High-speed observation of the piston effect near the gas-liquid critical point

Abstract

We measured high-speed sound propagation in a near-critical fluid using a ultra-sensitive interferometer to investigate adiabatic changes of fluids on acoustic timescales. A sound emitted by very weak continuous heating caused a stepwise adiabatic change at its front with a density change of order 10 −7 g/cm 3 and a temperature change of order 10 −5 deg. Very small heat inputs at a heater produced short acoustic pulses with width of order 10µsec, which were broadened as they moved through the cell and encountered with the boundaries. The pulse broadening became enhanced near the critical point. We also examined theoretically how sounds are emitted from a heater and how applied heat is transformed into mechanical work. Our predictions well agree with our data. Thermal equilibration in one-component fluids takes place increasingly faster near the gas-liquid critical point at fixed volume [1,2,3,4,5,6,7,8,9,10], despite the fact that the thermal diffusion constant D tends to zero at the criticality. This is because the thermal diffusion layer at the boundary expands and sounds emitted cause adiabatic compression and heating in the whole cell after many traversals in the container. This heating mechanism is much intensified near the critical point due to the critical enhancement of thermal expansion of the layer. If the boundary temperature T w is fixed, the interior temperature approaches T w on the timescale of the piston time [2],where L is the cell length and γ = C p /C V is the specificheat ratio growing near the critical point. This time is much shorter than the isobaric equilibration time L 2 /4D by the very small factor (γ − 1) −2 [11]. The previous experiments have detected only slow temperature and density changes in the interior region on timescales of order 1 sec. The aim of this letter is to report ultra-sensitive, high-speed observation of sound propagation through a cell filled with CO 2 on the critical isochore close to the critical point T c = 304.12K. We can detect density changes of order 10

Sound-driven fluid dynamics in pressurized carbon dioxide

“…enhanced thermalization. 17,18 In contrast to the piston effect, the frequency and acoustic pressure allow for direct manipulation of the time scale and the extent of phase separation. The tunability is also manifested by the possibility to induce phase separation at fixed positions in the sound field, i.e., at the pressure antinodes of a standing wave ͓Fig.…”

Thermoacoustic effects in supercritical fluids near the critical point: Resonance, piston effect, and acoustic emission and reflection

“…4, we plot the absolute value |R T | calculated from Eq. (3.12) vs the normalized frequency ωL/πc at ǫ = 10 −3 and 10 −4 , using the data for CO 2 in a Cu cell with L = 1cm [20]. It exhibits peaks at ω ∼ = nπc/L as expected, but its peak heights do not exceeds 1/2 due to the small factor 1−Z ∼ = 2W in the numerator in Eq.…”

“…For CO 2 in a Cu cell [20] we have a w = 3 × 10 3 ǫ 0.92 . The boundary temperature at x = 0 is fixed or δT (0) = 0 for a w → ∞, while the boundary is thermally insulating or (dδT /dx) x=0 = 0 as a w → 0.…”

“…This behavior is detectable only for an increase ofQ 0 within a time shorter than t ξ . Miura et al applied a stepwise heat flux withQ 0 = 0.183×10 7 to find a stepwise outgoing sound with δρ/ρ ∼ = 2.2 × 10 −7 for CO 2 , whereQ 0 is in cgs units (erg/cm 2 sec) [20]. Our theoretical expression (3.38) becomes δρ/ρ = 1.38 × 10 −13Q 0 with the aid of (∂T /∂p) s ∼ = T c /6.98p c for CO 2 [35].…”

“…The critical growth of the effusivity and the bulk viscosity of the fluid both serve to suppress the boundary damping, as confirmed experimentally and theoretically. Very recently, Miura et al [19,20] measured acoustic density changes with precision of order 10 −7 g/cm 3 in near-critical CO 2 on the acoustic time scale using a ultra-sensitive interferometer. They detected emission and traversal of sound pulses with width of order 10µsec, which were broadened as they moved through the cell and interacted with the boundary walls.…”