Flow characteristics of two-phase dispersed annular streams in heated tubes

Patterns and characteristics of flow through heated tubes are investigated on the basis of concepts of two-phase dispersed annular flow patterns [1] within the framework of the three-velocity and single-temperature equilibrium model, with flowrates of the mixture not too close to critical. Conditions for onset of burnout of the second kind, i.e., deterioration in the transfer of heat leading to an abrupt rise in the temperature of the heating surface, and, as such, associated with desiccation of the thin film of liquid on the wall [2, 3], are investigated. Hydraulic drag, the flowrate of liquid in the film, and the true steam content by volume are among the factors discussed. Two-phase flow patterns in dispersed annular flow are characterized by the combined motion of the three components of the mixture: vapor, the liquid wall film, and droplets. The assumption entertained is that each component of the mixture acquires its own velocity, and that the temperature of the mixture is equal, in each cross section through the channel, to the saturation temperature at the pressure prevailing in the particular cross section.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 78–88, July–August, 1973.     

Turbulent forced convective heat transfer in two-phase evaporating droplet flow through a vertical pipe

A physical model was developed to study heat transfer in turbulent dispersed flow at very high vapor quality in a vertical pipe by numerically solving the coupling governing differential equations for both phases. Major heat transfer mechanisms included in the model were the thermal nonequilibrium effects, droplet vaporization, droplet deposition on the duct wall and thermal radiative transfer. The predicted results indicated that vapor superheating is dominant for the cases with high wall superheat, otherwise droplet vaporization dominates the energy transport processes. Heat transfer during the droplet-wall interaction only exists at low wall superheat but in small amounts.     

A model for droplet entrainment in heated annular flow

The ability to accurately predict droplet entrainment in annular two-phase flow is required to effectively calculate the interfacial mass, momentum, and energy transfer, which characterizes nuclear reactor safety, system design, analysis, and performance. Most annular flow entrainment models in the open literature are formulated in terms of dimensionless groups, which do not directly account for interfacial instabilities. However, many researchers agree that there is a clear presence of interfacial instability phenomena having a direct impact on droplet entrainment. The present study proposes a model for droplet entrainment, based on the underlying physics of droplet entrainment from upward co-current annular film flow that is characteristic to light water reactor safety analysis. The model is developed based on a force balance and stability analysis that can be implemented into a transient three-field (continuous liquid, droplet, and vapor) two-phase heat transfer and fluid flow systems analysis computer code.     

Study on heat transfer performance of spray cooling: model and analysis

In consideration of droplet–film impaction, film formation, film motion, bubble boiling (both wall nucleation bubbles and secondary nucleation bubbles), droplet–bubble interaction, bulk air convection and radiation, a model to predict the heat and mass transfer in spray cooling was presented in this paper. The droplet–film impaction was modeled based on an empirical correlation related with droplet Weber number. The film formation, film motion, bubble growth, and bubble motion were modeled based on dynamics fundamentals. The model was validated by the experimental results provided in this paper, and a favorable comparison was demonstrated with a deviation below 10%. The film thickness, film velocity, and non-uniform surface temperature distribution were obtained numerically, and then analyzed. A parameters sensitivity analysis was made to obtain the influence of spray angle, surface heat flux density, and spray flow rate on the surface temperature distribution, respectively. It can be concluded that the heat transfer induced by droplet–film impaction and film-surface convection is dominant in spray cooling under conditions that the heated surface is not superheated. However, the effect of boiling bubbles increases rapidly while the heated surface becomes superheated.     

Effectiveness of a multi-channel volumetric air receiver for a solar power tower

In this study, the heat transfer performance of a multi-channel volumetric air receiver for a solar power tower was numerically analyzed. The governing equations, including the solar radiation heat flux, conduction, convection and radiation heat transfer for a single channel, were solved on the basis of valid related references and a methodology that can predict the temperature distribution of the receiver wall and the heat transfer fluid for specific dimensions and input conditions. Furthermore, a mathematical model of the effectiveness of the receiver was derived from an analysis of the temperature profiles of the wall and the heat transfer fluid. The receiver effectiveness as an appropriate criterion to assess economic feasibility regarding geometric size was investigated, as it would be applied to the design process of the receiver. The main parameters for the thermal performance simulations described in this paper are the air mass flow rate, receiver length and the influence of these parameters on the heat transfer performance from the viewpoint of receiver efficiency and effectiveness.     

Performance model of metallic concentric tube recuperator with counter flow arrangement

A performance model for counter flow arrangement in concentric tube recuperator that can be used to utilize the waste heat in the temperature range of 900–1,400°C is presented. The arrangement consists of metallic tubular inner and outer concentric shell with a small annular gap between two concentric shells. Flue gases pass through the inner shell while air passes through the annular gap in the reverse direction (counter flow arrangement). The height of the recuperator is divided into elements and an energy balance is performed on each elemental height. Results give necessary information about surface, gas and air temperature distribution, and the influence of operating conditions on recuperator performance. The recuperative effectiveness is found to be increased with increasing inlet gas temperature and decreased with increasing fuel flow rate. The present model accounts for all heat transfer processes pertinent to a counterflow radiation recuperator and provide a valuable tool for performance considerations.     

An investigation of thermally stratified turbulent channel flow with temperature oscillation on the bottom wall by large eddy simulation

Thermally stratified shear turbulent channel flow with temperature oscillation on the bottom wall of the channel is calculated to investigate the behavior of turbulent flow and heat transfer by use of large eddy simulation (LES) approach coupled with dynamic subgrid-scale (SGS) models. The objective of this study is to deal with the effect of the temperature oscillation on turbulent behavior of thermally stratified turbulent channel flow and to examine the effectiveness of the LES technique for predicting statistically unsteady turbulent flow driven by time-varying buoyancy force. To validate the present calculation, thermally stratified shear turbulent channel flow is computed and compared with available data obtained by direct numerical simulation (DNS), which confirm that the present approach can be used to predict thermally stratified turbulent channel flow satisfactorily. Further, to illustrate the effect of the temperature oscillation with different Richardson numbers and periods of the oscillation on turbulence characteristics, the phase-averaged mean value and fluctuation of the resolved velocities and temperature, and instantaneous velocity fluctuation structures are analyzed.     

不同壁面条件下液滴撞击铺展特性的模拟研究

液滴撞击不同润湿性壁面的传热流动问题在自然界和工业生产中广泛存在。研究采用CLSVOF方法,引入描述壁面润湿特性的动态接触角,并考虑液滴物性参数随温度的变化,建立液滴撞壁模型,模拟研究液滴撞击流动行为,通过与实验对比验证,确定模型有效性。在此基础上,对传热作用下考虑壁面润湿性的液滴撞击问题展开研究,探讨壁面传热作用对液滴撞击铺展特性的影响。研究表明,在撞击过程中,液滴先铺展后逐渐收缩,与静态接触角模型相比,采用动态接触角模型所得的液滴流动特性与实验结果更加吻合;随着接触角增大,液滴在撞壁初期不易铺展,随后则易于收缩;虽然固液传热作用会影响液滴铺展直径,但不改变液滴的运动趋势。     

Effects of radiation on turbulent natural convection in channel flows

A computational study has been carried out to analyse complex interaction of radiation with turbulent natural convective flow of dry and humid air in open-ended channels. Transient flow simulations are undertaken in the channel with one uniformly heated wall and adiabatic side walls for different values of emissivity of active walls with and without participating medium. To adequately present turbulence and radiation, a computational model included large eddy simulations for the turbulent flow coupled with discrete ordinates method for radiation transfer. Spectral line-based weighted-sum-of-grey-gases for the absorption properties of water vapour has been adopted. Complex three-dimensional vortical structures are identified which directly affect the temperature distribution on the heated wall. Including wall to wall radiation resulted in significant changes in the heat transfer, reaching 14 °C temperature drop at the hot wall with wall emissivity of 0.9. Mixing and cooling rates in this case were increased by up to 25%. Including gas radiation for the humid air with the water vapour molar fraction of 0.02 corresponding to saturated conditions at inlet temperature of 25 °C did not have a significant effect on the mean flow and temperature values comparing with wall to wall radiation. However, turbulent statistics have changed significantly resulting in a delayed transition to turbulence near the active wall of the channel and increased turbulent activity near the cold wall. The model developed in the present study is also applicable in fire management, where the aim is to reduce the damage that occurs when a PV module is exposed to high temperatures.     

Heat transfer characteristics of an annular turbulent impinging jet with a confined wall measured by thermosensitive liquid crystal

The present paper describes the heat transfer characteristics of an annular turbulent impinging jet with a confined wall. The local temperature distribution on the impingement surface was measured using a thermosensitive liquid crystal sheet and an image processor. The net heat flux was evaluated by considering the heat conduction in the heated substrate and the thermal radiation between an upper confining insulated wall and an impingement surface. Distributions of the temperature and Nusselt number on the impingement surface were captured in two-dimensional maps. Effects of the diameter ratio of the annular nozzle, the space between nozzle and impingement surface and the Reynolds number on radial distributions of the local Nusselt number were examined. Experimental formulas of the local Nusselt number were obtained in power-law expressions of r/r_p for the major and minor flow regions.     

Turbulent flow structure and heat transfer in an inclined bubbly flow. Experimental and numerical investigation

The effect of channel inclination on the variation in the wall shear stress and the heat transfer in a two-phase bubbly flow in a rectangular channel is experimentally and numerically investigated. The wall friction was measured using the electrodiffusion method and the temperature was measured by tiny platinum resistance thermometers. The model is based on the system of RANS equations with account for the back influence of the bubbles on the flow characteristics. Flow turbulence is calculated according to the model of transport of the Reynolds stress tensor components. It is shown that in the gas-liquid flow the angle of the channel inclination to the horizon can have a considerable effect on the friction and the heat transfer. The greatest friction and heat transfer values correspond to the angles of channel inclination ranging from 30 to 50°. In the inclined two-phase bubbly flow the shear stress enhancement on the wall amounts to 30% and that of the heat transfer to 15%. A friction and heat transfer reduction to 10 and 25%, respectively, is noticed in near-horizontal flows.     

Effect of secondary flow on droplet distribution and deposition in horizontal annular pipe flow

In horizontal annular dispersed pipe flow the liquid film at the bottom is thicker and rougher than at the top of the pipe. A turbulent pipe flow experiencing a variation of roughness along the pipe wall will show a secondary flow. Such secondary flow, consisting of two counter-rotating cells in the cross-section of the tube, can change the distribution of the droplets inside the pipe and their deposition at the wall. Here, we compare the behaviour of the droplets (dispersed phase) with and without secondary flow, using large-eddy simulations. It is shown that the presence of secondary flow increases the droplet concentration in the core of the pipe and the droplet deposition-rate at the top of the pipe.     

A numerical study of droplet motion/departure on condensation of mixture vapor using lattice Boltzmann method

Droplet motion/departure, which is governed by external force acceleration coefficient, droplet radius and surface wettability on solid surfaces under external forces such as gravitational force, play a significant role in characterizing condensation heat transfer, especially when high fractional non-condensable gases (NCG) present. However, due to the challenge in visualizing the vapor/steam velocity field imposed by droplet motion/departure, the detailed mechanism of droplet motion/departure on condensing surfaces has not been completely investigated experimentally. In this study, droplet motion/departures on solid surfaces under external forces and their interactions with steam flow are simulated using two dimensional (2D) multiphase lattice Boltzmann method (LBM). Large external force acceleration coefficient, droplet radius and contact angle, lead to large droplet deformation and high motion/departure velocity, which significantly shortens the droplet residual time on the solid surface. Our simulation shows that steam vortices (lateral velocity) induced by droplet motion/departure can greatly disturb the vapor flow and would be intensified by increasing external force acceleration coefficient, droplet radius, and contact angle. In addition, the location of vortex center shifts in the ascending direction with increase of these factors. The average lateral velocities induced by droplet motion/departure at various conditions are obtained. The mass transfer resistance is substantially reduced owing to the droplet motion/departure, leading to an enhanced heat flux. The experimental results are compared to validate the influence of droplet motion/departure on condensation heat transfer performance, especially for steam–air mixture with the presence of high fractional NCG.     

Development of a turbulent near-wall temperature model and its application to channel flow

A numerical study of fluid flow and heat transfer in a two-dimensional channel under fully developed turbulent conditions is reported. A computer program which is capable of treating both forced and natural convection problems under turbulent conditions has been developed. The code uses the high-Reynolds-number form of the two equation turbulent model(k-?) in which a turbulent kinetic energy near-wall model is incorporated in order to accurately represent the behavior of the flow near the wall, particularly in the viscous sublayer where the turbulent Reynolds number is small. A near-wall temperature model has been developed and incorporated into the energy equation to allow accurate prediction of the temperature distribution near the wall and, therefore, accurate calculation of heat transfer coefficients. The sensitivity of the prediction of flow and heat transfer to variations in the coefficients used in the turbulence model is investigated. The predictions of the model are compared to available experimental and theoretical results; good agreement is obtained. The inclusion of the near-wall temperature model has further improved the predictions of the temperature profile and heat transfer coefficient. The results indicate that the turbulent kinetic energy Prandtl number should be a function of Reynolds number.     

Heat transfer characteristics of developing gaseous slip-flow in rectangular microchannels with variable physical properties

The effects of variable physical properties on the flow and heat transfer characteristics of simultaneously developing slip-flow in rectangular microchannels with constant wall temperature are numerically investigated. A colocated finite-volume method is used in order to solve the mass, momentum and energy equations in their most general form. Various channel aspect ratios are studied at different Knudsen numbers. Simulations indicate that the constant physical property assumption can result in under/over-prediction of the local friction and heat transfer coefficients depending on the problem configuration. Density and thermo-physical property variations have significant effects on predicting flow and heat transfer characteristics in the developing and fully-developed regions. The degree of discrepancy varies for different cases depending on Knudsen number, aspect ratio and the temperature difference between the channel inlet and the wall. The results suggest that even low temperature differences can alter the friction and heat transfer coefficients considerably.     

Heat transfer in horizontal tubes during two phase natural circulation with presence of noncondensing gas

 The condensation process of steam inside horizontal tubes during natural circulation gains in importance regarding the reactor safety research for existing and future nuclear power plants. Experimental investigations due to the condensation process were realized with the rig HORUS to study the behaviour of water-steam-gas mixtures in horizontal tubes. The paper includes statements regarding the flow and heat transfer conditions inside the tube and the temperature distribution inside the small tube wall. The experiments showed a blockade of the heat transfer area with Nitrogen which is connected with an increasing primary pressure followed by a compression of the Nitrogen and a reentry of steam into the tube. The experiments serve for the creation of an experimental data base. A model development for calculation of the heat transfer is described. The model was implemented in the German thermal-hydraulic code ATHLET. The comparison of calculated data and the measured parameters of HORUS rig show the code improvement for the simulation of noncondensing gases. Received on 17 January 2000     

Numerical modeling of cryogenic chilldown process in terrestrial gravity and microgravity

In this paper, a numerical model is developed to predict the cryogenic chilldown process of a vertical tube for both terrestrial and microgravity conditions. The flow field is covered by four distinct regions, which are single-phase vapor region, dispersed flow region, inverted annular flow region, and single-phase liquid region. Heat transfer mechanisms are dictated by the flow patters. A two-fluid model is employed to analyze the dispersed flow region and the inverted annular film boiling region. Gravity effect on the chilldown process is also investigated. Model results indicate that film boiling heat transfer decreases with decreasing gravity level for the bottom flooding condition. The model results show a good agreement with the experimental data.     

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part II – Heat transfer with 35 local heaters

This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7 mm silicon evaporator composed of 135 microchannels with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. There were 35 local heaters and temperature measurements arranged in a 5 × 7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2 °C, inferring that the boiling process was very stable. For wall heat fluxes over 45 kW/m², the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of Thome et al. (2004) was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness.     

Convective heat transfer from molten salt droplets in a direct contact heat exchanger

This paper presents a new predictive model of droplet flow and heat transfer from molten salt droplets in a direct contact heat exchanger. The process is designed to recover heat from molten CuCl in a thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen production. This heat recovery occurs through the physical interaction between high temperature CuCl droplets and air. Convective heat transfer between droplets and air is analyzed in a counter-current spray flow heat exchanger. Numerical results for the variations of temperature, velocity and heat transfer rate are presented for two cases of CuCl flow. The optimal dimensions of the heat exchanger are found to be a diameter of 0.13 m, with a height of 0.6 and 0.8 m, for 1 and 0.5 mm droplet diameters, respectively. Additional results are presented and discussed for the heat transfer effectiveness and droplet solidification during heat recovery from the molten CuCl droplets.