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This work is aimed at contributing to the thermal analysis of slip flow through circular microducts, providing an analytical solution to the energy conservation equation for partially heated walls. A uniform wall heat flux (H2 boundary conditions) is considered on the heated perimeter of the cross section while the remaining arc length is assumed to be adiabatic. The gaseous flow is considered laminar, fully developed, in steady state condition, and forced convection. The temperature profile, wall temperature distribution, and Nusselt number are presented as functions of both the heated perimeter of the cross section and the Knudsen number, resorting to simple converging series of trigonometric functions. The proposed solution can be useful for the design of the microfluidic devices such as micro heat sinks and micro heat exchangers.

Questo lavoro di tesi di Dottorato di Ricerca intende approfondire alcuni aspetti peculiari del fabbisogno energetico degli edifici e della produzione energetica degli impianti alimentati da fonti rinnovabili mediante l’ausilio di codici di calcolo e di indagini sperimentali. Sono analizzate le prestazioni energetiche di un edificio ad uso uffici, di un impianto solare fotovoltaico grid-connected e di un impianto solare termico per la produzione di acqua calda sanitaria. This Doctoral Thesis aims at studying some specific features of energy use in buildings and energy generation by solar systems integrated in buildings. Energy performances of an office building, of a grid-connected solar photovoltaic plant and of a solar domestic hot water system are analyzed. The numerical analysis is supported by experimental investigations.

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu. In this work the fluid-dynamic behaviour of a micro T-junction has been investigated both numerically and experimentally for low Reynolds numbers (Re

In the literature some researchers highlighted that for a dilute gas in slip flow regime and in presence of a non negligible viscous heating, the analysis of the gas micro-convection has to be tackled by modifying the thermal boundary conditions to account for the shear work due to the slip at the wall. Although in the recent past a specific modified boundary condition has been proposed, theoretically justified and applied to investigate the effect of the shear work on the convective heat transfer in presence of dilute gases, in this paper is demonstrated that there is not a need of a modified boundary condition in order to take into account the effect of the shear work in the analysis of forced convection. In the present work by means of a comprehensive theoretical analysis is demonstrated that the modified boundary condition is useless for the analysis of the effect of shear work on the evaluation of the convective heat transfer coefficients in presence of a dilute gas with non-negligible viscous dissipation. Moreover, to evaluate the inaccuracy of the results obtained by using the modified boundary conditions the difference, in terms of Nusselt number, between the exact and the approximate solution has been numerically estimated for elliptical microchannels. The numerical outcomes point out that the adoption of the modified boundary condition leads to an underestimation or an overestimation of the Nusselt numbers depending on the values of Brinkman number and on the channel cross section geometry.

In this work, hydrodynamically and thermally fully developed gas flow through elliptical microchannels is numerically investigated. The Navier–Stokes and energy equations are solved by considering the first-order slip flow boundary conditions and by assuming that the wall heat flux is uniform in the axial direction, and the wall temperature is uniform in the peripheral direction (i.e., H1 boundary conditions). To take into account the microfabrication of the elliptical microchannels, different heated perimeter lengths are analyzed along the microchannel wetted perimeter. The influence of the cross section geometry on the convective heat transfer coefficient is also investigated by considering the most common values of the elliptic aspect ratio, from a practical point of view. The numerical results put in evidence that the Nusselt number is a decreasing function of the Knudsen number for all the considered configurations. On the contrary, the role of the cross section geometry in the convective heat transfer depends on the thermal boundary condition and on the rarefaction degree. With the aim to provide a useful tool for the designer, a correlation that allows evaluating the Nusselt number for any value of aspect ratio and for different working gases is proposed.