Turbulence loss

Turbulence loss The energy loss that takes place in a ventilation system through air turbulence. 

Turning vanes Curved vanes added to ductwork elbows in an attempt to ensure streamline flow and, by so doing, reduce turbulent losses. 

This represents the linear effect of tip speed and the square of impeller size on the flow of a specific impeller. Referring to Figure 12-46, very low flow coefficients for a specific type of centrifugal or axial flow machine cause excessive wall friction or leakage losses, and very high-flow coefficients tend to he subject to turbulence losses due to insufficient flow guiding. ... 

There are numerous sources of frictional losses for the airflow through the hood and ductwork to be overcome by the exhaust fan. The air entering the hood must be accelerated from a minimal velocity in the room to the velocity within the duct. Due to physical factors, there is always some turbulence created during this process, so that the pressure difference created by the fan must be sufficient to provide the desired airspeed in the duct and overcome the losses due to turbulence. In the specifications for a hood, the hood static pressure data provided are a direct measure of the total of the energy needed for acceleration and to overcome turbulence losses. The static pressure will be proportional to the square of the velocity of the air entering the hood face. 

The net result is a shrinkage of the turbulent domain beyond the laminar layer, we now find a buffer layer, of size increasing with the polymer concentration. It is then natural to expect that the turbulent losses will be reduced Lumley (1973) gave a detailed argument to show this. Note that the whole effect occurs at arbitrarily low c as soon as we add some polymer, the viscous limit shifts. 

The subject of Chapter 4 (originally the third lecture) is the problem of the reduction of turbulent losses by polymer chains in a solvent. The topic appears to be especially intricate in view of the coupling between the hydrodynamic aspects of turbulent flow and the viscoelastic behavior of chains in strongly perturbed conformations. The lecture followed closely a paper Towards a scaling theory of drag reduction published in 1986 by Professor de Gennes in Physica which is here reprinted by permission of the publisher. Because this lecture is the most tentative (and difficult) part in the series we have put it in the last chapter. 

The power consumption of an impeller is the product of the pumping capacity (circulation fiow rate) and the velocity head, which is directly related to shear rate and turbulence. Depending on the type and size of the impeller, either the flow or the turbulence can be favored [110]. Axial flow impellers usually produce a fluid motion that is downward at the central axis of the vessel and upward in the wall region. They are designed to produce a high flow/power ratio with little turbulent loss. The designs of axial-flow impellers are derived from three-blade propellers. Radial-flow turbines produce a radial fluid motion from the impeller to the wall, where the radial flow separates into an upper and a lower circulation loop. They are characterized by a relatively low flow/power ratio, with much of the energy dissipated by turbulence around the impeller. Radial-flow turbines have flat blades or a disk with flat blades. 

Poor performance can result from fan inlet eccentric or spinning dow, and discharge ductwork that does not permit development of hiU fan pressure. Sometimes inlet restrictions starve a fan and limit performance. To obtain rated performance, the air must enter the fan uniformly over the inlet area without rotation or unusual turbulence. This allows all portions of the fan wheel to do equal work. If more air is distributed to one side of the wheel, such as with an elbow on the inlet, the work performed by the lightiy loaded portions of the wheel is reduced and capacity is decreased by 5—10%. The use of an inlet box duct on a fan can reduce capacity by as much as 25% unless there are turning vanes in the duct. Use of the vanes reduces the capacity loss to around 5%. 

In practice, the loss term AF is usually not deterrnined by detailed examination of the flow field. Instead, the momentum and mass balances are employed to determine the pressure and velocity changes these are substituted into the mechanical energy equation and AFis deterrnined by difference. Eor the sudden expansion of a turbulent fluid depicted in Eigure 21b, which deflvers no work to the surroundings, appHcation of equations 49, 60, and 68 yields... 

I oise Transmission Reduction in HVACSystems. One common use of sound-absorbing treatment is to reduce noise transmission in heating, ventilating, and air-conditioning (HVAC) systems (6). The treatments ate used to reduce the transmission of fan noise and air turbulence noise through ducts into occupied spaces. Noise transmission reduction in duct systems is described in terms of insertion loss, the difference in sound power level or sound pressure level measured at a given location before and after installation of the treatment or sound attenuation, the reduction in sound power between two locations affected by a sound source. The units ate decibels. 

For a trumpet-shaped rounded entrance, with a radius of rounding greater than about 15 percent of the pipe diameter (Fig. 6-13Z ), the turbulent flow loss coefficient K is only about 0.1 (Vennard and Street, Elementary Fluid Meehanies, 5th ed., Wiley, New York, 1975, pp. 420-421). Rounding of the inlet prevents formation of the vena eontraeta, thereby reducing the resistance to flow. 

Expansion and Exit Losses For ducts of any cross section, the frictional loss for a sudden enlargement (Fig. 6-13c) with turbulent flow is given by the Borda-Carnot equation ... 

Trumpet-shaped enlargements for turbulent flow designed for constant decrease in velocity head per unit length were found by Gibson (ibid., p. 95) to give 20 to 60 percent less friclional loss than straight taper pipes of the same length. 

Miller Internal Flow Systems, 2d ed.. Chap. 13, BHRA, Cranfield, 1990) gives the most complete information on losses in bends and curved pipes. For turbulent flow in circular cross-seclion bends of constant area, as shown in Fig. 6-14 7, a more accurate estimate of the loss coefficient K than that given in Table 6-4 is... 

TABLE 6-4 Additional Frictional Loss for Turbulent Flow through Fittings and Valves ... 

Permanent pressure loss across a concentric circular orifice with radius or vena-contracta taps can be approximated for turbulent flow by... 

Permanent pressure loss across quadrant-edge orifices for turbulent flow is somewhat lower than given by Eq. (10-30). See Alvi, Sridharan, and Lakshmana Rao, loc. cit., for values of discharge coefficient and permanent pressure loss in laminar flow. 

---