Heat Transfer BOILING HEAT TRANSFERFor example, in a reactor, if the critical heat flux is exceeded and DNB occurs at any locationin the core, the temperature difference required to transfer the heat being produced from thesurface of the fuel rod to the reactor coolant increases greatly. If, as could be the case, thetemperature 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 transfercoefficient 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 towrite an equation for the curve that has been drawn through the data and to compareexperimental results with those obtained by analytical means. In the application of any empiricalequation for forced convection to practical problems, it is important for the student to bear inmind that the predicted values of heat transfer coefficient are not exact. The values of heattransfer 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 turbulentand laminar flow, the accuracy of a heat transfer coefficient predicted from any availableequation or graph may be no better than 30%.SummaryThe 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. Thebubbles are swept into the coolant and collapse due to the coolant being asubcooled liquid. Heat transfer is more efficient than for convection.• Bulk boiling occurs when the bubbles do not collapse due to the coolant beingat saturation conditions.• Film boiling occurs when the heat transfer surface is blanketed with steambubbles and the heat transfer coefficient rapidly decreases.• Departure from nucleate boiling (DNB) occurs at the transition from nucleate tofilm boiling.• Critical heat flux (CHF) is the heat flux that causes DNB to occur.Rev. 0 Page 43 HT-02

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