Thermal-wave resonant-cavity measurements of the thermal diffusivity of air: A comparison between cavity-length and modulation-frequency scans

Abstract

The application of a thermal-wave resonant cavity to thermal-diffusivity measurements of gases has been investigated. The cavity was constructed using a thin aluminum foil wall as the intensity-modulated laser-beam oscillator source opposite a pyroelectric polyvilidene fluoride wall acting as a signal transducer. Theoretically, cavity-length and modulation-frequency scans both produce resonance-like extrema in lock-in in-phase and quadrature curves. These extrema can be used to measure the thermal diffusivity of the gas within the cavity. It was found experimentally that one can obtain very accurate and reproducible measurements of the thermal diffusivity of the gas by using simple cavity-length scanning without any signal normalization procedure, rather than traditional modulation-frequency scanning normalized by the frequency-dependent transfer function of the instrumentation. By scanning the cavity length, the thermal diffusivity of room air at 299 K was measured with three-significant figure precision as 0.216_ 0.001 cm 2-s-1, with a standard deviation 0.5 %. Only two significant figure accuracy could be obtained by scanning the frequency: 0.22 __+ 0.03 cm 2. s-t, with a standard deviation of 14%. Cavity-length scanning consistently exhibited a much higher signal-to-noise ratio.

Diffusive-to-ballistic transition of the modulated heat transport in a rarefied air chamber

“…[12][13][14][15][16][17][18][19][20][21][22][23][24][25] The thermal-wave resonant cavity (TWRC) has demonstrated to be a simple, reliable, and accurate technique to study the heat transport in a large variety of fluids. [12][13][14] This photothermal technique involves the use a lock-in amplifiers and filters that allow us measuring very low signals even in highly noisy environments. In its simplest configuration, the TWRC consists of three parallel layers: the first one is a thin film called heater, which is heated up with a laser beam of modulated intensity; while the second one is a fluid sample supporting the propagation of the temperature oscillations induces by the heater towards the pyroelectric detector (third layer) that records the amplitude and phase delay of the thermal-wave signal, in the form of a voltage.…”

Monitoring of hydrocarbon vapor diffusion in air using a thermal wave interferometer

“…One of them is based upon the fact that the L dependence of Eq. ͑4͒ is such that their in-phase ͑IP͒ and quadrature ͑Q͒ signals exhibit maxima and minima such that their nth extrema are given by 3,4 L n…”

“…The main attractive aspect of this new photothermal technique, originally called thermalwave resonant cavity, and later, thermal wave interferometer ͑TWI͒ by some other authors, is that it is well suited for the investigation of the thermophysical properties of gases and liquids. This can be appreciated by the growing number of papers that have recently applied this new technique to the investigation of the thermal properties of gases [3][4][5][6] and liquids. 7 The reason the TWI technique is so attractive to the investigation of gases and liquids is evident from its operational principle, namely, the monitoring of the spatial dependence of intracavity thermal waves.…”

Photothermal techniques applied to thermophysical properties measurements (plenary)

“…47,48 In the case of gases, most of the investigations have used photoacoustic detection, whereas most of the works with liquids so far have used photoacoustic detection and remote photothermal refractive index modulation techniques, such as the photothermal phase shift technique, thermal lens, and the photothermal optical beam deflection. An alternative photothermal technique for measuring the thermal properties of gases was first discussed by Bennet and Patty 49 and later explored by Shen and Mandelis, [50][51][52][53] who succeeded in demonstrating the feasibility of the pyroelectric detection of thermal wave propagating across an air gap between a pyroelectric sensor and another material acting as the source of thermal waves. These authors showed that the thermal diffusivity of air could be measured with very good accuracy using this configuration for detection of thermal waves.…”