The phases of pool boiling, and the associated heat flux into the fluid, are a function of the excess temperature. The excess temperature, ∆T_c, is the temperature of the solid surface above the saturation temperature of the fluid.

As seen in this figure the heat flux, q_w, increases during natural convection and nucleate boiling, then decreases significantly during transition boiling. Following this, the heat flux steadily rises.

In stable film boiling a vapor film forms on the solid surface and insulates it. Consequently, the boiling surface becomes much hotter as the heat transfer coefficient diminishes while q_w rises.

The demonstration of the 4 phases of pool boiling will de done using the experimental setup sketched below.

Instead of water, the boiling fluid will be 3M’s heat transfer fluid FC-72 which has a saturation temperature of 56 C as shown on the included data sheet. This is a fluid commonly used for boiling demonstrations because of its low boiling point, and because it is safe for students to use because its vapors are non-toxic.

Before the actual experiment the fluid should be heated on a hot plate to a temperature just below its saturation temperature (bring the fluid to boiling then let it cool).

Since this fluid is quite expensive, and relatively hard to acquire, we have purchased only 250 grams, or about 150 ml. Thus, we will use a small 250 ml beaker for the experiment. We are purchasing a new beaker to assure clarity.

NiChrome wire is being purchased for this experiment. See the attached data sheets for specifications of various sizes and compositions of NiChrome wire. This is a high temperature wire with a high thermal resistance so as to get hot with smaller currents and to survive heating. If needed we will join it to lengths of copper wire for connection to the power supply used to provide the 1-5 amps at about 40 volts to produce the desired excess temperatures in the wire.

An ammeter on the power supply will provide an indication of the power input to the NiChrome wire. Additionally, a small precision resistor will be connected in-line to produce a voltage signal that can be recorded.

A laboratory thermometer will be used to record the temperature of the fluid.

This experiment should be documented with digital pictures and by a webcam to capture bubble and jet formation in the boiling process.

This is a simple demonstration of pool boiling. Objectives for this experiment include:

• Team work in setting up the experiment, taking data and documenting results.
• Observation of the visual indicators which are characteristic of each boiling phase.
• Observation of the changes in current to the wire as a function of the boiling phases.

The boiling experiment will be done during two lab periods. The first lab period will be a rehearsal of the lab setup including the video taping. A copper wire will be heated in water to simulate the actual experiment. The NiChrome wire and the FC –72 Heat Transfer fluid will be used in conduction the actual experiment in the 2^nd lab period.

Attached are example results obtained with a heated NiChrome wire in FC-87 fluid. This heat transfer fluid has a boiling point of 30 C, but it is no longer available. The progression from isolated bubbles through jets and columns leading to stable film boiling is tabulated along with the power input to the wire. The pronounced change in resistance resulting from insulation, and rapid heating of the wire at the onset of film formation is shown graphically. Thanks to the University of Houston’s Heat Transfer Laboratory for this data.