Mauro Mameli

Thermal Simulation of a Pulsating Heat Pipe: Effects of Different Liquid Properties on a Simple Geometry

The pulsating heat pipe (PHP) is essentially a two-phase heat transfer device for low heat power applications (heat sinks, electronic cooling, etc.). Although it is a simple, cheap, and flexible structure, it is ruled by very complex physics, and a robust, validated simulation tool is still missing. In the present work the basic numerical model by Holley and Faghri (2005) has been updated with the latest fluid properties database and with the latest nondimensional heat transfer correlations in order to make it suitable for different working fluids. Good agreement between numerical results and experimental data coming from a single-loop PHP operating with ethanol is shown and, using a single “tuning” parameter, that is, the liquid film thickness around a vapor slug, which needs to be further experimentally investigated, the final goal of building a design tool for the PHP construction and implementation is getting closer.

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.

Non-Uniform Onset of Nucleate Flow Boiling of R-134a Inside a Glass Minichannel

The present work is an experimental investigation of the incipient boiling of R134a inside a circular glass minichannel mounted horizontally and equipped with a series of transparent indium tin oxide heaters. The effects of heat flux input levels and refrigerant mass fluxes on the onset nucleate boiling process and on the saturated boiling heat transfer rate are quantitatively explored. The flow pattern visualizations, carried on by means of a high-speed camera, show that the nucleation process is oddly non-uniform: the first vapor bubbles are always generated on the upper side of the tube and lead to a first wall temperature drop. A further increase in the heat flux values results in an increased wall superheat until bubble nucleation also originates on the lower side of the tube, causing a second wall temperature drop. Finally, at higher heat input levels, the boiling process becomes uniformly distributed on the inner tube surface. This phenomenon occurred also after a 180° rotation of the glass tube, and, after a critical analysis of the potential origins, it remains presently unexplained. An evaluation of heat transfer coefficients for low vapor quality regimes is finally presented.

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.

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.