Boiling Heat Transfer to Liquid Nitrogen and to Liquid Helium
Boiling Heat Transfer to Liquid Nitrogen and to Liquid Helium
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Date
1960
Authors
Mann, Horace Tharp
Advisor
Shreeve, C.A. Jr
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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.