Investigation of low Global Warming Potential working fluids for a closed two-phase thermosyphon

Two-phase thermosyphons are devices offering very high thermal conductance. The study reported here examined two-phase thermosyphons of length 2200 mm and external diameter 15.9 mm. Potential applications include air to air heat exchangers with operating temperature ranges of 10-50 degrees C for the ambient (cold) side and 60-80 degrees C for the hot side. The work is prompted by the fact that R134a, used in similar units, will be subject to a ban in the future as it has a high Global Warming Potential. A shortlist of potential replacement fluids was drawn up, and considering the environmental, operating and storage conditions, and cost, five were selected for tests in representative thermosyphons. The results of the experimental work showed a water-5% ethylene glycol mixture was a suitable replacement fluid, although under certain conditions its performance was less than that of R134a. The tests also showed water alone can give the highest heat transfer, although it is not suited to the target temperature range, and methanol did not perform as well as R134a for most of the experimental range. A predictive model based on the equations published by ESDU International was developed. It was found to give good results for water, workable results for water-5% ethylene glycol, be of limited use for methanol and be unsuitable for R134a. (C) 2012 Elsevier Ltd. All rights reserved.