Design of Mixer for Suppressing Partial Dry-out Inside a Channel of a Plate Heat Exchanger During the Evaporation Heat Transfer
To improve the heat transfer efficiency of a plate heat exchanger (PHE), it is necessary to address the flow maldistribution that occurs during two-phase operation. In the case of R-1234ze(E), the liquid phase tends to accumulate near the gasket due to its higher density compared to the vapor phase, causing the two phases to separate before entering the channel. This separation results in partial dry-out along the vapor-preferred path, which reduces the effective heat transfer area and leads to an overall loss in thermal performance. To suppress this maldistribution without redesigning the plates themselves, a helical mixer was installed in the inlet pipe to rearrange the relative positions of the liquid and vapor phases. This approach allows for improved flow mixing with minimal cost, as it requires only an external modification. The mixer design was optimized using numerical analysis that evaluated both flow patterns and pressure-drop characteristics. Although the mixer alters the phase distribution within the inlet pipe, its influence on channel-level flow must also be confirmed; therefore, full PHE simulations and experiments were conducted. Experiments performed under low vaporquality conditions—typical in practical PHE applications—revealed that the mixer increased the heat transfer coefficient by approximately 5% without causing additional pressure drop. Surface temperature measurements indicated that the dry-out region was reduced, and part of the liquid was redirected toward the previously vapor-preferred path in the lower section of the plate. Additionally, a noticeable temperature drop appeared on the opposite side of the original dry-out zone, suggesting a more balanced overall distribution. These results demonstrate that the helical mixer effectively mitigates two-phase maldistribution and enhances the overall energy efficiency of the PHE.