Measuring Mass Flow rate of Air with respect to Car velocity
A relationship between the car speed and the average velocity (and mass flow rate) of the air flowing through the radiator core must also be developed. In order to develop this relationship, a test was performed where a radiator core was held outside the window of a car and a vane type anemometer was used to take an array of air velocity measurements behind the radiator. Using this array, an average velocity was determined. The average velocity of the air through the core can be used with the face area of the radiator core to determine the mass flow rate of the air that flows through the radiator core. This test was performed while the car was travelling at a range of speeds from 0-60 mph. Details are as follows.
- Test Radiator
- Vane Anemometer
There is no real setup required to perform this test. If the test is performed in the same manner as it was performed to collect the data for this senior project, three people would be required to collect data. One person will drive the car, then one person will hold the radiator outside of the window while another person held the anemometer behind the radiator to collect data.
Ideally, the test would be performed on the formula car with a fixture to hold the anemometer behind the radiator in different positions along the face of the radiator to collect an array of velocity values over a range of car speeds. This is the ideal due to the fact that it is possible that the relative air velocity is slightly different at different points around the car.
- The car should be driven at a constant
- Record the car
- One person will hold the radiator away from the car while another person holds the anemometer behind the radiator
- Record data for an array of velocities in different positions over the face of the
- Enter the data into the spreadsheet labelled “Core Air ”
The Excel program will find the average air velocity based on the array of air velocities measured. Then it will determine the mass flow rate of the air that flows through the core at each car speed based on Equation below.
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The equation is evaluated where ρ is the density of air. The density of air assumed to be constant and equal to its published value of 1.18 kg/m3 at 25˚C. A is the area of the face of the radiator core. The program will then plot both the average air velocity and the mass flow rate of the air through the core as a function of the speed of the car. The Excel program was used to develop this plot for sample data collected that was collected.
To find Air pressure drop across radiator
This test is the first of two tests to be performed on the wind tunnel in the Thermal Science Lab. A relationship between the average air velocity through the radiator core and the static pressure drop across the core will be established. This relationship will be used in two ways: first, it will be helpful in measuring the mass flow rate of air through the core in Test, and second, it will be helpful in choosing a fan to be used on the car. In order to develop this relationship, the radiator to be tested will be mounted in the test section, then a pitot-static tube will be used to determine the velocity of the air over an array of positions on the face of the radiator core. A liquid column manometer will be used to determine the pressure drop across the radiator.
- Wind Tunnel w/ Test Section
- Aluminum Foil Tape
- Test Radiator
- Saw Horse
- Inclined Liquid Column Manometer
- ¼” ID Flexible Plastic Tubing
- Pitot-Static Tube w/ Readout
- Square Rule
To prepare for data collection in Test, the radiator core needs to be covered with aluminium foil tape on both sides where it will not be contained within the wind tunnel ducting, as depicted in Figure. This to ensure that the core is airtight; air should only pass through the fins contained within the wind tunnel ducting. The face area tested will be used to predict the necessary face area of the radiator core to achieve adequate cooling on the formula car. If more fins are rejecting heat, the face area reference used in these tests will be meaningless and the necessary face area predicted will be wrong. Additionally, the aluminium foil tape
should not cover any part of the core that is contained within the ducting. Once the face area outside of the duct has been covered with aluminium foil so no air can escape the core, the radiator can be mounted in the test section, as depicted in Figure. The front end of the wind tunnel where air enters is on wheels while the back with the fan is stationary. The sawhorse should be positioned so that it will be underneath the radiator when the front end of the wind tunnel has been pushed against the radiator. If the sawhorse is too short, boxes or other materials can be used on top of the sawhorse to support the radiator in the test section. It can be observed in Figure that a box was used on top of the sawhorse to support the radiator in the proper vertical position. Once the proper vertical position is achieved, the front end of the wind tunnel can be pushed against the radiator core. Check around the foam gasket to be sure that no fins are visible.
Lastly, use two pieces of rubber tubing to connect the static pressure ports on the test section to the inclined liquid column manometer mounted on the wall behind the wind tunnel.
Connect one end of the tubing to the test section as depicted in Figure. The other end of the tubing will be connected to the barb fitting on top of the liquid column manometer. The tubing should be placed such that the static pressure port in front of the radiator is connected to the barb fitting on the left side of the manometer. The static pressure port behind the radiator will be connected to the barb fitting on the right side of the manometer. As a result, the higher pressure that exists in front of the radiator will push the height of the inclined liquid column down with resistance from the lower pressure on the other side of the column.
- Move the lever on the circuit breaker for the wind tunnel (located behind the wind tunnel) to the “On”
- Set the fan to a low speed using the variable frequency drive (VFD, located on the wind tunnel near the test section).
- Flip the switch on right on the VFD to the “Start”
- Insert the pitot-static tube into one of the holes on top of the test section and pull up so that the tip of the pitot-static tube is at the top of the duct. This will measure the velocity of the incoming/outgoing air at that point on the face. Use a square ruler to be sure the pitot-static tube is straight up and down and the tip is pointed directly towards the incoming airflow Record the velocity measurement in the Excel program using the spreadsheet labelled “Core Pressure ”.
- Use the ruler to move the pitot-static tube down 1-1/8 inches and measure the velocity at that point on the radiator face and record the measurement in the Excel
- Repeat Step 6 until 5 measurements have been taken in that
- Repeat Step 4 through Step 7 until measurements have been taken in each of the 5 holes along the top of the test
- Observe the difference in the difference in height of the two oil columns in the inclined liquid column manometer on the wall and record the value in the Excel
- Repeat Step 4 through Step 9 over a range of fan speed
The Excel program will use the values from each array of measurements to determine the average velocity of the air that passes through the core. From this average velocity, the mass flow rate of the air in the duct can be determined in the same way it was determined in Test using Equation. It will then plot both the velocity of the air in the duct and the mass flow rate of the air in the duct as a function of the pressure drop reflected by the change in height of the liquid column manometer.
This plot can be used as a calibration curve to determine the average velocity of the flow in the duct. This will be valuable in Test, so that mass flow rate at any fan speed can be determined using the liquid column manometer.
The spreadsheet for this test will produce another plot. This plot develops a relationship between the mass flow rate of the air through the core and the static pressure drop associated with it for the radiator core as a loss element. This information will be valuable in selecting a fan if the formula team determines a fan is necessary to keep the engine cool. The second plot generated using the Excel program with results from testing performed can be observed
Wind Tunnel Testing –
In the present work, an experimental investigation is carried out to analyze the heat transfer and pressure drop characteristics of a corrugated louvred fin and flat tube compact heat exchanger used as a radiator.
MAJOR COMPONENTS INVOLVED IN THE EXPERIMENTAL SETUP
The major components involved in the experimental test rig, are the boiler, centrifugal pump, blower, wind tunnel, flow control valve and the other instruments used for measurements. The details of each component are explained in this section.
The boiler is a horizontal cylindrical drum used for generating the hot water required to circulate through the radiator in the setup during the experiment. It is made up of mild steel plates. The diameter of the boiler is 1000 mm and the length 1300 mm. There are 12 three-phase electrical heaters 44 (each side 6 heaters) which are kept inside the boiler to heat the water present in it. Every heater has a maximum input of 6 kW and is controlled by variable transformers. The temperature of the water inside the boiler is sensed by the thermocouples.
- Centrifugal pump with motor
A centrifugal pump is a hydraulic machine driven by an electric motor. The pump sucks the hot water from the boiler and pressurizes it, and the pressurized hot water is supplied to the radiator through the hose.
- Wind Tunnel
The wind tunnel is a hollow duct and is trapezoidal in shape. The tunnel is made up of mild steel plates. One end of the tunnel is rectangular in the cross-section, equal to the frontal area of the radiator core (810 x 717 mm), and the other end of the tunnel is a square with a side of 730 mm. The tunnel is fixed with the damper (guide vane) casing, with the aid of a hollow square to a round transition piece, whose one end is a square with a side of 730 mm, and the other end is circular with a diameter of 1000 mm. The atmospheric air sucked by the blower, after extracting the heat from the radiator, flows through this tunnel
The centrifugal blower is placed behind the wind tunnel and is driven by an electric motor. The hot air leaving the wind tunnel is sucked by the blower and is let out to the atmosphere through the outlet duct. The mass of air sucked by the blower from the atmosphere is controlled by the dampers
Dampers are otherwise called guide vanes and are used for controlling the frontal air velocity of the radiator. The dampers are curved blades made up of mild steel plates arranged radially around the rotor, which is fixed inside the circular chamber (damper casing). This rotor shaft is connected with the blower shaft. By adjusting the gap between the dampers, the mass flow rate of air flowing through the tunnel can be varied. The position of the dampers (guide vanes) is controlled by an adjusting lever. These dampers are used for testing the performance of the radiator at different frontal air velocities
- Flow control valve
The flow control valve is used to control the mass of water flowing through the radiator. This valve is fixed to the pipe, through which hot water flows to the radiator from the centrifugal pump. This valve is used to test the performance of the radiator, by varying the mass flow rate of water.
Test Bench Set up
This air supply line contains a blower, dampers and the necessary instruments. The test rig also has provision for the necessary inlet condition for water, which is supplied through the tube (hot) side of the test unit. Water is supplied through a pipeline that starts from the boiler, followed by a centrifugal pump, flow control valve and converging pipeline to match the entry dimensions of the inlet configuration to be tested.
The frontal area of the radiator is connected to the rectangular duct of the cross-section equal to the frontal area of the radiator. The rear end of the radiator is fixed to the tunnel. The other end of the tunnel is connected to the damper casing. The damper casing is connected to the blower. The blower is connected to the outlet duct. The hot water from the boiler enters the radiator from the top, and the cold water from the radiator leaves from the bottom. When the blower is switched on, the atmospheric air flows through the radiator core and becomes hot. This hot air is let out to the atmosphere through the tunnel and the outlet duct. To minimize the heat lost to the surroundings, all the components in the experimental setup are insulated with a 10 mm thick glass wool layer.
The different instruments used to measure the temperature, pressure, and the mass flow rate of air and water, are elaborately discussed in this section.
The inlet and outlet temperatures of the air-steam mixture are measured using two pre-calibrated RTDs (pt-100) with an accuracy of 0.1°C, which are placed at the upstream and downstream of the test section. Also, the inlet and outlet water temperatures are measured by two pre-calibrated 49 RTDs(pt-100), with an accuracy of 0.1°C. These data signals are recorded individually and then averaged.
The air and water pressure drop across the heat exchanger are measured, using a pressure transducer calibrated to an accuracy of ±0.09%, and the manometer checks the accuracy of the measurements. To obtain the air side pressure drop measurements through the louvred array, more number of samples are taken and averaged for each air velocity. The specifications of the pressure sensors used
The velocity of the air driven by a centrifugal blower is measured, using the portable digital vane anemometer with an uncertainty of 0.14%. The velocity of air is measured at 12 points on the upstream side of the test radiator and then averaged. The volume of water flowing through the radiator is measured with a differential pressure flow meter with an average error of 0.5%.
The HP – Agilent 34970A, Data Acquisition System (DAS) is used for the continuous monitoring of temperature, with respect to time. The DAS has an accuracy of ±0.05% of the reading and a resolution of 0.01°C. The photographic view of the instruments used for the measurements
- The radiator to be tested is fixed in front of the wind tunnel in the experimental setup, and it is ensured that there are no air leakages.
- The heaters in the boiler are switched on, and once the water in the boiler reaches the required temperature of 90°C, both the centrifugal pump and the blower are made operative.
- With the aid of the flow control valve, the mass of water flowing through the radiator is adjusted. Also, the velocity of air flowing across the test radiator is measured with the aid of a digital vane anemometer.
- Similarly, with the aid of a damper adjusting lever, the required frontal air velocity is obtained.
- After the heat exchanger obtained the steady-state, the temperature, pressure and the mass flow rate of water are observed and recorded, using the Data Acquisition System (DAS).
- The same procedure is repeated for four different mass flow rates of water, 0.075 m3 /min, 0.090 m3 /min, 0.110 m3 /min and 0.135 m3 /min.
- For each mass flow rate of water, experiments are conducted for five different air velocities of 3.5 m/s, 4.5 m/s, 5.5 m/s, 6.5 m/s and 7.5 m/s.
- For each experiment, three trials are made, to ensure the repeatability of the readings.