DESIGN REQUIREMENTS AND HEAT TRANSFER PERFORMANCE OF A POWER FREQUENCY VIBRATING RING INDUCTION HEATER

Abstract

The aim of this project is to develop a novel concept of designing a non-intrusion type induction heater with self-oscillatory heating element. The performance of the heater is analysed in the following aspects: a) Stirring force; b) Output heating power and efficiency; c) Amplitude and frequency of vibration; d) Convective heat transfer. The heater is basically a modified single phase electrical transformer with a short-circuited secondary ring, The primary coil produces the alternating magnetic flux passing through the core to induce eddy current in the ring. The eddy current produces resistive heating effect and interacts with the leakage flux to agitate the ring. The material of the ring should be properly selected so that the ring is light and conductive enough to allow agitation. An electronic integral cycle controller was designed to control the oscillatory motion. For the preliminary investigation, a prototype was built with a tank volume of 1120 cu.mm. A prior knowledge of the leakage magnetic flux is required for the determination of agitation force. Finite element method was used to determine the magnetic flux density at the vicinity of the ring. The magnitude of force acting on the ring was found to be 1.87 N with applied voltage 230V. A modified transformer determine the maximum equivalent circuit was used to efficiency and the maximum output heating power with respect to ring resistance. A 0.90 m? ring yielded a maximum efficiency of 96.5%. Maximum power of 396 W occurred at ring resistance of 0.276 m? with applied voltage 230V. The vibration system can be modelled by a damped oscillation system with periodic excitation force. Theoretical analysis indicated occurrence of resonant oscillation which pro?=duced vigorous disturbance on the water. Experiments were conducted to support the analysis. As a result, the resonance had brought induced effective stirring and enhancement of heat transfer by convection. Analysis of effect of vibration on heat transfer was mainly based on experimental findings. Investigations were conducted to find the relationship between convective heat transfer coefficient and vibrational intensity (amplitude x frequency). Beyond certain critical value of vibrational intensity, the convective heat transfer coefficient was found to increase drastically; and the critical vibrational intensity varied with the applied voltage. The convective heat transfer coefficient with vibration could be as high as 19.3 times that without vibration. The experimental data was correlated by the dimensionless groups Nu and Rev.