Transient Hot Bridge Research Articles

Thermal Measurements on Polymeric Epoxy-Expandable Graphite Material

Combustion measurements, such as heat release rate, critical flux, time-to-ignition, ignition temperature, thermal inertia, and kinematics—activation energy as well as preexponential factor—on epoxy polymer (Prime™20LV) with expandable graphite (EG) inorganic filler of different weight percentage composites, are conducted using the Dual Cone Calorimeter, the thermogravimetric analysis (TGA), and Linseis (Germany) THB100 Transient Hot Bridge thermal conductivity analyser. The results indicate that increasing the amount of EG in polymer composite leads to reduction in the critical flux, the time-to-ignition, the ignition temperature, the thermal inertia, the average thermal conductivity, and the activation energy (from 159.1 ± 2.3 to 145.9 ± 3.1 kJ/mol for neat epoxy to 3 wt.% EG-epoxy) of the composite samples. There is, however, an increase in the heat of gasification with increasing EG content.

The thermal conductivity of glass-sieves: I. Liquid saturated frits

The thermal conductivity of eight liquid-saturated glass sieves of pore sizes between 1 μm and 500 μm is presented as measured at room temperature. The liquids used as saturants cover a range in thermal conductivity from 0.135 W m−1 K−1 to 0.598 W m−1 K−1. The experimental results are successfully fitted to a simple two-dimensional composite model for the heat transfer in porous media. The experimental runs were carried out using a transient hot bridge (THB) measuring instrument. Unique for a transient technique, the fast and reliable THB instrument is characterized by a constant probing depth. This feature is of special importance when seeking for potential convective heat transfer inside the pores of a saturated porous medium. Here, any onset of convection could neither be detected nor predicted for the measured range of pore sizes. Investigations on the volumetric specific heat of the saturated glass sieves furnish their individual porosities or enable to assure their actual degree of saturation. Measurement uncertainties in thermal conductivity and in specific heat are assessed to 3 %–5 % and to 5 %–10 %, respectively.

A study of the thermal conductivity of granular silica materials for VIPs at different levels of gaseous pressure and external loads

Fast and reliable methods for the determination of thermal properties of core materials for vacuum insulation panels (VIPs) are needed. It is of great importance to know the thermal performance of a VIP core at different levels of vacuum and external loads. In this study a new self-designed device, consisting of two cylindrical cavities connected to a Transient Plane Source instrument, is used to determine the thermal conductivity of low-density nanoporous silica powders, from atmospheric pressure down to 0.1mbar while applying different levels of external pressure up to 4bars. The study includes a brief theoretical discussion of methods. The TPS is validated through comparison with available data for commercial silica as well as through independent stationary measurements with a hot plate apparatus and with a Transient Hot Bridge method. The different materials illustrate clear but different trends for the thermal conductivity as a function of the level of vacuum and external pressure. The analysis of experimental results shows that the transient methods are less suitable for measuring the thermal conductivity of low-density silica powders, especially for the cases when the density is less than a limit at which the heat transfer by radiation becomes dominant compared to pure conduction.

New Transient Hot-Bridge Sensor to Measure Thermal Conductivity, Thermal Diffusivity, and Volumetric Specific Heat

A high sensitivity thermoelectric sensor to measure all relevant thermal transport properties has been developed. This so-called transient hot bridge (THB) decidedly improves the state of the art for transient measurements of the thermal conductivity, thermal diffusivity, and volumetric specific heat. The new sensor is realized as a printed circuit foil of nickel between two polyimide sheets. Its layout consists of four identical strips arranged in parallel and connected for an equal-ratio Wheatstone bridge. At uniform temperature, the bridge is inherently balanced, i.e., no nulling is required prior to a run. An electric current makes the unequally spaced strips establish an inhomogeneous temperature profile that turns the bridge into an unbalanced condition. From then on, the THB produces an offset-free output signal of high sensitivity as a measure of the properties mentioned of the surrounding specimen. The signal is virtually free of thermal emf’s because no external bridge resistors are needed. Each single strip is meander-shaped to give it a higher resistivity and, additionally, segmented into a long and short part to compensate for the end effect. The THB closely meets the specific requirements of industry and research institutes for an easy to handle and accurate low cost sensor. As the key component of an instrument, it allows rapid thermal-conductivity measurements on solid and fluid specimens from 0.02 to 100 W· m−1·K−1 at temperatures up to 250°C. Measurements on some reference materials and thermal insulations are presented. These verify the preliminary estimated uncertainty of 2% in thermal conductivity.