Sauro Filippeschi

Pulsated two-phase thermosyphons for electronic equipment thermal control

This work proposes some heat transfer pulsating devices as interesting alternative solution to the LHP (Loop Heat Pipe) and CPL (Capillary Pumped Loop). These devices, named pulsated two-phase thermosyphon (PTPT) by the authors, have been studied, classified and experimentally observed in previous works. In this paper the operating principles of the PTPT devices in comparison with LHPs or CPLs for thermal control applications are illustrated. Although the LHP and CPL operate in a steady state regime of heat transfer and PTPT operates in a stabilized periodic regime, some important comparisons are presented. In conclusion the application of a PTPT could become a possible cheap choice in future in case of the thermal management of electronic devices.

Theoretical analysis of screened heat pipes for medium and high temperature solar applications

A mathematical model is applied to study the cylindrical heat pipes (HPs) behaviour when it is exposed to higher heat input at the evaporator for solar collector applications. The steady state analytical model includes two-dimensional heat conduction in the wall, the liquid flow in the wick and vapour hydrodynamics, and can be used to evaluate the working limits and to optimize the HP. The results of the analytical model are compared with numerical and experimental results available in literature, with good agreement. The effects of heat transfer coefficient, power input, evaporator length, pipe diameter, wick thickness and effective pore radius on the vapour temperature, maximum pressure drop and maximum heat transfer capability (HTC) of the HP are studied. The analysis shows that wick thickness plays an important role in the enhancement of HTC. Results show that it is possible to improve HTC of a HP by selecting the appropriate wick thickness, effective pore radius, and evaporator length. The parametric investigations are aimed to determine working limits and thermal performance of HP for medium temperature solar collector application.

U-PHOS Project: Development of a Large Diameter Pulsating Heat Pipe Experiment on board REXUS 22

U-PHOS Project aims at analysing and characterising the behaviour of a large diameter Pulsating Heat Pipe (PHP) on board REXUS 22 sounding rocket. A PHP is a passive thermal control device where the heat is efficiently transported by means of the self-sustained oscillatory fluid motion driven by the phase change phenomena. Since, in milli-gravity conditions, buoyancy forces become less intense, the PHP diameter may be increased still maintaining the slug/plug typical flow pattern. Consequently, the PHP heat power capability may be increased too. U-PHOS aims at proving that a large diameter PHP effectively works in milli-g conditions by characterizing its thermal response during a sounding rocket flight. The actual PHP tube is made of aluminum (3 mm inner diameter, filled with FC-72), heated at the evaporator by a compact electrical resistance, cooled at the condenser by a Phase Change Material (PCM) embedded in a metallic foam. The tube wall temperatures are recorded by means of Fibre Bragg Grating (FBG) sensors; the local fluid pressure is acquired by means of a pressure transducer. The present work intends to report the actual status of the project, focusing in particular on the experiment improvements with respect to the previous campaign.

Recognition of wall materials through active thermography coupled with numerical simulations

In the framework of historical buildings, wall thickness as well as wall constituents are not often known a priori, and active IR thermography can be exploited as a nonintrusive method for detecting what kind of material lies beneath the external plaster layer. In the present work, the wall of a historical building is subjected to a heating stimulus, and the surface temperature temporal trend is recorded by an IR camera. A hybrid numerical model is developed in order to simulate the transient thermal response of a wall made of different known materials underneath the plaster layer. When the numerical thermal contrast and the appearance time match with the experimental thermal images, the material underneath the plaster can be qualitatively identified.

The U-PHOS experience within the ESA student REXUS/BEXUS programme: A real space hands-on opportunity

U-PHOS (Upgraded PHP Only for Space) is a project developed by a team of students from the University of Pisa with the goal to analyze and characterize the behavior of a Pulsating Heat Pipe (PHP), one of the most attractive two phases passive systems for thermal management in space applications. The PHP consists of a sealed serpentine capillary tube filled with a working fluid. The heat is efficiently transported by means of the combined action of phase change and capillary forces, so no extra equipment is required. The project aims at investigating the thermal response of such a device under a milli-gravity condition, in order to assess its effectiveness in space conditions. U-PHOS is one of the selected experiment of the REXUS/BEXUS programme, which allows European university students to carry out scientific and technical experiments on research rockets and balloons, thanks to a bilateral agency agreement between the German Aerospace Centre (DLR) and the Swedish National Space Board (SNSB) in collaboration with ESA. 19 students from the University of Pisa, with different backgrounds, compose the U-PHOS team. Students had the chance to completely design, build and test the experiment, which will flight up to space in March 2017. This paper intends to describe the work done by the students, their organization and how this experience empowered their careers, from both an academic and professional point of view.

Numerical Analysis of a Paraffin/Metal Foam Composite for Thermal Storage

In the last decade, the use of Phase Change Materials (PCMs) as passive thermal energy storage has been widely studied both analytically and experimentally. Among the PCMs, paraffins show many advantages, such as having a high latent heat, a low vapour pressure, being chemically inert, stable and non-toxic. But, their thermal conductivity is very low with a high volume change during the melting process. An efficient way to increase their poor thermal conductivity is to couple them with open cells metallic foams. This paper deals with a theoretical analysis of paraffin melting process inside an aluminum foam. A mathematical model is developed by using the volume-averaged governing equations for the porous domain, made up by the PCM embedded into the metal foam. Non-Darcian and buoyancy effects are considered in the momentum equation, while the energy equations are modelled with the Local Thermal Non-Equilibrium (LTNE) approach. The PCM liquefaction is treated with the apparent heat capacity method and the governing equations are solved with a finite-element scheme by COMSOL Multiphysics®. A new method to calculate the coupling coefficients needed for the thermal model has been developed and the results obtained have been validated comparing them to experimental data in literature.

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.

Experimental investigation on influence of porous material properties on drying process by a hot air jet

The drying process of porous media is a subject of scientific interest, and different mathematical approaches can be found in the literature. A previous paper by the same authors showed that the celebrated Martin correlation for hot air jet heat and mass transfer yields different degrees of accuracy (from 15% to 65%, increasing at high values of input power) if tested on different fabrics, the remaining conditions being the same. In this paper the fabric drying has been experimentally investigated more in depth. A dedicated experimental apparatus for hot jet drying was assembled and operated, in which a hot jet impinges perpendicularly onto a wet fabric. A calibrated orifice was adopted to measure the jet flow rate, with an accuracy better than 3%. The drying power was determined by continuously weighing with a precision scale a moistened patch exposed to the drying jet. The effect of the time of the exposure and the initial amount of water has been evaluated for each sample. During the hot jet exposure, the temperature distribution over the wet patch has been observed by an infrared thermo-camera. A mathematical model of water transport inside and outside the fabric was developed, in order to evidence the governing transport resistances. The theoretical predictions have been compared with the experimental results, and showed the necessity to modify correlations and models accounting for fabric properties.

Design and experimental analysis of a screened heat pipe for solar applications

This paper summarizes the design, the construction and the preliminary results of a transient and steady state investigation of the heat transfer mechanisms of a horizontal heat pipe (HP). The experiments are performed using a custom-made HP constituted by copper tube with outer diameter and length as 35 mm and 510 mm, respectively, with the inner surface covered by three layers stainless steel mesh wick (100 mesh/inch). Water is used as a working fluid. The evaporator section is heated by electrical resistances wrapped around the tube and the cooling system consists of an insulated water manifold with inner diameter of 39 mm, connected to chilled water bath to maintain the inlet temperature of the circulating cooling water at 25 °C for various heat loads (30-100 W). The aims of this activity is to obtain data to verify the steady state HP analytical model already presented by authors at a fixed filling volume and to determine the effect of the heat transfer load on the heat transfer performance of screen mesh HPs. The heat transfer coefficients are determined using thermocouples on the outer wall and within the core of the HP. The agreement between the analytical results and the preliminary experimental data appears to be very good.

Recent Research Progress in Solar Thermal Conversion Theory and Applications

1Department ofThermal Science and Energy Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China 2Department of Architecture and Built Environment, The University of Nottingham, Nottingham NG7 2RD, UK 3Department of Energy, System, Territory and Constructions Engineering, University of Pisa, 55126 Pisa, Italy 4School of Mechanical and Vehicle Engineering, Beijing Institute of Technology, Beijing 100081, China