Transport processes in thermal conductivity detectors.

摘要

Transport processes in the thermal conductivity detectors employed for gas chromatography to detect trace gases' concentrations are studied experimentally and theoretically. Although heat transport in these sensors occurs predominantly by conduction, heat convection also plays a role. Convective heat transfer is affected by fluctuations in flow-rate that are picked up by the detector as noise and adversely affect its sensitivity.; The effect of time-independent and time-dependent convection on thermal transport in cylindrical detectors is examined experimentally when the TCD is operating with a single component gas. Low frequency (around 0.01Hz) and moderate frequency (1 {dollar}sim{dollar} 10Hz) noise were detected. The low frequency noise is independent of the flow rate and it is present even in the absence of flow. The moderate frequency noise's amplitude increases as the flow rate increases. The low frequency noise is caused by fluctuations in the TCD's wall temperature while the moderate frequency noise is caused by the flow rate fluctuations.; The effect of convection on the performance of cylindrical and planar thermal conductivity detectors (TCDs) is investigated theoretically. For low Peclet numbers, an asymptotic theory is constructed to correlate between the TCD's power dissipation and the Peclet number and to explain certain experimental observations. Then, the flow equations for a gas with temperature-dependent properties are solved numerically. The TCD's response is calculated for both time-independent and time-dependent flows. The theoretical predictions are critically compared with experimental observations.; Another potential noise source in the TCD is thermoacoustic (TAC) waves. In TCD, when the colder gas encounters the hot filament, a sudden expansion of the gas occurs which, in turn, induces TAC waves. These waves may induce time-dependent heat transfer from the filament and thus adversely impact the thermal conductivity measurement.; To better understand the thermoacoustic phenomenon, the generation and transmission of planar thermoacoustic (TAC) waves by sudden heating of a boundary of a quiescent, semi-infinite or confined, gaseous medium is investigated theoretically. For step and gradual changes in the boundary temperature of a semi-infinite gas medium with Pr = 3/4, long and short-time asymptotes are derived for the pressure, velocity, temperature fields, and wall heat flux. Then the method of Laplace Transform with numerical inversion is used to solve the linearized equations for general wall heating conditions. By comparing the Laplace Transform predictions with the asymptotic results and with experimental data, the numerical scheme is verified and found to be highly accurate. The pressure, velocity and temperature waves are computed as functions of time and distance from the heated wall and the contribution of the TAC waves to the heat transfer process is assessed. The effect of the wall heating time-constant on the TAC waves is also examined. Finally, the non-linear equations are solved numerically to assess the effect of nonlinearities on the wave characteristics and to establish the conditions under which the linear approximation is adequate.