Calculate the maximum safe heat flux in the nucleate-boiling regime for water flowing at a velocity of 15 m/s through a 1.2-cm-ID tube 0.31-m-long if the water enters at 100 kPa pressure and 100°C saturated liquid.

Question

GIVEN

? Nucleate boiling of water flowing through a tube

? Water velocity (V) = 15 m/s

? Tube inside diameter (D) = 1.2 cm = 0.012 m

? Tube length (L) = 0.31 m

? Water pressure (p) = 1 atm

? Water temperature (Tw) = 100°C

FIND

? The maximum safe heat flux in the nucleate boiling regime (q?max)

ASSUMPTIONS

? Steady state

SKETCH



PROPERTIES AND CONSTANTS

From Appendix 2, Table 13, for water at 100°C, 1 atm



From Appendix 2, Table 35, for steam at 100°C: ?v = 0.5977 kg/m3

From Table 9.2: Surface tension at 100°C (s) = 0.0589 N/m

From Table 9.1: For water on copper, Csf = 0.0130

Answer 1

Assuming that by ‘safe’ we mean that the critical heat flux is not exceeded, we can use the Griffith

correlation, Figure 9.17. The critical pressure is Pc = 218.3 atm, P/Pc = 1/218.3 = 0.0046. From the

figure, the ordinate is therefore 6000



Since Ts = Tb, the parameter F simplifies to



The maximum heat flux is then given by