Experimental investigation on the influence of ejector geometry on the pull-down performance of an ejector-enhanced auto-cascade low-temperature freezer

•Effects of ejector geometry on EARC based low-temperature freezer were explored.•Dt and Lm are the critical parameters influencing the pull-down performances.•Malfunction of pressure lift at start-up phase was prone to occur at small Dm.•NXP seriously affects ejector performance instead of the pull-down rate Employing an ejector to recover the expansion work of the auto-cascade refrigeration cycle is a feasible method to improve the system performance. The system operation characteristics are closely relevant to the ejector geometry parameters. In order to obtain the critical structure parameters influencing the freezer's pull-down performance, experimental research was conducted on an ejector-enhanced auto-cascade refrigeration cycle applied in a low-temperature freezer. The impacts of the ejector nozzle throat, mixing chamber diameter and length, and the nozzle exit position on the system's pull-down and steady operation characteristics were explored. The experimental results illustrated that the cooling rate and the attainable freezing temperature were mainly influenced by the nozzle throat and mixing chamber length instead of the nozzle exit position and mixing chamber diameter. The nozzle throat diameter of 0.52 mm and the mixing chamber length of 25 mm was optimal concerning the fastest cool-down rate and lowest freezing temperature of -61.2 °C. At the early phase of the pull-down process, a small mixing chamber diameter would cause the ejector malfunction of the pressure lift. There was a worst nozzle exit position of slowing down the pull-down speed, rising the freezing temperature, and reducing the system COP and exergy efficiency at the given operations. The ejector yielded the maximum pressure lift ratio of 1.196 and the entrainment ratio of 0.523 at the optimal ejector geometries. This work would be helpful to guide the ejector structure optimization for the ejector-enhanced auto-cascade low-temperature freezers.