University of Minnesota
University of Minnesota: Department of Mechanical Engineering
http://www.me.umn.edu/

ME 4331: Boiling

Boiling Heat Transfer

Reading: Before you begin, read an introductory chapter on boiling heat transfer in an undergraduate heat transfer text. Also read the short description Boiling Heat Transfer Fundamentals

Experimental Setup: A Platinum wire is submerged in a liquid bath of HFE-7000 (a common dielectric coolant for electronic systems) whose temperature is kept at the saturation value (~ 34°C) by an electrical heater and is measured by a thermocouple. The wire is heated electrically using a DC power supply. The electrical power input to the wire can be determined by measuring the voltage drops (Vshunt, across the shunt resistor and Vwire across the wire) with a digital data acquisition unit. The average temperature of the wire is deduced from the electrical resistance of the wire, which is a function of its temperature. The resistance is computed, knowing the voltage and the current through the wire. The current is determined from the shunt resistor measured voltage and resistance.

The relation between resistance and temperature of the wire is obtained via a simple calibration. This calibration is performed in a bath of water that can be varied from room temperature to near-boiling. As the water is heated, measurements of the water temperature (which is the same as the wire temperature) are taken using the thermocouple. The resistance of the wire is also measured at these temperatures using a 4-wire resistance measurement technique. Using a least-squares fit, a straight line can be fit to this data. (The nice thing about platinum is that the resistance-temperature relationship is linear.) With this calibration in hand, the temperature of the wire can later be found by measuring its resistance and calculating temperature using this predetermined calibration equation.

Objective: Characterize the boiling curve of a platinum wire in HFE-7000.

In this lab, we suspend a platinum wire in a dielectric liquid HFE-7000 which is at its saturation temperature. Electrical current is passed through the wire. By way of Joule heating, the wire's temperature rises, causing heat transfer from the wire into the fluid. The fluid is at saturated temperature and, thus, additional heat will eventually cause boiling at the wire-fluid interface. As the current supplied to the wire is increased, the boiling passes through different boiling regimes, starting at the discrete nucleate bubble regime, followed by boiling with departing jets and columns, and finally to the film boiling regime. Film boiling is where a major portion of the wire is covered with a layer of fluid vapor. The heat transfer coefficient at this point becomes very low because heat transfer is first through (conduction as well as radiation) the vapor film to the liquid HFE-7000 beyond.

Determine whether the elements of the boiling curve can be reproduced in the lab. As each of the various processes (single-phase convection, isolated bubble regime, etc.) is created, document with data and sketches its peculiar characteristics. Describe, in detail, the heat transfer mechanisms present in each regime of the boiling curve. Determine the critical heat flux and evaluate the applicability of the Kutateladze correlation to the conditions of this test.

The complete boiling procedure will be video recorded using a digital video recorder. The relevant frames corresponding to critical points on the boiling curve will serve as visual illustration in the discussion and explanation of physical mechanism of boiling.

Experimental Procedure Requirements:

  1. Obtain 2 data points in the free convection boiling regime, 5 data points in isolated bubble nucleate boiling regime and 5 data points in the jets and the columns boiling regime that are equally spaced on the boiling curve. Note the typical coordinates of this curve when you choose your heat flux levels. Be careful not to use too large of power increments. Once film boiling is reached, boiling becomes very unstable and the wire becomes very hot.
  2. Obtain 2 data points within the film boiling regime.
  3. Obtain 12 or more data points as the power is reduced, starting at film boiling and ending when nucleate boiling stops.