An enhanced technique for hierarchical agglomerative clustering is presented. Classical clusterings suffer from non-uniqueness, resulting from the adopted scaling of data and from the arbitrary ...choice of the function to measure the proximity between elements. Moreover, most classical methods cannot account for the effect of measurement uncertainty on initial data, when present.
To overcome these limitations, the definition of a weighted, asymmetric function is introduced to quantify the proximity between any two elements. The data weighting depends dynamically on the degree of advancement of the clustering procedure. The novel proximity measure is derived from a geometric approach to the clustering, and it allows to both disengage the result from the data scaling, and to indicate the robustness of a clustering against the measurement uncertainty of initial data.
The method applies to both flat and hierarchical clustering, maintaining the computational cost of the classical methods.
Abstract
Computational Fluid-Dynamics (CFD) simulations are widely used for designing components and devices for heat transfer enhancement, and to this end the Reynolds Averaged Navier-Stokes (RANS) ...equations are often chosen, as they are computationally efficient. In this paper, several numerical simulations have been carried out on convective heat transfer of an air flow through a rectangular channel of 1:10 aspect ratio, 120 mm wide, 840 mm long. Numerical results have been compared to analytical values and experimental data. The configuration of the described numerical model will be used as starting point for detailed investigations of fluid-dynamic and thermal performances of ribbed channels in further analysis.
Abstract
In this paper, a Computational Fluid-Dynamics (CFD) analysis on heat transfer characteristic using Reynolds Averaged Navier-Stokes (RANS) equations on a rectangular channel with ribbed ...surfaces is presented. Several numerical simulations have been carried out on convective heat transfer of an air flow through a rectangular channel of 1:10 aspect ratio, 120 mm wide, 840 mm long, with 90° ribs. Ribs have a square cross section with 4 mm side and three different values of dimensionless pitch, namely, 10, 20 and 40. Results on convective global heat transfer coefficient, i.e., averaged over the whole channel, have been compared to experimental data obtained in a channel with the same geometry and boundary conditions. Agreement between numerical and experimental data is discussed for the three different pitches considered. As expected, comparisons show a decreasing reliability for increasing complexity. The aim of this work is to find a suitable configuration of a CFD model with RANS that will permit authors to apply it to the range of dimensionless pitches.
Different variables concur to the outdoor comfort assessment and, among them, the Mean Radiant Temperature (Tmr) represents the most challenging one to experimentally evaluate. According to the ...scientific literature, the most accurate method for in-field Tmr calculation is integral radiation measurements with three net radiometers. Nevertheless, net radiometers are expensive sensors and their implementation on large scale may be hindered by their price. To provide a cost-effective alternative some researchers proposed the use of globe thermometers. Globe thermometers are affordable sensors but typically exploited indoors, and their accuracy in urban settings is still under investigation.
The scope of this work is to provide information regarding the extent to which globe thermometers can substitute net radiometers in the evaluation of the Tmr in outdoor urban settings. To this purpose, an experimental comparison has been performed between the Tmr calculated using net radiometers, assumed as a reference, and using two different globe thermometers: a standard 150 mm black globe and a grey 50 mm globe.
The results revealed that the black globe tends to overestimate the Tmr while the grey globe mainly underestimates it. Moreover, the analyses evidenced a wide fluctuation in the profile of the Tmr calculated using globe thermometers, due to the rapid variation of meteorological parameters and the globes' long response time. To improve the fit between globe thermometers and net radiometers' response, regressive functions have been proposed, resulting in a significant enhancement of the adherence between Tmr calculated using the standard black globe's measurements and the reference Tmr.
•Tmr is key parameter for outdoor thermal comfort assessment in urban settings.•Globe thermometers cannot directly substitute net radiometers to assess the Tmr.•A regression equation is used to assess the Tmr from the standard globe thermometer.•In 90.4% of cases, the equation results in a negligible error for Tmr.•This result decreases greatly the cost and number of required sensors.
Numerical simulations are carried on to study the fluid-dynamical features of a smooth duct with aspect ratio of 10. The duct is operated with an incompressible, newtonian fluid, whose Reynolds ...number, computed over the hydraulic diameter and bulk velocity, ranges from 470 to 14500, encompassing laminar and turbulent flow. To capture the details of all the flow scales, Direct Numerical Simulations are performed, by means of a code developed at Politecnico di Milano. The adopted code is a finite-difference, structured grid solver, that includes a mass flow rate correction. The latter guarantees high accuracy in the calculation of unsteady flows, or during the transition to turbulent regime, and it allows to check the consistency of numerical results. Both global parameters -including the friction factor and the identification of the laminar-to-turbulent transition and local flow features, e.g., corner vortexes, are investigated and presented in this work. Preliminary analyses agree fairly well with literature data and with experimental results obtained at ThermALab of Politecnico di Milano. The final goal of this work, including a deep integration between the numerical and the experimental setup, is to carry on detailed investigations of the fluid-dynamical and thermal characteristics of ribbed ducts, in the perspective of heat transfer enhancement and pressure drop reduction.
The analysis of experimental results on heat transfer by forced convection in diverse ribbed ducts showed that different geometries lead to comparable thermal and fluid-dynamic performances. ...Moreover, no evident layout has been observed in data, and therefore a statistical clustering analysis is performed to detect the rationale, if any, underlying experimental results. A novel, ad-hoc developed technique is used to disengage the clustering from the data scaling and to account for the measurement uncertainty, consisting of an agglomerative procedure, based on the definition of dynamically-changing bounding boxes, whose size depends on the Nusselt number and the pumping power. Additional informations, such as the the relevance of the diverse geometric parameters and the persistence of similarity among configurations over a range of operating conditions, can be retrieved by means of the developed technique. The described method is applied to a large dataset, obtained during an experimental campaign carried on at ThermALab of Politecnico di Milano, aimed at identifying the Nusselt number and the friction factor for diverse-rib configurations in a large-aspect ratio channel with low-Reynolds flows. The considerations originated from of the results of the clustering analysis suggest the existence of an underlying structure, pointing to a possible unique parameter, termed "generalized blockage", which is possibly able to describe the global effect of the ribs geometry on forced convection.
•A new configuration of solar driven heat pump is proposed.•A yearly energy assessment is carried to assess the configuration.•The thermal power recovered by the ventilation allows heat pump COP ...increase by 5%.•The electricity produced by the PV tile is about twice the heat recovered.
Solar air heaters are seen as a promising option for reducing the building energy demand in particular when combined with heat pumps. In this work, a modeling tool to predict the performances of heat pump driven by solar heater is developed. In particular, the channel between the base of the roof and the tiles is considered. As a case study, a detached house in northern Italy is taken into consideration. Results show that the air temperature variation ranges from 2 °C to 20 °C leading to a heat recovered between 2 kW and 7 kW in winter and summer time, respectively. There is no significant variation of air temperature and heat recovered between the two considered orientation (N-S and E-W), whilst the integration of the PV cell in the tile reduces the thermal energy by 15%. The heat recovered enhances the heat pump COP, reducing the Primary Energy Consumption of a conventional heat pump by 5%, and achieved with limited additional costs. The impact of the electricity generated is more important also because the excess can be sold to the grid and not wasted as for the heat recovered in summer time.
In this work, laser colouring of commercially pure Ti (grade II) samples under three different gas atmospheres is shown and analysed. The laser colouring process was carried out by means of a ...ns-pulsed Yb-fiber laser operating at a wavelength of 1064nm with a pulse duration of 250ns, while the process gas composition was controlled by mixing an inert gas, i.e. argon, with oxygen in different proportions; the results are compared with the same processing in ambient air atmosphere. Colour appearance based on the interference phenomenon in a thin oxide film is explained with a simple analytical model. Hemispherical reflectance spectra of coloured specimens were measured in the visible wavelength range, and, from them, chromaticity values were calculated. The results showed that the employed gas mixtures increased the oxide layer thickness, allowing for a more rapid colour transformation. The annulled wavelength, expressed by the lowest reflectance wavelength, moved from near-ultraviolet region towards near-infrared with increased fluence, as expected from the interference model. Oxide thicknesses of specimens with different colours were measured to validate the analytical model, which was found to predict in a satisfactory manner the lowest reflectivity wavelengths in the visible spectrum.
•Laser colouring of titanium was applied under different process atmospheres.•Inert Ar was mixed with reactive O2 in different proportions and compared to processing in air.•Reflectance spectra were acquired on the produced coloured patches.•An analytical model was proposed linking oxide thickness to the annulled wavelengths.•Oxide thickness measurements and perceived colours were consistent with the developed model.
Workers involved in the production of Cd/As-based photovoltaic modules may be routinely or accidentally exposed to As- or Cd-containing inorganic compounds.
Workers' exposure to As and Cd was ...investigated by environmental monitoring following a worst-case approach and biological monitoring from the preparation of the working facility to its decommissioning. Workplace surface contamination was also evaluated through wipe-test sampling.
The highest mean airborne concentrations were found during maintenance activities (As = 0.0068 µg m(-3); Cd = 7.66 µg m(-3)) and laboratory simulations (As = 0.0075 µg m(-3); Cd = 11.2 µg m(-3)). These types of operations were conducted for a limited time during a typical work shift and only in specifically suited containment areas, where the highest surface concentrations were also found (laboratory: As = 2.94 µg m(-2), Cd = 167 µg m(-2); powder containment booth: As = 4.35 µg m(-2), Cd = 1500 µg m(-2)). The As and Cd urinary levels (As_u; Cd_u) were not significantly different for exposed (As_u = 6.11±1.74 µg l(-1); Cd_u = 0.24±2.36 µg g(-1) creatinine) and unexposed workers (As_u = 6.11±1.75 µg l(-1); Cd_u = 0.22±2.08 µg g(-1) creatinine).
Despite airborne arsenic and cadmium exposure well below the threshold limit value (TLV) when the operation is appropriately maintained in line, workers who are involved in various operations (maintenance, laboratory test) could potentially be at risk of significant exposure, well in excess of the TLV. Nevertheless, the biological monitoring data did not show significant occupationally related arsenic and cadmium intake in workers and no significant changes or differences in arsenic and cadmium urinary level among the exposed and unexposed workers were found.
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