A blowdown cryogenic cavitation tunnel and CFD treatment for flow visualization around a foil

Cavitation is one of the major problems in the development of rocket engines. There have been few experimental studies to visualize cryogenic foil cavitation. Therefore a new cryogenic cavitation tunnel of blowdown type was built. The foil shape is "plano-convex". This profile was chosen because of simplicity, but also of being similar to the one for a rocket inducer impeller. Working fluids were water at room temperature, hot water and liquid nitrogen. In case of Angle of Attack (AOA)=8°, periodical cavity departure was observed in the experiments of both water at 90℃ and nitrogen at -190℃ under the same velocity 10 m/sec and the same cavitation number 0.7. The frequencies were observed to be 110 and 90 Hz, respectively, and almost coincided with those of vortex shedding from the foil. Temperature depression due to the thermodynamic effect was confirmed in both experiment and simulation especially in the cryogenic cavitation.     

Lattice-Boltzmann simulation of induced cavitation in protruding structure

Pressure induced cavitation is a promising heat dissipation technology since it enables phase-change heat transfer at relatively low temperature. In this study, the sustainability properties of induced cavitation in protruding structure are studied by the Lattice Boltzmann Method. The simulation results suggest that a typical cavitation process is composed of four stages: 1st: cavitation appears in front of inflow liquid, 2nd: bubble shedding, 3rd: cavitation area gradually stretches towards downstream, 4th: cavitation area reaches equilibrium steady state. The flow pattern varies as the increase of inlet velocity. Only inlet velocity greater than “critical inlet velocity” can cavitation bubbles exist perpetually in protruding structure. The value of the critical inlet velocity decreases as the increase of width ratio and the resulting cavitation number falls to 1.8–2.1. The main mechanism for cavitation in protruding structure is pressure drop, rather than shear force.     

Factors affecting small axial cooling fan performance

Many factors such as outer diameter,hub ratio,blade numbers,shape and stagger angle affect the performance of small cooling fans.A small cooling fan was simulated using CFD software for three blade stagger angles (30.5°,37.5°,44.5°)and obtained the internal flow field and the static characteristics.Research indicated that the stagger angle has an obvious effect on the static characteristics of a fan.For flow rates below 0.0104 m3/s,total pressure is the greatest when the stagger angle is 37.5°;flow rates higher than 0.0104 m3/s,the total pressure is greatest when the stagger angle is 44.5° For the same flow rates,the velocity at inlet of pressure surface increases with increasing stagger angle,but the change of velocity on the suction surface is very small.For one model,vortices and the speed of revolution surfaces decrease with tip clearance increasing.But for other three models,increasing the stagger angle,the vortex intensity and speed of revolution surfaces at same height tip clearance increases,simultaneously,the position of vortex offset from the top of the rotor blade to the suction surface.     

Comparative study of high concentrating photovoltaics integrated with phase‐change liquid film cooling system

In high concentrating photovoltaic systems, thermal regulation is of great importance to the conversion efficiency and the safety of solar cells. Direct‐contact liquid film cooling technique is an effective way of thermal regulation with low initial investment. Tilt of solar cells is common in concentrating solar systems. An evaluation of direct‐contact liquid film cooling technique behind tilted high concentration photovoltaics was performed using both experimental and computational approaches. In the experiment, deionized water was used as the coolant at the back of simulated solar cells. Solar cell inclination of 0° to 75° with inlet water flow rate of 100–300 L/hour and inlet temperature of 30°C to 75°C were experimentally investigated. A two‐dimensional model was developed using computational fluid dynamics technique and validated by experimental results. The effects of inclination on average temperature, temperature uniformity, and heat transfer coefficient were discovered in this paper. The results indicated that 20° is the optimum angle for liquid film cooling. In addition, optimum inlet width, temperature, and velocity for inclination over 30° are 0.75 mm, 75°C, and 0.855 m/s, respectively.     

Numerical evaluation of the clearance geometries effect on the flow field and performance of a hydrofoil

The blade tip leakage flow with efficiency losses and cavitation phenomena is a concern for the low-head tidal power units. A simplified case of NACA0009 hydrofoil in a water tunnel is used to investigate the effects of tip clearance geometries including the foil tip shape and gap width on the flow features and foil performance. Steady non-cavitating simulations are implemented for a round tip foil and a sharp tip foil with two incidence angles (α = 10° and 5°) and different normalized gap width (τ). The minimum pressure is used to reflect the normalized vortex intensity (Γ*) and cavitation characteristics. The Γ*-τ curves at different streamwise positions show that the sharp tip foil generates relatively weaker tip leakage vortex with more flat curves, but its higher Γ* of tip separation vortex in wider gaps increases the risk of clearance cavitation. The flow features on a cross section inside the gap suggest that the sharp tip reduces the leakage flow losses and increases the velocity gradient due to the boundary layer separation. The lift coefficient is a little higher for the sharp tip foil than the round tip foil, with small differences for α = 5° but noticeable deviations for α = 10° especially within 0.3<τ < 1.     

Numerical investigation of unsteady aerodynamics of a Horizontal‐axis wind turbine under yawed flow conditions

In the present study, unsteady flow features and the blade aerodynamic loading of the National Renewable Energy Laboratory phase VI wind turbine rotor, under yawed flow conditions, were numerically investigated by using a three‐dimensional incompressible flow solver based on unstructured overset meshes. The effect of turbulence, including laminar‐turbulent transition, was accounted for by using a correlation‐based transition turbulence model. The calculations were made for an upwind configuration at wind speeds of 7, 10 and 15 m/sec when the turbine rotor was at 30° and 60° yaw angles. The results were compared with measurements in terms of the blade surface pressure and the normal and tangential forces at selected blade radial locations. It was found that under the yawed flow conditions, the blade aerodynamic loading is significantly reduced. Also, because of the wind velocity component aligned tangent to the rotor disk plane, the periodic fluctuation of blade loading is obtained with lower magnitudes at the advancing blade side and higher magnitudes at the retreating side. This tendency is further magnified as the yaw angle becomes larger. At 7 m/sec wind speed, the sectional angle of attack is relatively small, and the flow remains mostly attached to the blade surface. At 10 m/sec wind speed, leading‐edge flow separation and strong radial flow are observed at the inboard portion of the retreating blade. As the wind speed is further increased, the flow separation and the radial flow become more pronounced. It was demonstrated that these highly unsteady three‐dimensional aerodynamic features are well‐captured by the present method. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of the semi-analytical cavitation model to flows in a centrifugal pump

Visualization experiments are carried out to investigate the cavitating flows in a centrifugal pump at different flow rates. The cavity lengths under different pump inlet pressure are obtained. The semi-analytical cavitation model is improved since it is not suitable for predicting large cavities full of vapor. The improved cavitation model is used to numerically study the steady and rotating cavitation in the centrifugal pump. Compared with the Zwart cavitation model, the numerical results predicted by the semi-analytical model agree much better with the experiments, especially for large cavities. The cavity lengths at the suction side are overestimated during the simulations, especially by using the Zwart model. At low flow rates, the prediction of the rotating cavitation effect is weaker and the cloud shedding frequency is smaller than those in the experiments.     

转炉蒸发冷却器内喷嘴雾化介质选择的数值模拟

针对转炉蒸发冷却系统内气液外混双相流喷嘴雾化介质的选择,应用计算流体力学,通过FLUENT模块主要运用VOF模型在不同压力工况下对雾化介质为水蒸气和氮气时外混喷嘴的气液流动状态进行数值模拟,通过模型x=0截面的速度云图及矢量图观测计算模拟结果,并以条件雾化角θ、质量平均直径d_(MMD)和索太尔平均直径d_(SMD)为雾化质量的判断标准。所得水蒸气雾化介质下的平均θ为74.43°,d_(MMD)为32.14μm,d_(SMD)为64.99μm;氮气雾化介质条件下的平均θ为68.13°,d_(MMD)为24.48μm,d_(SMD)为65.10μm。对三个判断标准和雾化质量的相关性进行分析,最终建议气液压力比为2的条件下,雾化介质采用氮气。     

Experimental study on heat transfer augmentation in a double pipe heat exchanger utilizing jet vortex flow

This paper examines experimentally the effect of jet vortex technology on enhancing the heat transfer rate within a double pipe heat exchanger by supplying the heat exchanger with water at different vortex strengths. A vortex generator with special inclined holes with different inlet angles was designed, manufactured, and integrated within the heat exchanger. In this study, four levels of Reynolds number for hot water in the annulus (Re_h) were used, namely, 10,000; 14,500; 18,030; and 19,600. Similarly, four levels of Reynolds number for cold water in the inner tube (Re_c) were used, namely, 12,000; 17,500; 22,500; and 29,000. As for the inlet flow angle (θ), four different levels were selected, namely, 0°, 30°, 45°, and 60°. The temperature along the heat exchanger was measured utilizing 34 thermocouples installed along the heat exchanger. It was found that increasing the inlet flow angle (θ) and/or the Reynolds number results in an increase in the local Nusselt number, the overall heat transfer coefficient, and the ratio of friction factor. It is revealed that the percentage increase in the average Nusselt number due to swirl flow compared to axial flow was 10%, 40%, and 82% for an inlet flow angle of 30°, 45°, and 60°, respectively.     

柴油机模型喷孔内空化与空蚀特性试验研究

在真实喷油器空蚀试验基础上,利用X射线CT扫描检测到了喷嘴内微量空蚀。为获得空化对空蚀的具体影响,搭建了可同时获得喷孔内空化区域和空蚀区域及形貌的可视化试验装置,采用高速摄影技术和电镜扫描(SEM)技术,针对一个矩形喷孔内空化流动和空蚀特性进行试验研究。结果表明:喷射压力越高,喷孔内空化发展越充分,并出现了单相流、不完全空化流、超空化流和水力回流4种不同流态;云空化脱落呈现出一定的循环周期性,且喷孔下游空蚀区与云空化脱落溃灭区非常吻合。结合壁面附近空化区与液相区交界处存在的空蚀现象,推理云空化脱落溃灭和冷凝溃灭是喷孔内空蚀形成的两大因素。     

Investigation of coolant flow distribution and the effects of cavitation on water pump performance in an automotive cooling system

Cavitation is a well‐known phenomenon that causes performance losses in all kinds of hydraulic machinery, including automotive water pumps. The present study uses a coolant flow test rig to investigate cavitation in water pumps. The coolant flow rate was measured for various coolant temperatures and compositions. This study validates that cavitation occurs during the coolant warm‐up period, in which coolant temperature is typically below 80°C. Cavitation was also related to a drop in the water pump inlet pressure and driving torque. Based on the results from this study, it can be concluded that cavitation is affected by coolant temperature, engine speed, and coolant composition. Furthermore, it is found that the use of an electric water pump is effective for minimizing the pressure drop and driving loss of the pump. Copyright © 2008 John Wiley & Sons, Ltd.     

折弯弓形折流板换热器流动换热性能研究

为了提高折流板换热器的换热性能,改变了折流板换热器的折弯夹角和折流板间距,利用ANSYS Fluent对换热器壳程流体流动与换热过程进行模拟,分析了不同折流板折弯夹角α (110°,135°,170°和180°)、折流板间距（250,300和350 mm)和雷诺数（10 000,20 000和50 000）对换热器壳程压力、速度和温度的影响。结果表明：增大雷诺数对改善流动死区有很大的作用,雷诺数为50 000时的流动死区相对于雷诺数为10 000时面积减小较大;随着夹角α的减小,折流板背流侧的流动死区面积逐渐减小、换热器的表面传热系数和进出口压降力越大,夹角α为110°时出口温度最小、进出口压降最大,夹角α为135°时PEC最大且换热器综合性能最优;折流板间距增大,压力变化梯度减小,压差变化幅度减小,壳程出口温度变化不成正比关系,间距为300 mm时出口温度最低。     

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.     

Interface resolving two-phase flow simulations in gas channels relevant for polymer electrolyte fuel cells using the volume of fluid approach

With the increased concern about energy security, air pollution and global warming, the possibility of using polymer electrolyte fuel cells (PEFCs) in future sustainable and renewable energy systems has achieved considerable momentum. A computational fluid dynamic model describing a straight channel, relevant for water removal inside a PEFC, is devised. A volume of fluid (VOF) approach is employed to investigate the interface resolved two-phase flow behavior inside the gas channel including the gas diffusion layer (GDL) surface. From this study, it is clear that the impact on the two-phase flow pattern for different hydrophobic/hydrophilic characteristics, i.e., contact angles, at the walls and at the GDL surface is significant, compared to a situation where the walls and the interface are neither hydrophobic nor hydrophilic (i.e., 90° contact angle at the walls and also at the GDL surface). A location of the GDL surface liquid inlet in the middle of the gas channel gives droplet formation, while a location at the side of the channel gives corner flow with a convex surface shape (having hydrophilic walls and a hydrophobic GDL interface). Droplet formation only observed when the GDL surface liquid inlet is located in the middle of the channel. The droplet detachment location (along the main flow direction) and the shape of the droplet until detachment are strongly dependent on the size of the liquid inlet at the GDL surface. A smaller liquid inlet at the GDL surface (keeping the mass flow rates constant) gives smaller droplets.     

导叶开度对混流式水轮机转轮内部流态的影响

基于数值模拟方法,以梯级水光蓄互补联合发电系统示范工程中的猛固桥电站混流式水轮机为研究对象,对水轮机在不同特征水头、不同导叶开度工况下转轮叶片表面受力及流道内流场特性进行研究,分析导叶开度改变对水轮机转轮内流态的影响规律。结果表明：导叶开度对于混流式转轮进口流态的影响较大,导叶开度越小入流速度波动程度越大,来流对叶片头部冲击越大,在进口处产生严重的冲击损失;随导叶开度减小,转轮叶片表面等压线与进口边夹角增大,在上冠交接区域产生小三角区低压,使转轮流道出口更易产生空化损失。     

Flow separation in a diverging conical duct: Effect of Reynolds number and divergence angle

Fluid flows in passages whose cross-sectional area increases in the streamwise direction are prone to separation. Here, the flow in a conical diffuser fed by a fully developed velocity at its inlet and mated at its downstream end to a long circular pipe is investigated by means of numerical simulation. A universal flow-regime model was used to accommodate possible laminarization of flows having moderate-turbulent and transitional Reynolds numbers at the diffuser inlet. It was found that flow separation occurred for a diffuser expansion angle of 5° for inlet Reynolds numbers less than about 2000. This finding invalidates a prior rule-of-thumb that flow separation first occurs at a divergence angle of seven degrees. Results from the 10 and 30° simulations showed separation at all investigated Reynolds numbers. The largest streamwise length of the separation zones occurred at the lower Reynolds numbers.     

Asymmetrical Non-Uniform Heat Flux Distributions For Laminar Flow Heat Transfer With Mixed Convection In a Horizontal Circular Tube

Non-symmetric heat flux distributions in terms of gravity in solar collector tubes influence buoyancy-driven secondary flow which has an impact on the associated heat transfer and pressure drop performance. In this study the influence of the asymmetry angle (0°, 20°, 30° and 40°) with regard to gravity for non-uniform heat flux boundaries in a horizontal circular tube was investigated numerically. A stainless steel tube with an inner diameter of 62.68 mm, a wall thickness of 5.16 mm, and a length of 10 m was considered for water inlet temperatures ranging from 290 K to 360 K and inlet Reynolds numbers ranging from 130 to 2000. Conjugate heat transfer was modelled for different sinusoidal type outer surface heat flux distributions with a base-level incident heat flux intensity of 7.1 kW/m². It was found that average internal heat transfer coefficients increased with the circumferential span of the heat flux distribution. Average internal and axial local heat transfer coefficients and overall friction factors were at their highest for symmetrical heat flux cases (gravity at 0º) and lower for asymmetric cases. The internal heat transfer coefficients also increased with the inlet fluid temperature and decreased with an increase in the external heat loss transfer coefficient. Friction factors decreased with an increase in fluid inlet temperature or an increase in the external heat loss transfer coefficients of the tube model.     

Numerical Investigation on the Two Phase Flow Behaviors in Supercritical Water Fluidized Bed with Swirling Flow Distributor

ABSTRACT

Supercritical water fluidized bed reactor is a promising in the clean and efficient conversion of coal, and the distributor is one of the key component for the heat and mass transfer enhancement. However, the optimization study for the distributor in supercritical water fluidized bed reactor has been seldom conducted due to the special thermal properties of supercritical water. In this work, the swirling flow distributor was designed for its optimization for heat and mass transfer inside a supercritical water fluidized bed reactor. The swirling flow can be generated by the concentric circle or triangle type hole distribution in distributor with 0 or 45° intersection angle between the fluid inlet velocity direction and the distributor plane. The computational particle fluid dynamics (SCWFB) method, which has quick calculating speed and high accuracy, was used in this work to study the particle-fluid two-phase flow behaviors inside SCWFB with swirling flow distributors. Investigations were made to reveal the influence of the hole distribution type and intersection angle on the bed pressure drop and particle volume fraction characteristics. The results showed that the triangle type distributor with 45° intersection angle has the best fluidization performance. The conclusions drawn may has potential application for continuous and stable operation of supercritical water fluidized bed reactor for coal gasification.     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

NUMERICAL SIMULATIONS ON THE HYDRODYNAMICS OF A TURBULENT SLOT JET IMPINGING ON A SEMICYLINDRICAL CONVEX SURFACE

This study presents the numerical simulations of flow characteristics of a turbulent slot jet impinging on a semicylindrical convex surface. The turbulent-governing equations are solved by a control-volume-based finite difference method with power-law scheme, and the well-known k  ? ε turbulence model associated with the wall function is used to describe the turbulent behavior and structure.

While the width of the slot nozzle is fixed at 9.38 mm, the diameter of the semicylinder is at 150 mm, and air is the working medium, the adopted modifying parameters here include the Reynolds number of the inlet flow (Re = 6000 ~ 20000), jet to impingement surface spacing (y / w = 7 ~ 13), and the entrainment or wall boundary is employed nearby the convex surface. The numerical simulations of flow fields indicate that the velocity distribution of the free jet region departs from the center with increasing y / w. When we increase Reynolds number Re, the variation of the velocity on the convex surface becomes rapid, and the turbulent kinetic energy increases.