NO FORMAL REPORT IS REQUIRED FOR THIS LAB, SEE REQUIREMENTS AT END OF PAGE

Prelab

1. Print this datasheet and bring at least one copy per group to the lab.
2. Define the terms Fan Work and Kinetic Energy and explain how they relate to the lab below.

Theory

The first law energy balance equation is used to describe the change in energy from three parameters.

1. Change in enthalpy as a result of heat energy being added or removed from the system either mechanically or chemically.
2. Change in velocity (and thus, kinetic energy) of the mass of the system, either from acceleration or deceleration.
3. Change in elevation by raising or lowering some amount of the mass which would add gravitational potential energy to the system.

These three terms, describing energy of a system, are changed by the application or extraction of heat and/or work.

The combination of these factors results in the following relationship:

Q - W = (Δh + ½ v² + gΔz)

where:

Q is Heat (J)

W is Work (J)

h is Enthalpy (J/kg)

v is Velocity (m/s)

g is the gravitational constant (m/s²)

z is the change in elevation (m)

In the following lab, no heat energy is added, and the mass does not change in elevation. This means that only the change in air velocity and the heat added by the fan work need to be considered. The simplified formula as it pertains to this special case is then:

W = m (Δh +½ v²)

Note the addition of mass flow rate basis. Work, enthalpy, and velocity are all mass basis in this equation.

To determine the air's velocity, we will use a "hot wire anemometer". An anemometer is used to determine the speed of a gas flow, and this particular model measures the flow by tracking heat loss from a calibrated hot wire tip. The heat lost to convection is proportional to the speed of the air flow, and an initial calibration at a known zero flow rate allows it to extrapolate to a wide range of air flows. Since the initial velocity is zero, we can treat the exit velocity as the change in velocity. The work done by the fan will be measured by the electric power meters similar to earlier labs.

The mass flow rate is given by:

m = PvA / RT

where:

m is mass flow rate (kg/s)

P is exit pressure (room pressure) (Pa)

v is exit velocity (m/s)

A is exit pipe cross-sectional area (m²)

R is the Ideal Gas Constant (J/kg K)

T is the exit temperature (K)

You may use the dry air specific R, or the ideal gas constant with the appropriate mol to mass basis calculation.

You may use parameters in other units so long as appropriate conversion and cancellations are applied. These are suggested for convenience.

The remaining term, for enthalpy change as a result of changing gas properties, can be found from table, textbook, or electronics sources.

Problem

AP Engineering has been contracted to develop a system that rapidly dries fruits and vegetables. To do this, they've developed a system that forces air at a high velocity through a chamber where whatever is being dried can be stored. As a convenient consequence of the fan work done to force the air through the device, the internal cavity heats up. The combination of the heated environment and moving air effectively dries while avoiding cooking or overheating the food. Your team is tasked to determine the air velocity and steady state temperature as well as the energy consumed by the device. As a verification of reasonable data, you are expected to plug your data into the First Law Equation for Heat and Work and check for a reasonable balance.

Procedure

1. Start the blower motor as the operating temperature takes several minutes to stabilize.
   • Time 10 minutes to allow for the system to heat to a steady state.
2. Read and record the Barometric Pressure and Temperature in the lab room.
3. The Pipe diameter is 0.052m (~2").
4. Turn the power on for the Hot Wire Anemometer and wait for it to count down to 0 and then display 0.0.
5. Check it is displaying temperature in degrees C and velocity in m/s.
   • If not, press the C/F or UNIT button respectively for the correct units.
   • After the unit has self-calibrated, slide the protective sleeve at the end of the probe down to expose the fine sensing wire.
   • Do not use fingers or tools to touch the air velocity sensor. Do not blow on the wire. The meter may incur permanent damage if mistreated.
6. Place the Hot Wire Anemometer at the outflow with the white dot facing into the airflow and record the air flow and temperature.
   • Take several readings at the outflow in different spots and use the highest repeatable value for both temperature and velocity.
   • Be sure to measure very near the exit before the air has slowed down.
7. Return the protective sleeve to cover the air sensor and turn Anemometer off.
8. Read the power from the Wattmeter.
   • Press the Watt button to display power.
9. With the blower still running, cover the outlet pipe with your hand for a few seconds, note the change in power.

Results

1. Present the following lab data in a chart:
   • Barometric Pressure (Given in lab)
   • Room and Exit Temperature (Read with anemometer)
   • Gas Exit Velocity (Read with anemometer)
   • Initial Enthalpy, Exit Enthalpy, and Change in Enthalpy (you may use table A-21 in the textbook)
   • Exit Pipe Internal Diameter (Measured in lab)
   • Mass Flow Rate (calculated later)
   • Electric Power Consumed (From Kill-a-Watt^tm meter)
2. Calculate the mass flow rate of the gas.
3. Use this data in the special case of the First Law relation above, along with the electric work done by the motor to verify the relationship.
   • Explain any differences or deviations.

Questions

Why does the power drop when the outlet is blocked, and how does the first law help explain this?

NO FORMAL REPORT IS REQUIRED FOR THIS LAB, THE ABOVE RESULTS AND QUESTION ARE ALL THAT IS REQUIRED.