Numerical study of momentum and heat transfer in a pulsed plane laminar jet

In this paper, we propose numerical solutions for a two-dimensional pulsed plane jet in unsteady laminar regime. At the exit of the nozzle, the pulsating flow is imposed with a uniform temperature T₀ and a velocity u=u₀(1+Asin(ωt)). Two cases are considered: the free and the wall pulsed plane jet. For the wall jet case, the wall may either be considered adiabatic or subjected to a uniform temperature. Equations are treated with an appropriate finite difference method. The effect of the important governing parameters, such as the amplitude and the frequency of the pulsation, the Reynolds and Grashof numbers on the flow behavior are also investigated in detail. The results obtained show that the pulsation affects the flow in a vicinity region of the nozzle to reach the same asymptotic regime than the steady jet. The results also indicate that the initial development of the jet is considerably accelerated and the entrainment in the first diameters is enhanced.     

Experimental study on the unsteady laminar heat transfer downstream of a backwards facing step

The objective of this work is to study experimentally the unsteady heat transfer downstream of a backward-facing step in the 2-D laminar regime when the inlet flow is pulsated. To this aim, an experimental set-up has been prepared with water as the working fluid. The Reynolds number based on the hydraulic diameter of the inlet channel and average inlet velocity is 300. Inlet flow temperature is 30 °C and a region downstream of the step is heated up to 74 °C. Pulsation is achieved using a piston pump and heat transfer is studied up to a maximum pulsation Strouhal number of 1.2. The results obtained confirm previous numerical simulation work in the sense that pulsation could be used to partially recover the heat transfer efficiency that is lost in steady flow conditions downstream of a backward-facing step. It has also been confirmed that the behaviour of the averaged Nusselt number versus pulsation Strouhal number is of the resonant type. That is: the Nusselt number increases from the steady situation up to a certain value of the Strouhal number (0.41 in our case) and, then, it degrades as the frequency of the pulsation is further increased.     

Influence of a pulsation on heat transfer and flow structure in submerged impinging jets

An experimental investigation on pulsating impinging jets has been performed. The effect of the pulsation on the flow structure and heat transfer have been investigated. Frequency and amplitude were varied separately and the effect of each parameter was examined for different Reynolds numbers and nozzle-to-plate distances.The jet was found to become broader and the core jet length smaller with the pulsation. The reason for this behavior is that pulsation enhanced entrainment of air into the jet, which results in a change of mean velocity of the jet. Nevertheless, the behavior at lower frequencies (up to 140 Hz) is still quasisteady. This means that the amplitude of the pulsation behaves similar to the mean velocity of the jet, that the shapes of the velocity profiles are comparable to steady jets and that the jet behavior is independent of frequency.At moderate frequencies heat transfer is only affected by the pulsation when nozzle-to-plate distance and amplitude are large enough. At small nozzle-to-plate distances enhanced entrainment has no influence and no difference between steady and pulsating jets can be recognized. At large nozzle-to-plate distances entrainment increases and jet velocity reduces. This yields a reduction of heat transfer in the stagnation point of up to 50%.But besides of this effect of enhanced entrainment a theoretical limit could be determined, above which the jet is not anymore quasisteady. Above Sr = 0.2 heat transfer is affected by the pulsation also at small nozzle-to-plate distances. At this frequency boundary layer is also affected by the pulsation. This yields increased heat transfer coefficients at the stagnation point. For larger nozzle-to-plate spacings this effect is superposed by the reduction of heat transfer due to increased entrainment, resulting in a strong decrease of heat transfer coefficient.     

Dynamical Study of Pulsed Impinging Jet with Time Varying Heat Flux Boundary Condition

Pulsed jets in different configuration are potentially considered for enhancing transport phenomenon generally. Flow and temperature field in a pulsed impinging jet are simulated numerically by solving the governing equations using the control volume method. Ensemble Averaging Method as well as Phase Averaging has been employed for reporting the results in this study. In order to simulate a pulsating jet, inlet velocity profile was exerted as a time dependent sinusoidal and step signals. The results of this simulation showed an oscillatory jet could lead to an increase in jet development and its cross section with the wall and also a more uniform Nusselt profile would be obtained compared to the steady jet. For parametric investigations and extracting flow and thermal characteristics of a pulsed impinging jet, the effects of various parameters including flow frequency and amplitude and heat flux frequency were considered. It has been seen that Nusselt number varies by the changes in frequency, amplitude and the type of the excitation. It has been shown that the oscillating impinging jet has a better performance rather than the steady case when the excitation amplitude and frequency increase. Finally, it is also observed how a thermal field is going to respond with two pulsating inputs.     

Theoretical analysis of heat transfer in laminar pulsating flow

Pulsation effect on heat transfer in laminar incompressible flow, which led to contradictory results in previous studies, is theoretically investigated in this work starting from basic principles in an attempt to eliminate existing confusion at various levels. First, the analytical solution of the fully developed thermal and hydraulic profiles under constant wall heat flux is obtained. It eliminates the confusion resulting from a previously published erroneous solution. The physical implications of the solution are discussed. Also, a new time average heat transfer coefficient for pulsating flow is carefully defined such as to produce results that are both useful from the engineering point of view, and compliant with the energy balance. This rationally derived average is compared with intuitive averages used in the literature. New results are numerically obtained for the thermally developing region with a fully developed velocity profile. Different types of thermal boundary conditions are considered, including the effect of wall thermal inertia. The effects of Reynold and Prandtl numbers, as well as pulsation amplitude and frequency on heat transfer are investigated. The mechanism by which pulsation affects the developing region, by creating damped oscillations along the tube length of the time average Nusselt number, is explained.     

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.     

Heat Transfer from Pulsating Laminar Impingement Slot Jet on a Flat Surface with Inlet Velocity: Sinusoidal and Square Wave

Heat transfer from a pulsating laminar impingement slot jet on a flat surface was investigated numerically and experimentally. Inlet velocity was considered sinusoidal velocity and square wave velocity. Experimental studies were done only for the sinusoidal velocity state. An inverse heat conduction method, conjugated gradient method with adjoint equation, was used for the experimental estimation of the local heat transfer coefficient along the target surface. Effect of the square wave velocity of the laminar impingement slot jet was studied numerically. The results show pulsations in flow change flow patterns and the thermal boundary layer thickness because of the newly forming thermal boundary layer is extremely small each time the flow is resumed. Heat transfer rate in this state enhances due to pulsating inlet velocity in comparison with steady state. Heat transfer increases with increasing pulsation amplitude. Enhancement in mean heat transfer on the target plate for sinusoidal velocity is rather than square wave velocity.     

Laser doppler velocimetry investigation of the turbulence structure of axisymmetric diffusion flames

Measurements of mean velocity components, turbulent intensities, velocity probability density functions, power spectra and autocorrelation functions of axial velocity fluctuation, and spatial turbulence macroscale, are reported in a turbulent round jet flow, issuing vertically into stagnant air, in non-combusting and combusting situations. The fuel density (a mixture of methane and argon) is chosen to be equal to the cold flow gas density (a mixture of air and helium) in order to minimize cold fuel/cold gas mixture density difference effects on measured turbulence properties. The objectives are to study the influence of the combustion process on the turbulence structure of the combustible jet flows considered, and to provide data against which results of numerical prediction methods for such flows embodying various turbulence and combustion models can be compared, with a view to improving our understanding of relevant transport processes and on guiding modelling and prediction efforts of such flows. A one-dimensional laser velocimeter operating in forward scatter differential Doppler mode was used to obtain the measurements. Gas temperatures were measured by thermocouples. A visual study by schlieren photography has also been conducted. It is found that the existence of the flame suppresses turbulence in the upstream region of the jet flow and enhances it in the downstream region, where turbulence intensities are substantially higher than in the corresponding cold jet flow. However, the relative intensities, i.e. the ratio of the local turbulent intensity to the local mean velocity, are smaller in the jet diffusion flame and become comparable to relative turbulent intensities found in the cold jet flow in the downstream region of the flow. Turbulence in the jet diffusion flame is appreciably more anisotropic than in the corresponding cold jet in all regions of the flow, suggesting the eventual desirability of multi-stress models of turbulence for the prediction of such flames. The combustion process has been found to have also a marked influence on the turbulence macroscale. It is significantly smaller than in the cold jet flow in the upstream region and increases appreciably at downstream distances, the rate of this increase closely following the rate of temperature increase. The experimental results obtained will guide the development of an improved prediction method for such combusting systems.     

Turbulent impinging jet heat transfer enhancement due to intermittent pulsation

A numerical study was carried out of heat transfer under a pulsating turbulent slot impinging jet. The jet velocity was varied in an intermittent (on–off) fashion. The effects of the time-mean jet Reynolds number, temperature difference between the jet flow and the impinging surface, nozzle-to-target distance as well as the frequency on heat and mass transfer were examined. The numerical results indicate significant heat transfer enhancement due to intermittent pulsation of the jet flow over a wide range of conditions for both cooling and heating cases. Simulations of the flow and temperature fields show that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

双毛细管静电雾化模式及雾化特性实验研究

通过可视化实验研究了双毛细管静电雾化现象。以无水乙醇为介质，使用PDIA、PIV和高速摄影方法测量和分析了雾化模式和液滴的粒径分布、径向与轴向速度分布和锥角变化。结果表明，当改变流量与电压时，静电雾化呈现滴状、纺锤状、脉动射流、稳定锥射流、多股射流等多种雾化模式。因流量与电压的变化导致单位表面荷电量变化，进而变化了表面张力使得各处粒径大小发生改变，发现双毛细管雾化粒径分布规律呈近似M型，雾化状态越稳定粒径波动越小，且粒径远小于单毛细管雾化粒径以及雾化范围更广。液滴径向速度分布呈线性，雾化状态越稳定线性关系越强，液滴与毛细管末端距离对径向速度近无影响，流量对轴向速度影响远大于电压。液锥锥角在稳定锥射流模式最小，脉动模式锥角达到最大。     

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.     

Heat transfer between two parallel plates with laminar pulsating flow

The problem of heat transfer from two parallel plates of infinite width is formulated for the case where the flow between these plates consists of a periodic motion imposed on a fully developed laminar steady flow. The results indicate an increase in the heat transfer rate with pulsation. This increase is proportional to the amplitude of pulsation and inversely proportional to the Prandtl number.     

圆湍射流的轴对称大涡模拟

对空间发展的不可压缩圆湍射流进行了大涡模拟研究。在流动轴对称假定下,对Re数等于11300的圆浩射流流动进行数值模拟。大涡模拟很好地再现了圆湍射流中拟序结构非定常演化的前期过程,成功地捕获到了射流中Kelvin～Helmholtz不稳定性的触发与初级涡环的卷起及其第一次和第二次配对合并现象。但在流动轴对称假设下,大涡模拟不能模拟出湍流拟序涡环结构的破碎过程。对圆湍射流的轴对称大涡模拟结果进行长时间统计平均,能够预报出圆湍射流的核心区特征,但与圆湍射流的理论分析解和经典的实验数据对比发现,核心区后大涡模拟预报的流向速度降低缓慢。从控制方程的数学本质和拟序结构的物理机制上对圆湍射流在轴对称假设下产生上述大涡模拟结果的原因进行了分析与探讨。     

多工况下高水头水泵水轮机压力脉动特性分析

为研究多工况下高水头水泵水轮机内部的压力脉动特性,以某抽水蓄能电站水泵水轮机模型为例,采用SST湍流模型对非设计工况点下的水泵水轮机进行三维全流道非定常数值模拟,同时监测了固定导叶与活动导叶间、无叶区及尾水管处的压力脉动。结果表明,对于固定导叶与活动导叶之间的区域,水轮机工况下的压力脉动主频为叶片通过频率,而水泵工况下的最高扬程和最低扬程工况的主频分别为转频和叶片数通过的频率;对于无叶区,由于受到强烈的动静干涉效应,水轮机、水泵工况下的主频均为转轮叶片数通过频率,且脉动幅值较大;对于尾水管区域,直锥段处的频率分布规律与流量有关,水轮机小流量工况下,尾水管内主要为0.3倍转频的低频压力脉动,而水轮机大流量工况下,脉动频率主要以2.6倍转频为主。     

罗茨泵内部流场数值模拟研究

罗茨泵在运行中由于各种原因会出现振动大、噪音大、泄漏等问题,获得罗茨泵的流动信息对预知及解决故障问题具有非常重要的意义,建立罗茨泵流场数学模型,用CFD软件对其内部流场进行模拟,获得并分析内部介质的压力场、速度场、流量脉动曲线、进出口压差对流量影响。研究结果表明:两转子间隙处压差最大,形成较高涡流;排气口两侧有明显的涡流;出口流量在转子转动初始阶段达到峰值;进口流量和进出口压差成反比,进口流量的脉动系数和进出口压差成正比。验证了CFD数值模拟的准确性,为对罗茨泵的进一步研究和整体优化奠定了基础。     

Numerical Study of Heat Transfer of a Porous-Block-Mounted Heat Source Subjected to Pulsating Channel Flow

A numerical analysis was conducted to investigate the convective characteristics of pulsating flow through a channel with a porous-block-mounted heat source. Comprehensive time-dependent flow and temperature data are calculated and averaged over a pulsation cycle in a periodic steady state. The impacts of the Darcy number, pulsating frequency and amplitude, and porous blockage ratio are documented in detail. The results indicate that the periodic alteration in the structure of recirculation flows, caused by both porous block and flow pulsation, has a direct impact on the flow behavior in the vicinity of the porous block and on the heat transfer rate from the heater.     

Two-phase cooling characteristics of a saturated free falling circular jet of HFE7100 on a heated disk: Effect of jet length

Direct jet impingement boiling heat transfer operating at low flow rates is of great interest for the localized moderate heat fluxes from the targets with delicate mechanical structure, where the aggressive techniques such as high-speed jets are not suitable. Boiling heat transfer from an upward facing disk targeted by a falling jet was studied experimentally at different volumetric flow rates and various jet lengths. The working fluid was chosen to be the dielectric liquid HFE7100 and the heated spot was an 8-mm diameter disk. Using previous CHF correlations in their original form, valid at very low volumetric flow rates, results in large disagreements since it was found that variation in the jet length changes the boiling characteristics. It is demonstrated that although the circular hydraulic jump formation within the heater diameter may suppress the heat transfer under certain conditions, moving the jet closer to the target may significantly improve the boiling curves at the critical heat flux (CHF) regime. At low flow rates, the CHF increases as the jet length decreases while for moderate and high flow rates the boiling curves show approximately a universal behavior for different jet lengths. For such low flow rates, the effect of jet length on boiling curves was shown to be related to the variation of the cross section of the falling jet and the formation of hydraulic jump at radial distances smaller than the heater diameter. The current CHF results for different jet lengths are correlated by including the effect of jet length in the previous correlation proposed by Sharan and Lienhard.     

湿法烟气脱硫鼓泡流态化反应器流场的数值模拟研究

采用Fluent软件对鼓泡流态化反应器内气液两相流的流场进行了数值模拟,分别计算了三种不同工况下的流场分布状况。模拟结果表明:增大气体喷射速度和喷射管的插入深度都可以使气液两相混合更加充分,流场内较易形成涡旋结构,达到稳定流场的时间也比较短。     

Effects of pulsation on separated flow and heat transfer in enlarged channel

Numerical results of three-dimensional separated flow and heat transfer in an enlarged rectangular channel are presented in this paper.The expansion ratio and aspect ratio of the channel are 2.0 and 16.0,respectively.Reynolds number of the flow is 200 and it is over the critical Reynolds number.Over the critical Reynolds number,the flow in the symmetric channel becomes asymmetric and deflects to one side of the walls.Effects of the pulsating fluctuation at the inlet upon the flow in the channel are investigated.It is clarified that the inlet flow with a pulsating fluctuation of Strouhal number 0.05 and 0.10 strongly affects on the flow in the channel,and heat transfer on the walls is enhanced,especially on the wall surface covered with long separation bubble.On the other hand,the pulsation of St=0.0125 oscillates the shear layer more weakly than that of St=0.05,0.10 and the enhancement of heat transfer is smaller,though some vortices are shed from the vicinity of the side wall near the reattachment region.The oscillation of the main flow calms down gradually as the Strouhal number of the pulsation increases over 0.10.The influence of pulsation of St=0.20 on the flow is restricted in the near downstream of the step,and heat transfer on the walls is almost similar to that of the steady flow in the channel.     

蒸发液体射流表面三维不稳定色散方程的建立

讨论了蒸发黏性液体射流的不稳定性问题。假设环境介质的运动为势流运动,基态射流为流速恒定的平行流。对液体射流表面扰动采用简正模态分析法,并以贝塞尔函数的形式表达气相和液相控制方程的解,在射流边界上考虑液体蒸发速率对不稳定性的影响,建立了基于时间模式的射流三维不稳定扰动的色散方程,并对色散方程进行了无量纲化处理,论述了该色散方程对以往所建色散方程的兼容性,以此推进液体射流雾化机理的研究。