Helmholtz共振腔脉动流体强化换热的试验研究

采用试验研究的方法研究了流体力学不稳定性对强化换热的影响。水流经Helmholtz共振腔时被转变为脉动流体，脉动的水经单管换热器时被加热，测量不同条件下加装共振腔和不加共振腔时的换热系数。研究发现，加装了Helmholtz共振腔时换热系数明显提高约10％～40％。     

Helmholtz共振腔的设计及其脉动流体强化换热效果的试验分析

采用试验的方法分析了流体力学不稳定性对强化换热的影响。试验中，水流经Helmholtz共振腔被转变为脉动流体，脉动的水流经单管换热器被加热，试验测量了不同条件下加装共振腔和不加共振腔时的换热系数，结果发现加装Helmholtz共振腔后换热管的换热系数可以提高10％～40％。     

Helmholtz共振腔强化换热的试验研究

为研究脉动流体对强化换热的影响，设计Helmholtz共振腔并分别在加装和不加装共振腔的情况下进行对比试验，发现水流经共振腔后变成了脉动流体，脉动的水经过单管换热器后强化了换热，在一定的共振腔参数的配合下，换热系数提高约10％～40％。     

自激振荡脉冲射流强化换热实验研究

对自激振荡射流强化换热的机理进行了初步探讨，并对有无共振腔时的换热情况进行了对比实验。实验中发现，由Helmholtz共振腔产生的自激振荡脉冲射流增强了管内流体的掺混，破坏了边界层，从而达到了强化换热的目的。实验中还发现，当共振腔两端的压差增大时，将产生更为强烈的脉动流，明显地提高流体的紊流程度，从而强化了管内流动换热。     

自激振荡脉冲对流换热的实验研究

将Helmhotz共振腔应用于换热器来增强换热是一种新的强化换热方法。设计了一种换热效果较好的Helmhotz共振腔，并通过实验研究了Helmhotz共振腔对换热器的换热强化效果，分析了水力参数和结构参数对换热效果的影响，结果发现：对一定结构的共振腔，配以适当的水力参数，就可以产生自激振荡；对于同一结构的共振腔，水力参数不同，产生自激振荡的强弱也不同，随着压力的增加，自激振荡的强度也增加；将共振腔产生的自激振荡流引入换热器后，当自激振荡达到一定的强度时，能够破坏层流底层，从而可以强化换热；Helmhotz共振腔在绝大多数工况下能将管内换热系数提高10%-30%。     

自激振荡管壳式换热器强化传热的可行性分析

脉动流体能够使管壳式换热器换热系数得到提高,而自激振荡腔在一定的结构参数和运行参数下能够使流体产生脉动。在管壳式换热器前安装自激振荡腔,使流体流经自激振荡腔产生脉动流动,从而实现管壳式换热器的强化传热。分析了将自激振荡腔用于管壳式换热器强化传热可行性。     

利用流体脉动强化换热的试验研究

通过管内流体脉动的试验研究，分析脉动流体的水力参数和脉动特性对强化换热的影响规律。采用自行研制的自激振荡腔作为流体产生脉动的装置，并通过改变自激振荡腔的腔室长度和后喷嘴长度来达到调节流体脉动特性参数的目的。结果表明：流体的水力参数和自激振荡腔结构对流体脉动强化换热都有显的影响，随着流量或自激振荡腔腔室长度的增加，换热效果将增强；而后喷嘴长度则存在一个最优尺寸，在此处，换热效果最好。     

恒壁温下管内流体脉动流动对流换热的数值模拟

利用FLUENT软件,在流体进口速度发生周期性变化的情况下,对圆管内层流的流体流动与换热情况进行了数值模拟,分析了无因次脉动幅值、频率对对流换热系数、摩擦系数以及流场的影响,结果表明脉动流体会强化或弱化换热效果,阻力比无脉动时大,并且在流场中有与主流区流动方向相反的流动现象。     

弯尾管Helmholtz型无阀自激脉动燃烧器压力特性

建立了弯尾管Helmholtz型无阀自激脉动燃烧器实验系统,研究了燃烧室内的压力振荡特性,分析了尾管结构参数、热负荷和过量空气系数对燃烧室内压力振幅的影响.结果表明:所设计的弯尾管脉动燃烧器能产生稳定的脉动燃烧,脉动压力振幅较大、频率较低,压力振荡波形接近正弦曲线;压力均值和压力振幅沿90°弯尾管展开长度方向减小,弯尾管内的压力分布与1/4波形管分布接近;燃烧室内的压力振幅随尾管弯曲角度的增大而减小,弯曲位置在尾管出口处时的压力振幅较在尾管入口处时小;在不改变燃烧器结构参数的条件下,压力振幅随热负荷和过量空气系数的增大而增大,实验结果与理论预测值定性一致.     

管内脉动流强化换热研究进展

针对管内脉动流换热,本文全面介绍了常用的脉动源及其特点,脉动流对换热的强化与空化效应、往复流效应、层流-湍流过渡区非线性效应、场协同效应和旋涡演化等相关机理。本文初步总结了各种控制参数对脉动流强化换热的影响,并对各国课题组的代表研究成果进行了介绍。从目前的研究现状看,脉动流换热研究仍然存在很多争议,对其机理的认识仍然非常模糊,特别是湍流脉动换热。     

Natural Convection Heat Transfer in a Cubic Cavity Submitted to Time-Periodic Sidewall Temperature

The laminar unsteady natural convection in a cubic cavity is comprehensively studied here using a high accuracy temporal-spatial pseudospectral method. In this study, the cavity is filled with air and one of its sidewalls is submitted to sinusoidally varying temperature, while constant lower temperature is imposed on the opposing sidewall and other sidewalls are adiabatic. Computations are performed to explore the effects of several influential factors on the fluid flow patterns and heat transfer performances within the cavity, including Rayleigh number and the amplitude and period of pulsating sidewall temperature. Numerical results reveal that the heat transfer enhancement is complexly determined by the above influential factors, and the heat transfer resonance is observed in the case of a large Rayleigh number and amplitude of pulsating sidewall temperature. The three-dimensional effects on fluid flow patterns and heat transfer are discussed. Finally, the backward heat transfer is quantitatively studied.     

Numerical analysis of heat transfer in pulsating turbulent flow in a pipe

Convection heat transfer in pulsating turbulent flow with large velocity oscillating amplitudes in a pipe at constant wall temperature is numerically studied. A low-Reynolds-number (LRN) k–ε turbulent model is used in the turbulence modeling. The model analysis indicates that Womersley number is a very important parameter in the study of pulsating flow and heat transfer. Flow and heat transfer in a wide range of process parameters are investigated to reveal the velocity and temperature characteristics of the flow. The numerical calculation results show that in a pulsating turbulent flow there is an optimum Womersley number at which heat transfer is maximally enhanced. Both larger amplitude of velocity oscillation and flow reversal in the pulsating turbulent flow also greatly promote the heat transfer enhancement.     

Heat transfer enhancement with laminar pulsating nanofluid flow in a wavy channel

In this study, the heat transfer characteristics of Al₂O₃–water based nanofluids in a wavy mini-channel under pulsating inlet flow conditions are investigated numerically. The simulations are performed for nanofluid volume fractions, pulsating frequency and amplitude while the other parameters are kept constant by using control volume based cfd solver. The flow is both thermally and hydrodynamically developing while the channel walls are kept at a constant temperature. Results indicate that there is a good potential in promoting the thermal performance enhancement by using the nanoparticles under pulsating flow. Pulsation in nanofluids is a new idea for enhancement of heat transfer. Furthermore, the pulsating flow has an advantage to prevent sedimentation of nanoparticles in the base fluid. Results show that the heat transfer performance increases significantly with increase in nanoparticle volume fraction and with the amplitude of pulsation while the pulsation frequencies have a slight effect. In the pulsating flow conditions the combined effect of pulsation and nanoparticles is favorable for the increasing Nusselt number when compared to the steady flow case. The obtained results are given as dimensionless parameters.     

Numerical Study of a Shell-and-Tube Latent Thermal Energy Storage Unit Heated by Laminar Pulsed Fluid Flow

The present study aims to investigate the effect of the pulsed fluid flow on the thermal performance of a latent heat storage unit (LHSU). The storage unit consists of a shell-and-tube in which phase change material (PCM) occupied the shell space and the heat transfer fluid (HTF) flows in the inner tube. The present study is motivated by the need to intensify heat transfer and accelerate melting process in LHSU. A mathematical model based on the conservation equations of energy in both HTF and PCM has been developed. The finite volume approach was used for the discretization of equations. The developed model has been validated by comparing the obtained numerical results with experimental, analytical, and numerical data found in literature. The effects of the pulsation frequency and amplitude, the Reynolds and Stefan numbers on the thermal performance and behavior of the LHSU were investigated. The parametric study showed that the pulsating parameters (frequency and amplitude) affect the thermal performance of the LHSU. The results reveal reduction in the melting time for low pulsating frequency (less than 0.052) and high pulsating amplitude. For pulsating amplitude of 6 and pulsating frequency of 0.01, a reduction up to 13% (at Reynolds number of 500 and Stefan number of 0.16) was obtained. The results also showed that the Reynolds and Stefan numbers strongly affect the heat transfer rate, and the low melting time is obtained for high Reynolds and Stefan numbers.     

A study on the heat and mass transfer properties of multiple pulsating impinging jets

In order to explore the potential effect of unsteady intermittent pulsations on the heat and mass transfer rate of multiple impinging jets, a numerical study is performed on a two-dimensional pulsating impinging jet array under large temperature differences between jet flows and impingement wall when the thermo-physical properties can change significantly in the flow domain. Computational fluid dynamic approach is used to simulate the flow and thermal fields of multiple pulsating impinging jets. The numerical results indicate a significant heat transfer enhancement due to intermittent pulsation over a wide range of conditions. The oscillatory flow periodically alters the flow patterns in contrast to steady jets, which can eliminate the formation of a static stagnation point and enhance the local Nusselt number along the impingement wall between adjacent jets. Examination of the velocity field shows that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Flow and heat transfer of liquid plug and neighboring vapor slugs in a pulsating heat pipe

A model of fluid flow and heat transfer on liquid slug and neighboring vapor plugs in a pulsating heat pipe (PHP) is proposed. A new energy equation for the liquid slug is built by aid of Lagrange method. The shear stress term related with the fluid flow state is included in the motion equation of the liquid slug. A sensitive heat term is replaced by a phase change term in the energy equation of the vapor plug. Based on our analysis on the displacement variation of the liquid slug with time, it is known that the harmonic force acting on the liquid slug in PHPs is the pressure difference between the vapor plugs. The flow oscillation can be considered as a forced damping vibration of one degree of freedom system. The phase difference of the oscillating flow between with and without the gravity effect can reach 45°. The amplitude and angular frequency of flow oscillation is irrespective with the initial displacement of liquid slug. If the flow pattern remains strictly slug flow in the entire system, the contribution of the sensible heat exchange to the total heat transfer of the PHP is about 80%.