Department of Mechanical and Energy Engineering

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    Traversing Hot-Jet Ignition in a Constant-Volume Combustor
    (2014-04) Karimi, Abdullah; Rajagopal, Manikanda; Nalim, M. Razi
    Hot-jet ignition of a combustible mixture has application in internal combustion engines, detonation initiation, and wave rotor combustion. Numerical predictions are made for ignition of combustible mixtures using a traversing jet of chemically active gas at one end of a long constant-volume combustor (CVC) with an aspect ratio similar to a wave rotor channel. The CVC initially contains a stoichiometric mixture of ethylene or methane at atmospheric conditions. The traversing jet issues from a rotating prechamber that generates gaseous combustion products, assumed at chemical equilibrium for estimating major species. Turbulent combustion uses a hybrid eddy-breakup model with detailed finite-rate kinetics and a two-equation k-ω model. The confined jet is observed to behave initially as a wall jet and later as a wall-impinging jet. The jet evolution, vortex structure, and mixing behavior are significantly different for traversing jets, stationary centered jets, and near-wall jets. Pressure waves in the CVC chamber affect ignition through flame vorticity generation and compression. The jet and ignition behavior are compared with high-speed video images from a prior experiment. Production of unstable intermediate species like C2H4 and CH3 appears to depend significantly on the initial jet location while relatively stable species like OH are less sensitive.
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    Scaling Interface Length Increase Rates in Richtmyer– Meshkov Instabilities
    (2013-02) Kilchyk, V; Nalim, M. Razi; Merkle, Charles L.
    The interface area increase produced by large-amplitude wave refraction through an interface that separates fluids with different densities can have important physiochemical consequences, such as a fuel consumption rate increase in the case of a shock–flame interaction. Using the results of numerical simulations along with a scaling analysis, a unified scaling law of the interface length increase was developed applicable to shock and expansion wave refractions and both types of interface orientation with the respect to the incoming wave. To avoid a common difficulty in interface length quantification in the numerical tests, a sinusoidally perturbed interface was generated using gases with different temperatures. It was found that the rate of interface increase correlates almost linearly with the circulation deposited at the interface. When combined with earlier developed models of circulation deposition in Richtmyer–Meshkov instability, the obtained scaling law predicts dependence of interface dynamics on the basic problem parameters.
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    Assessment of Combustion Modes for Internal Combustion Wave Rotors
    (1999-04) Nalim, M. Razi
    Combustion within the channels of a wave rotor is examined as a means of obtaining pressure gain during heat addition in a gas turbine engine. Three modes of combustion are assessed: premixed autoignition (detonation), premixed deflagration, and non-premixed autoignition. The last two will require strong turbulence for completion of combustion in a reasonable time in the wave rotor. The autoignition modes will require inlet temperatures in excess of 800 K for reliable ignition with most hydrocarbon fuels. Examples of combustion mode selection are presented for two engine applications.
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    Expression and Activities of Matrix Metalloproteinases under Oscillatory Shear in IL-1-Stimulated Synovial Cells
    (2003) Sun, Hui Bin; Nalim, M. Razi; Yokota, Hiroki
    Matrix metalloproteinases (MMPs) are known to play a critical role in tissue disintegration, and an elevated level of MMPs is observed in synovium and synovial fluid of joints with rheumatoid arthritis. During joint movement, synovial tissue receives various mechanical stimuli, but effects of mechanical challenges on regulation of MMPs in rheumatic synovium are poorly understood. Focusing on cellular responses to oscillatory fluid shear in human synovial cells, we determined the expression of MMP-1 and MMP-13 by polymerase chain reaction and immunoblotting as well as proteolytic activities of total MMPs by a fibril degradation assay and zymography. The results revealed that ~0.5 dyn/cm 2 oscillatory shear at 1 Hz not only reduced an mRNA level and a protein level of MMP-1 and MMP-13, but it also decreased collagenase and gelatinase activities of total MMPs. Furthermore, the induction of the MMP expression and activities by interleukin-1 was suppressed by the oscillatory shear. Interestingly, the oscillatory shear upregulated the mRNA expression of TIMP-1 and TIMP-2. Our results support a potential role of oscillatory shear in regulating expression and activities of MMPs in the presence and the absence of proinflammatory cytokine.
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    Two-Dimensional Flow and NOx Emissions in Deflagrative Internal Combustion Wave Rotor Configurations
    (2003-07) Pekkan, K; Nalim, M. Razi
    A wave rotor is proposed for use as a constant volume combustor. A novel design feature is investigated as a remedy for hot gas leakage, premature ignition, and pollutant emissions that are possible in this class of unsteady machines. The base geometry involves fuel injection partitions that allow stratification of fuel/oxidizer mixtures in the wave rotor channel radially, enabling pilot ignition of overall lean mixture for low NOx combustion. In this study, available turbulent combustion models are applied to simulate approximately constant volume combustion of propane and resulting transient compressible flow. Thermal NO production histories are predicted by simulations of the STAR-CD code. Passage inlet/outlet/wall boundary conditions are time-dependent, enabling the representation of a typical deflagrative internal combustor wave rotor cycle. Some practical design improvements are anticipated from the computational results. For a large number of derivative design configurations, fuel burn rate, two-dimensional flow and emission levels are evaluated. The sensitivity of channel combustion to initial turbulence levels is evaluated.
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    Performance Enhancement of Microturbine Engines Topped With Wave Rotors
    (2006-01) Akbari, Pezhman; Nalim, M. Razi; Mueller, Norbert
    Significant performance enhancement of microturbines is predicted by implementing various wave-rotor-topping cycles. Five different advantageous cases are considered for implementation of a four-port wave rotor into two given baseline engines. In these thermodynamic analyses, the compressor and turbine pressure ratios and the turbine inlet temperatures are varied, according to the anticipated design objectives of the cases. Advantages and disadvantages are discussed. Comparison between the theoretic performance of wave-rotor-topped and baseline engines shows a performance enhancement up to 34%. General design maps are generated for the small gas turbines, showing the design space and optima for baseline and topped engines. Also, the impact of ambient temperature on the performance of both baseline and topped engines is investigated. It is shown that the wave-rotor-topped engines are less prone to performance degradation under hot-weather conditions than the baseline engines.
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    A Review of Wave Rotor Technology and Its Applications
    (2006-10) Akbari, Pezhman; Nalim, M. Razi; Mueller, Norbert
    The objective of this paper is to provide a succinct review of past and current research in developing wave rotor technology. This technology has shown unique capabilities to enhance the performance and operating characteristics of a variety of engines and machinery utilizing thermodynamic cycles. Although there have been a variety of applications in the past, this technology is not yet widely used and is barely known to engineers. Here, an attempt is made to summarize both the previously reported work in the literature and ongoing efforts around the world. The paper covers a wide range of wave rotor applications including the early attempts to use wave rotors, its successful commercialization as superchargers for car engines, research on gas turbine topping, and other developments. The review also pays close attention to more recent efforts: utilization of such devices in pressure-gain combustors, ultra-micro gas turbines, and water refrigeration systems, highlighting possible further efforts on this topic. Observations and lessons learnt from experimental studies, numerical simulations, analytical approaches, and other design and analysis tools are presented.
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    Thermal-Boundary-Layer Response to Convected Far-Field Fluid Temperature Changes
    (2008-10) Li, Hongwei; Nalim, M. Razi
    Fluid flows of varying temperature occur in heat exchangers, nuclear reactors, nonsteady-flow devices, and combustion engines, among other applications with heat transfer processes that influence energy conversion efficiency. A general numerical method was developed with the capability to predict the transient laminar thermal-boundary-layer response for similar or nonsimilar flow and thermal behaviors. The method was tested for the step change in the far-field flow temperature of a two-dimensional semi-infinite flat plate with steady hydrodynamic boundary layer and constant wall temperature assumptions. Changes in the magnitude and sign of the fluid-wall temperature difference were considered, including flow with no initial temperature difference and built-up thermal boundary layer. The equations for momentum and energy were solved based on the Keller-box finite-difference method. The accuracy of the method was verified by comparing with related transient solutions, the steady-state solution, and by grid independence tests. The existence of a similarity solution is shown for a step change in the far-field temperature and is verified by the computed general solution. Transient heat transfer correlations are presented, which indicate that both magnitude and direction of heat transfer can be significantly different from predictions by quasisteady models commonly used. The deviation is greater and lasts longer for large Prandtl number fluids.
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    Air-Standard Aerothermodynamic Analysis of Gas Turbine Engines With Wave Rotor Combustion
    (2009-09) Nalim, M. Razi; Li, H; Akbari, Pezhman
    The wave rotor combustor can significantly improve gas turbine engine performance by implementing constant-volume combustion. The periodically open and closed combustor complicates thermodynamic analysis. Key cycle parameters depend on complex gas dynamics. In this study, a consistent air-standard aerothermodynamic model with variable specific heat is established. An algebraic model of the dominant gas dynamics estimates fill fraction and internal wave compression for typical port designs, using a relevant flow Mach number to represent wave amplitudes. Nonlinear equations for thermodynamic state variables are solved numerically by Newton–Raphson iteration. Performance measures and key operating conditions are predicted, and a quasi-one-dimensional computational model is used to evaluate the usefulness of the algebraic model.
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    Transient Thermal Response of Turbulent Compressible Boundary Layers
    (2011-08) Li, Hongwei; Nalim, M. Razi; Merkle, Charles L.
    A numerical method is developed with the capability to predict transient thermal boundary layer response under various flow and thermal conditions. The transient thermal boundary layer variation due to a moving compressible turbulent fluid of varying temperature was numerically studied on a two-dimensional semi-infinite flat plate. The compressible Reynolds-averaged boundary layer equations are transformed into incompressible form through the Dorodnitsyn–Howarth transformation and then solved with similarity transformations. Turbulence is modeled using a two-layer eddy viscosity model developed by Cebeci and Smith, and the turbulent Prandtl number formulation originally developed by Kays and Crawford. The governing differential equations are discretized with the Keller-box method. The numerical accuracy is validated through grid-independence studies and comparison with the steady state solution. In turbulent flow as in laminar, the transient heat transfer rates are very different from that obtained from quasi-steady analysis. It is found that the time scale for response of the turbulent boundary layer to far-field temperature changes is 40% less than for laminar flow, and the turbulent local Nusselt number is approximately 4 times that of laminar flow at the final steady state.