Research Activities
Pool boiling experiments
Pool boiling has been one of the most investigated two-phase heat transfer phenomena over the last 100 years. It has been proposed to be applied as cooling method for electronic systems, chemical and nuclear reactors, laser generators, air conditioning equipment, etc. Nevertheless, the physical phenomena at the base of this process still remains unknown. Our research on boiling aims to isolate, observe, quantify, and predict these physical phenomena.
Pool boiling on metal foams
A metallic foam is a stochastic cellular structured material having open cells almost homogenous in size and shape, randomly oriented. Among all porous media investigated over the last years, metallic foams seem to be very promising for single-phase and two-phase heat transfer applications, having a large surface to volume ratio and porous density. They could enhance the boiling heat transfer coefficient (HTC) and maximize the critical heat flux (CHF) to extend the upper limit of the high efficient nucleate boiling process.
For more information: 10.1016/j.ijheatmasstransfer.2021.121451
Nanofluids pool boiling
The heat transfer behaviour of nanofluids has been extensively studied by many researchers since the late 1990s. For application of nanofluids under pool boiling conditions, some significant, though quite scattered, enhancements from 10% to 400% of the pool boiling critical heat flux (CHF) have been reported. However, there are also contrasting reports of large CHF degradation. Our research aims to rigorously investigate nanofluid pool boiling in order to clearly assess the potential of this technique for obtaining either higher CHF values directly or to produce coated surfaces, which can enhance the nucleated boiling performance.
Nano particles deposition
Nanofluid pool boiling could be an interesting technique to realize thin particle depositions for nucleate boiling enhancement. In this way it is possible to obtain some stable coatings that consist of a relatively uniform and dense sublayer of nanoparticles on top of which there is a secondary layer of additional nanoparticles that are more sparsely deposited. We characterize the surface topography of the samples before and after nanoparticle coating by means of measures of roughness parameters and of contact angle and by collecting images using a scanning electron microscope (SEM). Preliminary results confirmed these coated surface can improve the water pool boiling heat transfer and CHF.
Vaporization in mini-microfin tubes
Microfin tubes have been deeply investigated and used in many technical applications (e.g., air conditioning and refrigeration systems) since their first introduction in 1977. In fact, they potentially have many advantages with respect to smooth tubes, mainly when applied during refrigerant phase change. Also, a reduction in diameter leads to a reduction of the refrigerant charge, so these mini microfin tubes can provide more compact heat exchangers with a minimization of the whole system refrigerant hold-up, while maintaining high efficiencies. This latter feature could result attractive also to cope with the new and even more stringent national and international environmental regulations that limit the refrigerant charge inventory and its maximum GWP.
Vaporization on innovative composite materials
Composite materials combine light weight, exceptional strength, and stiffness with excellent refractory properties, making them the material of choice for severe-environment applications, such as atmospheric reentry, solid rocket motor exhaust, and disk brakes in high performance military and commercial aircraft, high speed trains, and racing cars. Emerging areas of applications include biomedical devices, aero-engine components, heating elements with 2000 °C temperature, and hardware for metal forming and glass making. They appear to be a viable option for future thermal management devices because they exploit interesting properties having a low density and a relatively high thermal conductivity and so they could be used in replacement of heavy copper spreaders to dissipate high heat fluxes while lowering the weight and volume of the heat sinks.
Contact angle measurements
The contact angle is the most important measurable characteristic to describe wettability, which plays a critical role in two-phase heat transfer. The wettability between fluid and surface influences the critical heat flux (CHF), the bubble contact diameter, and the nucleation frequency during pool boiling heat transfer. It also changes the heat transfer performance and the flow regime during the flow boiling phenomenon. In TEI Research Group we run contact angle measures of fluids on surfaces at room temperature.
Phase change materials (PCM) coupled with metal foams and/or nanoparticle
An ideal PCM should exhibit a high latent heat of fusion, and a high thermal conductivity; furthermore, it should be characterized by high density associated with a small volume change during the melting process and low vapor pressure in the melt. Nevertheless, no available substances able to fulfill all these requirements have been found in nature. Among the different solutions proposed, open-cell metal foams seem to be a promising way to enhance the heat transfer performance of the components that use PCMs, because they show high heat transfer area per unit of volume and high thermal conductivity. Furthermore, also the addition of nanoparticles can overcome the actual technological barriers for efficient latent heat storage, maximize the storable energy and drastically cut the costs.
For more information: 10.1016/j.applthermaleng.2022.119163
Numerical simulations both periodic and stochastic cellular materials
TEI Research Group members have the opportunity to work with an HPC system. It is an ultimate generation 192 parallel core cluster, having Intel E5-2640v3 2.6 GHz processors to run numerical CFD simulations. Thanks to this advanced hardware, it is possibile to afford also complex computational problems.
Metal additive manufacturing for heat transfer enhancement
In recent years, capabilities have been developed for 3D printing of metals with high thermal conductivity. The possibility to print pure copper and copper alloys opens new scenarios in thermal management by offering advantages for free-form design and high heat transfer performance. In the TEI research group, we estimate the friction factor and heat transfer coefficient in 3D printed channels for heat exchangers and refrigeration of components. The analysis of rough surfaces and X-ray computed tomography scan provide a better correlation to investigate the heat transfer performance.
For more information: 10.1016/j.icheatmasstransfer.2022.106128
High power latent thermal energy storage - Model segregation of salt hydrates
Sustainability depends also on energy management and storage, thus it is of paramount importance to find innovative, cost effective and efficient solutions to employ renewable resources. The innovative trait of applying X-ray Computed Tomography (XCT) technique to analyze PCMs could open new possibilities in the world of latent thermal energy storages (LTES), leading to a successful energy transition. XCT could eventually lead to the development of a model of the segregation phenomenon afflicting salt hydrates, which are usually cheaper than other PCMs helping to reduce the overall cost of LTES systems.
For more information: 10.1016/j.est.2023.106726
Lattice frame materials structures for electronic cooling
3D periodic structures can be easily manufactured via printing process. Such geoemtries can fastly increase the heat transfer surface area per volume unit and reduce the time for solidification and melting of an embedded PCM, for instance. The lattice frame structures are applied for the thermal management of high heat flux environments of small surface areas, such as electronic cooling devices.
For more information: 10.1016/j.ijthermalsci.2020.106376
Hybrid Latent Thermal Storages for electronic cooling
The increasing in high computing performance means better heat sinks for CPUs and mother boards. The experimental validation for new heat sinks design, that take into account emulsion-PCM copuling, is the key for the correct optimization of fins design. In TEI Research group, the performance of air heat sinks, improved with PCM matching, are tested for starting the optimization design of pins and fins of the heat sink to achieve the best performance in therms of friction factor and heat transfer coefficient.
Other areas of research
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Latent thermal energy storages for heat pumps.
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Optimization of metallic inserts for improving the thermal behavior of Latent Thermal Energy Storages (LTES).
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Two phase heat transfer and pressure drop of low GWP refrigerants in microfin and smooth tubes.