Boiling Heat Transfer to Liquid Nitrogen and to Liquid Helium

Abstract

Experimental data on free convection and nucleate boiling was taken in liquid nitrogen and liquid helium using platinum wires as heating elements. The results in liquid helium and particularly in liquid nitrogen were found not to agree with the generally accepted results in other liquids. In particular it was found that the transition from free convection to nucleate boiling would not take place until the temperature of the wire was much greater than that found for nucleate boiling. An "extended region" thus must be added to the free convection curve. This extended region did not reoccur in the reverse transition from nucleate boiling to free convection. It is usual to represent nucleate boiling heat transfer data to liquids as an equation of the form g/a = CΔT^n where g/ a is the rate of heat transferred per unit area, ΔT is the excess temperature of the heating surface and C and n are independent constants. There is universal agreement that 2.5 ≤ n ≤ 4 for all liquids. However, it is found for liquid nitrogen that n is not in this region but is about 11. It is then shown that C is a function of n and the equation in liquid nitrogen reduces to one with a single arbitrary constant of the form. g/a = exp^(10.25 - 2.45n) ΔT^n 5 ≤ n ≤ ∞ This equation represents a family of curves which intersect at the maximum observed value of g/a for . 0,008 inch wires. The existing mechanisms used to explain the high heat, transfer rates in nucleate boiling are reviewed and shown to be quantitatively invalid in liquid nitrogen. A "hot" molecule hypothesis is proposed, wherein a single hot molecule is assumed to supply all of the energy requirements for the growth of a bubble. It is shown that this hypothesis is invalid in itself but the calculations lead to an alternative hypothesis. This alternative hypothesis proposes that the excess energy stored in the bubble boundary acts as an energy sorting mechanism which must be present to keep a growing bubble from violating the laws of' thermodynamics. Experimental data is presented tor nucleate boiling from platinum wires in liquid helium. This data is also unusual but is more or less consistent with the results obtained in liquid nitrogen. An extended region is not, however, observed in liquid helium.