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Unformatted text preview: ce greatly affects heat transfer, and the
Rohsenow equation given above is applicable to clean and relatively smooth
surfaces. The results obtained using the Rohsenow equation can be in error by
100% for the heat transfer rate for a given excess temperature and by 30%
for the excess temperature for a given heat transfer rate. Therefore, care
should be exercised in the interpretation of the results.
Recall from thermodynamics that the enthalpy of vaporization hfg of a pure
substance decreases with increasing pressure (or temperature) and reaches
zero at the critical point. Noting that hfg appears in the denominator of the
Rohsenow equation, we should see a significant rise in the rate of heat transfer at high pressures during nucleate boiling. Peak Heat Flux
In the design of boiling heat transfer equipment, it is extremely important for
the designer to have a knowledge of the maximum heat flux in order to avoid
the danger of burnout. The maximum (or critical) heat flux in nucleate pool
boiling was determined theoretically by S. S. Kutateladze in Russia in 1948
and N. Zuber in the United States in 1958 using quite different approaches,
and is expressed as (Fig. 10–11)
·
q max Ccr hfg[ g 2 ( l )]1/4 (103) where Ccr is a constant whose value depends on the heater geometry. Exhaustive experimental studies by Lienhard and his coworkers indicated that the
value of Ccr is about 0.15. Specific values of Ccr for different heater geometries are listed in Table 10–4. Note that the heaters are classified as being large
or small based on the value of the parameter L*.
Equation 10–3 will give the maximum heat flux in W/m2 if the properties
are used in the units specified earlier in their descriptions following Eq. 10–2.
The maximum heat flux is independent of the fluid–heating surface combination, as well as the viscosity, thermal conductivity, and the specific heat of the
liquid.
Note that v increases but and hfg decrease with increasing pressure, and
·
thus the change in qmax with pressure depends on which effect dominates. The
·
experimental studies of Cichelli and Bonilla indicate that qmax incr...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
 Spring '10
 Ghaz

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