Ultra-High Temperature Thermal Conductivity Measurements of a Reactive Magnesium Manganese Oxide Porous Bed Using a Transient Hot Wire Method

The effective thermal conductivity of packed beds of magnesium-manganese oxide pellets is a crucial parameter for engineering Magnesium Manganese Oxide (Mg-Mn-O) thermochemical energy storage devices. We have measured the effective thermal conductivity of a packed bed of 3.66 ±0.516 mm sized magnesium manganese oxide (Mn to Mg molar ratio of 1:1) pellets in the temperature range of 300 to 1400°C. Since the material is electrically conductive at temperatures above 600°C, the sheathed transient hot wire method is used for measurements. Raw data is analyzed using the Blackwell solution to extract the bed thermal conductivity. The effective thermal conductivity standard deviation is less than 10% for a minimum of three repeat measurements at each temperature. Experimental results show an increase in the effective thermal conductivity with temperature from 0.50 W/m °C around 300°C to 1.81 W/m °C close to 1400°C. We propose a dual porosity model to express the effective thermal conductivity as a function of temperature. This model also considers the effect of radiation within the bed, as this is the dominant heat transfer mode at high temperatures. The proposed model accounts for micro-scale pellet porosity, macro-scale bed porosity, pellet size, solid thermal conductivity (phonon transport), and radiation (photon transport). The coefficient of determination between the proposed model and the experimental results is greater than 0.90.

Design, Construction and Commissioning of an Apparatus for Measuring the Thermal Conductivity of Geothermal Grouting Materials Based on the Transient Hot Wire Method

The aim of the present study is to develop an apparatus for the measurement of the thermal conductivity of geothermal grouting materials. The apparatus, named MCT, is designed and constructed as a direct application of a mathematical model of heat transference for conduction in an infinite homogeneous isotropic medium using a linear heat source of infinite length, infinitesimal radius and radial heat flow. This application is known as the transient hot wire method. The apparatus mainly consists of a hot wire, a power supply, a temperature sensor and a datalogger. The commissioning of the developed apparatus is carried out by means of the calibration of the temperature sensor, as well as measurements of thermal conductivity using four reference samples whose thermal conductivity is known. Each of the reference samples is formed of two solid rectangular prisms of the same material and of the same dimensions. MCT is precise and accurate. In good experimental conditions the uncertainty of the measurements is within 10%. In addition, the MCT apparatus is light and with reduced dimensions.

Analysis of the Transient Hot-Wire Method to Measure Thermal Conductivity of Silica Aerogel: Influence of Wire Length, and Radiation Properties

When the transient hot-wire method is used to measure the thermal conductivity of very low thermal conductivity silica aerogel (in the range of 10 mW/m·K at 1 atm) end effects due to the finite wire size and radiation corrections must be considered. An approximate method is presented to account for end effects with realistic boundary conditions. The method was applied to small experimental samples of the aerogel using different wire lengths. Initial conductivity results varied with wire length. This variation was eliminated by the use of the end effect correction. The test method was validated with the NIST (National Institute of Standards and Technology) Standard Reference Material 1459, fumed silica board to within 1 mW/m·K. The aerogel is semitransparent. Due to the small wire radius and short transient, radiation heat transfer may not be fully accounted for. In a full size aerogel panel radiation will augment the phonon conduction by a larger amount.