Natural convection of a magnetic fluid in concentric horizontal annuli under nonuniform magnetic fields

Natural convection of a magnetic fluid in concentric annuli was investigated experimentally. Two concentric cylinders were made of copper and placed horizontally. The temperature of the outer cylinder was kept at 15°C, and the inner cylinder was rapidly heated from 15°C to 25°C and held there. A thermosensitive liquid crystal was utilized for temperature visualization instead of flow visualization; temperatures on a central cross-section were also measured by thermocouples. A magnetic field was applied to the cylinders using a permanent magnet. The test liquid was a magnetic fluid with a 33% weight concentration of fine magnetic particles in a water carrier. Several kinds of experiments were carried out to clarify the influences of direction and the intensity of magnetic fields on the natural convection. When there was no magnetic field, ordinary natural convection was observed. When a magnetic field gradient was applied in the same direction as the gravity, a wall-temperature distribution was observed, as if an apparent gravity had increased; however, the clear influence of the magnetic field was not found. When a magnetic field gradient was applied in the opposite direction of the gravity, the reverse natural convection was observed. Consequently, even if the intensity of the applied magnetic field was small, it played an important role in natural convection and heat transfer of a magnetic fluid. It was recognized that natural convection of a magnetic fluid could be controlled by the application of a magnetic field.