Numerical investigation of heat transfer in a CO2 two-phase ejector

Abstract

In this paper, the influence of heat transfer in the walls of an R744 two-phase ejector on ejector performance was investigated. A numerical investigation was performed using a computational fluid dynamic (CFD) model of the R744 two-phase flow coupled with the heat transfer inside the ejector. An ejector equipped with thermocouple channels was designed and manufactured to investigate temperature distribution in the inner walls under boundary conditions typical for a refrigeration and air-conditioning application in a supermarket. The ejector was installed on the test rig to perform a test series that evaluated the outer walls of the ejector with and without insulation. The experimental results were used to validate the proposed CFD model, and a numerical investigation was performed to analyse the influence of heat transfer on ejector performance. The motive nozzle and suction nozzle mass flow rates accuracies were within ±7% and ±15%, respectively. In addition, the proposed CFD model predicted the wall temperatures with ±5 K accuracy for most of the validated points. The heat transfer coefficient of the R744 two-phase flow inside the ejector is presented. The non-adiabatic inner walls degraded ejector performance. The maximum reduction of the mass entrainment ratio reached approximately 13%.

Ejectors on the cutting edge: The past, the present and the perspective

“…Owing to the p-T relationship of CO 2 the ejector operating conditions are likely characterized by high temperatures and, thus, the heat losses at the walls might not be negligible. To this end, Haida et al [35] numerically and experimentally studied the influence of walls heat transfer of a CO 2 ejector on the ejector performances: the non-adiabatic inner walls degraded ejector performance (i.e., the mass entrainment ratio decreases up to 13%).…”

Non-equilibrium approach for the simulation of CO2 expansion in two-phase ejector driven by subcritical motive pressure

“…Moreover, the maximum reported inaccuracy was 52.0%. In this region of lower motive pressure, the HRM approach improved prediction accuracy to an average level of 20.2% and a maximum of 29.0% (Palacz et al, 2017a;Haida et al, 2018a). Nevertheless, as reported in the work of Haida et al (2018a), the largest underestimation of the m M N was located below 59 bar of the motive pressure, while in the operating range between 59 bar and 70 bar, the average accuracy was on the level of 6.5%.…”

An experimental investigation of performance and instabilities of the R744 vapour compression rack equipped with a two-phase ejector based on short-term, long-term and unsteady operations

“…Furthermore, additional six electric heaters each with a nominal power of 7.5 kW were integrated with the glycol tank for the MT evaporator section to enhance the cooling demand at very low evaporation temperature below -10 • C.A detailed description about the auxiliary loops can be found in Section 2.2. The main use case of the ejector-expansion rack presented in Figure 2 is to experimentally investigate the ejector performance for various ejector designs, i.e., vapour ejector [37], liquid ejector [38], or the bypass ejector [39]. The ejector section of the experimental rig is equipped with precise Coriolis-type mass flow meters, pressure sensors and temperature sensors installed at ejector primary and secondary inlets as well as at the ejector outlet.…”