Experimentálny výskum kavitácie v dýze veľmi malých rozmerov

This paper deals with defining of experimental methods and with description of an experimental setup used in research of fluid flows in jets and other orifices of very small scales. The jet used in the experiments has an axissymetric shape with circular cross-section. The experimental setup used for experiments consists of these basic parts: a device used for microscopic visualization and capturing of image of the liquid flow in small scale channels, a digital image data acquisition system and a system of image data manipulation, a system for parallel acquisition and manipulation with other experimental data.Príspevok sa zaoberá definovaním experimentálnych metód a opisom experimentálneho zariadenia na výskum prúdenia tekutiny v dýzach a clonách veľmi malých rozmerov (v zmysle hydraulickom). Dýza, prúdenie v ktorej je cieľom experimentálneho výskumu má osovo symetrický tvar s kruhovým prierezom. V ďalšom uvedená a krátko opísaná základná štruktúra experimentálneho systému: zariadenie na mikroskopickú vizualizáciu a snímanie obrazu prúdiacej kvapaliny v kanáloch veľmi malých rozmerov, systém digitálneho záznamu a ďalšieho spracovania video dát, systém na paralelný zber a záznam ďalších experimentálnych dát

Comparison of Y-jet and OIL effervescent atomizers based on internal and external two-phase flow characteristics

Presented paper focuses on spraying of two viscous liquids ( = 60 and 143 mPa·s) by two types of twinfluid atomizers with internal mixing. We compared the well-known Y-jet atomizer with the less known, “outside in liquid” (OIL), configuration of the effervescent atomizer. The required liquid viscosity was achieved by using the water-maltodextrin solutions of different concentrations. Both the liquids were sprayed at two gas inlet pressures (p = 0.14 and 0.28 MPa) and various gas-to-liquid ratios (GLR = 2.5%, 5%, 10% and 20%). The comparison was focused on four characteristics: liquid flow-rate (for the same working regimes, defined by p and GLR), internal flow regimes, Weber numbers of a liquid breakup (We) and droplet sizes. A high-speed camera and Malvern Spraytec laser diffraction system were used to obtain necessary experimental data. Comparing the results of our experiments, we can state that for both the liquids the OIL atomizer reached higher liquid flow-rates at corresponding working regimes, it was typical by annular internal flow and higher We in the near-nozzle region at all the working regimes. As a result, it produced considerably smaller droplets than the second tested atomizing device, especially for GLR < 10%

Unsteady Behaviour of the Effervescent Atomizer

The effervescent atomizer is a well-established type of the twin-fluid nozzle with internal mixing of fluids. It is popular for the ability to process highly viscous liquids, such as liquid fuels, into a fine spray with low gas consumption. This study aims to investigate the performance of the effervescent nozzle when spraying the liquids with a viscosity up to 308 mPa·s. The working parameters of the nozzle were defined by the mass flows ratio of the gas to the liquid (GLR =2.5 to 20 %) and the gas pressure at the nozzle inlet (Δp = 0.14 MPa). The spray quality was investigated by the laser diffraction system, measuring the spray drop sizes. The investigated nozzle was able to atomize all of the model liquids. However, the liquid viscosity increase led to the need to operate the nozzle with the larger gas consumption. The minimum GLR for the spraying of the liquid with the viscosity 308 mPa·s was 10 %, while the less viscous liquid (60 mPa·s) was processed with the GLR = 2.5 %. It was observed that the spray quality was, at the low GLRs, lowered by unstable nozzle work, caused by the presence of the plug flow in the mixing chamber of the atomizer

Experimental Research of Cavitation in Channels of Very Small Scales

This paper deals with defining of experimental methods and with description of an experimental setup used in research of fluid flows in jets and other orifices of very small scales. The jet used in the experiments has an axissymetric shape with circular cross-section. The experimental setup used for experiments consists of these basic parts: a device used for microscopic visualization and capturing of image of the liquid flow in small scale channels, a digital image data acquisition system and a system of image data manipulation, a system for parallel acquisition and manipulation with other experimental data

On the spray pulsations of the effervescent atomizers

The presented paper focuses on the comparison of the two effervescent atomizer configurations—the outside-in-gas (OIG) and the outside-in-liquid (OIL). The comparison was based on the spray pulsation assessment by different methods. The atomizers were tested under the same operating conditions given by the constant injection pressure (0.14 MPa) and the gas to the liquid mass ratio (GLR) varying from 2.5 to 5%. The aqueous maltodextrin solution was used as the working liquid (μ = 60 and 146 mPa·s). We found that the time-averaging method does not provide sufficient spray quality description. Based on the cumulative distribution function (CDF) we found that the OIG atomizer generated the spray with non-uniform droplet size distribution at all investigated GLRs. Exceptionally large droplets were present even in the spray which appeared stable when was analyzed by the time-averaging method

Comparison of the Viscous Liquids Spraying by the OIG and the Oil Configurations of an Effervescent Atomizer at Low Inlet Pressures

In this work we studied the influence of the fluid injection configuration (OIG: outside-in-gas, OIL: outside-in-liquid) on the internal flows and external sprays parameters. We sprayed the viscous aqueous maltodextrin solutions (μ = 60 mPa·s) at a constant inlet pressure of the gas and the gas to the liquid mass flow ratio (GLR) within the range 2.5 to 20%. We found that the fluids injection has a crucial influence on the internal flows. The internal flows patterns for the OIG atomizer were the slug flows, the internal flow of the OIL device was annular which led to the significant improvement of the spray quality: Smaller droplets, faster atomization, fewer pulsations

Stall identification methods in centrifugal compressor

The presented paper describes a method for detecting compressor stall precursors in a measured pressure signal using procedures arising from the chaos theory. The experiment was carried out on a scaled-down model of a compressor used in natural gas transportation. It was a single-stage centrifugal compressor with a vane diffuser. The presented method is based on the analysis of an attractor constructed using the time delay method from the pressure signal collected at the compressor outlet flange at a frequency of 25 kHz. Using a parameter called correlation dimension, we identified small changes in the dynamics of the measured signal before the onset of negative stall manifestations. In general, it was found that near the stall curve there were minor disturbances in the flow field in the compressor, due to which the correlation dimension decreased

Observation of development of cavitation damage

The paper presents a method developed and applied for observation of cavitation effects on material of different properties. It is intended for observation of cavitation effects of microorifices on selected materials. In the paper are presented some results obtained in our institute by applying of presented method

Visualization of cavitating micro jets

The paper deals with one experimental set up integrated for research of the cavitating micro flows, which is incipient behind the micro channel or micro discharge nozzle outlet port. Experimental system is integrated from three major systems: hydraulic circuit with installed discharge nozzle (or micro channel), subsystem for data acquisition and data processing (DAQ system) and vision system compound of high speed video camera and pulse light source with highfrequency repetition. First few results of experiments (parameters such as inlet pressure, downstream pressure were changed) is also discussed.

Multi-Exposure PIV Measurements of Velocity Fields in Sprays

This paper presents an approach to the use of the PIV method in the diagnosis of sprays generated by an effervescent atomiser. Due to the different density of the liquid phase depending on the distance from the nozzle, problems arise with the correct exposure of images for PIV analysis. The aim of the authors of this paper is to outline the possibility of solving this problem by composing a velocity field from partial measurements. To meet the objectives of the paper, in-house PIV equipment (hardware and software) was used rather than a commercial setup. This allowed for easier handling of the measured data and more sophisticated post-processing than offered by commercial products. It is clear from the results presented that, despite the fundamental differences in the optical properties of the spray particles, it is possible to obtain a velocity field from the discharge zone to the spray region with fine droplets. Moreover, it is possible to combine velocity measurements in the spray cone with measurements in the surrounding environment. Research background: Spray is an environment with an abundance of tracers for PIV analysis (droplets), but their density, size and shape vary dramatically with distance from the nozzle. The use of PIV can therefore be challenging due to the demands of this method for correct image exposure. Purpose of the article: Introduction of the application of the PIV method for environments with variable density and size of tracer particles Methods: PIV, image processing. Findings & Value added: By taking an appropriate approach to acquiring the source PIV images, it is possible to obtain information about the velocities throughout the spray cone as well as in the surrounding environment. The application of the proposed method requires a sufficiently large source data set (images) and sophisticated postprocessing. However, as a result, it is possible to obtain an overall view of the velocity field in the spray cone starting from the area behind the nozzle to the fine droplet region