Small Rectangular Channels Research Articles

Small circular- and rectangular-channel boiling with two refrigerants

Boiling heat transfer experiments were performed in a small circular channel ( d = 2.46) and a small rectangular channel ( d h = 2.40 mm) with Refrigerant 12. The channel sizes are representative of flow passages in compact evaporators. An experimental technique minimized test section end effects which can be appreciable relative to the heat transfer in these small channels. Local heat transfer results were obtained over a range of qualities up to 0.94, a mass flux range of 44–832 kg/m's, and a heat flux range of 3.6–129 kW/m 2. Saturation pressure was nearly constant, averaging 0.82 MPa for most of the testing, with some tests performed at a lower pressure of 0.51 MPa. Local heat transfer coefficients were determined experimentally as a function of quality along the length of the test section. Heat transfer rates were compared with results of previous experiments in which Refrigerant 113 in a 2.92-mm diameter small circular tube was used. The effects of channel geometry and fluid properties on heat transfer, as well as insights relative to heat transfer mechanisms in small channels, are presented. Results are presented for both nucleation- (wall superheat above 2.75°C) and convection-dominant boiling heat transfer regimes and compared with large-channel predictions. The heat transfer data for the two fluids were successfully correlated in the nucleation-dominant region.

Application of Chaos Theory in Identification of Two-Phase Flow Patterns and Transitions in a Small, Horizontal, Rectangular Channel

Various measurement tools that are used in chaos theory were applied to analyze two-phase pressure signals with the objective of identifying and interpreting flow pattern transitions for two-phase flows in a small, horizontal rectangular channel. These measurement tools included power spectral density function, autocorrelation junction, pseudo-phase-plane trajectory, Lyapunov exponents, and fractal dimensions. It was demonstrated that the randomlike pressure fluctuations characteristic of two-phase flow in small rectangular channels are chaotic, and governed by a high-order deterministic system. The correlation dimension is potentially a new approach for identifying certain two-phase flow patterns and transitions.

Determination and Characteristics of the Transition to Two-Phase Slug Flow in Small Horizontal Channels

Two-phase pressure drop and fluctuating static pressures were measured in a small horizontal rectangular channel (hydraulic diameter = 5.44 mm). The two-phase fluid was an air/water mixture at atmospheric pressure tested over a mass flux range of 50 to 2000 kg/m2˙s. Two-phase flow patterns were identified and an objective method was found for determining the flow pattern transition from bubble or plug flow to slug flow. The method is based on an RMS static pressure measurement. In particular, it is shown that the transition is accompanied by a clear and abrupt increase in the RMS pressure when plotted as a function of mass quality. Use of the RMS pressure as a two-phase flow pattern transition indicator is shown to have advantages over pressure-versus-time trace evaluations reported in the literature. The transition is substantiated by a clear local change in slope in the curve of two-phase pressure drop plotted as a function of either Martinelli parameter or mass quality. For high mass fluxes, the change in slope is distinguished by a local peak. Some degree of substantiation was found in previous work for both of the results (the RMS static pressure change and the local pressure drop change) at the transition to slug flow.

Interfacial mode interactions in horizontal gas—liquid flows

The behaviour of shear-generated interfacial waves in a cocurrent gas-liquid flow in a small rectangular channel is studied experimentally at conditions close to neutral stability. It is found that the linearly most unstable mode, which typically has a frequency of 8–10 Hz and a wavelength 1–4 cm, grows initially — followed immediately by the first overtone. Measurements of the bicoherence spectrum indicate that the overtone and fundamental are coherent in phase, which suggests that energy is transferred from the fundamental to the linearly stable first overtone. This energy transfer mechanism can stabilize the system, as evidenced by data, which shows that the first mode can saturate with a wave slope small as as 0.005. Theory based on weakly nonlinear mode-interaction equations suggests that this steady state should be stable at conditions close to neutral stability where only overtone modes are present. However, under more severe conditions, where the amplitude of the fundamental mode becomes sufficiently large, a subharmonic mode may be excited. The generation of the subharmonic, when it is linearly stable with respect to the flat film base state, can be interpreted as a linear instability of the steady state containing the fundamental and overtones. Modes that are sidebands (with wavenumbers = k ± δk) to the main peak may also occur. These can participate in interactions with low-frequency modes (i.e. δk) and thereby transfer energy to frequencies much below the fundamental. It is expected that all of these interactions play important roles in determining the wave spectrum of conditions far away from neutral stability.

Frictional pressure gradients in two-phase flow in a small horizontal rectangular channel

Frictional pressure gradients were measured in a two-phase flow of an air-water mixture in a rectangular channel with a cross section of 19.05 × 3.18 mm, for application to plate-fin compact heat exchangers of the plain fin type. The measurements were conducted with the channel in two horizontal orientations. Adiabatic flows of air-water mixtures near atmospheric pressure were used at mass qualities of 2.5 × 10 −5 to 1.0 and total mass fluxes of 50–2000 kg/(m 2 · s). Pressure gradients were measured with respect to two-phase flow patterns. Unique conditions were found at the transition from bubble or plug flow to slug flow. The data were compared to predictions of two widely used correlation equations. These equations did not adequately correlate the data, and a modified equation were developed for qualities larger than those occuring at the transition to slug flow. The modified two-phase flow pressure gradient correlation covers the parameters of interest for compact heat exchanger application of mass flux from 50 to 350 kg/(m 2 · s) and mass qualities greater than 0.05 (Martinelli parameter less than unity). An existing correlation was found to have good accuracy at mass fluxes above 400 kg/(m 2 · s).

Two-phase flow patterns and transitions in a small, horizontal, rectangular channel

Horizontal two-phase flow was studied in a small cross-sectional-area (19.05 × 3.18 mm) rectangular channel with both the short and long sides forming the base, resulting in aspect ratios of 6 and 1 6 , respectively. Adiabatic flows of air/water mixtures were tested over a large mass flux range (50–2000 kg/m 2 s). The full quality range was considered whenever experimentally achievable. Flow patterns were established and both mean and dynamic pressure measurements were used to accurately establish the plug/bubble-to-slug flow transition. This result, together with visual observations and supplemented with photographic data, was used to develop flow pattern maps. A comparison of existing flow pattern maps for circular pipes, capillary tubes and larger rectangular channels led to the conclusion that, while qualitative agreement exists, these maps are not generally applicable to the subject small rectangular channel on the quantitative basis.

Heat Transfer From a Small Heated Region to R-113 and FC-72

An experimental investigation of heat transfer from a small heated patch to a subcooled, fully developed turbulent flow is conducted. The test patch, approximately 0.25 mm long and 2.0 mm wide, is located on the floor of a small rectangular channel through which a coolant (R-113 or FC-72) is circulated. A thin film of Nichrome deposited on a quartz substrate serves as an integrated heater element and resistance thermometer. The maximum achievable heat flux with R-113, limited by the thermal decomposition temperature of the fluid, is 2.04 MW/m2 at a bulk velocity of 1.8 m/s and a high wall superheat of 80° C. The results obtained with FC-72 show large temperature excursions at the onset of nucleate boiling and a boiling hysteresis near the onset of nucleate boiling. These effects decrease with increasing velocity and/or subcooling. The heat flux at departure from nucleate boiling increases with increasing velocity and/or subcooling. A maximum heat flux of 4.26 MW/m2 at departure from nucleate boiling is observed.

Image Processing System for Measuring Surface Velocities

IMAGE processing techniques for measuring dynamic changes of surface velocities in rill flows are described. Surface velocities were determined by the movement of small wooden beads as they floated past a video camera. The system was evaluated for accuracy using a well defined velocity medium. Algorithms were then tested using water flows in a small rectangular channel. Results indicated that the system was an accurate and efficient technique for measuring surface velocities. The accuracy of the system was estimated as ± 10 mm/s.

Penetration of p‐xylylene vapor into small channels prior to polymerization

AbstractThe penetration of p‐xylylene gas into small rectangular channels has been studied by measuring the thickness of the formed poly(p‐xylylene) films as a function of penetration distance. The experimental values are related to the theoretical penetration curves under various process conditions. At low monomer and residual gas pressures, meeting the Knudsen conditions for wall‐to‐wall collisions of the monomer, a molecular flow model is valid which shows large penetration distances and a gradual decrease in film thickness. At increasing monomer pressures and corresponding deposition rates the molecular flow at the entrance is disturbed in favor of a more viscous type of flow. Consequently an accumulation of poly(p‐xylylene) has been observed in the first part of the channel. At increasing residual gas pressures the penetration can be described by a mass transfer model. Under our process conditions however, its ideal penetration characteristics could not be realized due to a visccus flow transition at the entrance of the channel. Polymerization of chloro‐p‐xylylene at low residual gas pressures showed a molecular flow controlled penetration, also at high deposition rates. Owing to its higher reactivity the penetration is less favorable than for p‐xylylene.

Experimental Investigation Of Small Single And Multiple Free Jets By Laser Raman Spectroscopy

CO2 and N2 jets issuing into air from small rectangular channels of 2 x 25 mm cross section were investigated. Concentration measurements were obtained by cw laser Raman spectroscopy; gradients larger than 10 vol. %/100 pm could easily be resolved; spatial resolution was better than 10-2 mm3 and the measured fluctuations of the Raman signals show satisfactory agreement with Poisson statistics. Axial and radial profiles are reported for single and multiple jets and comparison with theory is given, where possible.

Calculation of void volume fraction in the subcooled and quality boiling regions

The complex problem of void calculation in the different regions of flow boiling is divided in two parts. The first part includes only the description of the mechanisms and the calculation of the rates of heat transfer for vapour and liquid. It is assumed that heat is removed by vapour generation, heating of the liquid that replaces the detached bubbles, and in some parts, by single phase heat transfer. By considering the rate of vapour condensation in liquid, an equation for the differential changes in the true steam quality throughout the boiling regions is obtained. Integration of this equation yields the vapour weight fraction at any position. The second part of the problem concerns the determination of the void fractions corresponding to the calculated steam qualities. For this purpose we use the derivations of Zuber and Findlay {9}. This model is compared with data from different geometries including small rectangular channels and large rod bundles. The data covered pressures from 19 to 138 bars, heat fluxes from 18 to 120 W/cm 2 with many different subcoolings and mass velocities. The agreement is generally very good.