Close
There is of course no single answer to this question. There are, in fact, different criteria for judging the quality of a mixture:
1. Intrinsic quality of mixture
2. Time for procurement of mixture
3. Cost for procurement of mixture
1. Intrinsic Quality of mixture
We often define the quality of a mixture based on statistics helping to express the homogeneity of the mixture or the particle size obtained after emulsion for example. The satisfaction level of a mixture is often defined by a % of difference in a given volume of mixed products or by comparing a "batch" of production to another.
These statistical assessments are implemented through:
- Global measures: observation or detection of a tracer (dye) in the whole mixture made.
- Local measures: detection of a tracer by a local transducer (pH-metry, conductimetry, thermal…)
Indices based on averages and variances of concentration fields of tracers used allow judging the quality of mixtures observed.
2. Time for procurement of mixture
The mixing time is the time necessary for procuring the desired quality of mixture. It is necessary to define that time by taking into account a gap of acceptability compared to the ideal sought. It is interesting to note that for turbulent flow regimes the N.tm result is constant (N: agitator rotation speed, tm: mixing time), i.e. the mixture is still obtained after the same number of agitation rounds, whatever the dimensions of the equipment. It has often been observed that a "good mix", avoiding dead or poorly mixed zones, is obtained after a time tm = 3 to 5 tc. (tc : time of movement necessary for a fluid to make a complete rotation in the bowl)
3. Cost for procurement of mixture
The energy consumed to get the desired mixture quality, may be a criterion for assessment. It is as a fact more than one criterion for estimating mixer performance than the quality of the mixture itself. The consumed power depends on both the physical characteristics of the mixture itself, but also that of the mixer and its operating parameters. The expression of the agitation power in non-dimensional numbers leads to the definition of specific mixer values, such as the Power Number (NP), the Froude Number (Fr) or the Reynolds Number (Re).
