燃气轮机回流式燃烧室内两相流动数值模拟及分析

利用流体分析软件STAR—CD对某回流式燃气轮机燃烧室的内部流场进行了三维冷态数值模拟及两相流反应数值模拟。建立了燃烧室的三维计算几何模型及计算网格,计算了燃烧室的单相流场及喷雾两相流场。在计算中气相采用N-S方程求解,采用高雷诺数κ-ε湍流模型及SIMPISO算法;液相采用Lagrange法处理,采用颗粒群轨道模型。根据计算结果进行流动分析,为进一步进行燃烧室内部燃烧过程的数值计算分析及改善燃烧室的结构设计、降低排放奠定了基础。     

Numerical investigation of attached cavitating flow in thermo-sensitive fluid with special emphasis on thermal effect and shedding dynamics

This study aims to investigate the unsteady cavitation shedding dynamics flow around a NACA 0015 hydrofoil in thermo-sensitive fluid with thermal effect. The thermal effects are captured by a coupled solution of the continuity, momentum and energy equations, and the numerical results show a reasonable agreement with the available experiments. Time evolution process of the vortex structure is investigated. The ability and limitation of B-factor is analysed to evaluate the thermal effect, which can provide guidance for the further improvement of B-factor. The re-entrant jet and vortex structure show strong coherent relationship with the flow separation. The re-entrant jet promotes the formation of vortex structure, which in turn affects the separation flow of the flow field. Skin friction coefficient and boundary vorticity flux are applied to displace the flow separation on the hydrofoil surface. The results showed that the streamwise velocity decreases sharply in the vicinity of the collision between the re-entrant jet and the main stream, the skin friction streamline suddenly breaks off and separation or re-attachment line occurs. The intensity of the re-entrant jet inside the cavity becomes gradually weaker, the strength of the vortex is also weakened, which causes the skin friction coefficient in this region to be almost zero, and the phenomenon of flow separation and re-attachment is indistinct.     

Numerical and linear regression analysis on MHD two-phase flow in an asymmetric nonuniform channel

The flow through asymmetric nonuniform (convergent) channels with the effect of the magnetic field have a pronounced impact in engineering and biological fields such as chemical and food industries, blood flow through capillaries, and arteries, and so forth. With this motivation, the present study focuses on convective hydromagnetic particulate suspension flow in an asymmetric convergent channel under the heat generation effect. The numerical method is applied to solve the nondimensionalized equations governing the transport process of fluid and particle flow and its heat. To check the convergence of the computational results, a grid independence test has been performed. A comparison test has been made to validate the results and an admirable agreement is noticed with published results. Computation results are reported for the influence of emerging parameters on the fluid as well as particle velocity and temperature profiles through graphs and tables. A method of slope linear regression through data points is presented to study the impact of various parameters on skin friction and Nusselt number. The study pioneers the investigation on the significance of the combined influence of cross-flow Reynolds number and magnetic field on fluid and particle in the convergent channel and also reports its importance on drag coefficient and rate of heat transfer at the walls. It is perceived that a reduction in fluid velocity takes place with an increment in Magnetic parameter, Grashof number, and Reynolds number. An augmentation in fluid temperature is noted with an increment in Prandtl number and heat source parameter.     

Thermal analysis of plate condensers in presence of flow maldistribution

Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.     

Numerical analysis of the influence of two-phase flow mass and heat transfer on n-heptane autoignition

This paper investigates the influence of liquid fuel presence on the autoignition of n-heptane/air mixtures over a wide range of conditions encountered in internal combustion engines. To this end, evaporating droplet physics and skeletal chemistry mechanisms are simultaneously solved considering a homogeneous constant-pressure reactor. A skeletal mechanism is introduced to account for specific kinetics behavior in the Negative Temperature Coefficient (NTC) region. The impact of mass and heat source terms during evaporation is emphasized by comparing a two-phase flow scenario with a purely gaseous case. The competition between fuel vapor availability and the evaporation-induced gas temperature decrease is specific to two-phase flow autoignition. On the one hand, droplet evaporation delay restricts the gaseous local fuel/air equivalence ratio and consequently the kinetics runaway. On the other hand, temperature reduction due to evaporation may either reduce or enhance chemical reactivity, depending on the local thermodynamic conditions lying either inside or outside the NTC region. By simultaneously accounting for evaporation source terms and skeletal chemistry, we can reproduce the already experimentally observed transformation of the NTC region into a Zero Temperature Coefficient (ZTC) region depending on thermodynamic conditions and droplet size. The ZTC phenomenon appears when combustion heat-release starts before complete droplet evaporation. Since the ZTC behavior can be captured using the point source approach, in which droplets are considered only as zero-dimensional source terms of mass and energy, the present results pave the way for future exploration of NTC chemistry in sprays with a direct numerical simulation of discrete particles considering detailed chemistry and turbulent flows.     

Stability of two-phase stratified flow as controlled by laminar/turbulent transition

Flow pattern transition from stratified-smooth to stratified-wavy has been usually identified with a condition of neutral stability, where destabilizing effects are due to the inertia of the two-phases. It is shown that this is indeed the case when instability is approached with laminar gas phase. However, when the upper gas phase is turbulent, a destabilizing term appears due to dynamic interaction of the turbulent flow with the perturbed free interface. At the transitional range from laminar to turbulent flow regime the evolution of wavy pattern is not predicted by stability condition and coincides with the laminar/turbulent flow regime transition.     

A modified k–ε turbulence model for the simulation of two-phase flow and heat transfer in condensers

A modified k–ε turbulence model is developed in this study to simulate the gas–liquid two-phase flow and heat transfer in steam surface condensers. A quasi-three-dimensional algorithm is used to simulate the fluid flow and heat transfer in steam surface condensers. The numerical method is based on the conservation equations of mass and momentum for both gas-phase and liquid-phase, and mass fraction conservation equation for non-condensable gases. The numerical simulation of an experimental steam surface condenser has been conducted using the proposed modified k–ε turbulence model. The results obtained from the proposed model agree well with the experimental results and the results also show an obvious improvement in the prediction accuracy comparing with previous results where a constant value for the turbulent viscosity was used.     

Numerical simulation and performance analysis of horizontal-tube falling-film evaporators in seawater desalination

A comprehensive distributed parameter model for simulating the steady-state performance of a practical horizontal-tube falling-film evaporator has been developed and validated. This model is capable of predicting the distributions of thermal parameters in the tube-side and shell-side, which provide important information of heat and mass exchange processes. The fluid flow and heat transfer characteristics in tubes are analyzed in detail. The computational time is reduced significantly in comparison with the Computational Fluid Dynamics. Based on the numerical results, it is found that the steam is not evenly distributed in the horizontal tubes of each tube pass, which is favorable for parallel channels with uneven heat fluxes. The mass and heat flux of steam are mutually matched, indicating that the self-compensation characteristic appears among the tubes. In addition, the overall heat transfer coefficient reaches the maximum value of about 3300 W/m² K at the entrance region of each tube pass, and then decreases gradually along the flow direction. As liquid film falls downward from tube to tube, the liquid flow rate of seawater continually decreases from 0.063 kg/ms to 0.04 kg/ms with the corresponding salinity gradually increasing from 36 g/kg to 56 g/kg.     

发酵罐内固液两相流的数值模拟

以大连市某污泥处理厂为研究背景,以其厌氧发酵罐为研究对象,利用Fluent软件,采用RNG湍流模型、多相流中的混合模型对发酵罐内侧进式搅拌器对污泥与水固液两相的三维流动的影响进行数值模拟。通过模拟得到罐内速度场、固相浓度场的分布情况,分析对固相污泥的均匀分布产生影响的因素,对厌氧发酵过程中搅拌器的运行参数及结构参数的优化提供理论依据。     

Numerical and experimental investigation of two-phase flow in an electrochemical cell

In this study, a two-phase mathematical model is adapted to study void fraction distribution, flow field and characteristics of electrolysis process. The model involves transport equations for both liquid and gaseous phases. An experimental set-up is established to collect data to validate and improve the mathematical model. The void fraction is determined from measurement of resistivity changes in the system due to the presence of bubbles. It is observed that there is a good agreement between the numerical results and the experimental data.     

Numerical analysis of discrete phase induced effects on a gas flow in a turbulent two-phase free jet

The paper addresses numerical simulation of turbulent two-phase flow in a long vertical tube and turbulent two-phase free jet formed at the tube outlet, analyzing agreement between the numerical results and the results of corresponding experimental investigation carried out earlier.In the numerical analyses conducted, gas phase was modeled as an air flow (having a mass flow-rate in the range of 1.25–4.00 g/s), while the sand particles of two different sizes (0.25–0.30 and 0.8–1.0 mm) represented a discrete phase (particle to gas mass flow ratio of 0.72–4.08) in the two-phase flow considered. Gas-particle interaction was analyzed based on the gas velocities in the particle-laden two-phase flow and the particle-free gas flow, calculated and measured at various locations along the longitudinal axis and radius of the jet.Mathematical model of continuous phase flow was developed based on the single phase flow models, with certain corrections introduced to account for the effects of particles in the flow. In the simulation model developed, the flow analyzed was modeled as a two-phase mixture, with Eulerian simulation used to account for the gas phase behavior and the Lagrangian simulation modeling the particle movement in the two-phase flow considered. In order to appropriately close the system of time-averaged equations, k–ε turbulent model, deemed the most reliable, was used. Phase coupling i.e. fluid-particle interaction was modeled using the PSI-CELL concept. The results obtained via numerical simulation have shown a good agreement with the experimental data acquired.     

Numerical simulation of bubbly two-phase flow in a narrow channel

An advanced numerical simulation method on fluid dynamics - lattice-Boltzmann (LB) method is employed to simulate the movement of Taylor bubbles in a narrow channel, and to investigate the flow regimes of two-phase flow in narrow channels under adiabatic conditions. The calculated average thickness of the fluid film between the Taylor bubble and the channel wall agree well with the classical analytical correlation developed by Bretherton. The numerical simulation of the behavior of the flow regime transition in a narrow channel shows that the body force has significant effect on the movement of bubbles with different sizes. Smaller body force always leads to the later coalescence of the bubbles, and decreases the flow regime transition time. The calculations show that the surface tension of the fluid has little effect on the flow regime transition behavior within the assumed range of the surface tension. The bubbly flow with different bubble sizes will gradually change into the slug flow regime. However, the bubbly flow regime with the same bubble size may be maintained if no perturbations on the bubble movement occur. The slug flow regime will not change if no phase change occurs at the two-phase interface.     

Numerical study of convective heat transfer of nanofluids in a circular tube two-phase model versus single-phase model

Laminar convective heat transfer of nanofluids in a circular tube under constant wall temperature condition is studied numerically using a CFD¹ approach. Single-phase and two-phase models have been used for prediction of temperature, flow field, and calculation of heat transfer coefficient. Effects of some important parameters such as nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate have been investigated. The results of CFD simulation based on two-phase model were used for comparison with single-phase model, theoretical models and experimental data. Results have shown that heat transfer coefficient clearly increases with an increase in particle concentration. Also the heat transfer enhancement increases with Peclet number. Two-phase model shows better agreement with experimental measurements. For Cu/Water nanofluid with 0.2% concentration, the average relative error between experimental data and CFD results based on single-phase model was 16% while for two-phase model was 8%. Based on the results of the simulation it was concluded that the two-phase approach gives better predictions for heat transfer rate compared to the single-phase model.     

Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number (Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.     

Determining the two-phase friction coefficient in the transition between annular and fog flow

The dependence of the transition conditions between annular and fog flow on the parameters of a system with boiling flow are analysed in order to extend the applicability of a physical model of the two-phase flow recently proposed by the authors to calculate the friction coefficient. Having suitably represented the phenomenon, examination is made as to its influence on the analytical development of the model, the results obtainable from the latter being compared with those of the relations usually employed and with experimental indications, thus showing the broad field of validity of the formulae presented.     

Numerical simulation of three-dimensional gas/liquid two-phase flow in a proton exchange membrane fuel cell

Investigation into the formation and transport of liquid water in proton exchange membrane fuel cells (PEMFCs) is the key to fuel cell water management. A three-dimensional gas/liquid two-phase flow and heat transfer model is developed based on the multiphase mixture theory. The reactant gas flow, diffusion, and chemical reaction as well as the liquid water transport and phase change process are modeled. Numerical simulations on liquid water distribution and its effects on the performance of a PEMFC are conducted. Results show that liquid water distributes mostly in the cathode, and predicted cell performance decreases quickly at high current density due to the obstruction of liquid water to oxygen diffusion. The simulation results agree well with experimental data. Translated from J Tsinghua Univ (Sci & Tech), 2006, 46(2): 252–256 [译自: 清华大学学报]     

Measurement and analysis of dynamic instabilities in fluid-heated two-phase flow

Dynamic, density wave, stability experiments were performed using high-pressure boiling water and liquid heating. Liquid sodium at temperatures up to 500°C provided heat to boil water at pressures of 7–16 MPa and mass fluxes of 170–800 kg m⁻² s⁻¹. Water flowed inside a vertical tube, with a heated length of 13 m, and exited superheated in all tests. Special attention was given to the test procedure used to approach instability, and stability thresholds were determined. Experimental results were compared with predictions from the DYNAM computer code and two recent correlation equations. One of the equations was successfully modified to predict the experimental results well.     

Constructal design and thermal analysis of microchannel heat sinks with multistage bifurcations in single-phase liquid flow

Based on constructal theory, five different cases with multistage bifurcations are designed as well as one case without bifurcations, and the corresponding laminar fluid flow and thermal performance have been investigated numerically. All laminar fluid flow and heat transfer results are obtained using computation fluid dynamics, and a uniform wall heat flux thermal boundary condition is applied all heated surfaces. The inlet velocity ranges from 0.66 m/s to 1.6 m/s with the corresponding Reynolds number ranging from 230 to 560. The pressure, velocity, temperature distributions and averaged Nusselt number are presented. The overall thermal resistances versus inlet Reynolds number or pumping power are evaluated and compared for the six microchannel heat sinks. Numerical results show that the thermal performance of the microchannel heat sink with multistage bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sink with a long bifurcation length in the first stage (Case 1A) is superior. The usage of multistage bifurcated plates in microchannel heat sink can reduce the overall thermal resistance and make the temperature of the heated surface more uniform (Case 3). It is suggested that proper design of the multistage bifurcations could be employed to improve the overall thermal performance of microchannel heat sinks and the maximum number of stages of bifurcations is recommended to be two. The study complements and extends previous works.     

Numerical study of two-phase flow patterns in the gas channel of PEM fuel cells with tapered flow field design

In this study the air–water two-phase flow in a tapered channel of a PEMFC was numerically simulated using the volume of fluid (VOF) method. In particular, a 3D mathematical model of the fuel cell flow channel was used to obtain a reliable evaluation of the fuel cell performance for different taper angles and different temperatures and to calculate the total amount of water produced. This information was then used as boundary conditions to simulate the two-phase flow in the cell channel through a 2D VOF model. Typical operating conditions were assigned and the numerical mesh was constructed to represent the real fuel cell configuration. The results show that tapering the channel downstream enhances the water removal due to increased airflow velocity. In the rectangular channel no film formation is noted with a marked predominance of slug flow. In contrast, as the taper angle is increased the predominant two-phase flow pattern is film flow. Finally many contact angles have been used to simulate the effect of the hydrophobicity of a GDL surface on the motion of the water. As the hydrophobicity of a GDL surface is decreased the presence of film is more evident even for less tapered channels.     

Numerical investigation of two-phase laminar pulsating nanofluid flow in a helical microchannel

ABSTRACT

Numerical investigations on the thermal and hydraulic characteristics of pulsating laminar flow in a three-dimensional helical microchannel heat sink (HMCHS) model are performed using Al₂O₃-water-based nanofluid. The simulation is performed in the laminar regime for Reynolds number ranging from 6 to 25. The two-phase mixture model with modified effective thermal conductivity and viscosity equations is employed to solve the problem numerically. The detailed results for thermal and flow fields are reported for the effects of amplitude (1–3), frequency (5–20 rad/s), and nanoparticle concentration (1%–3%). The results indicate that the heat transfer performance improves significantly for sinusoidal velocity inlet conditions compared with steady flow conditions.