1,720,986 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 textCO and NO emissions from pellet stoves: an experimental study
No full textThe present work presents a report on an experimental investigation on commercial pellet stoves aimed to fully understand which parameters influence CO and NO emissions (NO2 is present in negligible quantities): tests are performed on different type of pellet stoves varying nominal power, combustion chamber size and combustion grid geometry.
After a brief review on the factors which influence the production of these pollutants, we present and discuss the results of the experimental tests aimed to ascertain how the geometry of the combustion chamber and the distribution of primary and secondary air, can modify the quantity of CO and NO in the flue gases. Experimental tests show that production of CO
is strongly affected by the excess air and by its distribution: in particular, it is critical an effective control of air distribution. In these devices a low level of CO emissions does require a proper setup to operate in the optimal range of excess air that minimizes CO production. We
have observed that the optimal range of operation can be enlarged as a consequence of proper combustion grid design.
NO emissions, on the other hand, are not a critical issue, since they are well below threshold enforced by law, are not influenced by the distribution of air in the combustion chamber, and their behavior as a function of air excess is the same for all the geometries that were investigated
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