The article describes an analytical model to calculate the stability of axial-flow compressor rotors. It illustrates how stability is related to the ratio of two times. The definitions of both times are obtained in a rational way for high-performance rotors subjected to non-small disturbances, and a theoretical stability margin coefficient is established. This coefficient determines the location of the stability line in the rotor map. The range of validity of this theoretical characterization is supported by several lemmas and theorems, whose proofs are given. These proofs also give physical meaning to the final analytical formula, connect the results with the surge and stall phenomena, and provide the widest area in the rotor map where normal operation can be sustained. The analytical results link the loading and flow coefficients, the tangential Mach number, and the basic rotor geometry. The stability model is validated with the actual data of transonic axial-compressor rotors. Errors lie in the 1-7 per cent range for cases accomplishing the premises of the theorems. Other empirical and numerical evidences that support the theory, as well as its limitations, are discussed. Due to its simplicity and robustness, the model can be easily used for multi-objective optimizations, preliminary performance calculations, and first estimations of axial solidity in new designs.

Recently, an analytical model to calculate the stability of an axial-flow compressor rotor has been presented in the scientific literature. The range of validity of that theoretical characterization was supported by several lemmas and theorems. One of the main results was the definition of a dimensionless coefficient for determining the location of the stability line in the rotor map. In this work the mathematical structure of that solution is studied. As a result of this detailed study, a new stability theorem and a new stability coefficient are obtained. This stability coefficient is an improvement of the previous one since it is physically and mathematically well defined in all the operational points of the compressor map. As a consequence, the new model is able to capture the stall inception for rotors and stators as well as the full characteristic curve (pressure rise versus mass flow rate) including rotating stall and possibly reverse flow. It is proved, as a consequence of the restriction imposed by the Stability Theorem, that each local component (rotor or stator) has its own instability point and its own post-stall characteristic curve. This theoretical criterion for predicting the averaged characteristic curve is in good accord with the experimental data. The stability coefficient is also verified for a compressor stage. Finally, the model is shown to provide an adequate quantitative and qualitative description of the averaged stall line giving a physical explanation of the mechanism involved in the instable region of the compressor map.

The paper presents a theoretical model of convective heat exchangers working with internal pulsating flows. It aims for a better physical understanding of the processes leading to a heat transfer enhancement inside these devices. When the frequency of the pulsation is increased, some geometries exhibit a maximum response, measured by its temperature rise, similar to those obtained in some dynamical resonant systems. The work explains the nature of this characteristic behavior and produces a simplified theoretical model that isolates the main physical features of the fluid dynamics involved. Two characteristic frequencies, measured by its Strouhal numbers, are theoretically found. The first one is associated with the spatial-averaged thermal response of the fluid near the wall and the second with the response of the velocity field. It is found, for a general device, that both Strouhal numbers and the maximum enhancement are mainly defined by the geometry of the device. Finally, the heat transfer enhancement of a straight channel, a backward facing step channel, and a two heated blocks inside an adiabatic channel are used to validate the model. Enhancements calculated with the present model are compared with the results reported in the scientific literature showing a good agreement for the tested cases.

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When considering economic aspects, recent work has shown that 100% hydrogen fueled airplanes will not be feasible for at least several decades. This paper analyzes some of the technological topics involved in the use of hydrogen as additional fuel in conventional kerosene fueled aircrafts. The addition of limited quantities of hydrogen does not require modifying aircraft shape or volume, and therefore, since it has no impact on its aerodynamics, could represent a starting point and smooth transition to 100% hydrogen use situation. An estimate of both engine and aircraft performances has been carried out in order to evaluate the benefits of this technological option. The addition of small quantities of hydrogen to a kerosene fueled gas turbine decreases its specific fuel consumption, no matter what the engine thrust. This decrease in specific fuel consumption will change the performance characteristics such as range, payload, operating empty weight (OEW) and fuel weight for a similar maximum take-off weight (MTOW) of the aircraft.