Investigation Into The Boiling Heat Transfer Performance Of Multiwalled Carbon Nanogube Suspension

There are many discrepancies and contradictions about the nanofluid boiling heat transfer performance and the roots. Some got the results that nanoparticles deteriorate the nucleate boiling heat transfer and presumed that the surface roughness becomes smoother due to deposition of the nanoparticles and the concurrent reduction of nuclei of vapor bubble. Others concluded that nanofluid wets the boiling surface easier than the base fluid, and that dryout is delayed by continuous supply of liquid. The author conjectures that the thermo-physical properties of nanofluid, deposition and fouling of nanopaticle, and change of the foul microstrucsture with time would be the key factors to determine the nanofluid boiling heat transfer behavior. The literatures on nanofluid boiling heat transfer monotonously focous on the nucleate regime and on critical heat flux (CHF). The post-critical regime has not yet touched upon. Relevant works have been thus conducted.Thermal conductivity and viscosity of the multiwalled carbon nanotube (CNT) nanofluid stabilized by dispersant were measured by transient hot wire method and Haake viscometer, respectively. Stability of the nanofluids formulated by concentrated acid mixture and by addition of dispersant was compared. Mechanisms underlying the modified thermal conductivity and rheology of the nanofluid were provided. Thermal conductivity and viscosity of the nanofluid change marginally. Microconvection due to nanoparticle, aggregation, and chemistry of the suspension impacts both parameters methoned above. Non-Newtonian behavior of the nanofluid is dominated by the aggregation.To clarify controversies about the boiling heat transfer characteristics of nanofluid, nucleate pool boiling experiments were performed on a 12 mm diameter copper surface utilizing aqueous gum Arabic (GA) solution of CNT. As compared with aqueous GA solution, nucleate boiling heat transfer coefficient of nanofluid is lowered at the same superheat and a reduced CHF extended to higher temperature difference between the fluid and the heating wall. Boiling heat transfer performance of water is deteriorated by addition of GA powder. Density of the fluid in the vicinity of the heating wall is locally increased and nuclei on the surface are fluctuating with depositing and fouling of the nanoparticles. Microstructure of the foul, involving diameter and length of the capillary and porosity, changes while boiling proceeds. The depositing progress gradually increases the density and thickness of the deposit, deactivating the larger nucleation sites and closing the capillaries. The heat transfer coefficient is reduced and eventually leveled off by the additional resistance of the deposits. In the neighborhood of the heating wall, since viscosity is markedly enlarged due to evaporation of the volatile component and local enrichment of GA, boiling heat transfer of aqueous GA solution is diminished.Characteristic boiling curve, determined by transient calorimeter method, was employed to gain insight into the boiling heat transfer performance of CNT nanofluid. Special interests have been focused on in the post-critical regime. Comparatively, CHF, transitional boiling heat transfer rate, and heat flux at Leidenfrost of water are augamented by dispersion of GA powder. CHF of nanofluid is higher than aqueous GA solution. It is observed that GA solution wets the copper sheet much easier than water does and that there is a trace of deposits on the copper sphere surface. Improved wettability by the presence of the dispersant renovates the heat transfer mechanism. And thus, heat transfer performance in the transitional boiling regime incorporating CHF and Leidenfrost point gets an improvement. CHF of the nanofluid is intensified by way of the changed bubbling mechanism.To explore the boiling characteristics, experiments were carried out on a two-phase closed thermosyphon with carbon nano-tube suspensions as working media. In contrast to typical thermosyphon, the one filled with a nano-particle suspension has adversely high boiling incipience temperature, temperature excursion, and thermal resistance. The carbon nano-tube suspensions deteriorate the performance of the gravity-assisted heat pipe. Measurements showed that suspending carbon nano-tubes to bulk water renders it enlarged surface tension. Changes of the interfacial properties lead to reduced active nucleation sites, density and departure frequency, and to enlarged bubble volume and coalescence readiness. Marangoni flow is induced by temperature and concentration gradients. From preferential evaporation of volatile component at the local interface arises mass transfer. All factors function together and result in high temperature excursion, evaporator wall temperature, and thermal resistance. Heat transfer for carbon nano-tube suspension filled gravity-assisted heat pipe deteriorates.Thermal conductivity of the nanofluid is strongly influenced by the aggregation of the nanoparticles. Depositing and fouling of the nanoparticles on the heating surface govern the boiling heat transfer performance of nanofluid. Aging etches the merits of nanofluid away relentlessly.