Lucio Araneo

Setting up a PDPA system for measurements in a Diesel spray

A PDPA system was set up, optimised and used to measure the time resolved characteristics of the droplets inside a spray produced by a common-rail diesel fuel injection system. Some preliminary tests are performed with gas flows to optimise the optical set-up. Parametric studies are performed to gain an understanding of the particle density limits of the system, and their dependence on PDPA system parameters. Then the diesel spray produced by a single-hole injector is measured, with the fuel pressure ranging from 500 to 1300 bar, gas density in the test chamber ranging from ambient conditions to 40 kg/m3. Fuel and gas temperature were 25oC. Beam waist size is reduced to the minimum value allowed by the optical stand-off of the spray enclosure. Receiver lens focal length is similarly reduced. Receiver slit width, which is found to have a dramatic effect on the detection of droplets during the injection period, was tested in the range from 100um to 25um. Tests performed with two different slit heights are tested, respectively 1mm and 50 µm, show that this parameter has m, show that this parameter has minimal effect on performance. PMT voltage (gain) is held to a moderately low value between 400 and 500 volt and the laser power between 400 and 800 mW in the green line. Optimum burst threshold is found to obtain the best quality data regardless of background level, which varies greatly in high-density pulsed sprays.

Correction method for droplet sizing by laser-induced fluorescence in a controlled test situation

The linear sizing technique is used to evaluate the accuracy of a new correction strategy to limit scattering matters. The ratio of the elastic scattered light with fluorescence emission over monodispersed microspheres homogenously mixed in water is measured. Three cells filled with different concentrations of these fluorescent particles are illuminated with a thin laser beam that is moved inside the cell from the front to the back. A comparison of the linear sizing technique with and without correction is presented and shows that the correction enables the measurement of higher concentrations; the limits of the correction method are also shown. These results could be a potential step to apply this technique to sprays.

Suppression pool testing at the SIET laboratory: experimental investigation of critical phenomena expected in the Fukushima Daiichi suppression chamber

In the unlikely event of a nuclear power plant long duration station black-out, as in the Fukushima Daiichi (1F) severe accident (SA), it was recognized that the suppression chamber (S/C) functions of heat sink and fission product (FP) scrubbing will degrade, resulting in the S/C pressure increase, reduction of the scrubbing efficiency and subsequent necessity of venting operations. Consequently, a relatively large amount of FPs, in particular highly volatile elements (e.g. CH3I), is likely to be dispersed into the environment. As a method to evaluate the degradation of the pool characteristics under discharge of steam and non-condensable gases through vent pipes and steam through different quencher geometries of make-up systems, an experimental campaign was recently started at the SIET research laboratory in Italy. Two different quencher geometries, representing vent pipes and the reactor core isolation cooling (RCIC) exhaust pipes in 1F2 and 1F3, were adopted. Several combinations of steam and air mass flow rates were tested to scale down the main conditions occurred during the 1F SA. Measurements of pool water temperature in different locations and visualization with high-speed camera represent the main outcome of the experimental activity. The preliminary results have demonstrated that a relatively small concentration of air in the steam flow is able to suppress the occurrence of chugging of the steam, with reduced mixing in the pool. Both RCIC quenchers adopted induced large chugging at the bottom of the pool which are effective to avoid temperature stratification, thanks to the large water recirculation and vertical mixing within the pool. At decreased subcooling, mixing in the pool ceases and the quenchers with holes disposed in the vertical direction, as in the RCIC exhaust pipe of the 1F unit 3, introduce intense stratification that drastically reduces the condensation efficiency of the S/C pool. Quencher of 1F2 RCIC does not present stratification possibly dependent on the distance of the pipe outlet to the pool floor. Given the reduced size of the pool compared to the plant scale, the observed phenomena should not be extrapolated for the whole S/C. The objective of the ongoing experimental activity is to construct a database based on the high-speed filming, measurements of major quantities such as water temperature, steam pressure and FPs concentration to foster the development of physical models for both lumped parameter SA codes and detailed computational fluid dynamics software, in an effort to enhance the understanding of the complex phenomena following the 1F accident.

An opto-structural method to estimate the stress-strain field induced by cell contraction on substrates of controlled stiffness in vitro

Purpose Mechanical properties of the extra-cellular matrix (ECM) such as stiffness mediate cell signaling, proliferation, migration, and differentiation. Within this context, we developed a method to estimate in vitro the stress-strain field induced by contraction of cardiovascular progenitor cells on substrates of controlled stiffness. Methods Two alginate-agarose hydrogels were polymerized and mechanically characterized under compression. The hydrogels showed different levels of stiffness, mimicking either normal or pathologic ECM of the cardiac tissue, with an average compressive equilibrium modulus of 3 and 25 kPa, respectively. To estimate substrate deformation induced by the adhering cells, fluorescent microspheres were included under the surface layer of the hydrogels as displacement trackers. The hydrogels were polymerized in multiwell plates and seeded with cells that were allowed to adhere for 24 hours. On the softer substrate, images of the substrate surface and the cells were acquired using time-lapse fluorescence microscopy. Image processing enabled tracking the microsphere movements and mapping local substrate deformation because of tensile stresses produced by the cells. The resulting tensile stresses could then be calculated from measured stiffness. Results and Conclusions The substrate strains ranged between a maximum contraction of –26.5% to a maximum stretching of 19.8%. The calculated stresses ranged between a maximum compression of –0.53 kPa to a maximum tension of 0.4 kPa (nN/μm2). These results may help to interpret experimental findings, showing important differences in cell morphology and expression of phenotypic markers, induced by culturing cells on substrates with different mechanical properties.

Visualization of Flow Patterns in Closed Loop Flat Plate Pulsating Heat Pipe Acting as Hybrid Thermosyphons under Various Gravity Levels

ABSTRACT A particular flat plate pulsating heat pipe (FPPHP), filled with FC72, is tested during the 62th and 64th ESA parabolic flight campaigns under vertical orientation. The FPPHP is made of a thin copper plate, in which a curved channel disposed with 11 U-turns is milled and closed on the top face by a transparent borosilicate plate. The particular characteristics is that the equivalent hydraulic diameter of the square channel (2.5 × 2.5 mm²) is above the working fluid capillary diameter on ground, inducing a stratification of the liquid/vapor phases under ground and hyper-gravity conditions, whatever the orientation. The energy transfer mode in such conditions is represented either by pure pool boiling inside the channels almost completely filled by the liquid phase or by an annular flow pattern inside the channels mostly filled by the refrigerant vapor. Instead, during the microgravity phases, the fluid regime naturally turns into a slug-plug flow pattern. During the transition from 1.8 g to 0 g a rapid dry-out may occur in some of the channels, followed by a similarly fast reaction of liquid plugs moving towards the evaporator from the condenser zone. Such stop-and-start motion events continue during the whole microgravity period, leading to strong temperature oscillations, but also to a still acceptable thermal performance of the device.

NATURAL GAS BURNERS FOR DOMESTIC AND INDUSTRIAL APPLIANCES. APPLICATION OF THE PARTICLE IMAGE VELOCIMETRY (PIV) TECHNIQUE.

This contribution presents some examples of the application of the particle image velocimetry (PIV) technique to domestic appliances and small-scale burners, with the aim of discussing relevant results together with problems encoun- tered. Combustion efficiency and pollutant emissions of gas burners are strongly influenced by the fluid dynamics of the mixture in both premixed and nonpremixed flames. For these reasons the Combustion Laboratory of Politecnico di Milano started using laser diagnostic techniques (LDV and PIV) many years ago. The first PIV application was on a premixed V-flame attached over a burner plate with rectangular twin slots, developed for domestic appliances. The autocor- relation of a double-exposed photograph was used to define the 2-D velocity flow field and high spatial resolution was obtained with a 1 : 1 magnification and a zoom Nikkor objective. More recently, a crosscorrelation CCD camera with a double-pulse Nd:YAG laser was used to characterize high-swirl flows under both nonreacting and reacting conditions. In the following, special emphasis will be given to the discussion of the most relevant results and the main problems encountered with PIV applications to the investigated cases.

Natural Gas Burners for Domesticand Industrial Appliances.

This contribution presents some examples of the application of the particle image velocimetry (PIV) technique to domestic appliances and small-scale burners, with the aim of discussing relevant results together with problems encountered. Combustion efficiency and pollutant emissions of gas burners are strongly influenced by the fluid dynamics of the mixture in both premixed and nonpremixed flames. For these reasons the Combustion Laboratory of Politecnico di Milano started using laser diagnostic techniques (LDV and PIV) many years ago. The first PIV application was on a premixed V-flame attached over a burner plate with rectangular twin slots, developed for domestic appliances. The autocorrelation of a double-exposed photograph was used to define the 2-D velocity flow field and high spatial resolution was obtained with a 1 : 1 magnification and a zoom Nikkor objective. More recently, a crosscorrelation CCD camera with a double-pulse Nd:YAG laser was used to characterize high-swirl flows under both nonreacting and reacting conditions. In the following, special emphasis will be given to the discussion of the most relevant results and the main problems encountered with PIV applications to the investigated cases.