NO FORMAL REPORT IS REQUIRED FOR THIS LAB, SEE REQUIREMENTS AT END OF PAGE
The first law energy balance equation is used to describe the change in energy from three parameters.
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:
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:
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:
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.
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.
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.