Effect of forced convection on thermal distribution in micro thermal conductivity detectors

To date, most models of micro-thermal conductivity detectors (mu TCDs) positioned in a micro channel have assumed a constant temperature along the entire length of the element. This was known to be an approximation used to make the problem tractable; however, the accuracy of this assumption was unknown. We have designed, fabricated and tested a new mu TCD for the purposes of measuring the physical changes in the thermal distribution on a mu TCD element with changing flow rate. This device shows, in contrast to the existing models for a thin film mu TCD, that due to constantly changing boundary conditions, there is effectively no thermal entry length where the gas obtains thermal equilibrium. Models of mu TCD operation cannot use the traditional constant temperature or constant heat flux assumptions and need to include joule heating as well as lateral thermal conduction to accurately simulate device operations.