A New Transient Hot Wire Thermal Conductivity Instrument for Use with Both Step Power and Ramp Power Forcing

Abstract

A new transient-hot-wire thermal conductivity instrument is described for use with non-electrolytic liquids. The initial state of the fluid of interest can be varied from ambient to 500 C and 200 atmospheres. This range of experimental conditions allows most hydrocarbon liquids to be studied up to either their critical point or their limit of thermal stability. The hot wire is 12.7 micrometers in diameter and 12 centimeters long. The hot wire is constructed of platinum, with all electrical connections silver-soldered, and has been proven reliable in a wide range of liquids including several unstable coal liquid fractions.

The transient hot wire experiment is controlled and monitored by a Rockwell AIM 65 microcomputer. The drive voltage to the hot wire is directly controlled by the computer through a 12-bit digital-to-analog converter. The fact that the drive voltage is directly under software control provides great flexibility in both the shape as well as the magnitude and timing of the forcing function. This has been used to advantage in this instrument to provide a choice of either a step-power or ramp-power forcing function. The resistance of the platinum hot wire is monitored with a 14-bit analog-to-digital converter connected to an amplified Wheatstone bridge system. The typical thermal conductivity experiment is one second in duration, and consists of 1000 measurements of temperature rise versus time. The data points are transferred to a mainframe computer over phone lines for detailed data analysis and permanent storage on magnetic tape.

The performance and accuracy of the instrument have been verified through a study of toluene with both ramp-power and step- power forcing. This provides a check of internal consistency through comparison of step and ramp results, as well as a check of accuracy through a comparison with the transient step-forced, and radiation-free steady-state parallel-plate data of other experimenters. This testing has shown that both the step and ramp forcing functions give comparable results, with an accuracy on the order of one percent. The thermal conductivity data obtained are believed to be nearly free of the effects of radiation due to use of a back extrapolation procedure to obtain the apparent thermal conductivity at zero time. The instrument has been utilized to study meta-xylene, methylcyclohexane, decahydronaphthalene, tetrahydronaphthalene, and 1-methylnaphthalene. In addition, the instrument has been used to study two well-characterized coal liquid materials, an SRC-I naphtha, and a Utah COED fraction. All data are from ambient to the limit of thermal stability, or to the critical point. The effect of liquid-phase compressibility on thermal conductivity is clearly resolvable by this instrument.