CFD-based ejector design to extend the operating range of ammonia/water absorption heat transformers
The early onset of cut-off conditions, referred to as operational limits, is one of the key factors hindering the adoption of ammonia/water absorption heat transformers (AHTs) in hot climates, particularly during peak summer periods. The relatively low cut-off temperature (30-35°C) arises from the coupling between the generator pressure and the pressure established at the condenser. At high ambient temperatures, condenser pressure increases, which limits the amount of ammonia that can be desorbed in the generator. One strategy to address this issue is to decouple the condenser and generator pressures, creating a system with three distinct pressure levels. A gas compressor could be used for this purpose; however, this solution is costly and may not be justified, as its operation would be limited to periods of high ambient temperatures. As a simpler alternative, gas ejectors can be employed. To operate the ejector, a portion of the evaporated ammonia is diverted from the evaporator outlet and used to entrain and compress the low-pressure vapor from the desorber, raising it to condenser pressure. This paper presents the methodology used to design and simulate an ejector integrated into an ammonia/water AHT system. A CFD-based optimization loop was developed to refine the geometry of the mixing chamber. This optimization framework is coupled with high-fidelity 2D CFD simulations that allow for a more reliable design process.