Condensation Flow Regimes Research Articles

Flow patterns and heat transfer in a square cross-section micro condenser working at low mass flux

Flow patterns and heat transfer were investigated during condensation of n-pentane in an air-cooled square cross-section micro condenser. The test section consisted of a borosilicate square microchannel, of inner and outer edges 553 μm and 675 μm, respectively, and of a length 208 mm. The transparency of the microchannel walls allowed the visualization of the phases distribution and the different condensation flow regimes. Different mass velocities ranging between 3 and 15 kg m−2 s−1 were imposed. Three main flow regimes were identified: annular regime, intermittent regime, and spherical bubbles regime. A specific experimental procedure was developed, based on bubbles tracking, in order to determine accurately the hydraulic and thermal parameters profiles in the isolated bubbles zone, such as the time-averaged void fraction profile α¯(z) and the time-averaged vapour quality profile x¯(z), according to the axial position in the microchannel. Thanks to energy balance, the time-averaged liquid temperature profile T¯l(z) in the isolated bubbles zone was determined. A significant temperature difference between the liquid and vapour phases was highlighted. Therefore, the latent and total heat fluxes released in this zone were quantified and compared to each other. Besides, the relationship between the void fraction and the vapour quality in the spherical bubbles zone was determined and compared to existing void fraction models. Finally, the bubbles detachment frequency was determined. A relationship between this frequency and the mass velocity was proposed.

Refrigerant Condensation Flow Regimes in Enhanced Tubes and Their Effect on Heat Transfer Coefficients and Pressure Drops

Flow regimes influence the heat and mass transfer processes during two-phase flow, implying that any statistically accurate and reliable prediction of heat transfer and pressure drop during flow condensation should be based on the analysis of the prevailing flow pattern. Many correlations for heat transfer coefficient and pressure drop during flow condensation completely ignored flow regime effects and treated flows as either annular or non-stratified flow or as stratified flow. This resulted in correlations of poor accuracy and limited validity and reliability. Current heat transfer coefficient, pressure drop, and void fraction models are based on the local flow pattern, though, resulting in deviations of around 20% from experimental data. There are, however, several inconsistencies and anomalies regarding these models, which are discussed in this paper. A generalized solution methodology for two-phase flow problems still remains an elusive goal, mainly because gas-liquid flow systems combine the complexities of turbulence with those of deformable vapor-liquid interfaces. The paper focuses on the state of the art in correlating flow condensation in micro-fin tubes and proposes flow regime-based correlations of heat transfer coefficient and pressure drop for refrigerant condensation in smooth, helical micro-fin, and herringbone micro-fin tubes.

Update on Condensation Heat Transfer and Pressure Drop inside Minichannels

The present paper reviews published experimental work focusing on condensation flow regimes, heat transfer, and pressure drop in minichannels. New experimental data are available with high (R410A), medium (R134a), and low (R236ea) pressure refrigerants in minichannels of different cross-section geometries and with hydraulic diameters ranging from 0.4 to 3 mm. Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi-empirical and theoretical models developed for conventional channels and models specifically created for minichannels. Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film.

A condensation heat transfer correlation for millimeter-scale tubing with flow regime transition

This study documents local convection heat transfer and flow regime measurements for HFC-134a condensing inside a horizontal rectangular multi-port aluminum condenser tube of 1.46 mm hydraulic diameter. The data is compared with condensation heat transfer correlations and flow regime maps from the literature. Existing correlations are found to overpredict both heat transfer and the stratified-to-annular flow regime transition velocity. Results of the experiments suggest that liquid drawn into the corners of the tube alter the phase distribution in the annular flow regime as well as stabilizing the annular flow regime at lower vapor velocities. To predict the heat transfer data, two correlations, each representing the physics of the specific phase distributions, are developed. A boundary layer analysis is applied for annular flow, in which the friction multiplier and dimensionless boundary layer temperature are evaluated specifically for this tube configuration. For stratified flow, documented film condensation and single-phase forced convection correlations are combined with straightforward void fraction weighting. Finally, a weighting correlation is successfully proposed to account for the all data regardless of the mix of flow regimes experienced. This weighting applies the result of a modified flow regime map developed from the flow visualizations. The final result is a practical correlation for the design of a condenser with millimeter-scale tubes.