Entropijska produkcija pri stacionarnom provodenju topline kroz stijenku cilindra s toplinskim izvorom

U radu je detaljno analiziran problem minimizacije entropijske produkcije za model stacionarnog provođenja topline u stijenci cilindra s toplinskim izvorom i nametnutim izotermnim rubnim uvjetima. Problem je riješen uvođenjem relevantnih bezdimenzijskih varijabli: bezdimenzijske izdašnosti toplinskog izvora, omjera vanjskog i unutrašnjeg polumjera cilindra, kao i omjera rubnih temperatura. Pomoću tih veličina i bezdimenzijskog polumjera izvedeni su analitički izrazi za temperaturno polje, te lokalnu i ukupnu entropijsku produkciju te je postavljen kriterij za postojanje minimuma entropijske produkcije. Rezultati provedene analize su obrazloženi te je prikazano područje vrijednosti omjera polumjera cilindra, te omjera rubnih temperatura, za određene vrijednosti izdašnosti toplinskog izvora, u kojem postoji minimum entropijske produkcije

Metafizika volje i pesimizam kod Schopenhauera : Diplomski rad

Rad ne sadrži sažetak

Metafizika volje i pesimizam kod Schopenhauera : Diplomski rad

Rad ne sadrži sažetak

Detailed analysis of the effect of the turbine and compressor isentropic efficiency on the thermal and exergy efficiency of a Brayton cycle

Energy and exergy analysis of a Brayton cycle with an ideal gas is given. The irreversibility of the adiabatic processes in turbine and compressor is taken into account through their isentropic efficiencies. The net work per cycle, the thermal efficiency and the two exergy efficiencies are expressed as functions of the four dimensionless variables: the isentropic efficiencies of turbine and compressor, the pressure ratio, and the temperature ratio. It is shown that the maximal values of the net work per cycle, the thermal and the exergy efficiency are achieved when the isentropic efficiencies and temperature ratio are as high as possible, while the different values of pressure ratio that maximize the net work per cycle, the thermal and the exergy efficiencies exist. These pressure ratios increase with the increase of the temperature ratio and the isentropic efficiency of compressor and turbine. The increase of the turbine isentropic efficiency has a greater impact on the increase of the net work per cycle and the thermal efficiency of a Brayton cycle than the same increase of compressor isentropic efficiency. Finally, two goal functions are proposed for thermodynamic optimization of a Brayton cycle for given values of the temperature ratio and the compressor and turbine isentropic efficiencies. The first maximizes the sum of the net work per cycle and thermal efficiency while the second the net work per cycle and exergy efficiency. In both cases the optimal pressure ratio is closer to the pressure ratio that maximizes the net work per cycle

Analysis of Exergy Destruction of an Evaporator or/and a Condenser

This paper presents the exergy destruction of an evaporator or a condenser or evaporator and condenser. The exergy destruction can be analytically calculated by using the same non-dimensional variables which have been used by its energy analysis. The one additional variable is the ratio of absolute input temperatures of both streams. This means that the given model enables the simultaneous calculation of heat transfer effectiveness and exergy destruction respectively. The exergy destruction is put in the ratio to the heat capacity of the weaker stream. The limited values of the exergy destruction are estimated, taking into account the zero and infinite values of relevant variables. Further, exergy destruction for the case when both streams change phase is calculated, which means that the appropriate heat exchanger is condenser and evaporator simultaneously. The given model is derived as a special case of the model for calculation of exergy destruction by parallel and counter flow heat exchangers respectively. For all presented cases the only temperature differences between the streams have been taken into account. This means that the effect of a frictional pressure drop of the streams on exergy destruction has been neglected

Entropy generation minimization in steady heat conduction through a cylinder wall with internal heat generation

U radu je detaljno analiziran problem minimizacije entropijske produkcije za model stacionarnog provođenja topline u stijenci cilindra s toplinskim izvorom i nametnutim izotermnim rubnim uvjetima. Problem je riješen uvođenjem relevantnih bezdimenzijskih varijabli: bezdimenzijske izdašnosti toplinskog izvora, omjera vanjskog i unutrašnjeg polumjera cilindra, kao i omjera rubnih temperatura. Pomoću tih veličina i bezdimenzijskog polumjera izvedeni su analitički izrazi za temperaturno polje, te lokalnu i ukupnu entropijsku produkciju te je postavljen kriterij za postojanje minimuma entropijske produkcije. Rezultati provedene analize su obrazloženi te je prikazano područje vrijednosti omjera polumjera cilindra, te omjera rubnih temperatura, za određene vrijednosti izdašnosti toplinskog izvora, u kojem postoji minimum entropijske produkcije.Minimization of the entropy generation in steady heat conduction through a cylinder wall with an internal heat generation and isothermal boundary conditions is considered in a detail. The problem is solved analytically by introducing the relevant dimensionless variables: the dimensionless rate of internal heat generation, the ratio of the outside to the inside radius, and the ratio of the boundary temperatures. By means of these values and the dimensionless radius, the analytical expressions for the temperature distribution, and the local and total entropy generation are derived. A necessary condition for the existence of a minimum of the entropy generation is presented and the results of the analysis are explained. The field of the values of the ratio of the outside to the inside radius and the ratio of the boundary temperatures, for certain values of the internal heat generation, in which minimum of the entropy generation exists is presented

Mathematical Model of Complete Vapor Compression Refrigeration System with Helical Coil Evaporator Flooded in the Water

Refrigeration cycle system modelling of the vapor compression experimental unit with the goal to predict the system performance of the cycle and geometry of the helical coil evaporator flooded in the water is presented in the paper. Design of the experimental unit is based on the commercially available scroll compressor and air cooled condenser. In order to determine the thermodynamic conditions of refrigeration cycle and heat transfer process in the evaporator the simulation model is developed. The model takes into account the specific data, dimensions and characteristics of the main components. Evaporation process, observed in three parts, and condensation process are described with appropriate heat transfer correlations. With two approximation functions, developed based on manufacturer data, the model of compression process is described. Results show relations between thermal resistance and geometrical quantities of evaporator with influence on the system performance. Analysis of thermal resistance shows that geometry of the evaporator may have important effect on the final design of these types of refrigeration applications

Exergy Analysis of an Air Conditioning Process

The exergy analysis of an air conditioning process in wintertime is presented in this paper. This process consists of mixing outdoor air with conditioned air, heating, humidifying, and reheating the air mixture. The air is heated in the heat exchangers by warm water at a temperature of 80/60 °C and humidified by spraying water at a temperature of 12 °C. The ratio of the mass flow rates of outdoor and conditioned air is varied, and it is upon this variable that the exergy analysis is performed. The five cases are analyzed with different outdoor air temperature values. It is shown that the heat transfer rate in the heat exchangers is the smallest at the ratio of mass flow rates of outdoor and conditioned air g1 = 0.45. The greatest exergy destruction is apparent in the heat exchangers, and the maximum exergy efficiency of the whole process ɛex = 0.71 is achieved at the ratio g1 = 0.45. According to the exergy criterion, this air conditioning is a relatively efficient process

Comparison of Finned Tube and Plate-Finned Heat Exchangers in Waste Heat Recovery

The use of waste heat recovery devices on mobile units (trucks and ships) is usually limited by the available space and the application of compact heat exchangers is recom¬mended for such purposes. The performance of the heat exchanger is defined by the optimized Rankine cycle (to achieve maximum power) and it depends on the mass flow and temperature of the flue gases and selection of the working fluid in Rankine cycle. An example of selection of the preheater performance (in case of water as the working fluid) is considered here, wherein the several surface types of finned tube and plate-finned heat exchangers are used, for which there are measured data of the heat transfer coefficient and friction factor. Heat exchangers are sized according to the criteria of maximum allowed velocity of the flue gases and shapes of the heat exchanger frontal area. The sized heat exchangers were compared with respect to the heat transfer coefficient, the area of the heat exchanger surface on the finned side, the volume of the heat exchanger and the pressure drop on both sides. From the comparison of the best plate-finned and the best finned tube heat exchangers it is concluded that in the recommended range of flue gases velocity (from 4 m/s to 6 m/s) the pressure drop at the gas side are similar (in the plate-finned heat exchanger it is in the range from 53.5 to 112 Pa and in the finned tube exchanger from 53.8 to 142.8 Pa), while the plate-finned exchanger has more than 50% smaller heat transfer area, compared to the finned tube one