Simulation of heat transfer of supercritical water in obstacle-bearing vertical tube

The heat transfer coefficient is very low at bulk temperatures higher than the pseudo-critical point,because the supercritical pressure leads to a vapor-like fluid.In this paper,the heat transfer downstream an obstacle-bearing vertical tube is simulated by the CFD code of Fluent 6.1,using an adaptive grid in the supercritical condition.The reliable results are obtained by the RNG k-ε model using the enhanced wall treatment.The blockage ratio and local temperature of obstacle affect greatly the heat transfer enhancement,and the resultant influence region and decay trend are compared with the existing equations.     

Forced convection condensation in the presence of noncondensable gas in a horizontal tube; experimental and theoretical study

To have a better understanding on forced convection condensation with noncondensable gas inside a horizontal tube, an experimental research and theoretical investigation were conducted under annular and wavy flow. The effects of noncondensable gas mass concentration, mixture gases velocity, pressure and inner wall sub-cooling on the condensation heat transfer have been analyzed. The results indicate that the local heat transfer coefficient increases with the increase of the mixture inlet velocity and pressure while decreases with the increase of the noncondensable mass fraction and wall sub-cooling. Based on the above conclusions, an empirical correlation for predicting the local heat transfer coefficient was proposed which showed a good agreement with the experimental data with an error of ±20%. Furthermore, a theoretical model using the heat and mass transfer (HMT) analogy method was developed including the suction effect. The heat transfer capacity for the film, gaseous boundary and convective heat transfer of the bulk gases were compared along the tube. Besides, the axial distribution of the bulk gases and liquid–gas interface temperatures inside the tube were analyzed. The present theoretical model fits better with the experimental data compared with Lee's and Caruso's models for stratified flow.     

Numerical simulation of heat transfer deterioration phenomenon in supercritical water through vertical tube

In this study, a numerical investigation of heat transfer deterioration (HTD) in supercritical water flowing through vertical tube is performed by using six low-Reynolds number turbulence models. All low-Reynolds models can be extended to reproduce the effect of buoyancy force on heat transfer and show the occurrence of localized HTD. However, most k–ε models seriously over-predict the deterioration and do not reproduce the subsequent recovery of heat transfer. The V2F and SST models perform better than other models in predicting the onset of deterioration due to strong buoyancy force. The SST model is able to quantitatively reproduce the two heat transfer deterioration phenomena with low mass flux which have been found in the present study.     

An investigation on heat transfer characteristics of different pressure steam-water in vertical upward tube

Within the range of pressure from 9 to 30 MPa, mass velocity from 600 to 1200 kg/(m² s), and heat flux at inner wall from 200 to 600 kW/m², experiments have been performed to investigate the heat transfer characteristics of steam-water two-phase flow in vertical upward tube. The outer diameter of the tube is 32 mm, and the wall thickness is 3 mm. Based on results, it was found that Dryout is the main mechanism of the heat transfer deterioration in the sub-critical pressure region. Near the critical pressure, when the heat transfer deterioration occurs, the steam quality of water is lower than that in the sub-critical pressure region, so that DNB is the main mechanism in this pressure region. At supercritical pressure, the heat transfer performance in circular channel is improved and enhanced. Heat transfer deterioration phenomenon is observed when the fluid bulk temperature approaches to the pseudo-critical value. Nusselt correlation of the forced-convection heat transfer in supercritical pressure region has been provided, which can be used to predict heat transfer coefficient of the vertical upward flow in tube.     

Simple heat transfer model for laminar film condensation in a vertical tube

A new physical model for estimating the liquid film thickness and condensation heat transfer coefficient in a vertical tube, considering the effects of gravity, liquid viscosity, and vapor flow in the core region, is proposed. In particular, for calculating the velocity profile in the liquid film, the liquid is assumed to be in Couette flow forced by the interfacial velocity at the liquid-vapor interface. The interfacial velocity is calculated using an empirical power-law velocity profile. The film thickness and heat transfer coefficient from the new model are compared with existing experimental data and the original Nusslet condensation theory. The new model describes the liquid film thinning effect due to the vapor shear flow and predicts the condensation heat transfer coefficient from experiments reasonably well.     

Measurement of heat transfer coefficients for steam condensation on a vertical 21.5-mm-O.D. tube in the presence of air

ABSTRACT

Governing the rate of heat transport by condenser tubes in the passive containment cooling system (PCCS), the steam condensation over a vertical cylinder in the presence of air was investigated experimentally. The main objective of this study was to explore if the condensation heat transfer coefficient relies on the tube dimension, which has been a variable missed in most condensation models or has been embraced without experimental demonstration under phase change environments. The mean heat transfer coefficient was measured in the condensation test facility named JERICHO (JNU Experimental Rig for Investigation of Condensation Heat transfer On tube). The outer diameter of the condenser tube used in this study was set to 21.5 mm. The measured heat transfer coefficients were compared to those obtained from the 40-mm-O.D. tube, and a multiplier to correct the variation of the heat transfer coefficient with the tube diameter was proposed for its application to Lee correlation. The proposed correlation was further validated against another set of experimental data obtained from a separate test facility housing the 31.8-mm-O.D. tube.     

Experimental and empirical study of steam condensation heat transfer with a noncondensable gas in a small-diameter vertical tube

An experimental study was performed to investigate local condensation heat transfer coefficients in the presence of a noncondensable gas inside a vertical tube. The data obtained from pure steam and steam/nitrogen mixture condensation experiments were compared to study the effects of noncondensable nitrogen gas on the annular film condensation phenomena. The condenser tube had a relatively small inner diameter of 13 mm (about 1/2-in.). The experimental results demonstrated that the local heat transfer coefficients increased as the inlet steam flow rate increased and the inlet nitrogen gas mass fraction decreased. The results obtained using pure steam and a steam/nitrogen mixture with a low inlet nitrogen gas mass fraction were similar. Therefore, the effects of noncondensable gas on steam condensation were weak in small-diameter condenser tubes.A new correlation was developed to evaluate the condensation heat transfer coefficient inside a vertical tube with noncondensable gas, irrespective of the condenser tube diameter. The new correlation proposed herein is capable of predicting heat transfer rates for tube diameters between 1/2- and 2-in. because of the unique approach of accounting for the heat transfer enhancement via an interfacial shear stress factor.     

Reflux condensation of flowing vapor and non-condensable gases counter-current to laminar liquid film in a vertical tube

An analytical model using the heat and mass transfer analogy approach was developed for local heat transfer in reflux condensation of flowing vapor and non-condensable gases counter-current to laminar liquid film in a vertical tube. The liquid film model was derived from the two-phase integral momentum equations for counter-current flow. The non-condensable gas effect was accounted for using the diffusion layer theory. The momentum, heat and mass transfer for the liquid and gas phases are coupled together by the shear stress, temperature and gas mass fraction at the two-phase interface, which together with the unknown vapor flow conditions at the outlet of the tube were solved iteratively. The model anticipates that vapor might not necessarily condense completely inside the tube. The root mean square of the theory's relative error is 30% compared with available experimental data. The model provides a mechanism for the safety analysis codes to evaluate reflux condensation in the presence of non-condensable gases.     

翅片管熔盐对流传热特性研究

高温熔盐在熔盐堆和太阳能等能源领域有广泛的应用前景。为研究熔盐在强化换热管中的强化传热效果,本文基于三元硝酸盐KNO_3-NaNO_2-NaNO_3(摩尔分数比为53%-40%-7%)与导热油的对流传热实验装置,根据相似理论使用导热油代替熔盐,对翅片换热管湍流区的对流传热特性开展了测量,流体雷诺数(Re)变化范围10 000-60 000。通过威尔逊分离法获得翅片管中湍流区的对流传热系数(ho)和努赛尔数(Nu),基于实验数据与Dittus-Boelter公式,拟合翅片管湍流区的对流传热关联式,实验数据与拟合公式的误差在-7.1%-7.5%之间。与传统对流传热关联式Dittus-Boelter公式对比进行强化传热效果评估,结果表明,翅片管的强化传热效果为光滑管的2.32-3.63倍。     

CFD analysis of flow and heat transfer of turbulent pulsating flow in a tube in rolling motion

The investigation of flow and heat transfer of turbulent pulsating flow is of vital importance to the nuclear reactor thermal hydraulic analysis in ocean environment. In this paper, the flow and heat transfer of turbulent pulsating flow is analyzed. The calculation results are firstly verified with experimental data. The agreement between them is satisfactory. The effect of spanwise and wall-normal additional forces is significant in small Reynolds number, and decreases with Reynolds number increasing. The rolling axis and rolling radius contribute slight to the flow and heat transfer. The effect of velocity oscillation period on the heat transfer is limited than that of Reynolds number and oscillating velocity Reynolds number. The traditional empirical correlations could not predict the flow and heat transfer of turbulent pulsating flow in rolling motion.     

对蒸发曳力模型的修正

建立了一套研究高温小球落水的工程热物理基础实验装置。在此装置上进行了一系列实验,证实了蒸发曳力模型的可信性．同时也发现其局限性：对蒸发曳力模型进行了修正,增加考虑了辐射热在液体内部和汽液交界面的分布及对流换热对小球落水阻力的影响,扩大了蒸发曳力模型的适用性。     

Research on the effect of Reynolds correlation in natural convection film condensation

Film condensation is a vital phenomenon in the nuclear engineering applications,such as the gas-steam pressurizer design,and heat removing on containment in the case of postulated accident.Reynolds number in film condensation can be calculated from either the mass relation or the energy relation,but few researches have distinguished the difference between them at present.This paper studies the effect of Reynolds correlation in the natural convection film condensation on the outer tube.The general forms of the heat transfer coefficient correlation of film condensation are developed in different flow regimes.By simultaneously solving a set of the heat transfer coefficient correlations with Remass and Reenergy,the general expressions for Remass and Reenergy and the relation between the corresponding heat transfer coefficients are obtained.In the laminar and wavefree flow regime,Remass and Reenergy are equivalent,while in the laminar and wavy flow regime,Remass is much smaller than Reenergy,and the deviation of the corresponding average heat transfer coefficients is about 30％ at the maximum.In the turbulent flow regime,the relation of Remass and Reenergy is greatly influenced by Prandtl number.The relative deviation of their average heat transfer coefficients is the nonlinear function of Reynolds number and Prandtl number.Compared with experimental results,the heat transfer coefficient calculated from Reenergy is more accurate.     

Numerical analysis of thermal-hydraulic behavior of supercritical water in vertical upward/downward flow channels

Investigations on the thermal-hydraulic behavior in the SCWR fuel assembly have obtained a significant attention in the international SCWR community. However, there is still a lack of understanding of the heat transfer behavior of supercritical fluids. In this paper, the numerical analysis is carried out to study the thermal-hydraulic behaviour in vertical sub-channels cooled by supercritical water. Remarkable differences in characteristics of secondary flow are found, especially in square lattice, between the upward flow and downward flow. The turbulence mixing across sub-channel gap for downward flow is much stronger than that for upward flow in wide lattice when the bulk temperature is lower than pseudo-critical point temperature. For downward flow, heat transfer deterioration phenomenon is suppressed with respect to the case of upward flow at the same conditions.     

Effect of noncondensable gas in a vertical tube condenser

An experimental study is performed to investigate the effects of noncondensable (NC) gas in the steam condensing system. A vertical condenser tube is submerged in a water pool where the heat from the condenser tube is removed by boiling heat transfer. The design of the test section is based on the passive condenser system in an advanced boiling water nuclear power reactor. Data are obtained for various process parameters, such as inlet steam flow rate, noncondensable gas concentration, and system pressure. Degradation of the condensing performance with increasing noncondensable gas is investigated. The condensation heat transfer coefficient and heat transfer rate decrease with noncondensable gas. The condensation heat transfer rate is enhanced by increasing the inlet steam flow rate and the pressure. The condensation heat transfer coefficient increases with the inlet steam flow rate, however, decreases with the system pressure. For the condenser submerged in a water pool with saturated condition, the strong primary pressure dependency is observed.     

Supercritical water heat transfer in vertical tubes: A look-up table

A new procedure for the prediction of wall temperatures in vertical tubes has been established in the framework of the development of the High Performance Light Water Reactor (HPLWR). The prediction of the wall temperature is accomplished by a look-up table for heat transfer in supercritical water. The look-up table lists the wall temperatures as a function of mass flux, heat flux, pressure, tube diameter, and bulk enthalpy. Based on an extended literature survey, experimental data for different conditions of upward flows in vertical smooth tubes are selected. To exclude data which exhibited local, buoyancy driven effects, a criterion of Jackson for deterioration of heat transfer is used to remove these data from further processing. An interpolation method is applied to assemble the tabulated grid points, based on published correlations for heat transfer in supercritical water. The look-up table covers a mass flux range of 700–3500 kg/m² s, a heat flux range of 300–1600 kW/m², a pressure range of 22.5–25 MPa, a diameter range of 8–20 mm and a bulk enthalpy range of 1200–2700 kJ/kg. Extreme combinations which required extrapolation of the data are excluded. The accuracy of the table in the vicinity of the pseudo-critical point is significantly higher than published correlations.     

Studies on steam condensation and particle diffusiophoresis in a heat exhanger tube

In this paper, studies on steam condensation and aerosol behaviour in a heat exchanger are presented. The heat exchanger is a model of one single tube of a Passive Containment Condenser (PCC) that is used in the European Simplified Boiling Water Reactor (ESBWR). A hot carrier gas containing nitrogen and steam with Ag and CsOH particles flows through the heat exchanger. The walls of the heat exchanger are cooled with a water jacket, thus causing steam condensation and diffusiophoretic particle deposition. The amount of condensed steam is measured, as well as the temperatures, particle mass concentrations and size distributions before and after the heat exchanger. The experiments are done with different proportions of steam and nitrogen in the carrier gas. Heat and mass transfer in the system are modelled with well known engineering correlations, producing results that agree nicely with the experimental results. The diffusiophoretic particle deposition velocity is shown to be proportional to the steam condensation rate, as expected.     

Measurement of heat transfer coefficients for direct contact condensation in core makeup tanks using holographic interferometer

The present work is to improve our understanding and analysis of direct contact condensation on the gravity injection of CMTs and to measure the heat transfer coefficients around steam bubbles using the holographic interferometer and high speed camera. The condensation regime map associated with the downward injection of steam into water through the steam pipe is investigated to understand the mechanism of the direct contact condensation. The present map shows that the boundary of chugging and subsonic jetting with the larger diameter pipe is shifted to the larger steam mass flux. Steam cavity mode, not found in the literature, and the unique mode of downward injection for the present geometry, is observed at the low subcooled water temperature. With the holographic interferometry and the high speed camera, the heat transfer mechanism for the direct contact condensation in CMTs is understood and the heat transfer coefficients are measured.     

Critical heat flux data in a vertical tube at low and medium pressures

AECL Research and École Polytechnique have been cooperating on the validation of the critical heat flux (CHF) look-up table (D.C. Groeneveld et al., Heat Transfer Eng. 7(1–2) (1986) 46–62). For low and medium pressures the values in the table have been obtained by extrapolation and curve fitting; therefore, errors could be expected. To reduce these possible extrapolation errors, CHF experiments are being carried out in water cooled 8 mm internal diameter (ID) tubes, at conditions where the data are scarce. This paper presents some of the experimental CHF data obtained for vertical up flow in an 8 mm ID test section, for a wide range of exit qualities (5–70%) and the exit pressure ranging from 5 to 30 bar. The experiments were carried out for heated lengths of 0.75, 1, 1.4 and 1.8 m. In general, the collected data show parametric trends similar to those described in the open literature. However, it was observed that for low pressure conditions CHF depends on the heated length; this dependence begins to disappear for exit pressure of about 30 bar. The CHF data have also been compared with predictions of well-known correlations (L. Biasi et al., Energia Nucl. 14(9) (1967) 530–536; R. Bowring, Br. Report AEEW-R789, Winfrith, UK, 1972; Y. Khatto and H. Ohno, Int. J. Heat Mass Transfer 27 (1984) 1641–1648) and those of the look-up table given by Groeneveld et al. For low pressures and low mass fluxes the look-up table seems to yield better predictions of the CHF than the correlations. However, for medium pressures and mass fluxes the correlations perform better than the look-up table; among those tested, Katto and Ohno's correlation gives the best results.     

Recent experiments for laminar and turbulent film heat transfer in vertical tubes

This study reviews experimental local heat-transfer data for laminar and turbulent film heat transfer for downward condensing films, under the influence of interfacial-waviness and shear–stress effects. Local laminar-wavy-film heat transfer and transition to turbulence are significantly influenced by local wave characteristics, which depend not only on the film developing length, but also on the film-formation method. The results demonstrate that the dimensionless film thickness, incorporating shear stress, provides a more appropriate length scale to estimate laminar-wavy-film heat transfer, as well as transition to turbulence. For turbulent films, a phenomenologically-based local heat-transfer correlation has been proposed, treating the near-wall and near-interface regions in series, to derive a ‘two-layer resistance model', based on the investigation of turbulence structure across sheared gas–liquid interfaces.     

Modeling condensation heat transfer on a horizontal finned tube in the presence of noncondensable gases

European designs for the next generation of nuclear reactors incorporate innovative passive systems in their containments to enhance heat removal by condensation under postulated accident conditions. These systems consist of several units of cross-flow finned tube bundles internally cooled with water. So far most of the studies that have been addressed to the issue of heat transfer onto finned surfaces under condensing conditions have involved refrigerants and pure vapor conditions. This study presents a model (HTCFIN) capable of predicting condensation of a cross-flow air–steam mixture onto a single horizontal finned tube. The comparison of HTCFIN predictions to the available databases shows its acceptable accuracy in a wide range of conditions and allows an interpretation of the influence of major variables acting on the scenario. As a consequence, HTCFIN model represents a step forward in the present theoretical capability to estimate heat transfer within containments of next generation of European reactors in the case of a hypothetical accident.