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Show Frequently, all three physical modelling techniques and at mathematical modelling technique is used to study a real problem. The techniques are described briefly below. 2.1.1 Water and Air Modelling least one industrial A model of the system to be studied is manufactured to an appropriate scale in clear acrylic plastic. Water is used as the working fluid for visual qualitative investigations. Air is then used to obtain quantitative values of the flow distribution and pressure drops, figure 1. 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 veloci ty as the prototype, whereas the ' same scale water model will require velocities of only one tenth of the gas velocity. This f~ct is one of the most important advantages of water models, since at low velocities flow visualization is easier and better. Aerodynamics plays a vital role in combustion and poor air distribution has been the cause of many a combustion problem and some spectacular failures - even today aerodynamics is often not taken fully into account when designing ductwork, windboxes etc., but yet again, the techniques have been around a long time. Water bead modelling for flow visualization can be traced at least back to the 1940's, and was in widespread use by research laboratories (1,2,3) in the 1960's and commercial use by at least two burner suppliers (4,5). Yet many furnace designers and users, including multi million dollar process companies are reluctant, even today, to pay for this simple technique to ensure correct air distribution and guard against aerodynamically induced combustion problems. Air modelling even pre-dates water/bead modelling and the techniques are frequently used together to study a system, water/bead for flow visualization and air modelling for quantitative results on the same system. These techniques are used to design windboxes and air ducting for water tube boilers, petrochemical heaters etc. 2.1.2 Acid/Alkali Modelling The flames encountered in boilers, furnaces, glass tanks, rotary kilns, etc. are of the turbulent jet diffusion type. To study the properties of such flames, the rules and methods of isothermal modelling of turbulent jet systems can be applied. An understanding of such jet systems and modelling procedures is necessary for the true representation of these flames. The earliest successful combustion modelling was the use of acid/alkali modelling which uses the mixing of a jet of aqueous alkali dosed with an indicator to represent the fuel, and dilute acid to represent the combustion air, figure 2. The technique, which was developed by Sir William Hawthorne (6) at MIT as long ago as 1939 when studying jet mixing, was first used to study industrial combustion by Ruhland (7), who examined flame characteristics in a cement kiln in the mid 1960' s. Further pioneering work by British Gas (8) and by Moles and Jenkins (9) at the University of Surrey, England, in the early seventies led to its application to other processes such as glass tanks, flash calciners, metal |