Numerical Study on Flow Heat Transfer Characteristics of Horizontal Tube Falling-Film Evaporator

As an efficient and energy-saving heat exchange technology, horizontal tube falling film evaporation has a great application prospect in refrigeration and air conditioning. The three-dimensional models of falling film flow evaporation outside horizontal single tube and inside evaporator were established, and the accuracy of flow and heat transfer simulation process was verified by comparison. For horizontal single tube, the results showed that total heat transfer coefficient was low and increased with larger spray density and evaporation temperature. The thickness of liquid film outside tube decreased gradually with the increase of tube diameter, and the total heat transfer coefficient of small tube diameter was significantly greater than that of the large tube diameter. The total heat transfer coefficient presented an increasing trend with larger liquid distribution height and density. In addition, the fluctuation of tube axial liquid film thickness distribution decreased with larger liquid distribution density. For evaporator, the results indicated that part of liquid refrigerant was carried into the vapor outlet. The temperature of tube wall and fluid presented a gradually rising trend in vertical downward direction, while tube wall temperature within the same horizontal and transverse row had little difference. The high-temperature zone on the outer wall of heat exchange tube moved towards the inlet and gradually decreased, and the outlet temperature of water in the tube also gradually decreased with the increase of refrigerant spray density. The local heat transfer coefficient of heat exchanger tube in the vertical direction presented a downward trend which was more obvious with the smaller spray density and it was obviously higher located in the middle of upper tube row and both sides of lower tube row for horizontal tube rows.

Pressure Drop of Liquid–Solid Two-Phase Flow in the Vertical Tube Bundle of a Cold-Model Circulating Fluidized Bed Evaporator

A cold-model vertical multi-tube circulating fluidized bed evaporator was designed and built to conduct a visualization study on the pressure drop of a liquid–solid two-phase flow and the corresponding particle distribution.Water and polyformaldehyde particle(POM)were used as the liquid and solid phases,respectively.The effects of operating parameters such as the amount of added particles,circulating flow rate,and particle size were systematically investigated.The results showed that the addition of the particles increased the pressure drop in the vertical tube bundle.The maximum pressure drop ratios were 18.65%,21.15%,18.00%,and 21.15%within the experimental range of the amount of added particles for POM1,POM2,POM3,and POM4,respectively.The pressure drop ratio basically decreased with the increase in the circulating flow rate but fluctuated with the increase in the amount of added particles and particle size.The difference in pressure drop ratio decreased with the increase in the circulating flow rate.As the amount of added particles increased,the difference in pressure drop ratio fluctuated at low circulating flow rate but basically decreased at high circulating flow rate.The pressure drop in the vertical tube bundle accounted for about 70%of the overall pressure drop in the up-flow heating chamber and was the main component of the overall pressure within the experimental range.Three-dimensional phase diagrams were established to display the variation ranges of the pressure drop and pressure drop ratio in the vertical tube bundle corresponding to the operating parameters.The research results can provide some reference for the application of the fluidized bed heat transfer technology in the industry.

Performance Assessment of a Shell Tube Evaporator for a Model Organic Rankine Cycle for Use in Geothermal Power Plant

The global energy demand increases with development and population rise. Most electrical power is currently generated by conventional methods from fossil fuels. Despite the high energy demand, the conventional energy resources such as fossil fuels have been declining and harmful combustion byproducts are causing global warming. The Organic Rankine Cycle power plant is a very effective option for utilization of low grade heat sources for power generation. In the Organic Rankine Cycle heat exchangers such as evaporators and condensers are key components that determine its performance. Researches indicated that shell tube heat exchangers are effectively utilized in this cycle. The design of the heat exchanger involves establishing the right flow pattern of the interacting fluids. The performance of these exchangers can be optimized by inserting baffles in the shell to direct the flow of fluid across the tubes on shell side. In this work heat exchangers have been developed to improve heat recovery from geothermal brine for additional power generation. The design involved sizing of heat exchanger (evaporator) using the LMTD method based on an expected heat transfer rate. The heat exchanger of the model power plant was tested in which hot water simulated brine. The results indicated that the heat exchanger is thermally suitable for the evaporator of the model power plant.

Design calculation of porous ceramics tube type dew point indirect evaporative cooler

To improve the wall surface hydrophilicity of a tube type indirect evaporative cooler,a new method adopting porous ceramics is proposed.This method realizes the combination of porous ceramics and the evaporative cooling technique.The design calculation of the porous ceramics tube type dew point indirect evaporative cooler are carried out from such aspects as the volumes and status parameters of the primary and secondary air,the cooler structure,the heat transfer of the solid porous ceramic tubes and the resistance of the cooler.The calculation results show that the design is reasonable.Finally,based on the design calculation,the porous ceramics tube type dew point indirect evaporative cooler is successfully manufactured.

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.

三维变形管MVR蒸发器用于纤维素生产末端废水零排放项目

蒸发器的换热性能对机械蒸汽再压缩(MVR)系统投资及运行均有着重要影响。MVR蒸发器的管内传热与压降性能在基于三维变形管和直圆管的情况下存在明显差异。建立了考虑盐水浓度修正的三维变形管MVR蒸发器性能准则关联式,设计一套用于纤维素生产末端废水零排放项目的新型蒸发器。应用实践表明,相对于传统直圆管的MVR蒸发器,基于三维变形管的新型MVR蒸发器节材节能效果显著,其在节省29%换热面积的情形下仍能完全满足工程实际需要。在现场调研期间MVR系统处理每吨废水的压缩机耗电量在18.81~23.78 kW∙h之间,体现出新型蒸发器高能效的技术特性,极具应用推广价值。

钛椭圆低肋横槽管外降膜蒸发流热特性

为提高钛椭圆低肋横槽管外降膜蒸发器传热效率,在试验验证基础上对椭圆系数E在1.0~2.0内管外液膜流热特性进行多相流数值模拟。结果表明:管外液膜分布均存在不同程度干斑,且液膜在临近干斑处较其他区域增厚明显,当E为1.2时液膜覆盖率最高;液膜沿轴/周向均减速铺展,当E为1.0和1.8时管上端液膜速度在无量纲轴向长度Z*为-0.25~-0.375及0.25~0.375之间骤减,液膜厚度沿轴向从喷淋口正下方位置向喷淋口远端变化趋势为先减小后增大;最大传热系数随着E增大而增大,管壁在液膜与干斑交界区域附近传热系数最高;平均传热系数先增大后减小,相较圆管,E为1.2和2.0时平均传热系数分别增大34.5%和15.7%;槽内汽含率分布与液膜分布呈相反规律,液膜整体汽含率随E的增大先增大后减小,槽内/外平均汽含率差值随E增加而增大。

强制对流下翅片管蒸发器表面结霜实验

翅片管蒸发器表面结霜是阻碍制冷系统高效运行的主要不利因素之一。利用恒温恒湿箱搭建强制对流下蒸发器翅片管表面结霜可视化实验平台,在环境温度0~8℃、相对湿度55%~75%及迎面风速0.8~2.4 m/s时,实时记录霜层动态生长过程,研究了环境温度、相对湿度和迎面风速对霜层生长特性及蒸发器换热性能的影响规律。结果表明:环境温度和迎面风速是影响蒸发器结霜的主要因素,结霜50 min,环境温度为0℃的霜层厚度比环境温度为8℃的提高了12.78%,迎面风速为2.4 m/s的霜层厚度比迎面风速为0.8 m/s的提高了14.66%,结霜量与换热量提高趋势相同。在结霜初期,相对湿度越大,换热量越大;结霜后期,相对湿度越小,换热量越大,并得到了换热量关于环境参数与时间的相关关系。

R32/润滑油在微通道蒸发器内的滞油特性及其毛细管流动特性试验研究

基于R32家用空调系统搭建R32/润滑油在平行流微通道蒸发器内的滞油特性及其毛细管流动特性测试平台。采用拆除称量法,测试不同入口油质量分数、质量流量及干度下的平行流微通道蒸发器内的滞油体积率,基于理论分析,建立能够反映流动特征的微通道蒸发器内的滞油量预测模型,并通过试验验证该仿真模型的准确性。另外,建立R32/润滑油毛细管内绝热流动的数学模型,经适当简化得到毛细管长度的计算公式,并通过试验测得不同尺寸、工况下的毛细管流量,验证模型的精度。最后,基于仿真模型得到的大量数据点,绘制R32毛细管诺模图。研究结果对R32毛细管的设计应用具有参考价值。

翅片管蒸发器传热性能的参数灵敏度分析

采用Sobol´全局灵敏度分析方法,计算出了各个影响参数对传热系数的一阶灵敏度和总灵敏度,探索和分析了单个及多个波纹翅片管参数对波纹翅片管蒸发器传热系数影响的程度和规律,并构建了传热系数与各个参数之间关系的响应面。结果:从灵敏度数值及响应面的分析皆显示:翅片间距对传热系数的影响最大,翅片厚度的影响次之,质量流量与工资热流密度对传热系数的影响相差不大,但大于管外径的影响;且翅片间距与其余各个因素之间的交互影响在一定范围内极其剧烈,其中翅片间距与翅片厚度之间的交互影响最大,又以质量流率与工质侧热流密度之间的交互影响次之;再者,垂直于风速方向的管间距与平行于风速方向的管间距的灵敏度数值皆为0,说明两者对传热系数的影响可以忽略不计。

R410A水平三维强化管蒸发换热实验

针对传热管内蒸发换热特性,开展了R410A在铜及不锈钢光管和涟漪纹管管内换热特性的实验研究,分析了部分流型对管内蒸发的换热机理。实验工况:饱和温度为6℃,制冷剂质量流速为100~200 kg·(m^(2)·s)^(-1),干度范围0.1~0.9。验证了光滑管内的流型,结果与Wojtan流型图的预测具有较好的一致性。实验结果表明:蒸发换热系数随质量流速增加而增加。换热系数与流型相关。压降主要取决于质量流速和干度。与光滑管相比较,传热强化率为1.27~1.96,沸腾压降增加了11%~36%。

含蒸发式稳定器模型燃烧室的液雾两相燃烧数值模拟

带蒸发式稳定器的燃烧室通过向稳定器蒸发管内供油的方式,改善稳定器下游的油雾分布,在低压和低油气比等极限工作条件下提高燃烧室的燃烧性能。然而由于蒸发管内存在燃油雾化、液滴碰壁飞溅以及蒸发等过程,难以直接对蒸发管内的物理过程进行数值模拟得到稳定器下游的油雾场分布。对于燃油蒸发管内部复杂的物理过程,根据蒸发管内部流动特性提出蒸发管雾化简化模型,并在欧拉-拉格朗日框架下,采用PaSR湍流燃烧模型结合动态自适应建表(ISAT)方法,对含蒸发式稳定器模型燃烧室在低压条件下的流动及两相燃烧开展数值模拟,得到的燃烧室横截面燃气温度以及燃烧效率的模拟结果和实验接近,表明提出的简化蒸发管雾化模型能够给出合适的稳定器下游燃油分布以及燃烧场计算。

单管多翅片排管蒸发器结构优化的数值模拟研究

为解决单管双翅片排管蒸发器翅片效率低、降温速率慢和原材料消耗量大的问题,通过模拟分析翅片间夹角对传热性能的影响,得到单管双翅片和四翅片的降温时间、温度场、速度场、传热系数和制冷量指标。结果表明,四翅片排管蒸发器下部翅片夹角越大,传热效果越好。单管四翅片排管蒸发器1的传热系数最大,相对于单管双翅片排管蒸发器提高了13.4%;单管四翅片排管蒸发器1的单位管材制冷量最高,相对与单管双翅片排管蒸发器提高了38.6%。研究为单管多翅片蒸发器结构优化提供了支撑。

DCC装置稳定塔复合蒸发式空气冷却器管束泄漏原因分析及对策

某石化公司2.2 Mt/a催化裂解装置稳定塔复合蒸发式空气冷却器(空冷器)自装置首次开工以来多次出现管束泄漏的情况。在对空冷器管束外壁垢样成分进行鉴定的基础上,对空冷器管束泄漏的原因进行了详细分析,可以归结为垢下腐蚀、缝隙腐蚀和Cl-腐蚀。针对上述原因,采取加强水质控制、升级管束材质、定期进行腐蚀检测和优化操作条件等措施,可以减缓空冷器管束腐蚀速率,延长空冷器运行周期。另外,针对在役及发生泄漏的空冷器管束,制定合理的监控运行措施和应急处置方案,可有效减轻管束泄漏程度,保证安全生产。

蒸发罐自动清洗机设计与实现

[目的]解决煮糖蒸发罐人工清洗效率低、清洗质量差,存在烫伤风险等问题。[方法]研发一种可在加热管阵上方定位和行走的煮糖蒸发罐自动清洗机,该清洗机携带有可喷射高压水流的喷洒头,喷洒头可伸至管内不同深度完成对罐室内各加热管内壁的全面清洗。根据清洗机的结构及工作流程,设计了加热管中心检测及路径规划程序、硬件控制系统,并根据不同规格蒸发罐的需求,完成了单、双清洗机构清洗机样机。[结果]单清洗机效率高于人工清洗,加热管均匀分布时,双冲洗机构样机效率可达到单冲洗机构样机2倍。[结论]该清洗机工作效率高、清洗效果好、运行安全平稳,满足自动清洗作业要求。

管翅式蒸发器析湿系数计算探讨

在分析当前管翅式蒸发器析湿系数2种计算方式优缺点的基础上,进一步提出了一种新的析湿系数工程计算方法。新的计算方法可以避免利用蒸发器出口状态参数来计算析湿系数,并提高计算精度。

椭圆型套管-管翅式蒸发式冷凝器传热传质性能及实验研究

介绍了一种椭圆型套管-管翅式蒸发式冷凝器,通过搭建的蒸发式冷凝器性能测试实验系统,研究该蒸发式冷凝器在不同循环水配比、不同地区环境以及“风冷”“水冷”“蒸发冷凝”和“全工况”4种不同运行模式下的传热传质性能与运行特性的变化规律。结果表明:在相同条件下“全工况”模式的热流密度高于“风冷”和“蒸发冷凝”两种模式,而“水冷”模式由于其传热面积远小于其它模式,所以其热流密度最高;“全工况”模式在保持配风量及循环水总量一定的条件下,热流密度随循环水配比的增大而先增大后减小,喷淋水与冷却水的流量最佳配比为2∶5;“全工况”模式在严寒干燥地区、寒冷地区以及夏热冬暖地区的总热流密度分别为5879.64、3964.28及2933.05 W/m^(2),因此不同地区环境运行的传热性能相差较大,该冷凝器“全工况”模式在空气干燥的地区,喷淋冷却水温度范围为13~18℃运行时传热性能较优。在相同的条件下,“蒸发冷凝”模式的水膜传质系数高于“全工况”模式,且两者均随喷淋密度的增大先增大后减小。

浅析板管蒸发冷却式无霜空气源热泵及其设计应用

在常规板管蒸发冷却式冷水机组设计的基础上,借鉴了热源塔热泵系统使用防冻液喷淋的防冻措施原理,将其应用在板管蒸发冷却式冷热水机组上,并对板管蒸发式换热器及系统进行设计改进,使其适用于冬季制热工况,从而形成板管蒸发冷却式无霜空气源热泵机组。机组夏季采用板管蒸发式换热器作为冷凝器,相比于空气源热泵降低了冷凝温度,提高了系统的性能系数。冬季使用防冻液喷淋,设备运行无除霜循环,系统供热稳定,大大提高了室内侧的热舒适性。试验研究了不同溶液再生方式下的机组性能,结果表明:利用过冷热再生模式进行溶液再生时机组性能最优,在制热量不衰减的情况下性能系数仅下降了2.5%~5.1%。机组适用于夏热冬冷地区及部分纬度较低的寒冷地区。

某船用锅炉对流蒸发管束泄漏原因分析

某船用锅炉对流蒸发管束频繁发生泄漏,采用宏观检测等方法对泄漏原因进行了分析。结果表明:由于给水中杂质元素过多,导致管内壁出现氧腐蚀,氧腐蚀坑积垢处长期处于炉水蒸发浓缩状态,持续产生垢下腐蚀。两种腐蚀共同作用导致泄漏。同时给出了锅炉运行维护的改进建议。

不同制冷剂对翅片管式蒸发器的影响模拟研究

蒸发器是影响制冷装置能效的核心部件,不同制冷剂使得换热器的效率表现不同。为研究不同制冷剂对蒸发器换热效率的影响,建立翅片管式蒸发器的模型。在翅片管式蒸发器中,利用Fluent软件对5种不同制冷剂进行换热模拟,得到蒸发器的速度场、压力场和温度场。结果表明:5种制冷剂中,CO_(2)制冷剂在翅片管式换热器中的换热效率最高。