刚性壁面法向振动对流动换热的影响

采用理论推导与数值模拟结合的方法,通过求解流动方程及能量方程得到壁面法向振动下流体流动特性变化规律,并分析振动对换热的影响。结果表明:层流状态下,低强度振动使壁面附近流体质点产生法向速度,但影响范围很小,对换热影响有限;随着振动强度的增大,流场逐渐转变为湍流,导致换热系数提升。通过数值模拟计算壁面平均努塞尔数随振动强度及来流速度的变化规律。结果表明:对于低速流体,当振动达到某一临界值后能增强换热效果,努塞尔数随振动强度增大而增加,最佳换热相位角在200°左右,低雷诺数下振动强化换热效果较为明显,对流换热系数最大可提升300%。     

振动对带肋通道内流动与传热影响的数值模拟

运用Fluent UDF动网格技术将二维流场中振动对带肋矩形直通道的流动与换热特性进行了数值模拟,分析了相同振动条件下入口速度对流动与换热的影响,以及相同振动强度下振幅和频率对流动与换热的影响.结果表明:相同振动条件下,Re越大,换热增长率越低,入口静压波动越大,且静压峰值相位相对于振动峰值相位逐渐超前;相同振动强度下,振幅越大,振动引起的换热波动越小,换热增长率越低;当振幅大于一个临界值时,振幅对换热的影响不明显;振幅越大,入口静压波动幅度越大;振动可以强化换热,但是随着Re和振幅的增大,局部区域会出现抑制换热的情况.     

紧凑满液型叉排管束蒸发换热器内水的沸腾强化换热特性

采用紧凑满液型蒸发换热器,利用水平传热管叉排管束狭窄空间内早期沸腾强化换热机理将中小热负 荷条件下的自然对流换热转化为旺盛核沸腾换热,换热性能大大优于传统的降膜式蒸发换热器。对水平传热管 管束在受限空间内沸腾强化换热进行实验研究,确认了紧凑满液式水平管蒸发换热器具有良好的换热性能,传 热管在管束中的位置对换热特性已经没有明显影响,随着压力增加,受限空间内沸腾强化换热强化效果显著增 加。     

折流板换热器壳程流场数值模拟与结构优化

基于多孔介质与分布阻力的概念,采用FLUENT软件对单弓形折流板换热器的壳侧流场进行了三维数值模拟,模拟结果与实验结果吻合较好。在此基础上针对折流板换热器壳程压降大、能耗高,存在传热死区等的缺点,提出了壳程流场的改进方案,通过数值模拟可以看到壳程流场改进后不仅具有压降低、场协同性能好、基本无传热死区等特点,而且在一定程度上还提高了管束抗流体诱导振动的性能。     

丁胞结构强化换热机理的场协同分析

采用数值方法计算了丁胞结构流道内对流换热过程，并运用场协同理论分析了丁胞结构强化换热的机理，分析了丁胞大小、深度以及Re等对换热过程的影响。结果发现，丁胞的前侧是换热弱化区，而后侧才是强化换热区，但总体表现为强化换热效果，在低Re条件下，Nu较普通流道高1．2～1．5倍，是一种较好的强化换热方式。     

直管振动流动传热特性数值模拟

通过Fluent软件求解二维雷诺时均Navier-Stokes方程,对整体振动二维直管内流场的传热进行数值模拟,分析了流体流速及振动参数对流场及温度场特性的影响。结果表明:振动会对直管内流场产生扰流;给直管施加振动后,直管换热强度也增加;随着换热管道的加长,换热效果随着振动频率和振幅的增大,会在低频率和低振幅的换热强度基础上下降,最多下降18.3%。     

肋片振动对带肋矩形通道内流动和换热影响的数值研究

基于Fluent动网格及UDF编程技术对二维流场中振动带肋矩形直通道的流动与换热特性进行了数值模拟,分析了振幅和频率对其换热特性的影响。数值计算表明,相比于静止的带肋矩形直通道的换热,振动对其换热有一定的影响,并且随着振幅和频率的提高,振动强化换热效果越显著;振动同时也使通道内的流场结构发生了改变,振幅和频率的提高都能使通道内的静压迅速地增加,振动时静止通道内两肋片之间尺度大小不一的两个漩涡随着振幅、频率的提高,漩涡尺度相继变小,直至最后都被主流带走。     

空气横掠矩形翅片椭圆管束换热规律的数值研究

采用Fluent软件对矩形翅片椭圆管束空气侧的对流换热情况进行了三维数值模拟，获得了不同流速下翅片表面温度分布，分析了迎面风速与换热系数之间的关系，随着速度的增大，空气侧的换热系数增加，并拟合了换热计算公式。同时分析了不同翅片间距对换热的影响因素，随着翅片间距的增大，空气侧换热系数增加，而且随着Rg数的增加，换热的强化更加明显。     

超声空化对太阳集热器换热效果影响的数值分析

应用Fluent软件数值模拟超声空化对太阳集热器中不同管形(圆管和波纹管)换热效果的影响,并利用场协同理论对数值模拟的结果进行分析,分析结果表明:超声波频率和声压幅值对圆管和波纹管的换热效果均有影响,超声波频率越小、声压幅值越大,越有利于强化换热;超声波在管内传播时场协同数比无超声波大,且场协同数随声压幅值的增大而增大,此时温度场与速度场的协同程度较好;考虑强化换热与流体减阻之间的关系可知声压幅值越小,管内换热的综合性能效果较好。     

空间螺旋式弹性传热管束模态与应力特性分析

弹性管束换热器利用流体诱导振动实现强化传热,弹性管束的结构优化、模态及应力特性等对弹性管束的使用寿命具有重要影响.基于此,提出一种新型空间螺旋式弹性传热元件,通过ANSYS有限元模拟技术,分析了空间螺旋管束的固有模态特性以及在一定振幅条件下管束的应力分布特性,并与现有的平面弹性管束进行比较.结果表明,在相同尺寸下,空间...     

流体横掠振动圆管湍流状态换热的场协同分析

从理论上推导了水流绕流振动圆管湍流状态下换热的壁面瞬时平均努塞尔数与场参数的表达式,并利用数值计算的方法对振动圆管湍流状态下的场协同原理进行了验证。结果表明,场协同理论同样适用于水流绕流振动圆管湍流状态下的换热。数值计算中,对振动圆管外部瞬时场参数取值域进行界定,并提出了域内参数离散平均的计算方法,利用动网格技术和udf编程方法对场参数取值进行计算,减小计算误差。     

Heat transfer enhancement analysis of a cylindrical surface by a piezoelectric fan

Piezoelectric fans consist of a thin flexible blade attached to a vibrating piezoelectric patch, and provide an effective means of enhancing the heat transfer in low convective regions. In this study, the characteristics of the three-dimensional heat and fluid flow fields generated by the vibrating fan are examined by performing numerical simulations and experimental measurements. In performing the simulations, the fluid domain is discretized using a dynamic meshing scheme to take account of the time-varying shape and position of the vibrating blade. The results show that two counter-rotating screw-type flow structures on either side of the blade appeared on either edge of the blade, and a pair of asymmetric vortex is formed around the fan tip. The experiment is conducted with a total of eighteen T-type thermocouples attaching to the cylindrical surface to measure the variation of temperature. The experimental and numerical results indicate that the piezoelectric fan improves the heat transfer coefficient by 1.2–2.4 times. Moreover, the augmentation of local heat transfer coefficient can be achieved by 2.85 times.     

小尺度涡流发生器强化传热特性及机理

对布置有不同高度的小尺度涡流发生器的矩形槽道进行了数值模拟,对其传热和流动特性进行了对比研究。分析了小尺度涡流发生器强化传热的特点和机理。从流体流动对温度场影响的角度来说明对流换热的物理机制。     

A 3‐D numerical heat and mass transfer model for simulating the vibration effects on drying process

The high‐energy costs in the drying process highlight the employment of vibration which is a promising way to intensify this process. This work presents a three‐dimensional (3‐D) numerical version that exhibits the coupled heat and mass transfer phenomena posed by the external vibrating flow of hot air during the drying process. In order to simulate 3‐D vibrating drying, a 3‐D unstructured control volume finite element method is developed to analyze the heat and mass transfer during unsaturated porous media drying. Numerical simulation results, which depict the effects of the external vibrating flow which forces the drying process to occur more rapidly and intensively than ever before, are presented and analyzed.     

Forced convective flow and heat transfer characteristics of aqueous drag-reducing fluid with carbon nanotubes added

A new kind of aqueous drag-reducing fluid with carbon nanotubes (CNTs) was developed. The new working fluid was an aqueous CTAC (cetyltrimethyl ammonium chloride) solution with CNTs added and has both special effects of drag-reducing and heat transfer enhancement. The thermophysical and rheological properties of the new working fluid were measured. An experiment was carried out to investigate the forced convective flow and heat transfer characteristics of conventional drag-reducing fluid (aqueous CTAC solution) and the new drag-reducing nanofluid in a test tube having an inner diameter of 25.6 mm. Results indicated that there were no obvious differences of the drag-reducing characteristics between conventional drag-reducing fluid and new drag-reducing nanofluid. However, there were obvious differences of the heat transfer characteristics between both fluids. The heat transfer characteristics of new drag-reducing nanofluid have strong dependences on the liquid temperature, the nanoparticle concentration and the CTAC concentration. The heat transfer enhancement technology of nanofluid could be used to solve the problem of heat transfer deterioration for conventional drag-reducing fluids.     

An investigation on the flow and heat transfer characteristics of nanofluids by nonequilibrium molecular dynamics simulations

ABSTRACT

The flow and heat transfer characteristics of nanofluids are investigated by nonequilibrium molecular dynamics simulations. Both the effect of chaotic movements of nanoparticle (CMN) on flow properties and its resulting heat transfer enhancement are analyzed. Results show that compared with the base fluid, the effective thermal conductivity of nanofluids is increased, and the increase ratio in the shear flow field is much higher than that in the zero-shear flow field. Based on the models built in this paper, the contributions of increased thermal conductivity and CMN to the heat transfer enhancement of nanofluids are 49.8–68.6% and 31.4–50.2%, respectively.     

A lattice Boltzmann simulation of enhanced heat transfer of nanofluids

Due to its distinctive characteristics nanofluid has drawn much attention from academic communities since the last decade. Compared with conventional fluids, nanofluid has higher thermal conductivity and surface to volume ratio, which enables it to be an effective working fluid in terms of heat transfer enhancement. Recent experimental works have shown that with low nanoparticle concentrations (1–5 vol.%), the effective thermal conductivity of the suspensions can increase by more than 20% for various mixtures. Although many outstanding experimental works have been carried out, the fundamental understanding of nanofluid characteristics and performance is still not sufficient. Much more theoretical and numerical studies are required. Over the past two decades, the lattice Boltzmann method (LBM) has experienced a rapid development and well accepted as a useful method to simulate various fluid behaviours. In the present study, the LBM is employed to investigate the characteristics of nanofluid flow and heat transfer. By coupling the density and temperature distribution functions, the hydrodynamics and thermal features of nanofluids are properly simulated. The effects of the parameters including Rayleigh number and volume fraction of nanoparticles on hydrodynamic and thermal performances are investigated. The results show that both Rayleigh number and solid volume fraction of nanoparticles have influences on heat transfer enhancement of nanofluids; and there is a critical value of Rayleigh number on the performance of heat transfer enhancement.     

A magnetic vortex generator for simultaneous heat transfer enhancement and pressure drop reduction in a mini channel

An active vortex generator is proposed for heat transfer enhancement in heat sinks and heat exchangers and removal of highly concentrated heat fluxes. It is based on applying a uniform magnetic field of permanent magnets to a magnetic fluid (ferrofluid) flowing in a heated channel. Numerical simulations are carried out for a 2 Vol% ferrofluid at different Reynolds numbers (150‐210) and magnetic field intensities (0‐1400 G) to investigate the possibility of simultaneous heat transfer enhancement and pressure drop reduction by the proposed method. Comparisons are also made with the other conventional vortex generators. Results indicate that the external magnetic field acts as a vortex generator that changes the velocity distribution, improves the flow mixing, and thereby increases the convective heat transfer. Surprisingly, the heat transfer enhancement is accompanied by a decrease of the friction coefficient due to the flow separation and decrease of the flow contact with the surface. It is also concluded that increasing the magnetic field intensity, decreasing the flow rate, and adding a second identical magnetic vortex generator have favorable effects on both pressure drop and heat transfer. A maximum of 37.8% enhancement of heat transfer with a 29.18% reduction of pressure drop has been achieved at the optimum condition.     

Effect of geometric configuration on the laminar flow and heat transfer in microchannel heat sinks with cavities and fins

A numerical simulation is performed to investigate the characteristics of flow and heat transfer in microchannels with cavities and fins. Nine microchannels with various shaped cavities and fins are presented and compared to the smooth microchannel. The effect of cavity and fin shapes on the flow field and temperature field is analyzed. Results show that the presence of cavity and fin can increase the heat transfer area, intensify mainstream disturbance, and induce chaotic advection, which result in obvious heat transfer enhancement. The shape of cavity or fin has a great influence on the hydrodynamic and thermal performance for such micro heat sinks. Based on the performance evaluation criterion (PEC), the overall performance of the microchannel is evaluated. The combination of cavities and fins leads to lower bottom temperature, lower net temperature gradient of fluid, and better heat transfer performance, which has the potential to meet the increased heat removal requirement.     

Theoretical study of vibrating effect on heat transfer in laminar flow

Green’s function method was adopted to study the problem of vibrating effect on heat transfer in laminar flow with constant flux and the influence of Prandtl number and the vibrating frequency on the heat transfer characteristics was investigated. The results show that the variation of the frequency leads to a different distribution of the unsteady velocity and temperature; with a lower frequency, the vibrating will weaken the heat transfer, but the heat transfer will be enhanced with a higher frequency. A lower Prandtl number leads to a strenuous variation of heat transfer.