Faculty

Publications

B.A. Wilhite, B. Blackwell, J. Kacmar, A. Varma and M. J. McCready. Origins of Pulsing Regime in Cocurrent Packed-Bed Flows. , Industrial and Engineering Chemistry Research, 44:6056-6066, 2005. view abstract The mechanism of the formation for cocurrent downflow pulse flow was studied experimentally in a packed-bed of inert spheres of 3, 6, and 8 mm using an air-water flow. By measuring the flow distance until pulses are observed, the spatial growth rate of convective disturbances within the pulsing-flow regime were determined. Observations indicate that pulses form from trickling-flow as the result of a global convective instability. Further, experiments indicate that an analogous transition exists for the formation of pulses from the dispersed bubble flow regime, except that pulses form as the flowrates are adjusted to become less severe. Existing global instability models based on averaged (dispersed flow) momentum equations were modified to explain experimental results. A key uncertainty in modeling pulse formation from trickle flow is the regularization (i.e., stabilization) force. Reexamination of this issue suggests some mechanistic inconsistencies with surface tension which had been used in previous studies. Consistent with the present experiments, it is proposed that gravity may be the primary restoring force. Incorporating gravity stabilization into the dispersed flow equations provides predictions that are at least as good as the previous models. A similar dispersed flow model is used to explain the bubbly flow to pulse transition. While predictions agree with experimental data for part of the range, model accuracy is limited by the accuracy of constitutive expressions for interaction forces between phases.

S. Gomes, M. J. McCready and A. E. Ostafin. Effect of oscillating fluid shear on solute transport in cortical bone. Journal of Biomechanics, 38:2337-2343, 2005. view abstract The consequences of an oscillatory fluid shear mechanism on nutrient transport in bone during physical activity and ultrasonic therapy are discussed. During movement, periodic stress on bone creates transient pressure gradients that circulate interstitial fluid through calcified bone. A transport model derived from oscillatory Taylor–Aris dispersion phenomena was used to predict a ratio of effective-to-molecular diffusivity, K =D; for solutes of varying sizes up to 50 nm in diameter, in pores filled with interstitial fluid and pericellular matrix. The magnitude of the estimated transport enhancement depended on the molecular size, pore dimension, applied frequency and the displacement of the fluid during pressurization. For oscillation frequencies and amplitudes corresponding to those experienced during normal human activity, transport enhancements of up to 100 fold are expected for molecules larger than 5 nm in diameter. Enhancements of up to one order of magnitude, due to ultrasound stimulations in the MHz frequency range, are also expected for 7-nm-sized solutes. No effects are anticipated for ions, whose molecular diffusion time is too fast relative to the oscillation frequency. This model is expected to be useful for understanding differences in bone growth as a function of type of movement or to develop new physical therapies.

B. Wilhite, X. Huang, M. J. McCready and A. Varma. Enhancing performance of three--phase catalytic packed-bed reactors. AIChE Journal, 47:2548-2556, 2001. view abstract Our previous theoretical work predicted the possibility of enhancing three-phase packed-bed reactor performance by operating in the pulsing-flow regime. This article deals with the experimental study of the beneficial effect of pulsing flow on reaction outcome. Hydrogenation of phenylacetylene, dissolved in n-tetradecane over Pt-alumina catalyst, was chosen as the experimental reaction system. This is a triangular reaction, with styrene and ethylbenzene as the desired intermediate and final products, respectively. With properly designed experiments, the reaction performance in pulsing flow and trickling-flow regimes was compared directly. The effects of process variables such as temperature, feed flow rates, and reactant concentration on reaction behavior were studied. A simplified model to describe the qualitative trends was also developed. Both experiments and calculations show that the yield of styrene is higher in pulsing flow than in trickling flow, which confirms the adantages of pulsing-flow operation predicted by the theoretical work.

B. Wilhite, X. Huang, M. J. McCready and A. Varma,. Phenylacetylene Hydrogenation in a three-phase catalytic packed – bed reactor: Experiments and Model. Chemical Engineering Science, 58:3465-3471, 2003. view abstract A mathematical model was developed for the cocurrent operation of a three-phase catalytic packed-bed reactor under both trickling and pulsing regimes. The local fluctuations of liquid–solid mass transfer, liquid flow rate and liquid holdup in unsteady pulsing flow were simulated as square-wave functions. The transport properties employed in the model were obtained using published correlations, while expressions for the intrinsic kinetics were taken from our previous work. The model results were found to be in good agreement with experimental data obtained from a laboratory scale reactor, and verified the advantage of pulsing flow over trickling flow.

M. R. King. D. T. Leighton and M. J. McCready. Stability of oscillatory two-phase Couette Flow: theory and experiment. Physics of Fluids, 11:833-844, 1999. view abstract The interfacial instability due to viscosity stratification is studied experimentally in a closed Couette geometry. A vertical interface is formed between two concentric cylinders with density-matched fluids of unequal viscosity. The outer cylinder is rotated with a time-harmonic motion, causing spatially periodic disturbances of the interface. The wavelengths and growth rates predicted by linear theory agree well with experimental results. Application of Fjo" rtoft's inflection point theorem shows the neutral stability curves to be consistent with an internal instability occurring in the less viscous phase. Because the standard Floquet theory yields only time-averaged growth rates, the instantaneous behavior of the system is examined numerically. This reveals the flow to be unstable to a disturbance which has a maximum that oscillates between the interface and a location within the less viscous fluid. Surprisingly, it is found that interfacial wave amplification originates with the internal disturbance, and is not directly caused by interfacial shear. This unsteady instability may explain the growth of waves in ''transient'' process flows, e.g., fluids encountering changing flow geometry. It is also demonstrated that in the long wave limit the problem of steady-plus-oscillatory plate motion is simply additive. This implies that it is possible to use oscillations to stabilize steady waves over a limited range of parameter values, but only when the less viscous phase is adjacent to the moving boundary.

Awards

Kaneb Teaching Award

Given on May 1, 2003 by University of Notre Dame

Special Presidential Award

Given on May 21, 2002 by University of Notre Dame

Courses

• CBE 20255 - Introduction to Chemical Engineering Analysis - This is the foundation course in chemical engineering. The principles of mass and energy conservation, which comprise fundamental physical laws are used with constitutive equations to analyze a var... more >
• CBE 40498 - Energy & Climate - This course integrates the principles of physical sciences and engineering as they pertain to energy, its sources and uses and the impact of these on the environment. The great majority of energy u... more >
• CBE 60598 - Energy & Climate - This course integrates the principles of physical sciences and engineering as they pertain to energy, its sources and uses and the impact of these on the environment. The great majority of energy u... more >

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