CUED - Cambridge University Engineering Department

    The influence of wake chopping on wet-steam turbine modelling

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    The formation of water droplets within condensing steam turbines is a complex process that occurs at supersaturated, nonequilibrium conditions and is influenced by the unsteady segmentation of blade wakes by successive blade rows. This is often referred to as 'wake chopping', and its effect on the condensation process is the subject of this paper. The practical significance is that thermodynamic 'wetness losses' (which constitute a major fraction of the overall loss) are strongly affected by droplet size. Likewise, droplet deposition and the various ensuing two-phase phenomena (such as film migration and coarse-water formation) also depend on the spectrum of droplet sizes in the primary fog. The majority of wake-chopping models presented in the literature adopt a stochastic approach, whereby large numbers of fluid particles are tracked through (some representation of) the turbine flowfield, assigning a random number at each successive blade row to represent the particle's pitchwise location, and hence its level of dissipation. This study contributes to the existing literature by adding: (a) a comprehensive study of the sensitivity to key model parameters (e.g., blade wake shape and wake decay rate); (b) an assessment of the impact of circumferential pressure variations; (c) a study of the implications for wetness losses and (d) a study of the implications for deposition rates

    Design considerations for high-power-density IPT pads using nanocrystalline ribbon cores

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    In inductive power transfer, ferrite cores present several drawbacks such as brittleness, low permeability and saturation point, and sensitivity to temperature variation. Other materials such as nanocrystalline alloys are being considered as substitutes. They offer a higher permeability and saturation point. Also, they are more robust and stable with temperature. This paper reviews the design considerations that should be taken into account when designing nanocrystalline cores for IPT applications. Bespoke designs are required to mitigate the eddy-current losses which arise due to the high conductivity of the material. A WPT3 pad, 11 kW, is designed and compared to and identical pad with ferrite cores. Using nanocrystalline ribbon cores, a higher coupling factor, 11%, was achieved. Also, a 2% improvement in efficiency was measured. This is attributed to the lower hysteresis losses and higher coupling factor. Finally, the saturation limits were tested for both materials. Results confirm that, with nanocrystalline ribbon cores, higher power ratings and power densities can be achieved

    Thermal-mechanical-electrical coupled design of multilayer energy storage ceramic capacitors

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    A combination of two-dimensional (2D) and three-dimensional (3D) finite element (FE) models of large size multilayer energy storage ceramic capacitors (MLESCCs) was established to simulate the distribution of internal electric field (IEF) under an applied electric bias after sintering process. The sintering stress calculated through thermal-mechanical-coupling was taken as the initial value for mechanical-electrical-coupling under an applied electric bias. The effect of geometric parameters, namely margin length, the gap between two internal electrodes, the thickness of covered layer and the internal electrode fillet radius, on distribution of electric field was studied. Numerical results indicate that the initial stress caused by sintering process would aggravate the non-uniformity of IEF but the adjustment of geometric parameters could improve such non-uniformity, which can serve as reference to the design of MLESCCs. A standard three-factor three-level full factorial design was introduced to pursue a more reasonable design ensuring both large capacitor and high-breakdown voltage

    Electronic structure, optical and dielectric properties of BaTiO<inf>3</inf>/CaTiO<inf>3</inf>/SrTiO<inf>3</inf> ferroelectric superlattices from first-principles calculations

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    The electronic structure, lattice vibrations, and optical, dielectric and thermodynamic properties of BaTiO3/CaTiO3/SrTiO3 (BT/CT/ST) ferroelectric superlattices are calculated by using first-principles calculations. After relaxation, the lattice parameters are in good agreement with the experimental and other theoretical values within an error of 1%. The band structure shows an indirect band gap with a value of about 2.039 eV, and a direct band gap of 2.39 eV at the Γ point. The density of states and the electron charge density along the [001] axis are calculated and show the displacement of Ti ions along the [001] axis. The strong hybridization between O 2p and Ti 3d contributes to the ferroelectricity of BT/CT/ST ferroelectric superlattices. The Γ modes are stable, while the vibration modes at A, M, R, and X points are unstable governing the nature of phase transition. The static dielectric tensor including the ionic contribution is calculated and the permittivity parallel to the optical axis is found to be almost eight times more than the permittivity vertical to the axis, exhibiting strong anisotropy. The thermodynamic enthalpy, the free energy, the entropy, and the heat capacity are also investigated based on the phonon properties

    Novel and simple patterning process of quantum dots via transfer printing for active matrix qd-led

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    © 2020 SID. The next generation of a self-emitting display requires precise and stable patterning techniques to shape Red, Green, and Blue pixels using quantum dots. In this study, we propose the novel and simple transfer printing process for the active matrix QD-LEDs

    An empirical model to evaluate the effects of environmental humidity on the formation of wrinkled, creased and porous fibre morphology from electrospinning

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    Abstract Controlling environmental humidity level and thus moisture interaction with an electrospinning solution jet has led to a fascinating range of polymer fibre morphological features; these include surface wrinkles, creases and surface/internal porosity at the individual fibre level. Here, by cross-correlating literature data of far-field electrospinning (FFES), together with our experimental data from near-field electrospinning (NFES), we propose a theoretical model, which can account, phenomenologically, for the onset of fibre microstructures formation from electrospinning solutions made of a hydrophobic polymer dissolved in a water-miscible or polar solvent. This empirical model provides a quantitative evaluation on how the evaporating solvent vapour could prevent or disrupt water vapor condensation onto the electrospinning jet; thus, on the condition where vapor condensation does occur, morphological features will form on the surface, or bulk of the fibre. A wide range of polymer systems, including polystyrene, poly(methyl methacrylate), poly-l-lactic acid, polycaprolactone were tested and validated. Our analysis points to the different operation regimes associated FFES versus NFES, when it comes to the system’s sensitivity towards environmental moisture. Our proposed model may further be used to guide the process in creating desirable fibre microstructure.</jats:p

    ACHIEVABLE DYNAMIC-RESPONSE FOR AUTOMOTIVE ACTIVE SUSPENSIONS

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    Unbalanced exchange flow and its implications for the night cooling of buildings by displacement ventilation

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    Passive ventilation of buildings at night forms an essential part of a low-energy cooling strategy, enabling excess heat that has accumulated during the day to self-purge and be replaced with cooler night air. Instrumental to the success of a purge are the locations and areas of ventilation openings, and openings positioned at low and at high levels are a common choice as there is then the expectation that a buoyancy-driven displacement flow will establish and persist. Desirable for their efficiency, displacement flows guide excess heat out through high-level openings and cooler air in through low-level openings. Herein we show that displacement flow cannot be maintained for the full duration of a purge. Instead, the flow must transition to an ‘unbalanced exchange flow’, whereby the cool inflow of air at low level is maintained but there is now a warm outflow and a cool inflow occurring simultaneously at the high-level opening. The internal redistribution of heat caused by this exchange alters the rate at which heat is self-purged and the time thought necessary to complete a purge. We develop a theoretical model that captures and predicts these behaviours. Our approach is distinct from all others which assume that a displacement flow will persist throughout the purge. Based on this enhanced understanding, and specifically that the transition to unbalanced exchange flow changes the rate of cooling and resultant emptying times, we anticipate that practitioners will be better placed to design passive systems that meet their target specifications for cooling
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