水平管降膜蒸发器的传热性能

在换热面积为2.375m2的水平管降膜蒸发试验平台上,采用5052铝合金管作为换热管,以实际海水为原料,进行了低温多效海水淡化中水平管降膜蒸发器传热性能研究试验。研究了料液喷淋密度、管外蒸发温度、总传热温差、海水盐度以及管内蒸汽中不凝气含量等因素对海水淡化过程降膜蒸发器总传热系数的影响。结果表明,在试验条件范围内,总传热系数随着料液喷淋密度和管外蒸发温度的升高而增加,随着传热温差的增大而降低;冷凝侧有不凝气存在时,总传热系数下降幅度较大;海水浓度对传热系数影响较小;在控制不凝气含量的条件下,传热系数在3500W/(m2?℃)以上。试验结果为海水淡化的工程设计和生产优化提供了依据。     

低温多效蒸发海水淡化系统热力分析

建立了喷射器低温多效蒸发海水淡化系统的数学模型,计算分析了各种温度损失随温度的变化,并研究了顶值盐水温度、蒸发器效数和动力蒸汽等参数对系统的造水比和生产单位质量淡水所需传热面积的影响。结果表明各种温度损失在末效蒸发器内显著增加;喷射器低温多效蒸发系统的热力特性明显优于多效蒸发系统;通过增加顶值盐水温度、蒸发器效数和动力蒸汽温度,可以实现系统的优化运行。     

海水盐度对海水沸腾传热及淡水产率的影响

对机械蒸汽压缩式(MVC)海水淡化中的沸腾传热与海水淡化性能进行理论分析。以竖直管式蒸发冷凝器建立具有不同盐度的海水核态沸腾传热模型,对海水的传热性能、热流密度、淡水产率以及海水淡化的能耗进行系统分析。为避免单位压缩功和淡水产率不能同时达到最优,设定单位压缩功不变。计算结果表明,海水盐度的增加对海水淡化的系统性能不利:海水盐度增加,沸腾换热系数αe、总传热系数、热流密度q和淡水产率m将有不同程度减小;增加沸腾压力和单位压缩功均有利于系统的传热性能,但单位压缩功的增大,增加了海水淡化的能耗。     

多相流蒸发法高浓缩率海水淡化系统的热力学性能分析

建立了顺流进料的多相流蒸发法高浓缩率海水淡化系统的数学模型,考虑了热力损失、压力损失、盐水含量变化引起的温差损失;计算分析了相同淡水产量下固体颗粒体积分数、预热量、首效加热蒸汽温度对系统的影响。结果表明,固体颗粒体积分数和首效加热蒸汽温度对各效蒸发器传热面积影响较大;预热量对生蒸汽耗量影响较大;固体颗粒的加入能够强化传热,减小各效蒸发器的传热面积;在无预热情况下,固体颗粒体积分数为13%时,蒸发器总传热系数比固体颗粒体积分数为3%时可增大6.71%,各效蒸发器传热面积减少4.97%,淡水成本减少1.22%。     

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。     

压力对水平管束外池沸腾传热性能的影响

研究了压力对水平管束沸腾传热性能的影响。实验结果表明，在真空条件下，管束的沸腾传热系数也象在常压条件下一样随操作压力的升高而增大，低压下的管束效应较高压时显著。     

海水与脱硬浓盐水用于电厂循环冷却水的对比研究

采用机力冷却塔与板式换热器模拟电厂循环冷却系统,分别用海水和脱硬浓盐水作为循环冷却水,考察了2种水对机力冷却塔和钛板式换热器性能的影响。结果表明,循环水量每增加1%,海水出塔水温升高0.04℃,蒸发损失量增加了0.006 m3/h,脱硬浓盐水出塔水温升高0.035℃,蒸发损失量增加了0.009 m3/h。水温每增加1℃,海水出塔水温升高0.13℃,蒸发损失量增加0.025 m3/h;脱硬浓盐水出塔水温升高0.1℃,蒸发损失水量增加0.027 m3/h。脱硬浓盐水传热系数随浓升高而缓慢增大,海水则随浓度升高而下降;同一浓度下,脱硬浓盐水作为循环水时,板式换热器换热效率随运行时间基本不变,海水则随运行时间延长而下降。     

吸收式热泵海水淡化系统性能与经济研究

提出了单效溴化锂吸收式热泵低温多效蒸发海水淡化系统,建立了系统数学模型。分析了不同溴化锂浓溶液浓度对系统的影响,并与不带吸收式热泵的低温多效蒸发海水淡化系统进行了性能与经济比较。计算结果表明:在本文的计算范围内,随LiBr浓溶液浓度的增加,发生器产生的饱和蒸汽温度降低,所需动力蒸汽量减少,造水比升高,海水淡化装置蒸发器总面积增加,综合效果是淡水成本增加。在相同条件下,与不带吸收式热泵的海水淡化系统相比,吸收式热泵海水淡化系统的淡水成本可节省16.5%。     

高浓缩倍率低温多效蒸馏海水淡化阻垢研究

模拟低温多效蒸馏装置水平管降膜蒸发条件,进行海水淡化高浓缩倍率条件下阻垢技术研究。实验利用滴定法和原子吸收分光光度计法,考察了不同操作条件下钙镁等易结垢离子浓度变化,并对形成的垢样收集处理后进行场发射扫描电子显微镜及能谱仪测试分析,得到垢物晶型形貌及其元素组成。结果表明:在蒸发温度40—80℃,盐水质量分数5%—8%(质量分数),阻垢剂添加量0—8 mg/L条件范围内,钙离子损失率随着蒸发温度和盐水质量分数的升高而增加,随着阻垢剂添加量的增大而减小。过程产生的垢主要成分为碳酸钙,另有少量氢氧化镁和硫酸钙。在提高海水浓缩倍率条件下,低温多效蒸馏装置结垢趋势更明显,可通过添加适量阻垢剂和定期药剂清洗来控制。     

低温多效蒸发海水淡化系统性能的实验研究

针对大型海水淡化工程设计关键技术,依托1 t·d-1低温多效蒸发海水淡化实验平台,考察了装置运行性能的稳定性,系统地研究了不同海水进料量及首效蒸汽温度对造水比、浓缩比和产品水质等关键参数的影响。结果表明:在实验条件范围内,随着首效蒸汽温度提高,海水的浓缩比先减小后增加,而首效温度对造水比的影响较小;在一定首效温度下,浓缩比随着海水进料量的增大而减小,而造水比随海水进料量的增大而增大。在实验范围内,产品水的固体总溶解浓度均低于5 ppm。小型海水淡化平台关键技术的实验研究为低温多效海水淡化系统在扩大化中的设计优化提供了参考和借鉴。     

低压条件下纳米流体的沸腾换热特性

在不同低压压力和不同纳米流体浓度下对光滑传热面上的水基纳米流体的池内沸腾特性进行了试验研究.纳米流体由平均直径50 nm的氧化铜粒子加入去离子水中组成,没有加入任何添加剂.研究主要针对7.2 kPa到100kPa的压力区间和0.1%到2%的质量浓度区间内压力和颗粒浓度对光滑表面沸腾换热特性的影响,研究结果表明:压力对纳米流体的沸腾换热特性有强烈影响,沸腾换热系数和临界热流密度(CHF)强化率随着压力的降低而大幅度增加.纳米流体浓度对沸腾换热系数和临界热流密度(CHF)有重要影响,并且在质量浓度约1%附近存在一个最佳颗粒浓度.研究结果显示由与去离子水相比,质量分数为1%,压力为7.2 kPa的纳米流体在光滑表面上的沸腾换热系数和临界热流密度都得到了显著提高.     

聚合物添加剂对流动沸腾传热与减阻的研究

研究了两种不同相对分子质量 (2 5 6× 10 4 和 85 5× 10 4 )的聚合物添加剂聚丙烯酰胺 (简称PAM )在垂直铜管内对水的流动沸腾传热的强化情况 ,研究了添加剂水溶液浓度、质量流率、热通量等对流动沸腾传热的影响 ,根据测得的稀溶液在垂直铜管内流动沸腾的总压降 ,用Martinelli分离流模型求得了相应的摩擦压降。结果表明在低热通量下 ,在适当的浓度范围内 ,两种不同分子量的PAM不仅可提高流动沸腾传热系数 ,还可降低流动沸腾的阻力。研究还表明 ,在相同操作条件下 ,在同样质量浓度下 ,两种不同分子量的PAM对水流动沸腾具有较为相近的强化传热与减阻作用     

Enhancement of solar water distillation process by surfactant additives

The presence of surfactant additives in water is found to enhance the boiling heat transfer significantly. The effect of using a surfactant as sodium lauryl sulfate (SLS) in relatively small dosages of concentration with a small size unit of solar water distillation process is investigated. Sun simulator (electric heater and variac transformer) is used to quantify the same input power for each experiment for the system instead of the sun. The experimental results show that a small amount of surfactant additive makes the top brine temperature (TBT) considerably higher, hence the freshwater product. Temperature curves for various surfactant concentrations are obtained and compared. The percentage of the increase in the system productivity is 0.7%, 2.5%, 4.7% and 7% at additive concentration equal to 50, 100, 200 and 300 ppm respectively. It is found that increasing the surfactant concentration more than 300 ppm not affecting the system daily productivity (DP) and TBT. Using surfactant concentration more than 400 ppm will decrease the DP by 6%.     

固体吸附作用对热管真空闪蒸水分离过程的影响

液滴的过热度是维持其蒸发的关键因素,直接决定了盐水分离设备的水分离率。提出和设计了一种集高效热管传热和固体吸附技术为一体的盐水分离装置,并对质量分数为3%的NaCl溶液进行水分离实验,结果表明：热管能以高热通量快速传递液体闪蒸后二次沸腾所需的热量,显著增加单位时间内的水分离容积速率;引入吸附床后,蒸发室压力在吸附过程前半段有明显降低,降幅约为0.4～1 kPa,即降低了液滴的沸点,提升了过热度,这为更低品位的热源在盐水分离方面的利用提供了新途径;降低热源温度,吸附剂的吸附能力相应降低,要达到同样的水分离速率,需增加吸附剂用量。     

水平管内R245fa/R141b沸腾换热特性的实验研究

针对新型混合工质R245fa/R141b，开展水平光滑管(外径10 mm)内工质沸腾换热特性的实验研究，对比纯工质与混合工质的换热性能及4种常用关联式的预测精度。结果表明：纯工质与混合工质的沸腾传热系数均随质量流速和热通量的增加而增大，随饱和压力的增加而减小；随干度的增加，沸腾传热系数均先增大后减小，即存在“过渡干度”，且混合工质的过渡干度大于纯工质；干度小于0.55时，混合工质传热系数小于纯工质；干度大于0.55后，混合工质的传热系数更高；随R245fa质量分数的增加，混合工质的沸腾传热系数增大。在所选关联式中，Gungor-Winterton关联式能准确地预测工质在光滑管内的沸腾换热特性，平均相对误差为16.67%。     

Influence of nanoparticle concentrations on flow boiling heat transfer coefficients of Al2O3/R141b in micro heat exchanger by direct metal laser sintering

Al2O3/R141b+Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investi-gate the influence of nanoparticle concentrations on flow boiling heat transfer of Al2O3/R141b+Span-80 in micro heat exchanger by direct metal laser sintering.Experimental results show that nanoparticle concentrations have significantly impact on heat transfer coefficients by homogeneity test of variances according to mathemat-ical statistics.The heat transfer performance of Al2O3/R141b+Span-80 nanorefrigerant is enhanced after adding nanoparticles in the pure refrigerant R141b.The heat transfer coefficients of 0.05 wt.%,0.1 wt.%,0.2 wt.%,0.3 wt.% and 0.4 wt.% Al2O3/R141b+Span-80 nanorefrigerant respectively increase by 55.0%,72.0%,53.0%,42.3% and 39.9% compared with the pure refrigerant R141b.The particle fluxes from viscosity gradient,non-uniform shear rate and Brownian motion cause particles to migrate in fluid especially in the process of flow boiling.This mi-gration motion enhances heat transfer between nanoparticles and fluid.Therefore,the heat transfer performance of nanofluid is enhanced. It is important to note that the heat transfer coefficients nonlinearly increase with nanoparticle concentrations increasing.The heat transfer coefficients reach its maximum value at the mass concentration of 0.1% and then it decreases slightly.There exists an optimal mass concentration corresponding to the best heat transfer enhancement. The reason for the above phenomenon is attributed to nanoparticles deposition on the minichannel wall by Scanning Electron Microscopy observation.The channel surface wettability increases during the flow boiling experiment in the mass concentration range from 0.2 wt.% to 0.4 wt.%.The channel surface with wettability increasing needs more energy to produce a bubble.Therefore,the heat transfer coefficients decrease with nanoparticle concentrations in the range from 0.2 wt.% to 0.4 wt.%.In addition,a new correlation has been proposed by fitting the experimental data considering the influence of mass concentrations on the heat trans-fer performance.The new correlation can effectively predict the heat transfer coefficient.     

三甘醇水溶液池沸腾传热

Boiling of water/triethyleneglycol (TEG) binary solution has a wide-ranging application in the gas processing engineering. Design, operation and optimization of the involved boilers require accurate prediction of boiling heat transfer coefficient between surface and solution. In this investigation, nucleate pool boiling heat transfer coef-ficient has been experimentally measured on a horizontal rod heater in water/TEG binary solutions in a wide range of concentrations and heat fluxes under ambient condition. The present experimental data are correlated using major existing correlations. In addition a correlation is presented for prediction of pool boiling heat transfer for the system in which the vapour pressure of one component is negligible. This model is based on the mass transfer rate equation for prediction of the concentration at the bubble vapor/liquid interface. Based on this prediction, the temperature of the interface and accordingly, the boiling heat transfer coefficient could be straightforwardly calculated from the known concentration at the interface. It is shown that this simple model has sufficient accuracy and is acceptable below the medium concentrations of TEG when the vapor equilibrium concentration of TEG is almost zero. The presented model excludes any tuning parameter and requires very few physical properties to apply.     

水平横槽纹管内氨沸腾换热及压降特性实验研究

对水平横槽纹管内氨沸腾换热及其压降特性进行了实验研究，获得了不同热流密度、不同质量流速和不同局部蒸汽干度下沸腾换热和压降的实验数据。实验结果表明，在本实验的参数范围内，与光滑管相比，横槽纹管在压降增加不大的情况下可使管内沸腾换热系数提高３０％～１５０％，横槽纹管的节距减小对传热强化的作用明显，而对槽深的影响不大。还给出了横槽纹管内氨局部沸腾换热和压降的无因次准则关系式。     

Boiling local heat transfer enhancement in minichannels using nanofluids

This paper reports an experimental study on nanofluid convective boiling heat transfer in parallel rectangular minichannels of 800 μm hydraulic diameter. Experiments are conducted with pure water and silver nanoparticles suspended in water base fluid. Two small volume fractions of silver nanoparticles suspended in water are tested: 0.000237% and 0.000475%. The experimental results show that the local heat transfer coefficient, local heat flux, and local wall temperature are affected by silver nanoparticle concentration in water base fluid. In addition, different correlations established for boiling flow heat transfer in minichannels or macrochannels are evaluated. It is found that the correlation of Kandlikar and Balasubramanian is the closest to the water boiling heat transfer results. The boiling local heat transfer enhancement by adding silver nanoparticles in base fluid is not uniform along the channel flow. Better performances and highest effect of nanoparticle concentration on the heat transfer are obtained at the minichannels entrance.     

Performance of a seawater desalination system coupled with a nuclear heating reactor under off-design operating conditions

In order to understand the performance of a large-capacity VTE-MED nuclear seawater desalination system thoroughly, the responding characteristics of the system to the off-design operating conditions were studied. The influence of scale deposit in tubes on the heat transfer areas needed and on the heat transfer coefficients with high top boiling temperature (TBT) was illustrated. The decrease of GOR, and also the damage to the normal operation of the system caused by temperature drop of feed brine because of seasonal variation, were explored, based on which a corresponding approach for adjusting feed brine flow rate was proposed. An optimal brine feed flow rate was also proposed to obtain the highest GOR under different heat loadings. The work could be helpful for the efficient and safe operation of nuclear VTE-MED desalination plants.