Boiling maximum heat flux

G. Hesse, Heat Transfer in Nucleate Boiling, Maximum Heat Flux and Transition Boiling, Int. J. Heat Mass Transfer (16) 1611-1627,1973. 

Figure 10-103A. Maximum heat flux boiling outside horizontal tubes kettle and internal reboilers. When using the estimate from this curve, a safety factor of 0.7 also should be used. (Used by permission Palen, J. W., and Small, W. M. Hydrocarbon Processing, V. 43, No. 11, 1964. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

In reboiler design, film boiling should be avoided. However, such rules-of-thumb as 10-12,000 Btu/(hr) (fti) maximum heat flux are frequently quite conservative. 

Similar results have been obtained by Bonilla and Perry 79>, Insinger and Bliss 801, and others for a number of organic liquids such as benzene, alcohols, acetone, and carbon tetrachloride. The data in Table 9.9 for liquids boiling at atmospheric pressure show that tile maximum heat flux is much smaller with organic liquids than with water and the temperature difference at this condition is rather higher. In practice the critical value of AT may be exceeded. Sauer et al.m] found that the overall transfer coefficient U for boiling ethyl acetate with steam at 377 kN/m2 was only 14 per cent of that when the steam pressure was reduced to 115 kN/m2. 

Figure 9.56. Effect of pressure on the maximum heat flux in nucleate boiling...

The heat transfer coefficient of boiling flow through a horizontal rectangular channel with low aspect ratio (0.02-0.1) was studied by Lee and Lee (2001b). The mass flux in these experiments ranged from 50 to 200 kg/m s, maximum heat flux was 15 kW/m, and the quality ranged from 0.15 to 0.75, which corresponds to annular flow. The experimental data showed that under conditions of the given experiment, forced convection plays a dominant role. 

The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

The maximum heat flux for stable nucleate boiling will, however, be less for a tube bundle than for a single tube. Palen and Small (1964) suggest modifying the Zuber equation for single tubes (equation 12.64) with a tube density factor. This approach was supported by Palen et al. (1972). 

If the vapor stream velocity exceeds this value, vapor cannot easily get away and thus a partial vapor blanketing (film boiling) may occur. This result is used to predict the maximum heat flux by relating the heat flux to the vapor velocity (see Sec. 2.4.4). 

Clinch, J. M., andH. B. Karplus, 1964, An Analytical Study of the Propagation of Pressure Waves in Liquid Hydrogen-Vapor Mixtures, Report IITRI-N-6054-6, IIT Research Inst., NASA-CR-54015. (3) Cobb, C. B., and E. L. Park, Jr., 1969, Nucleate Boiling—A Maximum Heat Flux Correlation for Corresponding States Liquids, Chem. Eng. Prog. Symp. Ser. 65(92) 188—193. (4)... 

Dhir, V. K., 1992, Some Observations from Maximum Heat Flux Data Obtained on Surfaces Having Different Degrees of Wettability, in Pool and External Flow Boiling, V. K. Dhir and A. E. Bergles, Eds., pp. 185-192, ASME, New York. (2)... 

The single tube values for hb have been correlated by equation 14.2, which applies to the true nucleate boiling regime and takes no account of the factors which eventually lead to the maximum heat flux being approached. As discussed in Volume 1, Chapter 9, equations for maximum flux, often a limiting factor in evaporation processes, have been tested by Palen and Taborek , though the simplified equation of Zuber is recommended. This... 

A test of Eq. (21), or (20), is shown in Fig. 14. The same maximum heat-flux data of Cichelli and Bonilla (C2) for three organic liquids which produced three separate lines by Rohsenow s correlation give a single line on the Forster-Zuber plot. The data of Kazakova (Kl) for water boiling at pressures varying from 10 to 80% of the critical pressure also fit the line nicely. 

Inasmuch as the critical AT corresponds to a maximum heat flux, this AT will permit a maximum duty for an evaporator, reboiler, or other boiling equipment. In practice, industrial equipment is designed to operate at slightly less than the critical A T. This gives a performance which is somewhat less than the optimum, but it provides insurance against exceeding the critical AT. 

Most researchers use "critical AT" to correspond to either of two conditions, maximum heat flux or maximum heat transfer coefficient. The two values are nearly identical for many systems. However, in some cases (boiling mercury, for example) the two are distinctly different. Evidence is needed to show which of these is more significant, i.e., which corresponds to burnout. 

Maximum heat fluxes are lower than in kettle reboilers. Because of boiling point elevations imposed by static head, vertical thermosiphons are not suitable for low temperature difference services. 

Rohsenow, W. M.. and P. Griffith Correlation of Maximum Heat Flux Data for Boiling of Saturated Liquids, AlChE-ASME Heat Transfer Symp., Louisville. Ky.. 1955. 

Water in a tank is to be boiled at sea level by a 1-cm-diameter nickel plated steel heating element equipped with electrical resistance wires inside, as shown in Fig. 10-16. Determine the maximum heat flux that can be attained in the nucleate boiling regime and the surface temperature of the heater in that case. 

SOLUTION Water is boiled at 1 atm pressure on a nickel plated steel surface. The maximum heat flux and the surface temperature are to be determined. 

This process can be nuclear or film type. In nuclear boiling, bubbles detach themselves quickly from the heat transfer surface. In film boiling the rate of heat transfer is retarded by an adherent vapor film through which heat supply must be by conduction. Either mode can exist in any particular case. Transition between modes corresponds to a maximum heat flux and the associated critical... 

---