Mini Channels Research Articles

An experimental study of flow boiling characteristics of carbon dioxide in multiport mini channels

Due to its unique combination of ecological and personal safety, carbon dioxide (CO 2), a natural fluid, is becoming one of the most promising alternative refrigerants for air-conditioning and refrigeration systems. This paper presents an experimental study of boiling heat transfer and pressure drop of CO 2 flowing in a multi-port extruded aluminum test section, which had 10 circular channels, each with an inner diameter of 1.31 mm. CO 2 was heated by hot water flowing inside copper blocks that were attached at both sides of the test section. Temperatures at the outer surface of the test section were measured using 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured using twelve heat flux sensors to obtain the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO 2 at the inlet and outlet of the test section were measured using two K-type thermocouples. The measurements were performed for pressures ranging from 3.99 to 5.38 MPa, inlet temperatures of CO 2 from −3.08 to 16.96 °C, heat fluxes from 10.1 to 20.1 kW/m 2, mass velocities from 131.4 to 399.0 kg/m 2 s, and vapor quality from 0.0 to 1.0. The results indicate that pressure drop along the test section is very small, two-phase CO 2 flow exhibits a higher heat transfer coefficient than that of the single-phase liquid or vapor flow. It is also shown that the mass velocity and the applied heat flux have significant effects on flow boiling heat transfer characteristics. The measured heat transfer coefficients were compared with correlations reported in the literature and large discrepancies are observed.

Experimental study on gas–liquid two-phase flow regimes in rectangular channels with mini gaps

Experimental investigations were conducted for adiabatic air/water two-phase flow in vertical rectangular channels. Three test sections with the narrow gaps of 0.3, 0.6 and 1.0 mm respectively were studied. The flow regimes were observed by using a high-speed video-camera and were identified by examining the video images. The flow regimes in channels with the gaps of 1.0 and 0.6 mm are similar to those found by previous studies, and can be classified into bubbly, slug, churn and annular flow. With decreasing the channel gaps, the transition lines for bubbly-slug, slug-churn and churn-annular flow shift to left in the flow regime maps with the superficial gas velocity as the horizontal coordinate and the superficial liquid velocity as the vertical coordinate. However, the flow regimes in the channel with the gap of 0.3 mm are dramatically different from the previous studies. Bubbly flow was never observed even at very low gas flow velocity. Cap-bubbly flow can exist in the channels stably. Due to the increased influences of the surface tension and friction shear stress in the mini channels, the liquid droplets adhere on the wall surface and are pushed by the flowing gas. The flow regimes can be classified into cap-bubbly, slug-droplet, churn and annular-droplet flow.

Porous medium model for two-phase flow in mini channels with applications to micro heat pipes

In this paper the porous medium concept is adopted to build up a fundamental model for two-phase flows inside mini channels. The capillary force which appears important for those problems is rigorously included in the model by following the same treatment as for porous media. Various applications of the developed model are demonstrated, among which are adiabatic co- and counter-current flows and heated forced and natural convective flows. Some fundamental results such as the two-phase pressure drop in a downflow, the flooding limit in a counter-current flow and dryout heat flux in natural convection boiling, are achieved and compared with previous studies available in the literature. With the main emphasis on the application to micro heat pipes, the model is used to predict the capillary limit of an operating micro heat pipe, to explain the liquid holdup phenomenon and to imply the onset and origin of the plug flow pattern. Future research needs are pointed out to refine this first version of a two-phase model and to apply it to additional practical applications.

Low conductance gramicidin A channels are head-to-head dimers of beta 6.3-helices

Weakly conductive, atypical channels were observed to form from highly purified Val1-gramicidin A in planar lipid bilayer membranes. The structure of these low-conductance channels (minis) was investigated by a detailed study of their channel forming characteristics. The possibility that minis originate from primary structural analogs or degradation products of gramicidin was considered and ruled out. In particular, spontaneous conductance changes in single channels demonstrated that minis can derive directly and reversibly from "standard" channels having the most common conductance level. The fraction of channels which are minis does not vary with changes in membrane gramicidin concentration, indicating that mini and standard channels have the same molecularity, that is, both are dimers. The mean lifetime of mini channels is only slightly shorter than that of standard channels, indicating that the six hydrogen bonds that stabilize the head-to-head dimer are minimally affected in minis. The fraction of channels which are minis is unaffected by the ionic strength, ionic composition, or pH of the bathing solution; it is also unaffected by the lipid composition of the bilayer. These findings are consistent with the hypothesis that minis arise from minor changes in the conformation of the Val1-gramicidin A molecule near the channel entrance or exit.