A review of wildland fire spread modelling, 1990-present, 1: Physical and quasi-physical models

In recent years, advances in computational power and spatial data analysis (GIS, remote sensing, etc) have led to an increase in attempts to model the spread and behaviour of wildland fires across the landscape. This series of review papers endeavours to critically and comprehensively review all types of surface fire spread models developed since 1990. This paper reviews models of a physical or quasi-physical nature. These models are based on the fundamental chemistry and/or physics of combustion and fire spread. Other papers in the series review models of an empirical or quasi-empirical nature, and mathematical analogues and simulation models. Many models are extensions or refinements of models developed before 1990. Where this is the case, these models are also discussed but much less comprehensively.Comment: 31 pages + 8 pages references + 2 figures + 5 tables. Submitted to International Journal of Wildland Fir

Computational study of self-heating ignition and smouldering combustion of carbon-rich porous media

Self-heating describes the tendency of carbon-rich porous media to spontaneously rise its temperature, driven by internal exothermic reactions and poor heat dissipation to the surroundings. The self-heating ignition of carbon-rich media can initiate fires, leading to energy waste, ecosystem degradation, damage to industrial facilities, and even loss of life. In this thesis, I computationally studied the transient evolution and mechanisms in self-heating ignition and smouldering combustion of coal and peat, two representative materials of importance in industry and nature respectively. Based on Gpyro, an open-source code, I built a computational model and validated it against laboratory experiments from the literature that cover two configurations, a wide range of sample sizes, and various coal sources. By integrating the model with a 2-step scheme that considers oxygen adsorption and heterogeneous combustion, I investigated the link between self-heating ignition and smouldering spread. The simulations revealed that the smouldering front originated near the hot boundary and then spread towards the free surface seeking oxygen supply. Including drying and desorption, a more complete 4-step scheme is obtained through inverse-modelling of microscale experiments and the simulations based on the 4-step scheme revealed that self-heating ignition at large scale is mainly driven by oxygen adsorption. Using a similar method, I then studied peat and produced a 3-step scheme that includes drying, biological reaction, and chemical oxidation for it. After validating the model against experiments in the literature, I simulated the self-heating ignition of large-scale peat layers for the first time. It was shown that biological reaction can trigger the acceleration of chemical oxidation, which eventually leads to smouldering fires. Overall, this thesis has provided a novel model that is able to predict a wide range of experiments of self-heating ignition for two carbon-rich porous media. The link between self-heating ignition and smouldering spread has been revealed, and the chemical multi-step effect at large scale has been analysed in detail, which improves our fundamental understanding of self-heating ignition and smouldering fires.Open Acces

Fully coupled CEM/CFD modelling of microwave heating in a porous medium

Computational results for the microwave heating of a porous material are presented in this paper. Coupled finite difference time domain and finite volume methods are used to solve equations that describe the electromagnetic field and heat and mass transfer in porous media. These equations are nonlinearly coupled through the dielectric properties which depend both on temperature and moisture content. By investigating the resonant behaviour in two-dimensional microwave cavities, the FD-TD scheme is validated. Validation of the microwave power distribution in 3-D microwave enclosures is compared with other numerical results available. 3-D temperature distribution in a biomaterial is validated against experimental results. Results using the proposed fully coupled approach are discussed and analyzed. The model is able to reflect the evolution of both temperature and moisture fields as well as energy penetration as the moisture in the porous medium evaporates. Moisture movement results from internal pressure gradients produced by the internal heating and phase change. The model is validated by comparison to some published results for simpler problems

Effect of curing conditions and harvesting stage of maturity on Ethiopian onion bulb drying properties

The study was conducted to investigate the impact of curing conditions and harvesting stageson the drying quality of onion bulbs. The onion bulbs (Bombay Red cultivar) were harvested at three harvesting stages (early, optimum, and late maturity) and cured at three different temperatures (30, 40 and 50 oC) and relative humidity (30, 50 and 70%). The results revealed that curing temperature, RH, and maturity stage had significant effects on all measuredattributesexcept total soluble solids

Porous materials in building energy technologies—a review of the applications, modelling and experiments

Improving energy efficiency in buildings is central to achieving the goals set by Paris agreement in 2015, as it reduces the energy consumption and consequently the emission of greenhouse gases without jeopardising human comfort. The literature includes a large number of articles on energy performance of the residential and commercial buildings. Many researchers have examined porous materials as affordable and promising means of improving the energy efficiency of buildings. Further, some of the natural media involved in building energy technologies are porous. However, currently, there is no review article exclusively focused on the porous media pertinent to the building energy technologies. Accordingly, this article performs a review of literature on the applications, modelling and experimental studies about the materials containing macro, micro, and nano-porous media and their advantages and limitations in different building energy technologies. These include roof cooling, ground-source heat pumps and heat exchangers, insulations, and thermal energy storage systems. The progress made and the remaining challenges in each technology are discussed and some conclusions and suggestions are made for the future research

Modelling local hygrothermal interaction between airflow and porous materials for building applications

Moisture related damage in buildings is a phenomenon which is familiar to most people. Most of the time it is spontaneously associated with damage due to liquid moisture transport such as plumbing leaks, rising moisture in walls, . . . Yet some materials and objects are so sensitive to moisture that they can already be damaged by water vapour transport through the air. This is especially true for culturally or historically valuable artefacts: even a small amount of damage (like small cracks, . . . ) is unacceptable for these objects. The reason for their high sensitivity for moisture related damage can be found in the used materials: these objects are typically composed of wood or other organic materials which strongly expand in function of the moisture content (and thus indirectly in function of the relative humidity). This means that subsequent fluctuations of the relative humidity in the air can result in an expansion and deformation of the object and the hereby induced tensions can lead to fractures or other damage phenomena (e.g. cracking of paint). To preserve these objects as good as possible it is hence extremely important to keep the relative humidity in the surrounding air as constant as possible. Art objects are typically stored and exhibited in historical (e.g. churches) or monumental buildings (e.g. museums). While it is now standard procedure to place a HVAC installation in large buildings, this was of course not the case for historical buildings. Yet due to the increased demand on thermal comfort, these historical buildings are, when retrofitted, more and more equipped with at least a permanent heating system. The intermittent use of a heating system however results in considerable temperature fluctuations and thus also in important relative humidity fluctuations. Due to the large volume of these buildings the temperature and relative humidity fluctuations will strongly vary in space: during heating the temperature above an air inlet will for example rise much stronger than the temperature in the rest of the building. Yet air flows in a building can also become very complex. It is hence not always possible to intuitively predict where the largest fluctuations will occur and it is even harder to estimate the magnitude of these fluctuations and the associated risk of moisture related damage. There is thus a need for a model that can predict the local fluctuations in air temperature and relative humidity and the associated hygric response of individual objects in order to predict the risk of local moisture related damage. This dissertation is the result of research conducted in search of suitable strategies to model local temperature and relative humidity fluctuations in the air and porous materials for the application in buildings. Chapter 1 starts with an elaborate introduction of moisture related damage induced by relative humidity fluctuations. Next a literature review on existing models and modelling techniques used in hygrothermal simulations for buildings is presented. This overview shows that at present the existing models can not be used for the applications aimed at in this work as they either do not offer the required level of detail or as there are limitations in the used physical models (e.g. only 2D, . . . ). It is also shown in this chapter that a combination of CFD flow simulation and a hygrothermal material model is best suited for the simulation of the local interaction between the air and the porous material. The second chapter focuses on the use of transfer coefficients for the modelling of the hygrothermal interaction between air and porous materials. If it would be possible to use transfer coefficients in the air model then the required computational power would drastically decrease. The study on transfer coefficients starts with an evaluation of the different definitions of the mass transfer coefficient. It is demonstrated that the use of vapour densities in this definition, results in a dependence of the mass transfer coefficient on the temperature difference between fluid and wall. This reduces the applicability of mass transfer coefficients in practice. Yet, when mass fractions are used in the definition, the value of the mass transfer coefficient no longer depends on the temperature difference and the applicability is strongly increased. Next these results are used to study the possibility of using the heat and mass (moisture) analogy to simplify the prediction of local mass transfer coefficients in buildings. It is shown that this is not feasible. The chapter is concluded by checking to which extent the local transfer coefficients remain constant during a transient process in a building environment. It is found that it is not possible to use constant transfer coefficients during long periods of time and that a possible advantage in calculation time associated with the use of transfer coefficients cancels since even in case of a transient moisture response with steady-state flow, the local mass transfer coefficients strongly vary. As it was shown that the use of constant transfer coefficients is not an option for the applications aimed at in this work, the choice was made to directly couple the CFD model and the hygrothermal material model. To reduce the extra computation time imposed by this coupling, the hygrothermal material model is integrated into the CFD solver. This means that no time consuming data exchange between two separate codes is necessary. Chapter 3 presents the combined heat and moisture transport equations which have to be solved by the extended solver. Chapter 4 describes how the transport equations introduced in Chapter 3 can be transformed into a discretized form and which numerical techniques are needed to assure the conservation of mass and energy. Implementation of these discretized transport equations into the CFD solver results in the coupled CFD - material model. The different solver settings are briefly discussed. The well functioning of the coupled model is proved by means of a verification and validation study. In Chapter 5 the newly developed model is applied to a case study: the microclimate vitrine. In such a vitrine valuable and fragile objects (e.g. paintings) are placed for protection against fluctuations in relative humidity. The operating principle of the vitrine is based on the fact that due to the hygric buffering of the object the relative humidity of the small amount of air in the vitrine is stabilized. To make a correct and detailed analysis of the effectiveness of such a vitrine it is of crucial importance to take the local interaction between the air and the object into account. Simulations performed with the new model confirm the stabilizing influence of the vitrine and offer an explanation for the non-intuitive phenomena experienced in practice for this type of vitrines. The new model clearly offers an added value when analyzing the risk of moisture related damage. Chapter 6 concludes this work by summarizing the most important conclusions and by looking forward to possible future research

External coupling of building energy simulation and building element heat, air and moisture simulation

The present challenge for the building industry is to provide buildings that are safe, comfortable, energy efficient and sustainable; creating a healthy and productive environment for users. Computational building performance simulation (CBPS) can play an important role to deal with this challenge, particularly for performance indicators related to the heat, air and moisture (HAM) at the whole-building scale. These indicators are currently assessed by a large number of programs with considerable uncertainty in their results, such as building energy simulation (BES) programs and building element heat, air and moisture (BEHAM) programs. This thesis poses the hypothesis that the lack of integration between these programs represents an important source of uncertainty in whole-building HAM simulation, which can compromise, in some circumstances, the accuracy of their results. In order to test this hypothesis, this thesis proposes, implements, verifies and validates protocols to integrate BES and BEHAM programs using external coupling. These protocols, which are based on literature review and theoretical analysis of the governing equations, are implemented in prototype computer programs using numerical simulation and inter-process communication routines. The prototypes are verified by a number of techniques developed in this thesis, such as the use of emulators, one-way coupling and self-coupling. Validation is carried out using analytical solutions, inter-model comparison and experimental results reported in the literature. Coupled BES-BEHAM simulations showed improvements in the accuracy when compared to stand-alone BES or BEHAM simulations. In order to identify cases where coupled BES-BEHAM simulations provide significant improvement in the results, the coupling necessity decision procedure (CNDP) is formulated. Capabilities of coupled BES-BEHAM simulations in combination with the CNDP are demonstrated by case studies, where some capabilities and deficiencies of stand-alone programs are also evaluated. This research concludes that coupled BES-BEHAM simulation provides a viable and reliable way to perform whole-building HAM simulation. A number of additional results are also provided in this thesis, such as the solution for several coupling features addressed in the coupling protocols, the verification techniques developed and the use of TCP/IP sockets for the communication between the programs

Co-simulation of building energy simulation and computational fluid dynamics for whole-building heat, air and moisture engineering

Building performance simulation (BPS) is widely applied to analyse heat, air and moisture (HAM) related issues in the indoor environment such as energy consumption, thermal comfort, condensation and mould growth. The uncertainty associated with such simulations can be high, and incorrect simulation results can lead to a design with adverse effects on health, comfort and functionality of space. In recent years, the use of BPS tools to predict and analyse the HAM behaviour of the indoor environment has grown significantly. Among these tools, Building Energy Simulation (BES) and Computational Fluid Dynamics (CFD) are recognized as potential tools for assessing HAM behaviour of the indoor environment, such as interaction of the HVAC system with convective heat and mass transfer. These tools have strong capabilities, but also some particular deficiencies in terms of boundary conditions, physical models and resolution in space and time. BES is mainly used to assess the thermal performance of buildings throughout the entire year. It is a powerful tool, but when compared to CFD tools it includes simplified air flow, heat and moisture transfer modelling. Detailed HAM modelling of the building indoor environment is possible with CFD. In CFD, however, the implementation of meteorological boundary conditions, the whole HVAC system modelling etc. are significantly less advanced than in BES. In this thesis, it is hypothesized that if used correctly, the combination of BES and CFD tools will increase the accuracy of HAM simulations of the indoor environment. The thesis first presents approaches for domain integration, relevant physical phenomena, interface variables, and coupling requirements. Then, it introduces a newly developed prototype, which integrates BES and CFD for high resolution HAM simulation of the indoor environment. Next, it describes the verification of the prototype. This is followed by the validation study of the prototype, which shows that the accuracy of the HAM simulation is enhanced. Finally its usage potential is illustrated by discussion of the results of real applications in the building industry

A co-operating solver approach to building simulation

This paper describes the co-operating solver approach to building simulation as encapsulated within the ESP-r system. Possible adaptations are then considered to accommodate new functional requirements

A review of technology of personal heating garments.

Modern technology makes garments smart, which can help a wearer to manage in specific situations by improving the functionality of the garments. The personal heating garment (PHG) widens the operating temperature range of the garment and improves its protection against the cold. This paper describes several kinds of PHGs worldwide; their advantages and disadvantages are also addressed. Some challenges and suggestions are finally addressed with regard to the development of PHGs