Ekkehard Lohse

Bio: Ekkehard Lohse is an academic researcher from Hamburg University of Technology. The author has contributed to research in topics: Thermal energy & Latent heat. The author has an hindex of 3, co-authored 5 publications receiving 23 citations.

Performance assessment of regularly structured Composite Latent Heat Storages for temporary cooling of electronic components

Abstract: Phase Change Materials (PCM) are very reliable cooling devices. However their low thermal conductivity is a major drawback for the widespread use in technical applications. This paper presents an approach to design Composite Latent Heat Storages (CLHS) combining the PCM with an aluminum frame-structure for transport of thermal energy. These new designs require a generalized assessment taking varying compositions into account. Thus different assessment parameters are presented based on material properties, heat storage composition and the transient thermal behavior. The material properties depend on the PCM, while the heat storage composition depends on the geometry and determines the systems weight. The transient assessment is based on FEM-simulations and shows the performance with respect to idealized reference heat storages. This allows the performance assessment of CLHS of different sizes and compositions with respect to important boundary conditions like volume or mass, that depend on the application.

Experimental analysis of regularly structured composite latent heat storages for temporary cooling of electronic components

Abstract: This study presents the experimental investigation of regularly structured Composite Latent Heat Storages. Solid–liquid Phase Change Materials have a low thermal conductivity, resulting in high temperature differences. This drawback is compensated by the combination with specially designed frame-structures made of aluminum to enhance the transport of thermal energy. A prototype is investigated experimentally on a test rig, where the heat load and temperatures are measured while the phase change process is observed optically, and compared to a solid block Phase Change Material.

Inherently Safe Looped Thermosyphon Cooling System for Aircraft Applications using Dielectric Fluid H-Galden

Abstract: This paper presents experimental results of an inherently safe liquid cooling system. A test rig is operated at Hamburg University of Technology in order to prove the concept and gather first data for electronics cooling in modern civil aircraft. The cooling system uses a dielectric working fluid for the natural circulation in a looped thermosyphon operating in both oneand two-phase mode. First the mass flow of this natural circulation system is investigated comparing the measurement data to calculations. After that the cooling performance of the system is evaluated by taking a closer look at the heat loads and corresponding temperatures. Finally heat transfer coefficients in the cold plate are calculated. The results are discussed with respect to the following parameters, which are varied in the test series conducted: heat load, the heat sink temperature and the system orientation. 1. INHERENTLY SAFE COOLING SYSTEMS The power densities of electronic components are increasing continuously, thus conventional air cooling systems are replaced with liquid cooling to remove the waste heat (Dietl et al., 2008). Liquid cooling systems can transfer much higher waste heat flux densities. State-of-the-art liquid cooling systems use a closed cooling loop basically consisting of a cold plate and a cooler connected with pipes. The liquid is circulated by a pump. In the cold plate the waste heat from the electronic components is transferred to the working fluid. This hot liquid is pumped to the cooler, where the waste heat is discharged in most cases to sink the ambient air acting as the final heat. 1.1 Objective in Aircraft Applications Crucial flight systems (e.g. avionics) need highly reliable cooling systems. In common civil aircraft these systems are air cooled using forced convection. The electronics are air-ventilated by fans. Nevertheless the cooling systems are designed to ensure a minimum cooling performance without fans for some time to allow a safe landing at the nearest airport. With increasing power densities of microprocessors and power electronics the waste heat flux densities increase and air cooling has to be replaced by liquid cooling systems like described above. The reliability is a critical issue for these active liquid cooling systems compared to the conventional air cooling. With a failure in a coolant pump, which can have many reasons, the liquid stops circulating and the electronics get overheated very quickly. Thus, analogically to air cooling, it is the aim of an inherently safe liquid cooling system to ensure a minimum cooling performance without a coolant pump. In this passive configuration the circulation of the working fluid has to be actuated by buoyancy forces, which result from density differences. In one-phase operation this is a critical issue, as the density does not vary much for most liquids. A significant change in density is achieved by evaporating the working fluid. Therefore two-phase operation is highly attractive for inherently safe liquid cooling and will be the main focus of this paper.

Experimental investigation of temporary electronics cooling with regularly structured composite latent heat storage

Abstract: This study shows results of the experimental investigation of regularly structured composite latent heat storage. Common solid–liquid phase change materials used as latent heat storages have a low thermal conductivity, which leads to high temperature differences inside large phase change material volumes. This drawback is compensated by the combination with specially designed frame structures made of aluminum to enhance the transport of thermal energy inside the regularly structured composite latent heat storage. A prototype is investigated experimentally on a test rig, where the heat load and temperatures are measured while the phase change process is observed optically. The experimental data are compared to a solid block phase change material heat storage and to the results of a transient Finite Element Method (FEM) simulation.

Experimental investigation of temporary electronics cooling with regularly structured composite latent heat storages.

Abstract: This study shows results of the experimental investigation of regularly structured composite latent heat storages. Common solid-liquid phase change materials (PCM) used as latent heat storages have a low thermal conductivity, which leads to high temperature differences inside large PCM volumes. This drawback is compensated by the combination with specially designed frame-structures made of aluminum to enhance the transport of thermal energy inside the regularly structured composite latent heat storage. A prototype is investigated experimentally on a test rig, where the heat load and temperatures are measured while the phase change process is observed optically. The results are compared to a solid block PCM heat storage.

Structure and thermal properties of expanded graphite/paraffin composite phase change material

Abstract: Different contents of expanded graphite (EG) composite phase change material (PCM) were prepared by the melt mixing method, taking paraffin as the PCM and EG as the supporting material. Phase compositions of EG, paraffin, and EG/paraffin composite were investigated using X-ray diffraction (XRD). Microstructures of EG and EG/paraffin composite PCMs with different EG contents were observed by a scanning electron microscope (SEM). Thermal properties, such as phase-transition temperature and latent heat of the materials, were determined by differential scanning calorimetry (DSC). Mass loss and thermal properties after 100 heating cycles were measured. The results show that physical absorption exists between paraffin and EG. EG is beneficial for the PCM composite to reduce leakage of paraffin, decrease the phase change temperature and latent heat, and strengthen the thermal stability. The solid–liquid phase change latent heat of materials is larger than that of the solid–solid one. The heating cycle ha...