Search

A prototype ejector chiller with nominal cooling power of 40 kW, designed according to the “CRMC” criterion, has been tested by our research group since 2011. The prototype has undergone several refinements and finally has been upgraded to R1233zd(E), which is non-flammable, has low-GWP and favourable thermodynamic properties, i.e. “dry-expansion” and moderate pressure at generator. In terms of saturation pressure curve, the new fluid is similar to R245fa, which was previously used in the prototype. Hence, R1233zd(E) has been basically used as a “drop-in” replacement. The ejector chiller was tested at different evaporation temperatures (2.5 to 10°C), typical of air conditioning applications. Saturation temperatures at generator from 95 to 105°C have been selected as representative of waste-heat recovery. The experimental findings prove that the CRMC ejector has a satisfactory performance with the new working fluid.

This paper numerically investigates the performance of a high capacity, industrial ejector chiller working with the non-flammable, low GWP refrigerant R1233zd(E). The test procedures adopted to acquire the experimental data are explained in part 1 of this work. Different turbulence models were examined in order to accurately predict the ejector off-design transition and critical pressure. A peculiar difference has been noticed between the numerical results obtained when initializing the CFD simulation from the solution of a case with lower or higher condenser pressure. This difference may partly explain the difficulties in the accurate predictions of the critical conditions reported by this and other works in the literature. A detailed discussion is presented on the nature and possible cause of this phenomenon, which is identified as typical of non-linear fluid dynamics.

Research activity on ejectors is ongoing at the University of Florence since the late nineties. The most important achievement is a 40 kW ejector chiller designed according to the “CRMC” criterion. The experimentally validated CFD simulations have given some hints about some possible improvements, i.e. refine the surface finish of the ejector, study the effect of heat transfer and improve the final part of the diffuser, which in its present shape does not produce a measurable compression. The prototype has been recently filled with low-GWP refrigerant R1233zd, as a drop-in replacement of previously used R245fa. Both fluids are “dry-expanding” and hence significantly easier to model in CFD simulations. Synthetic low-GWP refrigerants may be an option for ejector chillers, due to their ability to reach below-zero temperature and high volumetric refrigerant capacity. Some lessons learned with synthetic refrigerants can be transferred to the project of a steam ejector chiller, which remains one of our future targets. Herein we resume the principal findings gathered by means of experimental and numerical activity on our prototype and propose a few ideas for the future research.

Based on a numerical study of the water vapour absorption process in LiBr–H 2 O solution, for a laminar, gravity driven, viscous, incompressible liquid film, flowing over a horizontal cooled tube, irreversibilities related to fluid friction, heat transfer, mass transfer and their coupling effects have been locally and globally examined. The hydrodynamic description is based on Nusselt boundary layer assumptions. The tangential and normal velocity components, respectively obtained from momentum and continuity equations, have been used for the numerical solution of mass and energy transport equations in the two-dimensional domain defined by the film thickness and the position along the tube surface. Local entropy generation calculation can be performed referring to the calculated velocity, temperature and concentration fields. Results have been explored in different operative conditions, in order to examine comprehensively the impact of the various irreversibility sources and to identify the least irreversible solution mass flow-rate for the absorber. As a parallel, a refined understanding of the absorption process can be obtained. Considering absorption at the film interface and cooling effect at the tube wall, the analysis thermodynamically characterises the absorption process which occurs inside actual falling film heat exchangers and establishes a criterion for their thermodynamic optimisation. Results suggest the importance to operate at reduced mass flow rates with a thin uniform film. Meanwhile, tension-active additives are required to realise this condition.

The Constant Rate of Momentum Change (CRMC) criterion attempts to improve the design of supersonic ejectors, that can be used in heat-powered chillers for industrial or air-conditioning use. Moving from its original formulation, the CRMC design method can be advanced accounting for friction irreversibilities and real gas behaviour, as done in a previous work by our research group. Here we present an upgraded version of this analysis, supported by experimental data from a prototype chiller using R245fa as working fluid. The analysis is extended to other fluids (water, isobutane, 5 HFCs and 3 HFOs) whose performance is calculated on a wide range of heat source/sink temperatures. The existing literature, based generally on ideal gas simulations, suggests that water yields poor results in terms of COP. This paper shows that this result may be argued. Low GWP fluid HFO1233zd also gives good results.

In this work a thermodynamic analysis of a desiccant wheel is proposed to investigate and identify the optimum size and operating regime of this device. A steady state entropy generation expression, based on effectiveness parameters suitable for desiccant wheels operability, is obtained applying a control volume approach and assuming perfect gas behaviour of the binary air–vapour mixture. A new entropy generation number NL is defined using a minimum indicative value of the entropy generation SL,min and investigated in order to obtain useful criteria for desiccant wheels optimization. The effectiveness-NTU design method is employed by combining solution of thermal exchange efficiency for rotary heat exchanger with the characteristic potential method, under the conditions of heat and mass transfer analogy. The analysis is applied to a specific desiccant wheel and NL variation with NTU is explored under various operative conditions and wheels characteristics in terms of dimensionless velocity and flow unbalance ratio.

Ejector refrigeration has been studied at DIEF (Dipartimento di Ingegneria Industriale Firenze) since the ‘90s. Use of environmentally safe fluids (steam) was addressed. A two-stage prototype with cooling capacity 5 kW was optimized and built. Later, the CRMC prescription for the design of the supersonic diffuser was focused. By a gradual reduction of the fluid velocity and a continuous profile, the CRMC design promises a reduction of the normal shock that usually develops in the mixing chamber. A second 40 kWf prototype was designed in 2010 for an industrial partner (Frigel Firenze s.p.a.). The design procedure used a thermodynamic code accounting for real gas behavior. This code gives a first design of the mixing chamber and diffuser according to the CRMC criterion. It also gives an estimation of the friction loss along the diffuser. A comparison between different operating fluids was performed and resulted in the selection of R245fa. A first design of the ejector was manufactured in carbon fiber. The primary nozzle is mounted on a movable support, in order to change its axial position with respect to the mixing chamber. In terms of COP, first results were below the values predicted by the simulation code. Meanwhile a numerical simulation was in progress with FLUENT. From the first CFD results it was decided that the diffuser throat had to be moved forward from the primary nozzle exit, in order to allow a complete mixing between the primary and secondary flows, and enlarged, the losses encountered in the mixing process being higher than expected and hence the fluid density lower. This produced a second ejector design, which was manufactured and tested in 2012, showing improved performance. These results suggested a third design, with a further lengthened diffuser, which has undergone a complete testing campaign, allowing validation of the CFD results. The activity performed till now suggests that ejector refrigeration plants have a robust operation and can be easily manufactured at relatively low cost with off-the-shelf components, a part from the ejector itself, which however represents a small fraction of the system cost. However, the COP is lower with respect to absorption refrigeration and hence needs substantial improvement through detailed thermodynamic and CFD design optimization.

Optimisation is a design method to search the best value of the defined system's goal that can be expressed by an objective function to be minimised or maximised. A set of unknowns subject to constraints controls the values that the objective function can assume. A multi-variable approach enables one to address the optimisation of a thermodynamic system: the best working conditions of the system are different from those corresponding to component optimisation. Then we have to model the complete system to find an optimumty. Different objective functions are presented as optimisation criteria of the design data of a steam ejector cycle, keeping the same boundary conditions and convergence limits and using a numerical optimisation of the cycle published by the authors. The comparison between the results obtained with different objective functions is presented to show the influence of the function chosen on the system design. The comparison also shows that the choice of the objective function decisively influences the robustness of the numerical code results and the convergence performances of the code.

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.