1,721,034 research outputs found
On the optimum thermal design of longitudinal convective fin arrays
No full textThe paper analyzes the problem of the optimum thermal design of free and forced convection fin arrays composed of longitudinal fins with constant thickness. Two different optimization problems have been considered: the minimization of the weight for a given heat flow and the maximization of the heat flow for a given fin weight. Two different geometrical configurations of the finned array have been considered -closed array and open array- and a particular optimization method considering the number of fins of the array as discrete optimization variable, has been developed. The procedure for the optimization is given for the general case and a complete analytical solution of the problem, for the case with the tip approximated as being insulated, is developed. The paper contains several illustrative examples of the application of the proposed optimization procedure
Modello monodimensionale di flussi gas-particelle in condotti verticali a sezione variabile
No full textEXPERIMENTAL INVESTIGATION AND ANALYTICAL MODELING OF A CLOSED TWO-PHASE THERMOSYPHON WITH IMPOSED CONVECTION BOUNDARY CONDITIONS
No full textA closed two-phase thermosyphon operating with Refrigerant-11 and with imposed convection boundary conditions at the heated and cooled surfaces is experimentally investigated and analytically modeled with a lumped parameter model for varying working fluid temperatures. The thermosyphon exhibits different operational modes depending on the heating and cooling fluids' temperature difference. An increase of this temperature difference produces at first a maximum heat transfer rate identified with the flooding heat transfer limit. Beyond this limit the thermosyphon operation reverts to a different steady state through a transient non-equilibrium process. The new steady state, identified as the thermal blocking condition, produces a lower heat transfer capacity and is attributed to the simultaneous existence of: (1) a new flooding state near or at the exit of the adiabatic section, and (2) the dryout in the evaporator resulting from the transfer of liquid in the evaporator pool to the condenser during the transient process which leads to the thermal blocking condition. The limiting operational modes of the thermosyphon are modeled by a lumped parameter model that accounts for different geometrical configurations and liquid entrainment in the vapor. A comparison between the predicted and experimental heat transfer rates, prior and during the thermal blocking condition, demonstrates the model's utility to predict complex thermohydrodynamic processes in a thermosyphon
On Some Perspectives for Increasing the Efficiency of Combined Cycle Power Plants
No full textThe paper proposes an analysis of some possibilities to increase the combined cycle plant efficiency to
values higher than the 60% without resorting to a new gas turbine technology. Optimization of heat recovery
steam generator (HRSG) with the use of parallel sections and of limit subcritical conditions (up to
220 bar) is the key elements to obtain this result.
The HRSG optimization is sufficient to obtain combined cycle plant efficiencies of the order of 60%
while, joining HRSG optimization with the use of gas turbine reheat (postcombustion) and gas to gas
recuperation can lead the efficiency of the whole plant to the limit value of 65%. Results are proposed with
reference to a turbine inlet temperature of 1500 K, corresponding to those of usual commercial D–F series
gas turbine
The one-dimensional flow of homogeneous gas-particle mixtures in variable-area ducts with friction
No full textA general one-dimensional model for the steady adiabatic motion of gas-particle mixtures in arbitrarily oriented ducts with gradually varying cross-section and wall friction is presented. The particles are assumed to be incompressible and in thermomechanical equilibrium with a perfect gas phase, and the effects of their finite volume and of gravity are also taken into account. The equations of motion are written in a form that allows a theoretical analysis of the behaviour of the solutions to be carried out. In particular, the results of the application of the model of a procedure that permits the identification and the topological classification of the singular points of the trajectories representing, in a suitable phase space, the solutions of the set of equations defining the problem are described. This characterization of the singular points is useful in order to overcome difficulties in the numerical integration of the equations. Subsequently, a geometrical analysis is carried out which allows a study of the signs of the local variations of the flow quantities, and shows that some unusual behaviour may occur if certain geometrical and fluid dynamic conditions are fulfilled. For instance, in an upward motion it is possible to have a simultaneous decrease of velocity, pressure and temperature, while in a downward flow an increase of all these quantities may be found. It is also shown that conditions exist in which expansion and heating of the mixture may take place simultaneously, both in accelerating and decelerating flows. The model is applied to the study of upward motion in particular ducts, having converging-diverging and constant-diverging cross-sections; to this end the equations are integrated numerically by using the Mach number as the independent variable. The results show that even limited variations of the duct diameter may give rise to significant qualitative and quantitative variations in the flow conditions inside the duct and in the mass flow rate. Finally, an example is given of a subsonic downward flow in which a simultaneous increase of pressure, temperature and velocity occurs even in the case of a pure perfect gas
CONSIDERAZIONI SU CICLI CON IRREVERSIBILITA' MECCANICHE INTERNE [Consideration on the Cycles with Internal Irreversibilities of Mechanical Nature]
No full textStarting from the fundamental relations relying upon the first and second law of thermodynamics, the cycles with no overall external work are examined in this paper. The restricted conditions to which such systems must satisfy under pre-defined assumptions, are determined. Some of these cycles, which exhibit internal irreversibilities of mechanical nature (cicli dissipativi) are studied in detail; as an instance of application for cycles with a recirculating fluid, the case of the single-phase thermosyphon is analyzed
OSSERVAZIONI SU TUBI DI CALORE A MEDIA TEMPERATURA [Observations on Heat Pipes at Medium Temperatures]
No full textAfter a short review of the current status of studies on this subject, the paper describes an analytical optimization method. Experimental apparatus is described and results of tests of a heat pipe operating with water up to the temperature of about 150 C are presented. In particular, the effect of condenser parameters on the heat transfer is investigated
Thermoeconomic evaluation of the feasibility of highly efficient combined cycle power plants
No full textThe paper proposes an analysis of the feasibility of highly efficient combined plants. The aim of the paper is to discuss and analyze different strategies for the increase of the efficiency of the combined cycle power plants with respect to those usually proposed in the literature. Resorting to the optimization of the components, joined with the use of regeneration and postcombustion (reheat) in the topping cycle it is shown how the combined plant efficiency can rise well over the actually well known limit of 60%. The possibility of obtaining such a high efficiency value is confirmed also by the proposed thermoeconomic optimization, based on the minimization of the total cost of the plant per unit power, obtained referring to a common economic basis the cost of the exergy losses and the costs of the components. The feasibility of obtaining combined plant with efficiency higher than 62%, simply by best fitting the available technology and without waiting for meaningful technological improvement of the gas turbines, is demonstrated
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