1,720,991 research outputs found
Metodo di calcolo delle trasmittanze lineiche in pannelli prefabbricati in calcestruzzo alleggeriti
No full textOver 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
Valutazione della trasmittanza termica di pannelli prefabbricati: confronto tra metodi di calcolo
No full textA note on calculation of efficiency and emissions from wood and wood pellet stoves
No full textIn recent years, national laws and international regulations have introduced strict limits on efficiency and emissions from woody biomass appliances to promote the diffusion of models characterized by low emissions and high efficiency.
The evaluation of efficiency and emissions is made during the certification process which consists in standardized tests.
Standards prescribe the procedures to be followed during tests and the relations to be used to determine the mean value of efficiency and emissions. As a matter of fact these values are calculated using flue gas temperature and composition averaged over the whole test period, lasting from 1 to 6 hours.
Typically, in wood appliances the fuel burning rate is not constant and this leads to a considerable variation in time of composition and flow rate of the flue gas. In this paper we show that this fact may cause significant differences between emission values calculated according to standards and those obtained integrating over the test period the instantaneous mass and energy balances.
In addition, we propose some approximated relations and a method for wood stoves which supply more accurate results than those calculated according to standards. These relations can be easily implemented in a computer controlled data acquisition systems
Modeling operation mode of pellet boilers for residential heating
No full textIn recent years the consumption of wood pellets as energy source for residential heating has increased, not only as fuel for stoves, but also for small-scale residential boilers that produce hot water used for both space heating and domestic hot water.
Reduction of fuel consumption and pollutant emissions (CO, dust, HC) is an obvious target of wood pellet boiler manufacturers, however they are also quite interested in producing low-maintenance appliances. The need of frequent maintenance turns in higher operating costs and inconvenience for the user, and in lower boiler efficiency and higher emissions also.The aim of this paper is to present a theoretical model able to simulate the dynamic behavior of a pellet boiler. The model takes into account many features of real pellet boilers. Furthermore, with this model, it is possible to pay more attention to the influence of the boiler control strategy. Control strategy evaluation is based not only on pellet consumption and on total emissions, but also on critical operating conditions such as start-up and stop or prolonged operation at substantially reduced power level. Results are obtained for a residential heating system based on a wood pellet boiler coupled with a thermal energy storage.
Results obtained so far show a weak dependence of performance - in terms of fuel consumption and total emissions - on control strategy, however some control strategies present some critical issues regarding maintenance frequency
Experimental study of a pellet stove
No full textThe present work is a preliminary report of an experimental investigation of a commercial pellet stove aimed to fully understand which parameters influence its total efficiency. The long tem goal of this activity is to identify criteria for performance improvements of the stove. In the first part of the paper we present an analytic model of the thermal behaviour of the stove that accounts for the heat exchange between the flue gases, including flame, and the stove and between the stove and surrounding environment. Results of experimental tests are presented and discussed, comparing results with those predicted by the model. In the second part of the paper we present an experimental study of the influence on the combustion efficiency of the excess of air and of its distribution along the height of the furnace. Experimental tests show that the production of CO was very affected by both factors. The best results are obtained when the total amount of air supplied is about twice the stoichiometric quantity, for any geometry of combustion grid. In the last part of the paper we address the different behaviour of a pellet stove in real operating conditions compared with operating condition during a certification test according the standard UNI EN 14785, in which measurements of efficiency and emissions are performed when the thermal transient is certainly concluded. Instead, in real case, the stove works mainly in transient conditions and this affects both the global efficiency and the CO emissions
Influenza della relazione tra permeabilità e porosità sulla simulazione numerica della solidificazione di un lingotto
No full textA comparison of two porosity-permeability relations in numerical simulations of solidification of metal alloys
No full text???Defining entropy before heat, not viceversa??? in introductory thermodynamics: twentyfive-year teaching experience in engineering programs
No full textAs well as many other people, we have felt, both as students and as teachers, that some traditional approaches present ambiguities
and logical inconsistencies in the exposition of the basics of thermodynamics. Since the late ???80s we have adopted an approach developed over thirty years of course and research work at M.I.T.: rooted in the work of Hatsopoulos and Keenan [1], it has been presented in a systematic and detailed way by Gyftopoulos and Beretta [2]. On the basis of our teaching experience we believe that this approach is particularly suited for students attending engineering programs and our goal here is to underline the most important reasons of its success. In the paper we summarize and discuss how we have adapted the sequence of arguments proposed in [2, Chaps. 2-14] to meet the needs of undergraduate engineering students.
[1] G.N. Hatsopoulos and J.H. Keenan, Principles of General
Thermodynamics, Wiley, 1965.
[2] E.P. Gyftopoulos and G.P. Beretta, Thermodynamics.
Foundations and Applications, Macmillan, 1991, (Dover Edition, 2005)
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