Performance evaluation of a model thermocompressor using computational fluid dynamics

Thermocompressors are widely used in a large number of industries that use steam as their heating medium or as a power generating utility. They are devices that use the energy of a high pressure fluid to move a low pressure fluid and enable it to be compressed to a higher pressure according to the principle of energy conversion. They work like a vacuum pump but without usage of any moving part and so they can save energy. The performance of a thermocompressor highly depends on its geometry and operating conditions. This paper first describes the flow behavior within a designed model of a thermocompressor using the computational fluid dynamics code, FLUENT. Since the flow is turbulent and supersonic, CFD is an efficient tool to reveal the phenomena and mixing process at different part of the thermocompressor which are not simply obtained through an experimental work. Then its performance is analyzed by choosing different operating conditions at the boundaries and also different area ratios which is one of the significant geometrical factors to describe the thermocompressor performance. Finally, the effect of various nozzle exit plane diameters which cause different Mach numbers at the nozzle exit is investigated on the thermocompressor performance. The results indicate that these variables can affect both the entrainment ratio and critical back pressure. This device uses water vapor as the working fluid and operates at 7.5 bar motive pressure, 63°C and 80°C for suction and discharge temperatures, respectively

Simulation and measurement of condensation and mixing effects in steam ejectors

Vapour compression via ejectors has become a topic of interest for researchers in the field of air conditioning and refrigeration. Ejectors have the benefit of being extremely reliable with stable operation and no moving parts leading to essentially maintenance free operation. However, these devices typically have very low efficiencies due to the low entrained mass flow rate of the low pressure secondary stream relative to the high pressure primary stream mass flow rate. The entrainment of the secondary stream and mixing between the primary and secondary streams are therefore dominant features which require investigation. Entrainment and mixing typically occurs under conditions of compressible, turbulent flow with strong pressure gradients. Steam ejectors, which are the focus of the present work, have the added complexity of condensation effects which must be accommodated in modelling and simulation work. Condensation in the primary nozzle of steam ejectors alters the steam flow properties relative to properties derived from ideal gas modelling, which is sometimes used for steam ejector analysis work. By performing computational simulations for non-equilibrium wet steam flow in a representative primary nozzle, the altered steam jet properties that arise during the nozzle expansion process are demonstrated, via empirical correlations, to be of sufficient magnitude to affect the mixing rate, and thus the entrainment ratio, of steam ejectors. For the particular primary nozzle and flow conditions considered, it was estimated that these changes in steam properties would cause around 29% increase in the mixing layer growth rate for the wet steam case relative to the ideal gas case. To further explore the influence of wet steam mixing effects, the non-equilibrium wet steam computational simulation approach was then expanded to the case of a complete ejector. Under particular conditions for the choked flow ejector operation, results indicated that the non-equilibrium wet steam model simulates an entrainment ratio that is 10% higher than that for the ideal gas model. The non-equilibrium wet steam model also gives a higher critical back pressure by about 7% relative to the ideal gas model. Enhanced mixing layer growth, which arises due to steam condensation in the primary nozzle, was identified as the main reason for higher entrainment ratio of the ejector simulations using the wet steam model. Higher pitot pressure of the mixture at the diffuser entrance for the wet steam simulation was also identified as the reason for higher critical back pressure for the ejector relative to the case of ideal gas simulation. To estimate the relative significance of pressure-driven effects and mixing-driven effects on the secondary stream entrainment, ideal gas computational simulations were also performed. Under a fixed operating condition for the primary and discharge streams, the ejector entrainment ratio was more strongly influenced by the mixing effects at lower secondary pressure. For a particular ejector and associated operating conditions, about 35% of the ejector entrainment ratio was attributable to mixing effects when the secondary stream pressure lift ratio was 4.5, while this portion was reduced to about 22% when the secondary stream pressure lift ratio was 1.6. Given the significance of ejector mixing effects and the lack of consensus on the most appropriate model for turbulent mixing in steam ejectors, an experimental investigation was performed to provide direct data on the mixing of wet steam jets in steam ejectors for model development and validation of computational simulations. Pitot and cone-static pressures within a high pressure supersonic steam jet that mixed with low pressure co-flowing steam were obtained. Results from the non-equilibrium wet steam simulations were analysed to give values of pitot pressure and cone-static pressure values using both equilibrium and frozen-composition gas dynamic models. The equilibrium analysis appeared reasonable for the pitot pressure, whereas the frozen-composition analysis was a better approximation for the cone-static pressure. Differences between the experimental data and the wet steam computational simulations were in the vicinity of 25% at certain locations. The static pressures downstream of the nozzle exit were lower than the triple point, but energy exchanges associated with the transitions to and from the solid phase were not incorporated in the wet steam model. The development of such a model is required before definitive conclusions can be made regarding the accuracy of the turbulence modelling

Ejector primary nozzle steam condensation: area ratio effects and mixing layer development

Recent ejector simulations based on wet steam modeling give significantly different performance figures relative to ideal gas modeling, but the origins of such differences are not clear. This paper presents a numerical investigation of flow in the primary nozzle of a steam ejector to further explore the differences between ideal gas and wet steam analysis of ejector flows. The wet steam modeling was first validated using primary nozzle surface pressure data from three experiments reported in the literature. Ejector primary nozzles with area ratios (AR) of 11, 18 and 25 were then simulated using wet steam and ideal gas models. The wet steam simulations show that nozzle static pressures are higher than those for ideal gas model, and in the AR = 25 case, the static pressure is larger by a factor of approximately 1.7. In contrast, no significant difference exists between the nozzle momentum flux for both ideal gas and wet steam models, except the vicinity of the nozzle throat where nucleation occurs. Enhanced mixing between primary and secondary streams, which arises because primary stream condensation reduces compressibility in the mixing layer, is proposed as an explanation of the increased entrainment ratio observed in recent wet steam ejector simulations

Mixing layer effects on the entrainment ratio in steam ejectors through ideal gas computational simulations

Ejector entrainment ratios are influenced by both pressure-driven effects and the mixing between the primary and secondary streams, but the significance of each factor has not been identified in prior literature. This paper presents a computational simulation investigation of flow in a representative steam ejector to specify the contribution of mixing and pressure-driven effects to the overall ejector entrainment ratio under different operating conditions. The simulation of mixing layer growth was validated by using experimental data available in the literature, while the application of the computational method to the ejector flows was validated using static pressure distribution and entrainment ratio data in the particular experimental ejector arrangement. Simulation results show that under a fixed operating condition for the primary and discharge streams, at lower secondary pressure the ejector entrainment ratio is more strongly influenced by the mixing effects. For the particular ejector and the operating conditions considered herein, about 35% of the ejector entrainment ratio is due to mixing effects when the secondary stream pressure lift ratio is 4.5, while this portion is reduced to about 22% when the secondary stream pressure lift ratio is 1.6

Effect of mixing on the performance of wet steam ejectors

Steam ejector computational simulations using a wet steam model give higher entrainment ratios and higher critical back pressures for the ejector compared with the ideal gas model. This paper identifies the origin of these differences. Simulation results show that the wet steam model predicts an entrainment ratio for the choked flow ejector operation that is 10% higher than that for the ideal gas model. The wet steam model also gives a higher critical back pressure by about 7% relative to the ideal gas model with a closer agreement to experimental data for the unchoked ejector operation. Enhanced mixing layer growth which arises due to steam condensation in the primary nozzle is identified as the main reason for higher entrainment ratio of the ejector simulations using the wet steam model. The difference in the mixing layer growth rate between ideal gas and wet steam simulations is 21%, indicating enhanced entrainment for the wet steam model. Furthermore, the mixture at the start of the diffuser is shown to have a higher pitot pressure than in the ideal gas simulations and these elevated pitot pressures allow the ejector to operate in a choked mode to a higher critical back pressure