Air-jet method

Figure 4. Variation in attrition resistance as measured by the ASTM air-jet method for different aluminas and modified aluminas.

According to ASTM D 422 (2007) (AASHTO T 88 2013), materials passing through a 2.0 mm sieve are separated. A mass of test portion of approximately 100 g, if sandy soil is used, or 50 g, if clay and silt size, is placed in a 250 ml beaker. The sample is covered with 125 ml of stock solution (sodium hexametaphosphate solution, 40 g/L), stirred until is thoroughly wetted and allowed to soak for at least 12 or 16 h. At the end of soaking, the contents of the beaker are washed into a dispersion cup using distilled or demineralised water and dispersed for a period of 60 s in a mechanical stirring apparatus. An alternative air-jet method may be used for dispersion. 

Air-jet method In this method, yarn is led through the turbulent region of an air jet at a rate faster than it is drawn off on the far side of the jet. In the jet, the yam stracture is opened, loops are formed, and the structure is closed again. Some loops are locked inside the yam and others are locked on its surface. 

The oscillating jet method is not suitable for the study of liquid-air interfaces whose ages are in the range of tenths of a second, and an alternative method is based on the dependence of the shape of a falling column of liquid on its surface tension. Since the hydrostatic head, and hence the linear velocity, increases with h, the distance away from the nozzle, the cross-sectional area of the column must correspondingly decrease as a material balance requirement. The effect of surface tension is to oppose this shrinkage in cross section. The method is discussed in Refs. 110 and 111. A related method makes use of a falling sheet of liquid [112]. 

The value of the coefficient of turbulent diffusion, D, depends upon the air change rate in the ventilated space and the method of air supply. Studies by Posokhin show that approximate D values for locations outside supply air jets is equal to 0.025 m-/s. Air disturbance caused by operator or robot movement results in an increase in the D value of at least two times. Studies by Zhivov et al. showed that the D value is affected by the velocity and direction of cross-drafts against the hood face, and the presence of an operator e.g., for a cross-draft directed along the hood face with velocity u = 0.5 m/s with D = 0.15 m-/s (with the presence of an operator), an increase to = 1.0 m/s results in D = 0.3 m-/s. 

In rooms where energy is introduced primarily by supply air jets, air distribution methods are referred to as mixing type. With a perfect mixing-type air distribution, airflow pattern and air velocity at any point in the room are... 

Current mixing-type air distribution methods typically consider ventilation of the occupied zone with jets intercepting its upper boundary (e.g.. Fig. 7.6l2, c). Also, the occupied zone can be ventilated by the reverse flow produced as the supply jet degrades above the occupied zone level (Fig. 7.6h]. Mixing-type air distribution methods include air supply with jets projected vertically downward, inclined jets, jets directed vertically upward, and horizontal jets along room surfaces. 

Based on theoretical analyses, Koestel, Sheptelev," and Grimitlyn developed equations for velocities and temperatures in vertical heated and chilled air jets. The assumptions used by these authors are similar, and the method used is described in Koestel. The assumptions used in the analysis can be summarized as follows ... 

The first experimental data on confined air jets used for ventilation date back to 1939, when Baturin and Hanzhonkov studied air supply method with... 

The discussion of the interaction of air jets supplied at some angle to each other shows that application of the method of superposition of the interacting jets momentums and surplus heat to predict velocity and temperatures in the combined flow results in inaccuracy when two unequal jets are supplied at a right angle. A different approach was undertaken in the studies of interaction of the main stream with vertical directing jets. Ti i... 

Each method has its own design criteria, but common to most ol the methods is that air supply is located close to or inside the controlled zone and the exhaust openings are located inside the uncontrolled zone. The location and power of the buoyancy sources in relation to the supply air jets have a remarkable influence on the accumulations of heat, contaminants, and humidity within the room. 

The room airflows are controlled mainly by the supply or/ md circulation air jets using, for example, concentrated jets, ceiling air supply, or high-impulse nozzle systems. Examples of different methods are illustrated in Fig. 8.14. 

This short outiine suggests that it is difficult to find Optimal design para-meters for air curtains. CFD may provide a more effective design method for air curtains (see Chapter 13). There are some published articles applying CFD to air jets, but comparison with experimental data is lacking. 

The multienergy method is based on the concept that, if detonation of unconflned parts of a vapor cloud can be ruled out, strong blast is generated only by those cloud portions which bum under intensely turbulent conditions. Such cloud portions include, for instance, intensely turbulent fuel-air jets resulting from a high-pressure... 

These are processes in which the paint is used once only and the excess material is not returned to the main bulk. A typical example is the normal spray system in which the paint is fed to the spray gun, atomised by air jets and applied to the article as a stream of small droplets. The excess paint and overspray are deposited on the walls of the booth and are collected by various methods depending on the type of spray booth used . There are many modifications of the conventional spray system which include the following. 

Of the various mechanical properties of a formed catalyst containing zeolite, attrition resistance is probably the most critical. This is particularly the case for FCC catalysts because of the impact on the addihon rate of fresh catalyst, particulate emissions of fines and overall catalyst flow in the reactor and regenerator. Most attrition methods are a relative determination by means of air jet attrition with samples in the 10 to 180 xm size range. For example the ASTM D5757 method attrites a humidified sample of powder with three high velocity jets of humidified air. The fines are continuously removed from the attrition zone by elucidation into a fines collection assembly. The relative attrition index is calculated from the elutriated fines removed at a specific time interval. 

The 20-minute dwell time used is considerably greater than that used in commercial practice. For example, in the spreading of PVC plastisols on cloth, depending on the thickness of film applied and the method of heating, such as infrared or more recently hot air jets, the dwell time can be as low as 30 seconds. The temperature attained by the plastisol in these commercial processes can be as high as 190°C. In the study reported here, however, we had to choose a dwell time which would allow the attainment of an equilibrium over the whole temperature range being studied. 

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