Mathematical modeling and experimental validation of an absorption-driven multiple-effect evaporator

A mathematical model has been developed to simulate the operation of an absorption-driven multiple-effect evaporator. The model is based on mass and enthalpy balances and heat transfer rate equations of the various components of the system. The model has been validated by comparing model predictions to experimental results from the operation of a four effect absorption-driven falling film evaporator coupled with a two-effect regenerator which operated using solutions of sodium hydroxide as an absorptive medium. The model predictions are in satisfactory agreement with the experimental results.     

一种新型环路重力热管的数值模拟和性能优化

研究了一种新型的环路重力热管(LGHP),并提出一种基于VOF方法的简化两相流数值模型,能够有效地模拟热管蒸发器内部气泡的产生和干涸区的位置,从而研究蒸发器内部工质的流动特性。在数值模拟的基础上改进了蒸发器的流道结构,以延缓水平放置时蒸发器干涸区的出现。通过实验验证发现改进后蒸发器的传热性能得到了很大的提高,蒸发器中制冷剂的液相积存量得以提升,使其极限热通量(CHF)提高到140 kW/m²(加热功率2000.7 W),为改进前CHF的两倍。证明该简化模型在模拟干涸区位置方面具有准确性,可以为进一步的设计和改进平板蒸发器流道结构提供参考。     

Optimum surface area distribution of quadruple-effect evaporator in sugar juice evaporation process

The evaporation process in raw sugar manufacturing consists of juice heater, multiple-effect evaporator, and crystallizer. Two performance parameters of the evaporation process are the rate of processed sugar juice and the steam economy, defined as the ratio between water content of sugar juice entering the process to the amount of steam required to run the process. If the total surface area of the multiple-effect evaporator is fixed, the performance parameters are affected by the distribution of the surface area among the effects of the evaporator. Although the optimization of evaporator surface area distribution has been investigated in previous studies, they have considered only the multiple-effect evaporator, and overlooked interactions between the three components of the evaporation process. The current investigation proposes a model of the evaporation process that takes into account interactions between the three components of the process through mass and energy balances. The model is used to find the optimum surface area distribution that maximizes the rate of sugar juice processed by the system and the optimum surface area distribution that maximizes the steam economy are different.     

Development of a new model for multiple effect evaporator system

A new simplified scalable mathematical model, based on concepts of stream analysis, temperature paths and internal heat exchange, has been developed for synthesis of a multiple effect evaporator systems. In this model, fresh feed is assumed to be composed of product and number of condensate streams, which come out from different effects and these are treated as separate streams. For the present work a septuple effect flat falling film evaporator system, used for concentrating black liquor in an Indian Kraft Pulp and Paper mill, has been considered. This system is being operated under backward sequence with condensate-, feed- and product-flashing as well as steam splitting in first two effects. The set of linear algebraic equations for this model are self-generated through programming and is solved simultaneously using Gaussian Elimination Method with partial pivoting. Results of the present approach are validated with published model and industrial data.     

Modelling and Control of an Industrial Multiple‐Effect Evaporator System

A study has been conducted of the modeling and control of an industrial multiple‐effect evaporator system used for black liquor recovery. The developed model is a combination of phenomenological and empirical (neural networks) approaches, based on industrial data. An advanced model‐predictive control strategy, allowing for manipulated variable constraints, compares favorably with a classical PID‐based scheme.     

管束异型排列制冷蒸发器沸腾传热性能数值模拟

制冷蒸发器是制冷系统中的重要设备,其沸腾传热性能对整个系统的能源利用效率有着显著影响。文中以管束异型排列的制冷蒸发器为研究对象,通过FLUENT软件模拟计算,分析探讨了管束排列方式对蒸发器壳程流场及沸腾传热性能的影响。计算结果表明,在相同的流体进料量下,异型管束排列蒸发器的压力变化趋势与正方形排列的相似。在综合性能上,具有合理气流通道的异型管束排列蒸发器明显优于管束正三角形排列和正方形排列的蒸发器。     

机载冷源参数对蒸发循环系统性能的影响

为了深入研究机载蒸发循环系统的工作特性及为搭建仿真计算模型做准备,基于试验室已有设备和条件,以一台新型微通道换热器作为研究对象,以R134a为工质,通过控制压缩机转速和电子膨胀阀开度一定,分别调节冷源温度和流量,考察冷源温度和流量对压缩机入口过热度、热源出口温度和制冷量的影响,并基于试验数据搭建了针对该蒸发器的仿真计算模型。试验结果表明在试验工况下,冷源防冻液入口温度对系统性能的影响明显,随冷源入口温度升高,压缩机入口过热度逐渐降低、热源出口温度明显升高,但冷源流量对系统性能的影响不显著。基于试验数据,建立了蒸发器仿真计算模型,将该模型应用于蒸发循环系统模型中,对比试验数据和仿真计算数据,可知,该蒸发器仿真计算模型准确可靠,可用于针对该蒸发器的性能仿真计算。     

基于气提效应的竖管内降膜蒸发器性能研究：水力学特性

降膜蒸发器具有传热速率高、持液量小、无静液柱引起的沸点上升等特点,适用于小温差传热,但系统需要一个循环泵。本文提出采用蒸汽输送液体的新型降膜蒸发模式来实现无泵循环,使系统结构更为简单、紧凑,可靠性更高。利用现已广泛使用的弹状流输送理论建立输送单元数学模型,研究其水力学特性。结果表明本新型气提输送降膜蒸发器在蒸汽引入率为0.3%～5%时即可实现弹状流的稳定输送。对每一个输送流量,存在一个最大扬程和最小吸程。且在相同液相表观速度下,最大扬程对应最小吸程。     

机械蒸汽再压缩蒸发结晶系统模拟分析

提出了一种升膜(或降膜)蒸发器与强制循环蒸发器并用的机械蒸汽再压缩蒸发结晶系统,溶液在升膜(或降膜)蒸发器中蒸发至接近饱和,再进入强制循环蒸发器中蒸发结晶,可减少溶液循环功耗。并以处理量为1 t/h,质量浓度为10%碳酸钠水溶液为例,模拟分析了蒸发温度、压缩机压缩比对制热能效比COP等的影响规律,并以年总费用为目标函数进行优化计算。结果表明:(1)COP随蒸发温度升高稍有下降,随压缩比增大大幅减小,本模拟条件范围内的COP达15~50;(2)本优化条件下,年总费用随压缩比的增大先减小后增加,压缩比在1.3~1.4之间年总费用最低。     

基于(火用)的管式降膜蒸发器液位优化设定

在降膜蒸发器中,物料液位直接影响蒸发器分离室压力和出料温度从而影响加热蒸汽消耗,对蒸发器的优化操作十分重要。但实际生产中液位的设定值通常是一个较大的范围,难以优化运用。针对此问题提出了一种基于(火用)分析的管式降膜蒸发器液位优化设定方法。深入分析了液位高度对蒸发过程各参数的影响,基于实际生产数据,拟合得到了液位与其他参数间的关系模型。结合蒸发器物料平衡关系以及(火用)分析方法,建立了最大化(火用)效率的能耗优化模型。对优化模型求解得到了蒸发器(火用)效率随液位高度变化的关系曲线,最后计算了不同工况条件下的最优液位,为优化实际生产操作提供了参考。     

Development and characterisation of a vacuum flash evaporator for concentrating a heat sensitive aqueous peptide stream

Canesis Network Limited, in conjunction with Keratec Limited, has developed a patented process for the chemical extraction of soluble keratin proteins from wool. This extraction process results in the generation of several proteinrich streams, including a dilute aqueous based peptide stream. This peptide stream requires concentration, and evaporation has been found to be a convenient method for achieving this. The peptide-containing stream is sensitive to heat damage. The existing evaporator has been found to be far from ideal for the application, with the harsh evaporation conditions resulting in damage to the peptides and undesirable darkening of the peptide solution. To overcome this, and increase product throughput, a new evaporator has been designed and built for this application. This study discusses the design and construction of this new, purpose built, flash type evaporator to evaporate the peptide stream in a vacuum. The study also details investigations undertaken to assess the performance of the evaporator and characterise the evaporation peptide stream. The overall outcome of this study has shown that the peptide stream can be successfully concentrated by evaporation using a flash evaporator operating under vacuum. Damage to the peptide stream can be minimised by controlling the evaporation conditions. Optimal operating conditions for processing the peptide stream have been identified. Investigations into the heat damage of keratin peptides and the effects of heat damage on the peptide offer an exciting opportunity for future research work.     

机载补气增焓制冷系统动态仿真模型

补气增焓制冷系统具有系统能效比高、运转费用低等优势,被广泛应用于现代飞机的环境控制系统中。为了指导系统的设计与控制,开发了机载补气增焓制冷系统的动态性能仿真模型,并开展了实验验证。通过将补气压缩机工作过程分解,建立了基于物理机理的补气压缩机显式计算模型;通过制冷剂相区划分,建立了蒸发器和冷凝器的移动边界模型;并开发了基于质量引导的系统动态求解算法。实验验证表明,模型能够准确反映系统压力和温度的动态变化趋势;在系统动态响应时间内,模型对于系统压力和温度的时均预测偏差分别为2.55%和-3.29℃。     

A Process Integration based Approach for the Analysis of an Evaporator System

A scalable new mathematical model based on the principles of Process Integration has been developed for the analysis of multiple effect evaporator (MEE) systems. It uses the concepts of stream analysis, temperature path and internal heat exchange for the formulation of the model equations. In addition to the above concepts, the model also takes into account the variable physico‐thermal properties of steam/vapor, condensate and liquor, while simulating the MEE system. The present model consists of a set of linear equations and does not present any stability or oscillation problems during solution as is generally seen in the case of models that are based on sets of nonlinear equations. The model equations are automatically generated through a computer program and the model was run for three different liquor and flow sequences to prove its utility. The results obtained are compared with published models.     

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。     

HEAT TRANSFER AND FLUID FLOW IN A NONISOTHERMAL CONSTRAINED VAPOR BUBBLE

A Constrained Vapor Bubble (CVB) with a relatively large Bond number formed by partially underfilling liquid in an evacuated cavity is capable of high thermal conductance. Il operates on the principle of closed loop phase-change along with capillarity to circulate the working fluid. Analytical investigations were conducted to compare with existing experimental data. A steady-state fluid flow model combined with a two-dimensional heat transfer model was developed and solved to yield key operating parameters ( i.e., temperature and liquid meniscus curvature) of the CVB. The modeling results of the outside wall temperature in the evaporator were found to agree well with the measured experimental data. An area average heat transfer coefficient was used to characterize the heat transfer on the inside wall of the evaporator. The value of this heat transfer coefficient was found to increase with the heat flow rate. The fluid flow model with the heat transfer model in the evaporator to provide the energy balance was used successfully to fit the experimental curvature data. The mass flow rate in the bottom corners of the CVB was found to be higher than that in the top corners due to the gravitational body force.     

HEAT TRANSFER AND FLUID FLOW IN A NONISOTHERMAL CONSTRAINED VAPOR BUBBLE

A Constrained Vapor Bubble (CVB) with a relatively large Bond number formed by partially underfilling liquid in an evacuated cavity is capable of high thermal conductance. Il operates on the principle of closed loop phase-change along with capillarity to circulate the working fluid. Analytical investigations were conducted to compare with existing experimental data. A steady-state fluid flow model combined with a two-dimensional heat transfer model was developed and solved to yield key operating parameters ( i.e., temperature and liquid meniscus curvature) of the CVB. The modeling results of the outside wall temperature in the evaporator were found to agree well with the measured experimental data. An area average heat transfer coefficient was used to characterize the heat transfer on the inside wall of the evaporator. The value of this heat transfer coefficient was found to increase with the heat flow rate. The fluid flow model with the heat transfer model in the evaporator to provide the energy balance was used successfully to fit the experimental curvature data. The mass flow rate in the bottom corners of the CVB was found to be higher than that in the top corners due to the gravitational body force.     

蒸发器换热过程对ORC系统混合工质选择和运行工况的影响

在有机朗肯循环（ORC）系统其他部件的效率均相同的条件下，蒸发器的换热量越大，不可逆损失越小，系统的做功潜力就越大。由于混合工质的相变温度滑移与热源的温度变化具有较好的“匹配性”，采用混合工质为循环工质，并使用流耗散率来描述混合工质与热源流体换热过程中的不可逆损失。结合 T-Q 图来分析蒸发器中混合工质与热源流体换热过程，发现混合工质与热源流体换热曲线围成的面积为流耗散率，并且分别得出了蒸发器换热量最大与不可逆损失最小的条件，以此指导系统最佳混合工质和运行条件的选择。针对确定的热源条件，在蒸发器换热量达到最大的条件下，以不可逆损失最小为目标，建立了基于蒸发器性能的系统最佳混合工质和运行条件的选择方法。采用参考文献中热源条件，筛选出最佳混合工质为R600a/R134a(0.2/0.8)，在最佳蒸发器出口温度为365.75 K的运行条件下，换热量是文献中最佳混合工质的3.3倍，在系统其他部件的参数选择均相同的条件下，系统的净输出功率是文献中最佳混合工质的2.4倍。     

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。     

CO2-空气微通道蒸发器两相区熵产分析

微通道已成为换热器研究领域的热点,以CO₂微通道蒸发器为研究对象,建立了CO₂微通道蒸发器两相区内、外侧均有相变的熵产模型,通过建立的CO₂微通道蒸发器二维分布参数模型求解系统熵产数.分析CO₂与空气侧质量流率、空气入口温度及CO₂蒸发温度对系统熵产数的影响.结果表明:CO₂质量流率对系统熵产数影响很小;系统熵产数主要由CO₂与空气两侧温差传热引起;系统熵产数随空气入口温度的增大而增大,随CO₂的蒸发温度的增大而减小;随着空气质量流率的增大,系统熵产数增大,且蒸发温度越高,空气质量流率对系统熵产数的影响越大.     

Evaluation of the calcium diminution in the circulated brine of a BSS-MSF plant

Among the “seeding” processes which reduce or eliminate the risk of scaling in sea water evaporators, the barium sulfate seeding process “BSS” has been reported to possess the special advantage of reducing the calcium concentration of the circulated brine in a multi-stage flash evaporator with brine recirculation. Using a simple mathematical model, this Ca-diminution is made understandable. Two examples are calculated based on a BSS plant feasibility study published elsewhere*:the results are Ca-diminution by the factors 0.73 and 0.56 in a stationary state, which is reached after about ten times the theoretical travel time of one fluid element along the brine circulation path.