Analyzing the effects of airflow disturbances on measurement and control equipment positioned downstream and close to an air duct elbow--for the purpose of optimizing system performance using a CFD technique.
Measurement and control equipment positioned downstream and close to an elbow fitting within the disturbed airflow region in an air duct can lead to inaccuracies introduced to the control system. Graphical computational fluid dynamics (CFD) results form the basis of this research. A distance criterion was defined for optimizing the performance of equipment.
The effect of air velocity rates on downstream elbow disturbances on equipment was also investigated. Results obtained should provide the designer and manufacturer standard information on optimized locations for installations.
Limitations were identified where transition regions exist. Transition from a disturbed to a stabilized airstream region, where none of the equipment listed in this paper can be optimized for performance.
LOCATION OF EQUIPMENT USED IN MEASUREMENT AND CONTROL IN HVAC AIR DUCT SYSTEMS
Manufacturers of measurement and control equipment usually specify their recommendations when installing such equipment downstream, but close to an elbow duct fittings. ASHRAE Handbook--Applications (2007) gives recommendations on the installation of equipment downstream of an air duct elbow. For example, ASHRAE Handbook--Applications discusses the installation of in-duct static pressure sensors at least three equivalent duct diameters from an elbow (2007). Manufacturer's data shall be consulted if available. For discussions and analysis on the equipment listed in the sections "Equipment Performance Deteriorating Due to Airflow Disturbances" and "Equipment Performance Not Affected by Airflow Disturbances" refer to the section "Results and Analysis."
Equipment Performance Deteriorating Due to Airflow Disturbances
Examples of such equipment are listed below.
VCD = volume control damper, used to regulate airflow in an air duct system
APS = air pressure sensors, for pressure measurement of airflow in an air duct. Manufacturer recommendations on installation are usually followed (ASHRAE 2007).
AFS = airflow sensor, for measuring velocity or airflow in an air duct. Accuracy depends on how they are applied and where in a system they are located (ASHRAE 2009).
HS = humidity sensor, measures humidity of airflow in an air duct. Refer to Figure 1.
Equipment Performance Not Affected by Airflow Disturbances
Examples of such equipment are listed below.
TS = temperature sensor, measures temperature of airflow in an air duct system.
UVL = ultraviolet lamp, for air disinfection in an air duct system. ASHRAE Handbook-Systems and Equipment covers in-duct installation of UVL in detail, such as the importance of considering duct height and width (2008).
HD = humidifier, adds humidity to airflow in an air duct system. Refer to Figure 1.
CFD analysis was conducted using ANSYS CFX. The details of modeling are shown in Appendix 1. Figures 2 and 3 are general angled 3-D view. Figures 4 to A1 show plan views displayed against a scale, showing airflow disturbances affects downstream the air duct elbow fitting.
The velocity ranges 2.5, 6, and 10 m/s (8.2, 19.68, and 32.8 ft/s) were selected to cover for the recommendations given in ASHRAE Handbook--Applications on maximum recommended duct airflow velocity to achieve specified acoustic design criteria. For ducts located within occupied space (2007).
Incompressible flow was assumed since airflow velocities are relatively low (see assumptions in Appendix 1).
RESULTS AND ANALYSIS
In both cases, disturbances on pressure contours and velocity stream lines continued for at least 2 m (6.56 ft) from the beginning of elbow or 1.7 m (5.57 ft) from elbow takeoff point (elbow cross-sectional dimensions 0.3 x 0.25m [0.98 x 0.82 ft]). In this discussion, Figures 8 and 9 were selected as an example. Tables 1 and 2 show a summary of the discussions shown below in this section, and graphical results are shown in Figures 4 to 9. A criterion for optimized positioning of measurement and control equipment has been set in Tables 1 and 2. Selected examples are discussed in the sections "Equipment Listed in Section 'Equipment Performance Deteriorating Due to Airflow Disturbances'--Performance Improves when Located Away from the Highly Disturbed Airflow Region" and "Equipment Listed in the Section 'Equipment Performance Not Affected by Airflow Disturbances'--Performance Improves when Located in the Highly Disturbed Airflow Region."
Equipment Listed in the Section "Equipment Performance Deteriorating Due to Airflow Disturbances"--Performance Improves when Located Away from the Highly Disturbed Airflow Region
The VCD not fully utilized due to the noneffective usage of the entire VCD face. Referring to Figures 8 and 9, location (A-B), high disturbances begin to recede 2 m (6.56 ft) away from beginning of duct elbow fitting. Stabilization of airflow velocity stream lines and pressure stream lines begin at 4 m+ (13.12 ft+) from duct elbow. That is where airflow stream lines begin to fill the entire duct cross-sectional area.
The APS equipment placed in the highly disturbed region in Figure 9 (location A), will show higher pressure readings than the true duct pressure conditions. Placing such equipment at the bottom part (location B), will give lower and possibly negative pressure values. Figure 9 shows that high disturbances shown in red colour (or darker colors in black and white prints) begin to disappear at 4 m+ (13.12 ft+); whereas, pressure stream lines begin to stabilize filling the duct sectional area.
The AFS located in Figure 8 (location A) can indicate higher velocity readings, up to 18 m/s (59.0 ft/s), where the true velocity is 10 m/s (32.8 ft/s), inlet velocity. Equipment placed at the lower part (location B) of the duct can lead to low or zero airflow velocity. Disturbances begin to disappear 4 m+ (13.12 ft+), where the velocity stream lines begin to stabilize filling the entire duct cross-sectional area.
The HS can be affected by the pressure increases shown in location A in Figure 9, the red region. Placing HS equipment at the lower part of the duct (location B) below the disturbed region will give inaccurate result due to virtually no or low airflow. Disturbances begin to disappear 4m+ (13.12 ft+) where velocity stream lines begin to stabilize.
Equipment Listed in the
Section "Equipment Performance Not Affected by Airflow Disturbances"--Performance Improves when Located in the Highly Disturbed Airflow Region
The TS not was significantly affected, the process was considered as isothermal, and temperatures rarely fluctuate against such pressure or velocity increases. Considering the length of the air duct system. If the TS location is near the elbow fitting, then it is recommended to install within the disturbed region, location A in Figure 9, where denser stream lines provide better contact with the temperature probe. Location B in Figure 9 should not significantly impact performance as discussed in the beginning of this paragraph.
UVL, if required to install near the elbow, this can be advantageous. Highly disturbed airflow stream lines, Figure 9, locationA provide good exposure to UV rays. Placing the lamp at the upper region where airstream lines are denser, provides more concentrated UV disinfection to airstream-borne microbes. With a possibly smaller UVL concentrated on the closely dense airstream lines saves in costs of equipment/electricity and size.
The HD located at the higher disturbed region, Figure 9, location A, of duct can allow for an effective mixing of added humidity in the closely packed airstream lines. This is important if an air outlet is located close to the elbow.
Figures 8 and 9 indicate that the region 2 to 4 m (6.56 to 13.12 ft) represents the transition region where disturbances begin to receded and airflow stream lines begin to stabilize and fill the entire duct cross-sectional area. This transition region was not considered for optimizing the location of equipment for the reasons discussed in the sections "Equipment Listed in the Section 'Equipment Performance Deteriorating Due to Airflow Disturbances'--Performance Improves when Located Away from the Highly Disturbed Airflow Region" and "Equipment Listed in the Section 'Equipment Performance Not Affected by Airflow Disturbances'--Performance Improves when Located in the Highly Disturbed Airflow Region." Figure A1 shows the complete CFD model of the sketch shown in Figure A2.
Figure A3 in Appendix 2 explains the theory behind the disturbances shown in the CFD analysis. It was shown by Massey (1984), that the double spiral motion shown in Figure A3 may persist for a distance downstream as much as 50 to 75 times the pipe diameter. In this analysis, disturbances for the considered duct geometry begin to diminish at 4 m (13.12 ft) from the beginning of the elbow. However, the airflow disturbances continue further downstream the 4 m (13.12 ft) distance, as mentioned by Massey and confirmed in the CFD analysis. See Figures 8 and 9 as an example.
The CFD analysis has shown that the air velocity rates had no impact on the reduction of the extent of airflow disturbances created downstream of the duct elbow. The analysis was based on three inlet air velocity levels; 2.5, 6, and 10 m/s (8.2, 19.68, and 32.8 ft/s).
Effective positioning of measurement and control equipment can improve process performance and saves energy.
Equipment such as VCDs, APSs, AFSs, and HSs are recommended to be positioned at 4 m+ (13.12 ft+) downstream of an elbow air duct fitting.
Positioning a pressure or velocity sensor within less than 2 m (6.56 ft) from the air duct elbow fitting can lead to either higher or lower pressure/velocity readings if placed in the highly disturbed airflow region. The higher or lower readings depend on whether the sensor is placed in the denser airflow stream lines or the lower density stream lines.
Fittings such as TSs, UVLs, and HDs are recommended to be installed downstream of an elbow air duct fitting, within 2 m (6.56 ft) and within the maximum disturbed airflow region, benefitting from the denser airflow stream lines.
Further experimental work should be conducted. Repeating the analysis with other fluids is recommended, such as water piping systems. Further analysis is recommended with various air duct geometries.
Physics report on data fed to the CFD analysis. Domain Physics for CFX version 13.
Type Fluid Materials Air at 25[degrees]C (82[degrees]F) Material Library Fluid Definition Continuous Fluid Morphology
Steel wall duct assumed with a wall surface Roughness Constant = 0.5 or select a smooth wall surface if using ANSYS CFX software.
Settings Buoyancy Model Non Buoyant Domain Motion Stationary Reference Pressure 1.0000e+00 [atm] Heat Transfer Model Isothermal Fluid Temperature 2.5000e+01 [degrees]C (82 [degrees]F) Turbulence Model k epsilon Turbulent Wall Functi Scalable
Assumption: the Mach number can be used to determine if a flow can be treated as an incompressible flow. If Mach < 0.2-0.3 and the flow is (quasi) steady and isothermal, compressibility effects will be small and a simplified incompressible flow model can be used (Wikipedia 2014). Where Mach 1 = 340.3 m/s (1116 ft/s) at sea level and temperature 15[degrees]C (62[degrees]F).
See Figure A3.
The author would like to thank Dr. Lik F. Sim for checking the CFD model and providing access to the ANSYS CFX software.
ASHRAE. 2007. Chapters 37 and 47, ASHRAE Handbook-Applications. Atlanta: ASHRAE
ASHRAE. 2009. Chapter 7, ASHRAE Handbook--Fundamentals. Atlanta: ASHRAE.
ASHRAE. 2008. Chapter 16, ASHRAE Handbook--Systems and Equipment. Atlanta: ASHRAE.
Massey, B.S. 1984. Mechanics of Fluids, 5th ed. Van No-strand Reinhold Co. Ltd.
Wikipedia. 2014. Mach number.
Reza Ghias, Senior CFD Analyst, Southland Industries,
Rockville, MD: What was the validation criteria for CFD? What was the Re number impact on this study?
Ali Hasan: It was suggested in the paper that experimental work should be conducted. Experimental work is a way of verifying CFD analysis.
In general, CFD analysis can be relied on provided the following items are correctly arranged:
* model mesh
* boundary conditions
* turbulence model
* solver settings
Re number was not looked into since the aim was just stabilization of streamlines.
However, what can happen with Re is as follows:
Re = [density x velocity x hydraulic diameter of duct]/dynamic viscosity
The Re number, as the equation shows, is in fact the ratio of inertial forces to viscous forces. In high Re values, inertial forces dominate, and with low Re numbers, viscous forces dominate.
Parameters such as velocity and hydraulic diameter are user defined, whereas fluid density and dynamic viscosity are fluid specific but are programmed in the software package; however, to fix these values, fluid temperature must be entered. What is also interesting in the above equation is that the fluid temperature impacts the inertial and the viscous parts of the equation, even though temperature is not mentioned in the equation.
The Re numbers will be higher in the highly disturbed region when compared with the substantially stabilized airflow region.
Ali M. Hasan, CEng
Ali M. Hasan is a senior engineer with KEO International Consulting Engineers, Doha, Qatar.
Table 1. Results Obtained from the CFD Analysis--Equipment that can be Affected by the Airflow Stream Lines Conditions Equipment Optimum Location to form the Beginning of the Duct Elbow Fitting VCD 4 m+ (13.12+) APD 4 m+ (13.12+) AFS 4 m+ (13.12+) HS 4 m+ (13.12+) TS Downstream of elbow within 2 m (6.56 ft). Upper part of duct in the denser airflow stream This table shows a summary of the results generated by the CFD analysis discussed in the section "Results and Analysis" and the CFD-generated graphical results shown in Figures 4 to 9. The table summarizes the optimum positioning measurement and control equipment in an air duct system, relying on CFD analysis. The summarized results apply to the three air velocities: 2.5, 6, and 10 m/s (8.2, 19.68, 32.8 ft/s)-low, medium, and high velocities, respectively. Table 2. Results Obtained from the CFD Analysis--Equipment not Affected by the Airflow Stream Lines Conditions but Can Perform Better if Stream Lines Conditions are Considered Equipment Optimum Location to form the Beginning of the Duct Elbow Fitting UVL Downstream of elbow within 2 m (6.56 ft). Upper part of duct in the denser airflow stream lines. HD Downstream of elbow within 2 m (6.56 ft). Upper part of duct in the denser airflow stream lines. This table shows a summary of the results generated by the CFD analysis, discussed in the section "Results and Analysis" and the CFD/generated graphical results shown in Figures 4 to 9. The table summarizes the optimum-positioning measurement and control equipment in an air duct system, relying on CFD analysis. The summarized results apply to the three air velocities: 2.5 m/s, 6, and 10 m/s (8.2, 19.68, 32.8 ft/s)-low, medium, and high velocities, respectively.
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|Author:||Hasan, Ali M.|
|Date:||Jul 1, 2014|
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