Novel flow modulation method for R744 two-phase ejectors – Proof of concept, optimization and first experimental results

The cooling industry involves various essential applications, such as food preservation, medicine storage and air conditioning. However, its significant direct and indirect contribution to global warming is bound to increase in years to come, leading to the need for highly efficient cooling units using eco-friendly working fluids. Consequently, carbon dioxide (R744) is achieving resounding success as a refrigerant for various medium- and large-capacity applications, as some of the available expansion work is recovered with the aid of two-phase ejectors. However, its adoption is being limited for small-capacity solutions (e.g. condensing units) due to the current lack of a suitable flow modulation technique for two-phase ejectors installed in these units. Therefore, the goal of this work is to bridge this knowledge gap by formulating and experimentally proving an innovative flow control mechanism for two-phase ejectors, being based on the pulse-width modulation (PWM) of the refrigerant flow through the ejector. All the experimental evaluations were carried out at the compressor speed of 50 Hz, water temperature at the gas cooler inlet of 35 °C and R744 evaporating temperature of about −5 °C.

The first experimental data revealed that the high pressure can be controlled appropriately as well as varied from about 87 bar to 112 bar, demonstrating the effectiveness of the proposed technique. In addition, the effect of the muffler volume as well as the PWM period on the ejector and system performance were investigated. It was found that the influence of both the muffler volume and the PWM period was not significant. Compared to the solution employing the passive ejector (i.e. without flow modulation technique), the unit with the PWM ejector presented enhancements in coefficient of performance (COP) by more than 5% at the optimum operation conditions. It is worth mentioning that its today’s available competitors, i.e. needle-based ejector and vortex-based ejector, feature COP enhancements by 2%–4% as contrasted with the passive ejector. As benchmarked to the standard unit (i.e. with flash gas by-pass valve and without ejector), the PWM ejector could improve the COP by more than 10% at the optimal running conditions. Also, the results obtained suggest that at present the proposed solution should operate with a PWM period of 2 s and no mufflers. Finally, the PWM ejector is characterized by low cost, simplicity, low vulnerability to clogging and no practical size or application constraints.