Investigations of heat transfer at high temperatures (T > 600 °C) in ceramic sponges

填充床的颗粒或蜂窝状是提高化学工程设备（如反应器）效率的典型手段。填充床的优点包括高交叉混合和高比表面积，而蜂窝式产生较低的压降。填料床的缺点是由于两个颗粒之间的点接触造成的高压降和高热阻，而蜂窝不允许流体的交叉混合。在一种新的创新方法中，这些常用的结构将被海绵（也称为开孔泡沫）所取代。海绵是一种高孔隙度的网状结构。进一步的关键特性包括相对较低的压降、高比表面积和流体的高交叉混合。由于连续的固相热点几乎可以避免，并且可以实现均匀的温度分布。在海绵中结合了颗粒结构和蜂窝结构的优点，同时减少了这两种结构的缺点。为了确定化工设备的尺寸，需要可靠的两相（=有效）传热参数的相关性。过去，申请人成功地开发了忽略辐射的中等温度下的传热相关性和模型。对于高温应用，辐射必须作为热传导之外的附加传热机制包括在模型中。然而，只有少数出版物只有少数实验数据存在的文献处理这一主题。因此，该项目的目的是建立一个广泛的实验数据基础，研究不同类型的海绵（材料、细胞直径和孔隙率的变化），并扩展自身的相关性，以便提供可靠地计算中高温海绵传热的可能性。通常，传热模型是基于均匀或非均匀的方法。第一种方法认为海绵是具有叠加性质的准均质体系。在这里，必须知道有和没有流动的两相导热系数。后一种模型认为海绵是一个两相系统。在这里，为了描述传热，两个能量平衡被一个包含传热系数的项耦合。这两种方法将在本项目中进行，从而在高达约1000°C的温度下实验确定热导率和传热系数。为了建立基于辐射的传热模型，海绵的光学参数（透射率、发射率和反射率）将由傅里叶变换红外光谱测定。

Packed beds of pellets or honeycombs are typical means for enhancing the efficiency of chemical engineering appliences (e.g. reactors). Advantages of packed beds include a high cross mixing and a high specific surface area whereas honeycombs yield lower pressure drops. Disadvantages of packed beds are the high pressure drop and high thermal resistances due to point contact between two pellets while honeycombs do not allow cross mixing of the fluid. In a new and innovative approach, these commonly used structures are to be replaced by sponges (also called open-celled foams). Sponges are network structures of high porosities. Further key characteristics include a comparatively low pressure drop, a high specific surface area and a high cross mixing of the fluid. Due to the continuous solid phase hot spots can nearly be avoided and a homogeneous temperature distribution can be achieved. While advantages of pellet structures and honeycombs are combined in sponges, disadvantages of these two structures are reduced at once.To dimension chemical engineering equipment, reliable correlations for the two-phase (= effective) heat transfer parameters are required. In the past, the applicants successfully developed heat transfer correlations and models at moderate temperatures neglecting radiation. For high temperature applications, the radiation must be included in the models as an additional heat transfer mechanism beside heat conduction. However, only few publications presenting only few experimental data exist in literature dealing with this topic. Consequently, the aim of the project is to establish a wide experimental data basis investigating different sponge types (variation of material, cell diameter and porosity) as well as to extend the own correlations in order to provide the possibility of calculating heat transfer in sponges at moderate and high temperature reliably. Typically, heat transfer models are based on a homogeneous or a heterogeneous approach. The first considers the sponge as quasi-homogeneous system with superposed properties. Here, the two-phase thermal conductivity with and without flow must be known. The latter type of model considers the sponge as a two-phase system. Here, for describing heat transfer two energy balances are coupled by a term containing the heat transfer coefficient. Both approaches will be pursued in this project leading to the experimental determination of both the thermal conductivity and the heat transfer coefficient at temperatures up to about 1000 °C. For modeling heat transfer based on radiation, optical parameters (transmittance, emissivity and reflectance) of the sponges will be determined by Fourier Transform Infrared Spectroscopy.