Yeshayahou Levy

Jet Engine Model for Control and Real-Time Simulations

The main objective of this paper is development of a simple real-time transient performance model for jet engine control. A jet engine arrives to its most dangerous condition during transient operation that may be triggered by fast changes of the input fuel command signal. Thus, the control system specifications are formulated to specify the maximal variance of the fuel flow command (from idle to maximum power level) during transient maneuver. Linear and piecewise-linear techniques are not always convenient and appropriate for turbine engine controller design. An alternative quasilinear simple/ fast engine model is discussed in this paper. This model has maximum accuracy for maximal variance of the fuel flow input command in accordance to the jet engine control system specifications. The fast model is obtained using the Novel Generalized Describing Function, proposed for investigation of nonlinear control systems. The paper presents the Novel Generalized Describing Function definition and then discusses the application of this technique for the development a fast turbine engine simulation suitable for control and real-time applications. Simulation results are compared between the conventional and fast models and found to provide good agreement.

Spray Characteristics of Angled Liquid Injection into Subsonic Crossflows

The spray characteristics of angled liquid injection into subsonic crossflows were experimentally investigated. The experiments were conducted using water as the test liquid and included variations of the nozzle injection angle, air velocity of the crossflow, and liquid flow rate. An initial assessment of the spray characteristics performed with the aid of a visualization technique revealed that the overall jet penetration, breakup length, and atomization quality are significantly affected by the nozzle injection angle and, to a lesser extent, by the liquid-to-air momentum flux ratio. Subsequently, detailed droplet size and droplet velocities were obtained for various sprays by the use of phase Doppler anemometry. The data reveal that there is an evident longitudinal droplet velocity/diameter correlation for all droplets, with small droplets being faster than larger droplets. Furthermore, the droplet mean diameters diminish on increasing the nozzle injection angle but are affected only marginally by the liquid-to-air momentum flux ratio.

Investigation of a small solid fuel ramjet combustor

Experimental and analytical investigations of a small solid-fuel ramjet (SFRJ) combustor were conducted. A static test system with a 25-kW electrical air heater simulated the air temperature and pressure encountered in flight at a Mach number of 3 at sea level. The transparent polymethylmethacrylate fuel used in the tests permitted continuous video photography, revealing the local fuel-regression-rate behavior and the instantaneous ignition and combustion phenomena. The results demonstrated high combustion efficiency and indicated peculiar local and average fuel-regression-rate correlations. The analysis indicated that the specific conditions resulting from the low Reynolds number range in small SFRJ motors, in contrast to large combustors, enhance the effect of the sudden-expansion heat-transfer regime relative to the boundary-layer regime. 14 refs.

On the oscillatory behavior of laminar spray diffusion flames: experiment and theory

In this study an experimental, well-controlled parametric investigation of the behavior of an oscillating Burke-Schumann type spray diffusion flame is described. It is demonstrated that these unique spray-related oscillations occur at frequencies in the range 1–5 Hz and it is established that their genesis is due to a heat and mass transfer mechanism resulting from the presence of the droplets in the system. A complementary stability analysis of a one-dimensional model problem, containing the essential features of the experimental conditions, is performed. It is found that the correct order of magnitude of oscillations is predicted by the analysis, confirming a posteriori that the model is a satisfactory paradigm for examining the observed phenomenon.

An easy‐to‐implement differential evolution approach for multi‐objective optimizations

Purpose – To present and validate a new differential evolution (DE) method for multi‐objective optimization method.Design/methodology/approach – A new selection scheme was designed to replace the existing one in DE to enable DE applicable to either single objective or multi‐objective optimizations.Findings – The new method was validated with three simple multi‐objective optimization problems. The simulation results show that the approach is capable of generating an approximated Pareto‐front for each selected problem. The new DE method was used to optimize a prototype air mixer subject to two objective functions to be minimized. The results demonstrate that the new DE approach can handle this practical multi‐objective problem successfully.Originality/value – The new method is an easy‐to‐implement evolutionary method and has the potential for application for any complicated engineering optimizations.

A Laser-Doppler Investigation of the Flow inside a Backswept, Closed, Centrifugal Impeller

A laser-Doppler technique is successfully applied to measure the flow field inside a closed, backswept impeller, through a rotating window. Results show that, in contrast to the flow in many radial-exit impellers, the flow in the backswept impeller is stable and attached. Further, comparison with an open impeller demonstrates the fundamental difference in the flow fields near the shroud.

Experimental Investigation of a Novel Combustor Model for Gas Turbines

The present study describes the performance of a novel combustor model for gas turbines. The working principle of this combustor is based on the establishment of a large recirculation zone in the combustion chamber, where part of the inlet air is mixed with the combustion products. Efficient mixing is achieved by the use of high-velocity inlet air jets. Six different inlet air geometries have been analyzed under nonreacting and reacting conditions at atmospheric pressure. Laser-Doppler anemometry was employed to characterize the mean velocity and turbulent kinetic energy fields as a function of the air mass flow rate and geometry configuration. Measurements of mean gas species concentration (O 2 , CO 2 , CO, HC, and NO x ) at the model exhaust are reported as a function of the equivalence ratio for all configurations. The isothermal data revealed that the recirculation ratio is mainly a function of the geometry, with a minor dependence on the experimental conditions. Under reacting conditions, the data revealed that NO x emissions are low regardless of the combustor operating conditions and geometry. However, the air inlet configuration has a strong effect in combustor efficiency.

On the oscillation of combustion of a laminar spray

A spray combustor, with flow velocities in the laminar range, exhibits a unique operating mode where large amplitude, self-induced oscillations of the flame shape occur. The phenomenon, not previously encountered, only occurs when fuel is supplied in the form of fine liquid droplets and does not occur when fuel is supplied in gaseous form. Several flow mechanisms are coupled in such a fashion as to trigger and maintain the oscillatory motion of the flame. These mechanisms include heat transfer and evaporation processes, dynamics of two-phase flows, and effects of gravity (buoyancy forces). An interface volume, lying between the fuel nozzle and the flame was found to be the most susceptible to gravity effects, and postulated to be responsible for inducing the oscillatory motion. Heptane fuel was used in the majority of the tests.

Numerical Analysis of a Multiple Jet Impingement System

Jet impingement is known to provide higher heat transfer coefficients as compared to other conventional modes of single phase heat transfer. Jet impingement has been a subject of research for a long time. Single jets have been studied extensively for their heat transfer and flow characteristics. However, for practical usage, multiple jets (in the form of arrays) have to be used for increasing the total heat transfer over a given area. Most of the research on multiple impinging jets have focused on evaluating heat transfer correlations for such arrays in the turbulent regime (Re >2500). The focus of the present paper is on experimental investigation of a large array of impinging jets in the low Reynolds number regime (<1000) and subsequently numerically modeling the same array by using existing Computational Fluid Dynamics tools in order to study the physical phenomena within such a complex system. Different turbulence models were used for modeling the fluid flow within these impinging jets and it was found that the SST k-ω model is the most suitable. Results obtained from CFD analysis are in reasonable agreement with experimental values. It was observed that CFD simulations over predicted the Nusselt number and pressure drop when compared to the experimentally obtained values. It was also observed that the decrease in Nusselt number along the streamwise direction of the array was not monotonic. This could be due to the complex flow field resulting from interaction between the crossflow and the impinging jets in the wall jet region. It is anticipated that results obtained from the present work will provide greater insight into the flow behavior and the heat transfer mechanism occurring in multiple impinging jets.Copyright

Shock-Tube Ignition Study of Methane in Air and Recirculating Gases Mixture

To investigate the effect of recirculated combustion products on the combustion process of a typical gas-turbine engine, shock-tube and modeling investigation of ignition delay time was performed. Different mixtures of CH 4 -O 2 -Ar-N 2 -H 2 O were tested. Results showed that replacing N 2 by combustion product components, CO 2 and H 2 O, at concentrations and temperature range, which are typical for the flameless oxidation regime, did not affect the ignition delay time. The temperature range of the shock-heated samples was 1350-1800 K, and the pressure range was 5-10 atm. Modeling calculations were carried out with two mechanisms, and they both showed similar tendency. The correlation between the experimental data and calculations is discussed.