Title page for ETD etd-12698-1093


Type of Document Dissertation
Author van Gelder, Maarten F.
Author's Email Address mvgelder@vt.edu
URN etd-12698-1093
Title A Thermistor Based Method for Measurement of Thermal conductivity and Thermal Diffusivity of Moist Food Materials at High Temperatures
Degree PhD
Department Biological Systems Engineering
Advisory Committee
Advisor Name Title
Diehl, Kenneth C. Committee Chair
Agblevor, Foster Aryi Committee Member
Haugh, C. Gene Committee Member
Marcy, Joseph E. Committee Member
Wilson, James H. Committee Member
Keywords
  • high temperature
  • food material
  • thermal diffusivity
Date of Defense 1997-12-18
Availability unrestricted
Abstract

A THERMISTOR BASED METHOD FOR MEASUREMENT OF THERMAL

CONDUCTIVITY AND THERMAL DIFFUSIVITY OF MOIST FOOD MATERIALS

AT HIGH TEMPERATURES

by

Maarten F. van Gelder

Kenneth C. Diehl, Jr., Chairman

Biological Systems Engineering

(ABSTRACT)


The purpose of this research was to assess the suitability of

the thermistor based method for measuring thermal conductivity

and diffusivity of moist food materials at high temperatures.

Research focused on aspects of calibration, thermal contact in

solid food materials, natural convection in liquid media and

the performance in moist food materials at high temperatures.


Thermistor probes were constructed in house and calibrated in

three materials of known thermal conductivity and diffusivity,

water, glycerol, and a heat transfer fluid, HTF 500. With few

exceptions, the calibrated probe estimated thermal properties

with an error of less than 5%, over the range of thermal

properties spanned by the those of the calibration media. An

alternate calibration using two media was also investigated. It

was found to give better accuracy over a more limited range.

Thermal contact in potato and lean beef was investigated

through a comparative study that used a miniature line heat

source probe as a reference method. The food materials were

measured at 25, 50 and 100 °C. Good agreement was found between

the measurements with the line heat source probe and the bead

thermistor probe, indicating adequate thermal contact at the

thermistor probe.


The effect of fluid viscosity and the magnitude of the

temperature step on the occurrence of natural convection was

studied for aqueous solutions of a thickening agent. During a

sample time of 30 seconds, convection was absent in solutions

with a viscosity of 25 cp or greater, when measured with a

temperature step of 1.5 and 2.5 °C, and in solutions with a

viscosity of 50 cp or greater, when measured with a temperature

step of 5.0 °C. A Rayleigh number was defined to study the

notion of a critical Rayleigh number at the onset of

convection. This study found that when the Rayleigh number was

below 43, convection could not be demonstrated. For a Rayleigh

number of 84 and higher, convection was observed.


The performance at high temperatures in food materials was

studied through tests in tomato concentrate and in a liquid

food supplement. Tomato puree and tomato paste were sampled at

100, 130 and 150 °C. The thermal conductivity of tomato puree

at 100, 130 and 150 °C was measured as 0.638, 0.645 and 0.647

W/m°C respectively. The thermal diffusivity was 1.63, 1.64 and

1.62 10 -7 m 2 /s respectively. For tomato paste at 100, 130 and

150 °C, a thermal conductivity was obtained of 0.590, 0.597 and

0.534 W/m°C respectively. The thermal diffusivity was 1.63,

1.84 and 2.36 10 -7 m 2 /s respectively. With some notable

exceptions the results of this study agreed well with Choi and

Okos (1983). A liquid food supplement was also studied at 95

and 150 °C. The thermal conductivity of the food supplement

decreased with increasing solids content from 0.62 W/m°C at a

solids level of 15% to 0.41 W/m°C at a solids level of 50%.


The results of this study indicate that the thermistor based

method was suitable for measuring thermal conductivity and

diffusivity of moist food materials at high temperatures.

However, the type of thermistor used in the research, a glass

encapsulated thermistor, was too fragile for routine work. In

particular the high temperature use of the glass thermistor was

impacted by its susceptibility to fracture.

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