Heat transfer in boiling

Whilst heat transfer in convection can be described by physical quantities such as viscosity, density, thermal conductivity, thermal expansion coefficients and by geometric quantities, in boiling processes additional important variables are those linked with the phase change. These include the enthalpy of vaporization, the boiling point, the density of the vapour and the interfacial tension. In addition to these, the microstructure and the material of the heating surface also play a role. Due to the multiplicity of variables, it is much more difficult to find equations for the calculation of heat transfer coefficients than in other heat transfer problems. An explicit theory is still a long way off because the physical phenomena are too complex and have not been sufficiently researched. 

The cause of this is not only that there are many influencing quantities that play a role in boiling processes, but also different types of heat transfer depending on the flow configuration and superheating. These different types of heat transfer will be considered first, followed by an explanation of the physical fundamentals of boiling phenomena. The final part of this section will consist of the calculation of the heat transfer. 

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Hahne, E. and W. Grigull, Heat Transfer in Boiling, Academic Press/Hemisphere Publishing Go. (1977). 

Steiner, D., and E. U. Schlunder, 1977, Heat Transfer and Pressure Drop for Boiling Nitrogen Flowing in a Horizontal Tube, in Heat Transfer in Boiling. E. Hahne and U. Grigull, Eds., p. 263, Hemisphere, Washington, DC. (4)... 

Fig. 4.29 Heat transfer in boiling water at 100 °C on a horizontal heated surface, according to Jakob etal. [4.43], [4.45]. Curve a stagnant boiling region, curve b nucleate boiling region...

Heat transfer in boiling is more easily understood when we know how the vapour bubbles form on the hot surface. The following consists of a discussion of the formation and growth of vapour bubbles, with a subsequent explanation of the different types of heat transfer. 

Some basic terms and definitions from two-phase flow theory are required for the description of heat transfer in boiling. For this we will consider the section of a channel shown in Fig. 4.48, in which the gas and liquid are flowing. An annular flow is shown in the picture for the sake of simplicity, but the following terms are... 

In the calculation of the heat transfer in saturated boiling it is presumed that the heat transfer is predominantly determined by the bubble formation. In addition, convection also has some influence on the heat transfer, although this may only be small. Models and empirical equations start from the heat transfer in boiling in free flow. The influence of forced flow is taken into account with an extra term. This is based on the concept that the growth of the bubbles is only slightly influenced by the flow, as long as the bubbles are smaller than the superheated boundary layer. The heat transfer coefficient is a combination of two parts... 

Stephan, K. Bubble formation and heat transfer in natural convection boiling. In Hahne, E. Grigull, U. Heat transfer in boiling. Washington Hemisphere 1977, p. 3-20... 

V. M. Zhukov, G. M. Kazakov, S. A. Kovalev, and Y. A. Kuzmakichta, Heat Transfer in Boiling of Liquids on Surfaces Coated With Low Thermal Conductivity Films, Heat Transfer Sov. Res. (7/3) 16-26,1975. 

FIGURE 15.45 Zones of heat transfer in boiling (from Del Valle and Kenning [92], with permission from Elsevier Science). 

Rathjen, W. Straub, J. In Heat Transfer in Boiling, Hahne, E. Grigull,... 

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