• 제목/요약/키워드: Convective velocity

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

  • Li, Cong;Ito, Kazuhide
    • 국제초고층학회논문집
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    • 제1권2호
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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 ($U_{eq}$).

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

  • 진재식;이대영;강병하
    • 대한기계학회논문집B
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    • 제24권8호
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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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    • 제17권3호
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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.

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

  • 임종혁;이봉수;구자예
    • 대한기계학회논문집B
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    • 제28권10호
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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.

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

  • 박종길;천은지;정우식
    • 한국환경과학회지
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    • 제25권3호
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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.

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

  • 현정준
    • 천문학회지
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    • 제12권1호
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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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덤프 연소기에서의 연소불안정과 능동제어에 대한 연구 (Combustion Instability and Active Control in a Dump Combustor)

  • 안규복;;윤영빈
    • 한국추진공학회:학술대회논문집
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    • 한국추진공학회 2005년도 제25회 추계학술대회논문집
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    • pp.445-449
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    • 2005
  • 혼합 acoustic-convective 모드 연소불안정과 스피커를 이용한 능동제어의 가능성에 대한 연구를 수행 하였다. 유입속도, 연소실 길이, 당량비를 변화시켜 가면서 동압과 화염구조를 동시 측정하였다. 유입속도와 연소실 길이는 덤프 연소기에서의 와류 생존시간에 영향을 주기 때문에, 연소길 길이가 길어질수록 그리고 유입속도가 작아질수록 연소불안정의 주파수는 작아지고, 동압에서 얻어진 최대 전력스펙트럼밀도 또한 전반적으로 작아지는 경향을 보였다. 당량비에 따라 불안정의 강도와 주파수 특성도 변했는데, 당량비의 증가에 따라 불안정 주파수와 연소불안정 강도는 증가하는 경향을 나타내었다. 폐루프 방식의 제어를 통하여 스피커를 이용한 능동제어는 이러한 혼합 acoustic-convective 모드 연소불안정으로 발생하는 와류의 발전을 감소시킬 수 있음을 확인하였다.

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

  • 김병주;김건일
    • 설비공학논문집
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    • 제23권1호
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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
    • 한국해양공학회지
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    • 제24권1호
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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.

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

  • 노병국;권기정;이동렬
    • 한국소음진동공학회논문집
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    • 제16권10호
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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.