Inlet Flow Disturbances Research Articles

Hopf Bifurcations of Moore-Greitzer PDE Model with Additive Noise

The Moore-Greitzer partial differential equation (PDE) is a commonly used mathematical model for capturing flow and pressure changes in axial-flow jet engine compressors. Determined by compressor geometry, the deterministic model is characterized by three types of Hopf bifurcations as the throttle coefficient decreases, namely surge (mean flow oscillations), stall (inlet flow disturbances) or a combination of both. Instabilities place fundamental limits on jet-engine operating range and thus limit the design space. In contrast to the deterministic PDEs, the Hopf bifurcation in stochastic PDEs is not well understood. The goal of this particular work is to rigorously develop low-dimensional approximations using a multiscale analysis approach near the deterministic stall bifurcation points in the presence of additive noise acting on the fast modes. We also show that the reduced-dimensional approximations (SDEs) contain multiplicative noise. Instability margins in the presence of uncertainties can be thus approximated, which will eventually lead to lighter and more efficient jet engine design.

Three-Dimensional Thermocline Dynamics in Thermal Storage Tanks

In this work, a series of three-dimensional unsteady numerical simulations are performed to study the stability and interface dynamics of a thermocline-based lab-scale single tank Thermal Energy Storage system (TES). The stability of thermocline is analysed by introducing relatively cold fluid for a short period at the inlet of the TES. Numerical simulations are performed for three inlet flow disturbances (weak, medium and strong) and three stratification levels (sharp, moderate and large). The fluid injected at the inlet rolls-up and interacts with the thermocline which causes spatio-temporal disruption of the stable stratification inside the TES. It was found that the three-dimensional simulations bear some resemblance to the two-dimensional case but also show crucial differences. The propagation of the injected cold fluid and the subsequent interaction with the thermocline are analysed. A wide gamut of flow structures is identified inside the TES depending on the degree of stratification and level of disturbance. Finally, the oscillatory nature of interface and associated mixing mechanism are addressed. The simulation indicates that the oscillations at the interface are through the successive generation of countersign vorticity which retards/suppresses the propagation of the vortex ring. In the case of large interface, internal waves are generated by the periodic array of vortices which generates a standing wave pattern near the Brunt-Va ̈isa ̈la ̈ frequency‎.

Experimental study of transient behaviour of laminar flow in zigzag semi-circular microchannels

An experimental investigation is carried out to understand the hydrodynamics of single-phase water flow in a zigzag microchannel with a semi-circular cross-section (diameter 2mm). The present study covers Reynolds numbers in the laminar regime, under both steady and transient conditions. Time-resolved velocity measurements were made using high-speed micro-PIV techniques. The influence of inlet unsteadiness on flow characteristics is investigated. Through time-resolved velocity measurements, a transition from a steady flow to a time-dependent oscillatory flow having a few dominant frequencies and subsequently to a complex transient flow is observed in the wavy channel. However, the transition to a transient flow occurs at much lower Reynolds numbers (Rec∼215) than predicted numerically. The early transition is believed to be triggered by small inlet flow disturbances in the experiments, since the flow is very sensitive to the inlet conditions at sufficiently high Reynolds numbers. This hypothesis is supported by results from transient CFD simulations with fluctuating inlet boundary conditions.

Heat transfer in a three-dimensional rectangular cavity oriented at an angle to the free-stream

We consider a flow of a viscous incompressible heat-conducting fluid over a cubic cavity. The heat transfer on the bottom of the cavity rotated at an angle to the free stream is studied numerically. The numerical algorithm includes a finite-difference approximation in the spatial coordinates, a semi-implicit time integration method, and a modified version of an iterative stabilized method of biconjugate gradients with an algebraic multigrid preconditioner for solving the Poisson equation. The algorithm is designed for the use of multiprocessor computers. Two different inlet flow conditions are considered: a steady-state flow and a steady flow with superimposed periodic perturbations. In the first case, it is shown that the integral heat transfer rate increases monotonically with increase in the cavity rotation angle α. For α = 45°, the increase in the heat flux amounts to 20%. The presence of periodic disturbances may result in up to 3-fold increase in the integral heat transfer rate as compared to the case of the steady-state inlet flow. The enhancement of heat transfer occurs when the frequency of the inlet flow disturbances is close to the frequency of unstable modes of the mixing layer formed at the upper boundary of the cavity.

Combustion Oscillations in Bluff Body Stabilized Diffusion Flames With Variable Length Inlet

This paper describes the results of an experimental study to understand the influence of inlet flow disturbances on the dynamics of combustion process in bluff body stabilized diffusion flames of liquid petroleum gas and air. The results show the influence of weak disturbances created by the change in incoming pipe length on the amplitude of pressure oscillations and the phase angle between pressure and heat release. It is seen that the phase delay increases as the entry length increases. The rms value of pressure, however, generally falls with the increase in length. The phase angle is seen to be in the second quadrant, showing that the heat release oscillations damp the pressure oscillations. Therefore, the decrease in the phase angle results in the reduction in damping and hence an increase in pressure fluctuations. The dominant frequencies of combustion oscillations are found to be the low frequency oscillations, and the frequency of oscillations increases with a decrease in the inlet pipe length and an increase in the flow Reynolds number. It is suggested that such low frequency oscillations are driven by vortex shedding at the wake of the bluff body, which energizes the diffusion and mixing process.

Pressure boundary condition of the lattice Boltzmann method for fully developed periodic flows

The general periodic boundary condition for the lattice Boltzmann method has been modified to incorporate the pressure difference for fully developed periodic flows. The results demonstrated that, unlike other existing pressure boundary treatments, the proposed procedure/treatment does not generate nonphysical inlet and outlet flow disturbances while preserving the system periodicity. This method is readily applicable to a range of lattice Boltzmann simulations for systems with periodic electric potential and temperature fields.

Constrained Model Predictive Control of a Wet Grinding Circuit

This paper deals with the control of a grinding circuit by means of a constrained model predictive control. Under this formulation the sump box is used to attenuate, in a natural way, the effects of the inlet flow disturbances over the battery of hydrocyclones, achieving a smoother operation. An efficient linear programming algorithm solves the optimization problem posed by this approach. The proposed constrained MPC controller is compared to a control strategy based on locals GPC controllers using a tight level control around the sump box.

Experimental and Theoretical Studies of a Novel Venturi Lean Premixed Prevaporized (LPP) Combustor

A new gas turbine engine using a unique layout patented in Norway has a low-emission combustion system under development. The gas generator uses entirely radial rotating components and employs a dual entry LP radial compressor, a radial HP compressor, and a radial HP turbine. The power turbine is of a two-stage axial design, coupled to an epicyclical gear embedded in the exhaust duct. Several combustor concepts have been tested and evaluated during the development of the engine. The engine is targeted for marine, power generation, and train propulsion. For the marine and train application liquid fuel operation is needed, thus the primary focus in the development has been for a lean premixed prevapourised system. An interesting concept utilizing two venturi premixers has been studied intensively. By utilizing venturi premixers the following advantages can be achieved: (1) low overall pressure drop but high injector pressure drop and velocities in the mixing region (throat region), (2) high shear forces and drag imposed on the droplets enhancing droplet shedding and evaporation, and (3) excellent emission behavior at designated load conditions. Although these advantages can benefit gas turbine low-emission combustion, the challenges in using venturi premixers are: (1) venturis are susceptible to separation and thus flame stabilization within the venturi which is detrimental and (2) inlet flow disturbances enhance the tendency for separation in the venturis and must be minimized. Studies were launched to investigate a proposed combustor configuration. These studies included analytical studies, computational fluid dynamics (CFD) calculations of isothermal and combusting flow inside the combustor together with rig tests at atmospheric, medium, and full pressure. Finally, engine tests within the full operating range were conducted with very favorable emission figures for lean premixed prevaporized (LPP) operation. The system was capable of running at below 20 ppm NOx and CO, at elevated power for liquid fuel. Control of part load performance and emissions is by variable fuel staging of the two venturi stages. The paper highlights the features of the venturi combustor development and discusses the characteristics in terms of flow conditions and droplet motion, heat transfer, ignition delay time, and emissions.

Boundary conditions for unsteady supersonic inlet analyses

New bleed and compressor face boundary conditions have been developed to improve the accuracy of unsteady supersonic inlet calculations. The new bleed boundary condition relates changes in the bleed hole discharge coefficient to changes in the local flow conditions; the local bleed flow rate can more than double as a shock moves forward over a bleed band in response to inlet flow disturbances. The stability margin of the inlet is strongly dependent on the throat bleed configuration since the locally rapid increase in bleed flow has a strong effect on the motion of the normal shock. The new compressor face boundary condition accounts for changes in the unsteady flow conditions at the compressor face by specifying the compressor face corrected mass flow or Mach number either as a constant or as a linear function of the stagnation conditions. The effects of inlet flow disturbances on the flow at the compressor face are represented more realistically with this new boundary condition than with traditional fixed static pressure or mass flow conditions. Euler calculations of the dynamic response of an inlet flow to a flow disturbance at the compressor face with 20- and 90-deg throat bleed hole angles are reported. These results indicate that an extra margin of stability for the inlet is obtained with 90-deg bleed holes because the increase in bleed flow rate as the shock moves forward over a bleed band is much larger for 90-deg holes than for 20-deg holes.

A moving-boundary formulation for modeling time-dependent two-phase flows

A mathematical model describing the transient interactions in one-dimensional two-phase flows with heat transfer is presented. A moving-boundary refrigerant model is used to predict the position of the two-phase/vapor interface. A boundary immobilization technique is used to predict the temperature profile along the heat-exchanger wall. Typical results of an evaporator model, in terms of interface position and discharge superheat, are presented for inlet flow disturbances. The model is then used in an overall heat-pump simulation to predict cyclic performance. The results compare favorably to those obtained with a high-fidelity spatially dependent heat-pump model, but require significantly less computational effort.

Distributor effects in liquid fluidized beds of low‐density particles

AbstractThe effect of flow nonuniformities existing near the distributor of a liquid fluidized bed containing very low‐density particles (ρs = 1.05 g/mL) has been investigated experimentally. Immobilized enzyme or cell bioreactors operated as liquid fluidized beds often employ low‐density particles as the support matrix containing the immobilized biocatalyst. With poorly designed multihole distributors, dead zones can be created which seriously undermine the performance of the bioreactor due to the lack of nutrients. Design guidelines are provided for distributors to minimize the inlet flow disturbances, and hence eliminate potential distributor region problems. As a result, the dispersion model, which is normally used for design purposes, can then be applied.

The influence of inlet flow disturbances on transition of Poiseuille pipe flow

The influence of inlet flow disturbances on transition of Poiseuille pipe flow

Reduction of fan noise in an anechoic chamber by reducing chamber wall induced inlet flow disturbances

The difference between flight and ground static noise data has arisen in the past few years as a significant problem in fan jet engine noise testing. The additional noise for static testing has been attributed to inlet flow disturbances or turbulence interacting with the fan rotor. In an attempt to determine a possible source of inflow disturbances entering fans tested in the Lewis Research Center anechoic chamber the inflow field was studied using potential flow analyses. These potential flow calculations indicated that there was substantial flow over the wall directly behind the fan inlet. This flow near the wall anechoic wedges could produce significant inflow disturbances. Fan noise tests were run with various extensions added to the fan inlet ducting to move the inlet away from this back wall and thereby reduce the inlet flow disturbances. Significant noise reductions were observed with increased inlet length. Over 5‐dB reduction of the blade passage tone sound power level was observed between the shortest and longest inlet at 90% fan speed and the first overtone was reduced 9 dB. High‐frequency reductions in the broadband noise were also observed.