缩放管内脉动流传热性能研究

分析研究了缩放管内脉动流的传热性能,利用Fluent数值模拟脉动振幅、脉动频率和扩缩比对缩放管传热和沿程阻力的影响。研究结果表明:缩放管内脉动流的传热性能要优于缩放管内流体稳态流动,与缩放管内流体流动为稳态时传热强化了11.4%左右;缩放管内脉动流强化传热同时也增大了沿程阻力;对综合评价指标数的分析得到脉动流条件下缩放管的传热性能显著增强。     

基于场协同的缩放管管外强化传热数值模拟

为研究缩放管管外强化传热性能,利用CFD软件对缩放管和光滑管管外对流传热进行了数值模拟。模拟结果表明:两种管外换热系数都随Re增加而增加,同种工况下缩放管管外侧传热系数是光管的1.5~1.8倍,但随着Re的进一步增大,强化传热有所减弱。最后,应用温度场和速度场的协同理论分析强化传热机理,结果表明:缩放管管外速度场与温度场之间夹角较小,协同程度好,从而达到强化传热目的。     

内置转子圆管内三维流动与传热数值模拟

建立了内置转子圆管的三维流动模型,采用RNGk-ε湍流模型对内置不同截面转子的管内流场进行数值模拟,得到了流动特性和传热特性。研究表明:内置组合转子的管内流动是复杂的三维螺旋流动;转子管内流体的径向速度比光管内流体的径向速度大;流体在转子直径范围内的切向速度随半径的增大而增大,流体在转子与管内壁间的环向区域内存在明显的切向运动;转子截面的改进明显改善了管内流场的分布,提高了管内换热效率,同时增大了阻力系数。     

水平管喷淋式降膜蒸发及其强化的研究

本研究包括三个部分:(1)采用电加热方式研究水平管外降膜蒸发;(2)采用蒸汽加热方式研究水平管外降膜蒸发;(3)研究水平管内的蒸汽冷凝。本实验除了对光滑管进行传热研究以外,还采用多孔表面管以强化蒸发侧的传热,在管内壁插入线圈以强化冷凝侧的传热。研究结果表明,水平光滑管降膜蒸发时的传热系数比垂直光滑管以及池式沸腾管的都高得多。而多孔表面水平管的蒸发膜系数又比光滑水平管的高100～50%,其值高达40kW/m~2·℃。水平管外壁为多孔表面时,内壁附有线圈的总传热系数要比内壁为光滑面的高17%,而压力降只高20%。     

热量运输机理及在管内螺带强化传热的应用

在较小流速和较低粘度的假设条件下,通过对流体微元能量方程变换,提出热量运输的观点.据此观点分析影响对流传热的主要因素并发展场协同理论.对螺带插入换热管内的流体受力情况进行分析,指出各个力对流场的影响.结合热量运输的观点,提出改善管内流动阻力、提高综合传热效率的方法.运用数值模拟得到管内流体流动和传热特性的分布规律.分析表明螺带后光管长度大约500～600 mm时,此段光管具有最大的综合传热系数.分析还表明在较长换热管条件下,螺带插入长度有更大的调整空间,可根据实际情况调整螺带长度以获得较大的传热系数或者较低阻力损失,而综合传热系数依然保持较高水平.证实了在热量运输机理指导下开发新的强化传热结构的可行性.     

扭曲管性能数值模拟及公式拟合

扭曲管是一种高效的管程强化传热管。对这一管形的传热性能及流阻性能进行了深入地数值模拟研究。结果表明,与普通圆管相比,扭曲管由于其结构的特殊性,使得管内介质不断冲刷管壁,造成扭曲管管壁附近传热边界层厚度减薄的现象,提高了管内介质对流传系数,传热效果明显增强。此外,在相同雷诺数下,扭曲管的管内对流传热系数比圆管高30%,而综合性能高出30%~35%。在此基础上,对扭曲管的结构尺寸(截面比例ai/bi和螺距s)进行了研究,扭曲管结构尺寸对其传热及流阻性能的影响较大,在模拟范围内,截面比例ai/bi=2或螺距s=100 mm的扭曲管综合性能最优。最后,通过对数值模拟的数据进行处理,拟合出统一的扭曲管管内Nu和ΔP公式,对工程设计有较大指导意义。     

超临界二氧化碳管内湍流流动和传热的数值模拟

应用标准的k-ε模型对超临界CO2在竖直微通道管内的湍流流动和传热进行了数值模拟。采用SIMPLE算法，得到了各种条件下不同管径的速度、温度剖面分布规律以及对流换热系数h和Nu数。分析了相关参数对超临界CO2管内湍流流动和传热的影响，并将模拟结果与相关的理论和实验成果进行了比较，验证了模拟结果的正确性。     

内置扭带换热管三维流动与传热数值模拟

对自转扭带换热管内流体的运动进行了分析,根据流体在自转扭带管内的切向运动特点,提出将自转扭带等效虚拟于静止扭带的思路。建立内置螺旋扭带换热管流体流动的三维物理模型,采用大型CFD软件FLUENT6.0中的RNG k-ε模型对内置扭带换热管内的流动与传热进行了数值模拟,得到了内置扭带换热管流体流动的速度、压力、湍流强度场分布规律及传热特性。比较了静止、旋转及旋转等效虚拟静止扭带换热管的传热和阻力降特性,分析了不同螺距对强化传热和阻力降的影响。速度场的模拟值与激光测速仪试验值进行了比较,二者吻合较好。     

缩放管内脉动流动对流换热的数值模拟研究

通过数值模拟计算,研究在不同脉动频率、脉动振幅下的管内流体脉动对缩放管的传热和阻力的影响。研究结果表明:管内流体脉动能够强化缩放管的传热效果,相比稳态时,传热被强化了约为16%;管内流体脉动在一定条件下,也会增大缩放管的沿程阻力。     

倾角渐增波纹翅片管换热器空气侧流动与换热特性的数值模拟

提出了一种圆管倾角渐增波纹翅片的管翅式换热器,利用FLUENT软件对其空气侧的流体流动和换热过程进行了数值模拟,得到了翅片通道中心面上的温度场和压力场的分布情况及平均传热系数、努赛尔数与速度的关系。并将其强化传热效果与倾角均匀波纹翅片换热器进行对比分析,结果表明倾角渐增波纹翅片比倾角均匀波纹翅片的传热效果更好,更节能。     

冷却对换热器内湍流流动的影响

采用IFA300恒温式热线风速仪对换热器热气通道内的湍流流动进行了测试研究。利用一维探针测试的结果表明:传热边界条件对被测区域的量纲一平均速度的分布影响很小,而传热边界条件不同对湍流度的分布有一定的影响。结合两维探针的测试结果分析说明了边界受冷却时,近壁区气体体积变化是加剧湍流脉动的主要原因。     

超临界CO_2微细管内传热特性研究

考察超临界CO2微细管内冷却条件下流动和换热的细观信息(管内不同截面湍动能分布,无量纲速度和温度分布以及湍流雷诺数分布等),结果表明:近壁处湍动能的变化对传热有较大影响,微细管内换热关联式需考虑浮升力的影响。对管径d=0.5,0.3,0.1 mm时,超临界CO2的换热特性进行计算分析,回归出管径小于0.5 mm时,超临界CO2在冷却条件下竖直向上流动和竖直向下流动时的换热关联式,作为对超临界CO2研究的有效补充。     

斜截椭圆柱式涡流发生器强化传热的大涡模拟

对流体在放置斜截椭圆柱式涡流发生器矩形槽道内的流动与传热特性进行大涡模拟,得出流场中速度、温度与压力参数的瞬态变化特性,再现温度场、压力场及诱导旋涡的变化过程,并对流动结构及涡流发生器强化传热的机理进行分析。为验证大涡模拟计算结果的准确性,在相同条件下对未布置涡流发生器的空槽道分别采用湍流模型和大涡模拟进行对比计算,两者的计算结果符合较好。计算结果表明：流场中布置的涡流发生器可以诱导漩涡,而由其所诱导的流向涡对强化传热起主要作用。与相同条件下未布置涡流发生器的情况相比,局部对流换热系数可提高64%～105%,平均对流换热系数则可提高17%～36%;涡流发生器附近位置的对流换热系数提高幅度最大,传热面附近流体的流动状况及流动结构与传热密切相关。     

Enhanced tubing thermal performance for innovative MSF system

Existing MSF systems experience severe mechanical difficulties due to fouling deposited inside the bore of the condenser tubes. The idea of replacing smooth tubing with enhanced tubing is discussed for innovative design. Performance analysis between enhanced and plain tubes is investigated experimentally, using a simulated design test rig. The study is performed for corrugated and smooth resemblance aluminum-brass tubes of 1100 mm length and 23 mm bore. Pumping different coolants as steady transition/turbulent flow, fresh water and authentic brine are conducted to simulate actual environmental conditions. Three different flow velocities are examined: 0.1, 0.1645, and 0.2398 m/s. Comparing results of the brine with that of fresh water shows the effects of fouling on significantly lowering values of overall heat transfer coefficient versus time. The results confirm that the smooth tube has a greater fouling tendency than the corrugated tube. The effect of fouling is reduced as flow speed increases, and consequently, the asymptotic value of overall heat transfer coefficient U* increases. Values of U* for the enhanced tube are much higher than that of the smooth tube. The highest difference occurs at velocity 0.2398 m/s by a factor of about 1.5. Overall, by utilizing real brine, the results proved that heat performance of the employed corrugated tube is superior to the plain, over the studied time period, 140 hrs, and on the studied range of flow speeds.     

Laminar forced convective heat transfer to near-critical water in a tube

Numerical modeling is carried out to investigate forced convective heat transfer to nearcritical water in developing laminar flow through a circular tube. Due to large variations of thermo-physical properties such as density, specific heat, viscosity, and thermal conductivity near thermodynamic critical point, heat transfer characteristics show quite different behavior compared with pure forced convection. With flow acceleration along the tube unusual behavior of heat transfer coefficient and friction factor occurs when the fluid enthalpy passes through pseudocritical point of pressure in the tube. There is also a transition behavior from liquid-like phase to gas-like phase in the developing region. Numerical results with constant heat flux boundary conditions are obtained for reduced pressures from 1.09 to 1.99. Graphical results for velocity, temperature, and heat transfer coefficient with Stanton number are presented and analyzed.     

Thermal performance analysis of heat exchanger for closed wet cooling tower using heat and mass transfer analogy

In closed wet cooling towers, the heat transfer between the air and external tube surfaces can be composed of the sensible heat transfer and the latent heat transfer. The heat transfer coefficient can be obtained from the equation for external heat transfer of tube banks. According to experimental data, the mass transfer coefficient was affected by the air velocity and spray water flow rate. This study provides the correlation equation for mass transfer coefficient based on the analogy of the heat and mass transfer and the experimental data. The results from this correlation equation showed fairly good agreement with experimental data. The cooling capacity and thermal efficiency of the closed wet cooling tower were calculated from the correlation equation to analyze the performance of heat exchanger for the tower.     

发夹式换热器管束流致振动数值模拟研究

运用计算流体力学方法,采用ANSYS CFX软件对发夹式换热器的壳侧流场进行了三维数值模拟。流场计算中采用多孔介质模型对管束区域进行简化,分析了壳侧流场的速度分布,结果表明:直管段部分的流体湍流强度大于弯管段,且外层管束所在区域为高流速区,受流体冲刷严重。结合流场信息,通过功率谱生成随机激振力,采用ABAQUS软件模拟计算了湍流激振下管束的振动响应,结果显示管束的面外均方根位移远大于面内位移,且弯管部分的振动位移最大。该研究结果可为发夹式换热器的性能分析和优化设计提供参考和依据。     

Turbulent mixing flow characteristics of solid-cone type diesel spray

The intermittent spray characteristics of the single-hole diesel nozzle (d_n=0.32 mm) used in the fuel injection system of heavy-duty diesel engines were experimentally investigated. The mean velocity and turbulent characteristics of the diesel spray injected intermittently into the still ambient were measured by using a 2-D PDPA (phase Doppler particle analyzer). The gradient of spray half-width linearly increased with time from the start of injection, and it approximated to 0.04 at the end of the injection. The axial mean velocity of the fuel spray measured along the radial direction was similar to that of the free air jet within R/b=1.0-1.5 regardless of elapsing time, and its non-dimensional distribution corresponds to the theoretical velocity distributions suggested by Hinze in the downstream of the spray flow fields. The turbulent intensity of the axial velocity components measured along the radial direction represented the 20-30% of the Ū_cl and tended to decrease in the outer region. The turbulent intensity in the trailing edge was higher than that in the leading edge.     

Effect of guide wall on jet impingement cooling in blade leading edge channel

The characteristics of fluid flow and heat transfer, which are affected by the guide wall in a jet impinged leading edge channel, have been investigated numerically using three-dimensional Reynolds-averaged Navier–Stokes analysis via the shear stress transport turbulence model and gamma theta transitional turbulence model. A constant wall heat flux condition has been applied to the leading edge surface. The jet-to-surface distance is constant, which is three times that of the jet diameter. The arrangement of the guide wall near the jet hole is set as a variable. Results presented in this study include the Nusselt number contour, velocity vector, streamline with velocity, and local Nusselt number distribution along the central line on the leading edge surface. The average Nusselt number and average pressure loss between jet nozzle and channel exit are calculated to assess the thermal performance. The application of the guide wall is aimed at improving heat transfer uniformity on the leading edge surface. Results indicated that the streamwise guide wall ensures the vertical jet impingement flow intensity and prevents the flow after impingement to reflux into jet flow. Thus, a combined rectangular guide wall benefits the average heat transfer, thermal performance and heat transfer distribution uniformity.     

微通道内流体流动及换热特性的数值分析

采用Navier—Stokes方程与滑移边界条件联立的理论分析模型，对等壁温、等热流及无温度梯度工况下，气体在微通道中的流速分布、阻力系数变化趋势（Cf·Re）和传热特性（努塞尔数）进行了数值研究。结果表明：气体稀薄效应可显著减小管内的摩擦阻力和努塞尔数，增大气体流速；壁面的速度滑移和温度跳跃对微圆管内换热特性的影响相反，温度跳跃的影响更大；等热流加热与等壁温加热两种情况下，努塞尔数随克努森数的变化趋势明显不同。