Effect of hot leg length on the steady-state performance of single-phase natural circulation based tube-in-tube thermosyphon heat transport device with heat sink directly above the heat source

Thermosyphon heat transport device (THTD) is a compact natural circulation loop to enhance the heat transport distance and the power rating compared to existing devices like heat pipe. Hence it finds application in indoor solar cooktops and diverse passive fuel cooling systems to eliminate Fukushima type accidents in advanced nuclear power plants. THTDs for the nuclear application have the heat sink directly above the heat source in the same vertical leg. Practically no study is reported, even for simple rectangular loops with heat sink directly above the heat source in the same vertical leg. All reported studies for rectangular loops with vertical heater vertical cooler configuration are for the heat source in the vertical up leg and heat sink in the other vertical down leg. The main issue with a heat sink located directly above the heat source in the same vertical leg is that it leads to instability with recirculation loops (bidirectional flow in the cooler with the heavy coolant coming down along the cooled wall and light fluid rising along the adiabatic wall) in the cooler. Earlier reported studies indicated that at higher powers, the instability is suppressed. In this paper, another method of eliminating this instability was experimentally studied, which involved increasing the hot leg length and, thereby the elevation difference. The main finding is that the instability can be avoided if the hot leg length (equivalent to increasing the height) is increased. The same finding may hold good even for the rectangular loops with heat sink directly above the heat source in the same vertical leg. Besides, the study also revealed that the same one dimensional steady-state dimensionless flow equation applicable to simple rectangular loops is found to be valid for the tube-in-tube type THTD considered in the present work.