On heat and mass transfer using evaporating self-rewetting mixtures in microchannels
Introduction
Miniaturization brings challenges in heat dissipation for high power industrial devices. Various phase change heat transfer technologies such as microscale heat pipes for compact nuclear reactors [1], microchannel boiling for concentrator photovoltaics [2] and spay cooling for high-power electronic devices [3] are used to achieve highly efficient thermal management at small scale. However, small sizes means large confinement numbers [4], leading to confined bubbles [5] as well as persistent liquid films that are deleterious to further phase-change heat transfer enhancement.
To overcome difficulties in enhancing heat transfer for two-phase systems at microscale, one potential way is to exploit surface tension, since the surface tension force could dominate when the characteristic size of two-phase system decreases to microscale [6]. Typically, there are two kinds of methods making use of surface tension for better phase-change heat transfer. The first is to make solid microstructures which alter interactions between the solid wall and the liquid–vapour system. Additional capillary pressures from microstructures can be helpful for bubble [7] /droplet [8] motion when there is phase change. The second is to exploit surface tension properties of the liquid itself to increase mobility of the liquid. One successful example is a self-rewetting fluid. Self-rewetting fluids are dilute aqueous solutions of alcohols with the carbon atom number upwards from 4 (such as butanol, pentanol, and hexanol) [9] which exhibit a relationship between the surface tension σ and the temperature, T, which is not monotonic. The surface tension, σ, decreases with increasing T like normal liquids when T < Tc, i.e. the temperature is lower than a critical value, Tc, but when T > Tc, surface tension increases with increasing temperature. Thus the Marangoni convection is expected to overcome the other forces to drive the cold liquid to the hot region allowing for self-rewetting behaviour [10]. Pioneering studies of self-rewetting fluids were carried out at 2005 by Abe et al. [11], after which other studies revealed that self-rewetting fluids have proved beneficial for phase-change heat transfer such as heat pipes [12], microchannel boiling [13] and spray cooling [14] due to their unique surface tension properties. However, there is still a lack of fundamental experimental work that elucidates the mechanism of heat and mass transfer of self-rewetting fluids, especially at the liquid–vapour meniscus.
Menisci exist widely in solid–liquid-vapour microsystems, for example at the tips of confined bubbles, or corners of trapped liquid in microstructures, where most heat is transferred from solid to liquid film at the meniscus [15]. The local heat fluxes at the contact line area of the meniscus are found to be about 5.4–6.5 times higher than the mean input heat fluxes [16]. Moreover, when a mixture was used as the working fluid, the curvature gradient of an evaporating meniscus was found to be affected by both heat flux and fluid concentration [17], making for a complex physical phenomenon needing elucidation. In the present study, five liquids (ethanol, deionized water, butanol, 5% v/v ethanol/water mixture and 5% v/v butanol/water mixture) were used as working fluids during meniscus evaporation experiments in open microchannels. The heating was applied by Joule effect of a thin Tantalum film coated on a transparent test section having length of 20 mm and inner diameter of 1 mm. The corresponding wall temperatures varied with different liquids as well as different heating powers giving a range of 24 °C to 95 °C. We found that Marangoni flow in the self-rewetting fluid contributes to better heat and mass transfer in the evaporation process.
Section snippets
Test section
Three types of microchannels with 1 mm inner hydraulic diameter were used as the test sections, as shown in Fig. 1. Three channel types (a, b and c) were used. For Type a, the channel was manufactured from borosilicate glass having a round cross-sectional area with inner diameter of 1 mm and wall thickness of 0.1 mm. Type a channels were used to measure evaporation rate in the microchannel without heating. For Type b, the channel was the same size and material as Type a, but with outer surface
One-dimensional diffusion model for evaporation mass transfer
Vapour concentration near the liquid–vapour interface on the vapour side is always equal to the saturation concentration in an ambient environment under slow evaporation conditions. By assuming a linear concentration gradient along channel axis direction [15], one can calculate the one dimensional diffusion mass flux as follows:where J(h) is the mass flux along the channel in the axial direction, D is the diffusion coefficient of the vapour, and n is the molar
Evaporation rate
Experiment #1 was conducted to verify the above theory. The experimental results are shown in Fig. 3. Interface speeds of more volatile liquids like ethanol and butanol are faster than less volatile liquids such as water and alcohol/water mixtures, the evaporation curves of which almost coincide. The evolution of meniscus location can be well fitted with a power law, also shown in Fig. 3. Comparisons of meniscus interface speed of experimental results from equation (1) and the one-dimensional
Conclusions
In this study, meniscus evaporation heat and mass transfer in microchannels were compared for pure liquid (water, butanol, and ethanol), a non-self-rewetting mixture (ethanol/water 5% v/v) and a self-rewetting fluid (butanol 5% v/v) to reveal fundamental mechanisms of evaporation heat transfer for self-rewetting fluids.
The main conclusions are:
For the mass transfer process, a one-dimensional diffusion model was verified under adiabatic conditions but underestimated evaporation rates when
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
The author (KS) would like to acknowledge the support of the UK Engineering and Physical Sciences Research Council (EPSRC), through grant EP/N011341/1. The author (X. Yu) Would like to acknowledge the support of the China Scholarship Council (CSC).
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