• Title/Summary/Keyword: Reynolds number

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Reynolds Number Effect on Regenerative Pump Performance in Low Reynolds Number Range

  • Horiguchi, Hironori;Yumiba, Daisuke;Tsujimoto, Yoshinobu;Sakagami, Masaaki;Tanaka, Shigeo
    • International Journal of Fluid Machinery and Systems
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    • v.1 no.1
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    • pp.101-108
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    • 2008
  • The effect of Reynolds number on the performance of a regenerative pump was examined in a low Reynolds number range in experiment. The head of the regenerative pump increased at low flow rates and decreased at high flow rates as the Reynolds number decreased. The computation of the internal flow was made to clarify the cause of the Reynolds number effect. At low flow rates, the head is decreased with increasing the Reynolds number due to the decrease of the shear force exerted by the impeller caused by the increase of leakage and hence local flow rate. At higher flow rates, the head is increased with increasing the Reynolds number with decreased loss at the inlet and outlet as well as the decreased shear stress on the casing wall.

Prediction of Critical Reynolds Number in Stability Curve of Liquid Jet ( I )

  • No, S.Y.;Ryu, K.Y.;Rhim, J.H.;Lim, S.B.
    • Journal of ILASS-Korea
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    • v.4 no.1
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    • pp.55-61
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    • 1999
  • The first maximum point in the stability curve of liquid jet, i.e., the critical point is associated with the critical Reynolds number. This critical Reynolds number should be predicted by simple means. In this work, the critical Reynolds number in the stability curve of liquid jet are predicted using the empirical correlations and the experimental data reported in the literatures. The critical Reynolds number was found to be a function of the Ohnesorge number, nozzle lengh-to-diameter ratio, ambient Weber number and nozzle inlet type. An empirical correlation for the critical Reynolds number as a function of the Ohnesorge number and nozzle length-to-diameter ratio is newly proposed here. Although an empirical correlation proposed in this work may not be universal because of excluding the effects of ambient pressure and nozzle inlet type, it has reasonably agrees with the measured critical Reynolds number.

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Reynolds Number Effects on the Non-Nulling Calibration of a Cone-Type Five-Hole Probe for Turbomachinery Applications

  • Lee, Sang-Woo;Jun, Sang-Bae
    • Journal of Mechanical Science and Technology
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    • v.19 no.8
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    • pp.1632-1648
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    • 2005
  • The effects of Reynolds number on the non-nulling calibration of a typical cone-type five-hole probe have been investigated for the representative Reynolds numbers in turbomachinery. The pitch and yaw angles are changed from - 35 degrees to 35 degrees with an angle interval of 5 degrees at six probe Reynolds numbers in range between $6.60{\times}10^3\;and\;3.17{\times}10^4$. The result shows that not only each calibration coefficient itself but also its Reynolds number dependency is affected significantly by the pitch and yaw angles. The Reynolds-number effects on the pitch- and yaw-angle coefficients are noticeable when the absolute values of the pitch and yaw angles are smaller than 20 degrees. The static-pressure coefficient is sensitive to the Reynolds number nearly all over the pitch- and yaw-angle range. The Reynolds-number effect on the total-pressure coefficient is found remarkable when the absolute values of the pitch and yaw angles are larger than 20 degrees. Through a typical non-nulling reduction procedure, actual reduced values of the pitch and yaw angles, static and total pressures, and velocity magnitude at each Reynolds number are obtained by employing the calibration coefficients at the highest Reynolds number ($Re=3.17{\times}10^4$) as input reference calibration data. As a result, it is found that each reduced value has its own unique trend depending on the pitch and yaw angles. Its general tendency is related closely to the variation of the corresponding calibration coefficient with the Reynolds number. Among the reduced values, the reduced total pressure suffers the most considerable deviation from the measured one and its dependency upon the pitch and yaw angles is most noticeable. In this study, the root-mean-square data as well as the upper and lower bounds of the reduced values are reported as a function of the Reynolds number. These data would be very useful in the estimation of the Reynolds-number effects on the non-nulling calibration.

Effect of Flow Pattern of Coolant for Injection Mold on the Deformation of Injection Molding (사출금형 냉각수의 유동 패턴이 사출성형품의 변형에 미치는 영향)

  • Choi, Kye-Kwang;Hong, Seok-Moo;Han, Seong-Ryeol
    • Journal of the Korean Society of Manufacturing Process Engineers
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    • v.14 no.4
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    • pp.92-99
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    • 2015
  • The deformation of injection molding is seriously affected by injection molding conditions, such as melt and mold temperature and injection and holding pressure. In these conditions, the mold temperature is controlled by flowing coolant, which can be classified by the Reynolds number in the mold-cooling channel. In this study, the deformation of the automotive side molding according to the variation of the Reynolds number in the coolant was simulated by Moldflow. In the results, as the Reynolds number was increased, the mold cooling was also increased. However, when the Reynolds number exceeded a certain range, the mold cooling was not increased further. In addition to the Moldflow verification, the mold cooling by the coolant was simulated by CFX. The CFX results confirmed that the Reynolds number significantly influenced the mold cooling. The coolant, which has a high Reynolds number value, quickly cooled the mold. However, the coolant, which has a low Reynolds number value, such as 0 points, hardly cooled the mold. In an injection molding experiment, as the Reynolds number was high, the deformation of the moldings was reduced. The declining tendency of the deformation was similar to the Moldflow results.

A Study on the Development of Low Reynolds Number k-$\varepsilon$ Turbulence Model (저레이놀즈수 k-$\varepsilon$난류모형 개선에 관한 연구)

  • 김명호;신종근;최영돈
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.16 no.10
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    • pp.1940-1954
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    • 1992
  • Fine grid computations were attempted to analyze the turbulent flows in the near wall low Reynolds number region and the numerical analyses were incorporated by a finite-volume discretization with full find grid system and low Reynolds number k-.epsilon. model was employed in this region. For the improvement of low Reynolds number k-.epsilon. model, modification coefficient of eddy viscosity $f_{\mu}$ was derived as a function of turbulent Reynolds number $R_{+}$ and nondimensional length $y^{+}$ from the concept of two length scales of dissipation rate of turbulent kinetic energy. The modification coefficient $f_{\epsilon}$ in .epsilon. transport equation was also derived theoretically. In the turbulent kinetic energy equation, pressure diffusion term was added in order to consider low Reynolds number region effect. The main characteristics of this low Reynolds number k-.epsilon. model were founded as : (1) In high Reynolds number region, the present model has limiting behavior which approaches to the high Reynolds number model. (2) Present low Reynolds number k-.epsilon. model dose not need additional empirical constants for the transport equations of turbulent kinetic energy and dissipation of turbulent kinetic energy in order to consider wall effect. Present low Reynolds number turbulence model was tested in the pipe flow and obtained improved results in velocity profiles and Reynolds stress distributions compared with those from other k-.epsilon. models.s.s.

Influence of Reynolds Number and Scale on Performance Evaluation of Lift-type Vertical Axis Wind Turbine by Scale-model Wind Tunnel Tests

  • Tanino, Tadakazu;Nakao, Shinichiro;Miyaguni, Takeshi;Takahashi, Kazunobu
    • International Journal of Fluid Machinery and Systems
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    • v.4 no.2
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    • pp.229-234
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    • 2011
  • For Lift-type Vertical Axis Wind Turbine (VAWT), it is difficult to evaluate the performance through the scale-model wind tunnel tests, because of the scale effect relating to Reynolds number. However, it is beneficial to figure out the critical value of Reynolds number or minimum size of the Lift-type VAWT, when designing this type of micro wind turbine. Therefore, in this study, the performance of several scale-models of Lift-type VAWT (Reynolds number : $1.5{\times}10^4$ to $4.6{\times}10^4$) was investigated. As a result, the Reynolds number effect depends on the blade chord rather than the inlet velocity. In addition, there was a transition point of the Reynolds number to change the dominant driving force from Drag to Lift.

A study on the critical reynolds number of steady, oscillatory and pulsating flow in a straight duct (직관덕트내에서 정상유동, 진동유동과 맥동유동의 임계레이놀즈수에 관한 연구)

  • 박길문;봉태근
    • Journal of Advanced Marine Engineering and Technology
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    • v.22 no.1
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    • pp.16-20
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    • 1998
  • The critical reynolds number in a square-sectional straight duct is investigated experimentally. The experimental study for the air flow in a square-sectional straight duct is carried out to calssify critical Reynolds number on steady flow and unsteady flow. To calssify the critical Reynolds number we obtained velocity waveform by using a hot-wireanemometer and data acquisition system with photocorder.

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Numerical Study About the Effect of the Low Reynolds Number on the Performance in an Axial Compressor (저 레이놀즈 수가 압축기 성능에 미치는 영향에 대한 수치적 연구)

  • Choi, Min-Suk;Chung, Hee-Taeg;Oh, Seong-Hwan;Ko, Han-Young;Baek, Je-Hyun
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.32 no.2
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    • pp.83-91
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    • 2008
  • A three-dimensional computation was conducted to understand effects of the low Reynolds number on the performance in a low-speed axial compressor at the design condition. The low Reynolds number can originates from the change of the air density because it decreases along the altitude in the troposphere. The performance of the axial compressor such as the static pressure rise was diminished by the separation on the suction surface with full span and the boundary layer on the hub, which were caused by the low Reynolds number. The total pressure loss at the low Reynolds number was found to be greater than that at the reference Reynolds number at the region from the hub to 85% span. Total pressure loss was scrutinized through three major loss categories in a subsonic axial compressor such as the profile loss, the tip leakage loss and the endwall loss using Denton#s loss model, and the effects of the low Reynolds number on the performance were analyzed in detail.

EFFECTS OF THE LOW REYNOLDS NUMBER ON THE PERFORMANCE OF AN AXIAL COMPRESSOR (저 레이놀즈 수가 압축기 성능에 미치는 영향)

  • Choi, Min-Suk;Baek, Je-Hyun;Oh, Seong-Hwan;Ko, Han-Young
    • 한국전산유체공학회:학술대회논문집
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    • 2007.04a
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    • pp.138-141
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    • 2007
  • A three-dimensional computation was conducted to understand effects of the low Reynolds number on the performance in a low-speed axial compressor at the design condition. The low Reynolds number can originates from the change of the air density became it decreases along the altitude in the troposphere. The performance of the axial compressor such as the static pressure rise wag diminished by the separation on the suction surface and the boundary layer on the hub, which were caused by the low Reynolds number. The total pressure loss at the low Reynolds number was found to be greater than that at the reference Reynolds number at the region from the hub to 90% span. Total pressure loss was scrutinized through three major loss categories in a subsonic axial compressor such as profile loss, tip leakage loss and endwall loss using Denton's loss model, and effects of the low Reynolds number on the performance were analyzed in detail.

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Effects of the Low Reynolds Number on the Loss Characteristics in a Transonic Axial Compressor

  • Choi, Min-Suk;Oh, Seong-Hwan;Ko, Han-Young;Baek, Je-Hyun
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2008.03a
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    • pp.202-212
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    • 2008
  • A three-dimensional computation was conducted to understand effects of the low Reynolds number on the loss characteristics in a transonic axial compressor, Rotor67. As a gas turbine becomes smaller in size and it is operated at high altitude, the operating condition frequently lies at low Reynolds number. It is generally known that wall boundary layers are thickened and a large separation occurs on the blade surface in axial turbomachinery as the Reynolds number decreases. In this study, it was found that the large viscosity did not affect on the bow shock at the leading edge but significantly did on the location and the intensity of the passage shock. The passage shock moved upstream towards leading edge and its intensity decreased at the low Reynolds number. This change had large effects on the performance as well as the internal flows such as the pressure distribution on the blade surface, tip leakage flow and separation. The total pressure rise and the adiabatic efficiency decreased about 3% individually at the same normalized mass flow rate at the low Reynolds number. In order to analyze this performance drop caused by the low Reynolds number, the total pressure loss was scrutinized through major loss categories such as profile loss, tip leakage loss, endwall loss and shock loss.

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