An Experimentally Validated Model for Two-Phase Pressure Drop in the Intermittent Flow Regime for Circular Microchannels

Abstract

This paper reports the development of an experimentally validated model for pressure drop during intermittent flow of condensing refrigerant R134a in horizontal microchannels. Two-phase pressure drops were measured in five circular channels ranging in hydraulic diameter from 0.5 mm to 4.91 mm. For each tube under consideration, pressure drop measurements were first taken over the entire range of qualities from 100% vapor to 100% liquid. In addition, the tests for each tube were conducted for five different refrigerant mass fluxes between 150 kg/m2-s and 750 kg/m2-s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were then used to identify data that corresponded to the intermittent flow regime. A pressure drop model was developed for a unit cell in the channel based on the observed slug/bubble flow pattern for these conditions. The unit cell comprises a liquid slug followed by a vapor bubble that is surrounded by a thin, annular liquid film. Contributions of the liquid slug, the vapor bubble, and the flow of liquid between the film and slug to the pressure drop were included. Empirical data from the literature for the relative length and velocity of the slugs and bubbles, and relationships from the literature for the pressure loss associated with the mixing that occurs between the slug and film were used with assumptions about individual phase friction factors, to estimate the total pressure drop in each unit cell. A simple correlation for non-dimensional unit-cell length based on slug Reynolds number was then used to estimate the total pressure drop. The results from this model were on average within ±13.4% of the measured data, with 88% of the predicted results within ±25% of the 77 measured data points.

Boiling and Evaporation in Small Diameter Channels

“…They found distinct effects of diameter on flow regime transitions. This work is extended in a later paper, with a focus on intermittent flow [47]. A paper by Barnea et al [48], which is cited in [19], looks at air-water flows in 2 to 6 mm tubes.…”

Bounds on Two-Phase Frictional Pressure Gradient in Minichannels and Microchannels

“…For each tube investigated in this study, they took pressure drop measurements first over the entire range of qualities from 100% vapor (x = 1) to 100% liquid (x = 0) at five different refrigerant mass fluxes between G = 150 kg/m 2 .s and G = 750 kg/m 2 .s. Then, they used results from a previous work [13] to select the data that corresponded to the intermittent flow regime; generally, these points had x < 0.25. They used their previous pressure drop model for circular microchannels as the basis for the model presented for noncircular microchannels.…”

“…Garimella et al [13] developed an experimentally validated model for pressure drop during intermittent flow of condensing refrigerant R134a in horizontal microchannels. They measured two-phase pressure drops in five circular channels with d = 0.506, 0.761, 1.52, 3.05, and 4.91 mm.…”

Two-Phase Frictional Pressure Drop Multipliers for SUVA R-134a Flowing in a Rectangular Duct

“…Recently, there has been interest in understanding the pressure drop for refrigerant fluids, in particular for refrigerant systems where the coolant flows through small tubes or micro-channels [6][7][8][9][10]. Tran, et al [8] have considered small tubes as those under about 3 mm in diameter.…”

“…The general operational range of interest in most refrigerant systems encompasses pressures under 1 MPa and mass fluxes less than 600 kg/m 2 /s. SUVA R-134a, a substitute for Freon used commonly in air conditioning systems, has been investigated by several researchers in this area [8][9][10]. Knowledge of refrigerant fluid behavior at higher pressures and mass flux is also important for scaling studies between the refrigerant modeling fluid and pertinent steam-water applications [11].…”