水平管外降膜蒸发流动和传热特性数值模拟

建立三维模型并模拟了制冷剂R410A在水平管外的降膜流动和蒸发过程,探究了喷淋密度、热通量和布液孔偏离管轴心距离对降膜流动和传热的影响。结果表明：沿管周方向,液膜厚度和传热系数逐渐减小并趋于稳定,至管底处由于局部液体堆积,液膜增厚、传热系数降低;喷淋密度较小时,总传热系数随着热通量增加而降低,随着喷淋密度增加而显著提高;液膜Reynolds数达2000后,总传热系数随喷淋密度增加而缓慢提升并趋于平稳,此时热通量的增加会提升总传热系数;随着布液偏心距的增加,总传热系数先略微上升并趋于平稳,而后由于出现局部“干涸”和液膜堆积区域,总传热系数急剧下降;随喷淋密度的增加,总传热系数急剧下降的临界点会逐渐往大偏心距偏移。     

水平管外降膜蒸发流动和传热特性数值模拟

建立三维模型并模拟了制冷剂R410A在水平管外的降膜流动和蒸发过程,探究了喷淋密度、热通量和布液孔偏离管轴心距离对降膜流动和传热的影响。结果表明:沿管周方向,液膜厚度和传热系数逐渐减小并趋于稳定,至管底处由于局部液体堆积,液膜增厚、传热系数降低;喷淋密度较小时,总传热系数随着热通量增加而降低,随着喷淋密度增加而显著提高;液膜Reynolds数达2000后,总传热系数随喷淋密度增加而缓慢提升并趋于平稳,此时热通量的增加会提升总传热系数;随着布液偏心距的增加,总传热系数先略微上升并趋于平稳,而后由于出现局部"干涸"和液膜堆积区域,总传热系数急剧下降;随喷淋密度的增加,总传热系数急剧下降的临界点会逐渐往大偏心距偏移。     

降膜蒸发管内插往复螺旋强化传热实验研究

为有效解决降膜蒸发器加热管中传热效率与结垢问题,提出了一种降膜蒸发管内插往复螺旋强化传热技术,强化管内插螺旋运动与管壁碰撞过程。实验研究内插螺旋的结构参数、螺旋往复行程,以及热通量、蒸发压力以及溶液喷淋密度等工艺参数对降膜蒸发过程传热性能影响。实验结果表明,此技术的除垢防垢性能及传热性能优于空管及单纯的内插螺旋性能,在螺旋外径d=30 mm、螺距f=45 mm、丝径e=1.8 mm、往复行程H=100 mm时,其传热系数分别是空管和单纯的内插螺旋的2.08和1.26倍。通过对管内蒸发侧传热系数进行分析,总结得到与热通量、蒸发压力以及溶液喷淋密度相关的降膜蒸发传热系数关联式。     

多组分降膜蒸发传热计算方法研究

在降膜蒸发器设计过程中,现有的多组分降膜蒸发传热计算方法未考虑传质的作用,其计算结果与实际工程设计相比存在较大的偏差。文中探讨了多组分降膜蒸发传热机制,充分考虑降膜蒸发的传热传质耦合作用,提出了适用于多组分降膜蒸发传热计算的关联式。针对多组分物料物性随蒸发浓缩发生变化的特点,建立了降膜蒸发器蒸发传热的分段设计方法。完成了丙二醇水溶液降膜蒸发不同单元数的分段传热计算,计算结果表明:分段计算与不分段计算的管内传热系数与总传热系数的偏差率分别为23%—30%及12%—16%。随着计算单元数的增加,降膜传热系数偏差率的变化会逐渐减小,设计计算过程中可以根据精度要求确定最佳计算单元数。此研究工作为降膜蒸发器的工艺设计提供了参考。     

螺旋槽管降膜蒸发强化传热与结垢特性研究

降膜蒸发是海水淡化的一项主要技术。传热管是降膜蒸发器的核心部件。螺旋槽管是一种应用广泛的高效强化传热管,但在海水淡化中应用较少,缺乏相关的实验数据。为论证螺旋槽管用于海水淡化的可行性,首先在水平管降膜蒸发传热实验平台上,以自来水为工质,研究了螺旋槽管的强化传热特性,分析了喷淋密度、热通量、传热温差和蒸发温度对其传热的影响;然后借助低温多效蒸馏(LT-MED)海水淡化中试装置,在海水淡化实际工况下,运行测试180 d,研究了螺旋槽管的结垢特性。在本实验中,螺旋槽管传热系数较光管平均提高35%左右;其结垢厚度小于光管,结垢疏松多孔,易于清除。     

伞板式降膜蒸发器传热性能研究

对伞板式降膜蒸发器进行了合理的结构设计,并对此蒸发器的传热性能进行了实验研究。结果表明,伞板式降膜蒸发器在低温差下仍能保持较高的传热系数,得出了伞板降膜蒸发侧无因次传热系数的实验关联式并与竖直光滑管的传热性能进行了比较。同时对伞板式蒸发器进行了液膜破裂的实验研究,得出了液膜破裂条件的实验关联式并与竖直管的破断条件进行了比较,从而得出了此蒸发器在小流量下,液膜分布均匀不易破裂等特性。     

异形竖板降膜蒸发器浓缩菠萝汁的实验研究

讨论了异形竖板降膜蒸发器蒸发浓缩菠萝汁时的传热及其结垢性能,给出了总传热系数K与流量、糖度、温差以及蒸发温度的实验关系式。实验结果表明,异形板式降膜蒸发器适用于菠萝汁的蒸发浓缩,在食品工业上具有一定的应用前景。     

伞板式降膜蒸发器传热性能的实验研究

本文以水和5％～40％的碳酸钾溶液为工作介质，对伞板式降膜蒸发器的传热性能进行了实验研究，分析了影响伞板式降膜蒸发器传热的主要因素，并且得到了伞板降膜蒸发侧无因次传热系数的实验关联式。通过对液膜破裂的实验研究，证明了此结构的蒸发器在小流量情况下液膜不易破裂的特性。     

木质素悬浮液管式降膜蒸发传热研究

管式降膜蒸发器由于具有传热系数高、传热温差小等优点,已应用于多个工业领域,对管式降膜蒸发器的研究也成为近年来传热技术领域的一项重要课题.文中设计了一套管式降膜蒸发装置,以木质素悬浮液为工质,对影响降膜蒸发传热的因素进行了实验研究,通过冷膜实验测试了所设计的筛板分布器的性能,得出筛板分布器能较好地使物料在换热管内壁上成膜...     

新型碳酸钾降膜蒸发器的实验研究

介绍了一种新型降膜蒸发器--伞板降膜蒸发器,对伞板蒸发器进行了结构说明.以水和碳酸钾溶液为介质,对其传热进行了实验研究,得到了伞板降膜蒸发侧无因次传热系数的实验关联式;在小流量情况下,通过对液膜破裂进行研究,得出了液膜破裂的极限条件.这种蒸发器可实现多效蒸发,便于低品位余热的再利用,具有广阔的应用前景.     

The influence of liquid-side mass transfer on heat transfer and selectivity during surface and nucleate boiling of liquid mixtures in a falling film

Experimental studies using a falling film apparatus and a theoretical analysis of heat and mass transfer for mixtures lead to the following results.During nucleate boiling the separation effect, that is, the selectivity, and the heat transfer are influenced to a great extent by liquid-side mass transfer resistances. The selectivity diminishes significantly with increasing heat flux. The heat transfer coefficients for boiling mixtures can be much lower than for pure substances. For the calculations liquid-side mass transfer resistances were assumed to be the only reason for the reduction of both the selectivity and the heat transfer coefficients. No further physical explanations were needed.During surface boiling the reduction of the heat transfer coefficients is negligible for practical applications. The selectivity is mainly controlled by the thermodynamic equilibrium. The liquid-side mass transfer coefficients are of the same order of magnitude as found in physical absorption and absorption with chemical reactions, i.e. (2–5) × 10⁻⁴ ms⁻¹.The effect if liquid-side mass transfer resistances on heat transfer and selectivity during partial evaporation of the binary refrigerant mixture R11–R113 in a falling film apparatus was investigated by varying the heat flux, the film Reynolds number and the liquid composition. During surface boiling the reduction of the heat transfer coefficients in negligible for technical applications, because of the minor deviations from evaporation which are mainly controlled by thermodynamic equilibrium. Nevertheless, the liquid-side mass transfer coefficients, which can be determined by the measured vapour and liquid mole fractions, are of the same order of magnitude as in physical absorption and absorption with chemical reactions, i.e. β_ℓ = (2–5) × 10⁻⁴ m s⁻¹. The coupled heat and mass transfer during falling film evaporatation of mixtures, condensation and absorption [17,19] can be calculated with the same relationships as for the hydrodynamics of falling films.During nucleate boiling the selectivity diminishes significantly and heat transfer is influenced to a great extent by liquid-side mass transfer resistances. There is considerable deviation form evaporation controlled exclusively by thermodynamic equilibrium. The heat transfer coefficients α for the R11–R113 mixture as well as for ten other binary and two ternary mixtures could be calculated assuming the mass transfer resistances to be the only reason for the reduction of the heat transfer coefficient α during boiling of mixtures. No other physical explanations were needed. The calculation method is easily extendable to multicomponent mixtures, if the corresponding vapour-liquid equilibria are available.     

水平管降膜蒸发海水淡化的传热试验研究

以φ19×0.75的光滑铝黄铜管为研究对象,在多功能蒸馏海水淡化传热试验平台上进行了水平管降膜蒸发传热试验研究,分析了液体负荷、热通量、蒸发温度和传热温差等对水平管降膜蒸发总传热系数的影响.试验研究结果可为海水淡化工程提供设计依据,为传热管间液体流动及传热传质的数值模拟研究提供对比数据,为水平管蒸发器的优化设计提供技术...     

分离型热管蒸发段流动特性和传热特性的试验研究

对分离型热管管内蒸发段流动特性和传热特性进行了试验研究 :在保证热管工作效率及安全性的前提下 ,分离型热管蒸发段工质流动形式除单相液流和泡状流 (低热流密度时为弹状流 )外 ,在蒸发段上部约有 42 %～ 50 %的不稳定飞溅降膜区 ;合理充液率随热流密度的增加而减少 ;随着热流密度的增加 ,核态沸腾区及飞溅降膜区的换热系数均增加 ,蒸发段总换热系数也增加。     

竖直多孔管降膜蒸发传热实验

研究了内表面烧结型多孔管对降膜蒸发换热效果的影响。采用单管降膜蒸发器,由壳程的蒸汽加热管程的水降膜传热。在热通量q=13~90 kW·m^-2,传热温差ΔT=2.87~9.5℃,液体Reynolds数Re_L=4500~15000 范围内,求不同工况下管内降膜传热系数,并将其与相对应的光管换热性能进行比较。比较数据可知:多孔管的管内降膜传热系数是光管的2.03 倍,总传热系数是光管的1.78 倍,多孔管强化传热效果明显。     

T型翅片板喷淋式降膜蒸发器传热性能研究

采用喷淋降膜蒸发形式，将蒸发器的传热面用机械线切割方法加工成Ｔ型翅片板。对这种蒸发器进行了传热性能的研究，并与池式光板进行传热性能对比，实验结果表明，Ｔ型翅片板降液膜蒸发能在低温差下维持沸腾且有较高的给热系数。同时对沸腾传热的强化机理也进行了研究，并探讨了Ｔ型翅片板喷淋降液膜蒸发器具有优良传热性能时主要原因。     

水-水降膜蒸发器的模拟仿真和优化

由于具有换热性能突出、制冷剂充注量小等优点,降膜蒸发器已被广泛应用于海水淡化等行业。然而,由于存在液位控制和干斑效应等实际操作问题,在压缩式制冷系统中,降膜蒸发器的设计还需要进一步优化。为解决这个问题,构建了水-水降膜蒸发器的仿真模型。采用有限元方法,获得了沿管程的温度、热流变化情况。对于现有的四管程降膜蒸发器,在80℃蒸发沸腾和0.4 kg·s⁻¹的喷淋量下,为获得最大的换热量,进行了详细的模拟计算,基于模型的计算结果,建议采用两管程降膜、两管程满液的液位控制方式。本模型同时给出了沿管程换热时的传热系数分布情况,提出了满液降膜分界线,为后续的降膜蒸发器结构设计优化提供了新思路。     

Experimental study of falling film evaporation heat transfer outside horizontal tubes

The heat transfer process of falling film horizontal evaporation includes evaporation outside tubes and condensation inside tubes, the heat transfer coefficient of the former is about 50% of that of the latter. So the overall heat transfer coefficient is influenced mainly by the falling film evaporation outside tubes. An experimental study of falling film heat transfer outside horizontal tubes was carried out in order to show how the heat transfer coefficient is affected by different parameters such as flow density evaporation temperatures, temperature difference between wall and saturation water, and mass concentration of the seawater. Experiments were conducted using 14 mm outer diameter Al-brass tubes heated by internal electric heaters so that a uniform heat flux was generated on the outside surface of tubes. The results show that when flow density Γ varies between 0.013 kg/ms < Γ< 0.062 kg/ms, the heat stransfer coefficient of falling film evaporation outside horizontal tubes h increases with the increase in liquid feeding, evaporation boiling temperature and heat flux. h also increases with an increase in distributor height, however there is a maximum height in which any height above this. Besides, the amount of non-condensing gas has significant effect on h. The difference of heat transfer coefficient between freshwater and seawater is small. These results contribute to further improving the performance of heat transfer process and developing new evaporator.     

水平管外壁液膜流动状态及其对传热的影响

采用智能化薄膜厚度测试仪对水平管外液膜流动状态进行了研究 ,得到液膜厚度的几率分布和平均厚度 .液膜的平均厚度不仅随液体负荷的增加而加大 ,而且与管外壁不同角度有关 ;液膜的波动状态受制于液体负荷与管径的共同影响 .定义了量纲 1波高 ,并通过实验数据回归得到量纲 1波高的经验关系式 .还进行了初步的传热实验 ,得到水平管外蒸发侧传热膜系数随量纲 1膜厚和量纲 1波高的变化 ,为进一步揭示蒸发侧传热的决定因素提供实验依据