Nucleate and Convective Boiling

The convective and nucleate boiling heat transfer coefficient was the subject of experiments by Grohmann (2005). The measurements were performed in microtubes of 250 and 500 pm in diameter. The nucleate boiling metastable flow regimes were observed. Heat transfer characteristics at the nucleate and convective boiling in micro-channels with different cross-sections were studied by Yen et al. (2006). Two types of micro-channels were tested a circular micro-tube with a 210 pm diameter, and a square micro-channel with a 214 pm hydraulic diameter. The heat transfer coefficient was higher for the square micro-channel because the corners acted as effective nucleation sites. 

Fig. 4.31 Heat transfer coefficient in nucleate and convective boiling (qualitative)...

Essentially, except for once-through boilers, steam generation primarily involves two-phase nucleate boiling and convective boiling mechanisms (see Section 1.1). Any deposition at the heat transfer surfaces may disturb the thermal gradient resulting from the initial conduction of heat from the metal surface to the adjacent layer of slower and more laminar flow, inner-wall water and on to the higher velocity and more turbulent flow bulk water. 

Except at the lowest heat and mass fluxes, both nucleate boiling and convective boiling components were present. 

Depending on the type of boiling, we differentiate between evaporation, nucleate boiling and convective boiling. We will consider evaporation first. 

Whilst in nucleate boiling the heat transfer coefficient is chiehy dependent on the heat hux q and is barely dependent on the how velocity, curve b in Fig. 4.29, in convective boiling the heat transfer is determined by the how velocity or the mass hux m, with the heat hux having vitually no inhuence. This is shown by Fig. 4.31, in which the nucleate boiling and convective boiling regions are distinctly separate from each other. 

The variety of regimes during the forced convection boiling in tubes or ducts requires different correlations in order to determine the heat transfer coefficient related to the respective boiling mechanisms. The well-established correlations have been developed for nucleate boiling controlled heat transfer - when evaporation occurs at the inner tube surface - and convective boiling heat transfer - when evaporation occurs at the liquid film interface. 

In a boiler, with the continued application of heat, steam under pressure is produced via a combination of steam bubble formation (nucleate boiling) and direct evaporation at the steam-water interface (convective boiling), as shown in the sketch of different generated steam flow forms in Figure 1.1. 

Figure LI Steam generation from a heated surface, showing nucleate boiling, leading to bubbly, intermediate, annular and mist flow forms of convective boiling. Steam bubbles in water (a) leading to water droplets in steam (b).

Typically, FT boilers tend to have lower rates of overall heat-flux and lower steam/water quality, and nucleate boiling predominates. Water tube (WT) boilers tend to have higher heat fluxes and higher steam/water quality under these conditions, annular flow convective boiling tends to dominate. 

High heat-transfer rates at boiler surfaces promote rapid nucleate boiling and other forms of convective boiling, which in turn may cause steam blanketing. 

The conclusion to be drawn from the above examples and many others is that softness in a boiling system, preceding the boiling channel inlet, may cause flow oscillations of low frequency. It is probably the pressure perturbations arising from the explosive nature of nucleate boiling that initiates the oscillation, and the reduced burn-out flux which follows probably corresponds to the trough of the flow oscillation, as a reduction in flow rate always drops the burn-out flux in forced-convection boiling. 

The detail experimental study of flow boiling heat transfer in two-phase heat sinks was performed by Qu and Mudawar (2003b). It was shown that the saturated flow boiling heat transfer coefficient in a micro-channel heat sink is a strong function of mass velocity and depends only weakly on the heat flux. This result, as well as the results by Lee and Lee (2001b), indicates that the dominant mechanism for water micro-channel heat sinks is forced convective boiling but not nucleate boiling. 

Kandlikar SG (2006) Nucleation characteristics and stability considerations during flow boiling in micro-channels. Exp. Thermal and Fluid Science 30 441 47 Katto Y, Ohno H (1984) An improved version of the generalized correlation of critical heat flux for the forced convective boiling in uniformly heated vertical tubes. Int J Heat Mass Transfer 27 1641-1648... 

In forced-convective boiling the effective heat-transfer coefficient hcb can be considered to be made up of convective and nucleate boiling components h fc and h nb. 

Braver, H., and F. Mayinger, 1992, Onset of Nucleate Boiling and Hysteresis Effects under Convective and Pool Boiling, Engineering Foundation Conf. on Pool and External Flow Boiling, Santa Barbara, CA, pp. 1 14. (4)... 

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