Influences of the wavy surface inserted in the middle of a circular tube heat exchanger on thermal performance

Numerical investigations on flow topology, heat transfer behavior and performance evaluation in a circular tube inserted with various configurations of wavy surfaces, Inclined wavy surface (IWS), V-downstream wavy surface (VDWS), V-Upstream wavy surface (VUWS) are presented. The effects of the flow attack angles; 20°, 30°, 45° and 60° are studied for the Reynolds numbers, Re = 100-2000. The numerical results are compared with the smooth circular tube with no wavy surface and the previous works. It is found that the IWS, VDWS and VUWS can produce longitudinal vortex flow and impinging jet of the fluid flow like inclined baffle, V-downstream baffle and V-Upstream baffle, respectively, but give lower friction loss. The flow phenomena created by the wavy surfaces help to augment the heat transfer rate and thermal performance in the test tube. In the range studied, the order of enhancement for heat transfer rate is around 1.40-3.75, 1.60-6.25 and 1.30-5.80 times higher than the smooth tube for IWS, VDWS and VUWS, respectively. Moreover, the maximum thermal performance, presented in terms of the Thermal enhancement factor (TEF), is found to be about 1.60, 2.40 and 2.10, respectively, for IWS, VUWS and VDWS.     

Turbulent heat transfer characteristics in compound channels with gap

To examine the characteristics of turbulent flow and heat transfer about the compound channel with gap, the present study has performed a numerical analysis by using large eddy simulation (LES). Our first aim was to analyze the cause of pulsating flow by investigating the turbulent flow through the unsteady analysis of compound channel. Next, to understand the correlations between gap width and heat transfer enhancement, we tried to verify the enhancement effect of heat transfer when a gap exists within the compound channel. From this numerical analysis, we could see that the lateral velocity occurs periodically around the gap and this is developed in the pulsating flow. The pulsating flow has generated a strong turbulent flow mixing within the compound channel. And the turbulent flow mixing in the pulsating flow plays an effective role of enhancing heat transfer effect by making the fluid temperature uniformly within the compound channel.     

Experimental investigation of heat transfer enhancement in a circular duct with circumferential fins and circular disks

The characteristics of heat transfer and pressure drop for fully developed turbulent flow in a tube with circumferential fins and circular disks were experimentally studied. The various spacing and sizes of circumferential fins and circular disks were selected as design parameters, while the effects of these parameters on heat transfer enhancement and pressure drop were investigated. In order to quantify the effect of heat transfer enhancement and the increase of pressure drop due to the fins and disks in a tube, the Nusselt numbers and the friction factors for various configurations and operating conditions were compared to those for a corresponding smooth tube. The results showed that the heat transfer rate was significantly enhanced by increasing the height of circumferential fins and decreasing the pitch of circumferential fins. On the other hand, the influence of the disk size and the fin-disk spacing were not significant. Based on the experimental results, a correlation for estimating the Nusselt number was suggested.     

Axial slit wall effect on the flow instability and heat transfer in rotating concentric cylinders

The slit wall effect on the flow instability and heat transfer characteristics in Taylor-Couette flow was numerically studied by changing the rotating Reynolds number and applying the negative temperature gradient. The concentric cylinders with slit wall are seen in many rotating machineries. Six different models with the slit number 0, 6, 9, 12, 15 and 18 were investigated in this study. The results show the axial slit wall enhances the Taylor vortex flow and suppresses the azimuthal variation of wavy Taylor vortex flow. When negative temperature gradient exists, the results show that the heat transfer augmentation appears from laminar Taylor vortex to turbulent Taylor flow regime. The heat transfer enhancement become stronger as increasing the Reynolds number and slit number. The larger slit number model also accelerates the flow transition regardless of the negative temperature gradient or isothermal condition.     

再循环重力供液制冷系统换热性能可视化研究

建立了再循环重力供液制冷系统可视化试验台,在不同工况下对制冷系统进行实验研究,观察玻璃蒸发管内制冷剂的沸腾换热流动状态,研究分析重力供液蒸发器的传热特性。试验表明:再循环重力供液蒸发器内,制冷剂的沸腾换热出现了气泡流、气塞流、气弹流、分层流、波状分层流等流型。通过编程计算得到,经过修正的J.Chawla关联式和Kandlikar关系式分别在低温及高温工况下对沸腾换热系数有较好的预测,计算结果与试验值的偏差均在12.5%以内,采用两种关联式相结合的方法能较好地对重力供液蒸发器管内沸腾换热进行预测。     

The effect of cooling conditions on convective heat transfer and flow in a steam-cooled ribbed duct

This work presents a numerical and experimental investigation on the heat transfer and turbulent flow of cooling steam in a rectangular duct with 90° ribs and studies the effect of cooling conditions on the heat transfer augmentation of steam. In the calculation, the variation range of Reynolds is from 10,000 to 190,000, the inlet temperature varies from 300°C to 500°C and the outlet pressure is from 0.5MPa to 6MPa. The aforementioned wide ranges of flow parameters cover the actual operating condition of coolant used in the gas turbine blades. The computations are carried with four turbulence models (the standard k-?, the renormalized group (RNG) k-?, the Launder-Reece-Rodi (LRR) and the Speziale-Sarkar-Gatski (SSG) turbulence models). The comparison of numerical and experimental results reveals that the SSG turbulence model is suitable for steam flow in the ribbed duct. Therefore, adopting the conjugate calculation technique, further study on the steam heat transfer and flow characteristics is performed with SSG turbulence model. The results show that the variation of cooling condition strongly impacts the forced convection heat transfer of steam in the ribbed duct. The cooling supply condition of a relative low temperature and medium pressure could bring a considerable advantage on steam thermal enhancement. In addition, comparing the heat transfer level between steam flow and air flow, the performance advantage of using steam is also influenced by the cooling supply condition. Changing Reynolds number has little effect on the performance superiority of steam cooling. Increasing pressure would strengthen the advantage, but increasing temperature gives an opposite result.     

湍流边界层内钝体扰流的流动与传热特性

应用大涡模拟方法对小尺度开缝圆柱涡流发生器强化传热和流动减阻的机理进行研究。水平开缝圆柱置于充分发展湍流边界层内,分析不同间隙比对开缝圆柱尾流、湍流边界层拟序结构以及槽道底面流动与换热特性的影响。为验证所采用数值方法的准确性与可靠性,将矩形空槽道的计算结果与前人直接数值模拟结果及与采用相关准则关系式所得结果进行对比。计算结果表明：湍流边界层内钝体扰流的尾迹流与壁面边界层的相互作用能够显著提高槽道的换热性能。与未开缝的基准圆柱相比,间隙比小于2.0时,开缝圆柱通道的整体热性能较好;间隙比为2.0时,其综合性能系数最大;间隙比大于2.0时,整体热性能较差。与矩形空槽道相比,最大努塞尔数可提高17.45%,最小摩擦因数可减小4.94%。     

3D打印三层微通道冷板的综合特性研究

3D打印技术在快速成型和制造复杂结构零件方面具有巨大的优势.文中采用3D打印工艺制备了3种微通道散热器,分别是2种开放型的单层微通道和1种封闭型的三层微通道.对三层微通道散热器(Three-Layered Microchannel Heat Sink,TLMHS)的力学性能进行了压力测试.通过实验和数值仿真研究了50～...     

Experimental investigation of nasal airflow

The airway geometry of the nasal cavity is manifestly complex, and the manner in which it controls the airflow to accomplish its various physiological functions is not fully understood. Since the complex morphology and inaccessibility of the nasal passageways precludes detailed in-vivo measurements, either computational simulation or in-vitro experiments are needed to determine how anatomical form and function are related. The fabrication of a replica model of the nasal cavity, of a high optical clarity and derived from in-vivo scan data is described here, together with characteristics of the flow field investigated using particle image velocimetry (PIV) and flow visualization. Flow visualization is shown to be a capable and convenient technique for identifying key phenomena. Specifically the emergence of the jet from the internal nasal valve into the main cavity, how it impacts on the middle turbinate, and the large enhancement of dispersion that accompanies the initial appearance of flow instability are revealed as particularly significant features. The findings from the visualization experiments are complemented by PIV imaging, which provides quantitative detail on the variations in velocity in different regions of the nasal cavity. These results demonstrate the effectiveness of the cavity geometry in partitioning the flow into high shear zones, which facilitate rapid heat transfer and humidification from the nasal mucosa, and slower zones affording greater residence times to facilitate olfactory sensing. The experimental results not only provide a basis for comparison with other computational modelling but also demonstrate an alternative and flexible means to investigate complex flows, relevant to studies in different parts of the respiratory or cardiovascular systems.     

散热器翅片结构对流体流动及换热过程影响的数值仿真研究

用CFD软件FLUENT对散热器常用的平翅片和波纹翅片表面的流体流动及换热过程进行了数值模拟,获得了翅片表面的流场、温度场、压力场以及换热量、换热系数的变化规律。模拟结果表明在相同气流量的条件下,波纹翅片的压力损失比平翅片的大,平均表面换热系数及换热量均比平翅片的高,翅片的形状结构对流场分布和强化换热效果的影响较大。     

Viscous dissipation in micro-channels

Viscous dissipation effects in circular micro-tubes and rectangular micro-channels were investigated theoretically in a fully developed laminar flow region. From the energy conservation law, simple theoretical equations were derived for the evaluation of the viscous dissipation effect. The proposed equations were verified by comparing to other researchers’ experimental measurement and numerical results. Wall heat flux, fluid velocity, operating temperature, tube diameter, and working fluids’ effects on viscous dissipation were evaluated. It was found that viscous heating increases with decreasing aspect ratio in rectangular channels. The classical macro methods for heat transfer and pressure drop predictions agree well with the measurement and numerical predictions in micro-channels. Finally, a criterion for the evaluation of viscous dissipation was provided with the Brinkman number and heat balance concept.     

茹卡乌斯卡斯横掠错排管束实验模型的数值模拟

茹卡乌斯卡斯实验关联式在换热器的设计中有着广泛的应用。针对茹卡乌斯卡斯研究流体横掠错排管束流动与换热特性的实验段为原型,经过适当的简化,建立三维模型,运用大型CFD软件Fluent对该模型内流体的流动与换热特性进行了数值模拟研究。将数值模拟结果与公式计算结果进行对比,误差较小。通过考察特殊管排的局部换热特性,对模型壁面对换热的影响、末排管与中间管排的换热特性差异进行了分析;并将使用茹卡乌斯卡斯公式进行计算的误差与管排数的关系进行分析,在实际的设计计算中有一定的参考意义。     

Deep particle image velocimetry supervised learning under light conditions

Particle image velocimetry (PIV) is an important fluid visualization technology which extracts the velocity field from two successive particle images. Recently, some researchers have begun to use convolutional neural network (CNN) to tackle the PIV problem successfully. Some supervised learning methods make use of the PIV dataset with ground truth for network training. However, the existing dataset is composed of pairs of particle images under ideal light conditions and does not take into account the changes in actual experimental conditions. In this paper, we firstly generated a new and more challenging dataset called Light-PIV which fully simulates the change of the brightness of particle images in the real PIV experiment. Secondly, we present here a novel approach for fluid motion estimation which is based on an optical flow network LiteFlowNet. The proposed approach is verified by the application to a diversity of synthetic and experimental PIV images. We not only improve the structure, but also combine the traditional prior assumptions knowledge with the loss function to better guide the network training. The proposed approach is verified by the application to a diversity of synthetic and experimental PIV images. The experimental results show that our proposed method has advantages of high accuracy, obtaining detailed information and strong robustness in our PIV dataset compared with classical PIV methods such as HS optical flow and WIDIM, and even outperforms these existing approaches in some flow cases.     

矩形带肋通道中脉动流强化换热数值研究

为了提高燃气轮机叶片内部对流冷却,采用脉动蒸汽流取代稳态蒸汽流作为内冷介质。利用数值计算方法研究脉动蒸汽流在矩形带肋通道中的强化换热能力。通过与稳态蒸汽流的对比,分析了非稳态脉动流的脉动频率、脉动幅值和雷诺数对换热效果的影响。计算结果表明,脉动流对换热的影响取决于流体压力梯度的大小,当脉动的蒸汽处于逆压梯度区间,逆压梯度越大瞬态换热更强。而顺压梯度有稳定流体的作用,在顺压梯度时间区间,有瞬时的换热甚至低于稳态流体冷却的情况出现。     

Numerical approach for cocurrent stratified steam water flow in a horizontal configuration

The present study deals with the one-dimensional numerical approach in predicting the local flow properties for cocurrent stratified steam-water flow in a horizontal configuration. The turbulence-centered model, developed principally for gas absorption, has been modified and introduced for the condensation heat transfer coefficient using interfacial parameters, such as turbulent velocity and length scales. The calculated condensation rates, pressures and mean water layer thicknesses are in good agreement with Lim’s experimental data obtained from cocurrent stratified steam flow on a fairly thick layer of water. In addition, the approach was applied to the case of relatively thin liquid films, and the results were compared with Linehan’s experimental data. The comparison indicates that the one-dimensional numerical approach with the present condensation heat transfer correlation developed from the thick liquid data can be applicable to the prediction of the flow properties for thin liquid films.     

On the effective temperature and Reynolds number concept for a heated circular cylinder: commentary of the article by Baranyi et al.

Baranyi et al. [1] presented an experimental and numerical study, focusing on fluid dynamics and heat transfer of a heated circular cylinder in air cross flow. Although their paper interprets the results in a quite different manner to previous literature, their experimental results were quite consistent in some aspects with previous data in the literature. The purpose of this commentary is to eliminate any misunderstandings for readers of Ref. [1] and to point out that their data do reasonably confirm the effective temperature concept proposed in the literature.     

Experimental and numerical investigation of heat transfer characteristics in a square channel with various truncated ribs

Oblique ribs are widely applied to the internal cooling of turbine blades to promote the heat transfer between blade wall and coolant. In this study, the effect of several new types of truncated ribs on the heat transfer characteristics in 45° oblique rib channels is investigated experimentally and numerically. The numerical results obtained by the SST k-ω turbulence model agree well with the experimental data for the Reynolds number ranging from 10000 to 60000. The results indicate a significant entrance effect on the heat transfer in truncated rib channels. The numerical results show that ribs continuously truncated at 3.8 mm gives the best heat transfer performance among the newly truncated ribs. Compared with the original structure, the Nusselt number and heat transfer enhancement factor of newly truncated ribs increased by 24.6 % and 17.8 %, respectively. Concurrently, the friction factor is reduced by 5.1 %.

     

Laminar Heat and Fluid Flow Characteristic with a Modified Temperature-Dependent Viscosity Model in a Rectangular Duct

The present study proposes a modified temperature-dependent non-Newtonian viscosity model and investigates the flow characteristics and heat transfer enhancement of the viscoelastic non-Newtonian fluid in a 2:1 rectangular duct. The combined effects of temperature dependent viscosity, buoyancy, and secondary flow caused by the second normal stress difference are considered. Calculated Nusselt numbers by the modified temperature-dependent viscosity model give good agreement with the experimental results. The heat transfer enhancement of viscoelastic fluid in a rectangular duct is highly dependent on the secondary flow caused by the magnitude of second normal stress difference.     

Large eddy simulation of flow around a bluff body of vehicle shape

The turbulent flow with wake, reattachment and recirculation is a very important problem that is related to vehicle dynamics and aerodynamics. The Smagorinsky Model (SM), the Dynamics Subgrid Scale Model (DSM), and the Lagrangian Dynamic Subgrid Scale Model (LDSM) are used to predict the three-dimensional flow field around a bluff body model. The Reynolds number used is 45,000 based on the bulk velocity and the height of the bluff body. The fully developed turbulent flow, which is generated by the driver part, is used for the inlet boundary condition. The convective boundary condition is imposed on the outlet boundary condition, and the Spalding wall function is used for the wall boundary condition. We compare the results of each model with the results of the PIV measurement. First of all, the LES predicts flow behavior better than the k-ε turbulence model. When we compare various LES models, the DSM and the LDSM agree with the PIV experimental data better than the SM in the complex flow, with the separation and the reattachment at the upper front part of the bluff body. But in the rear part of the bluff body, the SM agrees with the PIV experimental results better than them. In this case, the SM predicts overall flow behavior better than the DSM and the LDSM.     

A study on performance analysis of the helically coiled evaporator with circular minichannels

In order to develop a compact evaporator, experiments that show characteristics of evaporating heat transfer and pressure drop in the helically coiled minichannel were performed in our previous research. This study was focused on the performance analysis of helically coiled heat exchangers with circular minichannels with an inner diameter=1.0 mm. The working fluid was R-22, and the properties of R-22 were estimated using the REFPROP program. Numerical simulation was performed to compare results with the experimental results of the helically coiled heat exchanger. As the heat transfer rate and pressure drop were calculated at the micro segment of the branch channels, the performance of the evaporator was evaluated. The following conclusions were obtained through the numerical simulations of the helically coiled heat exchanger. It showed good performance when the flow rate of each branch channels was suitable to heat load of air-side. The numerical simulation value agreed with experimental results within ± 15%. In this study, a numerical simulation program was developed to estimate the performance of a helically coiled evaporator. And, an optimum helically coiled minichannels evaporator was designed.