Stefanie Angel

Characterization of Air Flow Through Sintered Metal Foams

This study investigates air flow in metallic foams, which are produced by the SlipReactionFoamSintering (SRFS)-process. It was conducted as part of the collaborative research center (SFB) 561 “Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants”. The flow through a porous medium is analysed by Darcy’s equation with the Dupuit/Forchheimer extension. All measurements can be described very well by this equation and permeability and inertial coefficients are obtained for a large quantity of samples with different base materials and different porosities. A threshold porosity of 70 % is observed, above which the pressure loss starts sinking significantly with porosity. Additionally, it was found, that the permeability was anisotropic. Permeability is lower in the direction of gravity during foaming. Scattering in the data of the permeability and inertial coefficients versus the porosity is observed and discussed.

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

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.

Functional and Structural Characteristics of Metallic Foams Produced by the SlipReactionFoamSintering (SRFS)-Process

Highly porous open- cell materials on the base of various metals and alloys are of increasing interest as they combine structural and functional properties. There is a wide range of possible applications for such materials, e.g. as heat exchangers, filters or catalysts. A new and promising method to produce open- cell metallic foams on base of iron powder, low and high alloyed steel powders as well as nickel alloy powder is the SlipReactionFoamSintering (SRFS)- process. In comparison to other production processes of metallic foams, the SRFS- process provides several advantages: foaming at room temperature, allowing a very good process control by various parameters, foams of a great variety of metals are possible and a broad spectrum of properties is achievable.

Experimental Investigation of Heat Transfer and Pressure Drop in Porous Metal Foams

Metal foams made by the SlipReactionFoamSintering (SRFS)process are investigated. In these foams the pores are produced by a reaction between iron and a weak acid. The generated hydrogen forms pores in a metal powder slurry. These pores remain in the foam after sintering. Also secondary pores are found in these foams because of the sintering of the metal powder slurry. The metal powder base of the foams is Inconel 625 and Hastelloy B. Foam samples with a variety of different porosities of the two metals in the range of about 62 % to 87 % are used as well as samples made out of sintered metal powder which were not foamed with porosities of around 50 %. The motivation for this study is to use these foams as combustion chamber walls in gas fired power plants. By using porous walls effusion cooling can be applied to keep the wall temperatures low. Air is used as a fluid to study the flow characteristics of these samples. Their pressure drop with air at room temperature is measured in the range of velocities of up to around 1 m/s. The parameters characterizing the foams are obtained using the Darcy-Forchheimer equations resulting in the permeability and the inertial coefficients. The dependency on the porosity is discussed. The volumetric heat transfer is measured for the foams by a transient method based on an air flow with a sinusoidal temperature wave, which is attenuated by the sample. The obtained volumetric heat transfer coefficients are discussed and transferred to Nu-Re correlations. Correlations between the heat transfer coefficients and the pressure drop coefficients are made.

Characterisation of Flow and Heat Transfer in Sintered Metal Foams

Since sintered metal foam is an innovative material with promising properties such as high porosity, good thermal conductivity, beneficial mechanical properties like strength and weldability, it has been considered to be applied as an open porous wall element in combustion chambers of gas turbines. In this application, the foam serves as a functional material capable to lead cooling air through micro- and minichannels into the inside of the combustion chamber. This cooling technique also known as effusion cooling keeps the combustion chamber walls below critical temperatures and therefore enables the burning process to be more effectively operated at higher temperatures. For a proper design of the wall element, the temperature distribution along the path of the fluid inside the foam must be known. For an exact calculation of the temperature flow and heat transfer processes inside the foam must be known. Therefore in this study the permeability and heat transfer properties of the foam have been characterized experimentally. The methods are described and the results in terms of permeability coefficients, convective heat transfer coefficients and effective thermal conductivity are presented as functions of the foam’s porosity. The method of the calculation is described and finally, the results of the calculation are presented, showing that due to the fine grained structure of the foam, the heat transfer from the solid to the cooling fluid takes place in a thin layer close to the inner surface of the camber wall.