Thermal diffusion of water vapour in porous materials: Fact or fiction?

Abstract

The reliable evaluation of moisture transfer in porous materials is essential in many engineering applications, among which building science. One key aspect is a correct description of moisture flow phenomena and their transport potentials. While different issues can be debated in that respect, thermal diffusion of water vapour in porous materials -diffusion driven by temperature gradients -currently stands out, due to the contradictory findings on the topic. Thermal diffusion proponents uphold that, complementary to vapour pressure gradients, temperature gradients equally yield substantial diffusion. Thermal diffusion opponents, on the other hand, assert that these thermal transports are negligibly small. This paper resolves that contradiction. A critical analysis of the investigations supporting the occurrence of thermal diffusion reveals that all are flawed. A correct reinterpretation of all measurements allows concluding that no consistent nor significant thermal diffusion can be observed. This brings these investigations in line with their earlier opponents. This conclusion also agrees with thermodynamics, which confirms the actual existence of thermal diffusion, but also indicates its negligible magnitude. It can in conclusion be stated that thermal diffusion is of no importance for building science applications, leaving vapour pressure as the sole significant transport potential for the diffusion of water vapour in porous materials. KEYWORDSVapour diffusion, Fick's law, Soret effect, thermal diffusion, vapour pressure Postprint: Janssen H, 2011. Thermal diffusion of water vapour in porous materials: fact or fiction ?, International Journal of Heat and Mass Transfer, 54:1548-1562

A comment to “Assessment of temperature gradient effects on moisture transfer through thermogradient coefficient”

“…(Janssen 2011) was inspired by the contradiction between the different investigations on the topic. The thermal diffusion proponents (Kumaran 1987, Dahl et al 1996, Stephenson 2003, Peuhkuri et al 2008, Galbraith et al 1998 claim that temperature gradients, complementary to vapour pressure gradients, equally yield significant diffusion.…”

“…Thermal diffusion opponents (Galbraith et al 1998, Thomas 1999, Glass 2007, Baker et al 2009), on the other hand, state that such thermal transports are negligibly small. Thermal diffusion proponents hence allege that vapour pressure and temperature are both significant transport potentials for diffusion: (Janssen 2011): solely the binary diffusion of vapour and air in porous materials is studied, and thermal diffusion refers to diffusion driven by a temperature gradient in addition to diffusion driven by a vapour pressure gradient. The impact of these limitations with respect to Trabelsi et al's (2012) article is discussed further below.…”

“…The impact of these limitations with respect to Trabelsi et al's (2012) article is discussed further below. As prelude to and backdrop for the actual comment, the primary findings from (Janssen 2011) are shortly recapped first.…”

“…In that respect, thermal diffusion -diffusion of water vapour driven by temperature gradients -remains a hot topic. Based on a critical evaluation of earlier experiments and a theoretical analysis via irreversible thermodynamics, occurrence of significant thermal diffusion was recently invalidated by Janssen (2011), confirming vapour pressure as sole significant transport potential for diffusion. Nonetheless, Trabelsi et al (2012) again put a claim for thermal diffusion forward, built on their vapour diffusion model governed by vapour density and temperature gradients.…”

“…Trabelsi et al detect that the non-isothermal value is 14 % lower than its isothermal counterpart, and they therefore advocate reviewing the currently isothermal measurement protocol. As prelude to and backdrop for the concrete comment, the key findings from (Janssen 2011) are concisely summarised first: invalidation of the occurrence of significant thermal diffusion, and confirmation of the vapour pressure as sole significant transport potential. They are then verified once more by reassessing Trabelsi et al's measurements in function of vapour pressure and vapour permeability.…”