Determination of operation envelopes for closed, two-phase thermosyphons

Abstract

A concern in the design and operation of closed, two-phase therosyphons (CTPTs) is determining the initial filling ratio of the working fluid, as a function of CTPT dimensions, type and vapor temperature of working fluid and power throughput, to maximize performance, while avoiding potential dryout in the evaporator section. A one-dimensional, steady-state model is developed for determining the operation envelopes of CTPTs, in terms of the above parameters. The CTPT operation envelope is basically an enclosure with three boundaries. The lower boundary corresponds to when the liquid film thickness in the evaporator reaches a critical value—beyond it the liquid film could dry out—while the upper boundary corresponds to when the expanding liquid pool, due to boiling, fills the entire evaporator. The third and closing boundary corresponds to the counter-current flooding limit (CCFL) at the exit of evaporator. The correlation developed to calculate the expanded liquid pool height in the evaporator agrees with experimental data of acetone, ethanol and water to within ±8%. Also, the calculated upper and lower boundaries of the operation envelope for an ammonia CTPT are in excellent agreement with experimental data [1]. Calculations showed that increasing the CTPT diameter, evaporator length, or vapor temperature expands the operation envelope, while increasing either the condenser or the adiabatic section length only slightly changes the envelopes upper and lower boundaries.