THEORETICAL STUDIES ON FLASHING DRIVEN NATURAL CIRCULATION LOOPS BASED ON CARBON DIOXIDE

摘要

In this paper the results of theoretical studies carried out on carbon dioxide based natural circulation loopsrnfor refrigeration and air conditioning applications are presented. A mathematical model is developed tornsimulate the steady state performance of a rectangular loop, which operates either as a pure single phasernnatural circulation loop (NCL) or as a flashing driven natural circulation loop (FDNCL) depending upon thernoperating conditions. The flashing driven loop is modeled using both homogeneous as well as drift-fluxrnmodels. The results obtained from the mathematical model for a symmetrical, single-phase naturalrncirculation loop are compared with the analytical solution, and a good agreement is found between the two.rnEffects of design and operating parameters of the NCL on balanced mass flow rate are obtained. The looprndimensions are optimized based on the temperature drop across the heat exchangers and the mass of carbonrndioxide charged. The results show that compared to pure single-phase loop, a flashing driven loop yieldsrnhigher balanced mass flow rates and reduced temperature drop across the heat exchangers. For FDNCL, thernresults obtained using homogeneous flow model and drift flux model are found to be close, hence, thernmathematically simpler homogeneous flow model may be adequate for the present case.