An Expermentally Validated Model for Two-Phase Pressure Drop in the Intermittent Flow Regime for Noncircular Microchannels

Abstract

This paper reports the development of an experimentally validated model for pressure drop during intermittent flow of condensing refrigerant R134a in horizontal, noncircular microchannels. Two-phase pressure drops were measured in six noncircular channels ranging in hydraulic diameter from 0.42 mm to 0.84 mm. The tube shapes included square, rectangular, triangular, barrel-shaped, and others. For each tube under consideration, pressure drop measurements were taken over the entire range of qualities from vapor to liquid at five different refrigerant mass fluxes between 150 kg/m2s and 750 kg/m2s. Results from previous work by the authors were used to select the data that correspond to the intermittent flow regime; generally, these points had qualities less than 25%. The pressure drop model previously developed by the authors for circular microchannels was used as the basis for the model presented in this paper. Using the observed slug/bubble flow pattern for these conditions, the model includes the contributions of the liquid slug, the vapor bubble, and the transitions between the bubble and slugs. A simple correlation for nondimensional unit-cell length was used to estimate the slug frequency. The model successfully predicts the experimentally measured pressure drops for the noncircular tube shapes under consideration with 90% of the predictions within ±28% of the measurements (average error 16.5%), which is shown to be much better than the predictions of other models in the literature. The effects of tube shape on condensation pressure drop are also illustrated in the paper.

Prediction of two-phase pressure drop and void fraction in microchannels using probabilistic flow regime mapping

“…2.1 Two-phase flow mapping Garimella [1], Garimella et al [2], Coleman and Garimella [3], El Hajal et al [4], Thome et al [5], Didi et al [6], Zurcher et al [7][8], Mandhane et al [9], and Baker [10] have all described the importance of using flow pattern information in the determination of accurate two-phase flow models. Consequently, much attention has been directed towards developing two-phase flow regime maps.…”

“…Recently, some of the literature, such as Didi et al [6], seek to determine which existing pressure drop model is most accurate for a given flow regime. However, Garimella [1], Garimella et al [2], and Chung and Kawaji [13] have developed and presented similar flow regime based pressure drop models of the intermittent two-phase flow regime in microchannels, which attempt to incorporate the physics of the flow into the model. These models require the use of a flow regime map to determine the flow regime that exists and require flow visualization data on the slug rate.…”

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

“…Garimella et al [17] developed an experimentally validated model for pressure drop during intermittent flow of condensing refrigerant R134a in horizontal, noncircular microchannels.…”

Condensation Flow Mechanisms, Pressure Drop and Heat Transfer in Microchannels

“…Pressure drops across the test section were measured using a bank of three selectable differential pressure transducers, with an accuracy of ±0.25% of the span. Additional details of the pressure drop measurements, including validation procedures using singlephase gas and liquid pressure drop measurements, are available in Garimella et al [24,25].…”

“…An intermittent flow model for circular tubes was first developed by Garimella et al [25], with subsequent modifications to include the non-circular geometries shown in Figure 6 [24]. This model applies for the data points to the left of the corresponding transition lines shown in Figure 7.…”