Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part I--pressure drop characteristics

Two-phase pressure drop was measured across a micro-channel heat sink that served as an evaporator in a refrigeration cycle. The micro-channels were formed by machining 231 μm wide × 713 μm deep grooves into the surface of a copper block. Experiments were performed with refrigerant R134a that spanned the following conditions: inlet pressure of Pin = 1.44-6.60 bar, mass velocity of G = 127-654 kg/m2 s, inlet quality of xe,in = 0.001-0.25, outlet quality of xe,out = 0.49-superheat, and heat flux of q″ = 31.6-93.8 W/cm2. Predictions of the homogeneous equilibrium flow model and prior separated flow models and correlations yielded relatively poor predictions of pressure drop. A new correlation scheme is suggested that incorporates the effect of liquid viscosity and surface tension in the separated flow model's two-phase pressure drop multiplier. This scheme shows excellent agreement with the R134a data as well as previous micro-channel water data. An important practical finding from this study is that the throttling valve in a refrigeration cycle offers significant stiffening to the system, suppressing the large pressure oscillations common to micro-channel heat sinks.