Isokinetic flow

Because total air volume is the product of air-flow velocity (meters per second), orifice cross-sectional area (square meters), and time (seconds) any of these parameters could be changed in theory. In practice, a continuously variable orifice is difficult to build therefore, this value is fixed. The requirement for isokinetic flow will dictate the air-flow velocity. Therefore, only sample time can be independently controlled. 

Isokinetic flow can be achieved if the sampling system monitors the ambient wind velocity and adjusts the pumping speed as appropriate. Unfortunately, this system exacerbates the dynamic-range problem because the system samples faster in high winds, when the aerosol concentration tends to be higher. 

Aerosol monitor(s), automatic concentration range problems, 64 concepts, 63 data storage, 62 filter-paper tape, 66-67 flow conditions, varying, 65 isokinetic flow, 64 sample manipulation, 66 sampling time, 65-66... 

Air-dilute with necessary blowers, flow measurement, and control systems Use isokinetic sampling, refrigerated sample transport, and careful handling to minimize physical or chemical changes... 

Adjust the sample gas flow extracted (isokinetic sampling). 

Isokinetic Sampling I he sample gas partial volume flow must be extracted isokinetically to avoid aerodynamic separation effects and to ensure correct particle size distribution. Isokinetics means that the velocity and direction of the sample gas partial flow at the sample nozzle are the same as at the main gas stream. ... 

For isokinetic sampling, the doer sampling-point gas velocity has to be measured, and the corresponding sample gas flow calculated and adjusted. " Normally, a Pitot static tube is used for the measurement of duct gas velocity. 

Figure 5.84 Quality contours from isokinetic probe sampling of air-water flow in a 9-rod array. (From Schraub et al., 1969. Copyright 1969 by General Electric Co., San Jose, CA. Reprinted with permission.)...

Schraub, F. A., 1968, Isokinetic Sampling Probe Technique Applied to Two-Phase Flow, ASME 67-WA/FE-28, Annual Meeting, ASME, New York. (3)... 

In order to characterize the concentration of dust flowing inside a duct, a measured amount of dust must be extracted over a known period of time. This collection velocity must be the same as the internal duct flow velocity to avoid altering the distribution of dust particle sizes. In addition, a number of sample points over the entire duct cross sectional area is necessary to define the overall dust concentration. This method of sampling, known as gravimetric sampling under isokinetic conditions, was used to determine the dust concentrations at the various manufacturing areas in the Army Ammunition Plants. 

Figure 5.1 Closeup, overhead view of a plume released isokinetically into a turbulent boundary layer in an open-channel flow. Flow direction is from left to right.

The sample data presented in this chapter were collected for fairly simple flow conditions. The flow was a unidirectional open-channel flow without large-scale flow meander, and the release condition was isokinetic in the direction of the bulk flow. Thus, chemical filaments were advected by the bulk flow in the stream-wise direction, while turbulent mixing acted to expand the plume size and dilute the chemical concentration. Changes in the flow and release conditions lead to significant variation in the plume characteristics and structure. 

Several laboratory studies have contributed to our understanding of turbulent chemical plumes and the effects of various flow configurations. Fackrell and Robins [25] released an isokinetic neutrally buoyant plume in a wind tunnel at elevated and bed-level locations. Bara et al. [26], Yee et al. [27], Crimaldi and Koseff [28], and Crimaldi et al. [29] studied plumes released in water channels from bed-level and elevated positions. Airborne plumes in atmospheric boundary layers also have been studied in the field by Murlis and Jones [30], Jones [31], Murlis [32], Hanna and Insley [33], Mylne [34, 35], and Yee et al. [36, 37], In addition, aqueous plumes in coastal environments have been studied by Stacey et al. [38] and Fong and Stacey [39], The combined information of these and other studies reveals that the plume structure is influenced by several factors including the bulk velocity, fluid environment, release conditions, bed conditions, flow meander, and surface waves. 

Mass balance measurements for 41 elements have been made around the Thomas A. Allen Steam Plant in Memphis, Tenn. For one of the three independent cyclone boilers at the plant, the concentration and flow rates of each element were determined for coal, slag tank effluent, fly ash in the precipitator inlet and outlet (collected isokinetically), and fly ash in the stack gases (collected isokinetically). Measurements by neutron activation analysis, spark source mass spectroscopy (with isotope dilution for some elements), and atomic adsorption spectroscopy yielded an approximate balance (closure to within 30% or less) for many elements. Exceptions were those elements such as mercury, which form volatile compounds. For most elements in the fly ash, the newly installed electrostatic precipitator was extremely efficient. 

Laminar Flow Development. In a typical application, it is not generally possible to sample isokinetically. Equations governing the collection efficiency (e.g., equation 1) apply only under laminar flow conditions. It is considered necessary therefore to leave a length of the tubing surface at the entrance deliberately uncoated to permit laminar flow to fully develop before actual collection occurs. For a simple tube, the minimum inlet length, Lh necessary to fully develop laminar flow (within about 98%) is given by (34) ... 

Figure 1. Representation of an aircraft aerosol sampling inlet showing flow streamlines for subisokinetic, superisokinetic, and isokinetic sampling. Under subisokinetic conditions large particles will be oversampled under superisokinetic conditions small particles will be oversampled and under isokinetic conditions, the particle size distribution will be preserved.

In a wind tunnel, the direction and velocity of the airflow are predetermined, but in the field they are variable. This presents problems in the sampling of airborne particles, as discussed by May et al. (1976). If suck samples are used, the flow should be isokinetic, which requires that the sampler be aligned upwind and that the flow velocity into the sampler equals the wind speed. If these conditions are not met... 

The ideal condition for sampling is one in which the gas particle suspension is drawn into the instrument at a speed nearly equal to that of the external flow. Ideally, sampling should be done isokinetically, or with the sampler inlet velocity equal to the mainstream velocity. Only in the isokinetic case will the inertial deposition at the sampler tip be minimized and preferential size separation be small during sampling. 

If the probe is aligned with the flow and the sample velocity is equal to the stream velocity, sampling is said to be isokinetic, and the sample as collected should match the actual concentration. If sampling ve-... 

For gas-solids two-phase flow, owing to the presence of solids, measurement of gas velocity is difficult. Zhang (1990) designed an isokinetic momentum probe to measure gas velocity with reduced experimental error caused by the presence of the solids. Figure 30 shows measured results of local gas velocity under various operating conditions. 

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