• Energy Research

A simple, fast, and rather general procedure combining the superposition principle with a finite element method is proposed to deal with conjugate heat transfer problems. The method is employed to consider the wall conduction effect on heat transfer to fully developed laminar flow through a pipe whose exterior boundary is uniformly heated along a finite length. Results are given for two values of each of the four parameters determining the relative importance of axial conduction: the Peclet number of the fluid, the wall to fluid conductivity ratio, and the dimensionless thickness and length of the heated section of the pipe.

Laminar combined convection in horizontal circular ducts is investigated both numerically and experimentally, under uniform wall heating. A series of experiments for the heating of water in a long horizontal copper tube are simulated numerically in order to assess the reliability of the theoretical results. Peripheral and axial wall conduction effects, inherently present in the experiments, are accounted for in the numerical model. The cross validation of experimental and numerical data allows significant conclusions to be reached on conjugate conduction and convection with buoyancy effects in horizontal duct flows. Buoyancy is considered for values of the modified Rayleigh number,Raqo, up to 5·106; the forced convection contribution is considered for two values of the entry Reynolds number,Reo=500 and 1000.

AbstractResults are presented from an experiment on capillary rise of water in inclined small-bore (ϕ= 1.92mm) cylindrical tubes. A series of thick-walled tubes with different lengths and taken from the same glass rod was used, and the angle of inclination towards the vertical, α, was varied from 0° to 88°.Results indicate that the capillary rise progressively reduces for increasing α, contrary to Jurin's law predictions. It is observed further that the meniscus seems not to change in shape while varying the tube orientation. Possible consequences of the above observations on the statics of capillary tubes are commented. In the frame of the discussion, an alternative and more general approach for the derivation of the Jurin's law is proposed.

Abstract A combination of horizontal and vertical elements forming an eggcrate-type shading system is often used in the architectural design of building facades. An analytical-numerical method is presented to estimate the shading effect of such a structure on vertical windows of arbitrary azimuthal orientation. The effect of reflections from aggregate walls is accounted for in the analysis. Daily and monthly average solar radiation data are presented for shaded windows in Venice (45.40° northen latitude). The effects of the window geometry and the eggcrate projection are discussed.

I cool roofs sono tetti contraddistinti da elevata capacità di riflettere l’irradiazione solare incidente e, al contempo, di emettere energia termica nell’infrarosso. Essi possono fornire un’efficace soluzione al problema del surriscaldamento estivo dei singoli edifici e delle grandi aree urbane.In California e in altri Stati degli U.S.A. i cool roofs sono da molti anni oggetto di analisi tecnico-economiche estensive e, più recentemente, di attività di certificazione e regolamentazione. In Italia, invece, sebbene le condizioni climatiche di buona parte della penisola siano tali da rendere conveniente l’installazione di cool roofs, questi sembrano sconosciuti ai più. Studi in materia sono stati perciò avviati dal Dipartimento di Ingegneria Meccanica e Civile di Modena.In questa relazione si illustra il funzionamento dei cool roofs dal punto di vista fisico. In particolare, si analizzano gli effetti che le diverse proprietà emissive hanno sulle temperature raggiunte dal tetto e sul calore che lo attraversa. Si delineano inoltre i principali programmi di attestazione delle proprietà e di incentivazione avviati degli U.S.A. e la situazione di mercato.

Abstract Natural circulation of distilled water and FC43 has been experimentally investigated in a rectangular loop characterized by internal diameter of 30 mm and total length of 4.1 m. The aim of the present study is to analyse the influence of thermal boundary conditions on the flow regimes inside the pipes and on the stability of the system. The new aspect of the present research is the possibility of tuning the heat sink temperature in a range between −20 °C and +30 °C by means of a cryostat. This kind of analysis could be useful for the design of systems characterized by a wide range of environment temperatures, as for example for aerospatial applications. The other parameters investigated were the heat flux transferred to the fluid, which varied between 0.1 kW and 2.5 kW, and the thermo-physical properties of the working fluid. The system showed both stable and unstable behaviour. In particular, in the case of FC43 the loop was more unstable and it was characterized by higher velocities and frequencies compared to the case of distilled water. It was found that the stability threshold could be crossed by varying only the heat sink temperature, demonstrating the importance of this boundary condition on the dynamics of the system. Different flow regimes and fluid velocities were observed. In the case of steady-state flow, Vijayan’s correlation (Vijayan et al., 2000) was tested and found to give good agreement with experimental data. Linear stability analysis was made following the Vijayan’s model. In particular, the effect of heat sink temperature was considered in the dimensionless Stanton number based on the overall heat transfer coefficient at the heat sink. Finally, Ultrasound Pulsed Doppler Velocimeter (UPDV) was used on a natural circulation loop for the first time, and gave a preliminary validation of the traditional fluid velocity measurement method based on the frequency analysis.

Air-bubble entrainment produced by the impact of water drops on a liquid pool is investigated with the use of high-speed imaging. A wide range of drop volumes and impact velocities is considered to determine how the entrainment mechanisms change with varying the impact conditions. Five different entrainment regimes are distinguished on the basis of the observed flow phenomena. Their characteristic features are described in terms of bubble formation, crater evolution, jetting and secondary drop ejection. A regime map is reconstructed in the Froude-Weber space. Results obtained in the present study show good agreement with the phase diagrams reported in the literature and contribute to complete the scenario of the entrainment regimes. Quantitative data about the size and the residence times of the entrained nuclei are also presented.