Boiling heat-transfer fundamental

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 critical flux is reached at surprisingly low temperature differences around 20 to 30°C for water, and 20 to 50°C for light organics. 

The complex phenomena involved in heat transfer to a boiling liquid are discussed in Volume 1, Chapter 9. A more detailed account is given by Collier and Thome (1994), Westwater (1956, 1958), Rohsenow (1973) and Hsu and Graham (1976). Only a brief discussion of the subject will be given in this section sufficient for the understanding of the design methods given for reboilers and vaporisers. 

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W. M. Rohsenow, Boiling, in Handbook of Heat Transfer Fundamentals, W. M. Rohsenow, J. P. Hartnett, and E. N. Ganic eds., Chap. 12, McGraw-Hill Book Company, New York, 1985. 

Boiling heat transfer, which can be used for accurate temperature control of, for example, moving steel strip, has been discussed in Chap. 15 of this handbook, and reference is made to it for fundamentals of different types of boiling. Only four specific types of boiling conditions that are encountered in materials processing and manufacturing are discussed here. 

The multiplicity of phenomena characteristic of flow in heated micro-channels determined the content of the book. We consider a number of fundamental problems related to drag and heat transfer in flow of a pure liquid and a two-phase mixture in micro-channels, coolant boiling in restricted space, bubble dynamics, etc. Also considered are capillary flows with distinct interfaces developing under interaction of inertia, pressure, gravity, viscous and capillary forces. 

Veziroglu, T. N., and S. S. Lee, 1971, Boiling-Flow Instabilities in a Cross-Connected Parallel Channel Upflow System, Nat. Heat Transfer Conf., ASME Paper 71-HT-12, ASME, New York. (6) Veziroglu, T. N., S. S. Lee, and S. Kakac, 1976, Fundamentals of Two-Phase Flow Oscillations and Experiments in Single Channel Systems, NATO Adv. Study Inst. 1 423-466, Hannover, Germany. (6)... 

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. 

The fundamental dependence of the heat transfer coefficients on the quality is illustrated in Fig. 4.32. In regions of low quality x nucleate boiling occurs and the heat transfer coefficient is principally dependent on the heat hux. The quality increases downstream, and with that the how velocity also increases. The heat fed into the system is mainly transferred by convection from the tube wall to the vapour-liquid how. A conversion from nucleate boiling to convective boiling... 

In two-phase flow, the boiling temperature falls in the direction of flow as a result of the pressure drop. This results in a change in the driving temperature drop decisive for heat transfer along the flow path. Calculation of the heat transfer without simultaneous investigation of the pressure drop is therefore impossible. The fundamentals steps for this shall be explained in the following. 

M. Behar, M. Courtaud, R. Ricque, and R. Semeria, Fundamental Aspects of Subcooled Boiling With and Without Dissolved Gases, Proc. 3d Int. Heat Transfer Conf, AIChE, New York, vol. 4, pp. 1-11,1966. 

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