Heat Transfer
BOILING HEAT TRANSFER
For example, in a reactor, if the critical heat flux is exceeded and DNB occurs at any location
in the core, the temperature difference required to transfer the heat being produced from the
surface of the fuel rod to the reactor coolant increases greatly. If, as could be the case, the
temperature increase causes the fuel rod to exceed its design limits, a failure will occur.
The amount of heat transfer by convection can only be determined after the local heat transfer
coefficient is determined. Such determination must be based on available experimental data.
After experimental data has been correlated by dimensional analysis, it is a general practice to
write an equation for the curve that has been drawn through the data and to compare
experimental results with those obtained by analytical means. In the application of any empirical
equation for forced convection to practical problems, it is important for the student to bear in
mind that the predicted values of heat transfer coefficient are not exact. The values of heat
transfer coefficients used by students may differ considerably from one student to another,
depending on what source "book" the student has used to obtain the information. In turbulent
and laminar flow, the accuracy of a heat transfer coefficient predicted from any available
equation or graph may be no better than 30%.
Summary
The important information in this chapter is summarized below.
Boiling Heat Transfer Summary
•
Nucleate boiling is the formation of small bubbles at a heat transfer surface. The
bubbles are swept into the coolant and collapse due to the coolant being a
subcooled liquid. Heat transfer is more efficient than for convection.
•
Bulk boiling occurs when the bubbles do not collapse due to the coolant being
at saturation conditions.
•
Film boiling occurs when the heat transfer surface is blanketed with steam
bubbles and the heat transfer coefficient rapidly decreases.
•
Departure from nucleate boiling (DNB) occurs at the transition from nucleate to
film boiling.
•
Critical heat flux (CHF) is the heat flux that causes DNB to occur.
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