Machine Learning Algorithms for Flow Pattern Classification in Pulsating Heat Pipes

Owing to their simple construction, cost effectiveness, and high thermal efficiency, pulsating heat pipes (PHPs) are growing in popularity as cooling devices for electronic equipment. While PHPs can be very resilient as passive cooling systems, their operation relies on the establishment and persistence of slug/plug flow as the dominant flow regime. It is, therefore, paramount to predict the flow regime accurately as a function of various operating parameters and design geometry. Flow pattern maps that capture flow regimes as a function of nondimensional numbers (e.g., Froude, Weber, and Bond numbers) have been proposed in the literature. However, the prediction of flow patterns based on deterministic models is a challenging task that relies on the ability of explaining the very complex underlying phenomena or the ability to measure parameters, such as the bubble acceleration, which are very difficult to know beforehand. In contrast, machine learning algorithms require limited a priori knowledge of the system and offer an alternative approach for classifying flow regimes. In this work, experimental data collected for two working fluids (ethanol and FC-72) in a PHP at different gravity and power input levels, were used to train three different classification algorithms (namely K-nearest neighbors, random forest, and multilayer perceptron). The data were previously labeled via visual classification using the experimental results. A comparison of the resulting classification accuracy was carried out via confusion matrices and calculation of accuracy scores. The algorithm presenting the highest classification performance was selected for the development of a flow pattern map, which accurately indicated the flow pattern transition boundaries between slug/plug and annular flows. Results indicate that, once experimental data are available, the proposed machine learning approach could help in reducing the uncertainty in the classification of flow patterns and improve the predictions of the flow regimes

Postpartum Ovarian Vein Thrombosis

Postpartum ovarian vein thrombosis carries a significant risk of morbidity and mortality if not recognized early and managed appropriately

A Modular Rack for Shared Thermo-Fluid Dynamics Experiments in Reduced Gravity Environment

Abstract Parabolic flights represent an important tool for short space-related experiments under reduced gravity conditions. During the ballistic flight manoeuvres, the investigators have the possibility to operate their experiments, in a laboratory-like environment, where the level of gravity subjected to the experiments repetitively in a series of periods of reduced gravity, preceded and followed by periods of hypergravity. Aboard large aircraft, the duration of this phases varies from approximately 20 s for a 0g flight up to up to 32 s for a Martian g level. A parabolic flight rack able to host experiments concerning thermo-fluid dynamics, has been designed, realized and qualified during the ESA 66th Parabolic Flight Campaign. This microgravity research platform, is the first UK facility available for such investigations, providing a data acquisition system, cooling system and heating system compliant with Novespace requirements

Flow Characterization of a Pulsating Heat Pipe through the Wavelet Analysis of Pressure Signals

Pulsating Heat Pipes are two phase passive heat transfer devices characterized by a thermally induced two phase oscillating flow. The correct detection of the dominant frequencies of such oscillations is fundamental to fully characterize the device thermofluidic operation but the studies available in the literature are very heterogenous and results are often discordant. In this work, the concept of dominant frequency in Pulsating Heat Pipes is thoroughly discussed and defined analytically. The wavelet transform is used to characterize the fluid pressure signal in the frequency domain varying the heat power input at the evaporator and in the condenser zone of a full-scale Pulsating Heat Pipe tested in microgravity conditions. During the slug-plug flow regime, the dominant frequencies falls in the range 0.6–0.9 Hz, showing an increasing trend with the heat load input. The Cross-Correlation reveals that the two signals at the evaporator and at the condenser are very similar. Finally, the instantaneous angle of phase is calculated and lies between 310 and 360 deg. This value can be physically interpreted as a repeatable time shift between the two signals that can be used to evaluate the flow local mean velocity (0.09–0.13 m/s) constituting a valuable alternative to the visualization techniques

Developing flow pattern maps for accelerated two-phase capillary flows

The prediction offlow pattern transitions is extremely important to understand the coupling of thermal andfluiddynamic phenomena in two phase systems and it contributes to the optimum design of heat exchangers. Twophaseflow regimes have been extensively studied under controlled massflow rate and velocity. On the otherhand, less effort has been spent in the literature on the cases where theflow motion is purely thermally inducedand consequently the massflow rate or the velocity of the phases are not known a priori. In the present work,flow pattern transitions and bubble break-up and coalescence events have been investigated in a passive twophase wickless capillary loop, where the massflow rate is intrinsically not controllable. Modified Froude, Weberand Bond numbers have been introduced, considering the actual acceleration of thefluid and the length of thebubble as merit parameters for the transitions. The proposed nondimensional investigation was developed byanalysing experimental data obtained with ethanol and FC-72, as workingfluids, different heat input levels(from 9 to 24 W) as well as three different gravity levels (through a parabolicflight campaign). A new empiricaldiabaticflow pattern map for accelerated two-phase capillaryflows is presented, together with quantitativecriteria for the calculation of theflow regime transitions, defining the physic limits for the bubble coalescenceand break-up. This kind of new regime maps will be useful to the further development of comprehensive de-signing tools for passive two-phase wickless heat transfer devices

Start-up in microgravity and local thermodynamic states of a hybrid loop thermosyphon/pulsating heat pipe

A wickless passive two phase closed loop heat transfer device especially designed for a future implementation on the heat transfer host module of the International Space Station is tested in relevant environment on board a parabolic flight. The tube internal diameter (3 mm) is larger than the static capillary threshold evaluated in normal gravity for this working fluid (FC-72), leading the device to work as a loop thermosyphon on ground and in hyper-gravity conditions, and as a Pulsating Heat Pipe when micro-gravity occurs. Novel start up tests, where the heat load has been provided after the occurrence of microgravity, show that the 20 s microgravity period is enough for the device activation and, most important, that the device activation is purely thermally induced and not affected by the previous acceleration field. Two miniaturized pressure transducers and direct fluid temperature measurement via two micro-thermocouples, allow to provide a detailed insight on the fluid local thermodynamics states both in the evaporator and in the condenser zone during microgravity. It is shown that the two-phase fluid close to the evaporator and the condenser is subjected to several degrees (up to 5 K) of superheating or subcooling. The level of subcooling seems to increase with the heat input level both in terms of temperature difference and in terms of percentage time with respect to the whole microgravity period