بهینه سازی عددی یک گیاه اجکتور رفرجرت دو مرحله ای

Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.

The ejector is a well-known device since the early twentieth century. It was largely experimented and used in industrial process and plane propulsion. Recently, the increasing interest in environment safety is opening new possibilities in the refrigeration field, reintroducing the steam ejector refrigeration cycle in spite of low efficiencies compared with the traditional refrigeration plants. Many works about physical model formulation of the ejector integrated on the refrigeration thermodynamic cycle were carried out [1], [2], [3], [4] and [5] using the typical ejector pattern. Further attempts [6] to improve the ejector low performances modifying the traditional ejector scheme, were proposed. Therefore, the ejector refrigeration plant analysis and optimisation, including heat transfer irreversibilities, with an unusual ejector scheme seems interesting. 1.1. The ejector refrigeration cycle The ejector refrigeration cycle is a three-thermal refrigeration machine that can be considered as overlapping a motive and a refrigeration cycle (Fig. 1 and Fig. 2). The thermal power QG, supplied at pressure PG to the generator by the highest temperature thermal source is partially converted into work giving thermal power Q′C to the intermediate temperature thermal source throughout the condenser. The refrigeration cycle uses this work to produce the refrigerating effect at the evaporator, transferring thermal power QE from the low temperature thermal source to the intermediate temperature thermal sink. The overlapping appears at the condenser and at the ejector, that is the device that transfers work to the refrigerating fluid. The working points of the fluid, shown in Fig. 2, are related to ejectors sections (Fig. 3). The whole plant is considered as a steady_state open system that exchanges heat with the three thermal sources and consumes external work due to friction losses on the water side of the heat exchangers and to the pump transferring fluid from the condenser to the generator.

The code developed allows to obtain a coherent numerical simulation of the whole ejector refrigerating plant, including thermal irreversibilities. Results are in agreement with literature data [22], [23], [24] and [25] as Table 3 shows.The plant COP appears strongly depending on the condenser and evaporator temperatures. Simulation results show high ejector compression ratio with a very compact geometrical configuration but low entrainment ratio. Heat exchangers design obtained by numerical optimisation, shows very high scattering. This is due to the optimisation strategy used, but also to the very low influence of the considered pressure losses. The trend showed by the ΔTML, Kay's efficiency and the K factor at the evaporator, denotes high influence of this heat exchanger design on the plant COP. This suggests to search how to improve the evaporator performances for future plant analysis. It is interesting to underline that the best COP value is reached for very low heat exchangers efficiency at the condenser and at the generator. The best working system conditions are different from those corresponding to components optimisation. Then it is necessary to model the entire system to find an optimum.