Effect of particle size on surface-coating enhancement of pool boiling heat transfer

Abstract

The enhancement of pool boiling heat transfer by copper-particle surface coatings is experimentally investigated, using the wetting dielectric fluid FC-72. In one technique, loose copper particles are placed on a heated copper surface to provide additional vapor nucleation sites in the cavities formed at particlesurface and particle-particle contact points, thereby enhancing boiling performance over a polished surface. This 'free-particle' technique is benchmarked against the more traditional technique of sintering a fixed layer of copper particles to the surface to enhance boiling heat transfer performance. The effect of particle size on the heat transfer performance is studied for particle diameters ranging from 45 µm to 1000 µm at a constant coating layer thickness-to-particle diameter ratio of approximately 4. The parametric trends in the boiling curve and the critical heat flux are compared between the two techniques, and the dominant boiling mechanisms influencing these trends are compared and contrasted. High-speed visualizations are performed to qualitatively assess the boiling patterns and bubble departure size/distribution, and thus corroborate the trends observed in the boiling curves. The measured wall superheat is significantly lower with a sintered coating compared to the free-particle layer for any given particle size and heat flux. Performance trends with respect to particle size, however, are remarkably similar for both enhancement techniques, and an optimum particle size of ~100 µm is identified for both free particles and sintered coatings. The free-particle technique is shown to offer a straightforward method to screen the boiling enhancement trends expected from different particulate layer compositions that are intended to be subsequently fabricated by sintering.

Boiling Heat Transfer of Alumina Nano-Fluids: Role of Nanoparticle Deposition on the Boiling Heat Transfer Coefficient

“…The maximum CHF enhancement was obtained for 0.1% (by volume) nano-fluid to be about 100% while the boiling heat transfer coefficients deteriorated by increasing nanoparticle concentration in nanofluid. In another research, the enhancement of pool boiling heat transfer by copper-particle surface coatings is experimentally investigated by Sarangi and his co-workers using free-particle technique [13]. By using this technique, they manipulated the nucleation site densities using loose copper particles which led to enhancing the thermal performance of boiling mechanism.…”

Quantitative Evaluation of the Dependence of Pool Boiling Heat Transfer Enhancement on Sintered Particle Coating Characteristics

“…This requires either reducing or increasing the native packing density of the powder. The porosity of the coating can be reduced by controlling the sintering time and temperature to achieve the desired porosity [9]. In order to increase the porosity beyond the native packing density, the most commonly used method is lostcarbonate sintering (LCS) [29].…”

“…A model formulated by Ranjan et al [8], which considered the porous structure as being analogous to extended fins, showed that the optimum wick thickness occurred when the effective area enhancement was maximized. Sarangi et al [9] studied the effect of particle size variation on copper surfaces coated with sintered spherical copper particles, for particle sizes varying from 45 to 1000 lm. The sintered coatings were manufactured with a constant thickness-to-diameter ratio (d/d ¼ 4) and porosity of 30-40%.…”

“…Figure 1 shows a schematic diagram of the pool boiling test facility. An abbreviated description is provided here for this modified version of the facility used by Sarangi et al [9]. The base heater assembly consists of a copper heater block embedded in an insulation casing.…”

“…The copper block is inserted into a block of PEEK, a machinable, low thermal conductivity (0.28 W/m K) thermoplastic that C. A 0.8 mm-thick polycarbonate plate is attached to the top of the PEEK block; the copper block is aligned such that it protrudes just above the polycarbonate plate as shown. The vertical location of the thermocouple rake and the thickness of polycarbonate plate have been slightly altered from the original design [9]. The lower part of the copper block is insulated as described in Ref.…”