Empirical design criteria

When the relationship between the material flux and the parameters of the system can be calculated directly by solution of the appropriate differential equations, the criterion equation (2.7.30) has little significance. However, this is not possible in the great majority of practical systems, and thus the empirically determined criterial equation is of general validity for physically similar systems. It can form a basis for designing larger equipment on the basis of experiments with model systems. 

Equation (62) can be applied to derive an approximate criterion for flame stabilization by bluff-body stabilizers. Empirically, when the approach flow velocity Uj in the combustor exceeds a critical blowoff velocity Wi niax Ihe flame is blown downstream and can no longer be stabilized by the bluff body. A knowledge of is essential in the design of ramjet... 

Residence Time Theory. Residence time theory is based on the consideration of whether a particle will reach the cyclone wall in a given residence time. In the development of this theory, the distribution of all particles across the inlet is assumed to be homogeneous (Rietema, 1961). The cut size will be the size of those particles that enter the center of the inlet pipe and just reach the wall within the residence time. Using this theory coupled with extensive experimental test data, Rietema was able to estabhsh a set of empirical correlations and suggest a criterion (a characteristic cyclone number) for optimum design of hydrocyclones. 

In practical applications the designer or analyst may combine a failure criterion and knowledge of known defects and fracture energies to predict how and where failure is initiated and the subsequent progress of damage in the material. Such analyses usually have to be augmented by empirical data to model the complete failure process. 

An interesting observation is that, while geckos and some insects have adopted hairy tissues for robust and reversible adhesion, some other insects and tree frogs seem to have achieved this via smooth tissues. The main similarity of both designs is that the structured pad surfaces or particular properties of the pad materials guarantee a maximum real contact area with diverse substrates. Dahlquisfs criterion, which is based on empirical observations of pressure-sensitive adhesives, establishes an upper limit for Young s modulus ( 100 kPa) of materials with tack... 

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