de Hc Rick Lange

Streak interactions and breakdown in boundary layer flows

The objective of this paper is to show that the interaction of streamwise velocity streaks of finite length can lead to turbulent breakdown in the flat-plate boundary layer flow. The work is motivated by previous numerical and experimental studies of transitional flows where the high-frequency oscillations leading to turbulence are seen to form in the region of strongest shear induced by streaks in relative motion. Therefore, a model for the interaction of steady and unsteady (i.e., slowly moving in the spanwise direction) spanwise periodic streaks is proposed. The interaction of two subsequent streaks is investigated for varying collision parameters. In particular, the relative spanwise position and angle are considered. The results show that the interaction is able to produce both a symmetric and asymmetric breakdown without the need for additional random noise from the main stream. Velocity structures characteristic of both scenarios are analyzed. Hairpin and Λ vortices are found in the case of symmetr...

Sinuous breakdown in a flat plate boundary layer exposed to free-stream turbulence

In a flat plate boundary layer, perturbed with streaks, breakdown occurs due to a secondary instability acting on the streaks. An experimental study using a water channel with static turbulence grid revealed the presence of a sinuous secondary instability mode in the bypass transition process. Five sinuous instabilities are investigated in detail in the horizontal plane. The streamwise length scale of the sinuous instability is around 40δ300* and the spanwise scale equals around δ300*. Four main features are found in the underlying streak configuration and developing streak-streak interactions. Firstly, all instabilities arise in a streak configuration where two low-speed streaks are located at a small spanwise distance from each other. Patches of low-speed fluid (forming a discontinuity in the streak pattern) are present in the high-speed streaks surrounding the unstable low-speed streak. As a consequence of the streak-streak interactions at the discontinuities, vortices arise in a staggered configuratio...

Two-Dimensional Rotating Stall Analysis in a Wide Vaneless Diffuser

We report a numerical study on the vaneless diffuser core flow instability in centrifugal compressors. The analysis is performed for the purpose of better understanding of the rotating stall flow mechanism in radial vaneless diffusers. Since the analysis is restricted to the two-dimensional core flow, the effect of the wall boundary layers is neglected. A commercial code with the standard incompressible viscous flow solver is applied to model the vaneless diffuser core flow in the plane parallel to the diffuser walls. At the diffuser inlet, rotating jet-wake velocity pattern is prescribed and at the diffuser outlet constant static pressure is assumed. Under these circumstances, two-dimensional rotating flow instability similar to rotating stall is found to exist. Performed parameter analysis reveals that this instability is strongly influenced by the diffuser geometry and the inlet and outlet flow conditions.

Nonlinear Model Predictive Control Experiments on a Laboratory Gas Turbine Installation

Results on the feasibility and benefits of model based predictive control applied to a gas turbine are presented. For a laboratory gas turbine installation, the required dynamic simulation model and the real-time (nonlinear) model predictive control (MPC) implementation are discussed. Results on both model validation and control performance are presented. We applied a nonlinear MPC configuration to control the laboratory gas turbine installation and succeeded in a real-time implementation. Although the available computation time for prediction and optimization of the model limits the sample time, the advantages of MPC, i.e. constraint handling, and anticipation to future (set-point) changes are fully reached, and the control performance is good. Special attention is paid to the performance of the applied filter that compensates for inevitable mismatches between model and process measurements. In general, the opportunities of model based control of turbomachinery are promising.

Modeling of confined and unconfined laminar premixed flames on slit and tube burners

ABSTRACT A model is presented for laminar premixed Bunsen flames on slit and cylindrical burners burning in a surrounding atmosphere. A comparison between modeling and experimental results shows that the model can reproduce the experimental results within 10% accuracy. The influence of a surrounding atmosphere and burner curvature on the flame shape, flow field and mass transport in 2D laminar premixed Bunsen flames is also investigated. It is found that flames on cylindrical burners confined between similar flames have a smaller flame length and flame tip curvature than flames on slit burners with comparable dimensions. These effects are caused by the larger available expansion space (in radial direction) for the cylindrical flames. Furthermore, it is shown that the flames in a surrounding atmosphere are less curved than flames confined between other flames; the curvature of the tip is also smaller. These effects are explained by the fact that the confining flames push the mass flow and the flame front t...

Two-dimensional Methane/Air Flame

Abstract First results are presented of a new flame code for modeling transport processes in two-dimensional flames using a one-step reaction model. The code is applied to a study on the flame shape of methane/air flames on a triple-slit burner. The results compare reasonably well with experimental data. We expect further improvement when more accurate measurements become available. These measurements are necessary to come to a more definite conclusion on the reliability of the model.

Steam injection : analysis of a typical application.

A cardboard factory requires steam and electricity, which are produced in its own powerplant. Conventional cogeneration systems cannot cope with the large fluctuations in steam demand, inherent to the cardboard production process, while power demand remains almost constant. For this reason, two additional cogeneration systems, based on steam-injected gas turbines, were installed. Steam injection allows for a more flexible steam-production to power-production ratio. The effect of steam injection in these so-called Cheng Cycle systems is analysed by means of a steady-state model, which is based on construction drawings and available simulation data. Validation of the model with measurements shows good agreement of model behaviour with actual behaviour. With the model, the flexibility of the Cheng Cycle system is investigated and steam injection is analysed. It shows that steam injection should only be applied when excess steam has to be produced to meet the electricity demand. Although the analysis is based on an existing situation, the developed models and calculation methods may easily be extended to other systems.

Modal and non-modal stability of particle-laden channel flow

Modal and non-modal linear stability analysis of channel flow with a dilute particle suspension is presented where particles are assumed to be solid, spherical, and heavy. The two-way coupling between particle and fluid flow is therefore modeled by the Stokes drag only. The results are presented as function of the particle relaxation time and mass fraction. First, we consider exponentially growing perturbations and extend previous findings showing the potential for a significant increase of the critical Reynolds number. The largest stabilization is observed when the ratio between the particle relaxation time and the oscillation period of the wave is of order one. By examining the energy budget, we show that this stabilization is due to the increase of the dissipation caused by the Stokes drag. The observed stabilization has led to the hypothesis that dusty flows can be more stable. However, transition to turbulence is most often subcritical in canonical shear flows where non-modal growth mechanisms are responsible for the initial growth of external disturbances. The non-modal analysis of the particle-laden flow, presented here for the first time, reveals that the transient energy growth is, surprisingly, increased by the presence of particles, in proportion to the particle mass fraction. The generation of streamwise streaks via the lift-up mechanism is still the dominant disturbance-growth mechanism in the particle laden flow; the length scales of the most dangerous disturbances are unaffected, while the initial disturbance growth can be delayed. These results are explained in terms of a dimensionless parameter relating the particle relaxation time to the time scale of the instability. The presence of a dilute solid phase therefore may not always work as a flow-control strategy for maintaining the flow as laminar. Despite the stabilizing effect on modal instabilities, non-modal mechanisms are still strong in internal flows seeded with heavy particles. Our results indicate that the initial stages of transition in dilute suspensions of small particles are similar to the stages in a single phase flow.

Rotating Stall Characteristics in a Wide Vaneless Diffuser

Paper reports a numerical study on vaneless diffuser flow instability performed for the purpose of better understanding of rotating stall mechanism in radial vaneless diffusers. This analysis is restricted to the two-dimensional flow where effect of wall boundary layers is neglected. Numerical results reveal that a two-dimensional rotating flow instability similar to rotating stall occurs when critical flow angle is exceeded. They also show that the stability limit and the structure of a twodimensional rotating instability are influenced by the configuration geometry and inlet and outlet flow conditions. Good agreement with data from the literature is found for the stability limit and number and speed of propagating cells. Number of cells and their speed is somewhat higher than observed in experiments from literature. This might imply that inception point is caused by the core flow instability and that wall boundary layers are more determinative for the structure of rotating instability.

Flame cooling by a burner wall

A model is proposed to describe the structure of the cooling boundary layer between a cold burner wall and a flame near flash-back. Two-dimensional combustion equations are solved using a one-step chemical reaction model. The analytical solutions prove to be in good agreement with the results of a numerical study. Furthermore, the thickness of the cooling layer is estimated and appears to be in fair agreement with available experimental data. The understanding of the mass and heat transport processes in a flame near a cold burner wall is indispensable for the understanding of the flame stabilization process.