Critical heat flux of forced convective boiling in uniformly heated vertical tubes, with special reference to very large length-to-diameter ratios.

Abstract

The systematic nature of CHF in uniformly heated vertical tubes, including the condition of very large length-to-diameter ratios, are studied by analyzing the CHF data of R-12 at 1.47 MPa, obtained as many as 1051, for tube diameter d=3, 5 and 8 mm, length-to-diameter ratio l/d=50, 100, 200, 400, 600, 800, and mass velocity G=510-6055 kg/(m2·s). It is shown that CHF data in a tube of low l/d values with mixed inlet conditions are cannot be substituted for CHF data in a tube of high l/d values with subcooled inlet conditions. Those data obtained under subcooled inlet conditions are then analyzed on the basis of two typical, existing methods for the generalized correlation of CHF, disclosing that CHF values for tubes of very small diameters tend to deviate from standard values of CHF with increasing the value of l/d. A tentative method to predict the foregoing deviation of CHF values is also given.