Ann M. Mescher

Influence of pressure on the combustion rate of carbon

The influence of pressure on the combustion rates of carbon (or coal) particles is shown, by comparison of prediction with experiment, to be zero to minor in the temperature range studied. This result is contrary to the empirical ( n th order) assumption widely adopted in much of the literature that predicts a substantial pressure dependence at all temperatures. Two models were used in the comparison, and the results were compared with three independent experimental sets of data. These experiments were measurements of burning times of single coal particles by Tidona [19] at 1, 1.5, and 2 atm; reaction rates of char particles by Monson et al. [15] at 1, 5, 10, and 15 atm; and (noncritical) ignition temperatures of coal particles in the pressure range 0.4–1.7 atm [20]. The first model was based on the fundamental Langmuir-Nusselt-Thiele suite of theoretical equations in the form of the extended resistance equation (ERE) [35]. The second model combined the Nusselt BLD analysis with the empirical n th order assumption that the reaction rate at all temperatures is proportional to the n th power of the partial pressure of the oxygen concentration ( p ox n ) [2,3]. The ERE model was able to predict the structural form of the experimental results with adequate prediction of numerical values, particularly of the reaction rates measured by Monson et al. In particular, the ERE predictions and experiments jointly showed small to no dependency of the rates on pressure, contrary to the predictions of the empirical model. We conclude that the empirical model has no experimental support for the assumptions made and that fundamentally based equations can be developed or already exist that can be used to predict carbon combustion reaction rates at elevated or reduced pressure with acceptable confidence.

Remote Monitoring of Resin Transfer Molding Processes by Distributed Dielectric Sensors

Feed-forward adaptive control of resin transfer molding (RTM) processes is crucial for producing a high yield of usable parts for industrial applications. The enabling technique for this process is non-invasive monitoring of the fill-front position and the degree of cure of the resin as it is injected into the mold. Successful implementation of a sensing system capable of meeting these criteria will result in a high yield of composite parts that can be used for the next generation of aircraft. This article articulates the possibility of a hybrid sensing system for multiparameter monitoring during RTM processes. It addresses the fundamental engineering trade-offs between penetration depth and signal strength, discussing how to account for fringing electric field (FEF) effects present in the system. FEF effects hinder the measurement accuracy of the sensor system. This article describes how these effects are addressed using a mapping algorithm that is developed using numerical simulations of the experimental setup. The experimental setup utilizes a rectangular RTM tool and a water-glycerin mixture which simulates mechanical properties of epoxy resins, prior to cure. Modeling of the FEF effects helps to achieve high measurement accuracy of the fill front location.

UNSTEADY NATURAL CONVECTION OF AIR IN A TALL AXISYMMETRIC, NONISOTHERMAL ANNULUS

This article presents numerical predictions of axisymmetric natural convection within a tall annular cavity with an aspect ratio of 10 and a radius ratio of 0.6. The temperature of the bounding inner vertical cylinder is hot at its base and decreases linearly with height, while the outer cylinder is cold at its base and its temperature increases linearly with height. These boundary conditions promote a steady bi-cellular flow at low Rayleigh numbers. As the Rayleigh number is increased, the flow transitions to a time-varying state in which the interface between the two natural-convection cells starts to oscillate. If the Rayleigh number is further increased, hysteresis, subharmonics, multiple solutions, and a reverse transition back to a steady state are all predicted. Results predicting the onset of unsteady flow for the Cartesian case are also presented.

Investigation of Steady-State Drawing Force and Heat Transfer in Polymer Optical Fiber Manufacturing

The force required to draw a polymer preform into optical fiber is predicted and measured, along with the resultant free surface shape of the polymer, as it is heated in an enclosed cylindrical furnace. The draw force is a function of the highly temperature dependent polymer viscosity. Therefore accurate prediction of the draw force relies critically on the predicted heat transfer within the furnace. In this investigation, FIDAP was used to solve the full axi-symmetric conjugate problem, including natural convection, thermal radiation, and prediction of the polymer free surface. Measured and predicted shapes of the polymer free surface compared well for a range of preform diameters, draw speeds, and furnace temperatures. The predicted draw forces were typically within 20% of the experimentally measured values, with the draw force being very sensitive to both the furnace wall temperature and to the feed rate of the polymer.

Effect of unsteady natural convection on the diameter of drawn polymer optical fiber.

This paper presents experimental results showing the effect various natural convection heating regimes have on the diameter of drawn polymer optical fiber. The airflow, adjacent to the polymer, can be either laminar, oscillatory, or chaotic, depending on the imposed thermal boundary conditions at the furnace and iris walls. When subject to oscillatory and chaotic natural convection, the drawn fiber varies in diameter 2.5 to 10 times more than that measured under laminar heating conditions. Particle image velocimetry shows that unsteady natural convection occurs with the interplay between two asymmetric counter-rotating convective cells. This represents a significant instability mechanism, one that has not been previously identified.

A Methodology to Obtain a Desired Filling Pattern during Resin Transfer Molding

In Resin Transfer Molding (RTM), dry spot formation and air entrapment during the filling stage often lead to defective parts and high scrap rate. These problems are usually caused by improper design of inlet conditions and vent locations that prevent the Last Point to Fill (LPF) location from coinciding with the preset vent location. Use of direct filling simulation as a design tool for the RTM process often involves trial-and-error procedures in order to find the appropriate inlet conditions and locations as well as exit vent locations. This design procedure becomes complex when adesign involving multiple inlet gates is being considered, especially in large parts. There may also be uncertainty as to whether the final design (obtained using trial-and-error simulation procedures) is indeed the optimum design. This paper presents a methodology to design the RTM process with a desired filling pattern free of dry spots and knitlines. Unlike the traditional filling simulation that predicts the filling pattern using prescribed inlet conditions and the specification of the preform permeability field, this methodology calculates the optimum inlet conditions based on the specification of the desired filling pattern and the prescription of preform permeability. The use of this algorithm greatly enhances

The Effect of Spatially Correlated Roughness and Boundary Conditions on the Conduction of Heat Through a Slab

The nominally one-dimensional conduction of heat through a slab becomes two dimensional when one of the surfaces is rough or when the boundary conditions are spatially nonuniform. This paper develops the stochastic equations for a slab whose surface roughness or convective boundary condition is spatially correlated with correlation lengths ranging from 0 (white noise) to a length long in comparison to the slab thickness. The effect is described in terms of the standard deviation and the resulting spatial correlation of the heat flux as a function of depth into the slab. In contrast to the expectation that the effect is monotonic with respect to the correlation length, it is shown that the effect is maximized at an intermediate correlation length. It is also shown that roughness or a random convective heat transfer coefficient have essentially the same effects on the conducted heat, but that the combination results in a much deeper penetration than does each effect individually. In contrast to the usual methods of solving stochastic problems, both the case of a rough edge and a smooth edge with stochastic convective heat transfer coefficients can only be treated with reasonable computational expense by using direct Monte Carlo simulations.

Mechanism of carbon combustion: Relative influence of adsorption, desorption, and boundary layer diffusion as a function of pressure

Abstract These results show that common, pressure-independent coefficients can be used to predict oxygen-carbon reaction rates. Accurate agreement with experiment requires determination of the (Thiele) second effectiveness factor (e) which at this time can only be done by analysis of the experimental measurements of density changes with burn-off. This identifies prediction of the dependence of e on pressure as a future theoretical target. Independent evaluation of the terms governing the combustion show that, at the higher temperatures, adsorption has significant influence at atmospheric pressure, but its importance drops rapidly, to minor or insignificant, as pressure increases. Overall, the results show that, partly on account of offsetting effects of changes of e with pressure, governing the degree of internal penetration of the reaction, the influence of pressure on the reaction rate is second order or less.

Characterizing Combustion of Synthetic and Conventional Fuels in a Toroidal Well Stirred Reactor

The use of alternative/synthetic fuels in jet engines requires improved understanding and prediction of the performance envelopes and emissions characteristics relative to the behavior of conventional fuels. In this study, experiments in a toroidal well-stirred reactor (TWSR) are used to study lean premixed combustion temperature and extinction behavior for several fuels including simple alkanes, synthetic jet fuels, and conventional JP8. A perfectly stirred reactor (PSR) model is used to interpret the observed behavior.The first portion of the study deals with jet fuels and synthetic jet fuels with varying concentrations of added aromatic compounds. Synthetic fuels contain little or no natural aromatic species, so aromatic compounds are added to the fuel because fuel system seals require these species to function properly. The liquid fuels are prevaporized and premixed before being burned in the TWSR. Air flow is held constant to keep the reactor loading roughly constant. Temperature is monitored inside the reactor as the fuel flow rate is slowly lowered until extinction occurs. The extinction point is defined by both its equivalence ratio and temperature. The measured blowout point is very similar for all four synthetic fuels and the baseline JP8 at aromatic concentrations of up to 20% by volume. Since blowout is essentially the same for all the base fuels at low aromatic concentrations, a single fuel was used to test the effect of aromatic concentrations from 0 to 100%. PSR models of these complex fuels show the expected result that behavior diverges from an ideal, perfectly premixed model as the combustion approaches extinction.The second portion of this study deals with lean premixed combustion of simple gaseous alkanes (methane, ethane, and propane) in the same TWSR. These simpler fuels were tested for extinction in a similar manner to the complex fuels, and behavior was characterized similarly. Once again, PSR models show that the TWSR behaves similar to a PSR during stable combustion far from blowout, but as it approaches blowout and becomes less stable a single PSR no longer accurately describes the TWSR.This work is a step towards developing chemical reactor networks (CRNs) based on computational fluid dynamics (CFD) of the simple gaseous fuels in the TWSR. Ultimately, CRNs are the only realistic way to accurately perform detailed chemical modeling of the combustion of complex liquid fuels.Copyright

Sensitivity of the Human Comfort Equation and of Free Convection in a Vertical Enclosure as Examples of the Use of Global Sensitivity to Evaluate Parameter Interactions

Many models of engineering and scientific systems involve interactions between and among the parameters, stimuli, and physical properties. The determination of the adequacy of models for predictions and for designing experiments generally involves sensitivity studies. Good designs mandate that the experiments be sensitive to the parameters sought with little interaction between them because such interaction generally confuses the estimation and reduces the precision of the estimates. For design purposes, analysts frequently want to evaluate the sensitivities of the predicted responses to specific variables, but if the variables interact it is often difficult to separate the effects. Global sensitivity is a technique by which one can evaluate the magnitude of the interactions between multiple variables. In this paper, the global sensitivity approach is applied to the human comfort equation and to free convection in a rectangular enclosure. It is found that when occupants are uncomfortable, there is little interaction and that one can predict the effects of changing several environmental conditions at once by adding the separate effects. But when occupants are comfortable, there is a large interaction and the effects cannot be treated separately. Free convective heat transfer in an enclosure is a function of the Rayleigh number Ra and the aspect ratio H/W, and the flow field changes from unicellular to multicellular as Ra increases. There is a strong interaction for H/W≤ 2 but little for H/W≥2.