This paper reports on an experimental study to determine the conditions under which flow reversal occurs in an opposed forced convection - natural convection heat exchange system. Hot water, which flowed upward, was cooled in a double pipe heat exchange made from acrylic plastic pipe. The point at which flow reversal occurred in the hot water at the heat exchanger wall was determined by observing a dye trace which was generated by changing the pH of the hot water at small platinum electrodes imbedded in the wall. This pH change caused a colour change in the Thymol blue indicator dissolved in the hot water. Various lengths and diameters of heat exchangers were investigated.
The point at which flow reversal occurred was found to be correlated satisfactorily by the expression:

Gr = 0.045 Re^2.43

It was found that flow reversal was insensitive to the length-to-diameter ratio of the heat exchanger and the Prandtl number , at least over the range investigated.