Experiment 9 - Heat capacity measurement of foam powders The experimental approach to this experiment has changed since the last posting. The long list of difficulties associated with using the calorimeter justified changing the method to the one described below.

Click on Experiment Documentation for details of this experiment. Various methodologies for determining the specific heat of the three insulating powders supplied by the Cyrogenics Test Lab at KSC are first prototyped in Prof Higgins's home lab shown in the photo below. This lab is equipped with an Omega 6018 data acquisition module controlled by a LabView interface for multichannel thermocouple inputs which is calibrated by a Transmation Model 1045 Digital Calibrator. A Fluke model 8044A-AF military grade precision multimeter is used to read the thermistor resistance indicating the powder temperature during the test, a Blue M lab oven for heating the aluminum heater block, a Dell desktop computer for data logging and analysis along with various support instrumentation.

Prof Higgins' lab

Components of the Prototype Test Stack In the place of the calorimeter previously considered, is a test stack consisting of the aluminum heating block used to supply heat flux to a 78 gram copper sample holding cup which contains 45 grams of the sample under study. In this picture the sample powder is shown in the tin cup prior to being poured into the cup. Inside the cup the 30 K ohm thermistor is seen protected by fiberglass wire sleaves. Both the aluminum heater block and the copper cup are fitted with J-type thermocouples that are mechanically mounted with screws for secure, heat resistant attachment.

Prototype Experiment Setup This view shows the data connections to the test stack using the Fluke meter to measure the resistance change associated with the powder heating, and the thermcouple connections to record the corresponding temperature rise in the copper sample cup along with the cooling of the aluminum heater block. Not shown is the insulation used to minimize heat loss not passing into the sample cup. In this prototype run this unused heat dissipation was computed from correlations which yield Nusselt number for natural convection from the vertical sides and from the top surface not covered by the cup. In the actual experiment the cyrogenic blanket supplied by CTL will be used to focus the heat from the cooling aluminum block just to the cup.

Thermistor Table Shown here is the correspondence between thermistor resistance and temperature in C. Omega publishes a polynomial curve fit to this information which is used to convert the Fluke Ohm readings of the test powder to a powder temperature. A thermisitor is used here because of its fast response and high resolution. In the actual experiment a J-type thermocouple may be used for logging along with the other 2 temperature readings (block and sample can).

Lab Oven Used A Blue-M laboratory oven was used to heat soak the aluminum block to about 130 C for about 4 hours to achieve uniform temperature in the block.

Thermocouple welder A thermocouple welder especially suitable for small wire k-type thermocouples is made by extracting the carbon bar from a 6 volt carbon-zinc lantern battery then connecting the negative side of a large capacity, 6 volt sealed lead acid battery as shown. The positive side of the battery is then connected by alligator clip to within 5 mm of the twisted thermocouple wires to be welded. A good technique is to hold the thermocouple in a desktop vise then touch the carbon bar to the tip momentarily. With a little practice, a very fine ball-shaped weld is produced.

LabView Interface The LabView shown here was written to communicate with the Omega 6018 thermocouple module in order to control it, set the sampling rate, display the thermocouple temperatures and log this data to a selectable text disk file for later importing this data into Excel. Since National Instruments had no 6018 drivers, programming this interface began at the primative, Instrument Assistant, level. A student copy of LabView 8 was used along with the instrument driver library that included all the tools needed to start at the primitive level. Prior to each test the acquisition system is run for an hour to assure cold junction stability.

Methodology The heat capacity experiments performed using the above hardware follow this methodology: heat soak aluminum heater block to 140 C Pour known amount of test powder into sample cup verify all sensors are reporting reasonable data quickly construct test stack and wrap with cyrogenic blanket record temperatures in each element for 5 minutes analyze data to determine specific heat of powder

Analysis Excel is used to reduce the data. First, the text file written by the DAQ is imported onto the sheet. Then this data is examined to determine the time rate of change, dT/dt, for the aluminum block, sample cup and powder sample. Next the extraneous heat rate losses from the aluminum block are determined using natural convection formulas determined for vertical and horizontal metal surfaces. NuLave = 0.68 + 0.670RaL1/4/[1+(0.492/Pr)9/16]4/9 NuL = 0.54RaL1/4 Herein, NuL is Nusselt number based on length, RaL is Rayleigh number composed of the product of Grashof and Prandlt numbers and Prandlt number. Knowing Nusselt number yields the value of h, the heat transfer coefficient which quantifies the natural convection losses of the aluminum block using Newton's Law of Cooling. These were applied to the exposed aluminum block (uninsulated and not covered by the sample cup). Then an energy rate balance equated the rate of heat loss from the aluminum block directed into the sample cup to the rates of heat gain in the cup and in the powder. The only unknown in this balance is specific heat of the powder.

Planned Improvements
Planned improvments to this experiment include the following: accurate calibration of data acquisition system may return to thermocouple for powder temperature measurement insulate test stack to minimize extraneous heat loss use cylindrical heat block to match test sample