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Flow Visualization of Free Convection in a Vertical Cylinder of Water in the Vicinity of the Density Maximum M.F

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Flow Visualization of Free Convection in a Vertical Cylinder of Water in the Vicinity of the Density Maximum M.F CORE Metadata, citation and similar papers at core.ac.uk Provided by MURAL - Maynooth University Research Archive Library International Journal of Heat and Mass Transfer 47 (2004) 1175–1186 www.elsevier.com/locate/ijhmt Flow visualization of free convection in a vertical cylinder of water in the vicinity of the density maximum M.F. Cawley *, P. McBride Department of Experimental Physics, National University of Ireland, Maynooth, County Kildare, Ireland Received 24 February 2003; received in revised form 16 September 2003 Abstract Free convection in a vertical cylinder of water is studied in the vicinity of the density maximum at 4 °C. Results are presented both from cooling curve measurements (using a series of thermistors along the central axis of the cylinder) and from flow visualization experiments (using particle image velocimetry), and a comparison is made with simulations. The reversal of the overall flow direction as the temperature crosses the maximum density region gives rise to a plateau feature in the cooling curve, which may be explained by the formation of a rising toroidal structure along the column boundary. This feature effectively diverts flow away from the inner region of the cylinder, resulting in a temporary cessation of convective cooling within this region. Ó 2003 Elsevier Ltd. All rights reserved. 1. Introduction Hope [2]. Through a series of experiments, he proposed that water was at its most dense at a temperature be- Fluid density usually changes as a function of tem- tween 4.2 and 4.4 °C (Hope quoted his results using the perature in a reasonably linear manner. A notable Fahrenheit scale; he concluded that the density maxi- exception is liquid water. Pure water at a pressure of one mum lay between 39.5 and 40 °F). To circumvent atmosphere has a maximum density of 999.9720 kg mÀ3 objections, prevalent at that time, that the density at 4 °C [1]. Above this temperature the density of water anomaly was due to expansion or contraction of the decreases as the temperature is increased in a manner vessel in which the water sample was contained, the similar to other fluids. For temperatures below 4 °C, experiments devised by Hope did not rely on measure- however, the trend is reversed: density increases with ment of volume (and hence density) changes. Instead, increased temperature, giving rise to a maximum density Hope’s result relied primarily on measuring the tem- at the 4 °C point. The density profile for pure water at perature of the dense water which settled at the bottom 101 kPa (one atmosphere), in the vicinity of its density of a cooled vertical cylinder. By noting that a tempera- maximum, is shown in Fig. 1. There is a 0.013% increase ture inversion occurred, with warmer water settling be- in density between 0 and 4 °C, and a decrease in density low cooler–less dense-water, Hope concluded that the of a similar magnitude between 4 and 8 °C; the magni- water density anomaly existed independent of any tude of the density anomaly in the liquid state is there- complications due to changes in container volume. fore small compared to the anomalous contraction by The effects of the density anomaly may also be de- 8.3% in going from solid to liquid water (an anomaly tected in modifications to expected free convective flow. magnitude ratio of 640:1). Being driven by buoyancy forces, such free convection One of the earliest detailed studies of the liquid water flows are sensitive to small changes in the density of bulk density anomaly was carried out by Thomas Charles regions within the fluid. A density reversal will be par- ticularly disruptive to buoyancy-driven flow, as it will ultimately result in the reversal of the flow direction * Corresponding author. Fax: +353-1-7083313. (assuming the effect persists for a sufficient duration). In E-mail address: [email protected] (M.F. Cawley). this work we present results from a visualization study 0017-9310/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2003.09.028 1176 M.F. Cawley, P. McBride / International Journal of Heat and Mass Transfer 47 (2004) 1175–1186 Nomenclature g gravitational acceleration vector ðu; vÞ velocity components h height of cavity v velocity vector p pressure w width of cavity t time Greek symbols T temperature a coefficient of thermal diffusivity T reference temperature 0 b coefficient of thermal expansion T coolant temperature, cold c l dynamic viscosity T coolant temperature, warm h q density of liquid Tp;c plateau temperature, cooling curve q0 reference density Tp;h plateau temperature, warming curve Tq max temperature of density maximum of water undergoing free convection in a temperature 2. Experimental apparatus and procedure region bracketing the density maximum temperature. We follow the approach of Anselmi et al. [3]: water at A schematic diagram of the apparatus used in this 8 °C in a vertical cylinder is surrounded by fluid main- study is shown in Fig. 2(a), with details of the experi- tained at )2 °C, and the convective flow patterns are mental chamber shown in Fig. 2(b). Two domestic studied as the test sample cools through the 4 °C region refrigerator/freezer units are used to supply the coolant; (the subzero cooling temperature permits testing over one unit provides coolant at typically 8 °C (the warmer a wider temperature range bracketing the temperature temperature, Th), while the other is usually set to provide of density maximum; the anomalous density behaviour coolant at )2 °C (the colder temperature, Tc). A mixture of water persists into the supercooled region). In de Paz consisting of 20% ethylene glycol and 80% water is used et al. [4], Anselmi et al. [3], and Sonnino and de Paz [5], as the coolant. A series of solenoid valves (V1 to V4 in experimental observations were made of the tempera- Fig. 2(a)) and pumps (P1 and P2) under computer tures at various points along the central axis of the control are used to switch over from one temperature to cylinder. We repeat these measurements, and extend the other. Water, or a control fluid such as ethanol, is the study to include two-dimensional visualization of the placed in a vertical glass cylinder of inner diameter flow patterns using particle image velocimetry (PIV). 51 ± 1 mm and height 128 ± 1 mm (the axis of symmetry Such visualization studies permit a detailed explanation of the cylinder is aligned to within 0.02 rad of the ver- of the structure of the cooling curves. Results from a tical). The cylinder is filled with the fluid and the top is computational study of the free convective flow, which closed. This cylinder is enclosed within another glass confirm the experimental results, are also presented. cylinder of diameter 105 mm, and fluid at a fixed tem- perature (the coolant) is pumped continuously through the annular region between the cylinders. Three therm- istors (Betatherm, 2.5 mm diameter insulated sensor head, resistance of 5 kX at 25 °C) are mounted along the central axis of the inner glass cylinder, at heights of 32, 64, and 96 mm, to record the cooling curves (thermistor centers are positioned to ±1 mm, and temperature measurements are accurate to 0.1 °C). Apart from the glass construction, the apparatus is thus similar to that used by Anselmi et al. [3]. The concentric glass chambers and associated plumbing are mounted in the refrigerator compartment of one of the refrigerator/freezer units, and a double-glazed window is installed in the refriger- ator door to permit imaging of the convective flows. PIV is used to visualize the flow and provide quanti- tative information about the velocity field. This is a technique whereby the velocity field may be inferred Fig. 1. Density profile of pure water at 101 kPa in the vicinity from the motion of tracer particles in the fluid. The water of the density maximum. in the test chamber is seeded with polyamid spheres of M.F. Cawley, P. McBride / International Journal of Heat and Mass Transfer 47 (2004) 1175–1186 1177 V1 V2 (a) Experimental Chamber T water Th water c P1 V3 V4 P2 Circulating water outlets (b) Thermistor heights from bottom of inner chamber: 96mm 64mm 128mm Inner chamber 32mm 24mm 51mm Circulating water inlets 105mm Fig. 2. (a) Overview of apparatus––V1 to V4 represent solenoid valves under computer control, and P1 and P2 are pumps, (b) schematic diagram of the experimental chamber indicating positions of the temperature probes. 50 lm diameter and density 1.03 g cmÀ3 (Dantec The procedure used during a typical data acquisition Dynamics, part number PSP-50). A collimated sheet of sequence is as follows. Coolant at 8 °C is circulated white light is used to illuminate a vertical plane inter- through the annulus for a relatively long duration (at secting the central axis of the concentric glass cylinders. least one hour) so that the test fluid in the inner glass A CCD camera (768 · 576 pixels resolution) views the chamber reaches equilibrium at that temperature. The apparatus through the window at right angles to the temperature is recorded continuously at set points along illuminated plane. During the data acquisition, a PC the central axis of the test chamber, and the temperature records a sequence of images of the tracer particles. Each of the coolant in the annulus is also monitored. Shortly image is subsequently divided into interrogation win- before the coolant is exchanged, the PIV particles are dows of 32 · 32 pixels. The velocity of the flow in each dispersed throughout the test fluid by blowing gas window is calculated using cross-correlations between bubbles through a thin pipe inserted into the experi- corresponding windows in successive images.
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