Experimental Studies of Convection in a Single Stage Pulse Tube Refrigerator

Abstract

A single stage Pulse Tube Refrigerator has been designed, fabricated and operated with a 3kW GM type helium compressor with an indigenously developed rotary valve in the frequency range from 1.2 to 2.3Hz. The system produces a lowest temperature of 37.5K and a refrigeration power of 7.3W at 77K. The cold end temperature Tc of the Pulse Tube refrigerator depends on its orientation angle θ with respect to gravity vector and the increase in Tc arises due to the convection in the Pulse Tube. The lowest temperature occurs at θ = 0°. The increase in Tc with change in θ is small till about 70°. However, beyond this angle, Tc varies strongly and reaches a maximum value at θ ∼ 120° and decreases thereafter upto 180°.For a better understanding of convection phenomenon, detailed experimental studies have been performed on two pulse tubes with the same i.d. 13.3mm, but with lengths 250 and 300mm. The orientation dependence of Tc and the cooling power characteristics have been studied at different frequencies (1.2 Hz to 2.3 Hz) at two different pressures. The results have been analysed to obtain the experimentally observed heat flux due to convection, QconE.The studies show that these values are much larger than those theoretically calculated, QconT based on natural convection in the pulse tube. Due to the absence of a detailed theory to understand the experimental results, it is assumed that QconE is a sum of two parts, one arising from the natural convection and the other due to the forced convection. The latter is found to depend on the operating frequency. Empirical analysis has been carried out for the frequency dependent component of convection heat flux.