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We examined the effects of Prandtl number on three-dimensional mixed convection in a horizontal square duct with heated and cooled side walls numerically. Non-dimensional governing equations were solved for Re= 100, Pr = 0.1-10, a...
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We examined the effects of Prandtl number on three-dimensional mixed convection in a horizontal square duct with heated and cooled side walls numerically. Non-dimensional governing equations were solved for Re= 100, Pr = 0.1-10, and Ri = 36.44 by the SIMPLE method. The numerical results show that the swirl How was generated along the flow direction, and its pitch lengthened with the increase of Pr. We also examined the strength of swirl How using the swirl number, 5, and we discuss heat transfer behavior as it corresponded to the flow. Heat transfer was promoted by the swirl flow with all Pr, and the optimum value existed within these Pr.
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Swirling flows are of considerable practical importance. They are used for example to increase heat transfer in pipes or to stabilize flames at a distance from the injector unit by means of a central reverse flow that establishes ...
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Swirling flows are of considerable practical importance. They are used for example to increase heat transfer in pipes or to stabilize flames at a distance from the injector unit by means of a central reverse flow that establishes an inner recirculation zone of hot combustion products. The level of swirl is governed by a dimensionless parameter designated as the swirl number, which essentially quantifies the ratio of the axial component of the flow rate of angular momentum to the flow rate of axial momentum. This ratio controls to a great extent the structure of the swirling flow and its value determines whether an inner recirculation zone is established and whether a precessing vortex core (PVC) is formed in the flow. However, a major difficulty resides in calculating the swirl number from experimental measurements and over the last 50 years, several simplified formulas have been proposed to overcome this difficulty. The present study is aimed at using velocity and pressure profiles obtained by a large eddy simulation in a generic configuration to examine these simplified expressions and determine the conditions under which they may be applicable. The geometry comprises a cylindrical swirling injector, flush mounted in the back plane of a cylindrical cavity. Although the swirl number is in principle constant when the flow is established in a duct with a constant cross-section, provided that viscous forces at the wall are negligible, one finds that this quantity varies substantially if inadequate approximations are made. Among the many possibilities one concludes that two swirl numbers should be distinguished. The first corresponding to the original definition features conservation properties, but is difficult to properly calculate from experimental data. The second is a highly simplified formulation that is commonly used today but does not share the conservation properties of the first formulation. Recommended practices are provided on how each of these swirl numbers should be calculated. It is also shown that the other formulations yield values that notably differ from those provided by the original definition.
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This paper presents a discussion of the results and conclusions drawn from a series of experiments conducted to investigate the swirl flow that are generated by a three lobed helical pipe mounted within a laboratory scale pneumati...
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This paper presents a discussion of the results and conclusions drawn from a series of experiments conducted to investigate the swirl flow that are generated by a three lobed helical pipe mounted within a laboratory scale pneumatic conveying rig. The experiments employed Laser Doppler Anemometry (LDA) to quantify the strength of the induced vortex formations and the decay rates of the observed downstream swirl flows over a range of Reynolds number in the turbulent regime. Instantaneous point velocity measurements were resolved in three directions across regular measurement grids transcribed across parallel planes located at four distances downstream of the swirl inducing pipe section. The equivalent axial, radial and tangential velocities were subsequently computed at these grids points. The degree of swirl measured across each measurement plane was expressed in terms of a defined swirl number.
It was concluded that the three lobed helical pipe gave rise to a wall jet type of swirl whose rate of observed downstream decay is related to the Reynolds number of the upstream flow and the distance downstream of the swirl pipe. The decay rates for the swirl flows were found to be inversely proportional to the Reynolds number of the upstream flow. The swirl pipe was observed to create a redistribution of the downstream velocity field from axial to tangential, accompanied by a transfer of axial to angular momentum. The findings of this paper are believed to improve understanding to assist the selective use of swirl flow within lean phase particles pneumatic transport systems.
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Background This paper sets out to design a device for removing bubbles during the process of hemodialysis. The concept is to guide the bubbles while traveling through the device and eventually the bubbles can be collected. The des...
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Background This paper sets out to design a device for removing bubbles during the process of hemodialysis. The concept is to guide the bubbles while traveling through the device and eventually the bubbles can be collected. The design focuses on the analysis of various parameters i.e. inlet diameter, inlet velocity and size of the pitch. The initial diameters of Models 1 and 2 have thread regions of 6 and 10 mm, respectively. Parameters: Swirl number, Taylor number, Lift coefficient along with pressure field are also implemented.ResultsBased on computational fluid dynamics analysis, the bubbles' average maximum equilibrium position for Model 1 reached 1.995 mm, being greater than that of Model 2, which attained 1.833 mm. Then, 16,000 bubbles were released into Model 1 to validate the performance of the model. This number of bubbles is typically found in the dialysis. Thus, it was found that 81.53% of bubbles passed through the radial region of 2.20?±?0.30 mm. The appropriate collecting plane was at 100 mm, as measured from the inlet position along the axial axis. The Taylor number, Lift coefficient, and Swirl number proved to be significant parameters for describing the movement of the bubbles. Results were based on multiple inlet velocities. It is seen that Model 3, the improved model with unequal pitch, reached a maximum equilibrium position of 2.24 mm.ConclusionOverall, results demonstrated that Model 1 was the best design compared to Models 2 and 3. Model 1 was found capable of guiding the bubbles to the edge location and did not generate extra bubbles. Thus, the parametric study, herein, can be used as a prototype for removing bubbles during the process of hemodialysis.
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摘要 :
This paper sets out to design a device for removing bubbles during the process of hemodialysis. The concept is to guide the bubbles while traveling through the device and eventually the bubbles can be collected. The design focuses...
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This paper sets out to design a device for removing bubbles during the process of hemodialysis. The concept is to guide the bubbles while traveling through the device and eventually the bubbles can be collected. The design focuses on the analysis of various parameters i.e. inlet diameter, inlet velocity and size of the pitch. The initial diameters of Models 1 and 2 have thread regions of 6 and 10 mm, respectively. Parameters: Swirl number, Taylor number, Lift coefficient along with pressure field are also implemented.Based on computational fluid dynamics analysis, the bubbles' average maximum equilibrium position for Model 1 reached 1.995 mm, being greater than that of Model 2, which attained 1.833 mm. Then, 16,000 bubbles were released into Model 1 to validate the performance of the model. This number of bubbles is typically found in the dialysis. Thus, it was found that 81.53% of bubbles passed through the radial region of 2.20 ± 0.30 mm. The appropriate collecting plane was at 100 mm, as measured from the inlet position along the axial axis. The Taylor number, Lift coefficient, and Swirl number proved to be significant parameters for describing the movement of the bubbles. Results were based on multiple inlet velocities. It is seen that Model 3, the improved model with unequal pitch, reached a maximum equilibrium position of 2.24 mm.Overall, results demonstrated that Model 1 was the best design compared to Models 2 and 3. Model 1 was found capable of guiding the bubbles to the edge location and did not generate extra bubbles. Thus, the parametric study, herein, can be used as a prototype for removing bubbles during the process of hemodialysis.
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Experimental investigations are carried out with an objective of reducing impingement jet noise using coaxial swirl jets. Two nozzle-to-plate distances have been considered and the Mach number values ranged from 0.95 to 1.83. The ...
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Experimental investigations are carried out with an objective of reducing impingement jet noise using coaxial swirl jets. Two nozzle-to-plate distances have been considered and the Mach number values ranged from 0.95 to 1.83. The vane angles of the coaxial swir-lers ranged from 0 deg to 60 deg, corresponding to swirl numbers that ranged from weak to high. Flow visualization has also been conducted to understand the shock-cell structure for each case. The results indicate that swirl jets are highly efficient in the control of impingement noise. Transonic, screech, and impinging tones are completely eliminated by the swirl jets. A weak swirl seems efficient for lower Mach numbers while higher amounts of swirl are efficient at higher Mach numbers.
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This article reports on the effect of swirl intensity on the flow and combustion characteristics of pulverized olive waste (OW) burning. Numerical simulations are carried out using the computational fluid dynamics (CFD) Commercial...
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This article reports on the effect of swirl intensity on the flow and combustion characteristics of pulverized olive waste (OW) burning. Numerical simulations are carried out using the computational fluid dynamics (CFD) Commercial Software "Fluent ANSYS14" by choosing appropriate model parameters. The biomass furnace consists of a vertical cylinder equipped with a swirling co-flow injection burner. The particles of pulverized biomass (OW) are injected transversally into the furnace just near the co-flow exit, via four square-shaped injection nozzles. The non-premixed combustion model with mixture fraction/PDF model for turbulence-chemistry interactions is used. Standard k-epsilon turbulence model closure, discrete phase model (DPM) for tracking the motion of individual particle and P-1 radiation model for flame radiation inside the combustor are used in the numerical simulation. Four different swirl numbers S-n = 0.38, 0.95, 1.2 and 1.42 are used in this study in order to investigate the swirl intensity effect on the flow and combustion behavior. The results showed that the flow and the flame characteristics such as axial velocity, streamlines, gas temperature, flame length and CO2, CO and O-2 concentrations are affected by the swirl intensity.
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The present experimental study aims to investigate the combustion and emission characteristics of the flow through a low swirl injector (LSI). An experimental study was carried out on the flame structure, the temperature distribut...
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The present experimental study aims to investigate the combustion and emission characteristics of the flow through a low swirl injector (LSI). An experimental study was carried out on the flame structure, the temperature distribution and the exhaust emission of low swirl pre-mixed combustion under the condition of different swirl number and different fuel composition. In order to qualitatively analyze the flame structure, the velocity distribution of the non-reacting flow through the LSI was measured using the particle image velocimetry (PIV) technique. Experimental results indicated that: (i) the LSI can generate a blue lift-off "W" type flame which consists of four clusters of flames connected together and holds up a long yellow pulsating flame, (ii) the blue flame structure converts the "W" type flame into the "broom" type flame and the distance between the front of the flame and the nozzle shortens with increasing swirl number, (iii) there exist high temperature region flanked by two peaks on the temperature profiles in the blue flame while uniform higher temperature in yellow pulsating flame, (iv) the NOx and CO emission level of the LSI mainly depends on the gas composition and thermal load. (C) 2016 Elsevier Ltd. All rights reserved.
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Achieving considerable improvement in the ignition performance via flameless combustion is a big challenge. Obtaining uniform temperature field inside the combustor with ultra-low NO_x and CO emission as well as enhanced combustio...
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Achieving considerable improvement in the ignition performance via flameless combustion is a big challenge. Obtaining uniform temperature field inside the combustor with ultra-low NO_x and CO emission as well as enhanced combustion stability remains a challenge. In the present work, a flameless combustion technique is applied using a laboratory scale furnace under the swirling flameless combustion, using natural gas. Gaseous fuel is injected in the direction of the combustor axis and air is allowed to enter along the tangential and axial direction of the combustor to create swirling flow with recirculation as well as enhanced mixing of the hot gas near the fuel nozzle. The experimental results of temperature and emission of combustion process are presented. Three ratios of axial to tangential air entry were studied. These ratios were 7/3 (Case 1), 1 (Case 2), and 3/7(Case 3). Case2 shows the highest temperature followed by Case 1 and 3. The amount of NO_x and CO emission measured at the outlet is observed to be the lowest for Case 3 as a result of the high recirculation and swirling effect.
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The present article analyzes different definitions of the swirl number and their application to control the mixing process in the cylinder of direct injection diesel engines. We first of all discuss the origins of the existing met...
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The present article analyzes different definitions of the swirl number and their application to control the mixing process in the cylinder of direct injection diesel engines. We first of all discuss the origins of the existing methods used to control swirl, which are usually based on the presence of a stirring device, or rotometer, in the cylinder head. A new possibility is then proposed for determining the swirl number, based on equivalent angular velocity, and a comparison made of results obtained using the different techniques available for measuring swirl in motor cylinders.
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