mech lab final report
DESCRIPTION
Mech Lab Report FinalTRANSCRIPT
EnviRobot – Air Purification Testing
Crosby Laboratory
University of Maine
Orono, ME 04469
May 8, 2013
By: Emmanuel Marsh-Sachs
Kyle Staples
Josh Stubbs
Keith Pearson
Rebecca Hanks
Brian Farnsworth
Table of Contents
Introduction ................................................................................................. 3
Objectives ...................................................................................................... 3
Apparatus...................................................................................................... 3
Equipment ...................................................................................... 3
Instrumentation ............................................................................. 6
Theory ............................................................................................................ 7
Fluid Mechanics Calculations: ........................................................ 7
Measuring Flow in Ducts: ............................................................... 7
Air Filter Pressure Drop: ................................................................. 9
Uncertainty ................................................................................................... 9
Procedure .................................................................................................. 10
Results ......................................................................................................... 11
Conclusions ............................................................................................... 16
Introduction The goal of the EnviRobot capstone design project is to create several robotic attachment
systems that promote a healthy environment. These systems are designed to fit on a single
mobile robotic platform so only one may be used at a time. Three systems have been designed;
one to plant seeds, one to purify air, and one to acquire data about the environment. This
experiment involves testing the air purification system to ensure the air filter housing is
appropriate to process the airflow rate through the filter achieving maximum purification.
Objectives 1. Take velocity measurements across a cross-section of the duct at small increments,
starting from the duct wall and moving inward, mapping out the velocity profile. The
velocity measurements are to be taken in the straight portion of the duct.
2. Take these velocity measurements utilizing both a dirty filter and a clean filter.
3. Take static pressure differential measurements, using a manometer, for both a clean and
dirty filter. Compare the pressure drop through the clean filter with the manufacturer’s
posted value.
Apparatus
Equipment The apparatus that is to be used in the air purification experiment can be seen in Figure 1.
The volumetric flow rate of air through the duct is provided by the fan pictured on the left side of
the duct. The fan that was used is a DC blower which is to be controlled by sending a voltage
input to provide the desired flow rate through the duct. The inlet of the duct is circular to match
the outlet area of the fan. This circular cross-section expands into a rectangular cross-section for
the portion of the duct where the filter is contained. This is done to decrease the air velocity into
the required range of effectiveness for the filter. There is a straight portion of the rectangular
duct before the air filter to allow the flow, after expansion, to return to ordinary flow before
traveling through the filter. The length of this straight portion is equal to four diameters of the
duct inlet. After the air passes through the filter, it flows through a four inch straight duct, before
being contracted through a circular duct. A diagram of the experimental set-up can be seen below
in Figure 1.
Figure 1: 2-D Diagram of the Air Purification Experiment
The cross-sectional dimensions of the assembled filters are 8 in. x 8 in. This includes the
plywood encasing that is surrounding the filters. The actual filter median itself is only 5.5 in. x 6
in. Since the cross-sectional area of the duct is not equal to the cross-sectional area of the filter
median, spacer board was used to fill in the gap. This is illustrated in Figure 2.
Figure 2: Cross-Sectional Area of the Filter Assembled into the Duct
Duct Dimensions:
Duct Inlet 3 in. diameter
Expansion Duct Length 12 in.
Straight Duct Dimensions 8 in. x 8in.
Straight Duct Pre-Filter Length 12 in.
Pressure Tap Location Pre-Filter 2 in. before filter
Air Filter Dimensions 5.5 in. x 6 in.
Air Filter Depth 5.875 in.
Straight Duct Post-Filter Length 4 in.
Pressure Tap Location Post Filter 2 in. after filter
Duct Outlet 3 in. diameter
Contraction Duct Length 12 in.
Rubber Tubing
Two pieces of rubber tubing of diameter to accept a Pitot tube end and cut to a length of about 2
feet.
Rubber Stoppers
Nine rubbers stoppers, ½ in. diameter, were used to plug unused pressure taps.
Weatherproof Stripping
One roll of 3/8 in. diameter weatherproof stripping was used to seal air leaks around the holes
that were used during data collection.
Fan
Jabsco Model: 35515-0010
Fan Outlet 3 in. diameter
Fan Voltage 12 VDC
The fan airflow rate is described by the curve shown below in Figure 2.
Figure 3: Performance Curve for the fan. Displays Airflow vs. Current and RPM
Instrumentation
Hot-Wire Anemometer
Serial Number: Q641740 Model Number: HHF 2005 HW
Range: 40 to 3940 ft/min
Uncertainty: ±(10% + 1 ft/min)
Inclined Manometer
No Serial Number Listed
Range: 0 to 100 psi
Maximum Operating Temperature: 150°F
Uncertainty: ±.01 in. H20
Pitot Tubes
Two Pitot tubes were used in combination with the inclined manometer in order to obtain the
static pressure drop through the filter.
Ruler
C-Thru Metric
Uncertainty: ±1/16 in.
Theory
Fluid Mechanics Calculations: Velocity measurements are taken at each of the specified locations in the duct according to the
schematic in Figure 3. These velocity measurements must be converted into a volumetric flow
rate to ensure that the flow rate is within the manufacturer’s specified range for the filter. The
velocity measurements are first averaged in order to get two flow velocity values, one for before
the filter and one for after the filter. These values are then converted to a volumetric flow rate by
using equation 1 below.
Equation 1
Where:
Q = Volumetric Flow Rate (ft^3/min)
Vavg = Average Velocity (ft/min)
A = Filter Cross-Sectional Area (ft^2)
L = Filter Length (ft)
W = Filter Width (ft)
Measuring Flow in Ducts: In order to properly determine the volumetric flow through the duct, a method must be used that
encompasses the low flow rates seen at the duct wall as well as the higher flow rates in the
middle of the duct. The American Society of Heating, Refrigerating and Air Conditioning
Engineers (ASHRAE) recommends using the log-Tchebychev method to measure the air
velocity at a number of points at the same cross section of the duct and average the values. The
standard recommends points at which to measure the velocity. Those points are displayed in
Figure 4 below.
Holes are drilled through the duct in accordance to ASHRAE standards and sealed in order to
allow the placement of an anemometer probe into the duct at desired locations to take velocity
measurements for velocity profile mapping. Five holes are drilled along the top of the duct on
one cross-section before the filter and five holes are drilled on one cross section after the filter.
Since the height of the anemometer probe can be varied in the duct, these holes will allow
measurements to be taken at all of the required locations.
Figure 4: Velocity measurement locations inside the cross section of the duct
The velocity measurements taken at these points are necessary to create velocity profile graphs
of the flow. These profiles are plotted on a surface plot that shows a three-dimensional model of
flow through the duct. Measurements are taken along horizontal and vertical cross-sections of the
duct. These respective cross sections are shown below in Figure 5.
Air Filter Pressure Drop: The static pressure drop through the filter is measured by using an inclined manometer. This
pressure drop can be explained as a dissipation of energy as the air passes through the fibers of
the filter. The flow has to change direction in order to get past the fiber and then change again to
resume the original flow direction. This change in direction results in a momentum loss in the
airflow, which causes the pressure drop.
Uncertainty The uncertainty in the calculated value of the volumetric flow rate must be found in order to
accurately compare the values. With the relationship between the measured velocity and area of
the filter known, the Kline McClintock technique can be used to calculate the uncertainty. The
general form of the equation can be found in Equation 2.
[(
)
(
)
(
)
]
Equation 2
Where R is the result of the independent variables xn, wR is the uncertainty in the result and wn is
the uncertainty in the independent variables. The uncertainty in Q, the volumetric flow rate can
be found in Equation 3:
[(
)
(
)
(
)
]
Equation 3
The uncertainty in any measurement is the measure of how well a measurement can be taken
both repeatedly and accurately. The uncertainty in measuring velocity can be found from
OMEGA, the manufacturer of the hot wire anemometer. For the model used they indicate an
Figure 5: Horizontal and Vertical Cross-Sections of the Duct
uncertainty of 10% of the measurement taken plus the least significant digit. For the uncertainty
calculations, all of the measured velocity values have been averaged, and this value was applied
to the uncertainty equation.
The uncertainty for the ruler used to measure the cross sectional area of the filter has been
reported above as:
The partial differential equations are calculated for each contributing variable for use in the
uncertainty equation. The partial differential equation of Q with respect to the average velocity
is:
The partial differential equation of Q with respect to the length of the filter is:
The partial differential equation of Q with respect to the width of the filter is:
Plugging these partial differential equations into Equation 3 the uncertainty in the volumetric
flow rate can be calculated as:
The uncertainty in the measurement pressure drop through the filter is a direct result of the
inclined manometer used.
Procedure Preparation:
1. Soil filter by utilizing wood dust and other air particulates in the wood shop at Crosby
Lab.
Velocity Profile Modeling:
1. Place the anemometer probe shaft alongside the duct. Raise the probe to the first height
location from the bottom of the duct where a velocity measurement is to be taken
according to the log-Tchebychev diagram (.498 in.). Place a piece of tape on the
anemometer probe shaft where it is barely visible on top of the duct.
2. Repeat step one for each of the other four height locations in the measurement schematic.
3. Place the anemometer probe into the first tap location, down into the first height location
in the duct and turn on the anemometer.
4. Turn on the fan and record a velocity measurement.
5. Move the anemometer to the each of the other four heights in this tap location and take a
velocity measurement at each.
6. Repeat steps 3-5 at each of the other tap locations, including the other four on the pre-
filter side of the duct and the 5 tap locations after the filter.
7. Replace the clean filter with the dirty filter and repeat this process.
Static Pressure Drop Testing:
1. Place one Pitot tube in the first drilled hole before the filter and place another Pitot tube
in the first drilled hole after the filter. Make sure both are pointed in the direction of the
flow and oriented in a fashion so that the static pressure is taken rather than the total
pressure.
2. Seal the Pitot tubes in the duct with weatherproof striping.
3. Connect the ends of the Pitot tubes into the inclined manometer using the rubber tubing.
Make sure the Pitot tube before the filter is connected to the high-pressure end.
4. Turn on the fan and take a pressure measurement reading from the inclined manometer.
Record this measurement as the static pressure drop through the filter at this location.
5. Repeat steps 1- 4 at each of the drilled holes before and after the filter. There are 5
measurement locations in total.
6. Turn off the fan and replace the clean filter with the dirty one from preparation
7. Repeat steps 1-5 for the dirty filter.
Results In this section, surface plots will display a three-dimensional graph of flow velocity throughout
the duct. The x-axis corresponds to the length of the cross-section of the duct and the y-axis
corresponds to the height of the cross-section of the duct. The four plots display the velocity
profile for flow before the clean filter, after the clean filter, before the dirty filter, and after the
dirty filter. The velocity profile for flow before the clean filter can be seen below in Figure 6.
Figure 6: 3-D Surface Plot of Flow Velocity Through the Duct, Clean Filter, Pre-Filter
It can be seen that the peak velocity is measured in the bottom corner of the ductwork at high
values of x. The flow velocity also seems to be higher in the duct corners. The velocity in the
center of the duct is generally lower.
The velocity profile for flow after the clean filter can be seen below in Figure 7.
Figure 7: 3-D Surface Plot of Flow Velocity Through the Duct, Clean Filter, Post-Filter
The velocity profile exhibits an expected shape for airflow through a rectangular duct. The flow
has a higher velocity near the center of the duct and it is slower near the walls of the duct.
The velocity profile for flow before the dirty filter can be seen below in Figure 8.
Figure 8: 3-D Surface Plot of Flow Velocity Through the Duct, Dirty Filter, Pre-Filter
It can be seen that the peak velocity is measured in the top corner of the ductwork at high values
of x. The flow velocity also seems to be higher in the duct corners. The velocity in the center of
the duct is generally lower.
The velocity profile for flow after the dirty filter can be seen below in Figure 9.
Figure 9: 3-D Surface Plot of Flow Velocity Through the Duct, Dirty Filter, Post Filter
The velocity profile exhibits an expected shape for airflow through a rectangular duct. The flow
has a higher velocity near the center of the duct and it is slower near the walls of the duct. The
profile has a slightly more irregular surface than that of the profile for flow after the clean filter.
Overall the velocity surface plots exhibited some similar characteristics and some distinct
characteristics. One observation that was made was that the velocity was generally higher near
the duct walls and especially near the corners of the duct for flow before the filter. This is most
likely due to the fact that the cross-sectional area of the filter median was smaller than the cross-
sectional area of the duct. Plywood was used to fill in the gap between the two areas. This caused
the flow toward the walls to travel faster in order to travel through the filter median, which was
in the center of the duct.
The surface plots for flow after the filter show a more expected pattern for airflow through a
rectangular filter. The fastest flow is near the center of the duct, whereas the flow near the duct
walls has a lower velocity. In this case the no-slip condition is preserved. The surface plots for
the dirty filter had a more irregular shape than the surface plots for the clean filter. We believe
that this is because the air is either bouncing back off of the dust particles trapped in the filter
fibers or being redirected around these particles.
Volumetric Flow Data
Volumetric Flow Rate (CFM)
Pre-Filter Post-Filter
Clean Filter 30.72 26.8
Dirty Filter 36.33 26.3
Static Pressure Drop Data
Static Pressure Drop across the Clean Filter
y-position (in.) 2 6
x-position (in.) Static Pressure Drop (in. H2O)
0.488 0.48 0.48
1.88 0.46 0.46
4 0.45 0.45
6.12 0.44 0.44
7.512 0.44 0.44
Static Pressure Drop across the Dirty Filter
y-position (in.) 2 6
x-position (in.) Static Pressure Drop (in. H2O)
0.488 0.44 0.44
1.88 0.44 0.44
4 0.44 0.44
6.12 0.44 0.44
7.512 0.44 0.44
The static pressure drop across the dirty filter was measured to be lower than the static pressure
drop across the clean filter. This was most likely caused by the dirty filter not being soiled
enough to actually increase the pressure drop through the filter. The pressure drop would then be
expected to be the same for both filters. The discrepancy between these values can be explained
by the uncertainty in the inclined manometer used which is 0.01 in. H2O.
Conclusions After analyzing the results, a few conclusions can be made. First, it is optimal to use a filter
whose median’s cross-sectional area that is equal in size to the duct cross-sectional area. This is
preferred in an air purification system because it makes the most effective use of the space inside
the duct, since there is more surface area for the filter fibers. This also makes it very difficult to
assemble the filter into the duct without obstructing the flow, which is undesirable.
The method used to soil the dirty filter did not result in a higher pressure drop through the filter.
With no additional data regarding the filter’s cleanliness, no conclusions can be drawn from the
pressure drop data collected.
Another possible contribution to the irregular velocity profile is the length of the straight portion
of duct before the filter. ASHRAE recommends a straight portion of length equal to 8 to 10
diameters of the expansion inlet diameter, before passing air through the filter. Our ductwork
utilized a straight duct that was only 4 diameters. In conclusion it is wise to follow ASHRAE
standards while designing an air purification system.