Davide Ercole

Nano-PCMs for passive electronic cooling applications

The present work aims at investigating a new challenging use of oxide (TiO2, Al2O3, etc.) nanoparticles to enhance the thermal properties: thermal conductivity, specific heat, and latent heat of pure paraffin waxes to obtain a new class of Phase Change Materials (PCMs), the so-called nano-PCMs. The nano-PCMs were obtained by seeding different amounts of oxide nanoparticles in a paraffin wax having a melting temperature of 45°C. The thermophysical properties such as latent heat and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCM. Finally, a numerical comparison between the use of the pure paraffin wax and the nano-PCM in a typical electronics passive cooling device was implemented. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures, melting time and junction temperature were reported. Moreover, a comparison with experimental results was also performed.

HEAT TRANSFER BEHAVIOURS OF PARALLEL SQUARED CHANNEL SYSTEM FOR LATENT HEAT THERMAL ENERGY

Systems for the energy management and storage are broadly used in many industrial and commercial applications to supply thermal energy. The energy demand in these applications is not always constant and therefore a Thermal energy storage (TES) system represents a good opportunity to cover such problem. Lately TES systems for thermal applications, such as space and water heating, air-conditioning, cooling, etc. have received much attention. Solar energy is an important renewable energy source that is increasingly used. One of the main drawbacks in the solar energy application is its working periodic time. therefore, solar applications require TES system to store exceed energy and release it when it is demanded. Although the TES systems could be used for many applications, it is particularly recommended for solar systems. TES can improve the efficiency of a solar system because it works as a thermal buffer to cover a mismatch between the energy supply and energy demand. Various TES systems have been investigated for heating and cooling applications, industrial applications and power plants. In TES system, energy is stored by changing the temperature of a storage medium or employing the latent heat of Phase Change Material (PCM). For a TES system, it is important to have a charging velocity rather high, because for an assigned system size, it is possible to obtain greater amount of stored energy in less time. therefore, the heat transfer between the working fluid and the TES system should be improved by increasing the surface exchange area. A solution of this problem could be a hybrid system realized partially by PCM in solid matrix where a working fluid passes through. The system have a honeycomb shape, filled with PCM in checkerboard way. In the present work a computational investigation of transient thermal control device using Phase Change Material (PCM) is accomplished. The system is a honeycomb solid checkerboard matrix filled with Phase Change Material. The honeycomb is set of different parallel squared channels and half of them are filled with PCM and the others are passed through by the working fluid. Various configurations are investigated for different channels per unit of length (CPI), different heat fluxes and inlet velocities. A comparison between the direct honeycomb model and a porous medium model is made. The porous medium is modelled with the Darcy law and to evaluate the heat exchange between the solid and the fluid zones a Local-Thermal Non-Equilibrium assumption is used. The analysis have the aim of estimate an optimized configuration in term of channels per unit of length (CPI) as a balance between pressure drop and heat transfer rate inside the honeycomb system. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of melting time, temperature fields, stored energy as function of time are presented.

Nano-PCMs for electronics cooling applications

The present work aims at investigating a new challenging use of Aluminum Oxide (Al2O3) nanoparticles to enhance the thermal properties (thermal conductivity, specific heat, and latent heat) of pure paraffin waxes to obtain a new class of Phase Change Materials (PCMs), the so-called nano-PCMs. The nano-PCMs were obtained by seeding 0.5 and 1.0 wt% of Al2O3 nanoparticles in two paraffin waxes having melting temperatures of 45 and 55 °C, respectively. The thermophysical properties such as specific heat, latent heat, and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCMs. Furthermore, a numerical comparison between the use of the pure paraffin waxes and the nano-PCMs obtained in a typical electronics passive cooling device was developed and implemented. A numerical model is accomplished to simulate the heat transfer inside the cavity either with PCM or nano-PCM. Numerical simulations were carried out using the ANSYS-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures and melting time were reported and discussed.Copyright

A Numerical Analysis on a Compact Heat Exchanger in Aluminum Foam

A numerical investigation on a compact heat exchanger in aluminum foam is carried out. The governing equations in two-dimensional steady state regime are written in local thermal non-equilibrium (LTNE). The geometrical domain under investigation is made up of a plate in aluminum foam with inside a single array of five circular tubes. The presence of the open-celled metal foam is modeled as a porous media by means of the Darcy-Forchheimer law. The foam has a porosity of 0.93 with 20 pores per inch and the LTNE assumption is used to simulate the heat transfer between metal foam and air. The compact heat exchanger at different air flow rates is studied with an assigned surface tube temperature. The results in terms of local heat transfer coefficient and Nusselt number on the external surface of the tubes are given. Moreover, local air temperature and velocity profiles in the smaller cross section, between two consecutive tubes, as a function of Reynolds number are showed. The performance evaluation criteria (PEC) is assessed in order to evaluate the effectiveness of the metal foam.

Numerical Investigation on the Effect of Aluminum Foam in a Latent Thermal Energy Storage

In this paper, a numerical investigation on Latent Heat Thermal Energy Storage System (LHTESS) based on a phase change material (PCM) is accomplished. The geometry of the system under investigation is a vertical shell and tube LHTES made with two concentric aluminum tubes. The internal surface of the hollow cylinder is assumed at a constant temperature above the melting temperature of the PCM to simulate the heat transfer from a hot fluid. The other external surfaces are assumed adiabatic. The phase change of the PCM is modeled with the enthalpy porosity theory while the metal foam is considered as a porous media that obeys to the Darcy-Forchheimer law. The momentum equations are modified by adding of suitable source term which it allows to model the solid phase of PCM and natural convection in the liquid phase of PCM. Both local thermal equilibrium (LTE) and local thermal non-equilibrium (LTNE) models are examined. Results as a function of time for the charging phase are carried out for different porosities and assigned pore per inch (PPI).The results show that at high porosity the LTE and LTNE models have the same melting time while at low porosity the LTNE has a larger melting time. Moreover, the presence of metal foam improves significantly the heat transfer in the LHTES giving a very faster phase change process with respect to pure PCM, reducing the melting time more than one order of magnitude.Copyright