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Numerical simulation of heat and mass transfer in direct membrane distillation in a hollow fiber module with laminar flow

机译：层流中空纤维模块直接膜蒸馏传热传质的数值模拟

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The heat and mass transfer processes in direct contact membrane distillation (MD) under laminar flow conditions have been analyzed by computational fluid dynamics (CFD). A two-dimensional heat transfer model was developed by coupling the latent heat, which is generated during the MD process, into the energy conservation equation. In combination with the Navies–Stokes equations, the thermal boundary layer build-up, membrane wall temperatures, temperature polarization coefficient (TPC), local heat transfer coefficients, local mass fluxes as well as the thermal efficiency, etc. were predicted under counter-current flow conditions. The overall performance predicted by the model, in terms of fluxes and temperatures, was verified by single hollow fiber experiments with feed in the shell and permeate in the lumen.\ud\udSimulations using the model provide insights into counter-current direct contact MD. Based on the predicted temperature profiles, the local heat fluxes are found to increase and then decrease along the fiber length. The deviation of the membrane wall temperature from the fluid bulk phase on the feed and the permeate sides predicts the temperature polarization (TP) effect. The TP coefficient decreases initially and then increase along the fiber length. It is also found that the local Nusselt numbers (Nu) present the highest values at the entrances of the feed/permeate sides. Under the assumed operating conditions, the feed side heat transfer coefficients hf are typically half the hp in the permeate side, suggesting that the shell-side hydrodynamics play an important role in improving the heat transfer in this MD configuration. The model also shows how the mass transfer rate and the thermal efficiency are affected by the operating conditions. Operating the module at higher feed/permeate circulation velocities enhances transmembrane flux; however, the thermal efficiency decreases due to the greater heat loss at a higher permeate velocity. The current study suggests that the CFD simulations can provide qualitative predictions on the influences of various factors on MD performance, which can guide future work on the hollow fiber module design, module scale-up and process optimization to facilitate MD commercialization.
机译：通过计算流体动力学（CFD）分析了层流条件下直接接触膜蒸馏（MD）中的传热和传质过程。通过将MD过程中产生的潜热耦合到能量守恒方程中，建立了二维传热模型。结合Navies–Stokes方程，在反压下预测了热边界层的积聚，膜壁温度，温度极化系数（TPC），局部传热系数，局部质量通量以及热效率等。当前的流量条件。该模型预测的总体性能（通过通量和温度）已通过单个中空纤维实验进行了验证，外壳中进料，内腔中有渗透物。\ ud \ ud使用该模型进行的模拟可提供对逆流直接接触MD的见解。基于预测的温度曲线，发现局部热通量沿着纤维长度先增加后减少。膜壁温度与进料和渗透侧上的流体本体相的偏差预测了温度极化（TP）效应。 TP系数先降低，然后沿纤维长度增加。还发现，在进料/渗透侧的入口处，局部努塞尔特数（Nu）呈现最高值。在假定的工作条件下，进料侧的传热系数hf通常为渗透侧的hp的一半，这表明壳侧的流体力学在改善此MD构型的传热中起着重要的作用。该模型还显示了传质速率和热效率如何受到工作条件的影响。以较高的进料/透过液循环速度操作模块可提高跨膜通量；然而，由于在较高的渗透速度下热量损失较大，所以热效率降低。当前的研究表明，CFD模拟可以对各种因素对MD性能的影响提供定性预测，从而可以指导中空纤维组件设计，组件放大和工艺优化方面的未来工作，以促进MD商业化。

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