Boiling convective

Single-phase flow region at the inlet the liquid is below its boiling point (sub-cooled) and heat is transferred by forced convection. The equations for forced convection can be used to estimate the heat-transfer coefficient in this region. 

Sub-cooled boiling in this region the liquid next to the wall has reached boiling point, but not the bulk of the liquid. Local boiling takes place at the wall, which increases the rate of heat transfer over that given by forced convection alone. 

Dry wall region Ultimately, if a large fraction of the feed is vaporised, the wall dries out and any remaining liquid is present as a mist. Heat transfer in this region is by convection and radiation to the vapour. This condition is unlikely to occur in commercial reboilers and vaporisers. 

Saturated, bulk, boiling is the principal mechanism of interest in the design of reboilers and vaporisers. 

A comprehensive review of the methods available for predicting convective boiling coefficients is given by Webb and Gupte (1992). The methods proposed by Chen (1966) and Shah (1976) are convenient to use in manual calculations and are accurate enough for preliminary design work. Chen s method is outlined below and illustrated in Example 12.9. 

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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).

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

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. 

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. 

While dimensional analysis is often a useful tool for dealing with a problem, it has not yet been successful for studying this phenomenon, mainly because the fluid properties of importance in forced-convection boiling have not been identified. Burn-out correlations based on dimensional analysis have appeared, e.g., Griffith (G5), Reynolds (R2), Zenkevitch (Zl), Ivashkevich (12), Tong et al. (T6), but the fluid properties used in these cases have been chosen on the basis of various assumptions without any demonstration that the properties used were the correct ones. They have, in fact, been shown in recent work by Barnett (B5), (to be considered later) to be either incorrect or incomplete. 

Jens and Leppert (J4), and, more recently, Barnett (B2), have emphasized the need to distinguish between a temperature-controlled surface and a heat-flux-controlled surface when referring to boiling phenomena. Failure to observe the distinction has caused some confusion in the literature, particularly as further complications arise from differences that exist between pool boiling and forced-convection boiling. 

From here on, unless otherwise stated, we shall be considering only forced-convection boiling with a heat-flux-controlled surface. 

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. 

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