Study On Flow Distribution Characteristics And Heat Transfer Optimization For The Heat Exchangers And Its Manifold System In Large-scale Cold Box

As one of the core equipment in the heat exchange process, large-scale cold box is generally applied in air separation, megaton ethylene production, natural gas and coal liquefaction, etc. Given the growing size of the cold box and extreme operating conditions, flow distribution and heat transfer optimization have become the technical "bottleneck" in the development of the cold box. Under the support of the Major Science and Technology Project of Zhejiang Province "Research on the key technologies of the multiphase flow distribution and heat transfer optimization for the large-scale petrochemical cold box"(No.201OC11020), the flow distribution characteristics and heat transfer optimization of the plate-fin heat exchanger (PFHE) and its manifold piping system in the large-scale cold box were studied in this paper. Effects of operating conditions, structural factors and fluid properties on flow distribution and pressure drop of the PFHE and manifold structure were analyzed. The mutual coupling effects of the operating condition, flow hydrodynamic characteristic and heat transfer performance of the studied structures were investigated. Meanwhile, the influence of the flow nonuniformity on heat transfer efficiency and energy consumption of a single PFHE and multiple parallel PFHEs were further studied. Based on the results, corresponding optimization strategies were proposed in order to effectively improve the efficiency and performance of the cold box, which has certain guiding significance for the design, processing and manufacturing of the large-scale cold box. The main research content and innovations are as follows:(1) Numerical models of the PFHE and its manifold system in cold box were established by using the porous media theory, resistance coefficients of the PFHE and its fins were obtained. Based on the models, an experimental platform for measuring flow distribution of the PFHE and its manifold system was designed and implemented. A multiparallel-based flow separation and measurement method for two-phase flow in the PFHE was proposed. Effects of various operating conditions (velocity and gas-liquid ratio) on flow distribution and pressure drop of the PFHE and related manifold system were experimentally studied.(2) Effects of the inlet Reynolds number (Re), channel fins, and fluid dynamic viscosity on flow distribution and pressure drop of the PFHE were analyzed. Based on the results, a correlation among the flow distribution standard deviation parameter (STD), pressure drop and Reynolds numbers was obtained. Meanwhile, the velocity-temperature coupling effects on the flow distribution instability of the PFHE were particularly studied. It was found that when the fluid viscosity has negative exponential relationship with the fluid temperature, the flow distribution instability phenomenon, which was caused by the variation of the fluid viscosity due to heat transfer, was observed at a certain range of Re. Finally, a channel resistance-based optimization strategy for improving flow distribution of the PFHE was proposed, which is proved to be better than the traditional strategy by adding baffle inside the header of the PFHE.(3) Velocity and pressure distribution in the manifold system of the cold box were studied under steady-state and transient conditions. Meanwhile, effects of the PFHE and branch sizes on flow distribution and pressure drop of the manifold structure were investigated, and a phenomenon termed as adaptive dynamic balance phenomenon was observed. Transient hydrodynamics characteristics of the U-type and Z-type manifold structure were further studied under the increment, decrement and pulsation operating conditions. Based on the results, two novel strategies, named dynamic balance-based strategy and the dichotomy-based strategy, for the attainment of flow uniformity were proposed. It was verified that the strategies can greatly improve flow distribution of the studied manifold structure, and meanwhile cause negligible pressure drop variation.(4) A theoretical model was developed for determining the performance deterioration of the PFHE due to flow nonuniformity effect. Based on the model, the effects of the uneven flow distribution on the heat transfer performance and energy consumption of a single PFHE and multiple parallel PFHEs were studied. Given the contradictory relationship between the heat transfer performance and energy consumption, an analysis method to assess the heat transfer performance and energy consumption of a single PFHE and multiple parallel PFHEs was proposed. It was found that for the PFHE with higher number of transfer units (Ntu), its heat transfer performance is sensitive to the flow distribution. As the Ntu, heat load and Re increase, corresponding pressure drop of the optimum performance for a single PFHE and multiple parallel PFHEs tends to increase.