Air entrainment effect

In the absence of confinement, ambient air entrainment effects may cause unpredictable effects for hot air impingement the heat transfer performance may worsen by 20%-50% depending upon the flow rate, difference between the jet and the ambient temperature, and nozzle-surface spacing. 

The water redncer from the first sub-gronp introdnce usually low quantity of air (on the level 2-3 % by mass of cement) and are retarders those from the second sub-group have no air entraining effect but they retard setting too. The retarding action increases markedly when the addition of plasticizer is too high and attains for example 0.6% by mass of cement [362]. 

The air-entrainment effects are related to the workability of the fresh mix with higher slump the volume of air is increased, but for high slump (al50-175 mm) the mix may be too fluid to retain the air bubbles before hardening. The proper amount and nature of an air-entraining admixture is needed to obtain the required effects and this is usually established by testing. 

The test rig shown in Figure 5.10 is near to the real extruder situation. Controlling the discharge rate with a pair of rotating sponge rollers, avoids the assistance of momentum and air entrainment effects of standard tests, whilst using a redundant extruder hopper or a copy will replicate behaviour in a production extruder. 

The primary mechanisms responsible for most particle segregation problems are sifting, particle velocity, air entrainment, particle entrainment, and dynamic effects (14). 

Jets discharging dose to the plane of the ceiling or wall are common in ventilation practice. The presence of an adjacent surface restricts air entrainment from the side of this surface. This results in a pressure difference across the jet, which therefore curves toward the surface. The curvature of the jet increases until it attaches to the surface. This phenomenon is usually referred to as a Coanda effect. The attached jet or, as it is commonly called, wall jet, can result from air supply through an outlet with one edge coincident with the plane of the wall or ceiling fFig. 7.27). Jets supplied at some distance from the surface or at some angle to the surface can also become attached (Fig. 7.28)... 

A large one-sixth-scale model of the unloader hopper was selected so that flow patterns in the enclosure could be evaluated.Smoke was used to simulate the behavior of the lime dust in the enclosure. The lime drop from the clamshell was simulated by releasing coarse sand, thus modeling the flow patterns caused by the volume displacement and the air entrainment. The effects of local wind speed and direction on the enclosure were also simulated. 

Fig. 1.3 The effect of air entrainment on compressive strength of concrete containing a water-reducing air-entraining agent and a normal air-entraining agent.

The presence of a water-reducing admixture can alter the air content of concrete, either as a deliberate measure (the air-entraining water-reducing admixtures) or as a side effect of the material in lowering the surface tension of the aqueous phase. 

Air-entraining water-reducing admixtures require special consideration the presence of entrained air leads to a reduction in compressive strength, whilst the water reduction results in a compensatory increase in strength. The effect can be quantified, however, by considering the amount of entrained air in terms of an equivalent volume of water to calculate the (air and water)-cement ratio. This new factor can be used to estimate the expected strength from Fig. 1.37. 

The superplasticizers are a special category of water-reducing agents in that they are formulated from materials that allow much greater water reductions, or alternatively extreme workability of concrete in which they are incorporated. This is achieved without undesirable side effects such as excessive air entrainment or set retardation. 

Fig. 3.1 The effect of 300 freeze-thaw cycles (according to ASTM C666 Procedure B) on air-entrained (right) and plain concrete (left).

Numerous studies have been made on the effect of additions of air-entraining agents to cement pastes which enable an insight to be gained into the mechanism by which these materials produce the stable microscopic air... 

The effect that air-entraining agents have on the rheology of fresh cement pastes can be considered from the point of view of changes due to the admixture itself, and those due to the presence of entrained air. 

Fig. 3.5 Mixing apparatus for experinnents on air-entrained and non-air-entrained pastes (Bruere). ethoxylate, has almost no effect on the paste viscosity when no air is entrained.

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