Experimental Studv On Heat Transfer Of Oscillating Flow In The Heat Exchangers With Parallel Plate Channels

Regenerative heat engines and rgenerative refrigerators both rely on the oscillating working fluid to realize the conversion between heat and mechanical work. Heat transfer performance of the heat exchangers is one of the crucial factors which will affect the efficiency of regenerative heat engines and also refrigerators. So far, most of the heat exchangers working in oscillating flow have been designed by referring to the experimental correlations of unidirectional flow. Actually, it will lead to a distinct error as the oscillating flow is characterized by cyclic reversion of its fluid velocity direction, which obviously differs from the unidirectional flow. As one of the key components to improve the overall system’s efficiency, heat exchangers working in this particular flow field should be given high attention. And it will be of great importance to gain a relatively universal experimental correlation to help the design of heat exchangers in oscillating flow. According to the analysis of reported work, present research mainly focuses on the following two aspects:1. Experimental method based on energy balance analysis:After the determination of related dimensionless similarity criteria, an experimental apparatus has been built based on energy balance analysis to study the heat transfer of oscillating flow in heat exchangers with parallel plate channels. A data reduction is referred to the space-cycle averaged Nusselt number, which is used to summarize the heat transfer characteristics of oscillating flow.2. Experimental study on heat transfer of oscillating flow in heat exchangers with parallel plate channels:According to the operating conditions of hot-end (ambient-temperature end) heat exchangers employed in Stirling refrigerators, thermoacousitic refrigerators and pulse tube refrigerators, and also with the typical working fluid of helium, the dependence of Nusselt number on Reynolds number and Valensi number have been investigated. Diverse existing correlations for the Nusselt number have been analized by comparing with our experimental data. The impact of the compression and expansion effect indicatied by pressure ratio PR is studied in this work. It is shown that pressure ratio PR is another important parameter which is independent from the other six dimensionless similarity criteria. With Reynolds number, Valensi number and pressure ratio unchanged, the impact of the length of parallel plate channels on the heat transfer of oscillating flow is also investigated. The aspect ratio l/dh (ratio of the length to the hydraulic diameter of parallel plate channels) and the dimensionless displacement amplitude Ar (ratio of the peak-to-peak displacement of fluid parcel to the length of parallel plate channels) are both discussed in this work, and the corresponding comparison indicates that the aspect ratio is relatively more reasonable for geometrical similarity. Finally, a new experimental correlation is proposed to predict the heat transfer of lamilar oscillating flow of helium gas in heat exchangers with parallel plate channels. The applicable conditions of this new correlation have also been pointed out.