Flow and Heat Transfer Characteristics of Supercritical N-Decane in Adjacent Cooling Channels with Opposite Flow Directions

To ensure the safety of a scramjet, an arrangement scheme of adjacent regenerative cooling channels with opposite flow directions is adopted to decrease the maximum wall temperature. Based on extended corresponding-state methods, the flow and heat transfer characteristics of supercritical n-decane in cooling channels with same and opposite flow directions under a pressure of 3 MPa are comprehensively investigated in this paper. Compared to adjacent cooling channels with same flow direction, the local maximum wall temperature in adjacent cooling channels with opposite directions is notably reduced. Moreover, the effects of the heat flux and gravity on the development of flow field are analysed. A pair of recirculation zones is found close to the bottom wall of the cooling channels along the flow direction, the scale of which greatly expands with increasing heat flux. Once the heat flux density reaches a critical value, a phenomenon of flow asymmetry occurs. In addition, the small recirculation zones induced by the buoyancy force narrow when the gravity and heat flux directions remain the same, and the gravity effect could inhibit the generation of small-scale vortices and flow asymmetry.

Spectral graph theory efficiently characterizes ventilation heterogeneity in lung airway networks

This paper introduces a linear operator for the purposes of quantifying the spectral properties of transport within resistive trees, such as airflow in lung airway networks. The operator, which we call the Maury matrix, acts only on the terminal nodes of the tree and is equivalent to the adjacency matrix of a complete graph summarizing the relationships between all pairs of terminal nodes. We show that the eigenmodes of the Maury operator have a direct physical interpretation as the relaxation, or resistive, modes of the network. We apply these findings to both idealized and image-based models of ventilation in lung airway trees and show that the spectral properties of the Maury matrix characterize the flow asymmetry in these networks more concisely than the Laplacian modes, and that eigenvector centrality in the Maury spectrum is closely related to the phenomenon of ventilation heterogeneity caused by airway narrowing or obstruction. This method has applications in dimensionality reduction in simulations of lung mechanics, as well as for characterization of models of the airway tree derived from medical images.