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Description
Pressure Wave Refrigerators (PWRs), in principle, work with thermal separation through shock waves. The fundamental flow dynamics within a PWR closely resemble those in a shock tube. The sudden release of high-pressure gas in a tube or channel produces shock waves that move through the driven gas, and the associated rarefied waves reduce the pressure of the driver gas. A PWR typically comprises a rotating gas distributor that periodically injects high-pressure gas into one or more stationary receiving tubes. The produced shock wave heats the residual gas towards the dead end, while the rarefied waves produce refrigeration in the driver gas at the nozzle end. Upon connection to the outlet port, the refrigerated gas flows out of the receiving tube into a receiver. PWRs have several advantages, such as simple design, low cost, high reliability, low rotational speed, and high efficiency comparable to conventional turbo-expanders. PWRs are effectively used in applications such as gas separation, cryogenic grinding, and cryogenic refrigeration.
The performance of a PWR is influenced by several key factors, such as tube length, operating frequency, pressure ratio, damping of reflected shock waves, and heat generation through shock wave compression. Some works have reported the effects of these variables or issues. Reflected shock waves from the dead end of the receiving tube of a PWR may heat the fluid inside the tube and seriously affect the performance. A damping unit in the form of a large cylindrical tank, i.e. a dump tank, is usually fitted at the end of the receiving tube so that the reflected shock waves are attenuated and retained within the tank. However, very little information is available in the literature on the sizing of the dump tank.
Reported studies indicate that for a given system and other operating conditions, there are peaks in isentropic efficiency at some operating cycle frequencies. The frequency for the first peak efficiency is important in terms of the minimum speed requirement of the rotor. It also depends on the size of the dump tank. Therefore, in this work, we have also determined the first peak frequency under varying sizes of the dump tank.
The present simulation involves the solution of mass, momentum, and energy conservation equations by using a finite volume formulation in Ansys FluentTM platform. Realizable k-ε turbulent equations are solved in a 2-D model. The effects of the size of the dump tank in terms of the volume of the stationary receiving tube are studied for different tube lengths and pressure ratios. The model is validated with experimental data from existing literature. The studies indicate that with the size of the dump tank, initially, the performance of the PWR increases upto a limit and then decreases. For example, as the volume of the dump tank is increased from 5 times to 20 times the volume of the receiving tube, the isentropic efficiency increases from 65% to 74% for a stationary receiving tube length of 1.5 m and operating pressure ratio of 2. The studies also show that there is a strong relation between the first peak in efficiency and the size of the dump tank. For instance, the occurrence of the first peak reduces from 90 Hz to 80 Hz as the volume of the dump tank is increased from 5 times to 20 times the volume of the receiving tube. The results of this research are expected to be useful for the design and operation of a PWR.
| Submitters Country | INDIA |
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