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The last two decades have witnessed the shift of structural fire design from prescriptive approaches to performance-based approaches in order to build more advanced structures while reducing costs. However, it is recognised that the implementation of performance-based approaches requires several key elements that are currently not fully developed or understood. This research set out to address some of these issues by focusing on the development, validation and application of methodologies for accurate predictions of thermal responses of structures in fire using numerical methods. This research firstly proposed a numerical approach with the finite element and the discrete ordinates method to quantify the fire imposed radiative heat fluxes to structural members with cavity geometry. With satisfactory results from the verification and validation tests, it is used to simulate heat transfer to unprotected steel I-sections with symmetrical cavities exposed to post-flashover fires. Results show that the cavity geometry could strongly attenuate the radiative energy, while the presence of hot smoke enhances radiative transfer by emission. Average radiative fluxes for the inner surfaces of the I-sections are seen to increase with smoke opacity. In addition, the net radiative fluxes are observed to decrease faster for I-sections with higher section factors. This work also shows that the self-radiating mechanism of I-sections is important in the optically thin region, and existing methodologies neglecting these physics could significantly underpredict steel temperatures. The next focus of this work is to develop a thermal analysis framework dedicated to structures-in-fire modelling in the OpenSees (Open System for Earthquake Engineering Simulation) platform which has been developed towards a highly robust, extensible and flexible numerical analysis framework for the structural fire engineering community. The thermal analysis framework, which is developed with object-oriented programming paradigm, consists of a fire module which has incorporated a range of conventional empirical models as well as the travelling fire model recently developed elsewhere to quantify the fire imposed boundary conditions, and a heat transfer module which addresses non-linear heat conduction in structural members with the finite element method. The developed work has demonstrated good performance from benchmark problems where analytical solutions are available and from full scale tests with measured data. With the thermal analysis capability developed in this work together with the work by other colleagues to quantify the mechanical response at elevated temperatures, the extended OpenSees framework can be used to predict structural performances subjected to a wide range of re scenarios. This work uses OpenSees for a case study of a generic composite structure subjected to travelling fires. The latest work on travelling fire methodology for structural fire design has been implemented in the OpenSees framework. The work presented in this thesis is the first effort to examine both the thermal and structural responses of a composite tall building in travelling fires using OpenSees. Results from the thermal analysis show that travelling fires of larger sizes (e.g. burning area equal to 50% of the floor area) are more detrimental to steel beams in terms of more rapid heating rate, while those of smaller sizes (e.g. burning area equal to 4% of the floor area) burn for longer duration and thus are more detrimental to concrete slabs in light of higher peak temperatures. The results also show that fires of large sizes tends to produce higher through-depth thermal gradients in the steel beam sections particularly in neighbouring regions with the concrete slab. Due to less rapid heating rates but prolonged burning durations, smaller fires produce lower thermal gradients but with higher temperatures in the concrete slab particularly at locations far from the fire origin. The subsequent structural analysis suggests that travelling fires produce higher deflections and higher plastic deformations in comparison with the uniform parametric fires, particularly with smaller fire sizes producing more onerous results. The results seem to be more physically convincing and they challenge the conventional assumption that the post-flashover fires are always more conservative for structural performance.

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A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfillment of the requirements for the degree of Master of Science. March 28, 2013 / The search for solutions to the important differential equations arising in extended surface heat transfer continues unabated. Extended surfaces, in the form of longitudinal fins are considered. First we consider the steady state problem and then the transient heat transfer models. Here, thermal conductivity and heat transfer coefficient are assumed to be functions of temperature. Thermal conductivity is considered to be given by the power law in one case and by the linear function of temperature in the other; whereas heat transfer coefficient is only given by the power law. Explicit analytical expressions for the temperature profile, fin efficiency and heat flux for steady state problems are derived using the one-dimensional Differential Transform Method (1D DTM). The obtained results from 1D DTM are compared with the exact solutions to verify the accuracy of the proposed method. The results reveal that the 1D DTM can achieve suitable results in predicting the solutions of these problems. The effects of some physical parameters such as the thermo-geometric fin parameter and thermal conductivity gradient, on temperature distribution are illustrated and explained. Also, we apply the two-dimensional Differential Transform Method (2D DTM) to models describing transient heat transfer in longitudinal fins. Furthermore, conservation laws for transient heat conduction equations are derived using the direct method and the multiplier method, and finally we find Lie point symmetries associated with the conserved vectors.

Heat transfer at the nanoscale has been one of the primary concerns in the design of nanoelectronics and nanostructured materials for applications such as thermal management and thermoelectric energy conversion. This thesis examines the thermal transport in nanoscale thin films and two-dimensional (2D) materials using an optical pump-probe technique based on frequency domain thermoreflectance (FDTR). The design and implementation of a continuous-wave laser based FDTR system is described in detail. The system is extended to an imaging microscope capable of producing micrometer scale maps of several thermophysical properties simultaneously. An analytical formula, which accounts for experimental noise and uncertainty in the controlled model parameters, is derived to calculate the precision of thermoreflectance measurements. The FDTR system is used to study the anisotropic heat conduction in periodic nanoscale Mo/Si superlattices and a 2D material, graphene. The measured in-plane thermal conductivity values of the superlattices are in good agreement with calculations taking into account both electron and phonon thermal transport, using a phonon mean free path which depends on the Mo layer thickness. The measurement procedure of graphene is described in detail, including the sample preparation, sensitivity analysis, and parameter fitting. Various graphene flakes supported on SiO2 surfaces and atomically flat Muscovite mica surfaces are measured. The results show that the thermal conductivity of single-layer graphene can be improved by ~3 times by using a mica substrate compared to commonly used SiO2 substrates. In addition, comparison with the reported values of suspended graphene suggest that the out-of-plane flexural phonon modes may contribute at least 70% to the thermal conductivity of graphene. Finally, the thermal model is modified to include volumetric heating for the measurement of materials without a transducer layer. An amorphous silicon film deposited on fused silica and silicon substrates is measured to validate the model.

Two-phase flows with or without phase change are present in terrestrial and space applications like thermal control of satellites, propellant supply for launchers, and waste water treatment for space exploration missions. Flow boiling experiment with HFE7000 were conducted in a heated tube in vertical upward flow on ground and in microgravity conditions to collect data on flow patterns, pressure drops, heat transfers, void fraction. Void fraction measurements allowed to measure mean gas velocity and the liquid film thickness in annular flow. In microgravity condition, the liquid film thickness and the interfacial shear stress are significantly lower than in normal gravity. A detail analysis of the film structure was performed by image processing. The impact of gravity and liquid and vapour superficial velocities on the disturbance waves velocities and frequencies was investigated. Two different measurement techniques were used and compared to determine the heat transfer coefficient. For quality values greater than 0.2, HTC is not sensitive to gravity and is in good agreement with classical correlations of the literature. For qualities smaller than 0.1, in the subcooled nucleate boiling regime HTC is significantly smaller in microgravityconditions.

Condensation on banks of tubes is of considerable interest in the power, refrigeration and process industries, where large scale condensers form a significant proportion of plant capital costs. Since the pioneering paper by Nusselt in 1916, numerous investigations, both experimental and theoretical, have made great inroads into the understanding of the important physical factors effecting performance, including effects of vapour shear and condensate inundation on heat-transfer performance. Despite this there are still significant gaps in the knowledge and no single recognised design approach exists for condensers under all conditions. Purely theoretical models have shown some success in modelling condensation on single tubes under high shear regimes, but these have not been successfully extended to full tube banks. The present work begins by drawing together a comprehensive data base of experimental results from those available in the literature. This includes assessing the experimental accuracy of the data and organising it into a consistent format to allow detailed comparison with existing and future models. The resulting data base comprises 13 tube bank geometries, 7 test fluids and over 4000 individual data points. The data base was used to evaluate existing theoretical and empirical models, and highlighted the weaknesses therein. In particular, it was found that empirical approaches were limited to application (ie refrigeration or steam condensers), with development and validation being based on experimental data for single fluids or groups of fluids. When these models were compared to the more comprehensive data base described above their performance was poor. Consequently there is limited confidence in their extension to applications outside those they were developed for. A new empirical based model was then developed. The approach involved identifying relevant dimensionless groups to account for the various physical factors which may affect heat transfer during condensation on a bank of tubes and formulating these into an equation involving a number of initially unknown but empirically obtainable constants. An iterative scheme was then employed to eliminate those groups having little effect on the result while retaining those which proved to be more important. The resulting model predicted the majority of the experimental data base to around 12%. A subsequent parametric study 3 showed the correct dependence of heat transfer coefficient on factors such as vapour velocity and tube row. The thesis concludes with some suggestions for future work.