In turbulent flow, secondary random motions are superimposed on the principal flow and there is an exchange of fluid from one adjacent sector to another. More important, there is an exchange of momentum such that slow-moving fluid particles speed up and fast-moving particles give up their momentum to the slower moving particles and themselves slow down.
The factor that determines which type of flow is present is the ratio of the inertial forces to the viscous forces within the fluid. This ratio is expressed by the dimensionless Reynolds number as
| (1.6) |
where
| Re | = | Reynolds number |
| v | = | mean fluid velocity, m/s (in./s) |
| L | = | characteristic length (equal to diameter if a cross-section is circle), m (in.) |
| µ | = | dynamic fluid viscosity, Ns/m2 (lbm/in.2) |
| ρ | = | density of the fluid, kg/m3 (lbm/in.3) |
Fluid flows are laminar for Reynolds numbers up to 2100. A transition between laminar and turbulent flow occurs for Reynolds numbers between 2100 and 40,000, depending upon how smooth the tube junction is and how carefully the flow is introduced into the tube. Above Re = 40,000, the flow is always turbulent. Hence,
| Re | < | 2100 – laminar flow |
| Re | ≥ | 2100 < 40,000 – transition flow |
| Re | < | 40,000 – always turbulent |
The Reynolds number may be viewed another way:
The viscous forces arise because of the internal friction of the fluid. The inertia forces represent the fluid’s natural resistance to acceleration. In a flow with a low Reynolds number the inertia forces are negligible compared with the viscous forces, whereas in a flow that has a high Reynolds number the viscous forces are small relative to the inertia forces.
In foundry science fluidity is defined as the ability of the molten metal to flow easily before being stopped by solidification. Factors that affect the fluidity of molten metal are given in Table 1.1.
Table 1.1 Factors affecting molten metal fluidity
| FACTORS | DESCRIPTION |
| Viscosity | Viscosity is an internal property of a fluid that offers resistance to flow. If the temperature of liquid increases, the viscosity tends to decrease and the fluidity increases. |
| Surface tension | Surface tension is an effect within the surface layer of the molten metal that causes that layer to behave as an elastic sheet. Surface tension is caused by the attraction between the molecules of the liquid as a result of various intermolecular forces. A high surface tension of the molten metal decreases fluidity.Oxide films on the surface of the molten metal have a significant negative effect on fluidity. Fluxing processes in the heating of materials are used to reduce or eliminate oxidation and to improve the fluidity of surface metal layers. |
| Inclusions | Inclusions are slag or other foreign matter entrapped during molten metal casting. Metallic and nonmetallic inclusions have long been recognized as one of the most important quality issues for metal casting. The presence of inclusions is often the cause of decreased fluidity. |
| Composition | Composition is one of the main factors influencing fluidity. Small amounts of alloy additions to pure metals reduce fluidity. For example, among elements that decrease the casting fluidity of pure aluminum are Ti, Fe, Zr, and others. |
| Superheat | The difference between the melting temperature and the liquid temperature is also a very important factor influencing fluidity. Fluidity increases with increasing melt temperatures for given alloy compositions. The pouring temperature is often specified, rather than the superheat temperature, because it is easier to do so. |
| Rate of pouring | The slower the rate at which molten metal is poured, the lower the fluidity will be. |
| Mold material | Studies of the influence of the mold material have found that fluidity in fine grain sand is lower than in coarse sand. Significant metal penetration was observed in the coarse sand spiral, which resulted in an increase of the length of the spiral. |
| Heat transfer rate | If the heat transfer rate between the molting metal and the mold is reduced, fluidity increases. |
| Coating | An important function of mold coatings is to reduce the heating transfer rate between the flowing metal and the mold. |
The two tests commonly used to measure fluidity of the molten metal are the following:
•the spiral fluidity test, and
•the vacuum fluidity test.
Spiral test. The spiral test has been traditionally used in the foundry because it includes the effect of the mold material. The spiral test measures the length the metal flows inside a spiral-shaped mold.
Vacuum test. The vacuum test measures the length the metal flows inside a narrow channel when sucked from a crucible by a vacuum pump. Those tests are shown schematically in Fig. 1.3:
Fig. 1.3 A scheme of two fluidity tests: a) spiral test b) vacuum test.
The length of such casting, under standardized conditions, is taken as the fluidity index of that metal. The greater the length of the solidified metal, the greater is its fluidity.
1.5 SOLIDIFICATION AND COOLING OF METALS
After being poured into the mold, molten metal cools and solidifies. This section is primarily concerned about what happens after the metal actually is poured into a mold. A series of events and transitions takes place during the process of the solidification of the molten metal and its cooling to ambient temperature. These events greatly influence the size, shape, and chemical composition