Measurement of Key Pool Boiling Parameters in Nanofluids for Nuclear Applications

Abstract

Nanofluids, colloidal dispersions of nanoparticles in a base fluid such as water, can afford very significant Critical Heat Flux (CHF) enhancement. Such engineered fluids potentially could be employed in reactors as advanced coolants in safety systems with significant safety and economic advantages. However, a satisfactory explanation of the CHF enhancement mechanism in nanofluids is lacking. To close this gap, we have identified the important boiling parameters to be measured. These are the properties (e.g., density, viscosity, thermal conductivity, specific heat, vaporization enthalpy, surface tension), hydrodynamic parameters (i.e., bubble size, bubble velocity, departure frequency, hot/dry spot dynamics) and surface conditions (i.e., contact angle, nucleation site density). We have also deployed a pool boiling facility in which many such parameters can be measured. The facility is equipped with a thin indium-tin-oxide heater deposited over a sapphire substrate. An infra-red high-speed camera and an optical probe are used to measure the temperature distribution on the heater and the hydrodynamics above the heater, respectively. The first data generated with this facility already provide some clue on the CHF enhancement mechanism in nanofluids. Specifically, the progression to burnout in a pure fluid (ethanol in this case) is characterized by a smoothly-shaped and steadily-expanding hot spot. By contrast, in the ethanol-based nanofluid the hot spot pulsates and the progression to burnout lasts longer, although the nanofluid CHF is higher than the pure fluid CHF. The presence of a nanoparticle deposition layer on the heater surface seems to enhance wettability and aid hot spot dissipation, thus delaying burnout.

Nanotechnology for Advanced Nuclear Thermal-Hydraulics and Safety: Boiling and Condensation

“…Interestingly, this equation and the hydrodynamic theory equation are essentially the same, except for the parameter k, which according to Theofanous and Dinh [20] "for a well-wetting surface is smaller than for a poorly-wetting surface"; i.e., everything else being the same, a well-wetting surface will have a higher CHF than a poorly-wetting surface [26,50]. D) Bubble interaction theory, which postulates that CHF occurs when at high heat flux the bubble number and departure frequency become so high that the bubbles coalesce radially, thus preventing liquid access to the surface [Rosenhow and Griffith, [21]].…”

“…This emphasizes the role of the bubble wait time (∆τw) and departure time (∆τd), as well as the role of the surface conditions, including the contact angle, via the nucleation site density [26,50].…”

“…Review of the traditional models has led to the following list of parameters governing nucleate boiling and CHF [26] …”

“…Colloidal suspensions have shown intriguing thermal performances,particularly regarding the following four points: (1) increased thermal conductivity (~150%);(2) increased single-phase heat transfer coefficient (~60%);(3) increased critical heat flux with extended nucleate boiling regime (~200%);and (4) improved quenching efficiency. In addition, nanofluids exhibit a very significant enhancement (up to +200%) of the boiling Critical Heat Flux (CHF) at low nanoparticle concentration (the CHF is the limit of the most efficient heat transfer regime or nucleate boiling or the indirect limit of the above-mentioned attainable energy-source temperature) [26,77]. Owing to these factors, nanofluids are very attractive heat transfer fluids with regard to many applications.…”

“…In order to prove this, some researchers indirectly showed that the deposition layer of nanoparticles improved both surface wettability and capillarity,as shown in Figs. 4 and 5 [50,51,26]. [26] Recently, Park et al [25] investigated the effects of newly-introduced graphene nanomaterials on pool boiling CHF.…”