TEMPERATURE DEPENDENCY OF THE EFFECTIVE THERMAL CONDUCTIVITY OF NICKEL BASED METAL FOAMS

Abstract

This article presents experimental results of the thermal conductivity of sintered metal foams. The foams are intended to be used in advanced combined power plants, investigated by the cooperate research center ‘Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants’ SFB-561. Porous materials are required for the active cooling of the gas turbine combustion chamber wall by effusion cooling. For design purpose, knowledge of the thermophysical properties of this new material is required within the temperature range up to 1000°C. The investigated metal foams were manufactured by the Slip Reaction Foam Sintering (SRFS) process with porosity ranges from 0.55 to 0.85. The overall porosity may be divided in two parts. The primary porosity with pore size levels about 1-3 mm and a range form zero to 0.68 results from a metal-acid-reaction. These primary pores are embedded in a packed bed of sintered metal grains (<150µm), which is also porous. This secondary porosity with pore size levels around 20µm results in porosities of about 0.5. The thermal conductivity of cellular solids differs from that of their corresponding dense material. Therefore, the various pore size level effects contributing to the thermal conductivity are accounted for by introducing an effective thermal conductivity eff . For the determination of the effective thermal conductivity the Transient Plane Source Technique, also known as Hot Disk was employed. The thermal conductivity of the sintered packed bed without primary pores was determined up to 700°C and compared to similar materials. For the foams, eff was determined for a primary porosity of 0.68 up to 700°C. In this article, a dependency between the primary porosity and eff can be shown. The linear rise of eff up to 400°C can be due to the increase of the thermal conductivity of the solid phase. The measurements are validated by comparison of the received specific heat with values received by thermogravimetry measurements. The general applicability of the measurement method to heterogeneous materials such as metal foams is discussed and an outlook about further investigations is given.