Document Type
Thesis
Date of Award
6-30-1960
Degree Name
Master of Science in Chemical Engineering - (M.S.)
Department
Chemical Engineering
First Advisor
Jerome J. Salamone
Second Advisor
George C. Keeffe
Third Advisor
C. L. Mantell
Abstract
The purpose of this study was to investigate some new approaches to the problem of correlating heat transfer coefficients in nucleate boiling, A survey of previous work done in this field reveals much inconsistency in recorded data relating the heat transfer coefficient to other properties of the observed system, but general agreement has been reached, however, concerning the important influence of the temperature difference between the heating element surface and the boiling liquid. As this temperature difference is increased, the surface coefficient of heat transfer rises until a critical point is reached, marking the end of nucleate boiling and the dominance of film boiling as the primary mechanism of heat transfer.
Although several correlations are presently available, they are limited in use due to the following reasons: (1) they become inaccurate if applied to data near the critical point; (2) they do not predict the critical point; (3) they involve constants which must be determined empirically for every different combination of liquid and surface; and (4) the equations contain terms raised to varying exponents which are usually not whole numbers, therefore making them unwieldy for practical application. Two different approaches were used to develop relationships which would improve upon those already existing. In both oases, the data of fourteen runs, made by three different investigators, was used to develop and indicate the validity of the correlations.
In the first approach, it was assumed that the relationship between the heat transfer coefficient, h, and the temperature difference, t, could be more accurately described analytically by an equation of the type
log h d + et + ft2
than by the previously used form
log h = 3 +p log t
where d, e, f, 3 and p are constants dependent on the system properties.
Using the selected data, the five constants were determined by the method of least squares, and it was found that when the linear equation was applied to 100 points, the average error between computed and experimental values was 7.6, while for the parabolic equation it was 3.2%. In addition, differentiation of the latter equation enabled prediction of a critical point, although more data is necessary to prove conclusively the accuracy of this computed maximum h.
If values of the heat transfer coefficient at three temperature differences are known for a given liquid in a particular system, this correlation can be used to determine h at any other t. A simple graphical method is presented for computing the constants, in order to avoid the tedious least square calculations.
The second approach was based on the natural convection correlation
Nusselt number = B [(Grashoff number) (Prandtl number)] n
This relationship was modified by replacing, in the dimensionless moduli, those terms which have been found to have no influence on the nucleate boiling mechanism by dimensionally equivalent terms involving influential system properties.
By graphical methods, n was found to be 1.0 in all three systems, which reduced the relationship to the form
h =BσgPLc/μ[t/tb]m
where B,m = constants dependent on the heater surface only
σ = surface tension at normal boiling point
g = acceleration due to gravity
Pl = liquid density at normal boiling point
c = specific heat at normal boiling point
μ = viscosity at normal boiling point
tb = normal boiling point expressed in absolute temperature
This correlation, although not applicable near the critical point, requires only two values of the heat transfer coefficient and the corresponding temperature differences (not necessarily for the same liquid) to evaluate B and m, and in turn, h at any t. None of the published correlations can be applied in this manner.
Both of the correlations developed require further application of data in order to conclusively prove their usefulness.
Recommended Citation
Fefferman, Gerald Burt, "Heat transfer coefficients in nucleate boiling" (1960). Theses. 2951.
https://digitalcommons.njit.edu/theses/2951
