The heat absorption of onboard hydrocarbon fuel is an important means to solve the thermal protection problem of hypersonic vehicles, and the thermophysical properties of the fuel are the premise for designing the cooling channel. In this work, the viscosity of endothermic propellant endothermic hydrocarbon fuel-Tianjin University (EHF-TU) was experimentally measured in the range of pressure 3-7 MPa and temperature 300-605 K, while the pressure was 6-8 MPa and the temperature was 295-715 K for aviation kerosene RP-3. A special viscosity measurement device was designed, and the long and short parallel tube bank method was used to minimize the error caused by the local resistance. The viscosity measurement uncertainty of the two fuels was calculated in detail, and the accuracy of the experimental method is verified using n-decane. The viscosity of both fuels decreases with rising temperature and augments slightly with increasing pressure. The measurement results of this paper can provide basic support for the design of hydrocarbon fuel cooling channels for future hypersonic aircraft.

This study outlines the design and manufacture of three types of finned tube bundle heat exchangers (HX) for advanced aero-engines followed by a series of experimental evaluations on flow and heat transfer characteristics. Considering the compactness and reliability, the finned tube bundle HX is more suitable for aero-engines when the heat transfer unit is small-diameter serpentine tubes with the enhanced heat transfer area provided by fins. In this paper, the Logarithmic Mean Temperature Difference method (LMTD) was utilized for the design of a small- diameter (OD: 3.6 mm with 0.3 mm thickness) finned tube bundle HX. Mass reduction was achieved by perforating the fins and removing the support devices, respectively. The flow and heat transfer characteristics of HXs and the impact of the mass reduction schemes were evaluated based on a series of comparative experiments. Among them, the connectionless scheme can significantly improve the power-to-mass ratio, providing it certain application value despite causing considerable flow resistance. Subsequently, empirical correlations for the flow and heat transfer outside the tubes suitable for small-diameter finned tube bundle HXs were proposed based on the experimental results, with over 95% of the data falling within a 10% error margin. These correlations have substantially modified the previous one for large-diameter straight tubes, providing an important reference for the future design of HXs in aero-engine.

Variable cycle engine (VCE) is one of the most promising technologies for the next-generation aircraft, the matching of different components in the compression system is a key difficulty VCE faced. To investigate the component matching mechanisms in the VCE compression system, an advanced throughflow program is employed to calculate the characteristic lines of each component, and a zero-dimensional method is developed to capture the component performance deviation during the coupling working process. By setting the compressor stall and choke conditions as the boundary, the operation range of the compression system is first clarified, and the aerodynamic performance in the operation zone is discussed, thus providing a theoretical basis for optimization of the engine operating control scheme. Results show that the efficiency of the core flow is optimum at the left-bottom corner of the operation region, while the total pressure ratio peaks at the right-top area, hence a balance is needed when deciding the matching point. Regulations of component control parameters will change the position of the operation zone, as well as the corresponding aerodynamic performance. Decreasing the core driven fan stage rotating speed can improve the total bypass ratio, yet the total pressure ratio of the core flow will be decreased. Closing the core driven fan stage inlet guide vane can increase the total bypass ratio without changing the core flow aerodynamic performance significantly. The bypass ratio of the compression system can also be increased by increasing the fan stall margin or decreasing its rotating speed, both ways will decrease the total pressure ratio of the core flow. Results of the study will benefit the variable cycle engine design process in operation point evaluation and thermodynamic cycle optimization.

The printed circuit heat exchanger (PCHE) stands out as a promising candidate for thermal systems. However, experimental studies on the thermodynamic performance of airfoil-fin PCHE using supercritical pressure hydrocarbon fuel are scarce, and reliable methods for calculating the Nusselt number (Nu) remain underdeveloped. Therefore, this work presents an experimental investigation of the thermal-hydraulic performance of an airfoilfin PCHE, using supercritical pressure hydrocarbon fuel and water as working fluids. The correlations for Nu and friction coefficient (f) were developed, with deviations of +/- 8 % for f and +/- 20 % for Nu. Under laminar flow conditions, the airfoil-fin PCHE demonstrated a high f, being 2.93 times greater than that of the straight PCHE. The heat transfer superiority of the airfoil-fin PCHE compared to other channel types depends on Re. When Re > 334, the airfoil-fin PCHE shows the best heat transfer performance, exhibiting an average Nu that is 7.74 times higher than the straight PCHE, 3.15 times higher than the zigzag PCHE, and 1.65 times higher than the S-shaped PCHE. The overall thermodynamic performance of the airfoil-fin PCHE also improves with increasing Re. Additionally, a comparative assessment of six different Nu calculation and fitting methods has been conducted based on experimental results.

Three-dimensional blading is an efficient technique in compressor aerodynamic design, and its function mechanism in the cantilevered stator needs to be addressed. This paper focuses on the sweep and dihedral in the cantilevered stator and seeks to expose their effects through detailed flow field analysis. Results show that the forward sweep could alleviate the corner flow separation by preventing the accumulation of the secondary flow toward the corner region, resulting in stronger flow separation at the blade trailing edge; in summary, forward sweep with appropriate parameters could increase static pressure rise by 14.3%. The positive dihedral will carry the endwall flow to the upper-span sections, thereby reducing blade corner separation; hence, as much as 23.5% improvement in static pressure rise could be obtained with the appropriate dihedral. Moreover, the combination of a relatively large sweep height and a moderate sweep angle with a low dihedral height and a moderate sweep angle provides optimum aerodynamic performance; the static pressure rise coefficient sees an increment of 25.5% at the near stall point. An experiment is then performed to further validate the theory, which shows a 2% improvement in efficiency of 3D blading at small mass flow rates. However, the secondary leakage should be given attention at high mass flow coefficients, while the corner separation needs further elimination at small mass flow rates.

The variable cycle engine is distinguished by its highly adjustable compression system, whose aerodynamic characteristic is extremely complex. To explore the regulation range of a double bypass engine compression system, a multi-dimensional analysis method is developed, through which the coupling mechanism between the compressor component and the bypass is examined. The operation zones of the compressor components and the bypass system are proposed, and the operation range of the compression system is obtained by calculating the overlapping part of the operation zones. The results show that in the double bypass mode, there exists a minimum mode selector valve area and a minimum core driven fan stage stall margin that ensures a feasible bypass flow, the two parameters correspond to each other. Under the given fan and core driven fan stage conditions, the maximum value of the inner bypass ratio is restricted by the upper limit of the forward variable area bypass injector and the maximum Mach number in the total bypass, while the minimum value of the inner bypass ratio depends on the lower limit of the forward variable area bypass injector geometry and the system recirculation margin. The single bypass mode is a unique condition of the double bypass mode, as the operation zone of the compressor component degenerates from a two-dimensional surface to a straight line. There are multiple bypass states available in the single bypass mode, while the regulation range of the bypass ratio is jointly restricted by the operation range of the high pressure compressor and the aerodynamic boundary of the forward variable area bypass injector.

The development of the aircraft industry seeks an increase in compressor loading, bringing unique flow phenomena and design problems; thus, insights into the ultrahigh loaded compressor are in great need. To reveal the loss characteristics of the ultrahigh loaded subsonic axial compressors, four well comparable compressor stages are carefully designed with the loading coefficient varying from 0.41 to 0.65. A novel flow-based loss decomposition method is performed to investigate the variation of different kinds of losses (including blade profile loss, tip leakage loss, casing endwall loss, and hub endwall loss) with the change in compressor loading level and operating condition. Results show that the blade profile loss always occupies the largest part of the total loss. In rotor passages, the percentage of the blade profile loss at the design point is increased from 69% to 76% with the increase in the compressor loading. Meanwhile, the proportion of the tip leakage loss decreases as the loading increases. For a specific compressor stage, the total loss of the rotor passage tends to increase with the increase in stage pressure rise coefficient along the operation line, whereas the proportion of the blade profile loss is squeezed by the tip leakage loss. As for stator passages, the proportion of blade profile loss to the total passage loss is nearly constant along the compressor operating line, but increases from 79% to 90% with the increase in the compressor loading level. By correlating the losses with blade solidity, it was found that the increase in flow losses in the highly loaded compressor, i.e., the decrease in efficiency, stems mainly from the high blade solidity.

基于双外涵变循环发动机压缩系统,分析了多连通气动布局变循环压缩系统的匹配工作机制.一体化全三维数值模拟表明:变循环压缩系统各压缩部件与涵道及其调节机构之间由于多连通特征相对于常规发动机压缩系统具有更强的耦合工作特点,高效的外涵道流动是发挥变循环发动机性能优势的关键.涵道几何的调节不仅会改变其自身流动状态,还伴随着压缩部件气动性能的偏移,模式转换过程必须符合各涵道及调节机构之间的气动协调匹配.提出了适用于多连通变循环压缩系统的一体化变维度分析方法,将部件通流程序与涵道零维程序相结合,实现了部件-涵道耦合匹配关系的快速分析.基于变维度分析方法给出了单外涵模式部件与涵道共同约束下的压缩系统综合匹配可行域,旨在为变循环发动机的匹配设计提供理论依据.

Both the compressor performance and the 3D flows inside the stator passage are significantly impacted by the stator hub gap. The interplay between leakage flow and corner separation within a cantilevered stator of a highly loaded, low-speed axial compressor with a succession of stator hub gaps was examined numerically in this paper. Firstly, the simulated results were compared with the measured results, including the compressor characteristics, the 3D flow structures, and the flow fields at the stator outlet. The results revealed that the used CFD solver, as well as the corresponding setup, can reproduce the flow not only in terms of the trend along with the stator hub gap, but also in terms of the specific scale of the 3D flow structure. Hence, it is feasible enough to be applied in the present investigation. Secondly, the flow mechanisms of the interplay between the corner separation and the leakage flow with different stator hub gaps were analyzed. It was found that the velocity of the leakage flow is the key parameter that dominates the flow structures as well as the compressor performance. Additionally, a simple metric was proposed to be used to choose the optimum stator hub gap. By comparing our results with those from published research, this metric was proven to be feasible. Finally, it is also discussed how the stator hub gap affected the stator inlet flow and rotor performance. It is demonstrated that the stator passage flow blockage can affect the upstream flow field. As a result, the performance of the rotor tends to vary in the opposite direction to that of the stator.

The variable area bypass injector (VABI) plays a crucial role in variable cycle engines by regulating the flow mixing process in complex bypass ducts, and low-dimensional theoretical models are the key to revealing its working mechanism while estimating its aerodynamic performance. An improved VABI model using the control volume method is established, through which the feature parameters that determine the VABI aerodynamic performance are summarized. To acquire an accurate prediction of the injection ratio, a calibration item is introduced to the governing equations to consider the static pressure discrepancy on the mixing plane, and a numerical database is developed to obtain the calibration item. Results show that the aerodynamic parameters that determine the VABI performance include the bypass total pressure ratio, bypass backpressure, and the injection ratio, while the injection angle and the VABI opening area also influence the injection flow characteristics. The injection ratio is increased by reducing the bypass total pressure ratio, decreasing the bypass backpressure, and closing the VABI. Numerical validation shows that the calculation error of the improved model is generally below 3%. The improved VABI model is then validated by a well-arranged experiment, for which the annular flow is simplified into a rectangular duct flow with an error of less than 5%. The experimental validation also proves the accuracy of the model.

为了研究双外涵变循环压缩系统的涵道流动匹配规律,发展了涵道流动的计算模型,对压缩系统模式转换过程中外涵道倒流问题的发生机理和影响因素进行了深入分析,总结了压缩系统的倒流判断准则,并基于简化计算模型和全三维计算结果对倒流判据进行了验证.研究结果表明,对于一定的压缩系统匹配状态,存在一个决定涵道匹配状态的临界核心机驱动风扇级(Core Driven Fan Stage,CDFS)总压比,当CDFS的实际工作压比高于临界压比时,压缩系统第二外涵道将发生倒流,反之则系统不倒流.复杂涵道系统内的流动损失和堵塞等特性对临界CDFS总压比有显著影响,为了准确判断压缩系统的匹配状态,需要对其进行精确模化.基于简化模型得到的压缩系统倒流临界线可以推广至全三维状态,提出的倒流判断准则具有较高的可靠性和可行性.

We face the problem to determine whether an algebraic polynomial is nonnegative in an interval the Yau Number Theoretic Conjecture and Yau Geometric Conjecture is proved. In this paper, we propose a new theorem to determine if an algebraic polynomial is nonnegative in an interval. It improves Wang-Yau Lemma for wider applications in light of Sturm's Theorem. Many polynomials can use the new theorem but cannot use Sturm's Theorem and Wang-Yau Lemma to judge whether they are nonnegative in an interval. New Theorem also performs better than Sturm's Theorem when the number of terms and degree of polynomials increase. Main Theorem can be used for polynomials whose coefficients are parameters and to any interval we use. It helps us to find the roots of complicated polynomials. The problem of constructing nonnegative trigonometric polynomials in an interval is a classical, important problem and crucial to many research areas. We can convert a given trigonometric polynomial to an algebraic polynomial. Hence, our proposed new theorem affords a new way to solve this classical, important problem.

In order to clarify the influence of bypass configuration on the variation range of the bypass ratio in a variable cycle engine, as well as revealing the effect of bypass ratio adjustment on the engine aerodynamic performance, an integrated zero-dimensional bypass model is established based on a double bypass engine compression system. The bypass model employs the control volume method and is validated by the bypass flow database which is established using a novel high-accuracy simplified bypass system computational method. By relating the model parameters to the engine component operation points, evaluation of the bypass aerodynamic performance in the real engine environment is realized, and the feature parameters decisive for the bypass matching characteristic are proposed. The operating range of the bypass system is then disclosed using the bypass model, which is instructive for the aerodynamic design of the whole compression system. Results show that to increase the total bypass ratio, one can decrease bypass backpressure, reduce the core driven fan stage (CDFS) total pressure ratio, increase the mode selector valve (MSV) opening angle or open up the forward variable area bypass injector (FVBAI). Different methods tend to influence the bypass matching characteristics differently. The non-monotonicity characteristic of the dividing flow loss around the MSV will induce a multi-solution zone for the double bypass configuration and make it difficult to diagnose the system operating state. Moreover, the inner bypass will reach the critical condition before the outer bypass, yet the supercritical condition is not recommended as it requires unique bypass geometry and will bring about high flow loss.

By establishing a three dimensional model of a double bypass variable cycle compression system, the flow patterns and matching characteristics of each working component during mode transition are investigated using numerical simulation. Results show that transition from the single bypass mode to double bypass mode by opening the mode selector valve (MSV) alone would increase the fan operation point while decreasing that of the core driven fan stage (CDFS) and the high pressure compressor (HPC). It would also incur outer bypass flow recirculation and bring radial inflow distortions to the CDFS. Deviations of the fan aerodynamic performance lie mainly in its aft stage, while the HPC first stage undertakes most of the inlet distortion. Reducing the bypass backpressure during mode transition is an effective way to alleviate the outer bypass flow recirculation but would further choke the CDFS. Closing the forward variable area bypass injector (FVABI) could raise the CDFS matching point so as to improve the stator performance without influencing the outer bypass ratio. It is recommended to decrease the bypass backpressure and close FVABI simultaneously in the real transition process.

对双外涵变循环压缩系统进行了建模和全三维数值模拟,分析了模式转换过程中第二外涵道倒流对系统气动性能及部件匹配规律的影响.随着倒流流量的增大,变循环压缩系统的第一外涵道比和总涵道比提高,风扇总压比升高,CDFS与高压压气机的总压比降低.倒流主要影响风扇后面级的气动性能.倒流导致CDFS进口流场发生畸变,CDFS下游第一分流环局部流场严重恶化,应及时调节系统控制参数防止压缩系统失效.