Influence of orifice spacing on twin bubbles formation in shear-thinning fluids: Laser image measurement

A laser image system for investigating the influence of orifice spacing on twin bubbles formation in shear-thinning fluid was established. The bubbles formation process at two orifices could be directly visualized and real-time recorded through computer by means of He–Ne laser as light source using the beam expanding and light amplification technology. The shape and size of bubbles generating in carboxymethylcellulose (CMC) aqueous solutions were studied experimentally at the orifices spacing 1D_o, 2D_o and 3D_o (D_o orifice diameter). The results reveal that the minute bubbles can be imaged clearly and amplified without distortion in the higher concentration solutions, and therefore the shape and size of bubbles are obtained accurately. With the increase of orifice spacing, bubble instantaneous volume in shear-thinning fluid decreases at the radial expansion stage but then increases at vertical elongation period, and the deformation and deviation of bubbles forming goes down due to the reduced interaction of adjacent bubble.     

Experiments on two-phase flow distribution inside parallel channels of compact heat exchangers

Uneven distribution in heat exchangers is a cause of reduction in both thermal and fluid-dynamic performances. Many papers have dealt with single-phase flow and both flow distribution data and analytical or numerical models are available for header design. With regard to two-phase flow, phase separation in manifolds with several outlets is so complicated that, to date, there is no general way to predict the distribution of two-phase mixtures at header-channel junctions. The design of headers for new generation compact heat exchangers and multi-microchannel evaporators is still based on an empirical approach, as a number of variables act together: geometrical parameters and orientation of the manifolds and of the channels, operating conditions, fluid physical properties.     

DNS of viscoelastic turbulent channel flow with rectangular orifice at low Reynolds number

Direct numerical simulations of turbulent viscoelastic-fluid flow in a channel with a rectangular orifice were performed to investigate the influence of viscoelasticity on turbulence statistics and turbulent structures downstream of the orifice. The geometry considered is periodic rectangular orifices with 1:2 expansion. The constitutive equation follows the Giesekus model, valid for polymer (or surfactant) solutions, which are generally capable of reducing the turbulent frictional drag in a smooth channel. The friction Reynolds number and the Weissenberg number were set to 100 and 20-30, respectively. A drag reduction of about 20% was achieved in the viscoelastic flows. The onset Reynolds number for the transition from a symmetric to an asymmetric state was found to be shifted to higher values than that for the Newtonian flow. In the viscoelastic flow, the turbulent kinetic energy was decreased and fewer turbulent eddies were observed, as the Kelvin-Helmholtz vortices were quickly damped. Away from the orifice, quasi-streamwise vortices in the viscoelastic flow were sustained for a longer period, accompanied by energy exchange from elastic energy of the viscoelastic fluid to kinetic energy.     

Optimization of a heterogeneous catalytic hydrodynamic cavitation reactor performance in decolorization of Rhodamine B: Application of scrap iron sheets

A low pressure pilot scale hydrodynamic cavitation (HC) reactor with 30 L volume, using fixed scrap iron sheets, as the heterogeneous catalyst, with no external source of H₂O₂ was devised to investigate the effects of operating parameters of the HC reactor performance. In situ generation of Fenton reagents suggested an induced advanced Fenton process (IAFP) to explain the enhancing effect of the used catalyst in the HC process. The reactor optimization was done based upon the extent of decolorization (ED) of aqueous solution of Rhodamine B (RhB). To have a perfect study on the pertinent parameters of the heterogeneous catalyzed HC reactor, the following cases as, the effects of scrap iron sheets, inlet pressure (2.4–5.8 bar), the distance between orifice plates and catalyst sheets (submerged and inline located orifice plates), back-pressure (2–6 bar), orifice plates type (4 various orifice plates), pH (2–10) and initial RhB concentration (2–14 mg L^?1) have been investigated. The results showed that the highest cavitational yield can be obtained at pH 3 and initial dye concentration of 10 mg L^?1. Also, an increase in the inlet pressure would lead to an increase in the ED. In addition, it was found that using the deeper holes (thicker orifice plates) would lead to lower ED, and holes with larger diameter would lead to the higher ED in the same cross-sectional area, but in the same holes’ diameters, higher cross-sectional area leads to the lower ED. The submerged operation mode showed a greater cavitational effects rather than the inline mode. Also, for the inline mode, the optimum value of 3 bar was obtained for the back-pressure condition in the system. Moreover, according to the analysis of changes in the UV–Vis spectra of RhB, both degradation of RhB chromophore structure and N-deethylation were occurred during the catalyzed HC process.     

Study on bubble formation in non-Newtonian fluids by laser image technique

A laser image system for studying bubble formation at the orifice submerged in liquid was established. The process of bubble formation can be directly visualized and real-time recorded through computer by means of He–Ne laser as light source using the beam expanding and light amplification technique. The behaviors of bubble formation in polyacrylamide (PAM) aqueous solutions were investigated at temperature 293.15 K and orifice diameters 1, 1.5 and 2 mm, respectively, the chamber volume was 90 cm³ and the gas flowrate from 0.1 to 0.6 cm³/s. The influences of mass concentration of solution and orifice diameter on bubble detachment volume were investigated experimentally and the results show that bubble detachment volume increases with solution concentration and orifice diameter in the experimental range of this work.     

A parametrical study of disinfection with hydrodynamic cavitation

The physical and chemical conditions generated by cavitation bubbles can be used to destroy microorganisms and disinfect wastewater. The effect of different cavitation chamber designs and diverse operational parameters on the inactivation rate of Escherichia coli have been studied and used to understand the mechanisms involved in cell disruption.     

Simulations for gas flows in microgeometries using the direct simulation Monte Carlo method

Micro gas flows are often encountered in MEMS devices and classical CFD could not accurately predict the flow and thermal behavior due to the high Knudsen number. Therefore, the gas flow in microgeometries was investigated using the direct simulation Monte Carlo (DSMC) method. New treatments for boundary conditions are verified by simulations of micro-Poiseuille flow, compared with the previous boundary treatments, and slip analytical solutions of the continuum theory. The orifice flow and the corner flow in microchannels are simulated using the modified DSMC codes. The predictions were compared with existing experimental phenomena as well as predictions using continuum theory. The results showed that the channel geometry significantly affects the microgas flow. In the orifice flow, the flow separation occurred at very small Reynolds numbers. In the corner flow, no flow separation occurred even with a high driving pressure. The DSMC results agreed well with existing experimental information.     

Experimental study on variation of acoustical resonance frequency of duct with orifice depending on periodic motion

Recently, a linear compressor has been actively developed to improve the energy efficiency of home appliances, such as refrigerators. Unlike a reciprocating compressor, the suction part of a linear compressor is periodically moving. Therefore, the suction valve and the muffler constituting the suction part are periodically moving. However, up to now, there has been no research into the characteristics of the sound propagation in a periodically moving acoustic system. Thus, in this study, characteristics of sound propagation in a periodically moving acoustic system were investigated for the first time. Among a variety of acoustic filters used in a suction muffler, the change in the orifice impedance has been observed because this change is considered to be easily affected by periodically moving. Due to difficulty in measuring the orifice impedance in a periodically moving acoustic system, the change in the orifice impedance was predicted from the change in the input impedance of the suction muffler that included orifice. The experiments were carried out while changing the diameter and the pattern of orifice as well as length of the duct. As a result of experiments, the impedance of periodically moving orifice was changed depending on diameter, pattern of orifice and frequency band. Therefore, if periodically moving orifice was used to design a suction muffler in linear compressor or acoustic system, the change in the orifice impedance should be taken into consideration.     

The acoustic impedance of perforated plates under various flow conditions relating to combustion chamber liners

The absorption of sound by cavities lined with perforated sheets depends crucially on the impedance of the orifices in the sheets. Although the theory for that absorption in the absence of a mean flow was well-developed in 1926, the presence of either a ‘bias’ flow through the orifices, or of a flow ‘grazing’ the sheet and deflecting the acoustic jets, radically alters the absorption. There are many theoretical and experimental treatments of the various cases, some of which are reviewed here. However, there has been little attempt to show how these data relate to one another, and this is also undertaken. The frequency dependence of the impedance is here expressed in terms of a Helmholtz number and used as the prime parameter for comparison. Theories for the cases where the mean flow is negligible are naturally based on the viscous penetration depth, whereas those for bias flow have a Strouhal number as the main parameter and are independent of viscosity. It is found that there are major uncertainties in the impedance for higher Strouhal numbers, when the bias flow is small. A criterion for transition to the no-bias flow theory is proposed. Theories and correlations for grazing flow rationally feature a Strouhal number based on the friction velocity in the duct, since this determines the boundary layer characteristics, but there should be a smooth transition to the case where the grazing flow can be considered negligible. Criteria for this are also proposed, based on the available experimental data. When both types of flow are present, particularly when the grazing velocity is larger than the bias velocity, the available data are very limited.