ME 354 - Thermofluids Laboratory
Spring 1997

LAB 8 - Pool Boiling Heat Transfer 

Introduction 

Boiling is the term used to describe the phenomena that occurs when evaporation takes place at a solid-liquid interface. The process will occur when the surface temperature Ts exceeds the saturation temperature associated with the liquid pressure Tsat. Heat is transferred to the liquid from the surface according to Newton's law of cooling

       (1)

where  = heat flux at the surface

h = heat transfer coefficient

, excess temperature

Boiling is characterized by the formation of vapor bubbles, which grow and eventually disengage from the surface. The growth of the bubbles and their subsequent dynamics depend upon the excess temperature, the surface characteristics, and on the thermophysical properties of the fluid, the most important of which is the surface tension. The dynamics of the vapor bubble formation influence fluid motion near the surface which affects the heat transfer coefficient.

Boiling may transpire under a variety of conditions. In this experiment, pool boiling will be investigated. This term refers to a condition where the liquid is quiescent and the liquid motion near the surface is due to free convection and to mixing induced by bubble growth and subsequent detachment. Another definition for pool boiling refers simply to conditions in which the heated surface is submerged below a free surface of liquid. Saturated boiling is a condition in which most of the liquid is at a temperature slightly greater than saturation while at the liquid-solid interface there is a significant decrease in liquid temperature. Bubbles that form on the solid surface are driven upward by buoyant forces and eventually are transported to the gas above the free surface. Figure 1 depicts the typical temperature distribution in saturated pool boiling. Subcooled boiling occurs when the liquid temperature is less than the saturation temperature and the bubbles that are formed eventually condense in the liquid.

Figure 1. Saturated pool boiling temperature distribution

Saturated pool boiling has received a considerable amount of attention within the heat transfer community. In this experiment the primary interest is the boiling curve, which considers the effect of excess temperature on either the heat transfer coefficient or the heat flux. By studying the different modes, or regimes, of saturated pool boiling, an appreciation for the underlying physical mechanisms may be obtained. These regimes are identified in the boiling curve of Figure 2. The curve shown is for water, but similar trends are observable in other fluids. Equation 1 indicates that  depends upon both the heat transfer coefficient h and the excess temperature. The various boiling modes can be differentiated according to the value of excess temperature. The different regimes evident on the boiling curve include film boiling, transition boiling, nucleate boiling, and free convection boiling. A more complete description of pool boiling and the boiling regimes can be found in Holman (1990).

Figure 2. Typical pool boiling curve for water at 1 atmosphere

Objectives 

Pool boiling will be investigated in this exercise by immersing an aluminum cylinder, initially at room temperature, into a container of liquid nitrogen. The primary objective is to generate a pool boiling curve similar to that in Holman (1990), where h is given as a function of the excess temperature . In addition, the different boiling regimes are to be observed, including any interesting physical phenomena, and then reported and discussed in the laboratory report.

Experimental Apparatus 

A sufficient supply of liquid nitrogen, such that the aluminum cylinder may be completely immersed, will be contained in a glass dewar flask. Most dewar flask are made from metals; however, the glass flask will allow the boiling process to be observed. Each student group will be provided with a test cylinder which is approximately 1.5" in diameter and 1.5" in length. A single type-T thermocouple is imbedded in the aluminum cylinders. The cylinders will be suspended from a lab stand at a height that allows the cylinders to be fully immersed in the liquid N2.The thermocouple emf will be read by a Keithley Series 500 data acquisition system supplied with an Aim 6 thermocouple data acquisition card.

A computer program using SOFT500 has been written that will allow the data acquisition system to read the thermocouple over the course of the experiment. Time and temperature data will be taken at approximately 1 second intervals for about 12 minutes. The data should be written to a file on the B: drive, which, on the supplied 286 computer, requires a 3 1/2" low density diskette (EACH GROUP MUST BRING A FORMATTED LOW DENSITY DISKETTE TO COLLECT THEIR DATA).

The required thermophysical properties of aluminum are temperature dependent. Data for thermal conductivity and specific heat of aluminum have been fit to a 3rd order polynomial equation in temperature. The general equation has the form

      (2)

where T = temperature (°K). Table 1 lists the values of the constants for the two properties of interest.

Table 1. Constants used in Equation 2
Thermal Conductivity
Specific Heat
W/m/°C
kJ/kg/°K
c1
0.3893171E+03 
-0.3973402E+00 
c2
-0.2870259E+01 
0.1266628E-01 
c3
0.1367150E-01 
-0.4477521E-04 
c4
-0.2054480E-04 
0.5673365E-07 

The lumped-heat-capacity method may be implemented in order to determine the heat transfer coefficient as a function of excess temperature. The underlying assumption in this analysis of transient heat conduction is that the entire system is uniform in temperature or, in other words, that the internal resistance is negligible in comparison with the external resistance. Applying the first law of thermodynamics to the body being cooled results in

      (3)
Safety Precautions 

Each student must have safety glasses in order to participate in the laboratory exercise. Exercise caution around the liquid N2. Exposing skin to the cold temperature liquid may cause burns.

Procedure 
  1. Determine the local barometric pressure. Weigh the aluminum cylinder to used in the test. Determine the channel number the thermocouple is connected to on the Keithley data acquisition card.
  2. Start the data acquisition program and follow its instructions.
  3. Lower the cylinder into the liquid N2 and begin data acquisition.
  4. Record time and temperature data at 1 second intervals until the excess temperature nears zero and you are sure that the boiling regime is entirely free convection. While boiling is occurring, closely observe and record all phenomena of interest for the entire boiling curve. Particular attention should be directed toward observing the transition from film boiling to nucleate boiling that occurs very rapidly (in 2 or 3 seconds) and is accompanied by very rapid, noisy boiling.
  5. Remove the cylinder from the liquid N2. The program will write all of the data to your disk..

Data Reduction and Points of Interest 
  1. Construct the boiling curve for aluminum in liquid N2. Discuss the experimental boiling curve by comparing it to other empirical results in the literature.
  2. Discuss all of the phenomena that you observed for the various boiling regimes. Relate to the boiling curve produced from the data.
  3. Discuss the effect that the attached thermocouple has on the experiment. Suggest means of reducing these effects.
  4. Discuss the validity of the lumped-heat-capacity assumption for this heat transfer process. Try to use quantitative arguments for this discussion.

References 
  1. Holman, J.P., Heat Transfer, 7th Ed., McGraw-Hill Book Co., New York, 1990.
  2. Incropera, F.P. and Dewitt, D.P., Fundamentals of Heat and Mass Transfer, John Wiley & Sons, New York, 1990.
  3. Chappa, S.C. and Canale, R.P., Numerical Methods for Engineers, 2nd Ed., McGraw-Hill, New York, 1988.
  4. Yakowitz,S., and Szidarovszky, F., An Introduction to Numerical Computations, 2nd Ed., Macmillan Publishing Co., New York, 1989.
  5. Holman, J.P., Experimental Methods for Engineers, 4th Ed., McGraw-Hill Book Co., New York, 1984.