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通过CFD技术,分别对5种长短轴之比的椭圆管管内湍流和层流状态时的换热与流动进行数值研究,分析了流体流动状态和椭圆管长短轴之比对换热系数与流动阻力的影响,并根据数值计算结果拟合出湍流区椭圆管管内换热系数的准则关系式,最后绘制每种类型椭圆管的局部换热系数曲线。研究结果表明:数值计算结果与实验值吻合良好;采用当量直径的方法计算椭圆管内换热系数误差较大;随着雷诺数的增加,每种类型的椭圆管管内阻力系数逐渐减小;而在相同的雷诺数下,随着长短轴之比K的增大,管内阻力系数逐渐增加;每种类型的椭圆管具有类似的局部换热特性,即长半轴两端点处局部换热系数最低,而短半轴两端点处具有最大局部换热系数。 相似文献
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针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。 相似文献
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利用计算流体力学(Computational Fluid Dynamic, CFD)方法,针对连续螺旋折流板换热器建立物理模型和数学模型,在管侧介质为水和壳侧介质为原油条件下,研究不同原油流量及螺旋角对螺旋折流板换热器内部流场、换热性能及阻力性能的影响,并拟合了水油换热时螺旋折流板换热器的Nu、f与Re的关联式。结果表明:22°螺旋角的螺旋折流板换热器与其它较小螺旋角换热器对比,壳侧压降和换热系数逐渐减小,综合换热性能最佳。通过对壳侧原油为层流状态下的阻力系数和对流换热系数关系式进行拟合,更好地指导水-油连续螺旋折流板换热器的热力设计。 相似文献
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以二氧化碳为研究对象,应用k-ε方法对其在水平管内与管外水成垂直交叉冷却的换热进行了分析.用FLUENT软件模拟了超临界二氧化碳在8、10 Mpa,流量为3.4、6.8 g/s,管径6 mm,壁厚1.1 mm,长400 mm的管中流动的状况;计算了平均换热系数h、Nu和Re的变化;并将10 Mpa、3.4 g/s时数值模拟得出的换热系数与实验进行了比较和分析.得出等热流密度下壁面温度的变化情况,数值模拟的换热曲线和实验测量的结果具有相同的趋势,在准临界点处都达到最大值. 相似文献
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用稳态法进行了竖直平行平板间加扰流柱的抽吸式自然对流换热实验研究,全面考虑了包括辐射损失在内的各项修正,得到了精度不低于10%的经验公式。实验发现,对于不加扰流柱的平行平板通道,当温差大于50℃时,换热系数随间距的变化存在极值,小间距加扰流柱时,扰流柱对流动的阻滞起主要作用,换热系数低于无扰流柱时的值。 相似文献
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为研究管束内的换热规律与管间距的关系,通过分析不同物理模型的流场、换热系数,验证了针对横掠管束充分发展段采用周期性边界条件和对称性边界条件的合理性。采用FLUENT软件,对一定雷诺数范围内、不同的管间距横掠顺排管束周期性充分发展段模型的流动换热进行数值模拟。结果表明:密集排列管束的换热效果明显要比稀疏排列的管束大,且对于纵向管间距S_n/d=1.25的管束,当横向管间距取S_p/d=2.0左右时,管束间的换热达到最强,可以为提高换热器换热能力提供工程实际参考。最后将数值模拟结果与已有的经验公式以及实验结果进行比较,验证数值模拟方法的正确性。 相似文献
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对具有2,4,6根管的管排在大空间内的自然对流进行了数值模拟。参数范围为10^2〈Ra〈10^4,S/D=1.25,2,3,5,9,11,Pr=0.7,流态为层流。数值计算结果表明:Ra和S/D对换热都有重要的影响,在小间距时对流作用处于劣势,管排中的管处于换热削弱状态,随着间距的增大对流作用逐渐占优势,管排中的各管从下至上逐渐受到加强,管排中出现了涡,这种涡起着增强局部换热系数的作用。 相似文献
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A numerical simulation and experimental study were carried out for evaporation heat transfer of a falling water film on a smooth horizontal tube bundle evaporator. A laminar model and a turbulence model were respectively adopted to calculate the heat transfer coefficients of falling water film on horizontal heated tubes. The calculation zone on the heated tube was divided into the top stagnation zone and the lateral free film zone. The initial boundary conditions for the free film zone were determined from the calculated results of the stagnation zone. The modified wall function method was used for the turbulent flow. Comparisons between the experimental data and the numerical solutions by use of two flow models show that the experimental data lie between the laminar model solutions and the latter turbulence model solutions and that they are closer to the latter solutions. Finally, a simple dimensionless correction based on the numerical simulations is proposed for predicting the evaporation heat transfer of falling water film for actual engineering applications. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 42–55, 2002 相似文献
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On the Performance of Ag/Oil Nanofluids in Heat Transfer Enhancement in a Sinusoidal Tube: Constant Heat Flux Boundary Condition
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The present work provides an empirical investigation on the thermal characteristics of Ag/oil nanofluids flow inside a sinusoidal tube under a constant heat flux boundary condition. Ag/oil nanofluids have been prepared in low‐volume concentrations of 0.011%, 0.044%, and 0.171%. The average size of the nanoparticles was 20 nm. A heated coil was attached to the upper and lower surface of the tube that satisfied the constant thermal boundary condition of 204 W. The experiment has been pursued at low Reynolds numbers less than 160. A loop was designed to keep the flow hydrodynamically fully developed during the experiment. The test case was a sinusoidal tube. Upper and lower surfaces of the tube have been designed sinusoidally. Moreover, the width of the plates was long enough, so the problem was not considerably affected by the three‐dimensional releasing effect. Convective heat transfer coefficient and Nusselt number were calculated. It has been observed that based on the acquired data of the present work, convective heat transfer coefficient increased up to 23% for the best case (nanofluid with a volume concentration of 0.171%) compared to the base fluid. This happened while the rise of the friction factor was very low. In addition, a comparison between the new results and the previous work by authors showed the positive performance of sinusoidal tubes in increasing the convective heat transfer coefficient (the average increase was calculated to be about 82%) compared to the annular tube. 相似文献
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D. Ashtiani M.A. Akhavan-Behabadi M. Fakoor Pakdaman 《International Communications in Heat and Mass Transfer》2012
An experimental investigation on heat transfer characteristics of MWCNT-heat transfer oil nanofluid flow inside horizontal flattened tubes has been carried out under uniform wall temperature condition. Nanoparticle weight fractions were 0%, 0.1%, 0.2%, and 0.4%. The copper tubes of 14.5 mm I.D. were flattened and used as the test section of oblong shape with inside heights of 13.4 mm, 11.7 mm, 10.6 mm, and 8.6 mm. The nanofluid flowing inside the tube was heated inside a steam chamber to keep the temperature of the tube wall constant. The required data were acquired for laminar hydrodynamically fully developed regime. The effects of different parameters such as volumetric flow rate, nanoparticle weight fraction, and hydraulic diameter on the heat transfer behavior of the tested systems have been investigated experimentally. For a given flattened tube at a constant nanoparticle weight fraction, increasing volumetric flow rate results in heat transfer enhancement. In addition, as the tube profile becomes more flattened and the hydraulic diameter decreases, the heat transfer coefficient goes up at constant volumetric flow rate. Utilizing nanofluids instead of the base fluid, the heat transfer rate enhances remarkably. The higher the nanoparticles weight fraction, the more the rate of heat transfer enhancement. Finally, the results show that the amount of increase in heat transfer coefficient caused by employing nanofluid instead of the base fluid is comparable to what caused by flattening the tube. 相似文献
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M. Haghshenas Fard M. Nasr Esfahany M.R. Talaie 《International Communications in Heat and Mass Transfer》2010
Laminar convective heat transfer of nanofluids in a circular tube under constant wall temperature condition is studied numerically using a CFD1 approach. Single-phase and two-phase models have been used for prediction of temperature, flow field, and calculation of heat transfer coefficient. Effects of some important parameters such as nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate have been investigated. The results of CFD simulation based on two-phase model were used for comparison with single-phase model, theoretical models and experimental data. Results have shown that heat transfer coefficient clearly increases with an increase in particle concentration. Also the heat transfer enhancement increases with Peclet number. Two-phase model shows better agreement with experimental measurements. For Cu/Water nanofluid with 0.2% concentration, the average relative error between experimental data and CFD results based on single-phase model was 16% while for two-phase model was 8%. Based on the results of the simulation it was concluded that the two-phase approach gives better predictions for heat transfer rate compared to the single-phase model. 相似文献
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用两根内表面微结构不同的水平光滑管环状流区流动沸腾换热实验数据,采用叠加模型分别建立了流动沸腾换热关系式,并比较它们的抑制因子。结果表明,表面微结构对抑制因子有显的影响;当表面的平均凹腔半径较大时,抑制因子明显增大。表明表面微结构改变对流动沸腾换热能起到较好的强化作用。 相似文献
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Numerical simulation of mixed convective heat transfer of a molten salt in a square tube with a thermally conductive wall
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熔盐因具有传热能力强、工作温度高、使用温度广、系统压力低、经济适用等优点,成为太阳能热发电系统蓄热工质的理想选择。熔盐在实际应用中会因加热过程的非均匀性产生存在于固体表面和流体间的温差,造成流体工质中的密度梯度,因此出现重力导致的浮升力效应,其叠加到主流流动方向上即形成混合对流。管壁导热会对熔盐混合对流传热过程产生一定的影响。本文对熔盐在水平方管内非均匀加热条件下的单面加热的混合对流过程进行了数值模拟研究,在考虑壁厚的情况下研究了方管单面加热熔盐混合对流传热特性,分析了无量纲参数间的变化关系,并将结果与流型判定图和经典关联式进行对比。结果表明,非均匀加热时,浮升力效应会造成随流动距离增加主流核心区域的形状发生改变,且更加靠近加热壁面。Nu数随Re数、Ri数的增大而增大,局部Nux数随流动距离的深入先减小后增大。与忽略管壁导热数值模拟结果相比,主流核心区形状更加均匀,局部Nux更高且回升位置更加提前,流动特性和传热特性基本保持一致。 相似文献
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Jiancong Dong Tuo Zhou Xiaojiang Wu Jian Zhang Haojie Fan Zhongxiao Zhang 《热科学学报(英文版)》2018,27(6):592-601
This paper presented a coupled heat transfer model combining the combustion in the furnace and the ultra-supercritical(USC) heat transfer in the water wall tubes. The thermal analysis of the spiral water wall in a 1000 MW double reheat USC boiler was conducted by the coupled heat transfer simulations. The simulation results show that there are two peak heat flux regions on each wall of spiral water wall, where the primary combustion zone and burnt-out zone locate respectively. In the full load condition, the maximal heat flux of the primary combustion zone is close to 500 kW/m~2, which is higher than that in the conventional single reheat USC boilers. The heat flux along the furnace width presents a parabolic shape that the values in the furnace center are much higher than that in the corner regions. The distribution of water wall temperature has a perfect accordance with the heat flux distribution of the parabolic shape curves, which can illustrate the distribution of water wall temperature is mainly determined by heat flux on the water wall. The maximal water wall temperature occurs at the middle width of furnace wall and approaches 530°C, which can be allowed by the metal material of water wall tube 12Cr1MoVG. In the primary combustion zone, the wall temperatures in half load are almost close to the values in 75% load condition, caused by the heat transfer deterioration of the subcritical pressure fluid under the high heat flux condition. The simulation results in this study are beneficial to the better design and operational optimization for the double reheat USC boilers. 相似文献