| The explosion of airplane fuel tanks is one of the main causes of airplane accidents and is a major concern in the aviation industry.The on-board nitrogen generation and inerting system based on Hollow Fiber Membrane(HFM-OBIGGS)technology is an effective way to address the above problem and has been widely used in military and civilian aircraft models both domestically and internationally.The HFM-OBIGGS system uses enriched nitrogen gas(NEA)to flush the fuel tank headspace in order to control the oxygen concentration in the fuel tank to below 9% or 12%.Due to the need to consider issues such as strength,center of gravity,space,and ventilation,the structure of aircraft fuel tanks presents the characteristics of multiple bays and multiple configurations.Therefore,insufficient and uneven inerting areas often occur during the inerting process,resulting in some areas exceeding the oxygen concentration limit.In order to achieve comprehensive control of the oxygen concentration in the fuel tank headspace,an optimal inerting gas distribution scheme needs to be proposed.This not only requires an understanding of the inerting characteristics of enriched nitrogen gas with different flow rates and concentrations but also a deep exploration of the influence of the flow characteristics and organizational forms of enriched nitrogen gas in complex spaces on the concentration and uniformity properties.This article is based on the practical design issues of the HFM-OBIGGS system,focusing on the structural characteristics of large military and civilian aircraft fuel tanks.By combining the multi-index evaluation theory with the theory of inerting of fuel tanks with multiple bays,a flow optimization theory for multi-bay fuel tank inerting systems has been established.Using CFD numerical simulation methods,an analysis of the flow characteristics of enriched nitrogen gas has been conducted,and a set of universal principles for the flow optimization design of multi-bay fuel tanks has been proposed.These principles were applied to the optimal design of the inerting gas distribution scheme for the central wing fuel tank of the B747.Based on this,a scaled-down fuel tank experimental platform was constructed,and gas flow visualization experiments and oxygen concentration measurement experiments were carried out to verify the correctness of the proposed flow optimization theory.The specific contents include:(1)Based on the theoretical model of fuel tank inerting and flushing,a comprehensive analysis was conducted through the establishment of principles for evaluation methods,selection of methods,determination of indicators,and other aspects.Ultimately,a multi-bay fuel tank inerting and flushing theory based on entropy-weighted improved TOPSIS was proposed,providing a reliable evaluation theory for subsequent design and optimization of inerting systems.(2)Using numerical simulation method,a universally applicable multi-bay fuel tank model was established,and the airflow organization in the inerting space of singlebay and multi-bay fuel tanks was studied through multiple calculations.The results showed that for multi-bay fuel tanks,the optimal inerting effect was achieved when the "main channel" occupied as much space in the bay as possible,and the "single-channel airflow" scheme had better inerting effect than other schemes.Finally,based on this theory,a universally applicable airflow optimization design principle for multi-bay fuel tanks was proposed: the inlet and outlet should be placed on different straight lines as much as possible,and the inlet should be mainly located on one side of the outlet;the "single-channel airflow" scheme should be used as much as possible;the short-circuit phenomenon between the inlet and outlet should be avoided as much as possible;the fuel tank should be designed as an m-by-n multi-bay tank,and when m< n,the design of the main channel traveling along the long side of the tank is better than the design of traveling along the short side of the tank.(3)Based on the above design principles,six different airflow schemes,including the original inerting scheme of the central wing tank in the Boeing 747 multi-bay fuel tank,were designed and a three-dimensional mathematical model was established.Through comparison with experimental results using Pearson correlation coefficient,the similarity between the two types of data curves was calculated to be about 96%,verifying the correctness of the model.Then,using the CFD numerical simulation method,the speed and uniformity indicators of the oxygen concentration reduction for the six schemes were obtained,providing raw data for the multi-criteria optimization method based on improved entropy weighting TOPSIS theory.The comprehensive evaluation analysis of the six schemes showed that the "single-side offset" type scheme designed in this thesis had better overall inerting effect than the "double-side offset" type scheme,especially Scheme 5,which improved the comprehensive performance index by 22.67% compared with the original inerting scheme.This scheme not only reduces the inert gas flow demand but also has a more uniform distribution of oxygen concentration in the inerting space.In addition,if only the speed indicator is considered,its performance has improved by 2.97%,while the performance has improved by 27.78% when discussing the uniformity indicator.(4)The experimental platform was constructed,which included gas flow visualization experiments that verified the correctness of the proposed optimization design principles for gas flow,and also verified that the best inerting scheme can achieve the goal of improving the inerting effect of the original scheme.The evaluation data was obtained through measurement experiments of the oxygen concentration changes in the multi-bay fuel tank,and the six schemes were evaluated again using the entropy-weighted improved TOPSIS multi-index evaluation method.The feasibility of the optimization method used in this thesis and the credibility of the evaluation results were verified by comparing the evaluation results with the CFD numerical simulation results.The research findings in this thesis can provide reference for the design and optimization of aircraft multi-bay fuel tank inerting systems. |
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