An experimental and analytical (simulation) study on two-phase loop thermosyphons: Very small to very large systems

An experimental and analytical (numerical) study was carried out on the fluid flow, heat transfer and instabilities for five (5) small, medium and large scale two-phase loop thermosyphons. The five two-phase closed loop thermosyphons (TLTs) specially designed and constructed for the present study are one small scale loop (SSL: 5.4 mm ID & 1.2 m x 0.5 m; 150 W), two medium scale loops (MSL I: 0.095 mm ID & 0.423 m in width; 60 W and MSL II: 0.0202 m ID & 3 m x 2 m; 1,500 W) and two large scale loops (LSL I: 0.0202 m ID & 1.8 m x 2 m; 7,500 W and LSI II: 0.0202 m ID & 2.208 m x 10 m; 100,000 W). Experimentally, various parameters which would affect the operation of a TLT such as the total system temperature difference, saturation temperature of the working fluid in the system, coolant velocity over the condenser section, coolant temperature, the size of the condenser section, the amount of a working fluid in the system, the choice of working fluids, pressure drop, effectiveness and instability were investigated. Two simulation methods based on thermal resistance net work, lumped and sectorial, are presented. In the simplified model called here as the Lumped method, the evaporator section is dealt as one lumped boiling section, i.e., one empirical equation for a forced convective boiling. However, this model could not simulate the TLTs adequately because of its simplistic approach to cover all the heat transfer modes of the different flow regimes involved in the system by one equation. In the Sectorial method, all possible phenomena which would occur in the evaporator section due to the two-phase boiling process are considered in detail. Flow regimes (bubbly, slug, churn, annular and annular mist etc.), the flow transition between flow regimes and other two-phase parameters involved in two-phase flows are carefully analyzed. The results of two different simulation methods are compared with experimental results and the limitation of the computer simulation for such two-phase heat transfer systems as the TTLs studied is critically discussed. To complement the simulation study for five different scale of TTLs, an analysis was also made on the instability of the SSL as a separate study. (Abstract shortened by UMI.)