weight Construction (LWC) continuously increases its share in construction due to aesthetic and design versatility, as well as due to its very good mechanical, thermal, fire and anti-seismic behavior. Dry Wall Systems (DWS), which are part of LWC systems, are widely used in buildings not only due to their very good mechanical and anti-seismic behavior, but also due to their very good thermal behavior under fire conditions. The latter, is mainly due to the fact that the walls (gypsum boards, cement boards etc.), that DWS consist of, contain water in their crystal structure, which, under high temperature conditions, is evaporated, absorbing significant heat quantities from the fire, and thus, delaying the heat transfer through the assembly.
The main goal of this dissertation is the development of specialized computational tools, in order to accurately predict the variation of the physical properties and the transfer phenomena inside porous materials (structural elements), which are exposed to different temperature conditions. Moreover, these computational tools are designed in order to be able to be combined with detailed Computational Fluid Dynamic (CFD) codes. The frame for the development of these tools is based on an integrated study, which is presented for the first time in this dissertation, of the thermal behavior of a structural material which is exposed to different temperature conditions, and demands the thorough examination of this behavior at a micro-scale (micro-structure size level), meso-scale (structural element size level) and macro-scale (building size level) size level.
At first, as part of the micro-scale level study of the thermal behavior of structural elements, exposed to different temperature conditions, available computational methods from the open literature, which are used for the determination of the kinetic parameters of solid state reactions, are programmed. The developed computational tool can be used for post processing the Differential Scanning Calorimetry (DSC) measurements and for defining the kinetic parameters of a solid state reaction that may occur when a structural element is exposed to different temperature conditions.
The connection bond between the micro- and meso-scale size levels, when studying the thermal behavior of a structural element exposed to different temperature conditions, is the physical properties of the materials, which compose the element and need to be modeled. For this purpose, an integrated system of algebraic equations, that defines the physical properties of a porous material, is presented. In particular, the physical properties that are related to heat and mass transfer through a porous material are modeled, using this system of equations, as well as the kinetic parameters, obtained from the micro-scale level analysis. The above system is programmed and a computational tool named GPRO (Gypsum PROperties) is developed. GPRO is a general computational tool, which is capable of predicting the variation of physical properties of a porous material, exposed to different temperature conditions.
The next step of the integrated study of the thermal behavior of a structural element exposed to different temperature conditions is the study at a meso-scale level. During this dissertation, a computational tool, named HETRAN (HEat TRansfer ANalysis), is developed, in order to predict the thermal behavior of a structural element, composed of multi-layer building materials, exposed to different temperature conditions. HETRAN code has been developed to fill in the gap in existing computational tools regarding simultaneous heat and mass transfer in multi-layered porous materials, as it solves a system of partial differential equations, capable of describing the one dimensional simultaneous heat and mass transfer through porous materials. Finally, the meso-scale HETRAN code, takes into account the effect of the micro-scale level study, by using the physical properties, obtained from the GPRO computational tool.
The developed tools are validated and then used in order to simulate the different physical-chemical processes that take place inside commercial gypsum boards, which are basic elements of DWS, when they are exposed to high temperatures. Firstly, several DSC measurements are performed, through which the theoretical background of these processes is verified. Based on the DSC measurements, a simple system of algebraic equations is developed, for the determination of the initial composition of a commercial gypsum board and the energy absorbed or produced at the end of each process. Morerover, the developed computational tool for the definition of the kinetic parameters of a solid state reaction is used, in conjunction with the DSC measurements, in order to define the kinetic parameters of the most important processes that take place inside a gypsum board at temperatures up to 600oC. Predictions of each reaction progress are compared with experimental data, revealing an excellent agreement. It is established that the obtained kinetic parameters can accurately describe the physical-chemical processes, which take place inside a gypsum board at temperatures up to 600oC.
Thereinafter, these kinetic parameters are incorporated into GPRO code in order to simulate the variation of physical properties of commercial gypsum boards, exposed to elevated temperatures. The validation of GPRO is performed with experimental data available from the literature and experimental data measured during the dissertation. Results showed that the developed computational tool can accurately describe the variation of the physical properties of a commercial gypsum board, exposed to elevated temperatures, and confirms that it can be used for parametric studies, in order to improve the physical properties of a gypsum board.
Finally, HETRAN code is used, in conjunction with GPRO code, in order to simulate the thermal behavior of commercial gypsum board slabs and assemblies, which are exposed to elevated temperatures. Predictions of HETRAN code are compared with experimental data available from the literature, as well as with experimental data measured during the dissertation, revealing the very good accuracy of the numerical results. Furthermore, HETRAN code is used in order to assess the effect of several parameters, such as the heat and mass transfer mechanisms inside the gypsum board porous structure, the heating rate and the water vapor partial pressure, on the thermal behavior of gypsum boards under high temperature conditions. Results showed that the developed computational tool can accurately describe the thermal behavior of a commercial gypsum board or a DWS composed of gypsum boards under fire conditions. Thus, it contributes not only to the theoretical study of the different physical-chemical phenomena that take place inside a gypsum board when it is exposed to elevated temperatures, but also to the design process, parametric study and optimization of the material.
To sum up, the computational tools, which were developed in this thesis, are capable of describing the thermal behavior of commercial gypsum boards, at micro- and mecro-scale level. Thus, they can be used in order to give a clear picture of how a gypsum board behaves, during its exposure to high temperatures conditions.