Page 299 - gas transport in porous media

Chapter 17: Subsurface Flow Measurements
particular point. Multiple measurements of the point velocity across the plane of a
flow field can then be converted to the flow rate of the gas. 297
Thermal anemometers have a dynamic response at lower flow rates where small
differencesinappliedenergycanbemeasuredwithgreataccuracyandprecision. Mea-
surements of small energy changes are more easily made when lower total energy
is required to maintain temperature (i.e., the ability to measure small changes in a
low background). At higher flow rates these instruments respond with less resolution
because small energy changes are made at high total energy required to maintain
temperature. This is the ubiquitous problem of the measurement of a small signal
in a high background. At high flow rates where a very large amount of energy is
required to fend off the onslaught of heat stealing gas molecules, a different tech-
nique using a different physical property of the flow phenomenon may be more
appropriate.
Although the strength of thermal anemometers are their ability to measure low flow,
at very low flow rates the instruments also have limitations related to the measurement
technique that significantly affect their accuracy. At very low flow rates the heated
sensor can produce convective flow of the gas near the sensor. The magnitude of this
effect will vary with the amount of energy applied to the heated element, the size
and thermal conductivity of the element, and the location (proximity and direction)
of the background thermal sensor with respect to the heated element. Better response
can be obtained if the thermal sensor is out of the convective heat path. For many
commercial versions of the thermal anemometer, flow velocities near the sensor of
0.05 m/sec or greater are required to minimize the effects of this self-induced flow
although thermal sensors capable of measuring 0.005 m/sec are available (Martin
et al., 2002).
Another issue affecting the use of thermal anemometers is phase and compositional
differences within the bulk fluid whose flow is being measured. If there is a change
in the heat carrying properties of the fluid the sensor will respond differently. For
example, if the composition of the gas stream changes from mostly air to mostly
carbon dioxide, the flow sensor will produce different values for the same flow rate
because of the difference in heat capacity and conduction of the different compounds.
In addition, if the fluid is hovering between two different phase states (e.g., liquid
and gas) with significant phase transition energy, the sudden occurrence of a different
phase on the sensor can produce dramatically different results than if the fluid phase
is homogeneous. This last effect is common in subsurface systems as the gas phase is
nearly always close to water saturation and condensed water is a common occurrence
in both natural and anthropogenic flow structures. If a water droplet falls on a heated
sensor wire, the amount of energy required to evaporate the droplet and maintain the
temperature of the wire is much higher than even the most dramatic flow changes
encountered with single phase soil gas. Often the very high energy in a very short
time demanded by the sensor’s control mechanism can damage the delicate sensor
element, limiting the life of the thermal anemometer. To compensate for this effect,
more rugged, thicker sensor elements are made but these have slower response time
and less sensitivity because they require more energy to heat the larger sensor mass.