Flow Visualization, Critical Heat Flux Enhancement, and Transient Characteristics in Pool Boiling Using Nanofluids

Abstract

This paper presents the experimental outcome of a study of the pool boiling heat transfer characteristics of alumina and CuO nanofluid in distilled water using a 0.19 mm diameter NiCr wire. A series of experiments were conducted in order to visualize the flow, critical heat flux (CHF) enhancement, and transient characteristics of nanofluid. The boiling phenomenon was visualized using a 0.1 g/l concentration of alumina nanofluid. The average bubble diameter was measured and was found to increase with increased heat flux. The average bubble contact angle decreased from 69° during the initial stages of boiling to 33° at CHF. Massive vapour bubbles were observed on the test heater surface near the CHF, inducing vapour blankets and forming hot/dry spots. The increase in the CHF could be well explained by the hot/dry spot theory. Pool boiling experiments conducted using low volume concentrations of CuO-water nanofluid at atmospheric pressure in distilled water showed an increase in the CHF by 30 % at a 0.3 g/l concentration. The transient behaviour of nanofluid, examined by exposing the heater surface at a constant heat flux of 700 kW/m2, indicated CHF enhancement of 5.21 % to 6.77 % for the two time durations. Based on the experimental investigations, it was concluded that the CHF enhancement is due to nanoparticle coating, which changes the thickness of the surface as a function of time and surface wettability and corroborates the hot/dry spot theory.

Visualization of pool boiling heat transfer of magnetic nanofluid

“…The isolated bubble regime, mushroom cloud, and bubble coalescence are captured by images for a thorough understanding of the process of boiling and condensation. Also, Hegde et al experimentally investigated the pool boiling heat transfer characteristics of Al 2 O 3 nanoparticles in water. By visualization, it was shown the generation of massive vapor bubbles forming hot/dry spots which are at certain nucleation site.…”

Glass packing materials used for intensification of heat transfer at boiling on tubular surfaces

“…The literature also presents different possibilities for intensification of heat transfer at boiling by using so called static promoters such as different form of bars, equidistantly placed on the plane horizontal heating surfaces, as well as different material particles, immersed in the boiling liquid [11]. Some authors present experimental results of local heat transfer coefficients for boiling of different fluids in a small diameter smooth tube [15], or in pool boiling on nanostructured surfaces [16][17][18][19][20], or the effect of concentration of nanofluids on pool boiling [19].…”

“…Critical heat flux (CHF) is the condition where the vapor generated by nucleate boiling becomes so large that it prevents the liquid from reaching and re-wetting the surface. Pool boiling CHF is the point where nucleate boiling goes through a flow regime transition to film boiling with a continuous vapor film separating the heater and the liquid [20]. Boiling heat transfer coefficient was established for different experimental conditions inside a horizontal tube [21].…”