Heat Transfer Mechanism Of Oscillating Heat Pipe And Its Coupling Effect With Pulsating External Thermal Field

Much attention has been paid to the research on the oscillating heat pipe not only because of its excellent heat transfer performance but also because of its other advantages, such as simple wickless and flexible structure, fast startup, lower cost, and so on. In this paper, experimental investigation and theoretical approaches were conducted to investigate the heat transfer characteristics and enhanced methods by external thermal field of oscillating heat pipe. The main research work involved was illustrated as follows:1. An experimental system was established, and systematically experimental study was performed to achieve a better understanding of operating law of oscillating heat pipe with water and ethanol as working fluid under various operating conditions. The effect of filling ratio, inclination angle heating temperature, as well as startup characteristics and comparison between water and ethanol were given.2. Analytical simplified models for oscillating heat pipe employing the theory of extended meniscus, considering the effect of liquid film, disjoining pressure, surface tension and gravity, were developed in this paper. Governing equations of mass, energy and momentum conservation base on the Lagrangian method were presented. The governing equations were solved using explicit finite difference scheme to predict the thermal characteristics of oscillating heat pipe affected by inner diameter and inclination angle.3. Analytical simplified models for oscillating heat pipe by pulse heating was instablished to investigate the inner oscillating mechanism and heat transfer characteristics of the heat pipe by external pulse thermal field.4. Pulse heating adopted instead of continuous heating was proposed to enhance the heat transfer of oscillating heat pipe. This method was experimentally proved to be feasible. Experiment results showed that both the heat transfer rate and the effective thermal conductivity of pulse heating oscillating heat pipe were much bigger than that of the continuous in most cases. The heat transfer performance of oscillating heat pipe was variable with change of pulse duration and interval time, and both heat transfer rate and effective conductivity had a maximum value. Obviously, although the oscillating heat pipe performance is improved by pulse heating, the extent of improvement is influenced by pulse duration and interval time. Due to the combined action of internal and external thermal field, an optimal operating range may exist. Further investigation on the combine effect of external and internal thermal field is needed.5. Analysis on the wall temperature fluctuation was performed. A new parameter fA, pulsation factor, was defined. The heat transfer performance of oscillating heat pipe was determined by combined effect of frequency and amplitude of temperature oscillation. When the oscillating heat pipe is of an effective performance, the temperature frequency and amplitude are commonly of an appropriate value, which can be roughly estimated by the value of fA.6. Acoustica cavitation was proposed to enhance the heat trasnfer of oscillating heat pipe. Experiment results have proved that the acoustic cavitation is feasible to enhance the heat transfer performance of oscillating heat pipe. However, it was not always effective for every operating condition. That is, the extent of heat transfer enhancement is not proportional to the acoustic intensity. However, it is weakened in some cases, which is different from the research results reported before. It is implied that the role of acoustic cavitation upon on the oscillating heat pipe is special and further study on the combine effect of external and internal thermal field is needed.