Analysis of Hydrodynamics and Heat Transfer in a Thin Liquid Film Flowing over a Rotating Disk by Integral Method

An integral analysis of hydrodynamics and heat transfer in a thin liquid film flowing over a rotating disk surface is presented for both constant temperature and constant heat flux boundary conditions. The model is found to capture the correct trends of the liquid film thickness variation over the disk surface and compare reasonably well with experimental results over the range of Reynolds and Rossby numbers covering both inertia and rotation dominated regimes. Nusselt number variation over the disk surface shows two types of behavior. At low rotation rates, the Nusselt number exhibits a radial decay with Nusselt number magnitudes increasing with higher inlet Reynolds number for both constant wall temperature and heat flux cases. At high rotation rates, the Nusselt number profiles exhibit a peak whose location advances radially outward with increasing film Reynolds number or inertia. The results also compare favorably with the full numerical simulation results from an earlier study as well as with the reported experimental results

Modelling spontaneous combustion of coal

Spontaneous combustion of coal is an important problem in mining and storage, in terms of both safety and economics. This is because coal reacts with oxygen in the air and an exothermic reaction occurs, even in ambient conditions. The heat of the reaction accumulates and the reaction becomes progressively faster and thermal runaway may take place to the point of ignition. A detailed computer model has been developed to simulate a bulk-scale, one-dimensional test column. Predictions from this model can then be used to simulate full-scale storage conditions. Model predictions are verified by using the experimental results from the test column at the University of Queensland. A 2-m column is being used in this laboratory to conduct a practical test capable of providing reliable data on coal self-heating. Coal self-heating results produced with the 2-m column are consistent with theory. In particular, the hot spot development in test runs closely matches model predictions. Features of moisture transfer and hot spot migration are clearly visible, both in the model and in tests in the column. Under the specific conditions considered in this study, it is shown that a subbituminous coal can reach thermal runaway in 4.5 days. This result is confirmed by observations made at the mine site, where hot spots have been found to occur within this timeframe. The results obtained in this study indicate that there is a definite need to consider the influence of coal moisture on spontaneous combustion

Diminishing Discrimination in English Premier League Soccer

Racism still represents one of the biggest problems in soccer. Szymanski has proven that racial discrimination exists in English Premier League soccer. This paper shows diminishing discrimination against African origin and foreign players looking at a time period from 2001 to 2010

FORCE FED MICROCHANNEL HIGH HEAT FLUX COOLING UTILIZING MICROGROOVED SURFACES

Among other applications, the increase in power density of advanced electronic components has created a need for high heat flux cooling. Future processors have been anticipated to exceed the current barrier of 1000 W/cm2, while the working temperature of such systems is expected to remain more or less the same. Currently, the well known cooling technologies have shown little promise of meeting these demands. This dissertation investigated an innovative cooling technology, referred to as force-fed heat transfer. Force-fed microchannel heat sinks (FFMHS) utilize certain enhanced microgrooved surfaces and advanced flow distribution manifolds, which create a system of short microchannels running in parallel. For a single-phase FFMHS, a numerical model was incorporated in a multi-objective optimization algorithm, and the optimum parameters that generate the maximum heat transfer coefficients with minimum pumping power were identified. Similar multi-objective optimization procedures were applied to Traditional Microchannel Heat Sinks (TMHS) and Jet Impingement Heat Sinks (JIHS). The comparison study at optimum designs indicates that for a 1 x 1 cm2 base heat sink area, heat transfer coefficients of FFMHS can be 72% higher than TMHS and 306% higher than JIHS at same pumping power. For two-phase FFMHS, three different heat sink designs incorporating microgrooved surfaces with microchannel widths between 21 μm and 60 μm were tested experimentally using R-245fa, a dielectric fluid. It was demonstrated that FFMHS can cool higher heat fluxes with lower pumping power values when compared to conventional methods. The flow and heat transfer characteristics in two-phase mode were evaluated using a visualization test setup. It was found that at low hydraulic diameter and low mass flux, the dominant heat transfer mechanism is dynamic rapid bubble expansion leading to an elongated bubble flow regime. For high heat-flux, as well as combination of high heat flux and high hydraulic diameters, the flow regimes resemble the flow characteristics observed in conventional tubes. The present research is the first of its kind to develop a better understanding of single-phase and phase-change heat transfer in FFMHS through flow visualization, numerical and experimental modeling of the phenomena, and multi-objective optimization of the heat sink

Turbulent ‘stopping plumes’ and plume pinch-off in uniform surroundings

Observations of turbulent convection in the environment are of variously sus- tained plume-like flows or intermittent thermal-like flows. At different times of the day the prevailing conditions may change and consequently the observed flow regimes may change. Understanding the link between these flows is of practical importance meteorologically, and here we focus our interest upon plume-like regimes that break up to form thermal-like regimes. It has been shown that when a plume rises from a boundary with low conductivity, such as arable land, the inability to maintain a rapid enough supply of buoyancy to the plume source can result in the turbulent base of the plume separating and rising away from the source. This plume ‘pinch-off’ marks the onset of the intermittent thermal-like behavior. The dynamics of turbulent plumes in a uniform environment are explored in order to investigate the phenomenon of plume pinch-off. The special case of a turbulent plume having its source completely removed, a ‘stopping plume’, is considered in particular. The effects of forcing a plume to pinch-off, by rapidly reducing the source buoyancy flux to zero, are shown experi- mentally. We release saline solution into a tank filled with fresh water generating downward propagating steady turbulent plumes. By rapidly closing the plume nozzle, the plumes are forced to pinch-off. The plumes are then observed to detach from the source and descend into the ambient. The unsteady buoyant region produced after pinch-off, cannot be described by the power-law behavior of either classical plumes or thermals, and so the terminology ‘stopping plume’ (analogous to a ‘starting plume’) is adopted for this type of flow. The propagation of the stopping plume is shown to be approximately linearly dependent on time, and we speculate therefore that the closure of the nozzle introduces some vorticity into the ambient, that may roll up to form a vortex ring dominating the dynamics of the base of a stopping plume

A Study of Critical Phenomena in Krypton

A detailed experimental study of equilibrium critical phenomena in krypton was made. Using the method of angle of minimum deviation the refractive index was measured along the coexistence curve, along 16 isotherms above and along 11 isotherms below the critical temperature. The range of the temperature measurements in terms of t the reduced temperature difference from Tc was -6.8 x 10-2 ≦ t ≦ -5.7 x 10-5 and 3.8 x 10-5 ≦ t ≦ 4.8 10-2. The measurements were planned so that the region very near the critical point was covered in most detail. The refractive index was related to the density through the Lorentz-Lorenz relation. After proper weight assignment, the data were analyzed in terms of the asymptotic power laws. The following values of the critical parameters, exponents and coefficients were determined: Tc = 209.286 ± 0.010°K, Pc = 54.213 ± 0.003 atm., LLc = 0.070588 ± 0.000006, β = 0.3571 ± 0.0008, B = 1.840 ± 0.001 γ = 1.182 ± 0.008, Γ = 0.0835 ± 0.0011; γG’ =1.15 ± 0.01, ΓG’ = 0.021 ± 0.001, γL’ = 1.13 ± 0.01, ΓL’ = 0.025 ± 0.001; δ = 4.25 ± 0.25. The law of the rectilinear diameter was obeyed with its slope = 0.0918 ± 0.0004. The reduced chemical potential differences and the reduced density differences were calculated. The chemical potential was observed to show antisymmetry for -2 x 10-3 ≦ t &#60; 4.8 x 10-2 and -0.3 &#60; ΔLL &#60; 0.3. The data in this range were analyzed using Widom's equation of state and the closed-form(29) of h(x). The proposed equation was found to fit the experimental data very well. The predictions of the linear model(32) were also checked and were observed to be consistent with the experimental results.</p

Spatio-Temporal Linear Stability Analysis of Stratified Planar Wakes: Velocity and Density Asymmetry Effects

This paper explores the hydrodynamic stability of bluff body wakes with non-uniform mean density, asymmetric mean density, and velocity profiles. This work is motivated by experiments [S. Tuttle et al., “Lean blow off behavior of asymmetrically-fueled bluff body-stabilized flames,” Combust. Flame 160, 1677 (2013)], which investigated reacting wakes with equivalence ratio stratification and, hence, asymmetry in the base flow density profiles. They showed that highly stratified cases exhibited strong, narrowband oscillations, suggestive of global hydrodynamic instability. In this paper, we present a local hydrodynamic stability analysis for non-uniform density wakes that includes base flow asymmetry. The results show that increasing the degree of base density asymmetry generally has a destabilizing effect and that increasing base velocity asymmetry tends to be stabilizing. Furthermore, we show that increasing base density asymmetry slightly decreases the absolute frequency and that increasing the base velocity asymmetry slightly increases the absolute frequency. In addition, we show that increasing the degree of base density asymmetry distorts the most absolutely unstable hydrodynamic mode from its nominally sinuous structure. This distorted mode exhibits higher amplitude pressure and velocity oscillations near the interface with the smaller density jump than near the one with the bigger density jump. This would then be anticipated to lead to strongly non-symmetric amplitudes of flame flapping, with much stronger flame flapping on the side with lower density ratio. These predictions are shown to be consistent with experimental data. These comparisons support the analytical predictions that increased base density asymmetry are destabilizing and that hydrodynamic velocity fluctuation amplitudes should be greatest at the flame with the lowest density jump

Laboratory layered latte

Inducing thermal gradients in fluid systems with initial, well-defined density gradients results in the formation of distinct layered patterns, such as those observed in the ocean due to double-diffusive convection. In contrast, layered composite fluids are sometimes observed in confined systems of rather chaotic initial states, for example, lattes formed by pouring espresso into a glass of warm milk. Here, we report controlled experiments injecting a fluid into a miscible phase and show that, above a critical injection velocity, layering emerges over a time scale of minutes. We identify critical conditions to produce the layering, and relate the results quantitatively to double-diffusive convection. Based on this understanding, we show how to employ this single-step process to produce layered structures in soft materials, where the local elastic properties vary step-wise along the length of the material