Thermal Transport Flow Transducers

 

The thermal transport properties of a moving fluid can be used to design a specific class of transducers. A hot wire or film which is at a higher temperature than the flow medium where it is placed will experience a heat loss which varies approximately with the square root of the fluid velocity. The primary mode of this heat transfer is through convection. Radiation and conduction are normally negligible factors. The governing relationship for a hot wire placed in a gaseous stream is:

 

i2R /(Tw - Ta) = A + B(rV)1/2

 

where i is the instantaneous current, R the wire resistance, Tw is the wire temperature, Ta is the ambient temperature, A and B are constants, r is the gas density and V is the free stream velocity.

 

Two types of thermal transducers are commonly used. The first type measures the temperature drop of the heated element in the flow field and relates the temperature drop and relates it to the fluid flowrate; the second type keeps the sensor at a constant temperature and relates the current draw in the circuit needed to maintain the element temperature.

 

In rapidly fluctuating temperature situations the heat capacity of the heating element will be a source of error. There will be a lag in the response of the sensing element to the actual fluctuations. To a large extent it is possible to compensate for some of this lag through circuit design.

 

The use of a miniaturized sensors and electronics make it possible to design probes to fit on the end of catheter tubes. Alternatively sensor design can be incorporated into a needle for insertion in tissue for transcutaneous temperature measurement.

 

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Support for the development of this module was provided by the National Science Foundation and The Cooper Union for the Advancement of Science and Art.
 

Please send questions or comments to Professor Ron Adrezin or Professor Daniel Raichel.