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The results obtained using the rohsenow equation can

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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 (10-3) 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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