Analysis of the temperatures measured in very thick and insulating roofs in the vicinity of a chimney

Chimneys convey exhaust gas produced in domestic heat appliances to the external environment and to do this they have to pass through elements such as roofs and floors. If these elements are made up of flammable materials the fire hazard may occur. In some European countries the number of roof fire is very high and they affect also certified chimneys, that is, tested following the prescription of the related standards. The aim of this paper is to highlight that the certification procedure does not allow to test chimneys in the worst condition, therefore, chimney installed following the manufacturer prescriptions may in some cases cause the overheating and subsequent roof fire. To do this, experimental tests have been performed for measuring the temperature on roofs in the vicinity of a certified chimney. The results show that the certification procedure should be revised

Procedura per la determinazione sperimentale del coefficiente di riflessione solare

In questa memoria viene proposta una metodologia che permette di determinare il coefficiente di riflessione solare di una superficie riflettente mediante una serie di quattro misure eseguite con un piranometro a termopila, sensibile all’intero spettro della radiazione solare. Il valore misurato è quindi rappresentativo del comportamento della superficie di interesse in applicazioni che coivolgono la radiazione solare e il metodo è utilizzabile, per esempio, per gli specchi dei concentratori solari. In questo lavoro vengono presentate le basi teoriche del metodo, vengono illustrate la configurazione sperimentale e la procedura di prova e infine vengono mostrati e commentati i primi risultati sperimentali

Chimney in wood-frame houses: preliminary thermal analysis in relation to the fire hazard

This work is a preliminary numerical study performed with the CFD code FLUENT about some critical aspects occurring when a chimney is installed through wood roof-packages of great thickness. The UNI EN 1443 standard [1] imposes the conditions that a chimney must comply to be certified and the UNI EN ISO 1859 standard [2] describes how the prescribed tests have to be performed: in particular it prescribes the thickness of the floors as 13.2 cm and 23.2 cm. Since real wood roof-packages are characterized by greater thicknesses, up to almost 40 cm, the test configuration is very different from these real situations. This paper illustrates some 3D and 2D axisymmetric simulations on two certified elements, indicated as chimney A (T600-N1-G20) and chimney B (T400-N1-G00). The 2D axisymmetric simulations are performed with different internal diameter, different floor thicknesses and different smoke temperatures. The results lead to interesting considerations concerning the problem of chimneys installed through roof-packages of great thickness and concerning the thermal performance test

Metodo di calcolo delle trasmittanze lineiche in pannelli prefabbricati in calcestruzzo alleggeriti

Over the past years, in order to improve the thermal performance of building elements, rules of construction have been issued and new limits of respect have been enforced. An example is the value of the thermal trasmittance U, that producers of building elements have to declare and that must stay below prescribed thresholds. European standards propose two methods to calculate U. One of this method is simple, but its use is not always allowed, the other requires timeconsuming numerical simulations. In this paper we present the result of a numerical study aimed to propose simple correlations that avoid the need of numerical simulations to compute the thermal trasmittance of precast concrete panels

Point thermal transmittance of rib intersections in concrete sandwich wall panels

Concrete sandwich panels are widely used building elements. They are made by two reinforced concrete wythes separated by a layer of lightweight material: the central layer is inhomogeneous due to the presence of concrete ribs which tie the external wythes and act as thermal bridges. International Standards allow to evaluate the average thermal transmittance of concrete sandwich panels as a linear combination of the transmittance of the solid concrete ribs and of the lightened parts - calculated as if the temperature field were 1D - and linear and point thermal transmittances associated with thermal bridges. In a recent work we have addressed the problem of finding an accurate correlation for prediction of linear thermal transmittance values. The goal was reached upon use of a fast and accurate Spectral Element Method. In this work we complete our study investigating the point thermal bridges and determining the associated point thermal transmittance. Point thermal transmittances in sandwich panels are associated with the concrete rib intersections, like in the four panel corners, and require 3D numerical simulations for their evaluation: the computational effort required to approximate the point transmittance is much larger than that needed to estimate the linear one. For this reason we present and discuss a solution strategy based on the use of low-order polynomials (p = 4) on three grids of increasing refinement, starting from a very coarse one: results have been improved through an iterated application of Richardson extrapolation. This procedure assures a good trade-off between accuracy, as required by Standards, and computational cost. A dataset of 1080 point transmittance values is obtained varying systematically six geometrical and thermophysical parameters. A simple power law correlation in terms of a single variable depending on linear transmittance of the intersecting ribs is introduced and its accuracy assessed

Computation of linear transmittance of thermal bridges in precast concrete sandwich panels

Precast concrete lightened sandwich panels are widely used building elements. They are made by two concrete wythes separated by a layer of lightweight material: the central layer is inhomogeneous due to the presence of concrete ribs which tie the external wythe and act as thermal bridges. Computation of thermal transmittance of sandwich panels is clearly described in European Standards, but in many cases it requires numerical simulations to determine the linear transmittance ψ associated with lightweight material-concrete interfaces in the inhomogeneous layer. Although simple, these simulations represent a critical issue for many panel manufacturers and they would much rather prefer correlations to compute ψ. In this work we present a correlation based on an artificial neural network (ANN) to estimate linear transmittance values for current Italian sandwich panel production. Five input parameters are considered: rib width, lightweight material conductivity, and thickness of the three panel layers. To obtain the data which are necessary to train and test the ANN, a fast and accurate Spectral Element Method is used to solve Laplace equation in the neighbourhood of a rib. 5460 ψ values are collected which ensure an accurate network response

Computing the exergy of solar radiation from real radiation data on the Italian area

The decrease of fossil fuels availability and the consequent increase of their price, has led to a rapidly evolution of renewable market and policy frameworks in recent years. Renewable resources include solar radiation which is of considerable interest as it is inexhaustible, free and clean. In order to calculate how much work can be obtained from solar radiation, several methods have been proposed in the literature. The aim of this work has been to calculate the exergy content of solar radiation in Italy. To do this, we have analyzed real radiation data and we have treated direct and diffuse radiation separately. We have proposed a single exergy factor valid on the Italian area, which is to be applied to the total radiation measured on horizontal surface

Study of the heat transfer between chimney and roof by means of design of experiment (DOE) technique

The topic of this work is that of the wooden roof fires: the passage of a chimney through a thick and insulating wooden roofs could lead to the overheating of these latter. Roof fires involve also tested chimney, whose production and installation are regulated by several standards. The aim of this work is to study the heat transfer between chimney and floor: data here analyzed (floor maximum temperatures) has been obtained by means of simulations performed with the CFD code Fluent and it has been analyzed with the DOE technique, that it is usually used to design experimental campaign, but in this work has been used to quantify and to compare the factors impact on the observed phenomenon. The results of this work is the determination of a mathematical relationship by means of which to calculate the floor maximum temperature on varying floor and chimney characteristics, more precisely the floor thickness, the floor thermal transmittance, the smoke temperature and the distance between the chimney and the floor