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

SINGLE LOOP PULSATING HEAT PIPE WITH NON-UNIFORM HEATING PATTERNS: FLUID INFRARED VISUALIZATION AND PRESSURE MEASUREMENTS

Abstract. A novel Single Loop Pulsating Heat Pipe (SLPHP) filled at 60% filling ratio with pure ethanol, with an inner diameter of 2mm is tested in Bottom Heated mode varying the heating power. The system is designed with two sapphire tubes mounted between the evaporator and the condenser allowing simultaneous fluid flow high-speed visualizations and IR analysis. Furthermore, two highly accurate pressure transducers carry out local pressure measurements just at the ends of one of the sapphire inserts. Additionally, three heating elements are controlled independently, in such a way to heat up the device varying the distribution of the heating location at the evaporator. It is found that peculiar heating distributions promote the slug/plug flow motion in a preferential direction, increasing the overall performance of the device. Pressure measurements point out that the flow patterns are strictly related to the pressure drop between the evaporator and the condenser. Furthermore, the IR visualization highlights interesting phenomena related to the liquid film dynamics during the device operations, which represent a very useful information for future numerical modeling of Pulsating Heat Pipes

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

Effect of the Ambient Temperature on the Start-Up of a Multi-Evaporator Loop Thermosyphon

Abstrac