No description available.

In recent years the use of lithium ion batteries in hybrid and electric vehicles has increased enormously. Due to the high cost of lithium ion batteries, maximising their performance and reducing the rate of degradation is vitally important to vehicle manufacturers. There are many ways in which this can be done, however this thesis focuses on the thermal behaviour of lithium ion batteries and how thermal management can affect performance and levels of degradation. The importance of good experimental design is discussed, with a specific focus on thermal boundary conditions. In addition, some experimental examples are shown highlighting the impact that thermal boundary conditions can have when testing batteries. The thermal properties of cells were measured, and an approach based on Searle’s bar technique and using cells of different sizes was used in order to measure the thermal conductivity of different components and interfaces in a cell. This approach was successful in measuring the thermal conductivity through the layers of a cell, and the results showed a 32% discrepancy with often-used literature values, showing how important the measurement of thermal conductivity is when designing battery thermal management systems or parameterising coupled electrochemical-thermal models. The effect of temperature gradients on the performance of batteries was investigated, showing that a cell under a temperature gradient exhibits poorer performance compared to a cell at a temperature equivalent to the mean of the gradient. This result was reproduced using a simple data-driven model, which showed significant inhomogeneous behaviour of the different layers of the cell. Finally, the effects of cell surface cooling and cell tab cooling were investigated, reproducing two typical cooling systems that are used in real-world battery packs. For new cells using slow-rate standardized testing, very little difference in capacity was seen. However at higher rates, surface cooling led to a loss of useable capacity of 9.2% compared to 1.2% for cell tab cooling. After cycling the cells for 1,000 times, surface cooling resulted in a rate of loss of useable capacity under load three times higher than cell tab cooling. Understanding how thermal management systems interact with the operation of batteries is therefore critical in extending their performance. For automotive applications where 80% capacity is considered end-of-life, using tab cooling rather than surface cooling would therefore be equivalent to extending the lifetime of a pack by 3 times, or reducing the lifetime cost by 66%.

This work investigates the effect of bulk viscosity in one-, two-, and three{dimensional compressible fows via direct numerical simulation. The role of bulk viscosity in compressible turbulence is of increasing importance due to three applications: spacecraft descending through the Martian atmosphere, the thermodynamic cycle of solar-thermal power plant, and carbon capture and storage compressors. All three rely on the accurate description of turbulence in carbon dioxide, a gas with a bulk-to-shear viscosity ratio three orders of magnitude larger than for air. In these applications, invoking Stokes's hypothesis is questioned as the divergence of velocity is non-zero, implying a significant difference between mechanical and thermodynamic pressures. Results of a constantly forced velocity perturbation follow the same trend as that predicted by Landau's acoustic absorption coeffcient for suffciently high Reynolds numbers. Below an optimum Reynolds number, the damping effectiveness reduces by a different mechanism to that of Landau. Maximum damping is achieved at an acoustic Reynolds number equal to unity. Two-dimensional decaying turbulence at the bulk-to-shear viscosity ratio of carbon dioxide demonstrates that the magnitude of the dilatational production term is greatly enhanced and is strongly biased to negative values, reducing the generation of velocity dilatation compared to the zero bulk viscosity case. Compressible Couette flow at two Reynolds numbers and two bulk-to-shear viscosity ratios show minimal changes to mean flow quantities and the main terms of interest in the turbulence kinetic energy budget. Instantaneous views of the dilatational velocity field show that an intermediate range of scales are damped in accordance with Landau's acoustic damping coeffcient. At small scales, however, damping reduces and turbulent patterns are preserved.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

The UK’s advanced gas-cooled reactor power plants contain welded components which were not stress relieved following fabrication. The presence of weld induced residual stresses, in combination with the plant operating under creep conditions and the material’s low creep ductility has caused cracks to form during service due to a process known as reheat cracking. To investigate this cracking process and to develop assessment procedures to evaluate the structural integrity of such components in operation, fracture specimens are required to simulate this loading condition and subsequently perform crack growth studies in laboratory controlled conditions. Two new fracture mechanics specimen designs were proposed in this study: an electron beam (EB) welded compact tension, C(T), specimen and a wedge-loaded C(T) specimen. EB welding had previously been used as a fabrication process to manufacture C(T) specimens reconstituted from ex-service components. However in this study, this weld process was used specifically to introduce residual stresses in the specimens. Both specimen designs were fabricated using ex-service Type 316H austenitic stainless steel. Extensive residual stress measurements were made using the neutron diffraction, contour method and slitting techniques on EB welded and wedge-loaded C(T) specimens. This data was used to develop and validate numerical simulations of the fabrication processes enabling residual stress predictions to be made and stress intensity factors, which define the crack driving force, to be determined. Large initial stress intensity factors due to the residual stresses were determined as up to 22.2 MPam^(1/2) and 43.6 MPam^(1/2) for the EB welded and wedge-loaded C(T) specimens respectively. Accurate estimates of the weld residual stresses in the EB welded C(T) specimen required detailed weld simulations to be developed. These were created by following guidelines recently published in the R6 fracture assessment procedure for modelling arc welding processes. The stress predictions made by the weld simulations were in close agreement with the experimental measurements, which showed the advice in the R6 guidelines may be followed to produce accurate numerical simulations of EB welding. Creep crack growth (CCG) tests were conducted using these new specimen designs at 550°C for up to 1,300 h, under secondary and combined loading conditions, where large crack extensions of up to 5.4 mm occurred. Such large crack growth was achieved by pre-conditioning the material by uniform pre-compression prior to specimen fabrication. This process reduced the creep ductility of the material enabling CCG to occur during relaxation of the residual stresses. It was shown that this pre-conditioning process was necessary to perform such experiments, as the creep ductility in ex-service Type 316H stainless steel in the as-received condition can be high. The experimental measurements were used to validate crack growth predictions using the new revision of the R5 assessment procedure. Crack extensions were predicted using C(t) estimated using a reference stress based approach under secondary and combined loading conditions. This assessment was shown to be very conservative where no plastic deformation was assumed, as predicted crack lengths were up to 10 times larger than experimental measurements. By including the effects of crack tip plasticity in the assessment, estimates of the crack extensions were in close agreement with the experimental data. The creep crack growth rates were sensitive to the material’s creep ductility, which differed between the test specimens due to weld induced plasticity and variability of the microstructure in the ex-service material. Use of the upper bound crack growth properties is recommended to ensure conservative assessments are made. Damage models developed using the ductility exhaustion approach were also used to predict CCG. Crack length predictions in the wedge-loaded C(T) specimens were generally in good agreement with the numerical models. However non-conservative crack growth predictions were made in the EB welded C(T) specimens which was due to the presence of plasticity.