Experimental investigation on the oscillation suppression in a R134a two-phase natural circulation close loop singly connected to an accumulator

Abstract

Having the advantage of durability (simple in structure without mechanical pump), two-phase natural circulation (TPNC) loops were widely used, but still limited by its inherent instability that may lead to evaporator dry out (boiling crisis). Till now, the mechanism of the TPNC instability are not fully understood. In our previous experiment in a TPNC close loop with a singly connected accumulator (Wen et al., 2018) [1], two types of the density wave oscillations (DWO) were identified by characteristic frequencies; and the stable region between the two types of DWO can be expanded by reducing the subcooling. In the present research, we found an effective way to suppress the two types of the oscillation by increasing the resistance coefficient of connection pipe between the accumulator and the loop, or even eliminate the oscillation simply by shutting off the pipe connection. We inferred that the exchange of fluid between two compressible volumes (the accumulator and the two-phase section of the loop in the present case) is a necessary condition for the two types of the DWO. It was found that, the type 2 density wave oscillation would lead to the evaporator dry out, causing about 25% deterioration of the loop’s heat dissipation capacity as compared with the stable loop when the oscillation was suppressed. Also, scaling analysis has been performed.