Mechanical and Nuclear Engineering Faculty Research and Publications

Permanent URI for this collectionhttps://hdl.handle.net/2097/4756

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Open Access
X-ray Inspection Model Validation with Physical Dosimetry
(2023-06-30) Pfeifer, Michael P.; Simerl, Nathanael; Porter, John; McNeil, Walter J.; Bahadori, Amir A.
A non-functional printed circuit board assembly was developed using ULTIboard and a simple computational transport model was created for use with the Monte Carlo software MCNP. EBT3 film radiography was used to compare with Monte Carlo simulation to validate the board representation. The computational transport model was then revised to include connection pins, solder balls, highly attenuating internal structures, and copper trace distribution. Results from the revised model were compared to the EBT3 films to observe improvements to dose profiles. It was found that this method was useful in verifying the placement of components, as the dose profiles were observed to follow the same trends. The experiment was then repeated using XRQA2 films to achieve the same level of contrast with 1% the dose of EBT3 films. It was found that high contrast may be achieved using these films to identify major issues with the model geometry, at a cost of dose profile accuracy. A second validation method was applied to the model using 37 CaF2 thermoluminescent dosimeters (TLDs). TLD measurements were compared with the simplified and complex transport models to identify the features that have the greatest impact on simulation accuracy. The TLD calibration to CaF2 was found to be accurate within 5.6%, while calibration to dose in Si was found to be accurate within 4.7%. It was observed that the accurate representation of solder balls and proper modeling of highly attenuating internal structures had the greatest impact on simulation accuracy.
Open Access
Physical mechanisms for delaying condensation freezing on grooved and sintered wicking surfaces
(2022-08-15) Stallbaumer-Cyr, Emily M.; Derby, Melanie M.; Betz, Amy R.
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.
Open Access
Raw voxel data of frog tongue
(2022-01-20) Amare, Rohan
Open Access
A Structured Cleaving Mesh for Bioheat Transfer Application
(2020) Amare, Rohan; Bahadori, Amir A.; Eckels, Steven
The human body executes a complex control scheme in an attempt to maintain a constant core temperature. The hypothalamus acts as a thermostat that receives input signals from different parts of the body and responds with different control mechanisms to regulate heat exchange and create a homeothermic core. This entire process of receiving input signals and providing feedback response is called the human thermoregulation mechanism. Thermophysiological models such as the Fiala model [1], [2] are used to simulate this process to understand the response of the human body to its thermal surroundings. The Fiala and other models used in thermophysiological and human thermal comfort studies rely on stylized phantoms [3]. A parallel branch of research that deals with biomedical applications [4] and radiation dosimetry [5]–[6][7] makes use of computational human phantoms (CHPs). CHPs initially consisted of stylized phantoms, which were made using simple geometrical objects. With advancements in technology and available resources, the stylized phantoms were largely replaced by voxel phantoms and later by a family of hybrid phantoms. Voxel phantoms are generated from medical imaging data and provide a detailed and accurate representation of human anatomy. Hybrid phantoms are constructed using non-uniform rational B-spline (NURBS) surfaces and polygon mesh elements that provide a smooth surface for organs and the entire human body. Voxel phantoms present the challenge of a stair-step effect on curved surfaces, especially for smaller organs like eye balls, due to the voxel shape [4]. Tetrahedral mesh-based (TM-based) CHPs are used as a solution to overcome the challenges of hybrid and voxel phantoms [4], [8]. The present study focuses on using a structured form of the cleaving mesh to (1) generate a simulation domain directly from medical imaging data of the patient and (2) perform heat-transfer simulations using this domain. This method makes generation of a mesh from medical imaging data straightforward, without any need for conversion to other phantom surface representations [4], [8]. The remainder of the paper is organized into sections that outline the proposed methodology and its significance. Section II provides a short review of models used in human thermophysiological research. Challenges associated with use of these models point to the need for a model that is based on accurate representation of human anatomy.
Open Access
Characterization and numerical simulation of liquid refrigerant R-134a flow emerging from a flooded evaporator tube bundle
(2019-11-01) Asher, William E.; Eckels, Steven J.; eckels
The distribution of liquid droplets emerging from an evaporator tube bundle is characterized for refrigerant R-134a with a triangular tube arrangement with a pitch of 1.167. The purpose of this research was to improve understanding of the droplet ejection process to aid in design of evaporators typically used in larger chiller systems. A laser and camera system captured images of the evaporator headspace at varying conditions. Conventional shadowgraphy techniques were applied to recognize and match droplets for velocity calculations. The evaporator conditions varied with bundle mass fluxes of 20.3 and 40.7 kg s−1m−2, top-rows heat fluxes of 15.8 and 31.5 kWm−2, and outlet saturation temperatures of 4.4 and 12.8 °C. Conditions ranged from flooded to dryout of the top rows. Droplet number, size distribution, velocity, and liquid volume fraction are presented in the headspace above the bundle. A method to numerically duplicate the droplet loading in the headspace using CFD with a Lagrangian discrete-phase model is also presented and verified, providing a powerful design tool. Liquid distribution in the headspace is found to be a strong function of all varied properties, particularly mass flux, liquid level, and saturation temperature.
Open Access
Relationship between turbulent structures and heat transfer in microfin enhanced surfaces using large eddy simulations and particle image velocimetry
(2019-06-01) Li, Puxuan; Campbell, Matthew; Zhang, Ning; Eckels, Steven J.; eckels
Internally enhanced surfaces such as micro-fins are an important class of heat transfer enhancement in commercial applications. Many research papers discuss the design and optima of these surfaces. However, most previous studies have demonstrated only the macro relationship between the geometries of the micro-fins and heat transfer. The need for a deeper understanding of these fins arose from some currently unsolved problems that limit future development of enhanced surfaces. First, why are increases of heat transfer larger than area increases in micro-finned tubes in most cases? Second, why do internally micro-finned tubes typically have lower heat-transfer-enhanced ratios in laminar and transition flows? This work presents a novel method to analyze the detailed relationship between flow characteristics and heat transfer for one type of micro-fin. The goal of the paper was not to find a new Reynolds number-based correlation, but to find flow patterns responsible for heat transfer enhancement and understand the mechanisms that cause this. First, this paper introduces comprehensive experimental measurements including particle image velocimetry (PIV), measurement of the heat transfer coefficient and accuracy of pressure-drop measurements, all used to validate numerical approaches. Validated large eddy simulations (LES) are then used to predict flow characteristics and coherent structures (Q criterion). The numerical simulation includes both heat conduction in the metal structure and heat convection on the solid–fluid interface. Finally, the paper documents how the flow structures link with the enhancement of heat transfer in the micro-finned duct.
Open Access
Multi-objective heat transfer optimization of 2D helical micro-fins using NSGA-II
(2019-04-01) Mann, Garrett W.; Eckels, Steven; eckels
A numerical simulation of helical micro-fins is implemented in ANSYS Fluent 15. The model is scripted to automatically set up and execute given three input parameters: fin height, helix angle, and number of starts. The simulation results reasonably predict experimental pressure drop and heat transfer for multiple micro-fin geometries. A multi-objective parameter optimization is implemented based on the NSGA-II algorithm to estimate the optimal trade-off (Pareto front) between Nusselt number and friction factor of a micro-fin tube for 0.0006 < e/D < 0.045, 10 < Ns < 70, at Reynolds number of 49,013. The resulting Pareto front is analyzed and compared with several experimental data points. From the optimal results, a distinct difference in flow characteristics was identified between geometries above and below a helix angle of approximately 45°. How the Pareto front can be used to choose micro-fin geometries for different performance evaluation criterion scenarios is demonstrated. Optimal results from various existing correlations are also compared to the optimization results.
Open Access
A COMPARISON OF R-22, R-134A, R-410A AND R-407C CONDENSATION PERFORMANCE IN SMOOTH AND ENHANCED TUBES: PART I, HEAT TRANSFER
Eckels, Steven J.; Tesene, Brian A.; eckels
Local and average heat transfer coefficients during condensation are reported for R-22, R-134a, R-410a, and R-407c in one smooth tube and three enhanced surface tubes. The test tubes included a 3/8 inch (9.52 mm) outer diameter smooth tube, a 3/8 inch (9.52 mm) outer diameter micro-fin tube, a 5/16 inch (7.94 mm) outer diameter micro-fin tube, and a 5/8 inch (15.88 mm) outer diameter micro-fin tube. The local and average heat transfer coefficients were measured over a mass flux range of 92,100 lb/ft2 hr (125 kg/m2 s) to 442,200 lb/ft2 hr (600 kg/m2 s), and at saturation temperatures of 104 F (40 C) and 122 F (50 C). A comparison of the performance of the different refrigerants reveals that R-134a has the highest heat transfer performance followed by R-22 and R-410a which have similar performances. In general, R-407c had the lowest performance of the refrigerants tested. The micro-fin tube more than doubles the heat transfer coefficient compared to the smooth tube for all refrigerants at the low mass fluxes, but only increases the heat transfer coefficients by 50% at the highest mass flux tested. The measured heat transfer coefficients are also compared with a number of correlations for condensation.
Open Access
Heat Transfer Coefficients and Pressure Drops for R-134a and an Ester Lubricant Mixture in a Smooth Tube and a Micro-Fin Tube
(ASHRAE, 1998) Steven, Eckels J.; Doerr, Tom M.; Pate, Michael B.; eckels
This paper reports average heat transfer coefficients and pressure drops during evaporation and condensation of mixtures of R-134a and a 150 SUS penta erythritol ester branched-acid lubricant. The smooth tube and micro-fin tube tested in this study had outer diameters of 9.52 mm (3/8 in.). The micro-fin tube had 60 fins, a fin height of 0.2 mm (0.008 in), and a spiral angle of 18o . The objective of this study was: 1) to evaluate the effectiveness of the micro-fin tube with R-134a, and 2) to determine the effect of circulating lubricant. The experimental results show that the micro-fin tube has distinct performance advantages over the smooth tube. For example, the average heat transfer coefficients during evaporation and condensation in the micro-fin tube were 50% to 200% higher than those for the smooth tube, while the average pressure drops were on average only 10% to 50% higher. The experimental results indicate that the presence of lubricant degrades the average heat transfer coefficients during both evaporation and condensation at high lubricant concentrations. Pressure drops during evaporation increased with the addition of lubricant in both tubes. For condensation, pressure drops were unaffected by additions of lubricant.
Open Access
Radioactively driven colloids: A special case of anomalous diffusion
(2018-10-01) Wilson, Graham; Bahadori, Amir A.; Bindra, Hitesh; bahadori
A novel concept of self-propelled, radioactively driven colloids is introduced. The focus of this paper is on assessing the impact of alpha emissions on colloidal kinematics. Using Langevin dynamics and a random walk model, a theory has been developed to describe this motion. This theory shows a special case of anomalous diffusion. Numerical simulations have substantiated the theory. It is shown that alpha-particle emission can significantly affect the motion of colloidal particles, although a very short-lived radioisotope is required.
Open Access
Evaporation and Condensation of HFC-134a and CFC-12 in a Smooth Tube and a Micro-Fin Tube (RP-630)
(1991) Eckels, Steven J.; Pate, M.B.
Evaporation and condensation heat transfer coefficients were measured for smooth and micro-fin tubes with HFC-134a and CFC-12. Micro-fin tubes are internally enhanced tubes that are characterized by numerous small fins that spiral down the tube. For example, in this study, the micro-fin tube had 60 fins with a height of 0.2 mm and an 17° spiral angle. Heat transfer measurements were performed on 3.67 m (12 ft) long tubes with inside diameters of 8.0 mm (0.31 in). Test conditions varied from 5°C to 15°C for evaporation and 30°C to 50°C for condensation. The refrigerant mass flux was varied from 130 kg/m²·s (95,860 lb/ft²·h) to 400 kg/m²·s (294,000 lb/ft²·h). When HFC-134a was compared to CFC-12 at similar mass fluxes in smooth tubes, the evaporation and condensation heat transfer coefficients were about 40% and 25% higher, respectively. A more relevant comparison of heat transfer coefficients is at equivalent cooling (or heating) capacities. In this case, the HFC-134a heat transfer coefficients were about 10% higher than CFC-12 values for both evaporation and condensation. The micro-fin tube produced higher heat transfer coefficients and pressure drops for all conditions when compared to the smooth tube. For example, for HFC-134a, heat transfer enhancement factors (defined as the convective heat transfer coefficients for the micro-fin tube divided by the value for the smooth tube measured at similar conditions) varied from 1.5 to 2.5 during evaporation and from 1.8 to 2.5 duringcondensation. Pressure drop penalty factors (defined similarly to enhancement factors)for both refrigerants were usually less than the heat transfer enhancement factors. However, in the case of HFC-134a at the lowest temperature and highest mass flux, the penalty factor slightly exceeded the enhancement factor.
Open Access
An experimental comparison of evaporation and condensation heat transfer coefficients for HFC-134a and CFC-12
(1991) Eckels, Steven J.; Pate, M.B.; eckels
Experimental heat transfer coefficients are reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation. These heat transfer coefficients were measured in a horizontal, smooth tube with an inner diameter of 8.0 mm and a length of 3.67 m. The refrigerant in the test-tube was heated or cooled by using water flowing through an annulus surrounding the tube. Evaporation tests were performed for a refrigerant temperature range of 5–15°C with inlet and exit qualities of 10 and 90%, respectively. For condensation tests, the refrigerant temperature ranged from 30 to 50°C, with et and exit qualities of 90 and 10%, respectively. The mass flux was varied from 125 to 400 kg m−2 s−1 for all tests. For similar mass fluxes, the evaporation and condensation heat transfer coefficients for HFC-134a were significantly higher than those of CFC-12. Specifically, HFC-134a showed a 35–45% increase over CFC-12 for evaporation and a 25–35% increase over CFC-12 for condensation. Résumé On rapporte les coefficients de transfert de chaleur expérimentaux, pour le HFC-134a et le CFC-12, au cours d'un écoulement monophasique, à l'intérieur d'un double tube, et au cours de l'évaporation et de la condensation. Ces coefficients de transfert de chaleur ont été mesurés dans un tube lisse horizontal d'un diamètre intérieur de 8 mm et d'une longueur de 3,67 m. Le frigorigène dans le tube d'essai était chauffé ou refroidi par circulation d'eau dans un espace annulaire entourant le tube. Pour l'évaporation, les essais ont été effectués dans une plage de températures du frigorigène comprises entre 5 et 15°C, avec des qualités d'entrée et de sortie de 10 et 90% respectivement. Pour les essais de condensation, les températures du frigorigène étaient comprises entre 30 et 50°C, avec des qualités d'entrée et de sortie de 90 et 10% respectivement. Le flux massique a varié de 125 à 400 kg m−2 s−1 pour tous les essais. Pour des flux massiques similaires, les coefficients de transfert de chaleur d'évaporation et de condensation, pour le HFC-134a, étaient nettement supérieurs à ceux du CFC-12. Plus précisément, le coefficient de transfert de chaleur à l'évaporation du HFC-134a était de 35 à 45% supérieur à celui du CFC-12, et le coefficient de transfert de chaleur à la condensation supérieur de 25 à 35%.
Open Access
In-tube Heat Transfer and Pressure Drop of HFC-134a and Ester Lubricant Mixtures in a Smooth Tube and a Micro-Fin Tube: Part I Evaporation
(1994) Eckels, Steven J.; Doerr, T.M.; Pate, M.B.
In-tube heat transfer coefficients and pressure drops during evaporation are reported for mixtures of refrigerant R134a and a penta erythritol ester mixed-acid lubricant. The ester lubricant was tested at viscosities of 169 SUS and 369 SUS over a lubricant concentration range of 0% to 5% in both a smooth tube and a micro-fin tube. The average saturation temperature used was 1 deg C (33.8 deg F). Measurements were taken for the refrigerant-lubricant mixture over a mass flux range of 85 kg/m2.s (62,700 lb/ft2Ã h) to 375 kg/m2Ã s (276,640 lb/ft2Ã h) in test tubes with an outer diameter of 9.52mm (three/eighths in.). Heat transfer coefficients during evaporation increased at low concentrations of the 169-SUS ester lubricant and then dropped off at high lubricant concentrations in both the smooth tube and the micro-fin tube. The higher viscosity 369-SUS lubricant decreased the heat transfer coefficients in both tubes over the range of lubricant concentrations tested. Pressure drops during evaporation increased in both the smooth tube and the micro-fin tube with the addition of ester lubricant viscosity. The heat transfer coefficients for the micro-fin tube were 100% to 50% higher than those for the smooth tube, with the higher values occurring at low mass fluxes. Pressure drops in the micro-fin tube were 10 to 20% higher than those in the smooth tube.
Open Access
Oil-water flow regimes in 4-mm borosilicate glass and fluorinated ethylene propylene channels: Effects of wall wettability
(2019) Riley, G.A.; Bultongez, K.K.; Derby, Melanie M.
Formation water, found in oil deposits, is highly corrosive. By utilizing flow phenomena and surface tension forces in smaller channels (e.g., Eötvös number less than one), these fluids can be separated, thus altering corrosion and the pressure required for transport. This research investigates the effects of wall wettability on oil-water flow regimes and pressure drops. Oil-water flows were studied in 3.5-mm hydrophilic borosilicate glass and 4.0-mm hydrophobic fluorinated ethylene propylene (FEP) channels using Parol 100 mineral oil (i.e., density of 840 kg/m3 and viscosity of 0.0208 Pa s) and tap water (i.e., 997 kg/m3 and a viscosity of 0.001 Pa s). For these oil-water combinations, glass was water wetting (i.e., contact angle of 67° for a water droplet submerged in oil on glass) and FEP was water repelling (i.e., contact angle of 93° for a water droplet submerged in oil on FEP) under static conditions. Flow regimes and pressure drops were recorded for a range of oil superficial velocities [i.e., 0.31–3.7 m/s (glass) and 0.23–2.7 m/s (FEP)] and water superficial velocities [i.e., 0.080m/s–5.5 m/s (glass) and 0.060–5.5 m/s (FEP)]. Stratified, intermittent, annular, and dispersed flow regimes were observed in both tubes. Additional inverted and dual flow regimes were observed in the hydrophobic FEP; oil wetted the wall in inverted flows, and flow regimes occurred inside of another flow regime in dual flows (e.g., inverted-annular intermittent). The modified Weber number indicated whether the walls were wetted by oil, mixed oil and water, or water. Pressure drops were found to be correlated to the flow regime with increased pressure drops observed when oil fully or partially wetted the wall.
Open Access
Droplet departure modeling and a heat transfer correlation for dropwise flow condensation in hydrophobic mini-channels
(2018) Chen, Xi; Derby, Melanie M.
Droplet 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.
Open Access
Condensation heat transfer in square, triangular, and semi-circular mini-channels
(2012-01-12) Derby, Melanie M.; Lee, Hee Joon; Peles, Yoav; Jensen, Michael K.
Condensation heat transfer coefﬁcients in mini-channels were measured with smaller measurement uncertainties than previously obtained using three specially designed copper test sections. Single-phase experiments validated the approach. Data are reported for R134a in 1 mm square, triangular, and semicircular multiple parallel minichannels cooled on three sides. A parametric study was conducted over a range of conditions for mass ﬂux, average quality, saturation pressure, and heat ﬂux. Mass ﬂux and quality were determined to have signiﬁcant effects on the condensation process, even at lower mass ﬂuxes, while saturation pressure, heat ﬂux, and channel shape had no signiﬁcant effects. The lack of shape effects were attributed to the three-sided cooling boundary conditions. Because there was no signiﬁcant surface tension enhancement, the macro-scale Shah (2009) [26] correlation best predicted the data, with a mean average error (MAE) of 20–30% for all geometries.
Open Access
Mini-channel flow condensation enhancement through hydrophobicity in the presence of noncondensable gas
(2017-12-01) Chen, Xi; Morrow, Jordan A.; Derby, Melanie M.
Steam condensation is important for a broad range of industrial applications, including power generation and nuclear containment systems. The presence of noncondensable gases in these systems significantly reduces heat transfer, prompting the need for condensation heat transfer enhancement. Steam was condensed in the presence of nitrogen in hydrophilic and hydrophobic 1.82-mm rectangular mini-channels for a range of experimental conditions: steam mass flux (i.e., 35–75kg/m2s), steam quality (i.e., 0.3
Open Access
Vibration-Enhanced Droplet Motion Modes: Simulations of Rocking, Ratcheting, Ratcheting With Breakup, and Ejection
(2019-01-07) Huber, Ryan A.; Campbell, Matthew; Doughramaji, Nicole; Derby, Melanie M.
Power plant water usage is a coupling of the energy–water nexus; this research investigates water droplet motion, with implications for water recovery in cooling towers. Simulations of a 2.6 mm-diameter droplet motion on a hydrophobic, vertical surface were conducted in XFLOW using the lattice Boltzmann method (LBM). Results were compared to two experimental cases; in the first case, experimental and simulated droplets experi-enced 30 Hz vibrations (i.e.,60.1 mm x-direction amplitude,60.2 mm y-direction amplitude) and the droplet ratcheted down the surface. In the second case, 100 Hz vibrations(i.e.,60.8 mm x-direction amplitude,60.2 mm y-direction amplitude) caused dropletejection. Simulations were then conducted for a wide range of frequencies (i.e., 10–100Hz) and amplitudes (i.e.,60.018–50 mm), resulting in maximum accelerations of 0.197–1970 m/s2. Under low maximum accelerations (e.g.,<7 m/s2), droplets rocked upward and downward in rocking mode, but did not overcome the contact angle hysteresis and, therefore, did not move. As acceleration increased, droplets overcame the contact angle hysteresis and entered ratcheting mode. For vibrations that prompted droplet motion, droplet velocities varied between 10–1000 mm/s. At capillary numbers above approximately 0.0044 and Weber numbers above 3.6, liquid breakup was observed in ratcheting droplets (e.g., the formation of smaller child droplets from the parent droplet). It was noted that both x- and y-direction vibrations were required for droplet ejection
Open Access
Investigation of oil-water flow regimes and pressure drops in mini-channels
(2017-11-01) Bultongez, Kevin K.; Derby, Melanie M.
Oil-water flow regimes were studied in 2.1 mm and 3.7 mm borosilicate glass tubes; both tubes exhibit Eötvös numbers less than one and therefore surface tension forces may be more important in these mini-channels compared to larger diameter tubes. A closed-loop, adiabatic experimental apparatus was constructed and validated using water. This study focused on tap water and two mineral oils (i.e., Parol 70 and 100) with a density of 840 kg/m3 but a factor of two difference in viscosity. Experiments included a wide range of oil superficial velocities (e.g., 0.84–6.84 m/s for D = 2.1 mm and 0.27–3.30 m/s for D = 3.7 mm) and water superficial velocities (e.g., 0.21–7.69 m/s for D = 2.1 mm and 0.07–4.96 m/s for D = 3.7 mm). Stratified, annular, intermittent, and dispersed flow regimes were observed in both tubes, although the annular flow regime was more prevalent in the smaller tube. Pressure drops increased with decreasing tube diameter and were flow regime dependent. Flow maps were created for these mini-channels and equations adapted from Brauner and Maron (1999) were used to predict the flow regime transitions. The effects of viscosity were modest, although increased oil viscosity enhanced stability of oil-water flows.
Open Access
Condensation heat transfer on patterned surfaces
(2013) Chatterjee, Abhra; Derby, Melanie M.; Peles, Yoav; Jensen, Michael K.
An experimental study of condensation heat transfer was carried out on a 25.4mm diameter surface using steam as the condensing fluid. Three surface conditions were studied: hydrophilic, hydrophobic, and a surface with patterns of distinct hydrophilic and hydrophobic regions. The effects of inlet vapor velocity, mass flux, and hydraulic diameter on the heat transfer coefficients were investigated. The inlet vapor velocity was varied from about 0.05m/s to about 5m/s and the hydraulic diameter was varied from 4.5mm to 32.5mm. Depending on the surface condition, the heat transfer coefficients showed different responses to the varying parameters of the experiments. For the hydrophilic surface, the heat transfer coefficient was observed to be up to 2.5 times lower than that for the hydrophobic surface with all other parameters unaltered. On the other hand, the surfacewith a pattern of distinct hydrophobic and hydrophilic regions showed heat transfer coefficients that were higher than that of the hydrophilic surface and lower than that of the hydrophobic surface. In both the patterned and the hydrophobic surfaces, the heat transfer coefficient was observed to increase significantly with mass flux, while for the hydrophilic surface, the heat transfer coefficient was observed to be affected much less by the mass flux. In all cases, the heat transfer coefficients increased with increasing heat flux and decreased with increasing wall sub-cooling. The effect of average quality of the steam showed little effect on the heat transfer coefficients.