Jakob number

As shown in Section 2.2.5.1, a value of C d of 1.5 X 10-4 is recommended for sodium and a value of 4.65 X 10-4 for potassium (because of their respective modified Jakob numbers). Suffice it to say that the relationship between bubble size and detachment frequency in nucleate boiling of liquid metals is not yet well established, even though it is fundamental to a good understanding of such boiling process. 

The Jakob number (Ja) is the ratio of the sensible heat carried by a liquid to the latent heat of a bubble with the same volume,... 

Evaporation Jakob Number Ja = cpLAT / AH e Compare sensible to latent heat in liquid-vapor phase change Tio Sadhal [414]... 

The Jakob number is basically a measure of the importance of subcooling expressing, as it does, the change in the sensible heat per unit mass of condensed liquid in the film relative to the enthalpy associated with the phase change. The Jakob number is small for many problems, i.e the sensible heat change across the liquid film is small compared to the latent heat release. For example, for cases involving the condensation of steam, Ja, is typically of the order of 0.01. 

Solution. Using the water and vapor properties given in Example 11.1, the following value for the Jakob number is obtained ... 

In the situation being considered, the vapor density is very small compared to the liquid density so pt(pt - Pv) = p2 is a good approximation. This is usually true for the condensation of steam. The Jakob number is given by ... 

A 30-cm high by 150-cm wide plate is maintained at 5°C and is inclined at 45° from the vertical. Calculate the rate of condensation, the heat transfer rate, and the maximum film thickness when the plate is exposed to stagnant saturated water vapor at 20°C. Do die calculation both with and without the effect of film subcooling. What is the value of the Jakob number for this problem ... 

The use of a dimensionless Jakob Number, Ja, is generally considered for large concentration changes ... 

Fig. 4. Jakob number of maximum heat flux vs reduced pressure curves for ethanol and water (B7).

Within the bubble boiling regime, thermal induced disintegration occurs when the vapor pressure unbalances the equilibrium between surface tension, viscous forces and inertial forces. The nature of this mechanism is different from those observed onto cold surfaces, as it is triggered by combined effects induced by the liquid surface tensirm and the latent heat of evaporation, /ifg, and the analysis requires the use of dimensionless groups complementary to those in Table 8.1. The most important is the Jakob number, defined as/a = Cp(Tw — 7 sat)//tfg where Cp is the specific heat of the liquid. 

The Jakob number is relevant within the transition regime given the violent boiling occurring within the early instants after impact, before the lamella levitates. On the other hand, there is an evident relation of droplet morphology with the Weber number, as shown in Fig. 8.3. 

This is also true for the complete evaporation wave. A simple criterion for the molar specific heat needed for complete evaporation can be obtained by considering the limit of Eqns. 1-3 as Wi approaches zero. If complete evaporation occurs for this case, then the entire subsonic portion of the adiabat will correspond to complete evaporation. This will be the case if the Jakob number is greater than unity ... 

In contrast to the steady-state behavior, there are significant qualitative differences in start-up behavior depending on the test liquid and reservoir pressure. Three different modes of flashing initiation are observed, which, for the range of our experimental conditions, can be ordered according to the liquid superheat based on the reservoir pressure i.e., the Jakob number). Representative examples of the three modes, taken from high speed motion picture frames, are presented in Fig. 5. 

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