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.
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
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
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A long cylinder of 0.1-m-diameter has a surface temperature of 400 K. If it is immersed in a fluid at 350 K, natural convection will occur as a result of the temperature difference. Calculate the Grashof and Rayleigh numbers that will determine the Nusselt number if the fluid is
(a) Nitrogen
(b) Air
(c) Water
(d) Oil
(e) Mercury
GIVEN
• A long cylinder immersed in fluid
• Diameter (D) = 0.1 m
• Surface temperature (Ts) = 400 K
• Fluid temperature (T?) = 350 K
FIND
The Grashof number (Gr) and the Rayleigh number (Ra) if the fluid is
(a) Nitrogen (b) Air (c) Water (d) Oil (e) Mercury
ASSUMPTIONS
• The cylinder is in a horizontal position (the characteristic length is the diameter of the cylinder)
SKETCH
PROPERTIES AND CONSTANTS
The thermal expansion coefficients for oil and mercury can be estimated from
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Fill in the blank(s) with correct word
Calculate the pressure amplitude (in N/m2) of a 500 Hz sound wave in helium if the displacement amplitude is equal to 5.0 × 10^?8 m. (? = 0.179 kg/m3, v = 972 m/s.)
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