Daniele Testi

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

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.

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 dc 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 microscale. 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.

Augmentation of Heat Transfer on the Downward Surface of a Heated Plate by Ion Injection

Abstract:  The heat transfer characteristics of an electrohydrodynamically induced submerged impinging liquid flow are studied in a point‐plane configuration. The working fluid, HFE‐7100, is a weakly polar dielectric. Thermofluid‐dynamic conditions as close as possible to the microgravity ones are obtained by heating a confined, downward‐facing plate. Heat transfer coefficients on the plate can be stably enhanced by over 10 times, with negligible additional work, by means of the ion injection technique.

On Sustainable and Efficient Design of Ground-Source Heat Pump Systems

This paper is mainly aimed at stressing some fundamental features of the GSHP design and is based on a broad research we are performing at the University of Pisa. In particular, we focus the discussion on an environmentally sustainable approach, based on performance optimization during the entire operational life. The proposed methodology aims at investigating design and management strategies to find the optimal level of exploitation of the ground source and refer to other technical means to cover the remaining energy requirements and modulate the power peaks. The method is holistic, considering the system as a whole, rather than focusing only on some components, usually considered as the most important ones. Each subsystem is modeled and coupled to the others in a full set of equations, which is used within an optimization routine to reproduce the operative performances of the overall GSHP system. As a matter of fact, the recommended methodology is a 4-in-1 activity, including sizing of components, lifecycle performance evaluation, optimization process, and feasibility analysis. The paper reviews also some previous works concerning possible applications of the proposed methodology. In conclusion, we describe undergoing research activities and objectives of future works.

Induction of waves on a horizontal water film by an impinging corona wind

The electric wind produced by corona discharge of a high-voltage electrode in air is employed for destabilizing a horizontal water film. In wire-to-plane geometry, the phenomenon is characterised by current versus voltage curves and visual observations of the onset of free-surface oscillations. The effect of the following parameters is examined for both positive and negative coronas: distance between the wire and the film (S), film thickness (h), wire diameter (phi) and composition, applied voltage (HV), and relative humidity (RH). The free-surface destabilisation is retarded by increasing d and phi and is insensitive to h in the tested range. The onset of corona discharge is predicted by Peeks law and compared with the experimentally observed threshold. In negative corona discharge, the current values are higher and the film is destabilised at lower HV than in positive polarity. Humidity tends to decrease the corona current at a given HV. Correlations are proposed for the current-voltage curves, in terms of the mean electric field in the inter-electrode gap and of RH, satisfactorily agreeing with the experimental data. Both positive and negative corona currents turn out to be stable for days of operation. The power loss by corona discharge is in any case lower than 12 W. Wave induction on the liquid-gas interface can effectively enhance heat and mass exchange between the two phases.

Development of a High-Performance Heat Sink for the International Space Station: Hydraulic and Thermostructural Analysis

The possibility of exploiting an electrohydrodynamic (EHD) technique of heat transfer enhancement for obtaining a high-performance heat sink is investigated in this work. The proposed heat sink is an evolution of a water-cooled cold plate (CP), designed by Daimler-Benz Aerospace (DBA) for the International Space Station (ISS). The dielectric liquid considered for the design process is perfluorohexane, also known as FC-72, a widely-used refrigerant, space-qualified, and present on the Japanese Experiment Module (JEM) of the ISS. The mechanical interfaces for accommodation on the ISS have been taken into account, along with the dynamic loads typical of the critical launch phase, defined by a given power spectral density (PSD) curve. The thermostructural behavior of the CP has been evaluated by the finite element method (FEM). The numerical analysis has been validated on the reference DBA model, whose performance data are available. The hydraulic results are based on simple calculations of pressure drops in ducts, arranged in series or in parallel. The step-by-step conceptual evolution to the final design solution is described in detail. Also, it is explained how to employ the global thermal resistance reduction for decreasing the pumping power, increasing the heat load or reducing volume and weight of the CP.Copyright

Thermal Characterization of Energy Pile Dynamics

The heat transfer process in energy piles is strongly affected by the heat capacity of such foundation elements. This phenomenon is more pronounced for energy piles compared to borehole heat exchangers, because of the lower slenderness of the former compared to the latter, and involves axial thermal gradients. In literature, capacity effects of energy piles and their transient thermal performance have not been analysed in depth. Looking at such challenge, this paper investigates the dynamic thermal performance of energy piles at short-to-medium time scales. The work analyses the results of almost thirty 3D finite element simulations of an energy pile equipped with 3-U ducts by varying: (i) the velocity of the fluid circulating in the ducts, (ii) the slenderness ratio of the pile, (iii) the radial position of the ducts, and (iv) the boundary condition characterizing the uppermost surface of the model. Simulation results are analysed to identify for which times, geometries, and operative conditions the energy pile can be modelled with a 2D geometry, instead of a full 3D geometry. Our analysis highlights a limited relevance of the axial effects during the transient period in any tested configuration. These results are functional to the application of simplified analytical models and design criteria for energy piles.