Past Experimental Investigations of Bubbly Flow Applied to Centrifugal Nuclear Thermal Propulsion

This paper reports the results of experimental investigations on microbubbles emission boiling (MEB) in a microchannel and minichannel. MEB is a boiling phenomenon under high subcool and high heat flux condition. To understand the mechanisms and processes of MEB, pool boiling on a thin wire under subcooled condition was firstly performed. Under various subcooling condition, the experiments of subcool boiling were performed. From photographic observation, distributions of bubble diameters and their dependence on subcooling were investigated. To achieve better understanding of the process of MEB and to find the incipient subcooling of MEB, a sound of ebullition was recorded and analyzed. It was found that 30 to 40 K are the incipient subcooling of MEB on the thin wire. In the experiment of MEB in microchannel and minichannel, the special attention was paid to longitudinal transition of boiling behavior, since bulk liquid temperature increases in short length in case of subcooled flow boiling. Boiling curves for various channel height were obtained from experimental data, and pressure drop through whole channel was measured. In a minichannel, annular flow flushed out the whole channel from the downstream to the upstream and the pressure drop fluctuates, while in a microchannel, flow pattern was divided into two regions: bubbly flow in upstream and annular flow in downstream.

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Investigation of the combined effect of air pockets and air bubbles on fluid transients

Journal of Hydroinformatics ◽

10.2166/hydro.2017.018 ◽

2017 ◽

Vol 20 (2) ◽

pp. 376-392 ◽

Cited By ~ 3

Author(s):

Oscar Pozos-Estrada

Keyword(s):

Bubbly Flow ◽

Combined Effect ◽

Hyperbolic Partial Differential Equations ◽

Experimental Investigations ◽

Air Bubbles ◽

Two Phase ◽

Pressure Transients ◽

Air Pocket ◽

Experimental Apparatus ◽

Air Pockets

Abstract This paper presents numerical and experimental investigations of the combined effect on pressure transients of air pockets and homogenous water–air bubble mixtures. An air pocket can accumulate at a high point of a pipeline along the control section located at the transition between pipes with sub- and supercritical slope, forcing open channel flow conditions underneath the pocket that ends in a hydraulic jump at the downward sloping pipe. The turbulence action at the jump generates small air bubbles that are entrained and transported along the pipe producing a two-component bubbly flow within the continuous liquid phase. A numerical model is developed, combining the explicit–implicit scheme proposed by McGuire and Morris and the method of characteristics for solving the quasi-linear hyperbolic partial differential equations for transient two-phase flow expressed in conservation form. To verify the proposed model, an experimental apparatus made of PVC was used to carry out hydraulic transient experiments. Tests were conducted in a tank–pipe–valve system and a valve with a pneumatic actuator at the downstream end generated transients. Numerical results at the test section pipe compares favorably with experimental data. The results show that pressure transients are significantly reduced with increasing air-pocket volumes and bubbly flow air content.

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Effects of water–air mixtures on hydraulic transients

Canadian Journal of Civil Engineering ◽

10.1139/l10-066 ◽

2010 ◽

Vol 37 (9) ◽

pp. 1189-1200 ◽

Cited By ~ 5

Author(s):

Oscar Pozos ◽

Alejandro Sanchez ◽

Eduardo A. Rodal ◽

Yuri V. Fairuzov

Keyword(s):

Bubbly Flow ◽

Experimental Investigations ◽

Hydraulic Transients ◽

Two Phase ◽

Pressure Transients ◽

Air Content ◽

Air Pocket ◽

Pressurized Pipelines ◽

Air Pockets ◽

The Impact

The purpose of this study is to investigate pressurized pipelines and the potential effects on pressure transients of air entrained at the downstream end of large entrapped air pockets followed by a hydraulic jump in pressurized pipelines. The homogeneous two-phase flow model is used to simulate the transient response of the bubbly mixture after a pump shutdown. The results show that pressure transients are significantly reduced with increasing air-pocket volumes and bubbly flow air content. Experimental investigations were carried out to analyze the impact of different air-pocket volumes located at high points of pressurized pipelines. A case study of an existing pumping system was considered to exemplify the impact of the bubbly flow air content on hydraulic transients.

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Devolatilization of Molten Polymers During Multiphase Flow in a Double Screw Extruder

Volume 1: Fora, Parts A and B ◽

10.1115/fedsm2002-31023 ◽

2002 ◽

Author(s):

Ingo Gestring ◽

Dieter Mewes

Keyword(s):

Mass Transfer ◽

Flow Field ◽

Bubbly Flow ◽

Volatile Component ◽

Bubble Nucleation ◽

Experimental Investigations ◽

Complex Flow ◽

Screw Extruder ◽

Two Phase ◽

Flow Mechanisms

Devolatilization is a thermal separation process in order to remove low molecular solvents from mixtures of polymers. Extruders with partly filled devolatilization zones are often used for this process. The two-phase flow of the polymer and the evaporating monomers and solvents is complex due to free surfaces. In film flow and two-phase bubbly flow the polymer is heated by dissipation and cooled by evaporation of the low molecular solvent. Temperature and concentration fields are difficult to predict in extruders because of the complex flow field. Therefore the experimental investigations are carried out in special designed apparatus with a flow field similar to that in extruders and in a transparent double-screw extruder to investigate the different flow mechanisms. In order to nucleate bubbles of the volatile component the polymers must be supersaturated and some kind of deformation must exist. The bubble nucleation is shear induced. The changes in concentration during two-phase bubbly flow result in decreasing temperatures. The mass transfer rates are increased due to the large inner surfaces of the bubbles in the foam and so is the cooling by evaporation. The higher the foam expansion the better is the mass transfer.

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Visualization of Spray Structure at the Outlet of the Micro Orifices

ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels ◽

10.1115/icnmm2015-48304 ◽

2015 ◽

Cited By ~ 1

Author(s):

Morteza Ghorbani ◽

Ali Koşar

Keyword(s):

High Speed ◽

Bubbly Flow ◽

Injection Pressure ◽

Experimental Investigations ◽

Small Scale ◽

Spray Characteristics ◽

Visualization System ◽

Spray Formation ◽

Cavitation Phenomenon ◽

Visualization Techniques

Spray formation occurring at the outlet of short microchannels/micro orifices due to the cavitation phenomenon is of great importance in biomedical and engineering applications. The spray characteristics are affected dramatically by the flow regime in the micro orifice. If properties of the flow are identified in the outlet of the nozzle, the treatment of the spray can be predicted. These properties can be used as boundary conditions. The experimental investigations show that the cavitation phenomenon occurs in the orifice and strongly affects the spray characteristics. However, visualization of the spray at the outlet of the micro orifice is a challenging task, since the phenomena related to the spray are occurred in very small scale and also the region near to the micro orifice is not clear. Therefore there is an urgent need to new and advanced visualization techniques and measurement equipments. In this study, spray formation and atomization, bubble evolution at the outlet of a short microchannel of an inner diameter of 152 μm were experimentally studied at different injection pressures with the use of a high speed visualization system. High speed visualization was performed at four different segments to cover ∼15 mm distance beginning from the microchannel outlet to understand the spray formation mechanism. It was observed that cavitating bubbly flow is strongly affected by injection pressure. Up to an injection pressure of 50 bars bigger size droplets form at the outlet, while beyond 50 bar injection pressure, cavitation erosion of intensified cavitation becomes dominant leading to smaller droplet sizes and a more conical spray. The results showed a good agreement with previous studies. This energy could be exploited in several applications, where destructive effects of bubbly cavitating flows are needed.

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