Speaker
Description
The efficiency of vapor compression cycles (VCC) such as those used in aeronautical Environmental Control Systems (ECS) is heavily dependent on the performance of the evaporator. A critical technical challenge related to the latter is the maldistribution of two-phase refrigerant flow within the header and channels, which can lead to thermal losses of up to 30%. With the pressing transition from high-GWP refrigerants like R134a to low-GWP alternatives such as R1234ze(E), characterizing these flow behaviors in complex geometries is essential for revising and optimizing heat exchanger design.
The experimental setup comprises a full-scale VCC loop that operates with R1234ze(E), developed within the PANTTHER Clean Sky 2 project. It features a semi-hermetic reciprocating compressor and a plate-and-fin evaporator. The evaporator includes two passes of parallel channels on the refrigerant side and a single pass with wavy fins on the air side. The orientation of the evaporator header and channel can be modified, along with the inlet and outlet tube positions.
To quantify refrigerant flow maldistribution, an infrared (IR) thermography methodology monitors the front side of the evaporator first pass. A set of 1000 IR images is acquired at 50 Hz and time-averaged for each test condition. From these averaged images, 18 refrigerant channel profiles are extracted. A uniformity distribution coefficient (Φ) is calculated using the methodology established by Bowers et al., based on the estimated height at which superheated vapor is produced in each channel. This coefficient ranges from 0 (extreme maldistribution) to 1 (perfect uniformity).
The air outlet temperature is investigated using four thermocouple rakes installed on the air side of the evaporator outlet. These rakes contain 26 type K thermocouples that are positioned according to the ISO 3966 standard account for potential spatial temperature variations. By linearly interpolating the thermocouple measurements in the X and Y direction, a two-dimensional temperature contour of the air downstream of the evaporator is obtained. The uniformity of this temperature contour is as well evaluated by means of a mixing effectiveness criterion.
The work will focus on the comparison between the refrigerant maldistribution estimated using IR images (only considering first pass distribution), and the maldistribution estimated using the air temperature contour. The comparison will examine three specific evaporator orientations and inlet/outlet configurations. The effectiveness, limitations, and potential for IR thermography as a non-intrusive technique for maldistribution quantification, as well as correlation to the maldistribution quantified using air temperature contours will be discussed.
