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Hot wire anemometry
Hot wire anemometry is a technique used to measure time resolved flow in the cylinder. The basis for hot wire anemometry is the heat balance equations which can be applied to the anemometric signal to calculate the flow velocity. Hot wire anemometry experiments use a thin wire stretched between the tips of two prongs. The hot wire is introduced into the combustion chamber using a probe. The anemometric probes consist of a thin wire held across the tips of two prongs (there are a variety of different arrangements), the anemometric probes may also be operated as thermometric sensors (see fine wire resistance thermometer).
The probes consist of prongs insulated from the exterior by inserting them axially into ceramic rods encased in a steel pipe. The sensing element is a couple of mm long and typically 10 micrometers in diameter, though this is fairly large (poor frequency response) it must be strong enough to survive the particularly harsh environment of the combustion chamber. The criteria for the dimensions of the probe are based on, strength, spatial resolution and the frequency response of the prong and wire combination.
The heat exchange between the hot wire and the combustion chamber environment effect the output voltage of the anemometric signal. For hot wire anemometry it is necessary to measure both the gas temperature and prong tip temperature in addition to the anemometric signal. This is so that during flow measurements the thermal capacity and thermal inertia of the hot wire and prongs are taken into account and may be balanced by a feedback system when processing anemometric data. The feedback system keeps the mean temperature and resistance of the wire constant. Partially shielded probes may used to resolve any directional ambiguity in the signal. The schematic below shows a typical anemometric probe inserted into the combustion bowl of a diesel engine, z, r and s are the axial, radial and tangential velocity components respectively.
Ref: [C]
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