Karl Geisler
May 2002 (updated July 2005)
(figure numbers refer to accompanying PowerPoint file)
Design of Boiling Experiment for Undergraduate Heat Transfer Lab
Introduction
This document describes the design of a boiling experiment suitable for an undergraduate
heat transfer laboratory course. The experiment takes a new look at the traditional
water/platinum pool boiling demonstration. In this case, the dielectric liquid FC-87 will
be used. FC-87 is a member of the 3M Fluorinert Electronic Liquid product line.
Fluorinert liquids (chemically similar to Teflon) have a number of attractive
characteristics:
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high electrical resistivity and high dielectric strength
good materials compatibility, chemically inert
low toxicity
nonflammable
wide range of boiling points available
The electrical and chemical properties of the Fluorinert liquids eliminate
corrosion/fouling and electrical arc concerns with water systems. In addition, these
liquids tend to have poor thermal properties compared to water (Fig. 1)—a major
advantage in terms of the electrical design of the experiment. As Critical Heat Flux
(CHF) for the Fluorinert liquids is typically one tenth that of water, the entire range of
boiling regimes can be investigated at one tenth the electrical power requirements of a
similarly-designed water experiment. Due to the low electrical resistivity of Platinum
(1.0610-7m at 20C) the current requirements of these types of experiments are usually
quite high. Further, the low boiling point of FC-87 (30C) eliminates the need for much
of the hardware and bulk of water systems, as saturated conditions can be achieved only a
few degrees above room temperature. The physical and electrical design are discussed in
detail below.
Module Design
Figure 2 shows a schematic of the proposed experimental module, while Fig. 3 illustrates
the overall system. The basic module will be fabricated by machining a 3”3”2” pocket
in a solid block of acrylic (Lucite, Plexiglass). An acrylic plate will be bolted to the
block, sealed with an o-ring, to complete the enclosure. The transparent plate will allow
visualization of the boiling activity, while the machined back wall of the enclosure will
provide a suitable opaque background. While photographic visualization is not required
for the experiment, it would prove very useful for boiling regime identification. Ideally,
the students would have access to a digital camera and could incorporate captured images
in their lab reports. Acrylic’s insulating properties (thermal conductivity = 0.14W/mK)
will minimize thermal interactions with the environment. The platinum wire will be
supported by two brass rods. These rods will be fastened to a small acrylic “cap,”
allowing possible rod separations (wire lengths) of 1, 2, and 3cm. The bulk liquid
temperature will be measured by a thermocouple. A lamp with a standard 100W light
bulb, placed close to the front of the module, can be used to heat the module to the
saturation temperature of the liquid. (The liquid saturation temperature is only a few
degrees above room temperature.) This lamp can be located behind the module during
the experiments to help maintain the liquid temperature and assist with bubble
visualization. A water-cooled condenser will be connected to the module to recondense
vapor generated during the experiment and minimize evaporative losses. The condenser
can be cooled by cold tap water. The physical design of the module will allow horizontal
and vertical wire orientations by simply rotating the entire system 90 (Fig. 2 clockwise).
Electrical Design
The physical dimensions of the wire and the electrical resistivity of platinum drive the
electrical design of the experiment. Given the expected resistivity range of 110-7 to
1.510-7m, a wire diameter of 0.010” (0.254mm), and basic electrical relations, the wire
can be expected to dissipate 0-4W of power (heat) with supplied voltages and currents
ranging 0-1V and 0-10A, respectively. (A similar water experiment would require 2030A of current at the upper limit.) A digital DC power supply is recommended for
supplying precisely-controlled electrical power to the wire. An accurate voltmeter will
be needed to measure the power supply voltage as well as the voltage drop across the
shunt resistor to calculate the supply current.
Parts/Components List
The following table shows the parts and materials needed to construct the proposed
experiment.
FC-87
acrylic module
condenser
type-t thermocouple
liquid heaters
brass rods
miscellaneous wire and hardware
platinum wire
Omega #SPPL-010
0.010” diameter platinum wire
www.omega.com
10A shunt resistor
such as Empro Shunts #HA-20-50
Type HA DC Ammeter Shunt
Empro Mfg. Co., Inc.
(317) 823-4478
www.emproshunts.com
sales@emproshunts.com
main power supply (0-1V, 0-10A)
voltmeter
thermocouple voltage reader/ice reference
light source
camera (if desired)