Blow-off parameter

Fig. 13.9 Computed blow-off parameter as a function of cross-flow velocity.

Whereas there is no universally accepted specification for marketed natural gas, standards addressed in the United States are Hsted in Table 6 (8). In addition to these specifications, the combustion behavior of natural gases is frequently characteri2ed by several parameters that aid in assessing the influence of compositional variations on the performance of a gas burner or burner configuration. The parameters of flash-back and blow-off limits help to define the operational limits of a burner with respect to flow rates. The yeUow-tip index helps to define the conditions under which components of the natural gas do not undergo complete combustion, and the characteristic blue flame of natural gas burners begins to show yellow at the flame tip. These... 

The fuel-concentration limits for blow-off from flameholders are correlated by a parameter of the form Uo/dn, where Uo is the average flow velocity at blow-off, d is the projected width of the flameholder, and n is an empirical exponent. Recent work suggests that some dimension of the actual recirculation zone should replace the term dn (to which the size of the zone is related) (4, 81). Table IV summarizes the reported correlating parameters for the concentration limits of blow-off from flameholders. 

Table IV. Correlating Parameters for Concentration Limits of Blow-Off From Flameholders...

The effect of pressure on burner flames is related to the burner diameter the parameter Uo/pd correlates blow-off data for various fuel concentrations (40). In other words, for a given burner and mixture, the blow-off velocity is proportional to the pressure. Quite similar parameters serve for supported flames as well (Table IV). 

Interestingly, the high cycle of heat release or simply the hot spot in this case closely follows the vortex development. The main difference from the previous case was that the inlet velocity was much lower, while the equivalence ratio was higher. In other words, the convective time scale wtis less and the chemical time scale was higher. This suggests that one of the important parameters to consider in identifying the proper location of controlled fuel injection is the Damkohler number. Near the flame blow-off limit, however, one would expect the hot spot to lag the vortices slightly. 

Figure 11-6. Blow-off flow velocities as function of equivalence ratio at different discharge parameters.

Thus, even though the blow-off pressure does not stay constant as the blister grows, Eq. (29), measured values can be employed with corresponding values of the blister height to calculate the strength of adhesion continuously no other parameters are needed. 

Green, Laskowitz, McConnel Evaluation of Design and Processing Parameters for extrusion Blow Mold Pinch-Off Design... 

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