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Selected Papers from the Eighth International Conference on Multiphase Flow
No full textX110sciescopu
EFFECT OF NON-CONDENSABLE GAS AND WAVY INTERFACE ON THE CONDENSATION HEAT-TRANSFER IN A NEARLY HORIZONTAL PLATE
No full textAn experimental study was conducted to investigate the effect of non-condensable gas and wavy water film on condensation heat transfer. The experiment was performed in a nearly horizontal (4.1 degrees) square duct of 0.1 m height, 0.15 m width and 1.52 m length at atmospheric pressure. A water film in a steady thermal condition was injected to simulate the effect of a wavy interface on the condensation. The experimental data for the heat transfer coefficient and the interfacial structure of the wavy condensate were obtained along with the three parameters: air mass fraction, mixture velocity and film flow rate. When the interface is smooth, the heat transfer coefficients with or without non-condensable gas agree reasonably with the previous theories. The waviness of condensate film increases the heat transfer up to several tenths of a per cent.X1110sciescopu
The fabrication of a single glass microchannel to study the hydrophobicity effect on two-phase flow boiling of water
No full textHydrophobicity is an important parameter in a microscale two-phase thermal hydraulic system because the interfacial force has an influential role in this system. Previous research has been limited to the two-phase flow regime, with most results indicating that the surface hydrophobicity is an influential parameter. However, the hydrophobicity effect on flow boiling has not been studied, especially at the microscale. This is due to the coupled difficulties of the fabrication of the microchannel and the modification of its surface. This study develops the methods for the fabrication of a single glass microchannel and the formation of a hydrophobic surface. The flow boiling experiment in the microchannel is conducted using deionized water. The visualization of two-phase flow patterns was carried out using a high-speed camera and a long-distance microscope. Very clear images were obtained due to the high transparency of the microchannel material. Local dry patches were observed in the liquid film of elongated bubbles and a new flow pattern was found with the hydrophobic surface.X1110sciescopu
MODELING OF CONDENSATION HEAT-TRANSFER IN A REACTOR CONTAINMENT
No full textThis paper proposes a set of condensation models for forced and natural convection in the presence of a noncondensable gas. A simple model is derived by using the analogy between mass, momentum and energy transfer. The effects of a wavy interface are implemented in this model by using correlations for a rough wall surface. A two-dimensional condensation model using a κ-ε{lunate} model for the turbulent vapor-air flow was also developed to investigate the effect of two-dimensional flow and to provide a sound theoretical basis for the simple model. Each model is compared with the available 'eparate' effects' experimental data. The forced convection model is then compared to the Carolinas Virginia Tube Reactor (CVTR) integral test by using the vapor-air velocity predicted by a separate two-dimensional fluid dynamics model. The effect of countercurrent flow is also considered in this comparison. The natural convection model is also compared to the steady-state integral data of Tagami. The comparison shows good agreement with both sets of experimental data. © 1990.X1128Nsciescopu
Experimental study of the effects of flow acceleration and buoyancy on heat transfer in a supercritical fluid flow in a circular tube
No full textExperiments on turbulent heat transfer by supercritical CO2 in a vertical upward flow were conducted in a tube with an inner diameter of 4.5 mm. The experiments were performed for bulk fluid temperatures ranging from 29 to 115 degrees C, pressures ranging from 74.6 to 102.6 bar, local wall heat fluxes ranging from 38 to 234 kW/m(2), and mass fluxes ranging from 208 to 874 kg/m(2) s. The wall temperature distributions were significantly influenced by wall heat flux and mass flux. The wall temperature had a noticeable peak value when the wall heat flux was moderate and the mass flux was low. To determine the buoyancy and flow acceleration effects on heat-transfer characteristics, the ratios of the Nusselt numbers obtained from the experimental data and from a reference correlation are compared with Bo* and q(+) along the test section. To analyze the changes in the shear stress distribution due to flow acceleration and buoyancy effects, the ratios of the shear stress reduction to wall shear stress due to flow acceleration and buoyancy effects are derived from approximate considerations. A new heat-transfer correlation is proposed, which could be used to predict heat transfer phenomena in a vertical upward flow of a supercritical fluid. The correlation is assessed by comparison with various experimental data. (C) 2010 Elsevier B.V. All rights reserved.X112729sciescopu
Heat transfer in a supercritical fluid: Classification of heat transfer regimes
No full textBecause of the dramatic variation of physical properties with a modest change of temperature, no existing engineering correlation or models can accurately predict heat transfer of supercritical fluids. This paper seeks to classify the conditions where the existing models are applicable and to better understand these local heat transfer mechanisms. The first objective is the focus of this paper. FLUENT was employed to compute the wall temperatures for various heat flux and mass flux conditions and to be compared with experimental data. Because the model was developed for a wide range of flow conditions, it was necessary to make certain assumptions. The simulations showed a good agreement with high mass flux conditions, where buoyancy effects could be neglected. The FLUENT model, however, had difficulty predicting the localized low heat transfer rates seen in the combined condition of high heat flux and low mass flux. A new generalized parameter, dependent on the heat and mass flux, was developed to classify under which conditions this FLUENT standard model was applicable. This global Froude number can be used as the parameter to predict under which conditions the buoyancy effect will be dominant and lower heat transfer rates will occur.X119sciescopu
THE RELATION BETWEEN THE INTERFACIAL SHEAR-STRESS AND THE WAVE MOTION IN A STRATIFIED FLOW
No full textA model is developed to describe the interfacial shear stress at a wavy interface with wave characteristics. Experiments have been conducted with near-horizontal (4.1-degrees) and near-vertical (87-degrees) flat plates for air-water concurrent stratified flows. The interfacial shear stress and normal and flow directional interface velocities were measured. Assuming a simple relation between the gas velocity near the interface and film thickness, the turbulent shear stress due to the wave motion is expressed by the normal interfacial velocity and the relative gas velocity. The model developed agrees well with the experimental data for a wide range of horizontal and vertical stratified flow.X119sciescopu
Experimental study of the gravity effect on the distribution of refrigerant flow in a multi-pass condenser
No full textGravity in multi-pass condensers affects the refrigerant flow rate distribution, owing to the gravitational pressure drop that occurs mainly in the U-bend tubes in fin and tube condensers with horizontal tubes. This effect was studied using an experimental approach. A condenser with two 'nU' circuits was selected, and the temperature variation of the refrigerant side was measured and compared along each circuit. The critical air velocity, which indicated the initiation of the gravity effect, was found for a given refrigerant flow rate. As the air velocity increased beyond the critical air velocity, the gravity effect (or mal-distribution of the refrigerant flow) developed further. Similarly, the critical refrigerant flow rate was also determined for a given air velocity. As the refrigerant flow rate decreased below the critical refrigerant flow rate, the gravity effect also developed further. The gravity-affected region was shown in the table with rows of air velocities and columns of refrigerant flow rates, and expressed using a single parameter for a given refrigerant flow circuit. (C) 2004 Elsevier Ltd and IIR. All rights reserved.X111sciescopu
Modeling of condensation heat transfer for a PRHRS heat exchanger in a SMART-P plant
No full textA theoretical model using a heat and mass transfer analogy and a simple model using Lee and Kim's [Lee, K.-Y., Kim, M.H., 2008a. Experimental and empirical study of steam condensation heat transfer with a noncondensable gas in a small-diameter vertical tube. Nucl. Eng. Des. 238, 207-216] correlation were developed to investigate steam condensation in the presence of a noncondensable gas inside a vertical tube Submerged in pool water. Rohsenow's correlation was used to consider the secondary pool-boiling heat transfer. Both models were assessed with the experimental data of Oh and Revankar [Oh, S., Revankar, S.T., 2005a. Investigation of the noncondensable effect and the operational modes of the passive condenser system. Nucl. Technol. 152, 71-86; Oh, S., Revankar, S.T., 2005b. Effect of noncondensable gas in a vertical tube condenser. Nucl. Eng. Des. 235, 1699-1712; Oh, S., Revankar, S.T., 2005c. Complete condensation in a vertical tube passive condenser. Int. Commun. Heat Mass Trans. 32, 593-602; Oh, S., Revankar, S.T., 2005d. Analysis of the complete condensation in a vertical tube passive condenser. Int. Commun. Heat Mass Trans. 32, 716-727; Ch, S., Revankar, S.T., 2006. Experimental and theoretical investigation of film condensation with noncondensable gas. Int. J. Heat Mass Trans. 49, 2523-2534; Oh, S., Gao, H., Revankar, S.T., 2007. Investigation of a passive condenser system of an advanced boiling water reactor. Nucl. Technol. 158, 208-218] for low pressure and Kim [Kim, S.J., 2000. Turbulent film condensation of high pressure steam in a vertical tube of passive secondary condensation system. Ph.D. dissertation, Korea Advanced institute of Science and Technology] for high pressure, which were obtained from in-tube steam condensation with air in the pool water. These models predicted the data of Oh and Revankar well, but they slightly underestimated the data of Kim. The design of the Passive Residual Heat Removal System (PRHRS) condensation heat exchanger was evaluated with the theoretical model at real operating conditions (e.g., secondary pool-boiling, high system pressure). The PRHRS condensation heat exchanger designed was estimated to remove sufficiently the remaining heat in a reactor during a major accident. (C) 2008 Elsevier BLV. All rights reserved.X111sciescopu
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