Combined Visualization and Heat Transfer Measurements for Steam Flow Condensation in Hydrophilic and Hydrophobic Mini-Gaps

Citation: Chen X, Derby MM. Combined Visualization and Heat Transfer Measurements for Steam Flow Condensation in Hydrophilic and Hydrophobic Mini-Gaps. ASME. J. Heat Transfer. 2016;138(9):091503-091503-11. doi:10.1115/1.4033496.Condensation enhancement was investigated for flow condensation in mini-channels. Simultaneous flow visualization and heat transfer experiments were conducted in 0.952-mm diameter mini-gaps. An open loop steam apparatus was constructed for a mass flux range of 50–100 kg/m2s and steam quality range of 0.2–0.8, and validated with single-phase experiments. Filmwise condensation was observed in the hydrophilic mini-gap; pressure drop and heat transfer coefficients were compared to the (Kim and Mudawar, 2013, “Universal Approach to Predicting Heat Transfer Coefficient for Condensing Mini/Micro-Channel Flow,” Int. J. Heat Mass Transfer, 56(1–2), pp. 238–250) correlation and prediction was very good; the mean absolute error (MAE) was 20.2%. Dropwise condensation was observed in the hydrophobic mini-gap, and periodic cycles of droplet nucleation, coalescence, and departure were found at all mass fluxes. Snapshots of six typical sweeping cycles were presented, including integrated flow visualization quantitative and qualitative results combined with heat transfer coefficients. With a fixed average steam quality (x¯ = 0.42), increasing mass flux from 50 to 75 to 100 kg/m2s consequently reduced average sweeping periods from 28 to 23 to 17 ms and reduced droplet departure diameters from 13.7 to 12.9 to 10.3 μm, respectively. For these cases, condensation heat transfer coefficients increased from 154,700 to 176,500 to 194,800 W/m2 K at mass fluxes of 50, 75, and 100 kg/m2 s, respectively. Increased mass fluxes and steam quality reduced sweeping periods and droplet departure diameters, thereby reducing liquid thickness and increasing heat transfer coefficients

Comparison of mini-channel condensation heat transfer for R513A and R134a

There is increased interest in emerging, low global warming potential (GWP) refrigerants due to With tthe phase out of hydrofluorocarbon (HFC) refrigerants (e.g., R134a); and the need for lower global warming potential (GWP) replacements, there are many questions that still need to be answered about replacement refrigerants, including heat transfer performance. One promising alternative to R134a (GWP = 1300) is R513A (GWP = 573), which is a non-flammable mixture comprised of 44% R134a and 56% R1234yf by weight. While this low GWPR513A refrigerant is attractive due to its lower GWP and A1 flammability rating, heat transfer data are limited. Condensation experiments were conducted for R134a and R513A in a vapor compression loop using multiport (i.e., 9 parallel channels) extruded aluminum tubes; each channel’s hydraulic diameter was 0.72 mm. Experiments covered a range of average qualities (e.g., 0.2 \u3c x \u3c 0.8) and mass fluxes (e.g., 300 \u3c G \u3c 500 kg/m2s). Heat transfer coefficients and pressure drops are reported. The heat transfer coefficients of R513A were found to be very similar to R134a. Condensation data weare compared to the condensation heat transfer coefficient correlations developed by Shah (2009) and Kim and Mudawar (2013). The MAE of the Shah correlation for R513A was 22% and 6.5% for the Kim and Mudawar (2013) correlation for R513A

Permeability Analysis of Additively-Manufactured Wick Structures with Heat Exchanger Applications

Heat pipes and other heat transfer applications use capillary-driven liquid motion to enhance performance. This research uses water and a low surface tension fluid FC-40 to test additive-manufactured polymer wicks using a rateof-rise test. The rate-of-rise tests give a measure of the wicks’ performance capabilities as well as being able to calculate the wicks’ permeability and effective pore radius. Four wicks were measured having two different internal structures (i.e., 1.0 mm triangle and 1.75 mm square) and two external structures (i.e., layered and column). The 1.0 mm Triangle wicks performed better than their 1.0 mm Square counterparts for both water and FC-40. Both 1.0 mm Triangle wicks performed similarly for both water and FC-40, with the column wick (11.0 mm) performing better than the layered wick (8.98 mm). Using a least squares method from the rate-of-rise results, the permeability and effective pore radius of each wick were calculated for the 1.0 mm triangle layered wick, the 1.75 mm square layered wick, and the 1.0 mm triangle column wick. The 1.75 mm square column wick was unable to wick either liquid, so the permeability and effective pore radius were not able to be calculated. The permeability and effective pore radius for each wick were 3.00 um2 and 130.1 um, 0.95 um2 and 221.1 um, and 77.8 um2 and 1099 um, respectively. Some challenges involved with polymer additive manufacturing design and creation were also discussed

Droplet ejection and sliding on a flapping film

Citation: X. Chen, N. Doughramaji, A.R. Betz, M.M. Derby, Droplet departure and ejection on flapping films, AIP Advances, 7, 035014.Water recovery and subsequent reuse are required for human consumption as well as industrial, and agriculture applications. Moist air streams, such as cooling tower plumes and fog, represent opportunities for water harvesting. In this work, we investigate a flapping mechanism to increase droplet shedding on thin, hydrophobic films for two vibrational cases (e.g., ± 9 mm and 11 Hz; ± 2 mm and 100 Hz). Two main mechanisms removed water droplets from the flapping film: vibrational-induced coalescence/sliding and droplet ejection from the surface. Vibrations mobilized droplets on the flapping film, increasing the probability of coalescence with neighboring droplets leading to faster droplet growth. Droplet departure sizes of 1–2 mm were observed for flapping films,compared to 3–4 mm on stationary films, which solely relied on gravity for droplet removal. Additionally, flapping films exhibited lower percentage area coverage by water after a few seconds. The second removal mechanism, droplet ejection was analyzed with respect to surface wave formation and inertia. Smaller droplets (e.g., 1-mm diameter) were ejected at a higher frequency which is associated with a higher acceleration. Kinetic energy of the water was the largest contributor to energy required to flap the film, and low energy inputs (i.e., 3.3 W/m2) were possible. Additionally, self-flapping films could enable novel water collection and condensation with minimal energy input

Evapotranspiration from Spider and Jade Plants Can Improve Relative Humidity in an Interior Environment

Citation: Kerschen, E., Garten, C., Williams, K., & Derby, M. (2016). Evapotranspiration from Spider and Jade Plants Can Improve Relative Humidity in an Interior Environment. HortTechnology, 26(6), 803-810. doi: 10.21273/HORTTECH03473-16Plants in the interiorscape have many documented benefits, but their potential for use in conjunction with mechanical heating, ventilation, and air conditioning (HVAC) systems to humidify dry indoor environments requires more study. In this research, evaporation and evapotranspiration rates for a root medium control, variegated spider plants (Chlorophytum comosum), and green jade plants (Crassula argentea) were measured over 24 hours at 25% and 60% relative humidity (RH) and 20 °C to generate data for calculation of the leaf surface area and number of plants necessary to influence indoor humidity levels. Evaporation and evapotranspiration rates were higher for all cases at 25% RH compared with 60% RH. At 25% RH during lighted periods, evapotranspiration rates were ?15 g·h?1 for spider plants and 8 g·h?1 for jade plants. Spider plants transpired during lighted periods due to their C3 photosynthetic pathway, whereas jade plants had greater evapotranspiration rates during dark periods—about 11 g·h?1—due to their crassulacean acid metabolism (CAM) photosynthetic pathway. A combination of plants with different photosynthetic pathways (i.e., C3 and CAM combination) could contribute to greater consistency between evapotranspiration rates from day to night for humidification of interior spaces. Using the measured data, calculations indicated that 32,300 cm2 total spider plant leaf surface area, which is 25 spider plants in 4-inch-diameter pots or fewer, larger plants, could increase the humidity of an interior bedroom from 20% RH to a more comfortable 30% RH under bright interior light conditions

Droplet Coalescence and Freezing on Hydrophilic, Hydrophobic, and Biphilic Surfaces

Frost and ice formation can have severe negative consequences, such as aircraft safety and reliability. At atmospheric pressure, water heterogeneously condenses and then freezes at low temperatures. To alter this freezing process, this research examines the effects of biphilic surfaces (surfaces which combine hydrophilic and hydrophobic regions) on heterogeneous water nucleation, growth, and freezing. Silicon wafers were coated with a self-assembled monolayer and patterned to create biphilic surfaces. Samples were placed on a freezing stage in an environmental chamber at atmospheric pressure, at a temperature of 295 K, and relative humidities of 30%, 60%, and 75%. Biphilic surfaces had a significant effect on droplet dynamics and freezing behavior. The addition of biphilic patterns decreased the temperature required for freezing by 6 K. Biphilic surfaces also changed the size and number of droplets on a surface at freezing and delayed the time required for a surface to freeze. The main mechanism affecting freezing characteristics was the coalescence behavior.Citation: A. Van Dyke, D. Collard, M. M. Derby and A. R. Betz, "Droplet Coalescence and Freezing on Hydrophilic, Hydrophobic, and Biphilic Surfaces," Applied Physics Letters, 107, Issue 14, 201

Layered wicks enable passive transport of condensation out of cooling systems

Layered wicks enable passive transport of condensation out of cooling systems Nhicolas Aponte, Jordan Morrow, Gennifer Riley, Partha Chakraborty, Melanie M. Derby Department of Mechanical and Nuclear Engineering, Kansas State University Cooling systems, like condensers or cooling towers of a power plant, transfer heat out of a system. The cooling process often occurs through the condensation of water, which forms a liquid film that reduces heat transfer. This problem makes cooling systems larger and more costly. One approach to this problem is drop-wise condensation in which condensed water gathers in the form of droplets which can then run off, preventing the reduction of heat transfer caused by the liquid film. For this solution to be effective in industry, a hydrophobic coating would need to last over 10 years, which is difficult to achieve. The approach studied in this work uses the capillary/surface tension forces to passively transport water, which is applicable to removing liquid films from condensers. This is investigated by using a wick, which is a structure that enables the passive transport of water. In this project, we compare wicking structures with different porosity in order to design an effective wick for industrial use. The wicks used are an array of layered spheres bridged by cylindrical columns with calculated porosity of 0.35(Wick C), 0.42(Wick B), and 0.66(Wick A). The wicks are 3-D printed onto a test plate, which allows the fabrication of complex geometries. The effectiveness of the wicks is compared using the rate-of-rise method. For this method, the wicks are lowered into a water reservoir. The interactions between the wick and the water are observed and recorded under a high speed camera. Then, the height the water rises to within the wick is compared. The wicks printed for this project outline problems we did not account for. The small pore volume of the wicks made it difficult to clean out support material after being printed. Future wicks will be designed with a greater pore volume than that of Wick C. The success of this project could improve the heat transfer in space cooling systems and power plant condensers

Physical mechanisms for delaying condensation freezing on grooved and sintered wicking surfaces

Heat pipes are passive heat transfer devices crucial for systems on spacecraft; however, they can freeze when exposed to extreme cold temperatures. The research on freezing mechanisms on wicked surfaces, such as those found in heat pipes, is limited. Surface characteristics, including surface topography, have been found to impact freezing. This work investigates freezing mechanisms on wicks during condensation freezing. Experiments were conducted in an environmental chamber at 22 °C and 60% relative humidity on three types of surfaces (i.e., plain copper, sintered heat pipe wicks, and grooved heat pipe wicks). The plain copper surface tended to freeze via ice bridging—consistent with other literature—before the grooved and sintered wicks at an average freezing time of 4.6 min with an average droplet diameter of 141.9 ± 58.1 μm at freezing. The grooved surface also froze via ice bridging but required, on average, almost double the length of time the plain copper surface took to freeze, 8.3 min with an average droplet diameter of 60.5 ± 27.9 μm at freezing. Bridges could not form between grooves, so initial freezing for each groove was stochastic. The sintered wick's surface could not propagate solely by ice bridging due to its topography, but also employed stochastic freezing and cascade freezing, which prompted more varied freezing times and an average of 10.9 min with an average droplet diameter of 97.4 ± 32.9 μm at freezing. The topography of the wicked surfaces influenced the location of droplet nucleation and, therefore, the ability for the droplet-to-droplet interaction during the freezing process

Analyzing the carbon dioxide emissions of R134a alternatives in water-cooled centrifugal chillers using the life cycle climate performance framework

Introduction: To reduce greenhouse gases, the Kigali Amendment to the Montreal Protocol seeks a phasedown of hydrofluorocarbons. R134a alternatives were analyzed for use in a water-cooled chiller: R450A, R513A, R516A, R1234ze (E), R515A, and R515B.Methods: A thermodynamic model of the chiller was employed to calculate compressor power, an input to the life cycle climate performance (LCCP) framework to estimate total equivalent carbon dioxide emissions, CO2eq. Emissions were calculated for an 809 kW [230 Tons of refrigeration (RT) nameplate] water-cooled centrifugal chiller at constant cooling capacity using five power sources (i.e., coal, distillate fuel oil, natural gas, nuclear, and wind) for a median chiller lifetime of 27 years. Two chiller operating profiles were considered: one using operational data from a university campus and a second from literature based on the Atlantic Fleet operation.Results and discussion: When powered via fossil fuels, over 90% of emissions were due to the indirect emissions from energy; therefore, the global warming potential (GWP) of the refrigerant was not the primary contributor to the total CO2eq emissions. With natural gas, total LCCP emissions were reduced for R450A (7.8%), R513A (4.7%), R516A (9.4%), R1234ze (E) (10%), R515A (8.4%), and R515B (6.4%) compared to R134a for the university campus load profile. For the round-the-clock Atlantic Fleet profile, there were emission reductions for R450A (3.6%), R513A (0.25%), R516A (2.3%), R1234ze (E) (2.4%), R515A (1.5%) and R515B (2.4%) compared to R134a. When coupled with renewable energy, the indirect emissions from the chillers substantially decreased, and GWP-dependent leakage emissions accounted for up to 74% or 40% of emissions from R134a alternatives powered by wind and nuclear, respectively. For operation using the load profile from the university campus chillers, R450A had the highest coefficient of performance (COP) of 5.802, while R513A had the lowest COP (5.606). Tradeoffs between alternative refrigerants exist in terms of operation, temperature glide, size of heat exchangers, system design, flammability, cost, availability, and material compatibility. In terms of flammability, R134a, R513A, R450A, R515B and R515A are A1 (nonflammable) fluids. R450A and R516A also have temperature glides of 0.4 K and 0.056 K, respectively, which can affect condenser design. In terms of equipment modification (sizing), R513A require fewer modifications

Droplet departure modeling and a heat transfer correlation for dropwise flow condensation in hydrophobic mini-channels

Citation: Chen, X., & Derby, M. M. (2018). Droplet departure modeling and a heat transfer correlation for dropwise flow condensation in hydrophobic mini-channels. International Journal of Heat and Mass Transfer, 125, 1096–1104. https://doi.org/10.1016/j.ijheatmasstransfer.2018.04.140Droplet nucleation, growth, coalescence, and departure control dropwise condensation heat transfer. Smaller droplets are associated with higher heat transfer coefficients due to their lower liquid thermal resistances. Unlike quiescent dropwise condensation with gravity-driven droplet departure, droplet departure sizes in flow condensation are governed by flow-droplet shear forces and droplet-solid adhesive forces. This research models droplet departure, droplet size distributions, and heat transfer through single droplets under different flow conditions. Heat transfer through single droplets includes the thermal resistances at the vapor-liquid interface, temperature depression across the curved surface, conduction in the liquid droplet, and conduction through the surface promoter (e.g., Teflon). Droplet size distributions were determined for two ranges using the population balance method and power law function for small and large droplets, respectively. Droplet departure sizes (e.g., 10–500 µm) were derived using force balances between drag forces (obtained using FLUENT) and droplet-solid adhesive forces (determined using a third-order polynomial for contact angle distribution along contact line). The analytical model was compared to experimental flow condensation heat transfer data in a Teflon AF™-coated rectangular mini-gap with hydraulic diameters of 0.95 and 1.8 mm. The correlation was compared against experiments with a steam mass flux range of 35–75 kg/m2 s and quality of 0.2–0.9. There was good agreement between the model and experimental data; without any curving fitting, the mean absolute errors of the heat transfer correlation were 9.6% and 8.8% respectively for the 0.95-mm and 1.8-mm mini-gaps