Numerical analysis of heat transfer enhancement on steam condensation in the presence of air outside the tube

In loss-of-coolant accidents,a passive containment heat removal system protects the integrity of the containment by condensing steam.As a large amount of air exists in the containment,the steam condensation heat transfer can be significantly reduced.Based on previous research,traditional methods for enhancing pure steam condensation may not be applicable to steam–air condensation.In the present study,new methods of enhancing condensation heat transfer were adopted and several potentially enhanced heat transfer tubes,including corrugated tubes,spiral fin tubes,and ring fin tubes were designed.STAR-CCM+was used to determine the effect of enhanced heat transfer tubes on the steam condensation heat transfer.According to the calculations,the gas pressure ranged from 0.2 to 1.6 MPa,and air mass fraction ranged from 0.1 to 0.9.The effective perturbation of the high-concentration air layer was identified as the key factor for enhancing steam–air condensation heat transfer.Further,the designed corrugated tube performed well at atmospheric pressure,with a maximum enhancement of 27.4%,and performed poorly at high pressures.In the design of spiral fin tubes,special attention should be paid to the locations that may accumulate high-concentration air.Nonetheless,the ring-fin tubes generally displayed good performance under all conditions of interest,with a maximum enhancement of 24.2%.

Theoretical Research on R245fa Condensation Heat Transfer inside a Horizontal Tube

Numerical simulation on R245fa condensation inside an inner diameter of 8 mm horizontal tube is researched in this paper. The effect of variation in velocity, condensation temperature and superheat of inlet steam and variation in cooling water temperature on heat transfer coefficient are investigated as a parametric study. Condensation process of steam has been successfully modeled by applying a user defined function (UDF) added to the commercial computational fluid dynamics (CFD) package. By analyzing the corresponding condensate contours and the curves of local heat transfer coefficient, the relationships between condensation heat transfer coefficient and various parameters of R245fa inside horizontal tube are obtained. It shows that the heat transfer coefficient increases by the increase in velocity, condensation temperature and superheat of inlet steam and the decrease in cooling water temperature. The errors between the heat transfer coefficient of simulation result and model of Wang and Shah are within ±30%. The parametric study will provide the basis for designing efficient heat exchangers of R245fa.

A comprehensive comparison on vibration and heat transfer of two elastic heat transfer tube bundles

Elastic heat transfer tube bundles are widely used in the field of flow-induced vibration heat transfer enhancement. Two types of mainly used tube bundles, the planar elastic tube bundle and the conical spiral tube bundle were comprehensively compared in the condition of the same shell side diameter. The natural mode characteristics, the effect of fluid-structure interaction, the stress distribution, the comprehensive heat transfer performance and the secondary fluid flow of the two elastic tube bundles were all concluded and compared. The results show that the natural frequency and the critical velocity of vibration buckling of the planar elastic tube bundle are larger than those of the conical spiral tube bundle, while the stress distribution and the comprehensive heat transfer performance of the conical spiral tube bundle are relatively better.

A Numerical Study on Fully Developed Fluid Flow and Heat Transfer in a Spiral Finned Tube

In this paper, the standard k-ε two-equation model is adopted to numerically simulate fully developed fluid flow and heat transfer in a spiral finned tube within a cracking furnace for ethylene manufacturing. By variable transformation, the original 3-D problem is converted into a 2-D problem in spiral coordinates. The algorithm of SIMPLEC is used to study the fully developed fluid flow and heat transfer in the spiral finned tube at constant periphery temperature and constant axial heat flux. The computed results agree pretty well with the experimental data obtained from the industry. Further studies on the fluid flows and temperature profiles at different Reynolds numbers within straight and spiral finned tubes are conducted and the mechanisms involved are explored. It is found that with the spiral finned tube, pressure drop increases to a great extent whereas heat transfer tends to be decreased.

Numerical simulation of flow and heat transfer of n-decane in sub-millimeter spiral tube at supercritical pressure

The flow and heat transfer characteristics of n-decane in the sub-millimeter spiral tube(SMST) at supercritical pressure(p = 3 MPa) are studied by the RNG k-ε numerical model in this paper. The effects of various Reynolds numbers(Re) and structural parameters pitch(s) and spiral diameter(D) are analyzed.Results indicate that the average Nusselt numberNu and friction factorNu increase with an increase in Re, and decrease with an increase in D/d(tube diameter). In terms of the structural parameter s/d, it is found that as s/d increases, the Nu first increase, and then decrease. and the critical structural parameter is s/d = 4. Compared with the straight tube, the SMST can improve Nu by 34.8% at best, while it can improve Nu by 102.1% at most. In addition, a comprehensive heat transfer coefficient is applied to analyze the thermodynamic properties of SMST. With the optimal structural parameters of D/d = 6 and s/d = 4, the comprehensive heat transfer factor of supercritical pressure hydrocarbon fuel in the SMST can reach 1.074. At last, correlations of the average Nusselt number and friction factor are developed to predict the flow and heat transfer of n-decane at supercritical pressure.

Ammonia corrosion analysis of copper tubes and experimental research on non-copper improvement for heat exchanger in power plant

Ammonia corrosion in copper tube will affect the safety of boiler running in power plant. Therefore, no copper in heating system has become a technical orientation in heat exchanger reconstruction, This paper analyzes the condition and mechanism of ammonia corrosion occurring in copper tube used in coal-fired power plants. Using a general steam condensation testing equipment only for horizontal single tube, with water vapor and water as working fluid, on two types of steel tube with 2-side enhancement heat transfer, namely, a spirally fluted tube and a ratchet tube with internal spiral groove （RISG tube） which was developed recently, a set of experimental tests are conducted to investigate the characteristics of heat transfer and hydromeehanics. In order to compare easily, both one copper smooth tube and one steel smooth tube are also used in the experiment. The experimental results, which get from single horizontal tube, show that the overall heat transfer coefficient of steel spirally fluted tube are improved by 10%o to 17%, and that of the steel RISG tube（22%-28%） is better than steel spirally fluted tube, its flow resistance coefficient is only increased by 22% to 66% when compared with smooth tube. Based on a lot of experimental data, the steel spirally fluted tube and the steel RISG tube were applied in a low pressure preheater and an oil-cooler of some or other power plant respectively. The field testing results showed that their heat transfer coefficient with each types of enhancement heat transfer tubes were improved by 2.5% and 21%-45% comparing with copper smooth tube heat exchangers. Both basic and field experiment indicates that the steel tube with 2-side enhancement heat transfer is an ideal choice for heat exchanger reconstruction in no copper issue in power plants.

Thermal transfer experimental research of a horizontal tube evaporative condenser

The thermal resistances distribution in different wet-bulb temperatures, air velocities and spraying water densities were achieved by the experimental test. The fluctuation of the water film convection and the water-air interfacial thermal resistance were reviewed especially. In the distribution of thermal resistance, the rank of the thermal resistance proportion （from max to min） is air flow heat transfer resistance, heat transfer resistance between refrigerant and wall, water film convection resistance and wall heat transfer resistance. When the heat flux is constant, the total resistance lowers nearly along with the increasing of air flow and water spray density. But there are a best air flow value of 2.98 m/s and a best spray water density of 0.064 kg/（m ·s） respectively, if continue to increase them, condensation performance is not significantly improved any more. The test results are available to improve the evaporative condenser performance and the designing lever.

Experimental Study on Condensation Heat Transfer Characteristics of R410A in Short Vertical Tubes

An experimental study on condensation heat transfer of R410 A in short vertical tubes(8.02 mm ID and 10.7mm ID) was presented. Experiments were performed in eight short copper tubes length varied from 300 mm to 600 mm at mass fluxes range of 58–246 kg m-2s-1 and saturation temperature of 38℃. Effects of mass flux, tube length on condensation heat transfer coefficient were investigated. The distribution of temperature, thickness of condensate film and local condensation heat transfer coefficient along the tube were also analyzed.. It is indicated that the entrance effect played an important role in condensation heat transfer of vertical tube, and the influence of entrance effect on average condensation heat transfer coefficients will weaken with the length of tube in the experimental condensation. The experimental results were compared with four well known correlations available in literatures, and the Chen correlation shows good agreement with the experimental data but with ±40% deviation. A new modified condensation heat transfer correlation with 12.7% mean deviation was developed to predict the condensation heat transfer coefficients in short vertical tube based on the experimental data.

Heat Transfer Enhancement and Vortex Flow Structure in the Spirally Fluted Tubes

The turbulence kinetic energy and heat transfer performance of air in spirally fluted tube were numerically studied at a constant wall temperature with Reynolds number(Re)between 5000 and 45000.Furthermore,the flow dynamics and heat transfer performance of spirally fluted tubes with five different geometric parameters as well as the effects of separation vortex and swirling wake flow on heat transfer and flow resistance were analyzed.According to the results,heat transfer is enhanced mainly because the fluid hit the windward side of the flute,thus generating a strong turbulence kinetic energy to further reconstruct the boundary layer.The second reason is that the formation of the recirculation zone between the flutes disturbs the boundary layer caused by the flow separation.With the increase of flute depth ratio(L_(d)/D),the separation vortex will become stronger and larger on the leeward side of flute.The separation vortex will break the boundary layer and improve the heat transfer capacity which is accompanied with the increase of fluid resistance.As the flute pitch length ratio(L_(p)/D)decreases,the spiral flow is strengthened,and meanwhile more wake flow is generated.The spiral flow causes little impact on enhancing heat transfer but inhibits the development of the separation vortex and fluid pulsation;in addition,the fluid resistance is reduced at the same time.The maximum value of the average Nusselt number appears when Re=5000,L_(d)/D=0.25 and L_(p)/D=1.00,which is 2.53 times the value of smooth tube.In view of the whole range of Reynolds number,the thermal performance enhancement factor indicates that L_(d)/D=0.15 and L_(p)/D=1.00 are the optimal geometric design parameters.

Experimental Study on Condensation Heat Transfer Characteristics inside an Inclined Wave-Finned Flat Tube of Direct Air-Cooling System

In this paper,the condensation heat transfer characteristics of parallel flow and counter flow inside an inclined wave-finned flat tube is investigated experimentally.The condensation heat transfer coefficients are analyzed based on the experimental results.Results of experiments show that condensation heat transfer coefficient decreases as the temperature difference Δt=ts-tw increases and mass flow rate decreases.The parallel flow has a similar development with the counter flow,and the condensation heat transfer coefficient of counter flow is less than that of parallel flow under the same air cooling conditions.In addition,condensation heat transfer coefficient correlations are also obtained under experimental ranges.The calculations agree well with the measured data and the agreement is seen to be within ±4% for the parallel flow and ±5% for the counter flow.

Condensation heat transfer characteristics of vapor flow in vertical small-diameter tube with variable wall temperature

To explore the condensation characteristics of vapor flow inside vertical small-diameter tubes, the classical Nusselt theory is revised and an analytical model with variable tube wall temperature is established by considering the effect of surface tension exerted by condensate film bending as well as the effect of shear stress on vapor-liquid interface. The effects of various factors including tube wall temperature and gravityon flow condensation in small-diameter tubes are analyzed theoretically to show the heat transfer characteristics. Comparison with the experimental data indicates that the proposed analytical model is fit to reveal the fundamental characteristics of flow condensation heat transfer in vertical small-diameter tube.

螺旋扭曲扁管传热流阻性能及工艺计算方法研究

螺旋扭曲扁管是一种非标换热管,成熟的热交换器计算商业软件,如HTRI、Aspen EDR等目前均尚未提供对应的计算模型,现有计算方法存在不统一、偏差大等问题。以螺旋扭曲扁管为对象,通过数值模拟计算、实验测试对其进行传热及流阻性能研究,综合利用HTRI传热计算软件,建立了一套螺旋扁管传热及流阻性能的通用化计算方法。进行了通用化计算结果、数值模拟分析数据、实验测试数据的对比,验证了通用计算方法的准确性,对螺旋扭曲扁管的工艺设计具有指导意义。

加装螺旋导流板的双层铜管换热器传热性能分析

为提高螺旋铜管(SCT)换热器的综合传热性能,通过在换热器内加装螺旋形导流板,采用双向流固耦合计算方法,分析了不同入口流速和导流板安装位置下对换热器内部双层SCT的振动和传热性能的影响。数值结果表明:双层SCT换热器外层SCT的振动强度明显高于内层;无振动时内层SCT的传热系数高于外层,有振动时外层SCT的传热系数高于内层;随着入口流速增加,内外层SCT的振幅均增大,传热系数提高;导流板的安装位置会直接影响SCT的振动和传热特性,且在SCT呈现最大振动强度和最佳传热性能时的安装位置并不相同;对比振动强化传热参数Q PEC和综合传热性能因子R JF发现,双层SCT换热器的Q PEC和R JF均高于单层,两螺旋导流板分别安装在换热器的左、下位置时,Q PEC最大,安装在换热器的左、上位置时,R JF最大且比安装在左、下位置时最高增大了7.99%。该研究可为高性能SCT换热器的设计提供理论和数据支撑。

射流管插入深度对螺旋通道传热性能的影响

为了揭示射流管插入深度对螺旋通道传热性能的影响规律,采用数值模拟方法对不同相对插入深度(δ=0~0.625)下通道内流场特性和强化换热特性进行了分析。结果表明:随着δ的增大,射流核心区到达外壁面的距离变短,在射流入射位置处射流管两侧的二次涡范围扩大,流体对上下壁面的冲击作用增强;上、下传热壁面的局部努赛尔数Nu_(c)随着δ的变化规律基本相同,最佳传热位置(Nu_(c)最大的位置)随着δ的增大向外壁面靠拢;内壁面的Nu_(c)在射流管壁内侧范围随着δ的增大而减小,在外侧范围随着δ的增大而增大;射流管插入深度增加对螺旋通道外壁面传热性能提高的效果最为明显;截面努赛尔数Nu_(θ)随着螺旋角度γ的增大呈现3个变化区域,即入口区域、射流影响区域和出口区域,其中入口区域和出口区域的Nu_(θ)随着γ的变化保持稳定,但出口区域的Nu_(θ)比入口区域高19%左右,Nu_(θ)最大值位于射流入射点下游5°左右的位置;在螺旋角度γ=-15°~45°的范围内射流对传热的影响最大,传热的影响范围基本与射流管的插入深度无关;综合换热评价指标(PEC)随着δ的增加呈现先增大后减小的变化规律,δ=0.250是螺旋通道最佳的综合传热性能结构参数。

不同螺旋缠绕异形管换热器壳程性能数值研究

缠绕管的形状对螺旋缠绕管换热器的传热能力有重要影响,本文建立了具有不同截面形状的螺旋缠绕管换热器模型,数值研究了八种不同截面形状的缠绕管对螺旋缠绕管换热器壳程流动与传热性能的影响。相比于传统圆管,三叶管和水滴管对壳程流体造成的扰动较大,增强了螺旋缠绕管换热器的综合换热性能,而三叶管则具有最大换热能力。进一步分析了不同冲刷方式对螺旋三叶管换热器和螺旋水滴管换热器壳程性能的影响,结果表明:在相同的进口工况下,螺旋三叶管换热器壳程努塞尔数相比于传统圆管提高了11.8%~15.5%,阻力系数增大了41.9%~49.6%;螺旋水滴管换热器壳程努塞尔数相比于传统圆管提高了2.8%~4.3%,阻力系数增大了3.2%~6.8%;在雷诺数为2 603~5 206的工况条件下,管截面与流动方向夹角为0°的三叶管换热器综合换热性能最好,而管截面与流动方向呈逆流形式的水滴管换热器综合换热性能优于顺流形式。

R513A在水平螺纹管内冷凝换热性能与压降

为研究R513A在内螺纹管中的冷凝换热性能与流动特性,通过实验对R513A在外径为12.7 mm和9.52 mm的光滑管和内螺纹管中的冷凝换热系数和压降进行分析,实验工况为:质量流速50—270 kg/(m^(2)·s),冷凝温度33、35、38、40℃。结果表明:R513A与R134a在光滑管内的冷凝换热系数差值为-18.8%—-25.5%,在螺纹管内差值为-5.5%—0.8%,螺纹管对于R513A的管内冷凝换热效果的增强更为显著;R513A的压降比R134a低5.6%—17.8%;随着管径的减小,R513A的管内冷凝换热系数和压降均增大。Oliver关联式对所测试螺纹管的管内冷凝换热系数的预测精度最高,平均误差值约为14.56%;对于管内压降的预测,所选关联式的计算结果与部分实验值存在较大误差;新拟合的压降关联式对R513A在内螺纹管内冷凝的压降预测结果较好,95.32%的数据点在30%误差线之内。

水平微肋管内R513A冷凝换热特性研究

采用实验方法研究了R513A在光管和不同齿密度微肋管内的冷凝换热性能,并通过对比冷凝换热系数实验值与关联式的预测值,探究了现有关联式对R513A的适用性,实验在冷凝温度33、35、38℃,质量流速50~150 kg/(m^(2)·s)的工况范围内进行。实验结果表明:冷凝换热系数随质量流速的增大而增大,在低质量流速区域,微肋管的冷凝换热系数增长幅度更大;随着齿密度的增加,管内换热面积增大,但管内表面张力作用增强,相应的流动阻力增强,强化传热作用被削弱;与R134a相比,微肋管对R513A的强化传热效果更优;对于光管,Bashar关联式的预测效果最佳,Cavallini关联式与Dobson关联式在低质量流速区域的预测误差偏大;对于微肋管,Yu关联式的预测效果最理想,Cavallini关联式与Kedzierski关联式均低估了管内冷凝换热特性。以Kedzierski关联式为基础,拟合了新的修正关联式,修正关联式的预测值与实验值的一致性较好,其平均绝对误差与标准差分别为7.8%、8.5%。

换热器内逆向折流板对螺旋管传热性能的影响

为了提高换热器的综合传热能力,以逆向折流板螺旋管(RBST)换热器为研究对象,通过改变折流板与螺旋管的安装方式,获得了更高综合传热性能的RBST换热器;采用双向流-固耦合的计算方法,研究了不同流速和不同结构方案下流体诱导螺旋管振动对RBST换热器综合传热性能的影响,并探讨了螺旋管振动的有效性。结果表明:当RBST换热器内的折流板与螺旋管采用间隙安装时,螺旋管的振幅随入口速度的增加而增加,但其振动频率始终保持不变;螺旋管振动能够增强RBST换热器的综合传热性能,但是强化传热评价指标最高仅为1.0303,效果并不显著,且振动强化传热效果会随着入口速度的增加而削弱;当RBST换热器内的折流板与螺旋管采用无缝安装时,换热器综合性能评价指标最高为1.018,这既延长了螺旋管的使用寿命,又提升了换热器的综合传热性能。

倾斜微肋管内两相流动冷凝换热特性研究

用实验方法对倾斜微肋管内两相流动冷凝换热特性进行实验研究,主要分析管内传热系数在不同实验工况、微肋管中的变化趋势,以揭示各实验变量对管内换热特性的影响机理,获取管内传热系数理论模型。实验结果显示:微肋管内传热系数随质流密度和干度的增加而增大,随管倾斜角的增大呈现先升高后减小的变化趋势;相比于光滑管,40°微肋管和25°微肋管换热强化倍率随干度的增加而增大、随倾斜角的增加而减小。此外,现有关联式低估了大部分实验数据,其预测误差均随干度、肋片螺旋角等的升高而增大。基于实验数据所获取新关联式可对40°微肋管和25°微肋管表现出较好的预测效果,其平均预测误差小于7%。

R134a及其混合物管外凝结换热特性的试验研究

针对制冷剂R134a及其与R125的混合物,在光管及强化管外的凝结换热特性进行了试验研究。研究表明:R134a在光管外凝结换热系数的变化与Nusselt理论解相吻合;与光管相比,强化管外凝结换热系数更高,当热流密度为25 kW/m^(2)时,R134a的强化倍率为15.27,而R134a/R125在R125占比分别为6%、12%、18%下的强化倍率分别是7.45、5.74、5.38,R417A的强化倍率为6.69。对于光管,含R125的R134a混合物凝结换热系数随热流密度的增大而减小;对于强化管,则随热流密度的增大而增大。