• Title, Summary, Keyword: free-convective flow

Search Result 25, Processing Time 0.034 seconds

Effects of Boundary Conditions on Redevelopment of the Boundary Layer in a Backward-Facing Step Flow (후향단유동내 경계층의 재발달에 미치는 경계조건의 영향)

  • Kim, Dong-Il;Lee, Moon-J.;Chun, Chung-Hwan
    • Proceedings of the KSME Conference
    • /
    • /
    • pp.506-511
    • /
    • 2001
  • This paper presents how redevelopment of the boundary layer in a backward-facing step flow is affected by boundary conditions imposed on velocity at the inlet, top and exit of the flow. A two-dimensional, laminar, incompressible flow over a backward-facing step with an open top boundary has been computed by using numerical methods of second-order time and spatial accuracy and a fractional-step method that guarantees a divergence-free velocity field at all time. The inlet velocity profile above the step is of Blasius type. Along the top boundary, shear-tree and Dirichlet conditions on the streamwise velocity were considered and at the exit fully-developed and convective boundary conditions were examined. (The vertical velocity at all boundaries were assumed to be zero explicitly or implicitly.) From the computed flow fields, the reattachment on the bottom side of shear layer separated from the tip of the step and succeeding redevelopment of the boundary layer were investigated.

  • PDF

The study of predictive performance of low Reynolds number turbulence model in the backward-facing step flow (후방계단유동에 대한 저레이놀즈 수 난류모형의 예측성능에 관한 연구)

  • Kim, Won-Gap;Choe, Yeong-Don
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.20 no.5
    • /
    • pp.1661-1670
    • /
    • 1996
  • Incompressible flow over a backward-facing step is computed by low Reynolds number turbulence models in order to compare with direct simulation results. In this study, selected low Reynolds number 1st and 2nd (Algebraic Stress Model : ASM) moment closure turbulence models are adopted and compared with each other. Each turbulence model predicts different flow characteristics, different re-attachment point, velocity profiles and Reynolds stress distribution etc. Results by .kappa.-.epsilon. turbulence models indicate that predicted re-attachment lengths are shorter than those by standard model. Turbulent intensity and eddy viscosity by low Reynolds number .kappa.-.epsilon. models are still greater than DNS results. The results by algebraic stress model (ASM) are more reasonable than those by .kappa.-.epsilon. models. The convective scheme is QUICK (Quadratic Upstream Interpolation for Convective Kinematics) and SIMPLE algorithm is adopted. Reynolds number based on step height and inlet free stream velocity is 5100.

PREDICTION OF FREE SURFACE FLOW ON CONTAINMENT FLOOR USING A SHALLOW WATER EQUATION SOLVER

  • Bang, Young-Seok;Lee, Gil-Soo;Huh, Byung-Gil;Oh, Deog-Yeon;Woo, Sweng-Woong
    • Nuclear Engineering and Technology
    • /
    • v.41 no.8
    • /
    • pp.1045-1052
    • /
    • 2009
  • A calculation model is developed to predict the transient free surface flow on the containment floor following a loss-of-coolant accident (LOCA) of pressurized water reactors (PWR) for the use of debris transport evaluation. The model solves the two-dimensional Shallow Water Equation (SWE) using a finite volume method (FVM) with unstructured triangular meshes. The numerical scheme is based on a fully explicit predictor-corrector method to achieve a fast-running capability and numerical accuracy. The Harten-Lax-van Leer (HLL) scheme is used to reserve a shock-capturing capability in determining the convective flux term at the cell interface where the dry-to-wet changing proceeds. An experiment simulating a sudden break of a water reservoir with L-shape open channel is calculated for validation of the present model. It is shown that the present model agrees well with the experiment data, thus it can be justified for the free surface flow with accuracy. From the calculation of flow field over the simplified containment floor of APR1400, the important phenomena of free surface flow including propagations and interactions of waves generated by local water level distribution and reflection with a solid wall are found and the transient flow rates entering the Holdup Volume Tank (HVT) are obtained within a practical computational resource.

An Experimental Investigation on Combined Convective Heat Transfer of NonNewtonian Fluids (비뉴톤유체의 복합대류 열전달에 관한 실험적 연구)

  • 김용진
    • Transactions of the Korean Society of Mechanical Engineers
    • /
    • v.19 no.7
    • /
    • pp.1770-1779
    • /
    • 1995
  • A combined convective heat transfer study for non-Newtonian fluids was experimentally performed in uniformly heated horizontal tubes with laminar flow in the thermal entry region. Velocity profiles were fully developed at the entrance of the heated sections in the tubes. Aqueous solutions of sodium carboxymethylcellulose(CMC ) were used; their behavior showed a reasonably good fit into the power-law model, .tau.=K.gamma.$^{n}$ . The test sections were made of copper with inside diameters of 3.23 cm and 5.042 cm and lengths of approximately 300 cm. Most experimental runs displayed noticeable secondary flows caused by buoyancy ; when present, secondary flows caused significant increase in the rate of heat transfer over the purely forced-convection case. A correlation, which relates the rate of heat transfer for flows with temperature-dependent properties, free convection effects, and non-newtonian effects, was suggested.

Influence of the Wake Behind Rectangular Bars on the Flow and Heat Transfer in the Linear Turbine Cascade (사각주 후류가 선형터빈익렬의 유동 및 열전달에 미치는 영향에 관한 연구)

  • Yoon, Soon Hyun;Sim, Jae Kyung;Woo, Chang Soo;Lee, Dae Hee
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.23 no.7
    • /
    • pp.864-870
    • /
    • 1999
  • An experimental study Is conducted in a four-vane linear cascade in order to examine the influence of the wake behind rectangular bars on the flow and heat transfer characteristics. Flow and heat transfer measurements are made for the inlet Reynolds number of 66000(based on chord length and free-stream velocity). Turbulent intensity and stress are measured using a hot-wire anemometer, and to measure the convective heat transfer coefficients on the blade surface liquid crystal/gold film Intrex technique is used. Each of experimental cases is characterized by the unsteadiness measured at the entrance of the cascade. The wake behind the rectangular bars enhances the turbulent motion of the flow in the cascade passage. It also promotes the boundary layer development and transition. The results show that heat transfer coefficients on the blade surface increase with increasing unsteadiness.

Numerical Study on Characteristics of Ship Wave According to Shape of Waterway Section

  • Hong Chun-Beom;Lee Sang-Min
    • Journal of Mechanical Science and Technology
    • /
    • v.19 no.12
    • /
    • pp.2263-2269
    • /
    • 2005
  • The ship wave phenomena in the restricted waterway were investigated by a numerical analysis. The Euler and continuity equations were employed for the present study. The boundary fitted and moving grid system was adopted to enhance the computational efficiency. The convective terms in the governing equations and the kinematic free surface boundary condition were solved by the Constrained Interpolated Profile (CIP) algorithm in order to solve accurately wave heights in far field as well as near field. The advantage of the CIP method was verified by the comparison of the computed results by the CIP and the Maker and Cell (MAC) method. The free surface flow simulation around Wigley hull was performed and compared with the experiment for the sake of the validation of the numerical method. The present numerical scheme was applied to the free surface simulation for various canal sections in order to understand the effect of the sectional shape of waterways on the ship waves. The wave heights on the side wall and the shape of the wave patterns with their characteristics of flow are discussed.

Flow Visualizations and Hot-Wire Measurements on Air Flow in Two Different Neonate Incubators

  • Kim, Young-Ho;Yoo, Seoung-Chool;Kwon, Chi-Ho
    • Journal of Mechanical Science and Technology
    • /
    • v.15 no.7
    • /
    • pp.1051-1060
    • /
    • 2001
  • Flow visualizations and hot-wire measurements on the inside flows of two different incubators are presented in this paper. An anatomically-correct neonate model was fabricated using the rapid prototyping machine, based on the 3-D scanned data. The result showed that air flow in the incubator was affected not only by the air circulation system but also by the design of incubator chamber. Large rotating motions were located around the corners of free space. A number of small eddies were found in regions of high shear flow, in areas such as that between the air inlet and the neonate. But, these small eddies were found to be stationary at that locations. Those small eddies might interfere with convective and evaporative heat transfers from the neonate. This study has led to a better understanding of flow mechanism in an incubator chamber and provided the guidance needed for the advancement of improved computational fluid dynamic models.

  • PDF

Free Convective Transition of Intermediate Prandtl-Number Fluids in a Wide-Gap Horizontal Annulus (넓은 수평 환형 공간에서의 중간 Prandtl수 유체의 자연 대류의 천이)

  • Yoo, Joo-Sik
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.24 no.2
    • /
    • pp.169-176
    • /
    • 2000
  • Natural convection in a wide-gap horizontal annulus is considered, and the transition of flows and the bifurcation phenomenon are investigated for the fluids with Pr=0.2 and 0.3. At Pr=0.2, a bicellular flow pattern is observed at high Rayleigh number, and the solution is unique. At Pr=0.3, both the steady unicellular and bicellular flows exist above a certain critical Rayleigh number. For the fluids of Pr=0.2, the bicellular flow can be obtained by the impulsive heating of the inner cylinder, but it is not obtained from the zero initial condition for Pr=0.3. Hysteresis phenomena have not been observed. A transition from a bicellular flow to a unicellular flow occurs for Pr=0.3.

A Study on the Flame Stability of Porous Ceramic Burner (다공성세라믹버너의 화염안정화에 관한 연구)

  • Lee, Do-Hyung;Yun, Bong-Seok
    • Journal of Power System Engineering
    • /
    • v.20 no.4
    • /
    • pp.12-18
    • /
    • 2016
  • Typical boiler system consists of combustion chamber and heat exchanger in one housing, therefore the size of boiler system is large and the heat exchanging efficiency becomes low. At these boiler systems, because the combustible mixture fires as free flame in the combustion chamber, consequently the combusted hot gas heats the heat exchanger only as conductive and convective heat transfer. The present Porous Ceramic Burner concept is that combustion process is occurred at the gaps of the porous ceramic materials, and the heat exchanger is placed in the same porous materials. Therefore we can reduce the boiler size, and we can also use radiative heat transfer from ceramic material with conductive and convective heat transfer from combusted gas throwing the porous materials. The purpose of this study is to search the flame stability ranges at different fuel flow rate and excess air ratio burning in the $Al_2O_3$ ceramic balls. We found out the stable excess air ratio range on given combustion intensity. And we can get clean porous ceramic combustion results compared with free flame.

Influence of the Unsteady Wake on the Flow and Heat Transfer in a Linear Turbine Cascade (비정상 후류가 선형터빈익렬의 유동 및 열전달에 미치는 영향에 관한 연구)

  • Yun, Sun-Hyeon;Sim, Jae-Gyeong;Kim, Dong-Geon
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.25 no.2
    • /
    • pp.164-170
    • /
    • 2001
  • The influence of unsteady wake on the flow and heat transfer characteristics in a four-vane linear cascade was experimentally investigated. The unsteady wake was generated with four rotating rectangular plates located upstream of the cascade. Tested inlet Reynolds number based on chord length was set to 66,000 by controlling free-stream velocity. A hot-wire anemometer system was employed to measure turbulent velocity components. For the convective heat transfer coefficients measurement on turbine blade surface, thermochromic liquid crystal and gold film Intrex were used. It was found that the unsteady wake enhances the turbulent motion in the cascade passage and accordingly promotes the development and transition of boundary layer. It was found that the heat transfer coefficients on the blade surface increase as the plate rotating speed increases. However, the increasing of heat transfer coefficients is not significant in the case that Strouhal number is higher than 0.503.