Experimental study on flow and stability characteristics of horizontal heater and cooler natural circulation loop equipped with air injection

Several studies have emphasized the role of air injection in a heated liquid column to enhance heat transfer. In recent years, this strategy has been implemented in the natural circulation loop (in specific vertical heater vertical cooler design) to improve the heat transfer fluid (HTF) flow rate. However, the same concept can also be employed in a horizontal heater horizontal cooler (HHHC) configuration to stabilize the loop in addition to the HTF flow rate augmentation. Hence, in the present experimental study, an HHHC loop (L-t/D = 298) is equipped with air injection at the vertical legs' top, middle and bottom positions. Initially the loop behaviour is compared with analytical predictions and found good agreement. Due to the selected loop geometry, the loop was stable for the entire power input range. Later, the effect of air injection rate and its position is studied by subjecting the loop to 40, 80, and 120 cc/min air flow rate. For all combinations of injection position and injection rate, void buoyancy driven flow is dominated over density gradient driven flow. At the bottom position, due to the greater elevation encountered by the air bubbles, their size remains relatively smaller, causing them to be discharged at the center of the vertical leg, unlike the other positions. This phenomenon leads to a minimum oscillation in Delta P. It also results in maximum flow rate enhancement as the longer liquid column experiences the bubbling effect (higher void buoyancy). At the bottom, when air is injected at 40 cc/min, the flow rate increases 5.2 times compared to the reference case. For 80 cc/min and 120 cc/min, the increment is 6.5 and 8.9 times. However, the top injection is not recommended as the amplitude of Delta P oscillation is high, and the flow rate enhancement is low. The present study uses both Delta P and Delta T methods to calculate HTF flow rate. Though the magnitude of both complemented each other, the trend deteriorated marginally for top air injection because of improper mixing of the HTF. Further, the thermal efficiency of the loop is evaluated, which yields 70%-83% and 80%-97% for without and with air injection respectively. As visualized, the bubble position and growth phenomena deteriorated the degree of flow enhancement concerning top and middle injection. As far as loop stability is concerned, irrespective of heat input, the flow direction follows the direction of air injection even after the supply is stopped. This tendency is advantageous in a stable loop as introducing air injection can always impose the counterflow effect at the cooler end. The flow visualization of air injection and loop flow direction is also undertaken to make the reader understand the flow dynamics better.

Automated summary

This study investigates the effect of air injection in a horizontal heater horizontal cooler configuration to stabilize the loop and increase the flow rate. The results show that with proper injection position and rate, the flow rate can be significantly improved while maintaining a stable loop behavior.