Slip flow in elliptic microducts with constant heat flux

This paper outlines a numerical model for determining the dynamic and thermal performances of a rarefied fluid flowing in a microduct with elliptical cross-section. A slip flow is considered, in laminar steady state condition, in fully developed forced convection, with Knudsen number in the range 0.001-0.1, in H1 boundary conditions. The velocity and temperature distributions are determined in the elliptic cross-section, for different values of both aspect ratio γ and Knudsen number, resorting to the Comsol Multiphysics software, to solve the momentum and energy equations. The friction factors (or Poiseuille numbers) and the convective heat transfer coefficients (or Nusselt numbers) are calculated and presented in graphs and tables. The numerical solution is validated resorting to data available in literature for continuum flow in elliptic cross-sections (Kn = 0) and for slip flow in circular ducts (γ = 1). A further benchmark is carried out for the velocity profile for slip flow in elliptical cross-sections, thanks to a recent analytical solution obtained using elliptic cylinder coordinates and the separation of variables method. The Poiseuille and Nusselt numbers for elliptic cross-sections are discussed. The results may be used to predict pressure drop and heat transfer performance in metallic microducts with elliptic cross-section, produced by microfabrication for microelectromechanical systems (MEMS)

Influence of Outdoor Air Conditions on the Air Source Heat Pumps Performance

Abstract The purpose of the present work is the investigation of the effect of the outdoor air temperature and relative humidity on the performance of an air heat pump, when the reverse-cycle defrosting is considered. The frost formation process has been analyzed by developing a simplified model which relates the number of defrost cycles to the outdoor air conditions. Moreover the energy consumption due to the defrosting has been taken into account in the evaluation of the heat pump performance. The results, carried out for many Italian sites, point out that the outdoor air conditions play an important role in determining the amount of defrost cycles; however the frost formation is mainly affected by the relative humidity. The analysis highlights also that the defrosting contribution on the heat pump performance is not negligible when the heat pump that operates in wet weather, although cold; in these conditions the hourly COP may be reduced by up to 20%. However, this effect becomes less relevant, but not negligible, when the seasonal heat pump performance is evaluated; the maximum decrease of SCOP, observed for the all analyzed cases, is less than 13%

Parameter estimation approach to the thermal characterization of intumescent fire retardant paints

Intumescent paints are widely used as passive fire retardant materials in the building sector. They swell on heating to form a highly insulating char, protecting steel members. Intumescent coatings for use in buildings are typically certified according to the standard cellulosic fire resistance test. This test is expensive, often non-representative of realistic fire conditions, and not enough versatile to gather detailed performance information on the response of reactive coatings. A promising approach, that could offer a helpful tool to the engineering community involved in fire safety, is found in the modelling of the behaviour of the intumescent coating. Under this approach, the knowledge of the equivalent thermal conductivity of the intumescent material is a fundamental issue, since it represents the main parameter that allows predicting the thermal protecting capability of the layer. The purpose of this paper is to optimize an estimation procedure intended to the restoration of the equivalent thermal conductivity of intumescent layers. The thermal stress is activated by the action of a cone calorimetric apparatus, while the estimation procedure is based on the inverse heat conduction problem approach under steady state assumption, where the temperature values measured at some locations inside the layer during the expansion process are used as input known data. This procedure was successfully applied to steel samples protected with an intumescent paint; the estimated equivalent thermal conductivity of the layer results to temperature dependent while the initial thickness of the paint does not seem to have a great effect

Energy need and generation in buildings: predictions and experimental data.

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

Effect of Floor Geometry on Building Heat Loss via the Ground

This paper analyzes the heat loss from an insulated slab on the ground, focusing on the influence of floor geometry on thermal processes in the ground. The calculation model includes the vertical and horizontal structures of the building; the foundation is also included. A building with a rectangular floor is considered; the ratio between the sides of floor (defined as aspect ratio) changes from 0 to 1. The thermal analysis is carried out resorting to a finite element code, validated in accordance with the requirements of International Standard ISO 10211. Numerical results show that floor geometry has a significant influence on steady-state ground global heat transfer coefficient; ranging from a narrow rectangular floor to a square one the steady-state ground global heat transfer coefficient decreases by about 15%. The effects of the perimeter insulation are also investigated; depending on the insulating layer thickness, the decrease of the heat transfer coefficient ranges from 8% to 13%. A comparison with the results obtained by applying the International Standard method ISO 13370 is also presented

Asymmetrical heating in rarefied flows through circular microchannels

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