L. Consolini

Mechanisms of Boiling in Micro-Channels: Critical Assessment

Numerous characteristic trends and effects have been observed in published studies on two-phase micro-channel boiling heat transfer. While macro-scale flow boiling heat transfer may be decomposed into nucleate and convective boiling contributions, at the micro-scale the extent of these two important mechanisms remains unclear. Although many experimental studies have proposed nucleate boiling as the dominant micro-scale mechanism, based on the strong dependence of the heat transfer coefficient on the heat flux similar to nucleate pool boiling, they fall short when it comes to actual physical proof. A strong presence of nucleate boiling is reasonably associated to a flow of bubbles with sizes ranging from the microscopic scale to the magnitude of the channel diameter. The bubbly flow pattern, which adapts well to this description, is observed, however, only over an extremely limited range of low vapor qualities (typically for quality less than 0.01–0.05). Furthermore, at intermediate and high vapor qualities, when the flow assumes the annular configuration and a convective behavior is expected to dominate the heat transfer process, the experimental evidence yields entirely counterintuitive results, with heat transfer coefficients often decreasing with increasing vapor quality rather than increasing as in macro-scale channels, and with a much diminished heat flux dependency compared with would be expected. In summary, convective boiling in micro-channels has been revealed to be much more complex than originally thought. The present review aims at describing and analyzing the boiling mechanisms that have been proposed for two-phase micro-channel flows and confronting them with the available experimental heat transfer results, while highlighting those questions that, to date, remain unanswered.

On the Prediction of Heat Transfer in Micro-Scale Flow Boiling

Xu et al. have recently published a set of results for boiling heat transfer measurements in a multi-channel micro-scale evaporator for flow boiling of acetone in triangular cross-section channels (hydraulic diameter of 155.4 mm). In the present collaboration, we assess our current capability to predict this independent flow boiling data set with a fluid not in the original database and also much smaller in size using the phenomenological three-zone model of Thome, Dupont, and Jacobi. The method models boiling in small diameter channels in the elongated bubble/slug flow regime. The boiling data falling in this regime are identified here using a new micro-scale flow pattern map proposed by Revellin in order to utilize only test data corresponding to the elongated bubble flow mode. The decrease of the measured wall temperature due to the heat spread by longitudinal conduction through the heat sink was investigated through a finite differences analysis. In addition, a data reduction procedure different than that one used by Xu et al. was used and, consequently, some differences in the heat transfer behavior were found. Based on the present database, a new set of empirical parameters for the three-zone model was proposed. The conjugated effect of flow pattern and bubble/slug frequency on the heat transfer coefficient was also investigated.

Heat Transfer in Confined Forced-Flow Boiling

Remarkably different behaviors are found when comparing micro-scale flow boiling heat transfer data by distinct authors, even under similar experimental conditions. Such differences are almost certainly related to the complexity of confined forced-flow boiling. Certain aspects of the phenomenon, which are negligible in the macro-scale, become surprisingly relevant when the system size becomes small. From the results reported in the literature on the thermal-fluid features of evaporating flows in small channels, the following study presents a discussion concerning convective boiling heat transfer, highlighting the aspects that are characteristic to confined two-phase flows.

Void Fraction and Two-Phase Pressure Drops for Evaporating Flow over Horizontal Tube Bundles

A study has been performed to assess the accuracy of several existing void fraction and pressure drop correlations for diabatic, two-phase, shell-side flow over a horizontal tube bundle. The void fraction predictions have been applied in the computation of the static and momentum components of the two-phase pressure drop and have been compared to measured data for diabatic, vertical, up-flow through a twenty-tube bundle evaporator for refrigerants R-134a, R-410A, and R-507A. The tests were made with bundles comprised of plain tubes, low-finned tubes, and enhanced boiling tubes. The recent correlation of Feenstra et al. [5] has been found to perform quite well. The void fraction predictions have also been compared to data for flow with oil in the refrigerant. The results show that the static pressure drop is much lower with oil due to the foaming of the refrigerant/oil mixture. Finally, by back calculating the expected frictional pressure drops from the measured data, the investigation has evaluated the leading method for predicting frictional losses for two-phase flow over tube bundles, and a new one has been proposed, applicable to low-mass velocities that are typical of flooded evaporators.

Microscale Adiabatic Gas–Liquid Annular Two-Phase Flow: Analytical Model Description, Void Fraction, and Pressure Gradient Predictions

The study is devoted to the modeling of microscale adiabatic gas–liquid annular two-phase flow. The turbulent diffusion of momentum in the annular liquid film is assumed to be governed by the conditions near the channel wall, in analogy with single-phase turbulent bounded flow. This allows the universal velocity profile for single-phase turbulent flow to be extrapolated to the annular liquid film for the prediction of the local velocity. Conservation of mass applied to the liquid film allows the calculation of the average liquid film thickness, which in turn yields the void fraction. Once the void fraction is known, conventional one-dimensional, two-fluid modeling can be applied to predict all the relevant hydrodynamic parameters, an approach applied previously to macrochannel two-phase flow that in the present article is extended to microchannels. In the article, the analytical model is described and applied to an experimental database containing about 1100 data points for refrigerants R-134a and R245fa flowing through three horizontal circular glass microchannels of inner diameters 0.52 mm, 0.80 mm, and 1.0 mm, respectively. The database includes the pressure drop, mass flow rate, and vapor quality and covers operating pressures from 155 to 877 kPa, mass fluxes from 277 to 2026 kg m−2 s−1 and vapor qualities from 0.07 to 0.92. In particular, the analytical results regarding the void fraction are shown to compare favorably with macroscale empirical correlations extrapolated to microchannels, while the two-phase friction factor is successfully correlated using just one dimensionless flow parameter (defined as the ratio of a liquid film Reynolds number to a gas core Weber number), allowing a satisfactory prediction of the measured pressure gradients.

Mechanisms Of Boiling In Microchannels: Critical Assessment

Numerous characteristic trends and effects have been observed in published studies on two-phase micro-channel boiling heat transfer. While macro-scale flow boiling heat transfer may be decomposed into nucleate and convective boiling contributions, at the micro-scale the extent of these two important mechanisms remains unclear. Although many experimental studies have proposed nucleate boiling as the dominant micro-scale mechanism, based on the strong dependence of the heat transfer coefficient on the heat flux similar to nucleate pool boiling, they fall short when it comes to actual physical proof. A strong presence of nucleate boiling is reasonably associated to a flow of bubbles with sizes ranging from the microscopic scale to the magnitude of the channel diameter. The bubbly flow pattern, which well adapts to this description, is observed however only over an extremely limited range of low vapor qualities (typically for x <0.01–0.05). Furthermore, at intermediate and high vapor qualities, when the flow assumes the annular configuration and a convective behavior is expected to dominate the heat transfer process, the experimental evidence yields entirely counter intuitive results, with heat transfer coefficients often decreasing with increasing vapor quality rather than increasing as in macro-scale channels, and with a much diminished heat flux dependency as would be expected. In summary, convective boiling in micro-channels has revealed to be much more complex than originally thought. The present review aims at describing and analyzing the boiling mechanisms that have been proposed for two-phase micro-channel flows, confronting them with the available experimental heat transfer results, while highlighting those questions that, to date, remain unanswered.

Prediction of Critical Heat Flux in Microchannels

An overview of the state-of-the-art of predicting critical heat flux during saturated flow boiling in microchannels is presented. First, a selection of experimental results is described for single channels and for multi-channels in parallel, including non-circular channel shapes. Next, the various empirical methods for predicting CHF are presented and discussed. Then, the theoretically based model of Revellin and Thome for microchannels, including prediction of CHF under hot spots, is described and discussed. Finally, some overall comments on the status of CHF modeling and experi-mentation are provided.