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.

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.

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.