•Progress in understanding of dropwise condensation (DWC) reviewed.•Modeling and experimental studies in external and internal DWC considered.•Applications for DWC explored and potential impact ...assessed.•Need for accurate heat transfer measurements and improved modeling highlighted.•Promoting durable DWC under industrial conditions remains a challenge.
Dropwise condensation (DWC) is a heterogeneous phase-change process where vapor changes to liquid in the form of discrete drops on an unwetted surface. The heat transfer coefficients associated with DWC are significantly higher than those of filmwise condensation (FWC) αDWC ≫ αFWC. Therefore, there is considerable interest in DWC, which could have prospects for a variety of industrial applications such as thermal power and process plants, space-conditioning and refrigeration, electronics cooling, and others. However, there have been discrepancies between the results from different researchers in this field. It was only in the late 1960s that dropwise condensation mechanism and theory started becoming formalized. This paper presents focuses on the literature on the theory, underlying mechanisms, experiments and applications of dropwise condensation. Insights gained from this review and assessment are also used to identify the need for accurate heat transfer measurements and improved modeling of DWC.
•Condensation heat transfer experiments on R-245fa/n-pentane zeotropic mixture.•Pressure drops and heat transfer coefficients measured.•Experimental results compared with correlations in the ...literature.•Mass transfer degradation of heat transfer coefficient evaluated and discussed.•Physics-based model developed to predict heat transfer coefficient (AAD ~8%).
Zeotropic mixtures are under consideration to replace pure working fluids for use in Organic Rankine Cycles for power generation from low-grade heat sources. An experimental investigation of the condensation heat transfer and frictional pressure drop of a zeotropic mixture of R245fa and n-pentane in smooth horizontal tubes was conducted. These measurements were made over mass fluxes ranging from 150 to 600 kg m−2 s−1, operating pressures from 122 to 610 kPa, and nominal vapor qualities ranging from 0.05 to 0.95. Results from this experimental study are compared with the common zeotropic modeling approaches using various correlations from the literature.
•Assessment of working pairs for adsorption heat pumps conducted.•Thermodynamic and heat transfer screening methods developed.•Screening methods are applied to over 100 potential working ...pairs.•Recommendations for working pairs in cooling and heating mode made.
Screening analyses based on thermodynamic and heat transfer principles are conducted for adsorption heat pumps to enable the comparison of working pairs on the basis of common figures of merit. After a broad survey of working pairs in the literature, 110 are analyzed for cooling mode operation, while 81 are analyzed for heating mode operation. The analyses are conducted at operating conditions based on American Heating and Refrigeration Institute (AHRI) standards. Working pairs with ammonia as the refrigerant and activated carbon as the adsorbent are found to perform well in the heating mode and yield compact systems for both modes. Certain activated carbon+ethanol working pairs are found to perform well in the cooling mode, and some metal-organic framework+ethanol pairs perform well thermodynamically in heating mode. Based on these assessments, working pairs are recommended for both modes.
An experimental and analytical study on the performance of a compact, microchannel water-carbon dioxide (CO
2) gas cooler was conducted. The experimental results addressed in Part I of this study are ...used here in Part II to develop an analytical model, utilizing a segmented approach to account for the steep gradients in the thermodynamic and transport properties of supercritical CO
2. The model predicted gas cooler heat duty with an average absolute deviation of 7.5% with varying refrigerant and water inlet conditions. The segmented model reveals that near the pseudo-critical point, there is a significant local decrease in refrigerant-side thermal resistance, which yields a sharp increase in local heat duty. The impact of this spike on gas cooler performance is analyzed. Results from this study can be used to predict the effect of changing geometric parameters of the heat exchanger without the need for expensive prototype development and testing.
•Construction of a low heat duty, low mass flux facility to measure condensation heat transfer.•Experiments conducted at different mass fluxes, channel geometry and working fluids.•Heat transfer ...coefficient is higher for a square geometry compared to circular.•R245fa has higher heat transfer coefficient than R134a and R1234ze(E) at the same conditions.•R1234ze(E) is drop-in replacement for R134a in condensation applications with heat transfer penalty < 10%.
Microchannel condensers have the potential to improve system level efficiency by increasing the heat transfer coefficient because of the small hydraulic diameters. In addition, they offer a high surface area-to-volume ratio, reducing the overall system size and fluid inventory. Understanding condensation in microchannels is essential for efficient component design, especially with the need for new low Global Warming Potential (GWP) working fluids. Condensation of refrigerants R134a, R245fa and R1234ze(E) in circular (Dh = 1.55 mm) and square (Dh = 0.98 mm) microchannels over a range of mass fluxes (50 < G < 200 kg m−2 s−1), saturation temperatures (30 < Tsat< 50°C), and wall subcooling (1.7 < ΔT < 5K) is investigated in this study. It is found that at high mass fluxes (G ≥ 150 kg m−2 s−1), R245fa has a higher heat transfer coefficient and pressure gradient than both R1234ze(E) and R134a, which have comparable heat transfer coefficients at similar conditions. In the circular channel, for a mass flux range of 50 < G < 150 kg m−2 s−1 and Tsat = 30°C, the heat transfer coefficients for R245fa, R134a, and R1234ze(E) varied between 2.3 and 6.7 kW m−2 K−1, 2.5 and 4.7 kW m−2 K−1, and 2.2 and 5.4 kW m−2 K−1, respectively.
•Effect of surfactants on two-phase flow studied using flow visualization.•Air-water mixture with 1-octanol as surfactant tested for conditions of bubble absorbers.•Flow regime transitions not ...affected, but flow structures are different.•Interfacial area increases, while bubble velocity decreases.•Expect heat and mass transfer enhancement due to increased interfacial area.
The effect of surfactants on vertical gas-liquid flow is experimentally investigated in a 12.7 mm diameter tube at conditions relevant to an ammonia-water bubble absorber. The characteristics of two-phase flow are studied using an air-water mixture, both with and without the addition of 1-octanol as the surfactant. High-speed videography is used to study the flow patterns and quantify interfacial areas and bubble velocities. Novel computer vision-based methods are used to analyze and quantify these flow parameters. The addition of 1-octanol results in enhancement in interfacial area due to the prevention of bubble coalescence leading to many small diameter bubbles. Measured values of interfacial area are compared with predictions from correlations in the literature, and agreement and differences are interpreted and discussed. The bubble velocity is measured by object tracking using the optical flow method. Surfactants lead to a decrease in bubble velocity and increase in the residence time. These are surmised to be due to the shear stresses caused by the non-uniform concentration distribution of surfactant along the bubble surface. Overall, the addition of surfactants can lead to appreciable enhancement in heat and mass transfer rates due to their effect on interfacial areas and residence times.
In this paper, (Part II of a two-part study), an investigation of void fractions for condensing flows of refrigerant R-134a in minichannels is presented. Custom digital image processing techniques ...were used to quantitatively analyze these flows in transparent minchannel geometries (square, rectangular and circular) with 2 < Dh < 4.91mm. A total of 140 void fraction values in the intermittent flow regime (slug and plug flow), the wavy flow regime, and an intermittent-wavy overlap region were measured. A new model was developed based on these measured values and predicts void fraction better than correlations from the literature.
► Void fractions for condensation of refrigerant R-134a in minichannels investigated. ► Image processing is used to analyze condensation for 2 ≤Dh ≤ 4.91 mm. ► Void fractions for intermittent and wavy flows, and an overlap region are obtained. ► A new model for void fractions for condensing flows in minichannels is developed.
•Passive enhancement of ammonia-water absorption using surfactants.•Surfactant selection criterion developed, and screening analysis presented.•Preferred additives are 500 PPM of 1-octanol or ...2-ethyl-1-hexanol.•Heat and mass transfer model predicts absorber UA increases ~30% upon addition of surfactant.
Surface-active agents or surfactants have the potential to substantially enhance heat and mass transfer in ammonia-water absorption by reducing the surface tension of the working fluid. The enhancement is caused by interfacial turbulence at the vapor-liquid interface that results from surface tension gradients. A surfactant selection criterion is developed based on the plateau value of surface tension, critical concentration, and the critical Marangoni number required to initiate interfacial turbulence. Based on this criterion, surface-active agents, and their ideal concentrations for the enhancement of ammonia-water absorption are recommended. The preferred additives are found to be 500 PPM of 1-octanol or 2-ethyl-1-hexanol. A heat and mass transfer model is developed to predict the performance of a falling-film absorber due to the addition of surfactants at conditions representative of an absorption heat pump. The model indicates that the overall conductance of the absorber is improved by ~30% by the addition of surfactants. The results from this work can guide intensification of various coupled heat and mass transfer processes using surfactants.
