Supersonic moist air flow with condensation in a wavy wall channel

The characteristics of Prandtl-Meyer expansion of supersonic flow with condensation along a wavy wall in a channel are investigated by means of experiments and numerical analyses. Experiments are carried out for the case of moist air flow in an intermittent indraft supersonic wind tunnel. The flow fields are visualized by a Schlieren system and the distributions of static pressure along the upper wavy wall are measured by a scanning valve system with pressure transducers. In numerical analyses, the distributions of streamlines, Mach lines, iso-pressure lines, and iso-mass fractions of liquid are obtained by the two-dimensional direct marching method of characteristics. The effects of stagnation temperature, absolute humidity, and attack angle of the upper wavy wall on the generation and the locations of generation and reflection of an oblique shock wave are clarified. Futhermore, it is confirmed that the wavy wall plays an important role in the generation of an oblique shock wave and that the effect of condensation on the flow fields is apparent.     

Numerical investigation of fluid flow and heat transfer characteristics in a helically-finned tube

In order to investigate the characteristics of flow and heat transfer rate in a Helically-finned tub (HFT), we used continuity, momentum and energy equations under a steady, three-dimensional and incompressible fluid flow assumptions. For the performance metrics, we considered the Darcy friction factor, Colburn j-factor, volume goodness factor and area goodness factor of the HFT. We could also evaluate the effect of geometry parameters on the results of local pressure coefficient, fluid vorticity and Nusselt number of the HFT. We carried out the CFD calculation for a range of laminar flow (Re = 100) and turbulent flow (Re = 2000 and 10000). In a laminar and turbulent flow regime, the friction factor increases with increasing the each geometric parameter. While the Colburn j-factor decreases as increasing these geometric parameters. Consequently, the thermal performance of HFT is poorer than that of single straight circular tube type because of having a small volume and area goodness factor as increasing the Reynolds numbers.     

Jet flow characteristics of sinusoidal wavy nozzles

The flow characteristics of jets issued from a sinusoidal nozzle with in-phase and 180° out-of-phase exit configurations were investigated using PIV (particle image velocimetry) and flow visualization techniques. The experiments were carried out at a Reynolds number of about 6300 based on the mean width of the jet nozzle. Compared to a normal rectangular jet, the sinusoidal nozzle jets have smaller velocity deficits as the flow goes downstream. In addition, the turbulence intensity is suppressed in the horizontal center plane. For the case of in-phase wavy nozzle jet, the length of the potential core exhibits small variations along the lateral direction, while the 180° out-of-phase wavy nozzle jet shows large lateral variation in the length of potential core. The turbulent kinetic energy of the 180° out-ofphase nozzle jet also shows sinusoidal variation in the horizontal planes. Large-scale vortices shed from the sinusoidal edge of the nozzle interact strongly and migrate toward the center plane as the flow develops downstream.     

Direct numerical simulation of flow characteristics and heat transfer enhancement in a rib-dimpled cooling channel

The present study reports the numerical investigation on the flow characteristics and heat transfer enhancement of the rib-dimpled channels. Two geometric variables were considered: the rib angle, θ, and the length between the rib center and the dimple rim, l. Nine cases were investigated by combining three different rib angles with three different lengths. Direct numerical simulations were conducted with a Reynolds number of 2800. As θ and l changed, the flow characteristics of the rib-dimpled channel were altered, which lead to different characteristics in the flow mixing and heat transfer rate. The span-wise rotating flow and the up-wash counter rotating vortices played an important role in the augmentation of heat transfer rate. The rib-dimpled channel with l = 0.15 and θ = 70° showed the maximum increase of 32 % in the volume goodness factor, in comparison with the general dimpled channel.

     

Numerical study of fluid flow and convective heat transfer characteristics in a twisted elliptic tube

We investigated the flow and heat transfer characteristics in a Twisted elliptic tube (TET). The effects of geometry parameters such as the aspect ratio and number of rotations in the TET were analyzed comparatively using three-dimensional (3-D) numerical simulation. We also solved numerically the conservation equations of continuity, momentum, and energy in the TET. Fully developed flow in the TET was modeled using the realizable k-ε turbulence model and steady incompressible Reynolds-averaged Navier-Stokes (RANS) equations. The simulation was performed for Reynolds numbers of 100, 1000 and 10000. The pressure drop and the heat transfer of the TET were assessed in terms of the Darcy friction factor and Colburn j-factor, and overall performance was evaluated using the area and volume goodness factors.

     

Experimental study of the flow between triangular ribs in a square channel

An experimental study of the flow features of a channel with triangular ribs was carried out using particle image velocimetry technique. The experiments were performed for three Reynolds numbers, including 2900, 8400 and 15000, and were based on the hydraulic diameter of the channel and the mean velocity. The mean and instantaneous velocity fields and turbulence statistics were also depicted in detail.

     

A study on the flow characteristics of butterfly valve with baffles

Journal of Mechanical Science and Technology - In the industry, globe valves, which are commonly used to precise control the flow rate along with opening and closing flow in pipes, are technically...     

CFD analysis of effects on fluid flow resistance of metallic wavy structures

Microscale wrinkles can be utilized to enhance the characteristics of products. For example, a shark skin having riblet surface can move faster underwater, minimizing fluid flow resistance. We introduce a novel approach to fabricate microscale metallic wrinkles using a soft lithography and electroforming process; we also evaluated the effects of wrinkles by computational fluid dynamics (CFD) analyses. For the generation of metallic wrinkles, a UV-curable resin, NOA68T was used to fabricate a master wrinkling pattern by mechanism of compressive forces on a skin of the weakly polymerized resin layer. The master pattern was molded and replicated as metallic wrinkles on a surface via soft lithography and electroforming processes. To understand the advantages of a wavy surface on reduction of flow resistance, we carried out two- and three-dimensional (2D and 3D) CFD analyses. The drag coefficient of a wrinkled 2D square model was decreased about 17.1 %, and a 3D real wrinkle model showed 4.9 to 7.3 % reduction of it compared to without wrinkle models. We believe that it is possible to reduce the fluid flow resistance using wavy surfaces that can easily be generated selectively or wholly on an arbitrary surface.

     

典型波纹翅片单元流动与传热特性的数值研究

以两种典型的波纹翅片单元为研究对象,在合理简化条件下给出了物理模型和数学模型,通过流固界面的传热耦合,对不同进口风速下波纹翅片单元的流动及传热和阻力特性进行了数值研究.通过对传热系数,Nu数、压降以及涡量分布的对比分析,结果表明:人字形翅片的传热性能优于波浪形翅片,而流动阻力性能却没有明显的劣势,其主要原因是翅片流场中涡流的产生与耗散存在差异.     

Magnetic field effect on fluid flow characteristics in a pipe for laminar flow

The influence of a magnetic field on the skin friction factor of steady fully-developed laminar flow through a pipe was studied experimentally. A mathematical model was introduced and a finite difference scheme used to solve the governing equations in terms of vorticity-stream function. The model predictions agree favourably with experimental results. It is observed that the pressure drop varies in proportion to the square of the product of the magnetic field and the sine of the magnetic field angle. Also, the pressure drop is proportional to the flow rate. This situation is similar to what applies in the absence of a magnetic field. It is found that a transverse magnetic field changes the axial velocity profile from the parabolic to a relatively flat shape. At first, the radial velocity rises more rapidly and then gradually decreases along the pipe until falling to zero. A numerical correlation can be written for the considerable distance required for the new axial velocity profile to establish. Owing to the changes taking place in the axial velocity profile, it exhibits a higher skin friction factor. The new axial velocity profile asymptotically approaches its limit as the Hartmann number becomes large.     

Numerical and experimental study on a channel mixer with a periodic array of cross baffles

In this study we show an enhanced mixing effect with a simple channel having a periodic array of cross baffles. We performed numerical compulation to obtain the steady flow field within the channel at low Reynolds numbers by using a commercial code, ANSYS CFX 10.0. A visualization experiment was also conducted to validate our numerical results qualitatively. In evaluating the mixing performance, we employed the Lyapunov exponent. It was shown that the visualized mixing pattern was in a good agreement with that numerically given. Our Liapunov exponent distribution in the space also demonstrates that the proposed channel design indeed exhibits a chaotic stirring at low Reynolds numbers. Our design is thus assumed to be applicable to designing a microchannel mixer with enhanced mixing effect     

A numerical study of the fluid flow and heat transfer around a single row of tubes in a channel using immerse boundary method

The analysis of the time-dependent and two-dimensional fluid flow and heat transfer around a single row of tubes in a channel is performed numerically. Due to its fundamental significance and practical importance, aerodynamic and heat transfer characteristics of tube bundle have been paid great attention by many researchers. In the present study, the immersed boundary method is applied by using a Cartesian grid system. Numerical solution for the governing equations of mass, momentum and energy conservation is obtained with the finite volume method. To validate the numerical approach with the immersed boundary method, the results have been compared with published data. The generation and evolution of vortical structures, wake interactions and their effects on the drag, lift and heat transfer are analyzed at different Reynolds numbers. The effect of hydraulic boundary layer development on the fluid flow and heat transfer is also investigated.     

Investigation of a novel Couette flow with cylindrical baffles

The high-precision measure instrument for flow velocity is essential for industrial applications because the high-precision velocity can well reflect the physical characteristic of the flow. A restricted laminar Couette flow with cylindrical baffles, using a synthetic heat conduction liquid, was designed to obtain a steady vortex flow and wider work scope, according to Couette flow and Suspension flow characteristics. The heat transfer mechanism was investigated with a laminar flow model by the Fourier law. The research indicates that the heat transfer enhancement is related to the Temperature Boundary Layer (TBL). The TBL is affected by the Velocity Boundary Layer (VBL). The TBL thickness and Nusselt number (Nu) have a dependent relationship. The Reynolds number (Re) and the gap between the baffle and plate wall (Δh/h) can further affect Nu. The vortex flow generated by Couette flow can significantly enhance the heat transfer performance by a double spiral structure, which can rapidly mix heat fluxes and make the temperature converge to uniform. There is a sensitive and stable relationship between flow velocity and heat transfer. Notably, it is linear when Δh/h or Re is small, which can be used to design a high-precision thermal flow velocity meter.     

Characteristics of fluid flow and heat transfer in a fluidized heat exchanger with circulating solid particles

The commercial viability of heat exchanger is mainly dependent on its long-term fouling characteristic because the fouling increases the pressure loss and degrades the thermal performance of a heat exchanger. An experimental study was performed to investigate the characteristics of fluid flow and heat transfer in a fluidized bed heat exchanger with circulating various solid particles. The present work showed that the higher densities of particles had higher drag force coefficients, and the increases in heat transfer were in the order of sand, copper, steel, aluminum, and glass below Reynolds number of 5,000.     

Numerical simulation of fluid flow and heat transfer in a plasma spray gun

A computational fluid dynamics (CFD) simulation for analyzing fluid flow patterns in a plasma spray gun is presented in this study. It is coupled with a heat transfer simulation of the plasma spray gun. Based on CFD and heat transfer theory, the numerical model of the nozzle in the plasma spray gun is developed, and the coupled simulation of the flow fluid and heat transfer is carried out with the semi-implicit method for pressure-linked equations (SIMPLE) method. Local turbulence, which will lead to appearance of a static-water region, is found at the front corner of the cooling channel in the nozzle. The locations insufficiently cooled are found in the wall near the heat source and in the gasket in the rear of the nozzle. Then, cooling processes with different parameters of cooling water are analyzed. The optimal velocity and direction of cooling water, which efficiently cool the nozzle and improve the service life of the plasma jet, are obtained .     

Large-eddy simulation of fluid flow and heat transfer in a mixing tee junction

The temperature fluctuation caused by thermal striping phenomena of hot and cold fluids mixing results in cyclical thermal stress fatigue failure of the pipe wall. Mean temperature difference between hot and cold fluids was often used as thermal load in previous analysis of thermal fatigue failure, thereby the influences of the amplitude and frequency of temperature fluctuation on thermal fatigue failure were neglected. Based on the mechanism of flow and heat transfer which induces thermal fatigue, the turbulent mixing of hot and cold water in a tee junction is simulated with FLUENT platform by using the Large-eddy simulation(LES) turbulent flow model with the sub-grid scale(SGS) model of Smagorinsky-Lilly(SL) to capture the amplitude and frequency of temperature fluctuation. In a simulation case, hot water with temperature of 343.48 K and velocity of 0.15 m/s enters the horizontal main duct with the side length of 100 mm, while cold water with temperature of 296.78 K and velocity of 0.3 m/s enters the vertical branch duct with the side length of 50 mm. The numerical results show that the mean and fluctuating temperatures are in good agreement with the previous experimental data, which describes numerical simulation with high reliability and accuracy; the power spectrum density(PSD) on top wall is higher than that on bottom wall(as the frequency less than 1 Hz), while the PSD on bottom wall is relatively higher than that on top wall (as the frequency of 1-10Hz). The temperature fluctuations in full mixing region of the tee junction can be accurately captured by LES and can provide the theoretical basis for the thermal stress and thermal fatigue analyses.     

Numerical simulation of fluid flow and heat transfer in a plasma spray gun

A computational fluid dynamics (CFD) simulation for analyzing fluid flow patterns in a plasma spray gun is presented in this study. It is coupled with a heat transfer simulation of the plasma spray gun. Based on CFD and heat transfer theory, the numerical model of the nozzle in the plasma spray gun is developed, and the coupled simulation of the flow fluid and heat transfer is carried out with the semi-implicit method for pressure-linked equations (SIMPLE) method. Local turbulence, which will lead to appearance of a static-water region, is found at the front corner of the cooling channel in the nozzle. The locations insufficiently cooled are found in the wall near the heat source and in the gasket in the rear of the nozzle. Then, cooling processes with different parameters of cooling water are analyzed. The optimal velocity and direction of cooling water, which efficiently cool the nozzle and improve the service life of the plasma jet, are obtained .     

折流板换热器壳程流体流动与传热特性数值模拟

根据流体动力学和计算传热学理论,建立了折流板管壳式换热器计算模型,运用CFD技术对换热器壳程流体的流动与传热问题进行了三维数值模拟,得到了不同壳程进口雷诺数Re条件下换热器壳程流体的流场和温度场。对数值模拟结果进行分析,以总传热系数h,壳程总压降Δp以及单位压力损失下的传热系数h/Δp作为换热器性能的衡量标准,分析了不同折流板间距和不同折流板圆缺高度时管壳式换热器壳程总传热系数h、总压降Δp以及h/Δp随壳程进口雷诺数的变化规律。结果表明:随着壳程进口流速的增大,换热器壳程总传热系数和总压降增大、h/Δp减小;在壳程流体流量不变的情况下,结合单位压力损失下的传热系数h/Δp,适当减小折流板间距或减小折流板圆缺高度,可提高换热器的换热性能。     

An experimented study on the flow characteristics in a three-dimensional cylindrical branching channel

In this paper, we report on an experimented study on incompressible Newtonian flow in a three-dimensional cylindrical branching channel. The flow configuration studied in the present investigation is such that a fully developed laminar flow enters an abruptly expanded cylinder and the flow leaves this cylinder by two identical cylindrical outlet branch pipes. Representative velocities in the flow field were recorded by LDV (Laser Doppler Velocimetry) measurements, and volume flow rate from each outlet branch pipe was measured. Flow visualization in representative symmetric planes was also carried out. Based on results of the present investigation, the flow field in the three-dimensional cylindrical branching channel was clarified within the range of laminar flow. The characteristics of the branch flow rate were obtained and showed that there exist two distinct domains of strong asymmetric flow distribution from the outlet branch pipes, depending upon Reynolds numbers. It was further observed that the flow became time periodic as the Reynolds number was increased.     

The effect of suction and injection on unsteady flow of a dusty conducting fluid in rectangular channel

In the present study, the unsteady Hartmann flow of a dusty viscous incompressible electrically conducting fluid under the influence of an exponentially decreasing pressure gradient is studied without neglecting the ion slip. The parallel plates are assumed to be porous and subjected to a uniform suction from above and injection from below. The fluid is acted upon by an external uniform magnetic field which is applied perpendicular to the plates. An analytical solution for the governing equations of motion is obtained to yield the velocity distributions for both the fluid and dust particles.