The present work is an experimental study of oscillatory flow and heat transfer in a simulated Stirling engine regenerator. In particular the effect of high pressure and high frequency zero-mean velocity oscillating flow on the internal heat transfer coefficient between working fluid and solid matrix in the regenerator was studied.
Using the same configuration as in the regenerator of NASA’s Stirling Space Power Demonstration Engine (SPDE), an experimental apparatus was designed. The working conditions in the test section are similar to those used in SPDE: the working fluid is Helium; the operational pressure is up to 15 MPa (2,200 PSI); operation temperatures are 630 K and 315 K at the hot and cold end respectively; and the oscillating frequency can be varied to 105 Hz. The porous medium used as the regenerator is SS 304 “Brunsmet” feltmetal with 0.0254 mm (0.001") wire diameter, manufactured by Brunswick Cooperation. The digital thermal lag compensation technique has been developed and used to measure the high frequency gas temperature.
The results of the study indicate that: (1) Inside the porous medium, the temperature of both fluid and solid matrix are changing periodically with the same frequency as the oscillating flow but with a temperature phase lag between the two phases. (2) Due to the high frequency oscillating flow in the regenerator, the heat transfer coefficient, h, and hence the Nusselt number, Nu, are of the order of 1×104 W/m2⋅k and 10, respectively. These values are a significant increase as compared to what they would be for unidirectional flow passing through similar porous material, where Nu is usually in the order of 1×10-2 to 1×10-1 with corresponding Reynolds number. These results agree with the prediction made by a computer code, HFAST, which indicated that the maximum instantaneous heat transfer coefficient h in the SPDE regenerator will be 33,750 W/m2⋅K. (3) The oscillating frequency of the flow has a large influence on both temperature variation and heat transfer coefficient while the pressure effects only the heat transfer coefficient. Because it is a very complex relationship and not easy to be quantified, the relationship between the oscillating flow and heat transfer in the porous metal regenerator is presented illustratively. And the transient Nusselt number is found to be a function of the oscillating frequency, f. (4) The regenerator has very high effectiveness, around 97-99.5%, and both oscillating frequency and working pressure have little influence on the effectiveness.
With all of the information we obtained from the experiments, we conclude that the Stirling engine regenerator and the engine should be run at the conditions of higher pressure, if material allowed, and oscillating frequency around 80-90 Hz.