RELATIVE THERMAL PERFORMANCE OF SUPERCRITICAL CO_2, H_2O, N_2, AND He CHARGED CLOSED-LOOP THERMOSYPHON-TYPE HEAT PIPES

摘要

Compared with water, CO_2's relatively high density at high temperatures and pressures makes it significantly more efficient as a working fluid for trans-critical power generation, and also, much smaller physically sized pipes and turbo machinery can be used. As such this paper considers the use of CO_2 as the working fluid for closed and closed-loop two-phase thermosyphon-type heat pipes. It also compares the relative performance of CO_2 compared with superheated and supercritical water, helium, nitrogen, and sodium. A time-dependent (transient) computer simulation program of a typical closed-loop thermosyphon has been developed. The simulation model requires the disctretization of the loop into a series of control volumes, the application of the equations of change (conservation of mass, momentum, and energy) to each control volume, and the solution of the set of nonlinear partial differential equations so generated using an explicit numerical formulation technique and appropriate properties functions and equations of state, for closure. The assumptions made include one-dimensional and quasi-equilibrium flow conditions and upwind differencing. The results of this analysis are collated in a tabular and graphical form thereby allowing the relative performance and scaling characteristics of the various working fluids to be readily established. It is also envisaged that such information will be useful in the design of high performance heat transfer systems, especially in the light of a greater awareness of and the ever-increasing imperative for sustainable and pollution-free energy and heat transfer solutions.