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The study of two-phase flow instability is of interest in all fields encountering flow boiling. Compact systems relying on flow boiling due to its superior transport capabilities of heat and mass are prone to several modes of flow instability, which adversely affect the safety and performance of systems. As such, understanding of the flow instability is critical to the design and operation of compact flow boiling systems. In current paper, the instability caused by channel-to-channel interaction is investigated experimentally in a test section with 6 parallel small rectangular channels having hydraulic diameter of Dh = 526.2 μm by using deionized water as working fluid. Boiling leads to very asymmetrical flow distribution within the 6 channels, which results in the simultaneous existence of different flow regimes along the transverse direction of test section. Time traces of pressure drop for different channels and operation conditions are analyzed to explore the properties of channel-to-channel interaction by using spectral analysis. Bubble dynamics including nucleation and growth give rise to flow fluctuations in individual channel, and the time intervals between flow fluctuations are caused by a series of time-scale self-convolutions of bubble duration. Based on the aforementioned mechanism, the time intervals between flow fluctuations can be predicted statistically by the gamma distributions.

The design of a nuclear thermal propulsion (NTP) reactor based on low-enriched uranium (LEU) requires additional moderator elements in the core to physically meet the critical requirements. This design softens the core energy spectrum and can provide more thermal neutrons for the fission reaction, but the heat transfer characteristics between the fuel and moderator assembly are more complex. Aiming at the typical LEU unit design, the heat transfer mathematical model is established using the principle of heat flow diversion and superposition. The model adopts the heat transfer relationship based on STAR-CCM+ simulation rather than the empirical expression used in the past literature to improve the applicability of the model. The heat transfer coefficients in the proposed model are evaluated under different Reynolds numbers and thermal power. The deviations between the proposed model and CFD simulation are analyzed. The results show that the calculation of the heat transfer coefficient between the proposed model and the CFD simulation maintains a good consistency, most of which are within 10%. It may provide a reliable and conservative temperature estimation model for future LEU core design.