Optimizing the geometry of tube-in-tube Thermosyphon heat transport device from instability Perspective: A computational study

• Thermosyphon heat transport device – a compact natural circulation loop. • Applications – passive decay heat removal in nuclear power plants. • Flow instability in the form of recirculation loops. • The center-line elevation difference beyond 0.8 m eliminates instability. • Increasing area ratio beyond 1.75 leads to instability. • Irrespective of geometry, all instabilities vanish at higher heater powers. Since the Fukushima disaster, there has been a significant surge in the adoption of infinite time passive decay heat removal systems. In this context, the thermosyphon heat transport device (THTD) has distinct advantages over natural circulation loops (NCLs). Previous studies proved that the THTD experiences flow instability characterized by recirculating loops under specific geometric and operational conditions. However, identifying the critical value for these parameters remains unresolved. Therefore, the present study deals with the geometric optimization of THTD through computational (CFD) simulations. Initially, the CFD model is validated with earlier experimental data, which resulted in a minimal deviation of −2 % to 5 %. The parameters for geometric optimization of THTD are height of the system (H), the center-line elevation difference between the source and the sink (Δ z ), heater length ( L h ), cooler length ( L c ), hot leg length ( L hl ), and area ratio (AR) between hot leg and cold leg, under the heater power range of 100 W to 1000 W. The impact of these geometric and operational variations on flow instability is evaluated by observing the presence of recirculation loops, which are identified by analyzing velocity vectors and contours. The key findings of the present study are: flow instability in the form of a single recirculating loop sweeping radially along the cross-section of the annulus, thereby leading to radial flow maldistribution; a center-line elevation greater than 0.8 m eliminates the flow instabilities; interchanging heating and cooling lengths for a given center-line elevation doesn’t alter the criteria for instability; Heater power of 100 W leads to instability irrespective of center-line elevation; the optimal cooling length can be considered as 5 % to 25 % of the total height of the THTD, whereas the optimal hot leg length can be considered as 25 % to 45 % of the total height; and the HTF flow rate increases with a rise in the area ratio until AR=2, after which a flow anomaly occurs, rendering the system unstable. In addition to these aspects, this study also suggests optimal geometric features of THTD to reduce instability and attain perfect unidirectional flow. By integrating findings from current research and past studies, the design methodology for conventional THTD is comprehensively presented to the scientific community.