Dmitriy M. Markovich

3D velocity measurements in a premixed flame by tomographic PIV

Tomographic particle image velocimetry (PIV) has become a standard tool for 3D velocity measurements in non-reacting flows. However, the majority of the measurements in flows with combustion are limited to small resolved depth compared to the size of the field of view (typically 1 : 10). The limitations are associated with inhomogeneity of the volume illumination and the non-uniform flow seeding, the optical distortions and errors in the 3D calibration, and the unwanted flame luminosity. In the present work, the above constraints were overcome for the tomographic PIV experiment in a laminar axisymmetric premixed flame. The measurements were conducted for a 1 : 1 depth-to-size ratio using a system of eight CCD cameras and a 200 mJ pulsed laser. The results show that camera calibration based on the triangulation of the tracer particles in the non-reacting conditions provided reliable accuracy for the 3D image reconstruction in the flame. The modification of the tomographic reconstruction allowed a posteriori removal of unwanted bright objects, which were located outside of the region of interest but affected the reconstruction quality. This study reports on a novel experience for the instantaneous 3D velocimetry in laboratory-scale flames by using tomographic PIV.

Helical modes in low- and high-swirl jets measured by tomographic PIV

ABSTRACT We report on a parallel study on properties of large-scale vortical structures in low- and high-swirl turbulent jets by means of the time-resolved tomographic particle image velocimetry technique. The high-swirl jet flow is featured by a well-established bubble-type vortex breakdown with a central recirculation zone. In the low-swirl flow, the mean axial velocity, while intermittently acquiring negative values, remains positive in the mean but with a local velocity defect immediately downstream from the nozzle exit, followed by a spiralling vortex core system and its eventual breakdown. Measurements of the 3D velocity fields allowed direct analysis of the azimuthal/helical modes via Fourier transform over the azimuthal angle and proper orthogonal decomposition (POD) analysis in the Fourier space. A precessing vortex core is detected for both swirl cases, whereas the POD analysis showed that the one originating in the bubble-type vortex breakdown is much more energetic and easier to detect.

Coherent Structures in a Turbulent Swirling Jet Under Vortex Breakdown. 3D PIV Measurements

The current study reports on spatial structure of a global mode of self-sustaining oscillations in a turbulent swirling jet under vortex breakdown conditions. Ensembles of 2D and 3D velocity fields were measured by stereoscopic and tomographic PIV systems, respectively, and were analysed via proper orthogonal decomposition. For the 2D PIV, the spatial resolution was sufficient to resolve most of the turbulent kinetic energy of the turbulent flow. The resolution in the case of tomographic PIV was lower, but the 3D instantaneous velocity fields unambiguously revealing that the global mode corresponds to a spiralling structure, counter-winded to the direction of the jet swirl.

Bubbly free and impinging jets under forced flow conditions: experimental study by means of PIV/PFBI

Jet flows are extensively used in various practical applications. Presently, the development of technical equipment where jets are employed is connected with the improvement and optimization of different methods of flow control. In the paper, an experimental investigation of the turbulent structure of forced bubbly free and impinging jets was carried out by means of PIV and PFBI techniques. PIV was applied to measure velocity distributions and turbulent characteristics in the continuous phase, while PFBI approach was applied to visualize bubbles in the flow and evaluate their sizes. The flow was studied at the Reynolds number of 12,500 and three void fractions β = 0, 1 and 2% for forced conditions St = 0.5. The mean air bubble diameter was estimated to be roughly 0.8 mm for all β. It was revealed that in the free jet the air bubbles and flow pulsations reduces substantially the longitudinal dimension of the jet core. In two-phase flow with forcing distribution of turbulence kinetic energy was similar to one-phase case but maximum value was two and a half times higher then for one-phase unforced jet. In the impinging jet flow, the bubbles produced a maximum of the turbulence kinetic energy near the wall, which increased two and a half times in forced conditions.

Cooling the vertical surface by conditionally single pulses

You Sprays with periodic supply of the droplet phase have great opportunities to control the heat exchange processes. Varying pulse duration and frequency of their repetition, we can achieve the optimal conditions of evaporative cooling with minimization of the liquid flow rate. The paper presents experimental data on studying local heat transfer on a large subcooled surface, obtained on the original setup with multinozzle controlled system of impact irrigation by the gas-droplet flow. A contribution to intensification of the spray parameters (flow rate, pulse duration, repetition frequency) per a growth of integral heat transfer was studied. Data on instantaneous distribution of the heat flux value helped us to describe the processes occurring on the studied surface. These data could describe the regime of “island” film cooling.

PREFACE: DROPS, BUBBLES, AND THIN FILMS, SPECIAL ISSUE HONORING PROFESSOR OLEG KABOV

On January 10th, 2016, Professor Oleg Kabov, a world renowne d expert in the field of interfacial heat and mass transfer and the founding Editor-in-Chief of the journal Interfacial Phenomena and Heat Transfer (IPHT), celebrated his 60th birthday. To mark the occasion, several of his colle agues conceived the idea of a special issue to celebrate his achievements and to provide an overview of some recent de velopments in the research areas pioneered and/or enhanced by Professor Kabov. We review his contributions to these areas here while also providing some biographical information. Oleg Kabov graduated from Tomsk Polytechnic Institute (now National Tomsk Polytechnic Research University) in 1978, and in the same year he moved to Novosibirsk to join th e laboratory of Professor S.S. Kutateladze at the Institute of Thermophysics. For many decades, this institu te has been one of the leading heat and mass transfer research centers in the world, in part due to the pioneering c o tributions of Kutateladze and his collaborators toward the understanding of physical mechanisms of boiling and con densation. Other important research areas developed at the Institute of Thermophysics include studies of waves o n liquid films, vortex flows, heat and mass transfer in disperse systems, phase transformation at cryogenic tempe ratur s, detonation, rarified gas dynamics, and synthesis of advanced materials. Professor Kabov defended his PhD the sis at the Institute of Thermophysics in 1987 under the direction of Professor I.I. Gogonin by working on proble ms of film condensation on arrays of finned tubes. The photograph in Fig. 1 (left) goes back to the early days at the K utateladze laboratory. The colleagues at the Institute of Thermophysics quickly recognized Oleg’s scientific potent ial. In 1987, at the age of just 31, he became the head of a new laboratory for heat transfer enhancement, see Fig. 1 (ri ght). Professor Vladimir E. Nakoryakov, an outstanding scientist who was the Director of the Institute of Thermophy sics between 1986 and 1997, proposed and strongly supported the establishment of this laboratory. In search of novel research directions for his laboratory, P rofessor Kabov turned his attention to studies of gravitydriven liquid films flowing over heated surfaces. This work re sulted in a remarkable discovery (Kabov, 1998). While

Symmetry in the problem of wave modes of thin viscous liquid layer flow

Abstract The equations in conservative form for modelling nonlinear waves on a liquid film flowing down a vertical plane have been investigated. It has been found out that the equations with boundary conditions are invariant under parity transformation in the extended computational domain. The steady-state travelling solutions are numerically shown to have the detected symmetry for moderate Reynolds numbers. The use of this symmetry for the numerical solution of the problem by Galerkin methods significantly increases the efficiency of calculations.

Cavitation on a scaled-down model of a Francis turbine guide vane: high-speed imaging and PIV measurements

Cavitation on two symmetric foils, a NACA0015 hydrofoil and a scaled-down model of a Francis turbine guide vane (GV), was investigated by high-speed visualization and PIV. At small attack angles the differences between the profiles of the mean and fluctuating velocities for both hydrofoils were shown to be insignificant. However, at the higher angle of incidence, flow separation from the GV surface was discovered for quasi-steady regimes including cavitation-free and cavitation inception cases. The flow separation leads to the appearance of a second maximum in velocity fluctuations distributions downstream far from the GV surface. When the transition to unsteady regimes occurred, the velocity distributions became quite similar for both foils. Additionally, for the GV an unsteady regime characterized by asymmetric spanwise variations of the sheet cavity length along with alternating periodic detachments of clouds between the sidewalls of the test channel was for the first time visualized. This asymmetric behaviour is very likely to be governed by the cross instability that was recently described by Decaix and Goncalves [8]. Moreover, it was concluded that the existence of the cross instability is independent on the test body shape and its aspect ratio.