Thermo-Technical Laboratory

The thermo-technical laboratory at the Fraunhofer IFAM Dresden provides excellent equipment for the measurement of thermophysical material and transport parameters as well as characteristic thermal and flow parameters of various materials and material composites. In combination with the many years of experience of our employees of the Energy and Thermal Management business unit, we are also able to solve your individual measurement requirements.

The parameters examined include, e.g.:

• density and specific heat capacity,
• thermal conductivity, heat transfer coefficients in gas and liquid flows and evaporating or condensing media, and
• permeabilities and pressure loss coefficients during flow.

In addition to solids with isotropic properties, composite materials (layer, fibre or particle composites) with direction-dependent characteristics, porous structures and numerous other samples - especially phase change materials - can also be analyzed.

The basic equipment of the laboratory includes:

• stationary and non-stationary measuring methods for determining the thermal conductivity at variable temperatures,
• equipment for the generation of tempered gas or liquid flows,
• test installations for the cyclic loading and unloading of heat storage elements,
• evaporator test stands, and
• efficient measuring and data acquisition systems for temperatures, pressures, speeds and other measured variables.

In addition to the use of existing laboratory equipment, we are happy to develop test apparatus for the implementation of special customer-specific measuring tasks.

Thermal conductivity and heat capacity of materials are important parameters, especially for the thermal management. Measuring them as accurately as possible requires high-quality measurement technology and many years of experience in conducting and evaluating the measurements.

At the Fraunhofer IFAM Dresden, two plate apparatuses are available for measuring the thermal conductivity of metallic and non-metallic materials (also porous or composite materials). A stationary heat flow is generated through the samples, from which the thermal conductivity can be determined in comparison with known samples (reference method) or by evaluating an energy balance (absolute method). Temperatures from room temperature up to 450 °C can be set, a rough vacuum or selected gas atmospheres can be generated or defined sample contact pressures can be realized, although not all options can be combined with each other.

The unsteady hot-disk method is not only suitable for solids, in which flat sensors are positioned between two solid samples or even in a liquid or a particle bed. The temperature or thermal conductivity of the sensor environment (= sample) is calculated from the relationship between heat output and the temperature profile of the sensor over time. The system is preferably used at room temperature and is additionally equipped with a small volume heat capacity measuring cell.

The Thermal Analysis Laboratory at the Fraunhofer IFAM Dresden also has further measuring methods for thermal conductivity and capacity.

For many applications in the field of heat transfer, heat storage and heat management, knowledge of the thermal and fluidic properties of (composite) materials is of great importance. These include essentially:

• the determination of heat transfer coefficients with forced convection during the flow through cellular metal structures (metal foam, metal fiber, metal wire structures) or during the flow over/around components with functionalized surfaces (e.g. project HeatCNT),
• the measurement of heat transfer coefficients with free convection on functionalized surfaces (e.g. structured evaporator surfaces),
• the measurement of permeabilities or pressure loss coefficients in or on structures through or over flowed structures.

For this purpose, the thermal engineering laboratory is optimally equipped to generate defined flow conditions in channels or on free surfaces in the form of

• Air flows with specified flow velocity (fan or mass flow controller) and temperature (electric air heater up to max. 20 kW heat output),
• Water flows up to 5 litres/min (Coriolis flow controller) with temperatures up to 95 °C (electric instantaneous water heaters up to max. 20 kW heating capacity), and
• Thermal oil flows (adjustable pump) up to max. 350 °C in a closed circuit.

Furthermore, modern flow, velocity, pressure and temperature measurement technology is available to characterize the flow conditions. The required coefficients are either determined from direct measured variables or with the help of suitable energy balances.

Thermoelectric modules allow the direct conversion of heat into electrical energy without moving parts and are therefore an ideal technology for e.g. using waste heat. The thermoelectric modules developed at the Fraunhofer IFAM Dresden or by external partners can be characterized in the thermo-technical laboratory in two ways:

• The dynamic behaviour of the modules provides information about their mechanical properties under alternating thermal stress, which, for example, causes the modules to warm up and cool down when a thermoelectric generator is started up and shut down. Temperature gradients are cyclically generated in the modules by electrical heating or air cooling in a specially developed test facility [(1) in the picture], which correspond to the practical operating conditions. Subsequently, the optical or mechanical testing of the modules takes place.

• The stationary behaviour of the modules provides information about the electrical power that can be generated when coupled to the flow of a heating medium (hot air up to 600 °C and higher) and a cooling medium (cold air or cooling water up to 90 °C). The thermal efficiency of the thermoelectric modules can be calculated from these measurements in combination with a thermal conductivity measurement.