Abstract
Although two-phase flow instabilities are attributed to be one of the impediments for achieving high heat flux in boiling systems, most of their fundamental characteristics remain uncharted. In particular, pressure drop oscillations and density wave oscillations are two types of dynamic two-phase flow instabilities that can cause large variations in pressure and temperature. Under particular working conditions, both oscillations have been observed to interact, resulting in long-period pressure drop oscillations with superimposed short-period density wave oscillations. However, in this situation, the amplitude of the density wave oscillations is typically larger than the corresponding to a pure density wave oscillation. Here, we show that a compressible volume in the system, essential for the occurrence of pressure drop oscillations, plays a major role in amplifying the amplitude of the superimposed density wave oscillations.
Highlights
Flow boiling is characterized by a complex interplay of hydrodynamic and thermal effects that can exhibit flow instabilities
We show that the compressible volume in the surge tank upstream of the test section plays a major role in controlling the amplitude of the superimposed density wave oscillations
It is possible to see shortperiod superimposed oscillations in the evolution of the pressure. This shows that the occurrence of the superimposed density wave oscillations during pressure drop oscillations does not depend on the presence of a compressible volume
Summary
Flow boiling is characterized by a complex interplay of hydrodynamic and thermal effects that can exhibit flow instabilities. Using linear frequency domain stability models, Yin et al (2006) showed that pressure drop oscillations occur due to the compressible volume at the heater inlet, while density wave oscillations are not affected by the latter.[14] In summary, in the previous studies, the superimposed oscillations have been attributed either to the complete boiling during pressure drop oscillations,[12] or sudden change of the mass flux in the orifice valve during the pressure drop oscillations[13] or the interaction of the oscillations.[14] In this letter, we show that the compressible volume in the surge tank upstream of the test section plays a major role in controlling the amplitude of the superimposed density wave oscillations
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