Unforeseen delays in procuring the required components and obtaining the safety authorisation have occurred. Consequently, the initial project plan has been restructured into the three main work packages (WPs), for which the description of the research activities and main achievements is provided here below.
WP1: Design, construction and validation of the cryogenic chilldown test rig using liquid nitrogen
A new experimental facility has been realised in this WP. As shown in Figure 2, it consists of an open-loop cryogenic transfer line operating with liquid nitrogen. In particular, 180 litres of liquid nitrogen are stored in a cryogenic tank at 20 bar. The tank is connected to the experimental transfer line via a cryogenic hose, and a pressure regulator is connected to the tank to set the discharge pressure downstream of the tank at a maximum of 3 bar. Then, an on-off cryogenic valve is opened, allowing the liquid nitrogen to flow into the transfer line, which is composed of SS316 tubes of 1/2 inch diameter. The liquid nitrogen flow first crosses the test section (currently it's a 40 cm long SS316 tube of 1/2 inch diameter equipped with 9 thermocouples installed on the external wall to monitor the temperature of the test section) and, then, a vaporiser (nominal installed power equal to 7.2 kW) before being vented outside using an external ventilation line. The vaporiser is a 1.8 m aluminum pipe (external diameter equal to 2 inches) equipped with 12 strip heaters (600 W each, 240 VAC) arranged in groups of three elements controlled via 4 dedicated temperature PID controllers, which keep the wall temperature of the vaporiser at 175 °C during the investigation. The cryogenic transfer line is insulated using polyurethane tube insulators (2 cm thickness), except the test section, which is enclosed in a vacuum chamber for thermal insulation (vacuum level of about 1 Pa). The vaporiser is insulated using ceramic wool insulation (2 cm thickness). Along the transfer line, there are 3 checkpoint stations to monitor pressure and temperature; also, a safety valve is installed to release the fluid when the pressure in the transfer line exceeds 4.5 bar. Between the vaporiser and the venting, a mass flow meter records the mass flow rate within the transfer line, while a cryogenic needle valve is used to regulate the mass flow rate. The entire facility is intended for cryogenic quenching experiments. The venting line is already available on the external wall of the building and has been used in the past for the venting of flue gases (combustion experiments).
Finally, liquid nitrogen chilldown tests were performed to validate the facility and define all the data processing routines to quantify the two main performance parameters (see Figure 3): the chilldown time (about 60 s) and the chilldown efficiency (about 36.7%).
WP2. Development of high-resolution surface thermography based on temperature-sensitive coatings.
In this WP, high-resolution surface thermography based on temperature-sensitive coatings (TSCs) has been developed. In this regard, the investigation has been structured into two steps.
As first step, we develop four main TSC formulations based on four fluorescent dyes: Ruthenium(II) dichlorotris(1,10-phenanthroline) hydrate (from now on referred to as Ru(phen)3Cl2), Europium(III) thenoyltrifluoroacetonate trihydrate 95 %(from now on referred to as Eu(TTA)) and Platinum(II) meso-tetra(pentafluorophenyl)porphine (from now on referred to as Pt(TFPP)) and CsPbBr3 halide perovskite quantum dots (CsPbBr3-QDs). Each of them is mixed with suitable binders and solvents: Polyacrylic acid (PAA) or polymethyl methacrylate (PMMA) as binders; ethanol or toluene as solvents. They are coated (final thickness < 3 μm) on the surface to be thermally sensed; an optical cage system is installed to illuminate the deposited TSC with a 1.1W UV lamp via a long-pass dichroic beam slitter with a cut-on wavelength of 490 nm. The light emitted by the TSP coating is recorded with a high-speed camera equipped with a macro lens 105 mm set at f/2.8 aperture, together with a 72 mm extension tube. Figure 4 shows the emission spectra of the four TSC formulations. The development of the technique (including the data processing routines from raw camera image dataset to temperature/heat flux/heat transfer coefficient datasets) is performed in the more comfortable temperature range 15 °C - 120 °C. The experimental assessment of the defined formulations is then performed via pure conduction and pool boiling tests through two separate investigations. Both investigations used electrically heated coated metal foil and consisted of a calibration study and a dynamic response analysis, aiming at a steady-state and dynamic performance assessment of TSC-derived thermography. The pure conduction study focuses on improving the spatial resolution to about 18 µm/px at fixed temporal resolution (500 µs), and evaluates the impact of denoising on the thermal resolution (kept below 130 mK). The pool boiling study focuses on improving the temporal resolution from 500 µs to 100 µs. Figure 5 shows a typical transient dynamics captured by TSC-based thermography in comparison with high-speed infrared thermography.
The second step was the application of the developed TSC-based thermography to a lower temperature range, i.e. from -40 °C to 15°C. A new investigation is performed for this purpose, by keeping the resolution parameters high: acquisition speed at 3 kHz (330 µs of temporal resolution); spatial resolution at 35 µm/px. In particular, the investigation assesses the steady-state and the dynamic performance of Ru(phen)3Cl2, Eu(TTA) and CsPbBr3-QDs based TSCs. A steady-state calibration study is first carried out for a performance assessment in terms of thermal sensitivity, accuracy and resolution (see Figure 6). Results show that the colder, the better (higher thermal sensitivity, higher accuracy, higher thermal resolution). Then, droplet impact tests on supercooled surfaces are performed to evaluate the dynamic response. The temperature transients during the droplet impact are nicely captured (see Figure 7), enabling quantification of temperature/heat flux/heat transfer coefficient dynamics.
WP3. Boiling heat transfer enhancement using cryogenic simulants
This WP explores boiling heat transfer enhancement via femtosecond laser texturing using cryogenic simulants. Such a choice is based on heat transfer similarity (cryogenic chilldown is a reverse transient boiling) and fluid similarity (in terms of low surface tension, high wettability, low boiling point and low latent heat). In this regard, tests have been carried out using two experimental setups for pool and flow boiling testing, respectively. In pool boiling with the cryogenic simulant HFE-7100, the role of high-aspect-ratio femtosecond-laser-textured grooves is investigated. As shown in Figure 8, the spacing between two consecutive grooves significantly impacts the bubble dynamics in terms of artificial nucleation bubble coalescence control. With respect to the bare surface (blue curve denoted as FS), results show that a huge +100% enhancement of the boiling heat transfer coefficient is obtained by reducing the groove spacing to 100 µm (green curve denoted as TS1).
In flow boiling, using the fluid Novec 649 as cryogenic simulant, the effect of conical cavities (see Figure 9) is investigated in both horizontal channel orientations, taking into account the role of the cavity aspect ratio. In particular, two aspect ratio (AR) configurations are assessed: low aspect ratio (LAR) with AR=2.5); high aspect ratio (HAR) with AR=7.5. Results are shown in Figure 10: deepening the conical cavity and increasing the aspect ratio improves the heterogeneous nucleation, producing a significant +225% heat transfer coefficient enhancement at the boiling onset due to the easier bubble nucleation; by increasing the heat flux and moving towards nucleate boiling regimes, the heat transfer coefficient enhancement is still noticeable (+60%). Furthermore, the test cell is designed to host a metallic foil electrically fed to produce joule heating and operate as the boiling surface. This allows the use of high-speed IR thermography (this is a novelty aspect!), enabling new heat transfer dynamic characterisation (in space, time and frequency) of flow boiling via advanced modal decomposition techniques.
