Heat transfer study of the closed two-phase thermosyphon with inner tube

The issue concerning the dynamic response of closed two-phase thermosyphon with inner tube inserted inside evaporator creating natural circulation system is getting on increasing degree of attention due to its relevance during startup. Thermosyphons transient operation for startup from ambient condition to steady state is considered a stringent necessity for vital applications such as electronic, solar, geothermal and even nuclear reactors safety systems. This typically returns to the need to keep the temperature within certain limits before reaching critical conditions. Also, a greater understanding of the thermosyphon and its transient behavior is needed.Transient thermal-hydraulic analysis of the closed two-phase water/copper thermosyphon with inner tube is theoretically and experimentally presented. The main objective of the current study is to develop a theoretical model that can predict the dynamic behavior of the double-tube evaporator by tracing various transient parameters during operation from start up to steady state condition.The model numerically describes thermosyphon of double tube evaporator (double tube thermosyphon DTT) by seven coupled sub-models. These sub-models are: heater, wall of thermosyphon, fluid in the riser hot channel inside evaporator, vapor core in adiabatic and condenser sections, liquid in the down-comer of thermosyphon, the condensate film in the condenser, and finally the cooling water of the condenser. The mathematical model is derived by applying the energy balance for each section and sub-section in addition to continuity, and momentum equations for the liquid and vapor in different sections of double tube thermosyphon (DTT). The model simulates the thermosyphon by several differential equations which are solved to predict the various parameters. A computer program is designed to solve these differential equations by an explicit finite difference method. To validate the predicted theoretical model results, an experimental investigation has been conducted. An experimental setup is constructed from: thermosyphon main tube of 42 mm inner diameter, 4 mm thickness and 1950 mm long. The evaporator and condenser section lengths are 1100 mm and 400 mm respectively, while the adiabatic section is 450 mm long. An inner tube made of poly propylene is installed inside the evaporator. The inner tube has outer diameter of 34 mm with thickness 3.5 mm and 1100 mm length. This structure forms a hot channel with 4 mm annular gap width, where the natural flowing liquid circulated between inner and outer tubes. The heat source is simulated by 4 electrical coils while the heat sink is directly performed by a cooling process at condenser section, using cooling water of 0.0456 kg /s.