Experimental study on quenching of a small metal sphere in nanofluids

The objective of this research is to systematically investigate the quenching characteristics of a hot sphere in nanofluids. The experiments are carried out with a small (9.5 mm) stainless steel sphere with initial temperatures near 1000 °C. Alumina nanofluids and deionized water are tested at low volume concentrations (less than 0.1 % by volume) and saturated conditions (100 °C). The results show that the quenching behavior in nanofluids is nearly identical to that in pure water. Moreover it is found that some nanoparticles accumulate on the sphere surface during the quenching process. Such accumulation of nanoparticles on the surface promotes the destabilization of the vapor film in subsequent quenching experiments, thus accelerating the return to nucleate boiling at higher temperature than that in the clean surface case. NOMENCLATURE Tcenter Temperature at the center of the sphere, °C TMHF Temperature at the minimum heat flux point, °C Tmax_slope Temperature at the maximum negative slope point, °C INTRODUCTION A number of recent investigations on boiling of nanofluids showed that such engineered fluids can effectively delay departure from nucleate boiling (DNB) with respect to pure fluids [1-9]. It was found that the DNB heat flux enhancement is closely related to nanoparticle deposition, which significantly roughens the heater surface by changing the initially smooth surface to one with peaks and valleys. Moreover the deposition of oxide nanoparticles like alumina and titania significantly enhances the affinity (or wettability) of the cooling liquid to the surface. These changes in the surface alter the boiling heat transfer characteristics, e.g., they increase the value of the critical heat flux. Park et al. [10] performed quenching experiments of a hightemperature sphere in alumina nanofluids to investigate the effect of the nanoparticles on film boiling heat transfer. They observed an interesting phenomenon: after a quenching experiment of a sphere in nanofluids, the unwashed sphere quenched more rapidly through transition boiling, apparently bypassing the film boiling mode. This result suggested that nanoparticle deposition on the sphere surface prevents formation of a stable vapor film around the sphere, which consequently promotes a more rapid quenching. However, the main focus in that study was to explore the effect of nanoparticles on film boiling heat transfer, not quenching. At MIT we are investigating the use of nanofluids in safety systems for Light Water Reactors [11], and quenching phenomena play an important role in ensuring the coolability of the nuclear fuel following loss of coolant accidents in such reactors. Specifically, because during an accident the fuel can be initially very hot (>700°C), its rewetting occurs slowly through the development of a quench front which advances upward in the reactor core. The speed of the quench front and thus the peak temperature reached during the reflood transient depend on a combination of factors including film boiling heat transfer, wettability of the fuel surface by the coolant, and localized axial conduction within the cladding near the quench front. The use of nanofluids could afford a significant increase of the quench speed for two reasons. First, boilinginduced deposition of nanoparticles on the surface greatly enhances surface wettability. Second, deposited nanoparticles of high-conductivity material (e.g., alumina) improve localized axial conduction in the cladding near the quench front. The enhanced wettability especially is expected to increase the minimum heat flux temperature (Leidenfrost point) for the cladding and promote its rapid cooling. As a first step in the feasibility assessment of nanofluids for use in nuclear reactor accidents, the quenching characteristics of a small metallic sphere in nanofluids are studied. Nanofluids with 0.001, 0.01, and 0.1% volume concentrations of alumina nanoparticles are tested at saturated conditions (100 °C) under atmospheric pressure. In this paper, the transient cooling curves (temperature vs time) during the entire quenching process in distilled water and in nanofluids are investigated. *Corresponding author: Email: [email protected] 2 Copyright © 2008 by ASME EXPERIMENT Experimental Setup Figure 1 shows the details of a test sample for the quenching experiment. The test sample consists of a metal sphere, a thermocouple to record the temperature at the center of the sphere, and a reinforcing precision tube to mechanically support them. The reinforcing tube is connected to a 9.5 mmdiameter connecting tube via a tube fitting. A stainless steel sphere of 9.5-mm diameter is used as the quenching sample. The sphere is drilled to the center having a hole stepped from 0.9 mm to 0.5 mm in diameter, as shown in Fig. 1. A 0.5 mm-diameter K-type sheathed ungrounded thermocouple is inserted to the bottom of the hole by friction fitting. The installation of the thermocouple via friction fitting ensures a good thermal contact with the sphere, and thus minimizes its response time so that the rapidly changing temperature history of the sphere is acquired correctly. A reinforcing tube of 0.6 mm ID and 0.89 mm OD is inserted between the 0.5 mm-diameter thermocouple and the 0.9 mmdiameter hole to mechanically support the test sample. A staking technique – hitting the edge with a sharp tool – is used to connect the tube and the sphere (See Fig. 1). The reinforcing tube is a path of conduction heat loss during the experiments, which should be minimized. In the present experiment the ratio of the diameter of the reinforcing tube to that of the sphere diameter is very low (0.09), so conduction losses are negligible. In addition, the length of the tube is one of the important design parameters because vigorous boiling on the surface when quenching occurs causes vibration of the sphere, which may disturb formation of the stable vapor film around the sphere. Based on an analysis of mechanical vibration of the rod-sphere system, the proper length of the precision tube is determined to be 20 mm. Figure 2 shows the schematic of the experimental setup for the quench test. It consists of the test sample, the radiant furnace, the air slide, the quench pool, and the data acquisition system. Figure 1 Details of the test sample A radiant furnace having a maximum temperature of 1500 °C is used to heat the test sample. A DC power supply (25V, 150A) is used to power the furnace. A B-type sheathed thermocouple is mounted inside the furnace to monitor the temperature. A pneumatic air slide moves the test sample between the furnace and the quench pool with the stroke length of 200 mm. Pressurized air near 600 kPa is used to operate the slide. The average downward velocity of the sample is about 0.5 m/sec, which is measured from a visualization of the moving sphere using a high speed camera. Resultantly the time to convey the sample from the furnace to the pool is about 0.4 sec. A 4-way solenoid valve is adopted to change the direction of the slide. The quench pool is 95 mm × 95 mm rectangular vessel having depth of 150 mm, which has an effectively infinite thermal capacity with respect to the sphere. It is made of Pyrex glass for visual observation of the quenching phenomena. The quench pool is placed on a hot plate with the maximum power of 800 W. The temperature of the quench pool is maintained with the feedback control of the hot plate and a Pt-100 ohm RTD sensor immersed in the quench pool. The side wall is insulated to minimize buoyant convection due to heat loss through the wall. A HP agilent 34980A data acquisition system and a PC are used for gathering and storing temperature data from the thermocouple within the sphere. The temperature data is acquired at a rate of 10 Hz which is fast enough to monitor the temperature history of the quenched sphere. Nanofluids In the present study, water-based nanofluids with alumina nanoparticles are selected, as these particles are most widely used in the previous investigations of heat transfer in nanofluids. Nanofluids with the desired nanoparticle concentrations are prepared by diluting the concentrated nanosolution purchased from Nyacol with distilled water. The vendor-specified particle size is 50 nm. The nanofluids with 0.001, 0.01, and 0.1% volumetric concentrations of the nanoparticles are tested at saturated temperature under atmospheric conditions. Figure 2 Schematic of quenching test facility