Low Cost True Monofiber Optical Probe for Local Void Fraction Measurements in Minichannels

Two phase flow inside minichannels is one of the most investigated research topic at present. The measurement of the flow rate parameters is fundamental to characterize the flow pattern and its evolution over time. This paper shows that an optical technique, well-known for large diameter pipes, can be applied to mini channels with a laminar mass flow rate. In particular, a Y-junction mono-fiber optic system with a chamfered tip probe has been built and tested. This method is applied to the local void fraction measurement in a copper capillary pipe with internal diameter of 2 mm and external diameter of 3.00 mm. Different probes have been developed and tested. The accuracy of the method depends on the size, the shape of the tip and on the tip distance from the pipe centre. Different distances and liquid flow rate have been tested. The two-phase flow pattern is also visualized and recorded by a high speed camera (FASTEC Troubleshooter 16000 fps) and post processed with an image analysis technique. A good agreement between the optical and the video signal has been observed

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

Effect of the application of an electric field on the performance of a two-phase loop device: Preliminary results

In the last decade, the continuous development of electronics has pointed out the need for a change in mind with regard to thermal management. In the present scenario, Pulsating Heat Pipes (PHPs) are novel promising two-phase passive heat transport devices that seem to meet all present and future thermal requirements. Nevertheless, PHPs governing phenomena are quite unique and not completely understood. In particular, single closed loop PHPs manifest several drawbacks, mostly related to the reduction of device thermal performance and reliability, i.e. the occurrence of multiple operational quasi-steady states. The present research work proposes the application of an electric field as a technique to promote the circulation of the working fluid in a preferential direction and stabilize the device operation. The tested single closed loop PHP is made of a copper tube with an inner tube diameter equal to 2.00 mm and filled with pure ethanol (60% filling ratio). The electric field is generated by a couple of wire-shaped electrodes powered with DC voltage up to 20 kV and laid parallel to the longitudinal axis of the glass tube constituting the adiabatic section. Although the electric field intensity in the working fluid region is weakened both by the polarization phenomenon of the working fluid and by the interposition of the glass tube, the experimental results highlight the influence of the electric field on the device thermal performance and encourage the continuation of the research in this direction

Closed Loop Pulsating Heat Pipes: Ground and Microgravity experiments

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Numerical Simulation of a Capillary Pulsating Heat Pipe in Various Gravity Conditions

In the last two decades a new concept of capillary heat pipe without wick structures, commonly known as Pulsating Heat Pipe (PHP), entered the domain of the two-phase passive heat transfer devices. The thermal-hydraulic behavior of this mini-channel with alternate heating and cooling zones, evacuated and partially filled with a working fluid, mainly depends on the interplay between phase change phenomena, capillary and gravity, if present, which may assist or damp the fluid motion. Numerous are the attempts to simulate PHPs complex behavior, but only a few of them are capable of complete thermal-hydraulic simulations; in addition, none is able to predict the effects of various gravity levels. Nevertheless, validated numerical simulations can constitute useful tools to complete and support experimental studies, and to help the design of new and better performing PHPs. Thus, a novel lumped parameters numerical code for the transient thermo-hydraulic simulation of PHPs has been developed and validated. It consists of a two-phase separated flow model where capillary slug flow is assumed a priori. A complete set of balance differential equations accounts for homogeneous and heterogeneous phase-changes, as well as thermal and fluid-dynamic phenomena. This novel model shows a very good quantitative and qualitative prediction capability not only when computing the correct measured equivalent thermal resistance, but even when reproducing the experimental trend of temperature when transient conditions are applied. This paper presents the comparison between numerical and experimental data, for a copper PHP (I.D./O.D. 1.1mm/2.0mm) filled with FC-72 tested experimentally in micro-gravity (58th Parabolic Flight Campaign), and hyper-gravity conditions (ESA SYT!2013 Programme

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

An Innovative Enhanced Wall to Reduce the Energy Demand in Buildings

Energy saving in buildings is one of most important issues for European countries. The 40% of the total European energy consumption is due to building Heating and conditioning. Although in the last years many studies have been carried out in order to reach the zero-consumption house by means of passive solar heating, ventilation or thermal insulation, the energy rate due to passive solar heating could be further enhanced. This paper proposes a method for increasing the energy rate absorbed by opaque walls by using a two phase loop thermosyphon connecting the internal and the external façade of a prefabricated house wall. The evaporator zone is located on the outside face and it is irradiated by the sunlight while the condenser zone is placed on the internal face and releases heat to the domestic environment. The temperature differences between the internal and external wall facades are lower than 30 K and the heat fluxes at the evaporator change during the day from 2 up to 7 x 104 W/m2 K. The thermosyphon has been preliminary designed and implanted into a wall for a prefabricated house in Italy. A thermal model of building equipped with the thermosiphon wall has been used in order to evaluate the impact in terms of energy saving and thermal comfort in a real prefabricated low consumption house. The transient behaviour of the building has been simulated day by day during the winter period by using the EnergyPlusTM software. This solution enhances the thermal comfort of the building by keeping the indoor temperature close to the thermal comfort standard for most of the day. The annual saving on the heating energy is higher than 50% in the case of a low consumption buildin

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