Wall Superheat

The boiler designer must proportion heat-absorbing and heat-recovery surfaces in a way to make the best use of heat released by the fuel. Water walls, superheaters, and reheaters are exposed to convection and radiant heat, whereas convection heat transfer predominates in air preheaters and economizers. The relative amounts of these surfaces vary with the size and operating conditions of the boiler. 

Models for Prediction of Incipient Boiling Heat Flux and Wall Superheat... 

Table 6.2 shows that the ratio of qo /pUmC changes relatively weakly in the range of wide variation of heat flux and inlet flow velocity. The wall superheat significantly depends on heat flux. The data of Bergles and Rohsenow (1964) are shown in Fig. 6.1. This dependence is close to ATs,onb onb- results presented by... 

Fig. 6.3 Dependence of wall superheat on heat flux. Experiments performed by Qu and Mudawar (2002) in rectangular parallel micro-channels 231 pm wide and 713 pm deep...

Figure 6.4 shows the relation between the heat flux and the wall superheat at the ONB position obtained by Hino and Ueda (1975) in the range of the largest cavity radius r x = 0.22—0.34 pm. Experimental points show that the wall superheat at the ONB position was practically independent of the mass flux and the inlet subcooling. The lines shown in this figure represent the values of Sato and Matsumura (1964), and Bergles and Rohsenow (1964). The wall superheats reported by Hino and Ueda (1975) were much greater than those predicted by Eq. (6.9). 

Fig. 6.4 Relation between heat flux and wall superheat at the position of incipient boiling. Reprinted from Hino and Ueda (1975) with permission...

The wall superheat in Eq. (6.9) depends on saturation temperature Ts and consequently on pressure Ps ... 

Fig. 6.6 Dependence of dimensionless wall superheat Ar Qj g = AroNs/Argi g on dimensionless pressure = Ps/ s- Hsu (1962), 2 Bergles and Rohsenow (1964), 3 Thom et al. (1965), 4 Jens and Lottes (1951)...

On the other hand, Jens and Lottes (1951) and Thom et al. (1965) presented empirical correlations that showed another dependence of wall superheat on pressure. Dependence of the dimensionless wall superheat = AroNB/A7Qj,jg on... 

Comparison of wall superheat predicted by classical kinetics ofnucleation to experimental results... 

The wall superheat that corresponds to bubble formation in liquid flow can be estimated using an approach that is not connected to the mechanism of bubble formation. Such tentative estimation makes it possible to consider only the low level of wall superheat. According to Kays and Krawford (1993) the temperature distribution in turbulent flow and Pr 1 is... 

Dependence of wall superheat on inlet fluid velocity For D<, 7b,onb s and... 

Under these conditions the wall superheat depends weakly on the Reynolds number (Kennedy et al. 2000). 

For some kind of surfactant solutions boiling incipience was accompanied with hysteresis and the wall superheat up to 24 K was observed. 

Experimental and analytical studies showed that wall superheat significantly depends on the heat flux. This dependence is close to ATs.onb qnb Wall superheat corresponding to nucleate boiling may be calculated using Eq. (6.9). 

Empirical correlation (6.14) by Bergles and Rohsenow (1964) is recommended when taking the dependence of wall superheat on pressure into aceount. It agrees fairly well with the prediction of theoretical analysis based on the Hsu (1962) model. 

The temperature distribution in the capillary slot is presented in Fig. 10.18. These data show the wall superheat influence on temperature fields in liquid and vapor domains. In these cases, significant heterogeneity of temperature fields is observed. 

The results of calculations of the Nusselt number are presented in Fig. 10.19. Here also the data of the calculated heat transfer by the quasi-one-dimensional model by Khrustalev and Faghri (1996) is shown. The comparison of the results related to one and two-dimensional model shows that for relatively small values of wall superheat the agreement between the one and two-dimensional model is good enough (difference about 3%), whereas at large At the difference achieves 30%. 

Effect of superheat (Tw — 7 a() on hc All correlations for hc that have been presented so far, with the exception of Eq. (2-155), indicate that hc should vary inversely as the one-fourth power of the wall superheat (AT), if all other things are equal. The last condition, however, cannot hold if AT is varied. The result is that hc shows an appreciably larger dependence on the superheat (varies inversely as much as the one-third power). 

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