Numerical investigation of entropy generation and heat transfer of pulsating flow in a horizontal channel with an open cavity

In this study, the entropy generation and the heat transfer of pulsating air flow in a horizontal channel with an open cavity heated from below with uniform temperature distribution are numerically investigated. A numerical method based on finite volume method is used to discretize the governing equations. At the inlet of the channel, pulsating velocity is imposed for a range of Strouhal numbers St_p from 0 to 1 and amplitude A_p from 0 to 0.5. The effects of the governing parameters, such as frequency and amplitude of the pulsation, Richardson number, Ri, and aspect ratio of the cavity, L/H, on the flow field, temperature distribution, average Nusselt number and average entropy generation, are numerically analyzed. The results indicate that the heat transfer and entropy generation are strongly affected by the frequency and amplitude of the pulsation and this depends on the Richardson number and aspect ratio of the cavity. The pulsation is more effective with the aspect ratio of the cavity L/H= 1.5 in terms of heat transfer enhancement and entropy generation minimization.     

Heat transfer characteristics in micro-fin tube equipped with double twisted tapes： Effect of twisted tape and micro-fin tube arrangements

An experimental study was carried out to investigate the influence of double twisted-tape inserts (DTs) in micro-fin tubes (MFs) on heat transfer, friction factor and thermal performance factor characteristics of the compound devices in the following configurations: (1) twisted tapes acted in the same direction (for co-swirl) while MF and twisted tapes acted in the same (parallel) direction (MF-CoDTs:P), (2) twisted tapes acted in the same direction (for co-swirl) while micro-fin tube and twisted tapes acted in opposite directions (MF-CoDTs:O) and (3) twisted tapes acted in opposite directions for counter swirl (MF-CDTs). The MF alone and the MF equipped with a single twisted tape in parallel/opposite arrangement were also considered for comparison. The experiments were conducted for the flows with the Reynolds numbers between 5 650 and 17 000, under uniform heat flux condition. The experimental results indicate that MF-CDTs induce stronger swirl/turbulence flow, resulting in higher heat transfer rate, friction factor and thermal performance factor than other combined devices. The thermal performance factors associated with the use of MF-CDTs were found to be higher than those associated with the uses of MF-CoDTs:P, MF-CoDTs:O and MF alone up to 9.3%, 6.5% and 56.4%, respectively. The empirical correlations developed using the present experimental data for the Nusselt number, friction factor and thermal performance factor are also reported.     

Parameters determining water temperature of a proglacial stream: The Phelan Creek and the Gulkana Glacier,Alaska

The heat budget of a proglacial stream, Phelan Creek, from the Gulkana Glacier, Alaska, is estimated by using the hourly time series of stream water temperature, discharge and meteorology obtained in the summer of 2006. As an analytical result, the net shortwave radiation and bed friction occupied 32.1 and 38.2% of the total thermal input to the stream, respectively. The time series of the water temperature were simulated by a deterministic model with the coupling of the heat budget and the thermal advective diffusion equation. The simulated result is agreeable to the observed one with the Nash‐Sutcliffe efficiency (NASH) of 0.747 and the root‐mean‐square error (RMSE) of 0.236°C over the observation period. However, under condition that the rainfall of more than 1 mm h^?1 occurs continually for more than a day, the simulation was less reasonable with NASH = 0.225 and RMSE = 0.226°C. This is probably because the relatively warm subsurface flow input to the stream channel from the non‐glacial area. Copyright © 2009 John Wiley & Sons, Ltd.     

南水北调中线工程总干渠冬季大流量输水 水温调控措施

为保障南水北调中线工程冬季输水安全，利用建立的总干渠全线一维水温模型，针对 350、280、210 和 150?m3/s?这 4 种不同输水流量方案，模拟分析各方案在强冷冬、冷冬、平冬和暖冬等典型冬季气象条件下总干渠 冬季输水保温效果，在此基础上提出总干渠冬季无冰盖大流量输水水温调控措施。结果表明：针对不同典型冬季 气象条件，在有限条件下采取无冰盖大流量输水调度运行方式，可不同程度缩短输水流动时长、减少沿程水体热 量损失，从而达到保温效果；在各典型冬季年份气象条件下，随着总干渠输水流量增大，沿程水温比降相应减缓， 明渠末端水温相应提升；在冬季气象条件适宜情况下，总干渠冬季采取无冰盖大流量输水水温调控措施，既有利 于减小水温降幅，同时也可有效解决冬季输水能力受限问题；建议根据总干渠中短期气温预报和水温预测情况， 在冷冬、平冬和暖冬等典型冬季气象条件下尽可能采取较大输水流量输水模式，在强冷冬气象条件下可分时段、 分渠段采用无冰盖大流量输水模式。     

高纬度地区渠道无冰盖输水的冰情控制研究

中国北方高纬度地区有一些渠道工程,因其建设年代早,受规划之初经济条件限制,未考虑冰期运行,仅在非冰期输水。随着经济发展,目前亟需实现冬季输水。但是受结构上的限制,这些渠道抗冰冻能力差,无法采取冰盖下输水或冰水二相流输水,通过加设保温盖板,防止渠道结冰,既可解决用水矛盾,又可节省工程改造投资,具有巨大的经济效益和社会效益。本研究根据传热学理论对加设保温盖板后的渠道水体失温过程进行理论分析,并在此基础上建立了设置保温盖板的一维渠道冰期输水数学模型,对加设保温盖板的冰期输水过程进行了模拟,分析了保温材料厚度、输水流量、出库水温等对冰期输水的影响,论证渠道冬季加设保温盖板输水的可行性。并提出了切实可行的冬季输水方案。研究成果对于冰期渠道安全输水和渠道改造都有一定的借鉴意义。     

THREE-DIMENSIONAL NUMERICAL SIMULATION OF THERMAL-HYDRAULIC PERFORMANCE OF A CIRCULAR TUBE WITH EDGEFOLD-TWISTED-TAPE INSERTS

Three-dimensional numerical simulations and experiments were carried out to study the heat transfer characteristics and the pressure drop of air flow in a circular tube with Edgefold-Twisted Tape (ETT) inserts and with classic Spiral-Twisted-Tape (STT) inserts of the same twist ratio. The RNG turbulence model for mildly swirling flows, the enhanced wall treatment for low Reynolds numbers, and the SIMPLE pressure-velocity method were adopted to simulate the flow and heat transfer characteristics. Within the range of Reynolds number from 2 500 to 9 500 and the twist ratio y from 5.4 to 11.4, the Nusselt number of the tube with ETT inserts is found to be 3.9% ? 9.2% higher than that with STT inserts, and the friction factor of the tube with ETT inserts is 8.7% ? 74% higher than that of STT inserts. The heat enhancement is due to higher tangential velocity and asymmetrical velocity profile with the increase and decrease of the periodic velocity within an edgefold length. It is found that main factors affecting the heat transfer of ETT inserts are the twist angle and the gap width between the tube and inserts. A larger twist angle leads to a higher tangential velocity, and larger Nusselt number and friction factor. The thermal-hydraulic performance slowly decreases as the twist angle increases. The gap width between tube and inserts has a significant influence on the heat transfer, while little influence on pressure drops. The thermal-hydraulic performance increases in average by 124% and 140% when the gap width reduces from 1.5 mm to 1.0 mm and 0.5 mm. The larger the gap width, the higher velocity through the gap will be, which would reduce the main flow velocity and tangential velocity. So a small gap is desirable. Comparing experimental and numerical results at variable air flow and tube wall temperature, the numerical results are found to be in a reasonable agreement with the experiment results, with difference of the Nusselt number in a range of 1.6% ? 3.6%, and that of the friction factor in a range of 8.2% ? 13.6%.     

LAMINAR FLOW AND HEAT TRANSFER IN THE DEVELOPING REGION OF HELICAL SQUARE DUCTS

Simultaneous development of the laminar flow and heat transfer in helical square ducts was numerically studied. The governing equations were written in an orthogonal helical coordinate system and fully parabolized in the axial direction. Results were found out over a wide range of the governing parameters. Two axial velocity entries were taken into account. The development of secondary flow, axial velocity and temperature distribution for the large Dean number were examined in detail and the effects of different governing parameters on the friction factor and the Nusselt number were annlyzee. Many new and interesting conclusions were reached. The present results reveal the nature of fluid flow and heat transfer in the developing region of helical square ducts.     

LARGE EDDY SIMULATION OF FREE SURFACE TURBULENT CHANNEL FLOW WITH HEAT TRANSFER

1. INTRODUCTIONFreesurface turbulent flow exists widely in engineering. Due to difficulties in measurement and simulation, very little has been understood about the structures and transport mechanism of turbulence near the freesurface. Komori and Ueda…     

The viscoelastic effects on thermal convection of an Oldroyd-B fluid in open-top porous media

The effects of two viscoelastic parameters on the thermal convection of a viscoelastic Oldroyd-B fluid in an open-top porous square box with constant heat flux are investigated. The results show that the increase of relaxation time is able to destabilize the fluid flow leading to a higher heat transfer rate, while the increase of retardation time tends to stabilize the flow and suppress the heat transfer. The flow bifurcation appears earlier with the increase of the relaxation time and the decrease of the retardation time, resulting in more complicated flow patterns in the porous medium.     

引水流量对引水渠道中水内冰演变影响的数值模拟

水电站引水渠道中的冰害会影响水电站的安全运行,危害当地人民的人身安全。为开展引水渠道中水内冰演变的研究,建立了三维非稳态欧拉两相流模型,动量方程考虑了相间曳力、升力、虚拟质量力,热量传递方程中考虑了冰水之间的热量传递以及太阳辐射、有效辐射、蒸发热损失、水面对流热损失等。模拟分析了不同引水流量下温度及冰体积分数的沿程分布,结果表明：冰温沿程逐渐升高;水温沿程先降低后升高、沿程出现最低点。冰体积分数沿程存在突变点,且突变点在水温最低点附近。随着引水流量减小,产冰起始位置向渠道上游移动,但最终的产冰量和冰体积分数逐渐减小。通过与黄河上游河道中水内冰演变计算结果对比,验证了模型的可靠性。     

小颗粒堆积多孔体传热特性实验研究

对平行平板间填充小颗粒后的换热实验结果进行了归纳和分析，发现板间距不变，平均Nu数不随颗粒直径单调变化，说明存在最佳填充层数，并对此进行了初步分析。实验表明，小颗粒填充可以得到较好的换热强化效果，而流动阻力却远小于微颗粒填充情况，因此有可能利用小颗粒填充方法在常规条件下强化平板间的换热。     

表面活性剂减阻流体传热与阻力关系试验研究

针对在水中添加表面活性剂可以出现阻力系数减少而同时传热性能发生变化的现象，采用测压差装置和热电偶测温系统分别对二维流道内不同的减阻流体的减阻性能、传热性能进行了试验，研究了浓度、配比、雷诺数和热流密度对减阻流体传热与阻力系数之比值的影响。结果表明，表面活性剂减阻流体的传热系数考尔朋（Colburn）因子与阻力系数之比和水的情况有较大的差异；当减阻流体的减阻率稳定时，随着雷诺数的增大，考尔朋恩因子与阻力系数比值缓慢下降；达到临界雷诺数后，考尔朋因子与阻力系数之比值波动较大，并且比值迅速增大到接近没有添加减阻剂的水平。     

窄边非加热区对窄缝流道内流场和温场影响的数值模拟

本文运用商用流体动力学软件CFX数值模拟了非加热区对窄缝流道内流场和温场的影响。研究表明,窄缝内横截面中心线上的速度呈梯形分布,窄边附近流速较低,中间高;宽边完全加热时,流体不足以带出加热板发出的热量,使得角部出现热集中现象;而在角部设计有非加热区后,角部热集中现象得到解决,并且非加热区的存在并没有降低主流的速度。     

Natural convective heat transfer and nanofluid flow in a cavity with top wavy wall and corner heater

A numerical analysis of natural convection of nanofluid in a wavy-walled enclosure with an isothermal corner heater has been carried out. The cavity is heated from the left bottom corner and cooled from the top wavy wall while the rest walls are adiabatic. Mathematical model has been formulated using the single-phase nanofluid approach. Main efforts have been focused on the effects of the dimensionless time, Rayleigh number, undulation number, nanoparticle volume fraction and length of corner heaters on the fluid flow and heat transfer inside the cavity. Numerical results have been presented in the form of streamlines, isotherms, velocity and temperature profiles, local and average Nusselt numbers. It has been found that nanoparticle volume fraction essentially affects both fluid flow and heat transfer while undulation number changes significantly only the heat transfer rate.     

冰期明渠水温模型

正确模拟明渠水温是分析冰情及优化调蓄水库布局的基础.基于历史气象资料,本文提出了水体与大气热交换的线性化模型和非线性模型,这些模型包含了太阳辐射、长波辐射、蒸发和对流热交换等主要因素.建立了水体与混凝土衬砌渠道热交换的参数化模型,考虑了衬砌结构、导热性能和地温等因素的影响.在此基础上,建立了明渠一维流动水温模型,提出了...     

NUMERICAL STUDY OF VORTEX SHEDDING FROM A SQUARE CYLINDER AND ITS EFFECTS ON HEAT TRANSFER OF MIXED CONVECTION

1 .　INTRODUCTIONForseveraldecades ,thenumericalandexperi mentalinvestigationofvortexsheddingfromaheated cooledbluntbodyinthemixednaturalandforcedconvectionregimeshasbeenasubjectofgreatinterestamongappliedmathematicians,fluiddynamicistsandheattransferanalysistowingtoitsnumerousengineeringapplicationssuchasthecoolingofelectroniccomponents ,thecoolingofhotwireanemometersandtheliftenhancementasattributedtotheMagnuseffects .Literatureonvortexsheddingfromcircularcylinderinthemixedconvectionregi…     

The mechanical energy equation for total flow in open channels

The mechanical energy equation is a fundamental equation of a 1-D mathematical model in Hydraulics and Engineering Fluid Mechanics. This equation for the total flow used to be deduced by extending the Bernoulli＇s equation for the ideal fluid in the streamline to a stream tube, and then revised by considering the viscous effect and integrated on the cross section. This derivation is not rigorous and the effect of turbulence is not considered. In this paper, the energy equation for the total flow is derived by using the Navier-Stokes equations in Fluid Mechanics, the results are as follows：（1） A new energy equation for steady channel flows of incompressible homogeneous liquid is obtained, which includes the variation of the turbulent kinetic energy along the channel, the formula for the mechanical energy loss of the total flow can be determined directly in the deduction process.（2） The theoretical solution of the velocity field for laminar flows in a rectangular open channel is obtained and the mechanical energy loss in the energy equation is calculated. The variations of the coefficient of the mechanical energy loss against the Reynolds number and the width-depth ratio are obtained.（3） The turbulent flow in a rectangular open channel is simulated using 3-D Reynolds averaged equations closed by the Reynolds stress model（RSM）, and the variations of the coefficient of the mechanical energy loss against the Reynolds number and the width-depth ratio are discussed.     

Numerical simulation of thermal convection of viscoelastic fluids in an open-top porous medium with constant heat flux

This paper makes a numerical study of the buoyancy-driven convection of a viscoelastic fluid saturated in an open-top porous square box under the constant heat flux boundary condition. The effects of the relaxation and retardation times on the onset of the oscillatory convection, the convection heat transfer rate and the flow pattern transition are investigated. It is shown that a large relaxation time can destabilize the fluid flow leading to an early onset of the thermal convection and a high heat transfer rate, while a large retardation time tends to stabilize the flow and suppress the convection onset and the heat transfer. After the convection sets in, the flow bifurcation appears earlier with the increase of the relaxation time and the decrease of the retardation time, resulting in more complicated flow patterns in the porous medium. Furthermore, with the increase of the ratio of the relaxation time to the retardation time, the fluid may be blocked from flowing through the open-top boundary, which may be caused by the viscoelastic effect. Finally, the comparison of our results with those under isothermal heating boundary conditions reveals that the heat transfer rate corresponding to a constant heat flux boundary is always higher.     

Experimental Investigation on Heat Transfer Enhancement in Composite Porous Media

The method of composite porous media with mini-longitudinal channels at the surface and with beads packing between plates was put foward to improve the integated performance of flow and heat transfer in porous media. The experimental results in the corresponding porous media were reported and analyzed. The experiments indicate that with proper matching of the particle diameter dp, the mini-channel width w, the channel depth d and the distance between plates δr the heat transfer in the composite porois media is enhanced and flow resistence reduced compared with those of no mini-longitudinal channels at the surface. So this is an effective method to improve the integrated performance of flow and heat transfer in porous media.     

Thermodynamic analysis for a third grade fluid through a vertical channel with internal heat generation

This paper makes the thermodynamic analysis in forced convective flow of a third grade fluid through a vertical channel. Due to the reactive nature of the fluid, the effect of internal heat generation is considered and assumed to be a linear function of temperature. The coupled nonlinear dimensionless ordinary differential equations governing the fluid flow are solved by using the Adomian decomposition method(ADM). The effects of various physical parameters such as third grade material parameter, buoyancy parameter and heat generation parameter on the thermal structure of flow are presented and discussed.