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.

A new experimental method was proposed: the enthalpy difference method to measure the isobaric specific heat capacity of RP-3 at 6-8 MPa, 338.1-788.3 K. According to the calculation of the uncertainty of the relevant experimental apparatus, compared with the normal measurement method, the enthalpy difference method can reduce the uncertainty from 5.17 to 3.65%. The results were verified by using n-decane. The results showed that compared with National Institute of Standards and Technology (NIST), the average error of the experimental results of n-decane was 1.8%. Experimental results show that the measured peak isobaric specific heat capacity of RP-3 in this range is 3.93 kJ/kg (slightly less than the true value of the substance). As the pressure increases, its peak value gradually decreases until it becomes flat.

Based on the demands of compact heat exchangers and micro cooling channels applied for aviation thermal protection on aero-engines, the elbow local flow resistance characteristics for supercritical pressure aviation fuel RP-3 flowing in adiabatic horizontal serpentine tubes with the inner diameter of 1.8 mm and the mass flux of 1179 kg/(m2·s) were experimentally studied. The long-short-tube method was used to obtain the elbow pressure drop from the total serpentine tube pressure drop, and the effects of system pressures (P/Pc = 1.72–2.58) and geometry parameters including bend numbers (n = 5–11), bend diameters (D/d = 16.7–27.8), and bend distances (L/d = 20–60) on elbow pressure drops and local resistance coefficients are analyzed on the basis of the thermal physical property variation. The results show that both the increase in the elbow pressure drop and the decrease in the local resistance coefficient with temperatures speed up at the near pseudo-critical temperature region of T > 0.85Tpc. And the growth of the elbow local pressure drop could be inhibited by the increase of system pressures, while the local resistance coefficient is slightly affected by pressures. The influence of bend diameters on the local resistance coefficient is mild when D/d is larger than 22.2 in the premise of fully developed flow in straight tubes. Furthermore, a piecewise empirical correlation considering the bend diameter and physical property ratio is developed to predict the elbow pressure drop of the serpentine tube and optimize the layout of the cooling tube system on aero-engines.

This research investigates the design, manufacture and heat transfer characteristic evaluation of three types of heat exchangers (HXs) serving for modern aero-engines. In the field of heat transfer system, previous research has concentrated on large-diameter tubes, whereas estimation for heat transfer in small-diameter tubes commonly relies on correction factors. However, small-diameter heat exchangers offer extensive applicaiton prospects in aero-engines utilizing cooled cooling air (CCA) technology, benefiting from their high heat transfer efficiency and lightweight. Hence, directly investigating compact small-diameter HXs is significant. In this paper, three analogous serpentine tube bundle HXs with different tube diameters (OD: 2.2/1.8/1.4 mm with 0.2 mm thickness) were designed by the Logarithmic Mean Temperature Difference method (LMTD). A series of comparative experiments were conducted to explore the effects of the tube diameter and flow rates on the airside heat transfer under different fuel flow conditions. The results indicate that in the case of turbulent fuel flow, decreasing the tube diameter assists in enhancing total heat transfer, whereas the trend is opposite in laminar fuel flow. In addition, during turbulent fuel flow, heat transfer capacity rises with the increase of flow rates on both sides, while variations in air flow rate have minimal impact on airside heat transfer in laminar fuel flow. Finally, the calculated heat transfer rate exceeds the experimentally measured value by no more than 15%, with this deviation escalating as the reduction of tube diameter. Subsequently, an airside Nu-Re correlation suitable for smalldiameter HXs is fitted based on experimental data, with 94.1% of the data points locate in the 5% error band, providing guidance for the design and validation of aero-engine HXs in future.

This paper presents a novel air-fuel heat exchanger used for the cooled cooling air technology in aero-engines. The helical tube heat exchanger weighing 1.1 kg with an area density of 214 m2/m3 can cool the hot air down 260 K at the air flow of 0.3 kg/s with relative airside pressure drop less than 0.6%. Empirical correlations by multiplying a constant of 1.06 and 0.837 can well predict the pressure drop and convective heat transfer coefficient respectively for hot gas cross-flowing helical tube bundles. Furthermore, in the tube failure tests to simulate the fuel control system fault and flight mode conversion, the straight tube can work continuously for more than 360 s under the extremely high tube wall temperature of over 1200 °C in the heat flux sudden increase test, while the bent tube could continue to glow brightly for 30 min. Coke morphology and chemical composition analysis revealed that increased thermal stress caused by gradually thicker coke layer takes a major cause in the heat flux sudden increase test. The secondary flow in bent tubes can effectively improve the convective heat transfer performance and reduce the formation of coke deposition, thereby improving the tube working life.

The long-term demand forecast for annual national electricity and energy consumption plays a vital role in future strategic planning, power system installation programming, energy investment planning, and next-generation unit construction. Three machine learning algorithms of BP-NN, MLR, and LS-SVM were chosen for training forecasting models, with the data on population, GDP, mean temperature, sunshine, rainfall, and frost days in 1993–2019 serving as the input variables. The total data were divided by 70% into the training set (1993–2011) and 30% into the test set (2012–2019), in chronological order. RMSE, MAPE, and MaxError were adopted as the performance criteria. The statistical results show that the gross population of the UK increases year by year from 1993 to 2020. The GDP generally increases before 2007 but has a decline, and then varies with a large amplitude afterward. The electricity and energy consumption of the UK generally increase from 1993 and reach a peak around 2005. Afterward, a decline occurs basically year by year until 2019. The simulation results reveal that all three models predict well on the training set but have some overestimation on the test set. The LS-SVM model has the best forecasting performance among the three models on the training set. The results show that it is feasible to use machine learning algorithms to predict the future electricity and energy consumption of a country based on past economic and livelihood data. In this way, economic decision-makers can rely on the predicted values to make a well-founded layout for future energy construction and investment to avoid waste or a shortage of resources.

Introducing a pre-cooling process into the air intake of a hypersonic turbine engine can reduce the temperature of the air entering the compressor and increase the available pressure ratio and the engine thrust. To study the thermodynamic performance of the precooler, the structure and heat exchange of the precooler have been investigated in detail, and a segmented thermodynamic calculating model for an involute precooler using high heat sink hydrocarbon fuels as the cold source is established. Results suggest that the thermodynamic calculation for a precooler must be performed in segments when both cold and hot fluids experience large temperature variations. The influence of the fuel mass flow rate, air outlet temperature, and precooler structural parameters on the thermodynamic performance of the precooler are studied. A tremendous amount of micro heat exchange tubes lead to almost laminar flows in the tube. With the increase of the fuel flow rate, the cooling capacity of the precooler is enhanced, and the weight is reduced, but the fuel after heat absorption might not be totally used for combustion, resulting in thrust waste. Lowering the air outlet temperature helps improve the engine thrust performance, but will increase the precooler weight and air pressure loss. When the transverse and longitudinal pitches of the tube bundle both are 1.5 times of the tube diameter, compared with the staggered arrangement, the air-side convection heat transfer capacity for the in-line arrangement deteriorates, and the precooler weight and air pressure loss are both larger. The transverse and longitudinal pitches of the tube bundle have complicated influence on the thermodynamic performance of the precooler. This work can provide strong support for the design, verification, and performance analysis of the tube bundle precooler with similar structures in the future.

Based on the demands of compact heat exchangers and micro cooling channels applied for aviation thermal protection, the flow resistance characteristics of aviation kerosene RP-3 were experimentally studied in a vertically downward circular miniature tube with an inner diameter of 1.86 mm at supercritical pressures and constant heat fluxes. A long and short tube method was used to accurately calculate the frictional pressure drop, and experimental conditions are supercritical pressures of 4 MPa, mass flow rates of 2–4 g/s (i.e., mass fluxes of 736–1472 kg/(m2∙s)), heat fluxes of 100–500 kW/m2, and inlet temperatures of 373–673 K. Results show that the sharp variations of thermophysical properties, especially density, have significant influences on frictional resistances. Generally, the frictional pressure drop and the friction factor increase with increasing inlet temperatures, and this trend speeds up in the relatively high-temperature region. However, the friction factor has a sudden decline when the fuel outlet temperature exceeds the pseudo-critical temperature. The frictional pressure drop and the friction factor basically remain unchanged with increasing heat flux when the inlet temperature is relatively low, but increase quickly when the inlet temperature is relatively high. Besides, a larger mass flux yields a higher pressure drop but does not necessarily yield a higher friction factor. Finally, an empirical friction factor correlation is proposed and shows better predictive performance than those of previous models.

This study explores the design, analysis, and air pressure drop assessment of three analogous air–fuel heat exchangers consisting of thin serpentine tube bundles intended for use in high Mach number aero-engines. In high speed flight, the compressor bleed air used to cool high temperature turbine blades and other hot components is too hot. Hence, aviation kerosene is applied to precool the compressor bleed air by means of novel air–fuel heat exchangers. Three light and compact heat exchangers including dozens of in-line thin serpentine tube bundles were designed and manufactured, with little difference existing in aspects of tube pitches and outer diameters among three heat exchangers. The fuel flows inside a series of parallel stainless serpentine tubes (outer diameter: 2.2, 1.8, 1.4 mm with 0.2 mm thickness), while the air externally flows normal to tube bundles and countercurrent with fuel. Experimental studies were carried out to investigate the airside pressure drop characteristics on isothermal states with the variation of air mass flow rates and inlet temperatures. Non-isothermal measurements have also been performed to research the effect of heat transfer on pressure drops. The experimental results show that inlet temperatures have significant influence on pressure drops, and higher temperatures lead to higher pressure drops at the same mass flow rate. The hydraulic resistance coefficient decreases quickly with Reynolds number, and the descent rate slows down when Re > 6000 for all three heat exchangers. Additionally, the pressure drop on heat transfer states is less than that on isothermal states for the same average temperatures. Moreover, the pressure drop through heat exchangers is greatly affected by attack angles and transverse pitches, and an asymmetric M-shaped velocity profile is generated in the cross-section of sector channels.

为解决飞行器高马赫数飞行时,用于冷却航空发动机涡轮叶片和其它高温部件的压气机出口空气温度过高的问题,设计加工了一种以外涵空气为冷源的螺旋管式空气-空气换热器用于预冷冷却空气.换热器由48根材料为不锈钢321,外径4mm,壁厚0.5mm的螺旋管组成,重量1.91kg,传热面积密度106m2/m3.在常温工况下,实验研究了换热器管内和管外流体的阻力特性,高温条件下试验验证了换热器设计点性能,压气机出口空气温降达188K,功重比4.9kW/kg.高温条件下实验研究了两侧空气流量分别单独变化时换热器的热动力性能,拟合了管外换热经验关系式.研究结果可用于未来相似结构换热器的设计.

To meet the demand of thermal protection in aero engines, this paper presents a novel compact cross-flow air-air heat exchanger based on the Cooled Cooling Air (CCA) technology. This novel air-air heat exchanger consisting of 4 × 10 serpentine tubes (4.4 mm I.D., 5.0 mm O.D., stainless steel type 321) was designed using the classic Logarithmic Mean Temperature Difference (LMTD) method. Experimental verification has been done to research the hydraulic and heat transfer performance of the heat exchanger. The results show that the 1.48 kg serpentine tube air-air heat exchanger can cool the high pressure compressor bleeding air by 200 K at the mass flow rate of 0.05 kg/s using bypass duct cold air. Comparisons between calculated and experimental data have been done and good agreement between them was obtained in both flow resistance and heat transfer characteristics. Thus, the LMTD method could be well adopted in designing compact air-air heat exchanger for aero-engines. A new empirical heat transfer coefficient correlation for the tube outside is obtained using Wilson plot method, and it can be helpful designing heat exchanger with similar structures. This research is a great proof of CCA’s feasibility in terms of theory and practice.

Based on CCA technique and design concept of compact helical tube heat exchanger, this article carried out a series of experiments to study the flow and convective heat transfer characteristics for air flowing through helical-tube bundles under uniform heat fluxes. The single helical tube outer diameter d(0) is fixed at 2.2 mm and the spatial structure of various helical-tube bundles remain the constant. The experimental results show that the flow resistance is generally influenced by helical diameter ratio D/d(0) a and slightly affected by variation of helical pitch. Furthermore, the Nusselt number will increase by 30% with the decrease of helical diameter ratio D/d(0) at the range from 13.64 to 9.10. For constant helical diameter ratio D/d(0), the heat transfer coefficient increases with the increase of helical pitch ratio P/d(0) at the range of 4.55-6.82, but reduces at the range of 6.82-9.10. In addition, temperature difference correction factor is exploited to modify the heat transfer caused by non-uniform temperature field. At last, two empirical correlations are proposed to summarize the friction factor and convective heat transfer coefficient for air flowing through helical-tube bundles. (C) 2018 Elsevier Ltd. All rights reserved.