Duct flow

Table 5. Correlations for Heat-Transfer and Darcy Friction Coefficients for Noncircular Laminar Duct Flow ...

Reynolds number. Rcfj= FIGURE 9.46 Friction factor f of duct flow, ... 

Capture efficiency can also be measured by first estimating workspace emission rates and local exhaust emissions. The local exhaust emission rate equals the duct concentration (mass/volume) multiplied by the duct flow rate (volume/time). The workspace emission rates can be calculated using appropriate mass balance models and measured ventilation rates and workspace concentrations. Capture efficiency is the ratio of duct emission rate to total emission rate (duct plus workspace). ... 

In air ducts, the measurement of the local air vekxiity is used to determine the flow rate in the duct. The duct flow is usually more stable and the flow direction under better control than in the room space. Different types of disturbances in the ductwork, such as bends, tees, or dampers, will influence the nature of the flow and cause swirl and other problems in velocity measurement. 

There is also a standardized method based on the estimation of the flow rate on one measurement point only, In this method the velocity probe is placed in the duct so that the measured local velocity is equal to the mean axial velocity. In fully developed turbulent duct flow, this distance from the wall... 

The previous methods are mainly used to measure duct flow. When measuring flows on supply or exhaust terminals, different methods are used. The measurement on exhaust terminals is simple to carry out, as the velocity field near the terminal is relatively constant, with no steep gradients or swirls. In the case of a grill, traversing across the terminal surface using a suitable velocity instrument is a good alternative. A suitable instrument for most cases is the vane anemometer. 

Separation occurring due to contractions or expansions in duct flow. 

Note that for straight duct flow at constant cross-section, the total and static pressures decrease together (constant resistance). At the contraction section, the total pressure decreases very litde, but static pressure is converted to velocity pressure, because static and velocity pressures are mutually convertible. At the sudden enlargement, the process of changing the velocity pressure to static pressure is inefficient, and a total pressure loss occurs. AtJ the static pressure is 0, and the total pressure is the velocity pressure as the gas stream leaves the duct. 

Most plate heat exchanger designs fall into the viscous flow range. Considering only Newtonian fluids since most chemical duties fall into this category, in laminar ducted flow the flow can be said to be one of three types ... 

BOOTHROYD, R. G. Trans. Inst. Chem. Eng. 44 (1966) T306. Pressure drop in duct flow of gaseous suspensions of fine particles. 

Kostic M (1994) On turbulent drag and heat transfer reduction phenomena and laminar heat transfer enhancement in non-circular duct flow of certain non-Newtonian fluid. Int J Heat Mass Transfer 37 133-147... 

Duct flows, like steady two-dimensional flows, are poor mixers. This class of flows is defined by the velocity field... 

Duct flows can be converted into efficient mixing flows (i.e., flows with an exponential stretch of material lines with time) by time- modulation or by spatial changes along the duct axis. One example of the spatially periodic class is the partitioned-pipe mixer (PPM). This flow consists of a pipe partitioned with a sequence of orthogonally placed rectangular plates (Fig. 5a). The cross-sectional motion is induced through rotation of the pipe with respect to the assembly of plates, whereas the axial flow is caused by... 

D time periodic flows and spatially periodic duct flows can produce chaos. A necessary condition for chaos is crossing of streamlines. 

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. 

Of the three operations in building B-46, higher dust concentrations were generated by the demachining operation. Fairly constant concentrations were found across the duct. This can be attributed to the high duct flow velocities. 

The charge density appears to be approximately proportional to the peak mass flow rate (duct flow rate, Q, times the maximum dust concentration) in the duct. 

Figure 3.18 Generic process reactor alternatively equipped with PAT sensors or probes, shown at typical deployment locations at different reactor heights. TOS analysis again results in rejection of all options presented. The critique leveled against incorrect PAT sensor deployment in ducted flow (Figure 3.16) applies in full force for reactor sampling as well.

Figure 3.19 Generic ducted flow sampling solution completely respecting all TOS principles (elimination of IDE and lEE). It is a prerequisite that appropriate flushing/cleaning measures (always problem-dependent) eliminate potential IPE in the bypass extractor segment.

Fig. D-5 shows an external compression air-intake designed for optimized use at Mach number 2.0. Fig. D-6 shows a set of computed airflows of an external compression air-intake designed for use at Mach number 2.0 (a) critical flow, (b) sub-critical flow, and (c) supercritical flow. The pressures at the bottom wall and the upper wall along the duct flow are also shown. Two oblique shock waves formed at two ramps are seen at the tip of the upper surface of the duct at the critical flow shown in Fig. D-6 (a). The reflected oblique shock wave forms a normal shock wave at the bottom wall of the throat of the internal duct. The pressure becomes 0.65 MPa, which is the designed pressure. In the case of the subcritical flow shown in Fig. D-6 (b), the shock-wave angle is increased and the pressure downstream of the duct becomes 0.54 MPa. However, some of the airflow behind the obhque shock wave is spilled over towards the external airflow. Thus, the total airflow rate becomes 68% of the designed airflow rate. In the case of the supercritical flow shown in Fig. D-6 (c), the shock-wave angle is decreased and the pressure downstream of the duct becomes 0.15 MPa, at which the flow velocity is stiU supersonic.

H. Muller-Mohnssen, Direct Determination of Apparent Slip for a Ducted Flow of Polyacrylamide Solutions, J. Rheol., 31(4) 323-336 (1987). 

All pressure-driven, rectilinear duct flows, whether Newtonian or non-Newtonian, undergo transition to turbulence when the transition parameter of Hanks, defined by... 

The fact that S is independent of duct flow rate can be illustrated as follows. Suppose the fire is generating particulates at a constant rate, at some point during a test, the exhaust fan speed is adjusted so that the duct flow rate is doubled. As a result additional ambient air is drawn into the hood and the concentration of particulates in the exhaust is reduced by 50%. The extinction coefficient is therefore also cut in half and the smoke production rate is unchanged. 

For fully developed duct flows in which it can be assumed that fhe fluid properties are constant, the form of the velocity and temperature profiles do not change with distance along the duct, i.e., considering the variables as defined in Fig. 2.12. if the velocity and temperature profiles are expressed in the form... 

Internal flows of the type here being considered occur in heat exchangers, for example, where the fluid may flow through pipes or between closely spaced plates that effectively form a duct Although laminar duct flows do not occur as extensively as turbulent duct flows, they do occur in a number of important situations in which the size of the duct involved is small or in which the fluid involved has a relatively high viscosity. For example, in an oil cooler the flow is usually laminar. Conventionally, it is usual to assume that a higher heat transfer rate is achieved with turbulent flow than with laminar flow. However, when the restraints on possible solutions to a particular problem are carefully considered, it often turns out that a design that involves laminar flow is the most efficient from a heat transfer viewpoint. 

Therefore, as was the case with fully developed pipe flow, the velocity profile in fully developed plane duct flow is parabolic. 

This equation describes the temperature distribution in fully developed laminar plane duct flow when the wall heat flux is a constant. It can be written in terms of the specified wall heat flux. qw, by noting that when Eq. (4.77) is used to give the value of dT dy y = w in Eq. (4.71), the following is obtained ... 

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