Design And Fabrication Of Thermal Conductivity Measuring Equipment

ABSTRACT

This work is on building of thermal conductivity measuring equipment. In this paper, thermal conductivity equipment was designed using low cost and readily available industrial materials. This is done with a view to encouraging indigenous local ingenuity derived from the knowledge of basic classroom physics theory and its adaptability to science and technological training. The insulator in the system was a combination of clay and saw dust as fibre which formed the composite material. The effectiveness of the insulation was found to be 49.65%. The electrical system was most simple with a control On-Off switch. A light battery-operated digital temperature indicator was used to record temperatures at various locations in the equipment. The designed and constructed conductivity measuring equipment was used to determine the thermal conductivity of brass and mild steel. The conductivity of mild steel specimen at 100^oC was 50.2W/mK, and the measured conductivity of brass at the same temperature was found to be 120W/mK. This compared favourably well with literature value of 109-159W/mK. The equipment performed as commercially available ones and could measure the thermal conductivity of metal to the range of ±10% error.

 CHAPTER ONE

1.0                                                      INTRODUCTION

1.1                                        BACKGROUND OF THE STUDY

Energy studies require the knowledge of the value of many thermophysical properties. Values of these properties for a variety of substances and materials are available [Eckert et al, 2019]. However, for new materials which appear regularly, it is important to be familiar with some basic methods of measuring these properties.

Thermal conductivity is seen as a thermal and transport property [. Incropera, 2002]. Most thermal property measurements involve a determination of heat flow and temperature gradient. Heat flow is usually measured by making an energy balance on the device under consideration.

Incropera and DeWitt [2002] stated that thermal conductivity may be classified as transport property since it is indicative of energy transport in a fluid or solid. In gases and liquids, the transport of energy takes place by molecular motion while in solids, transport of energy is by free electrons and lattice vibration.

Marechal and Devisme [2010] worked on thermal diffusivity of building materials using periodic signals method. The apparatus used was the same as that employed for measuring conductivity by guarded hot plate technique. They showed that this method was particularly suitable for measurements on humid materials since the temperature of the samples varies only by a few degrees.

Bouchard on using the Angstrom’s method similarly found the thermal conductivity of a brass rod to be 133±84 and 163±35W/mK, respectively for the first and third harmonic of the heat wave by applying the Fourier analysis on the temperature data collected which compared with the literature value of 128W/mK [Bouchard, 2000].

Studying the thermal conductivity measurement of geothermal cementing systems, Gutierrez and Paredes [Bouchard, 2000] compared experimentally the classical Line-source and the Jaeger methods. Determining the effective thermal conductivities of six Mexican cementing systems used in geothermal well completion compared within the temperature range of 28 to 200^oC, they reported an experimental thermal conductivity uncertainties of 4 and 11.85%, for the Line-source and Jaeger methods, respectively.

Similarly, Huang and El-Genk [2014] performed an experiments to measure the thermal conductivities of two-particle sizes of alumina (Al₂O₃) powder and molded Min-K insulation materials in vacuum. Correlating the thermal conductivity data as a function of temperature from 340 to 900K, within ±10 and ±5% for Al₂O₃ and Min-K, respectively, they showed that the Min-K thermal conductivity was the lowest and least dependent on temperature while its conductance was approx. 2-3 orders of magnitude higher than the lateral conductance of molybdenum multifoil insulation.

Conversely, Ingenhousz’ conductometer as reported by Muller [2017] was used to compare the thermal conductivities of solids coated with wax layer by showing the inequality of the capacity of different bodies to transmit heat. He observed that the thermal conductivity of copper was best while the least conductor among the five rod materials tested was wood since the rate at which the wax melted within the length of the rods exposed to the same source of heat indicates the best conductor.

Further, Owate et al [2007], similarly performed an experiment in their paper titled “A device for thermal conductivity measurement in a developing economy” to determine the thermal conductivities of copper, aluminum and brass materials using a system they designed, constructed and tested. The system was a modified form of Smith’s thermal conductivity apparatus which has been widely applied in normal laboratory. The results of the experiment obtained show that the measured thermal conductivity values of copper, aluminum and brass were 397.4 ± 2.2, 238.0 ± 1.3 and 110.2 ± 1.2 Wm^-1K^-1, respectively which compared statistically and relatively well with other standard values in the order of 396, 236 and 109 Wm^-1K^-1, respectively for the materials. Hence, the analysis shows that the device can be reproduced for thermal conductivity measurements in a developing laboratory experimental environment.

Again, since the temperature to be measured in the system was of paramount importance and significance, thermocouple reference information materials as it relates to temperature-voltage curves and thermocouples environmental limits were consulted to select the best suitable thermocouple capable of measuring the temperatures at any point along the specimen within the range of the design. In-view of these, Iron-Constantan (J- Curve), Chromel-Constantan (E-Curve), Chromel- Alumel (K-Curve), Platinum-Rhodium (S and R-Curve), Tungsten-Rhenium (C-Curve), and Copper-Constantan (T-Curve), respectively were considered and the best among them selected for the design.

Based on the fore-going, Copper-Constantan thermocouple was selected and used in the experimental testing of this work. The Copper-Constantan thermocouple (+ve Copper wire and –ve Constantan wire) as posited by Wikipedia was recommended for use in mildly oxidizing and reducing atmospheres up to 400^oC and suitable for applications where moisture is present. It is also recommended for low temperature work unlike other thermocouple alloys considered where their operating temperatures are usuable in the various ranges of 870^oC to 2760^oC that demanded high temperature applications. Hence, the homogeneity of the component wires of the Copper-Constantan thermocouple can be maintained better than the other base metal wires. Thus, errors due to the non- homogeneity of wires in zones of temperature gradients is greatly reduced. The thermocouple selected was directly connected to a digital display unit (DDU) which enables the user to read a temperature at will.

1.2                                               PROBLEM STATEMENT

The constant want for increasing heat transfer for a range of applications is one of the most complicated challenges faced by thermal engineers. With the innovation of technologies, heat transfer at higher rates and efficiency from small cross section areas or over low temperature difference are causing a rise in demands. As a consequence of the wide range of thermal properties there is no single measure method which can be used for all thermal Conductivity measurements. Consequently, over the past decades a wide variety of techniques for the enhancement of heat transfer has been suggested, where the most well-known and promising methods are briefly described in this paper. The study describes the steady state and transient methods for measuring thermal conductivity for different temperature ranges

1.3                                 AIM AND OBJECTIVES OF THE STUDY

The main aim of this study is to design and fabricate a thermal conductivity measuring equipment using locally made available materials. The objectives are:

1. To build the conductivity measuring device
2. To determine the thermal conductivity of brass and mild steel

• To study the thermal conductivity of different materials

1.4                                         SIGNIFICANCE OF THE STUDY

Engineering education would be purposeful if only engineering and basic sciences can be applied to practical situations without the drudgery of memorizing the formula or imagining what a particular scientific equipment looks like. This study will be of great benefit to technical institutions, schools, universities and the student involved in learning how to apply theory in practice. It will also provide a basic conceptual ideology on how the equipment is designed.

1.5                                             RESEARCH METHODOLOGY

In the course of carrying this study, numerous sources were used which most of them are by visiting libraries, consulting journal and news papers and online research which Google was the major source that was used.

1.6                                     PROJECT ORGANIZATION

The work is organized as follows: chapter one discuses the introductory part of the work,   chapter two presents the literature review of the study,  chapter three describes the methods applied, chapter four discusses the results of the work, chapter five summarizes the research outcomes and the recommendations.