• Title/Summary/Keyword: Convective velocity

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Numerical Analysis of Convective Heat and Mass Transfer around Human Body under Strong Wind

  • Li, Cong;Ito, Kazuhide
    • International Journal of High-Rise Buildings
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    • v.1 no.2
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    • pp.107-116
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    • 2012
  • The overarching objective of this study is to predict the convective heat transfer around a human body under forced strong airflow conditions assuming a strong wind blowing through high-rise buildings or an air shower system in an enclosed space. In this study, computational fluid dynamics (CFD) analyses of the flow field and temperature distributions around a human body were carried out to estimate the convective heat transfer coefficient for a whole human body assuming adult male geometry under forced convective airflow conditions between 15 m/s and 25 m/s. A total of 45 CFD analyses were analyzed with boundary conditions that included differences in the air velocity, wind direction and turbulence intensity. In the case of approach air velocity $U_{in}=25m/s$ and turbulent intensity TI = 10%, average convective heat transfer coefficient was estimated at approximately $100W/m^2/K$ for the whole body, and strong dependence on air velocity and turbulence intensity was confirmed. Finally, the formula for the mean convective heat transfer coefficient as a function of approaching average velocity and turbulence intensity was approximated by using the concept of equivalent steady wind speed ($U_{eq}$).

Effect of the Hydraulic Boundary Layer on the Convective Heat Transfer in Porous Media (유동 경계층이 다공성물질내 대류 열전달에 미치는 영향)

  • Jin, Jae-Seek;Lee, Dae-Young;Kang, Byung-Ha
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.24 no.8
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    • pp.1119-1127
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    • 2000
  • Convective heat transfer in a channel filled with porous media has been analyzed in this paper. The two-equation model is applied for the heat transfer analysis with the velocity profile, considering both the inertia and viscous effects. Based on a theoretical solution, the effect of the velocity profile on the convective heat transfer is investigated in detail. The Nusselt number is obtained in terms of the relevant physical parameters, such as the Biot number for the internal heat exchange, the ratio of effective conductivities between the fluid and solid phases, and hydraulic boundary layer thickness. The results indicate that the influence of the velocity profile is characterized within two regimes according to the two parameters, the Biot number and the conductivity ratio between the phases. The decrease in the heat transfer due to the hydraulic boundary layer thickness is 15% at most within a practical range of the pertinent parameters.

Forced Convective Boiling of Refrigerant-Oil Mixtures in a Bundle of Enhanced Tubes Having Pores and Connecting Gaps

  • Park, Ji-Hoon;Kim, Nae-Hyun;Kim, Do-Young;Byun, Ho-Won;Choi, Yong-Min;Kim, Soo-Hwan
    • International Journal of Air-Conditioning and Refrigeration
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    • v.17 no.3
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    • pp.81-87
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    • 2009
  • The effect of oil on convective boiling of R-123 in an enhanced tube bundle is experimentally investigated at $26.7^{\circ}C$ saturation temperature. The enhanced tube had pores (0.23 mm diameter) and connecting gaps (0.07 mm width), which had been optimized using pure R-123. The effects of oil concentration (0 to 5%), heat flux (10 to $40\;kW/m^2$), mass velocity (8 to $26\;kg/m2^s$) and vapor quality are investigated. The oil significantly reduces the bundle boiling heat transfer coefficient. With 1% oil, the reduction is approximately 35%. Further addition of oil further reduces the heat transfer coefficient. The data are also compared with the pool boiling counterpart. The reduction in the heat transfer coefficient is smaller in a bundle (convective boiling) than in a pool (single-tube pool boiling), with larger difference at a smaller heat flux. Similar to pure R-123 case, the effects of mass velocity and vapor quality are negligible for the convective boiling of R-123/oil mixture.

Vaporization Characteristics of Supercritical Hydrocarbon Fuel Droplet in Convective Nitrogen Environments (유동이 있는 초임계 질소 환경에서 탄화수소 연료 액적의 기화 특성)

  • Lim Jong-Hyuk;Lee Bong-Su;Koo Ja-Ye
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.28 no.10
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    • pp.1279-1287
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    • 2004
  • The vaporization characteristics of a liquid heptane droplet in a supercritical nitrogen flow are numerically studied. The transient conservation equations of mass, momentum, energy, and species are expressed in an axisymmetric coordinate system. The governing equations are solved time marching method with preconditioning scheme. The modified Soave-Redlich-Kwong equation of state is employed for taking account of real gas effects such as thermodynamic non-ideality and transport anomaly. Changing the convective velocity and ambient pressure, several parametric studies are conducted. The numerical results show that the two parameters, Reynolds number and dimensionless combined parameter(${\mu}$s/${\mu}$d)(equation omitted), have influence on supercritical droplet vaporization.

Case Study on the Physical Characteristics of Precipitation using 2D-Video Distrometer (2D-Video Distrometer를 이용한 강수의 물리적 특성에 관한 사례연구)

  • Park, Jong-Kil;Cheon, Eun-Ji;Jung, Woo-Sik
    • Journal of Environmental Science International
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    • v.25 no.3
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    • pp.345-359
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    • 2016
  • This study analyze the synoptic meteorological cause of rainfall, rainfall intensity, drop size distribution(DSD), fall velocity and oblateness measured by the 2D-Video distrometer(2DVD) by comparing two cases which are heavy rainfall event case and a case that is not classified as heavy rainfall but having more than $30mm\;h^{-1}$ rainrate in July, 2014 at Gimhae region. As a results; Over the high pressure edge area where strong upward motion exists, the convective rain type occurred and near the changma front, convective and frontal rainfall combined rain type occurred. Therefore, rainrate varies based on the synoptic meteorological condition. The most rain drop distribution appeared in the raindrops with diameters between 0.4 mm and 0.6 mm and large particles appeared for the convective rain type since strong upward motion provide favorable conditions for the drops to grow by colliding and merging so the drop size distribution varies based on the location or rainfall types. The rainfall phases is mainly rain and as the diameter of the raindrop increase the fall velocity increase and oblateness decrease. The equation proposed based on the 2DVD tends to underestimated both fall velocity and oblateness compared with observation. Since these varies based on the rainfall characteristics of the observation location, standard equation for fall velocity and oblateness fit for Gimhae area can be developed by continuous observation and data collection hereafter.

SOME PROBLEMS IN THE STELLAR CONVECTION ZONE - ON THE PRESSURE EQUILIBRIUM ASSUMPTION - (환성(恒星)의 대류층(對流層)의 문제점(問題点) - 압력평형(壓力平衡)의 가정(假定)에 관하여 -)

  • Hyun, J.J.
    • Journal of The Korean Astronomical Society
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    • v.12 no.1
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    • pp.1-5
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    • 1979
  • The usual assumption of the pressure equilibrium between the convective elements and the surrounding fluid has been dropped, and the effects of the pressure perturbation of the convective element on its velocity and T perturbation have been estimated.

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Combustion Instability and Active Control in a Dump Combustor (덤프 연소기에서의 연소불안정과 능동제어에 대한 연구)

  • Ahn Kyu-Bok;Yu Kenneth;Yoon Young-Bin
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2005.11a
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    • pp.445-449
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    • 2005
  • The mixed acoustic-convective mode combustion instability and the possibility of combustion control using a loudspeaker to these instabilities were studied. By changing inlet velocity, combustor length and equivalence ratio, the dynamic pressure signals and the flame structures were simultaneously taken. The results showed that as the combustor length increased and the inlet velocity decreased, the instability frequency decreased and the maximum power spectral densities of the dynamic pressures generally decreased. The instability frequency could be affected by an equivalence ratio over the operating conditions. From the data of close-loop control, as the loudspeaker may work out-of-phase with the natural instability, the optimum time-delay controller was confirmed to be able to reduce the vortex shedding from the mixed acoustic-convective mode combustion instability.

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Convective Boiling Two-phase Flow in Trapezoidal Microchannels : Part 1-Pressure Drop Characteristics (사다리꼴 미세유로의 대류비등 2상유동 : 1부-압력강하 특성)

  • Kim, Byong-Joo;Kim, Geon-Il
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.23 no.1
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    • pp.87-94
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    • 2011
  • Characteristics of two-phase pressure drop in microchannels were investigated experimentally. The microchannels consisted of 9 parallel trapezoidal channels with each channel having $205\;{\mu}m$ of bottom width, $800\;{\mu}m$ of depth, $3.6^{\circ}$ of sidewall angle, and 7 cm of length. Pressure drops in convective boiling of Refrigerant 113 were measured in the range of inlet pressure 105~195 kPa, mass velocity $150{\sim}920\;kg/m^2s$, and heat flux $10{\sim}100\;kW/m^2$. The total pressure drop generally increased with increasing mass velocity and/or heat flux. Two-phase frictional pressure drop across the microchannels increased rapidly with exit quality and showed bigger gradient at higher mass velocity. A critical review of correlations in the literature suggested that existing correlations were not able to match the experimental results obtained for two-phase pressure drop associated with convective boiling in microchannels. A new correlation suitable for predicting two-phase friction multiplier was developed based on the separated flow model and showed good agreement with the experimental data.

Experimental Study on Nonlinearity Characteristics Near the Free Surface in the Regular Wave Condition

  • Choi, Hae-Jin;Jung, Kwang-Hyo;Suh, Sung-Bu;Jo, Hyo-Jae;Choi, Han-Suk
    • Journal of Ocean Engineering and Technology
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    • v.24 no.1
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    • pp.1-9
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    • 2010
  • A series of experiments employing particle image velocimetry (PIV) technique was conducted to produce benchmark wave kinematics data for regular waves having four different wave slopes in 2-D wave tank. Water velocities and accelerations near the free surface of regular waves were computed from image pair obtained by PIV systems. With the measured wave velocity field, the wave accelerations were computed using a centered finite difference scheme. Both local and convective components of the total accelerations are obtained from experimental data. With increasing the wave slope, the horizontal velocity and the vertical accelerations near the wave crest obtained by PIV technique became larger than theoretical results, which are well-known phenomena of the wave nonlinearity. It is noted that the relative magnitude of convective acceleration to the local acceleration became larger with increasing wave slope.

Rapid Cooling Mechanism Utilizing Acoustic Streaming Generated by Ultrasonic Vibrations (초음파 진동에 의해 발생된 음향유동을 활용한 급속냉각 메카니즘)

  • Loh, Byoung-Gook;Kwon, Ki-Jung;Lee, Dong-Ryul
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.16 no.10 s.115
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    • pp.1057-1066
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    • 2006
  • Acoustic streaming Induced by longitudinal vibration at 30 kHz is visualized for a test fluid flow between the stationary glass plate and ultrasonic vibrating surface with particle imaging velocimetry (PIV) To measure an increase in the velocity of air flow due to acoustic streaming, the velocity of air flow in a gap between the heat source and ultrasonic vibrator is obtained quantitatively using PIV. The ultrasonic wave propagating into air in the gap generates steady-state secondary vortex called acoustic streaming which enhances convective cooling of the stationary heat source. Heat transfer through air in the gap is represented by experimental convective heat transfer coefficient with respect to the gap. Theoretical analysis shows that gaps for maximum heat transfer enhancement are the multiple of half wavelength. Optimal gaps for the actual design are experimentally found to be half wavelength and one wavelength. A drastic temperature variation exists for the local axial direction of the vibrator according to the measurement of the temperature distribution in the gap. The acoustic streaming velocity of the test fluid in the gap is at maximum when the gap agrees with the multiples of half wavelength of the ultrasonic wave, which are specifically 6 mm and 12 mm.