An Experimental Study on the Orifice Nozzle System that Generates Micro-bubbles by Self-suction of Air with a Recirculating Flow

Unsteady turbulent flame evolution in non-premixed combustion has been computationally investigated using large eddy simulations. A simple coaxial combustion chamber, subjected to highly unsteady, turbulent recirculating flow is considered, following the experimental study of Owen et al. (Proc. Combust. Inst., vol. 16, 1976, pp. 105–117). Large-scale flame fluctuations, reported in the above experimental study, such as pulsating flames in swirling and non-swirling conditions, were identified here in our computation. New criteria for flame three-dimensional inhomogeneity are suggested and implemented in the present study, providing the ability to quantify the flame unsteadiness. Using this technique, it is shown that local, large quenched regions develop in the flame’s mixing area and rotate continuously, even when swirl is not imposed on the inlet. However, this rotation appears to be disordered, abruptly changing its direction. On the other hand, our study shows that when swirl is imposed on the inlet, a larger quenched region is identified, rotating in steady ordered rotation in the direction of the imposed swirl. In addition, large-scale radial flame fluctuations are increased downstream with the increase of swirl number. Consequently, significant correlations between radial and circumferential flame fluctuation frequencies were retrieved. Proper orthogonal decomposition analysis reveals coherent flame structures of five dominant modes that contain most of the energy in the fluctuating flame. A simplified analytical stability model is derived and implemented here to assess the hydrodynamic contribution to the flame instability; it is shown that radial fluctuations are excited by circumferential perturbations in the mixing region, providing new insight into the mechanism responsible for the onset of radial fluctuations. The computed radial flame fluctuation spectrum is predicted well using the linear stability analysis. Thus, our findings may therefore be applicable to a large class of non-premixed turbulent combustion problems.

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Experimental study of turbulent flow and heat transfer behaviors over a micro-rib-dimple-structured surface

Journal of Turbomachinery ◽

10.1115/1.4050946 ◽

2021 ◽

pp. 1-27

Author(s):

Kuan Zheng ◽

Wei Tian ◽

Peng Zhang ◽

Yu Rao ◽

Hui Hu

Keyword(s):

Heat Transfer ◽

Experimental Study ◽

Flow Field ◽

Thermal Efficiency ◽

Turbulent Mixing ◽

Heat Transfer Performance ◽

Transfer Performance ◽

Structured Surface ◽

Recirculating Flow ◽

Rib Structure

Abstract An experimental study was conducted to characterize the evolution of turbulent boundary layer flow over a micro-rib-dimple-structured surface. In addition to measuring the surface pressure distribution and detailed flow field inside the dimple cavity, the heat transfer performance over the rib-dimpled surface was investigated using transient liquid crystal thermography. The flow field measurements were correlated with the heat transfer measurements to elucidate the underlying physical mechanism of the improvement in thermal efficiency due to the micro-rib structure. It was found that, compared to the dimpled surface, the micro-rib structure induces a stronger downwash flow and acts as a tabulator to enhance the turbulent mixing of the downstream flow, which significantly restricts the flow separation and the recirculating flow inside the dimple cavity. The dominant flows inside the dimple cavity are the downwash and successive upwash flows, which significantly enhance the turbulent mixing and consequently, improve the heat transfer performance over the rib-dimpled surface. The measurements of the pressure loss and heat transfer performance indicated that the rib-dimpled surface has an overall thermal efficiency approximately 12%−16% higher than that of the dimpled surface owing to the micro-rib structure.

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Experimental study on the restoration from a tilt-azimuth series for improvement in resolution

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010013955x ◽

1995 ◽

Vol 53 ◽

pp. 636-637

Author(s):

Norio Baba ◽

Norihiko Ichise ◽

Syunya Watanabe

Keyword(s):

Experimental Study ◽

Spherical Aberration ◽

Incident Beam ◽

Optical Parameters ◽

Beam Tilting ◽

Illumination Method ◽

Restoration Method ◽

Illumination Mode

The tilted beam illumination method is used to improve the resolution comparing with the axial illumination mode. Using this advantage, a restoration method of several tilted beam images covering the full azimuthal range was proposed by Saxton, and experimentally examined. To make this technique more reliable it seems that some practical problems still remain. In this report the restoration was attempted and the problems were considered. In our study, four problems were pointed out for the experiment of the restoration. (1) Accurate beam tilt adjustment to fit the incident beam to the coma-free axis for the symmetrical beam tilting over the full azimuthal range. (2) Accurate measurements of the optical parameters which are necessary to design the restoration filter. Even if the spherical aberration coefficient Cs is known with accuracy and the axial astigmatism is sufficiently compensated, at least the defocus value must be measured. (3) Accurate alignment of the tilt-azimuth series images.

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