Walter Grassi

Motivation and results of a long-term research on pool boiling heat transfer in low gravity

Abstract This paper summarises the main results of a long-term research, begun more than ten years ago, about the influence of gravity and electric fields on heat transfer. After a somehow detailed analysis of the impact of heat transfer on modern society and of the motivation of low gravity research on this subject, the authors describe their own research work on the effects of gravitational and electric forces on single-phase convection and pool boiling. This research has the twofold aim to investigate the basic mechanisms of convective heat transfer, without the masking effect of gravity, and to identify methods to make free convection possible also in the absence of buoyancy, for space applications. It has been experimentally shown that the application of an external electric field generally enhances the heat exchange between a heated wire and a liquid pool. The single-phase heat transfer coefficient is improved, the nucleate boiling region is extended to higher heat fluxes, by increasing the critical heat flux, CHF, as well as the heat transfer rate in film boiling is augmented. The convective heat exchange is an increasing function of gravity, therefore heat transfer generally deteriorates in low gravity. The application of a sufficiently intense electric field restores the same value of heat transfer coefficient and critical heat flux measured on earth, thus demonstrating the progressive overwhelming of the electrical force on the buoyancy one.

Experimental study on rising velocity of nitrogen bubbles in FC-72

Abstract In this work, the rising velocities of gas bubbles in a still liquid are measured and compared with available theories. In order to separate the mechanical effects from the thermal and mass exchange ones in bubble dynamics, adiabatic two-phase flow conditions were established by injecting gas (nitrogen) bubbles in a fluoroinert liquid (FC-72) at ambient temperature and pressure through an orifice (about 0.1 mm diameter) drilled on a generatrix of a horizontal tube. Bubble size, aspect ratio, detachment frequency, velocity and frequency of shape oscillations were measured by processing of high speed video images (at 1500 fps). A sensible steady oscillation of velocity, with a amplitude up to 20% of the mean value, was evidenced after the initial acceleration region. This oscillation was well correlated with the one in aspect ratio, thus providing evidence of the separate influence of this last parameter on drag coefficient. Available correlations did not give fully satisfactory results in predicting the mean rising velocity, showing a general tendency to underprediction. Sensible wake effects were excluded. Finally, the frequency of shape oscillation and the mean aspect ratio were compared with available models, evidencing their limitations.

Effect of an Externally Applied Electric Field on Pool Film Boiling of FC-72

The paper reports the results obtained during an experimental study on film boiling on wires in the presence of an externally imposed electric field. The arrangement allowed achieving various combinations of pressure and subcooling independently of the environmental conditions. The test section was an electrically heated platinum wire with diameters of 0.1 and 0.2 mm. The working fluid was FC-72. The results showed that two different film boiling regimes, separated by an additional boiling transition, can exist in the presence of an electric field. The first regime, at low wire superheat, was strongly influenced by the electric field, showing a remarkable heat transfer enhancement with increasing voltage. The second regime, at higher superheat, was weakly dependent on the field strength and almost coincident with the zero field one. These results are analogous to the ones previously obtained using R113 as working fluid. The reasons for the occurrence of the transition were investigated. A simple model of the effect of the electric field on the interface behavior was developed, taking into account the variation of the thickness of the vapor layer with the increase of the heat flux, thus providing a possible explanation for the occurrence of the transition.

Nucleate pool boiling in the presence of an electric field: Effect of subcooling and heat-up rate

• The purpose of the paper is to experimentally assess the effect exerted by subcooling degree and heat-up rate (i.e., the rate of increase of thermal flux) on the phenomenon of nucleate boiling in the presence of an electric field. To this aim, an experimental facility was set up to investigate pool boiling on a heated platinum wire of 0.2-ram diameter. The working fluid was R-113 (C2C13F3). A cylindrical electric field (up to 21 MV/m at the heater surface) was imposed. The power was increased along a linear ramp, at a rate ranging from 2.5 mW/s to 6 W/s. Subcooling degrees up to 22 K (Ja = 0.146) were investigated. The results of zero-field measurements are in agreement with previous ones in the literature. The effect exerted by the electric field consists mainly of a strong increase in critical heat flux, whereas, for a given heat flux, the nucleate boiling performance can be either weakly degraded or slightly enhanced. The critical heat flux was found to remain nearly constant with increasing transient velocity up to a given heat-up rate and then to increase with it. The effect of subcooling on nucleate boiling is quite complex but, in any case, quite weak in the entire investigated range.

Heat Transfer Enhancement by Electric Fields in Several Heat Exchange Regimes

Abstract:  With the present article, the authors summarize over 15 years of work dedicated to studying the effects of the electrical and the gravitational force fields on two‐phase and single‐phase thermofluid dynamics. Results obtained on several microgravity platforms are presented and the role played by the electric field in the different heat transfer processes is analyzed. In particular, the regimes of nucleate boiling and film boiling are treated. Also, the parameters controlling the liquid–vapor interface instability and the main consequences regarding the critical heat flux (CHF) are outlined. In the final section, the promising technique of ion injection for efficient heat transfer enhancement in single‐phase liquids is described, together with the main results obtained under different flow regimes and geometries. The following dielectrics were compared: R113, Vertrel XF, and FC‐72 for the pool boiling research; again FC‐72 and HFE‐7100 for the single‐phase convection heat transfer experiments.

Heat Transfer Correlations for Turbulent Mixed Convection in the Entrance Region of a Uniformly Heated Horizontal Tube

Flow of perfluorohexane in the entry region of a uniformly heated horizontal cylindrical duct was studied in a regime of weakly turbulent mixed convection. Heat transfer coefficients were measured at five cross sections along the heated length and various values of flow rate and heat flux were imposed. A different thermofluid-dynamic behavior was observed between the upper and lower sides of the pipe. Correlations of the Nusselt number for developed and developing flow were proposed, showing agreement with the experimental data within the 10% deviation band.

Heat transfer augmentation by ion injection in an annular duct

The thermofluid dynamic effects of ion injection from sharp metallic points added perpendicularly to the inner wire of a short horizontal annulus were experimentally investigated. A dielectric liquid (FC-72 by 3M) was weakly forced to flow in the duct, which was uniformly heated on the outer wall. A d.c. voltage as high as 22 kV was applied to the inner electrode, while the heated wall was grounded. Both the laminar and the turbulent mixed convection regimes were obtained, varying the imposed flow rate. Once an electric field is applied, the flow is dramatically modified by the jets of charged particles, which transfer their momentum to the neutral adjacent ones. Different injection strengths were obtained on the emitters, because the shape of the point tips was not controlled at the micro-scale. Nusselt number distributions were obtained azimuthally and longitudinally, monitoring the wall temperatures. In all cases, heat transfer turned out greatly enhanced in the proximity of the emitters, without a significant increase in pressure drop through the test section and with a negligible Joule heating, making this technique very attractive for application in compact heat exchangers.Copyright

Pool Film Boiling Experiments on a Wire in Low Gravity

Abstract: This paper reports preliminary results for pool film boiling on a wire immersed in almost saturated FC72 recently obtained during an experimental campaign performed in low gravity on the European Space Agency Zero‐G airplane, (reduced gravity level 10−2). This is part of a long‐term research program on the effect of gravitational and electric forces on boiling. The reported data set refers to experiments performed under the following conditions: (1) Earth gravity without electric field, (2) Earth gravity with electric field, (3) low gravity without electric field, and (4) low gravity with electric field. Although a decrease of gravity causes a heat transfer degradation, the electric field markedly improves heat exchange. This improvement is so effective that, beyond a certain field value, the heat flux is no longer sensitive to gravity. Two main film boiling regimes have been identified, both in normal and in low gravity: one is affected by the electric field and the other is practically insensitive to the field influence.

ELECTRIC FIELD EFFECTS ON FILM BOILING ON A WIRE

Experimental results concerning pool film boiling on a wire under the action of an externally imposed electric field are reported. The working fluid was saturated R113 at atmospheric pressure, the heaters were platinum wires of 0.2, 0.3, and 0.6 mm diameter. An electrostatic field of cylindrical geometry was created around the wire by means of a 60-mm-diameter cylindrical cage in order to assess electrohydrodynamic (EHD) effects on boiling. Voltages up to 15 kV d.c. were applied. The results showed that two different film boiling regimes, separated by an additional boiling crisis, can exist in the presence of an electric field. The first regime, at low wire superheat, is strongly influenced by the electric field, showing a remarkable heat transfer enhancement with increasing voltage. The second one, at higher superheat, is weakly dependent on the field strength and almost coincident with the zero field one. The two film boiling regimes are separated by a transition characterized by a hysteresis cycle, sim...

Analysis of thermodynamic losses in ground source heat pumps and their influence on overall system performance

The present work aims at identifying the relative influence of GSHP subsystems (viz. ground source, earth heat exchangers, heat pump unit, pumping devices) on the overall efficiency and the limits to which technological improvements should be pushed (because, beyond these limits, only minor benefits may be achieved). To this end, an analysis of thermodynamic losses is conducted for a case study, followed by a sensitivity analysis on the heat pump unit thermal performance. Primary energy consumptions of nine configurations with different combinations of ideal and real subsystems are compared. The completely ideal system is used as the reference to normalize energy consumptions and obtain a dimensionless efficiency parameter. The results show that – when a proper design methodology is employed – the performance of the borehole heat exchangers slightly affects the overall efficiency. On the contrary, the thermal response of the ground and the thermal and hydraulic performances of the heat pump unit are key factors. Finally, a sensitivity analysis is conducted by increasing the heating and cooling efficiencies of the heat pump device.