A BOILING HEAT-TRANSFER PARADOX

An instructive experiment for observing the Leidenfrost phenomenon is presented. The experiment, suitable for an undergraduate experimental course, consists of introducing a copper body at room temperature into liquid nitrogen and observing its temperature history. The experiment is then repeated with the body covered by a thermal insulating material, observing that the body reaches thermal equilibrium much more rapidly in the second case. This apparent paradox greatly motivates the students, who need to understand the different regimes of boiling heat transfer to resolve it. The paper also contains an approximate method to determine the insulator thickness that gives the minimum cooling period.