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.     

Two-Phase Frictional Pressure Drop and Flow Boiling Heat Transfer for R245fa in a 2.32-mm Tube

Experimental two-phase frictional pressure drop and flow boiling heat transfer results are presented for a horizontal 2.32-mm ID stainless-steel tube using R245fa as working fluid. The frictional pressure drop data was obtained under adiabatic and diabatic conditions. Experiments were performed for mass velocities ranging from 100 to 700 kg m^?2 s^?1, heat flux from 0 to 55 kW m^?2, exit saturation temperatures of 31 and 41°C, and vapor qualities from 0.10 to 0.99. Pressures drop gradients and heat transfer coefficients ranging from 1 to 70 kPa m^?1 and from 1 to 7 kW m^?2 K^?1 were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity, and vapor quality. Five frictional pressure drop predictive methods were compared against the experimental database. The Cioncolini et al. (2009) method was found to work the best. Six flow boiling heat transfer predictive methods were also compared against the present database. Liu and Winterton (1991), Zhang et al. (2004), and Saitoh et al. (2007) were ranked as the best methods. They predicted the experimental flow boiling heat transfer data with an average error around 19%.     

Two-Phase Flow and Heat Transfer across Horizontal Tube Bundles‐A Review

This paper presents a state-of-the-art review of two-phase flow and flow boiling across horizontal tube bundles. The review covers studies related to the dynamic aspects of two-phase flow on the shell side of staggered and in-line tube bundles for upward, downward, and side-to-side flows (i.e. the evaluation of void fraction, two-phase flow behaviors and pressure drops). Heat transfer experimental work and heat transfer prediction methods on tube bundles in cross-flow for plain, low-fin, and enhanced boiling tubes are also covered. The proposed flow pattern maps and semi-empirical correlations for predicting void fraction and frictional pressure drop are critically described. These prediction methods are generally based on experimental results for adiabatic air-water flows, and noticeable discrepancies are revealed in the results provided by them. This study reveals that before now, there were no heat transfer prediction methods that can be recommended as a general design tool. Finally, this study suggests further research focusing on the development of representative databanks and prediction methods.     

Critical Heat Flux of R134a and R245fa Inside Small-Diameter Tubes

This article presents new experimental critical heat flux results under saturated flow boiling conditions for a macro-/microscale tube. The data were obtained in a horizontal 2.20-mm inside diameter stainless-steel tube with heating lengths of 361 and 154 mm, R134a and R245fa as working fluids, mass velocities ranging from 100 to 1500 kg/m²-s, critical heat flux from 25 to 300 kW/m², exit saturation temperatures of 25, 31, and 35°C, and critical vapor qualities ranging from 0.55 to 1. The experimental results show that critical heat flux (CHF) increases with increasing mass velocity and inlet subcooling but decreases with increasing saturation temperature and heated length. The data also indicated a higher CHF for R245fa when compared with R134a at similar conditions. The experimental data were compared against four CHF predictive methods and the results of the comparisons are reported.     

Saturated flow boiling heat transfer and critical heat flux in small horizontal flattened tubes

This paper presents experimental results for flow boiling heat transfer coefficient and critical heat flux (CHF) in small flattened tubes. The tested flattened tubes have the same equivalent internal diameter of 2.2 mm, but different aspect height/width ratios (H/W) of ¼, ½, 2 and 4. The experimental data were compared against results for circular tubes using R134a and R245fa as working fluids at a nominal saturation temperature of 31 °C. For mass velocities higher than 200 kg/m²s, the flattened and circular tubes presented similar heat transfer coefficients. Such a behavior is related to the fact that stratification effects are negligible under conditions of higher mass velocities. Heat transfer correlations from the literature, usually developed using only circular-channel experimental data, predicted the flattened tube results for mass velocities higher than 200 kg/m²s with mean absolute error lower than 20% using the equivalent diameter to account for the geometry effect. Similarly, the critical heat flux results were found to be independent of the tube aspect ratio when the same equivalent length was kept. Equivalent length is a new parameter which takes into account the channel heat transfer area. The CHF correlations for round tubes predicted the flattened tube data relatively well when using the equivalent diameter and length. Furthermore, a new proposed CHF correlation predicted the present flattened tube data with a mean absolute error of 5%.     

Experimental study of nucleate boiling of halocarbon refrigerants on cylindrical surfaces

The present paper reports the results of an experimental investigation of saturated pool boiling of halocarbon refrigerants on cylindrical surfaces of different materials. Experiments covered a wide range of reduced pressures and heat fluxes, being carried out on copper, brass and stainless steel surfaces with different finishing conditions. The obtained results are discussed with regard to the controlled physical and operational parameters of the investigation. An empirical correlation is proposed in terms of reduced pressures. The performance of the correlation can be deemed adequate, considering that it compares well with experimental results of different authors.     

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.     

State-of-the-art of two-phase flow and flow boiling heat transfer and pressure drop of CO2 in macro- and micro-channels

A comprehensive review of flow boiling heat transfer and two-phase flow of CO₂ is presented that covers both macro-channel tests (diameters greater than about 3 mm) and micro-channel investigations (diameters less than about 3 mm). The review addresses flow boiling heat transfer experimental studies, macro- and micro-scale heat transfer prediction methods for CO₂ and comparisons of these methods to the experimental database, highlighting the various limitations of current approaches and the divergence of some data sets from others. In addition, two-phase flow pattern results available in the literature are summarized and compared to some of the leading flow pattern maps, showing significant deviations for CO₂ from the maps prepared for other fluids at lower pressures. Available two-phase pressure drop data for CO₂ are also compared to leading prediction methods.     

New prediction methods for CO2 evaporation inside tubes: Part II—An updated general flow boiling heat transfer model based on flow patterns

Corresponding to the updated flow pattern map presented in Part I of this study, an updated general flow pattern based flow boiling heat transfer model was developed for CO₂ using the Cheng–Ribatski–Wojtan–Thome [L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside horizontal tubes, Int. J. Heat Mass Transfer 49 (2006) 4082–4094; L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, Erratum to: “New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside tubes” [Heat Mass Transfer 49 (21–22) (2006) 4082–4094], Int. J. Heat Mass Transfer 50 (2007) 391] flow boiling heat transfer model as the starting basis. The flow boiling heat transfer correlation in the dryout region was updated. In addition, a new mist flow heat transfer correlation for CO₂ was developed based on the CO₂ data and a heat transfer method for bubbly flow was proposed for completeness sake. The updated general flow boiling heat transfer model for CO₂ covers all flow regimes and is applicable to a wider range of conditions for horizontal tubes: tube diameters from 0.6 to 10 mm, mass velocities from 50 to 1500 kg/m² s, heat fluxes from 1.8 to 46 kW/m² and saturation temperatures from ?28 to 25 °C (reduced pressures from 0.21 to 0.87). The updated general flow boiling heat transfer model was compared to a new experimental database which contains 1124 data points (790 more than that in the previous model [Cheng et al., 2006, 2007]) in this study. Good agreement between the predicted and experimental data was found in general with 71.4% of the entire database and 83.2% of the database without the dryout and mist flow data predicted within ±30%. However, the predictions for the dryout and mist flow regions were less satisfactory due to the limited number of data points, the higher inaccuracy in such data, scatter in some data sets ranging up to 40%, significant discrepancies from one experimental study to another and the difficulties associated with predicting the inception and completion of dryout around the perimeter of the horizontal tubes.