Experimental investigations of periodic airflow reversal during forced-air cooling of Apples: Impacts on cooling rate and uniformity

Forced-air cooling of apples is typically a unidirectional flow process which often leaves the apples downward of the flow passage at elevated temperatures causing non-uniform temperature distributions at the process end. Previously, package vent and flow parameter modifications have often been investigated to improve these non-uniform temperatures and cooling characteristics with more focus on cooling rate improvements rather than cooling uniformity. This work analyzed the effects of periodic airflow reversal on cooling characteristics (i.e., cooling rate, uniformity, and energy consumption) and highlighted the challenges faced while using it as an optimization strategy. Compared to conventional designs available in the market, an improved vent design is proposed and used for comparing different flow reversal strategies at three constant cooling velocities (i.e., 1 m/s, 2 m/s, and 3 m/s). Also, the overall heterogeneity index (OHI) based on surface heterogeneity maps and energy consumption based on fan power and seven-eighth cooling times (SECT) were used to quantify the whole process uniformity and conspicuously compare different strategies. The results show that the periodic flow reversal improves precooling time, energy consumption, and cooling uniformity within the package and fruit for all flow velocities. Especially for higher velocities (i.e., 3 m/s) generally preferred for faster cooling, a significant improvement (i.e., 43.89% reduction in OHI with three reversals) in cooling uniformity was observed with more flow reversals. In addition, it is observed that OHI calculations provide a superior understanding of the cooling uniformities and properly handle experimental irregularities when compared with the other methods, which might show different results for different experiments. Therefore, for a process with intermediately seizing or warming periods like the present study, OHI can be an appropriate choice to compare the cooling uniformities. Finally, some cooling strategies and guidelines for utilizing flow reversals are proposed to achieve more uniformities at different flow velocities.

Automated summary

Forced-air cooling of apples often leads to non-uniform temperature distributions due to unidirectional flow which leaves the apples at elevated temperatures. Past studies on improving cooling characteristics have focused more on cooling rate improvements, and this work analyzed the effects of periodic airflow reversal on cooling characteristics and highlighted the challenges faced when using it as an optimization strategy. The results show that periodic flow reversal improves cooling uniformity, energy consumption, and precooling time for all flow velocities, making it a valuable strategy for achieving uniform cooling.