Stagnant boiling

The vaporization at the surface acts as a heat sink, which could be imaginarily replaced by another process, for example radiation. As the vaporization takes place at the surface, we speak of stagnant boiling. By its nature this process belongs to the phenomena of convection in closed spaces. Heat transfer coefficients from the heated surface to the liquid are formed with the driving temperature difference t 0 — i L, where 0(l is the wall temperature of the heated surface and is the liquid temperature. As the liquid temperature i9L is not known in advance... 

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...

If a(q) from (4.73) or (4.74) and (4.75) are graphically illustrated, two straight lines are produced, if the ordinate and abcissa have logarithmic scales. Two clearly separate areas are obtained, one for stagnant the other for nucleate boiling, as shown in Fig. 4.29, which reproduces the measured values from Jakob et al. [4.44], [4.45],... 

Uniform internal energy u " = 6 MW/m3 is generated in a long, vertical, cylindrical fuel element (D = 5 cm, k = 20 W/m-K) immersed in a stagnant water pool at TM = 40 °C Assuming the water is pressurized (no boiling), determine (a) the surface temperature of the fuel element and (b) the center temperature of the fuel element, (c) Repeat (a) and (b) for a water flow with Vm = 5 m/s. 

Boiling heat transfer A liquid-to-vapor phase change at a heated surface, categorized as either pool boiling (occurring in a stagnant liquid) or flow boiling. 

The effects of heat transfer on corrosion rates depend on the s>fstem. If corrosion is under activation control the presence of heat transfer might not have much effect. On the other hand, if corrosion is controlled by diffusion of a species, such as oxygen, to the surface, then heat transfer may greatly change the corrosion rate. There are three possible causes. First, a temperature difference between the wall and bulk solution may affect the solubility and diffusion coefQcient of the diffusing species. Second, boiling at the surface can increase turbulence or increase diffusion. Third, heat transfer in the absence of Quid Qow, as in a stagnant tank, can cause natural convection currents that enhance mass transfer [79]. 

Figure 10-4 shows how the refrigerant system was changed. The reactor (evaporator) was revamped from stagnant boiling in the tubes to forced... 

Water (cont) stagnant hot water boiled and controlled at temperature below boiling 71 1042 5 ... 

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