Razi Nalim

Analysis of Flow Processes in Detonative Wave Rotors and Pulse Detonation Engines

This study is aimed to make a systematic comparison between performance of pulse detonation engines (PDE) and detonative wave rotors, using a quasi-one dimensional numerical model. The model incorporates almost all major losses including viscous, heat transfer, port mixing, gradual opening and closing of channels losses. Two scenarios of (a) instantaneous detonation and (b) deflagration to detonation transition (DDT) for each engine are considered and flow field of these cases are compared with each other. To make flow field comparisons, the same rotor length, fuel distribution, inlet port size, and frequency are used for both engines. As expected, the outlet flow of the detonative wave rotor is shown to be more uniform than of the PDE, due to its fast rotation and large number of channels. This is more acceptable to turbine blades for gas turbine applications and may provide the combustor for new generation of aircraft engines. Higher pressure gain is produced in the detonative wave rotor configuration due to the mixture pre-compression by a hammer shock. Other advantages of the wave rotor approach are discussed.

Analytic Aerothermodynamic Cycle Model of the Combustion Wave Rotor in a Gas Turbine Engine

Design and evaluation of the wave rotor as a combustion device requires estimates of the performance as a function of operating conditions and gasdynamic processes occurring inside the rotor channels. An analytical model based on a combination of thermodynamic and gasdynamic approaches is established for assessing quantitatively the influence of various design parameters and operating conditions on the wave rotor performance. Performance results expressed by specific cycle work, thermal efficiency, and specific fuel consumption are calculated as functions of several design parameters. The results indicate that significant performance improvements are possible using combustion wave rotors, making this technology desirable for next generation of gas turbines and propulsion systems.

Shock-Flame Interaction Modeling in a Constant-Volume Combustion Channel Using Detailed Chemical Kinetics and Automatic Mesh Refinement

More efficient and powerful gas turbine engines can be designed using constant-volume combustors that may involve ignition of a combustible mixture using a hot gas jet, subsequent flame and pressure-wave propagation, and their interactions. Accurate prediction of three-dimensional transient turbulent combustion is computationally challenging. To resolve propagating turbulent combustion, predict ignition, and track pressure waves accurately requires techniques to minimize the numerical cell count and kinetics calculation times. This study of shock-flame interaction (SFI) used detailed chemistry that includes low-temperature ignition reactions. Computational cells with similar temperatures and composition were grouped as ‘zones’ where kinetics are solved only once per zone per time step, using average values of species concentrations and thermodynamic properties for that zone. This avoids expensive kinetic calculations in every computational cell, with considerable speedup. A relatively coarser initial mesh was refined selectively and automatically, based on predicted velocity and temperature gradients, tracking propagating pressure waves and flames. The time step is variable, limited by the local speed of sound, to ensure accurate wave propagation. These techniques, previously validated for non-premixed, premixed and multiple-fuel turbulent combustion in industrial IC engines, are applied to study SFI during premixed combustion in a long constant-volume combustor.© 2013 ASME

Wave-Rotor Pressure-Gain Combustion Analysis for Power Generation and Gas Turbine Applications

A wave-rotor pressure-gain combustor (WRPGC) ideally provides constant-volume combustion and enables a gas turbine engine to operate on the Humphrey-Atkinson cycle. It exploits pressure (both compression and expansion) waves and confined propagating combustion to achieve pressure rise inside the combustor. This study first presents thermodynamic cycle analysis to illustrate the improvements of a gas turbine engine possible with a wave rotor combustor. Thereafter, non-steady reacting simulations are used to examine features and characteristics of a combustor rig that reproduces key features of a WRPGC.In the thermodynamic analysis, performance parameters such as thermal efficiency and specific power are estimated for different operating conditions (compressor pressure ratio and turbine inlet temperature). The performance of the WRPGC is compared with the conventional unrecuperated and recuperated engines that operates on the Brayton cycle. Fuel consumption may be reduced substantially with WRPGC introduction, while concomitantly boosting power. Simulations have been performed of the ignition of propane by a hot gas jet and subsequent turbulent flame propagation and shock-flame interaction.Copyright

Single-Tube Simulation of a Semi-Intermittent Pressure-Gain Combustor

This work is aimed to investigate the fundamental combustion and reignition process in semi-intermittent pressure-gain combustors for gas turbine applications. A combustion-torch ignition method is used to simulate reignition in one tube of a pressure-gain combustor by employing burned gas produced in a pre-chamber combustor. Numerical flow and combustion simulations are performed to understand and guide preliminary experimental results. The computational fluid dynamics code StarCD® is used to predict internal flow and combustion upon attempted ignition by a hot gas jet. This study provides improved understanding of the complex, sub-millisecond processes involved: transient supersonic jet mixing, ignition, highly turbulent flame propagation, and shock-flame interaction in near-wall region. The results are useful for successful design of rotary pressure gain combustors or internal combustion wave rotors under various operating conditions.Copyright

Traversing Hot-Jet Ignition in a Constant-Volume Combustor

Hot-jet ignition of a combustible mixture has application in IC 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 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 pre-chamber that generates gaseous combustion products, assumed at chemical equilibrium for estimating major species. Turbulent combustion uses a hybrid eddy-break-up 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.Copyright

Two-Dimensional Flow and NOx Emissions in Deflagrative Internal Combustion Wave Rotor Configurations

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.Copyright

Cooling Challenges of Modern Truck Diesel Engines

Efficient cooling system designs are required for the modern diesel truck engine to meet new standards of increased efficiency and reduced emissions. Often, emissions reduction requires substantial cooled exhaust gas recirculation (EGR) to decrease peak combustion temperatures. This extra heat rejection imposes additional costs on the cooling system, and may not comply with application space constraints. Space and cost constraints require minimization of EGR cooler size and the risks from coolant boiling and exhaust condensation, while restraining growth in radiator frontal area, pumping power, and fan power. These objectives are usually contradictory, and a careful optimization is needed. This paper examines the effect of a coolant flow rate and peak temperature on these objectives, in parallel-flow and counter-flow arrangements of EGR cooler systems. It is concluded that these systems are likely to be inadequate, and alternative configurations may be necessary.Copyright

Workshop: Project-enhanced learning in engineering science education

Early drop out and poor retention rates are a major challenge to engineering education, which in many institutions have prompted a focus on improved first-year experiences. Retention and contributing learning challenges persists into the middle years, particularly when students confront the first engineering science courses in their major field. Students often perceive these courses as too abstract, intended to weed them out, and not meaningfully connected to their professional aspirations. A proven approach to improve student learning, self-efficacy, motivation, and retention is the use of active learning, including problems and projects [1-4]. Despite evidence of the benefits of active learning, engineering schools and faculty members have inadequate incentives to experiment with non-traditional approaches [5].

Computational Study of Fuel Injection in a Large-Bore Gas Engine

Emission controls in stationary gas engines have required significant modifications to the fuel injection and combustion processes. One approach has been the use of high-pressure fuel injection to improve fuel-air mixing. The objective of this study is to simulate numerically the injection of gaseous fuel at high pressure in a large-bore two-stroke engine. Existing combustion chamber geometry is modeled together with proposed valve geometry. The StarCD® fluid dynamics code is used for the simulations, using appropriate turbulence models. High-pressure injection of up to 500 psig methane into cylinder air initially at 25 psig is simulated with the valve opened instantaneously and piston position frozen at the 60 degrees ABDC position. Fuel flow rate across the valve throat varies with the instantaneous pressure but attains a steady state in approximately 22 ms. As expected with the throat shape and pressures, the flow becomes supersonic past the choked valve gap, but returns to a subsonic state upon deflection by a shroud that successfully directs the flow more centrally. This indicates the need for careful shroud design to direct the flow without significant deceleration. Pressures below 300 psig were not effective with the proposed valve geometry. A persistent re-circulation zone is observed immediately below the valve, where it does not help promote mixing.Copyright