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Show The kinematic viscosity of hot furnace gases is about 12 times that of cold air and 120 times that of cold water. So if, for the purpose of dynamic similarity, equal Reynolds numbers are to be maintained in the model and in the prototype a 1:12 scale air model will require the same gas velocity as the prototype, whereas the same scale water model will require only one tenth of the gas velocity. This fact is one of the most important advantages of water models, since at low velocities flow visualization is easier and better. The success of modelling depends entirely on ensuring that there is similarity between the processes investigated in the model and the processes occurring in the full size installation. Geometrical similarity requires a constant factor to be maintained for every linear dimension in the model and the prototype. Mechanical similarity may be static, kinematic or dynamic. Static similarity requires that the model and the prototype undergo similar elastic or plastic deformations when corresponding stress systems are applied, whereas kinematic similarity means that there is a velocity scale ratio which is maintained constant between corresponding points in the model and the prototype. Dynamic similarity requires that the force ratios causing acceleration of masses in the corresponding systems are maintained. If the Reynolds number is sufficiently large, and the system investigated is fully separated flow (jet flow), as is the case in the region of a turbulent jet flame, it is not necessary to maintain the same Reynolds number in both model and prototype, provided that the Reynolds number is maintained in the turbulent regime. In cases where the process investigated is controlled by turbulent mixing (as it is in most furnaces)the molecular transport processes in such flows can be neglected and there is, therefore, no need to maintain equal Prandtl or Schmidt numbers in the model and prototype. The effects of buoyancy are ignored in cases of high Froude numbers and the reproduction of the prototype value in the model for this number is not necessary. -2- |