Film Heat Transfer Coefficient Research Articles

Improvement in heat transfer and flow pattern of sprayed falling on horizontal tubes with rib structure for a sewage source heat pump

The falling film flow and heat transfer characteristics are critical to improve the performance of spray heat exchangers in a sewage source heat pump (SSHP), which is influenced by the operating and structural parameters. Therefore, considering the unique flow patterns of wastewater falling film caused by the oily content, and the influence of heat exchange tubes with surface structures on flow and heat transfer, two types of heat exchanger tubes with axial and circumferential ribs were designed and the falling flow over tubes were investigated through CFD method using VOF model. The falling film flow pattern, liquid film coverage and heat transfer coefficient (HTC) outside the ribbed heat exchange tubes were investigated, and the influence of rib structures and Re were analyzed. It was found that oily wastewater liquid film spreaded differently depending on the rib structures, and circumferential ribs of case 2 improved the flow pattern by increasing film coverage, which obviously affected the tube HTC. At higher Re, the ribbed tubes displayed higher potential in better heat transfer performance. Therein, the circumferential ribbed tubes showed highest average HTC, with up to 31.9 % higher than smooth tubes, which can be further enhanced by increasing Re. This study provides a foundation for enhancing the heat transfer performance of spray heat exchangers in sewage source heat pumps through the design and modification of tube surface structures.

A prediction model based on data-driven method for velocity and heat transfer coefficient of falling-film liquid on horizontal tube

The prediction of liquid film velocity and heat transfer coefficient of horizontal-tube falling-film flow is important for the investigation and enhancement of the flow and heat transfer behavior. To date, the prediction mainly relies on semi-empirical-semi-theoretical formulas based on data fitting. This prediction method has many limitations and assumptions on working conditions, and the error is large when it is extended to practical applications. In this paper, a support vector regression machine prediction method based on sine cosine algorithm optimization is proposed to improve the prediction accuracy of the liquid film velocity and heat transfer coefficient. And the database was established by conducting horizontal-tube falling-film flow and heat transfer experiments. Results show that the support vector regression model has the best prediction performance with small sample size in comparison with other four machine learning methods and traditional correlation based on Least Square method. The RMSEs of liquid film prediction model and heat transfer prediction model are only 0.0281 and 0.0117 respectively. And the MAPEs are 8.58 and 2.63 respectively. The liquid film velocity and heat transfer coefficient were accurately predicted based on relevant parameters. The importance of these parameters was also analyzed.

Numerical study of falling film flow and heat transfer on the horizontal tube with a porous coating layer

Porous surfaces often exhibit exceptional performance in liquid spreading and heat transfer, making them promising candidates for enhancing falling film heat transfer on solid surfaces. This paper delves into the flow and heat transfer characteristics occurring within falling films on horizontal tubes coated with a porous layer. To achieve this, a three-dimensional numerical model that couples the Volume of Fluid (VOF) method with porous media considerations was employed. The investigation comprehensively covers film spreading behavior, flow patterns, film velocity, film thickness, temperature distribution, and heat transfer coefficients, for both plain and porous tubes. Furthermore, we delve into the effects of key parameters, such as porosity, layer thickness, and material properties, on the overall heat transfer process. The results demonstrate the accuracy of our model in reproducing film thickness and heat transfer coefficients within falling films on horizontal porous tubes. When comparing plain tubes to porous ones, some key differences emerge. Specifically, the liquid film on porous tubes spreads more slowly, leading to increased film thickness and fewer occurrences of dry patches. Additionally, porous tubes result in lower film velocity and a more uniform temperature field compared to plain tubes. Interestingly, the findings reveal that smaller porosities yield thicker films and higher heat transfer coefficients, particularly when the porosity is below 0.4. Moreover, thicker porous layers result in thinner films, higher wetting ratios, and increased heat transfer coefficients. Among the materials we tested, aluminum and copper porous layers exhibit the highest heat transfer performance, while steel demonstrates the lowest.

Experimental and numerical study on the falling film flow process on the outer wall of dome cylinder

The passive containment cooling system (PCCS) is crucial for ensuring the safety of nuclear power plants, and the thickness and temperature of the falling film on its outer wall are important parameters that determine the heat and mass transfer performances. In this paper, a compact liquid film thickness and temperature simultaneous measurement system was developed to provide a measurement method, which is based on the absorption spectroscopy technology, and then the measurement accuracy was verified by using a calibration tool. The results showed that the mean relative error of the liquid film thickness and temperature measurements from the known values were about 3.4% and 0.4%, respectively. On this basis, this system was used to study the falling film process on the outer wall of the dome cylinder analogous to the containment under different working conditions and combined with numerical simulation for comparison. The results showed that the two methods have good consistency. The variation of the liquid film heat transfer coefficient for various operating conditions was further analyzed. It was found that the average thickness of the falling film is positively correlated with the liquid flow rate, but negatively correlated with the temperature and heat transfer coefficient. In summary, this system can accurately study the falling film process on the outer wall of the dome cylinder, which is expected to accurately analyze the heat and mass transfer process in the PCCS and provide data support for its design and optimization.

Numerical simulation of condensation heat transfer characteristics of R134a at high vapor velocity under different angles

Heat pipe is widely used as an efficient built-in heat exchanger for thermal management of high pulse and heat flux power supplies. Enhanced heat dissipation at the condensing side is an important issue in heat pipe research, where vapor velocity is an essential factor affecting the condensing heat transfer characteristics. This study adopted the CFD method to numerically simulate the condensation heat transfer of R134a flowing in a smooth pipe. The flow condensation heat transfer characteristics, such as film thickness and heat transfer coefficient (HTC), were investigated under vapor velocities v = 20–100 m/s and tube inclinations β =-90–90° Geometric and numerical heat transfer modeling is established in high vapor velocity flow condensation. The error of numerical simulation results and model validation is 10%. The HTC increases rapidly with the change in vapor velocity under 60 m/s, increasing the rate to 0.4. When the vapor velocity exceeds 60 m/s, the growth trend slows to 0.15, 37.5% of the original increase. At high vapor velocities above 60 m/s, the effect of pipe inclination on condensation heat transfer coefficient can be almost negligible. A modified empirical correlation suitable for high vapor velocities is proposed, which has higher predictive accuracy when the vapor velocity exceeds 80 m/s.

A Scalable Heat Pump Film with Zero Energy Consumption.

Radiative cooling is an effective technology with zero energy consumption to alleviate climate warming and combat the urban heat island effect. At present, researchers often use foam boxes to isolate non-radiant heat exchange between the cooler and the environment through experiments, so as to achieve maximum cooling power. In practice, however, there are challenges in setting up foam boxes on a large scale, resulting in coolers that can be cooled below ambient only under low convection conditions. Based on polymer materials and nano-zinc oxide (nano-ZnO, refractive index > 2, the peak equivalent spherical diameter 500 nm), the manufacturing process of heat pump film (HPF) was proposed. The HPF (4.1 mm thick) consists of polyethylene (PE) bubble film (heat transfer coefficient 0.04 W/m/K, 4 mm thick) and Ethylene-1-octene copolymer (POE) cured nano-ZnO (solar reflectance ≈94% at 0.075 mm thick). Covering with HPF, the object achieves 7.15 °C decreasing in normal natural environment and 3.68 °C even under certain circumstances with high surface convective heat transfer (56.9 W/m2/K). HPF has advantages of cooling the covered object, certain strength (1.45 Mpa), scalable manufacturing with low cost, hydrophobic characteristics (the water contact angle, 150.6°), and meeting the basic requirements of various application scenarios.

Experimental Study of a Novel Non-Packing Closed Evaporative Cooling Tower with Vertical 3D Deformation Tubes

The closed evaporative cooling tower (CECT) is widely used in the field of industrial cooling. At present, most CECTs still mainly adopt the horizontal-tube falling-film cooling method. In this paper, a novel vertical CECT using 3D deformation tubes is developed. To investigate the vertical surface falling-film evaporative cooling effect of this novel cooling equipment, a traditional horizontal CECT was modified to produce a prototype of vertical non-packing CECT. The cooling performance of the novel vertical CECT has been investigated and compared to the previous traditional horizontal CECT by experimental method. The results show that the convective heat transfer coefficient of the water film outside the tube was increased by 5.87~12.95% and the overall cooling performance was increased by 7.31% on average. This indicates that the cooling load can be increased by changing the traditional horizontal-tube falling-film evaporative cooling method to the vertical falling-film evaporative cooling method. Moreover, the heat flux of the novel vertical CECT decreases by about 7% when the wet bulb temperature increases by 1 °C under the test range of wet bulb temperature, which indicates that the ambient wet bulb temperature has an obvious influence on the cooling load. The research results can provide reference for the optimization design of the CECT.

Research on heat transfer capability of liquid film in three-phase contact line area

• Heat transfer studies were carried out in the three-phase contact line region using the TDTR method with high spatial and temporal resolution. • The experimentally measured results of the overall heat transfer coefficient of the n-octane liquid film in the evaporating thin-film region can reach 439 kW/(m 2 ·K). • A bidirectional heat transfer model for the TDTR experiment was built during data processing. • The usability of the TDTR method in microscopic liquid film heat transfer studies is demonstrated by numerical calculation. The three-phase contact line area plays a vital role in the heat dissipation of micro-devices due to its intense heat transfer capacity. However, the scale of the three-phase contact line area is small, and experimental studies are mainly used to observe the film profile. Few experiments can characterize this area with high spatial resolution, making it a well-known challenge to make a quantitative assessment of the limit of heat transfer capacity. In this work, we use the time-domain thermoreflectance system to measure the liquid film heat transfer capacity in the contact line region. The effective thermal conductivity of the liquid film obtained from the experiment can well reflect the heat transfer capacity of the liquid film at different positions and can be used to calculate the evaporating heat flux and the overall heat transfer coefficient of the liquid film. We also established a theoretical model and performed numerical calculations compared with the experimental results. The results show that the liquid film has the most vital heat transfer capacity in the evaporating thin-film region. The overall heat transfer coefficient can reach ∼439 kW/(m 2 ·K), and the evaporating heat flux of the liquid film can reach ∼1.8 × 10 6 W/m 2 . This work provides a new idea for the experimental study of the three-phase contact line area, offers experimental support for theoretical research, and lays a foundation for revealing the heat and mass transport mechanism in the three-phase contact line area.

Film thickness and heat transfer characteristics of R1233zd(E) falling film with nucleate boiling on an inclined plate

Falling film evaporation is an effective heat transfer method for achieving high heat flux and wide-area cooling with less refrigerant. As the film thickness and wall superheat increase, nucleate boiling occurs in the liquid film. In this case, the superficial liquid film thickness becomes thicker due to the existence of vapor bubbles. Although such superficial film thickness is an important parameter for expressing the film structure, the liquid film thickness of hydrofluorocarbon and hydrofluoroolefin refrigerants under nucleate boiling conditions has not yet been reported. This study experimentally investigated the liquid film thickness and heat transfer coefficients on an inclined plate with the width of 50 mm and the length of 75 mm. R1233zd(E) was used as the working fluid at a saturation temperature of 20°C. The film Reynolds number at the inlet was varied from 4.4 × 102 to 2.7 × 103, and the heat flux was varied from 5.6 to 138 kW/m2. As a result, the mean film thickness under adiabatic conditions increased when the flow rate increased or the inclination angle decreased from 30° to 15°, and ranged from 0.14 to 0.37 mm. The values were in good agreement with preexisting theories and correlations. Under heating conditions without boiling, the film thickness was almost equal to that under adiabatic conditions, and the heat transfer coefficients were slightly higher than the typical heat transfer correlation of the saturated falling film of water. With nucleate boiling in the high heat flux range, the superficial film thickness reached 1–3 mm at the location where vapor bubbles existed. The frequency of the thickness exceeding 1 mm increased with the heat flux as nucleate boiling became more active; thus, the average thickness also increased. The heat transfer coefficients under the developed nucleate boiling conditions were dominated by heat flux and were in good agreement with the existing pool boiling correlation.

Film thickness, interfacial waviness and heat transfer during downflow condensation of R134a

• Condensation tests are performed with R134a in a 3.38 mm tube. • Simultaneous film thickness and heat transfer measurements are carried out. • The experimental results of R134a are compared to those of R245fa. • The interfacial waves inception is delayed for R134a due to the high vapor density. • Models for condensation heat transfer, film thickness and void fraction are assessed. Coupled measurements of liquid film thickness and heat transfer during condensation inside minichannels are rare in the literature due to the technical difficulties in performing accurate and non-intrusive measures. The availability of such experimental data is fundamental for better understanding the occurring physical mechanisms and for the proper design of heat exchangers. To cover this gap, in the present work liquid film thickness and heat transfer coefficients have been simultaneously measured during vertical downflow condensation inside a 3.38 mm inner diameter channel. Condensation tests have been run with refrigerant R134a at mass flux between 30 and 150 kg m −2 s −1 and 30 °C saturation temperature. The instantaneous liquid film thickness and interfacial waves are detected by combining two complementary optical methods: shadowgraph technique (applied to a sequence of flow pattern images recorded by a high speed camera) and chromatic confocal imaging. In the data analysis, the experimental results of R134a have been discussed in comparison to R245fa condensation data taken during a previous experimental campaign at much lower pressure. On average, R134a displays higher liquid film thickness than R245fa and the appearance of high-amplitude disturbance waves, which promote the thinning of the liquid film and enhance the heat transfer, is detected at higher mass flux for R134a as compared to R245fa. The liquid film thickness is found to decrease with the increasing mass flux for both fluids, except from 30 to 50 kg m −2 s −1 due to the transition between shear stress-driven and gravity-driven downflow condensation. The accuracy of several well-known models for the prediction of heat transfer coefficient, film thickness and void fraction has been assessed with comparison to the experimental data of both R134a and R245fa.

Study on the Flow Boiling Heat Transfer Characteristics of the Liquid Film in a Rotating Pipe

A three-dimensional numerical model is established to study the flow boiling heat transfer characteristics of the liquid film in a rotating pipe, and the effectiveness of the model is verified by a comparison between the numerical results and the experimental results. The effects of rotational speed, heat flux, and Coriolis force on the characteristics of heat transfer of the rotating liquid film are investigated. The conclusions are drawn as follows: (1) The convection of the rotating liquid film is enhanced while the nucleate boiling in the rotating liquid film is inhibited by the increase in the rotational speed; (2) With the influence of these two factors, the heat transfer coefficient increases with centrifugal acceleration increasing from 20 g to 40 g, then decreases with centrifugal acceleration increasing from 40 g to 120 g; (3) The turbulent intensity of the flow with Coriolis force is obviously increased compared to that without Coriolis force when the centrifugal acceleration ranges from 20 g to 80 g, which shows no increase at higher centrifugal accelerations when the turbulence is sufficiently strong. The Coriolis force also has an impact on the heat transfer coefficient of the liquid film, which should not be ignored when studying the boiling heat transfer of a rotating liquid film.

Determination of Film Cooling Effectiveness and Heat Transfer Coefficient Simultaneously on a Flat Plate

In this paper, flat plate film cooling with two rows of compound angle cylindrical film cooling holes was investigated. A data processing method was evaluated which could determine the film cooling effectiveness and heat transfer coefficient simultaneously from the transient wall temperature data. The method was based on solving an inverse problem of the one-dimensional transient heat conduction equation. To evaluate the performance of the method, wall temperature data were obtained using the known film cooling effectiveness and heat transfer coefficient data as the convection boundary condition. Then, the method was applied to calculate the film cooling effectiveness and heat transfer coefficient based on the wall temperature data. Different blowing ratios, heat transfer coefficients, mainstream temperatures, and material thermal conductivities were investigated. In general, the data and calculation were in good agreement. It was found that the error decreased when the heat transfer coefficient increased and the material thermal conductivity decreased. The percentage error of the span-wise averaged film cooling effectiveness was mainly between 0% and 10%, and the percentage error of the span-wise averaged heat transfer coefficient was mainly between 0% and 4%.

Experimental study of steam-air mixture thermal stratification during the dropping of containment pressure

The passive containment cooling system (PCCS) as a passive technology has been applied in Generation Ⅲ nuclear power plants. Experiments were conducted in a scale model to analyze the thermal stratification in containment during the containment pressure dropping. Results indicate that the obvious thermal stratification occurs in the containment during the transient-state heat transfer process but not during the steady-state heat transfer process. For the transient-state process, the steam-air mixture temperature increases by 2 °C for each 100 mm increase in the elevation direction of the pressure tank. The nonuniform distribution of air in containment causes thermal stratification, which is more pronounced for lower air mass fraction and higher initial containment pressure. The thermal stratification effect on the condensation heat transfer process is assessed via diffusion layer heat transfer and film heat transfer to evaluate its effect. The diffusion layer heat transfer coefficient (HTC) is strongly affected by thermal stratification, whereas no effect is observed on the film HTC. This leads to the total condensation HTC at the top of the heat exchanger is approximately 3–5 times than that at the bottom in the first 100 s of the containment pressure dropping process. To improve the overall heat transfer performance of PCCS, approaches that enhance the heat transfer capacity at the bottom of the heat exchanger should be considered for prioritization, especially the methods of steam diffusion heat transfer capacity.

Numerical Modeling of Atmospheric Rime Ice Accretion on an Airfoil Using an Eulerian Approach

AbstractThis paper presents an approach to numerically simulate the inherently unsteady rime ice accretion problem on a two-dimensional airfoil and elucidate the associated variations under different icing conditions. The airflow field and the water impingement on the airfoil are obtained based on an Eulerian two-phase model. A dynamic mesh strategy is employed to unsteadily account for the changes in the ice profile and its impact on the air and droplet flow by continuously reconstructing the computational grid at each time-step through smoothing and layering mechanisms. All main icing modules including the airflow field, droplet trajectory, icing thickness profile, and mesh management are fully coupled within the same computational framework without resorting to any external tools. Classical icing theory is employed to model the rime ice roughness, and it is assumed that the ice accretes in a direction normal to the airfoil surface. The governing Reynolds-averaged Navier–Stokes (RANS) conservation equations along with the energy and continuity equations are solved to produce the velocity and temperature fields. A convective film heat transfer coefficient is computed based on the surface heat flux and a recovery temperature which takes into account the dissipative heat release in the boundary layer in the vicinity of the airfoil surface. With the implemented strategy and calculating the convective heat transfer coefficient, the water film thickness is also calculated along with the ice shape. The model is validated by comparing the local collection efficiency distribution and ice shape with experimental data, and the results show that the implemented approach provides acceptable predictions of ice accretion profiles and rates.

A comprehensive review on computational studies of falling film hydrodynamics and heat transfer on the horizontal tube and tube bundle

Falling film evaporation is a promising technology widely applied in refrigeration, desalination and beyond applications owing to the great advantages. However, the liquid film hydrodynamics and heat transfer performance are studied insufficiently. Compered with experiments, numerical simulations are economical and efficient especially in the study of liquid film, through which some microscopic understandings are expected to be extracted. In this paper, a comprehensive review for a large pool of computational studies about falling liquid film flow and heat transfer on the horizontal tube and tube bundle is presented. Computational methods of liquid–gas two-phase flow and mass transfer model, as well as relevant researches were first introduced. Then, the studies on the falling film hydrodynamics, film thickness, sensible heat transfer, flow pattern, evaporation and boiling outside the single tube, tube bundle, special-shaped tube and entire evaporator studied with 2D and 3D models are respectively reviewed in order. Then, the investigations on the falling film breakout and dryout are reviewed. And then, the numerical predictions of falling film thickness and heat transfer coefficient are involved. Afterthat, the benchmark data for falling film numerical simulation are summarized. Finally, some future needs and recommendation relating to crucial technologies that must be solved are proposed.

Experimental and Numerical Study of Hydrodynamic and Heat Transfer Characteristics of Falling Film Over Metal Foam Layered Horizontal Tube

AbstractA novel nonintrusive technique based on an air-coupled ultrasonic transducer was used to study the hydrodynamic behavior of falling film over metal foam layered horizontal tube. Copper foam having a porosity of 90.5%, brazed over a copper tube of 25.4 mm diameter was used in this study. Falling film thickness distribution in the circumferential direction and the dynamic characteristics of falling film were studied in the falling film Reynolds number range of 356–715, and at a tube spacing of 5 mm and 15 mm. The falling film characteristics over metal foam layered horizontal tubes were compared with that over a plain horizontal tube surface. Heat transfer studies of falling film over metal foam layered tube were studied in an evaporator of a multi-effect desalination system by experiment. It was observed that the falling film heat transfer coefficient was enhanced 2.7 times by the application of metal foam over the plain horizontal tube. The measurements obtained from hydrodynamic and heat transfer studies were compared with the predictions made by a computational model and were found to be in good agreement. Metal foam properties required for the computational model were obtained using a microcomputed tomography-based study.

Overall cooling performance evaluation for film cooling with different winglet pairs vortex generators

Previous studies have shown that the film cooling effectiveness can be improved by using a winglet pair vortex generator upstream or downstream of the film hole, but these studies merely focused on rectangular and delta winglet pairs and the overall performance evaluation has no heat transfer results. In this study, the overall film cooling performance of a cylindrical film hole with four types of winglet vortex generators upstream of the film hole was numerically investigated. The numerical simulation with the realizable k-ε turbulence model was performed by commercial software Fluent, solving steady Reynolds-average Navier-Stokes equations. The film cooling effectiveness with vortex generators is one or more times higher than that without the vortex generators owing to the downwash vortices generated by the vortex generators. The downwash vortices mitigate the negative effect of the counter-rotating vortex pair. There is a competitive mechanism between the downwash vortices and the counter-rotating vortex pair, so the beneficial effect of the vortex generators will weaken with the increase of the blowing ratio. The heat transfer coefficient of film cooling with vortex generators is higher, with a maximum augmentation of 11.45%. The heat transfer distributions of different vortex generator configurations are similar. Results prove that the case with the rectangular winglet pair vortex generator is the optimal configuration since it provides 0.08–0.29 higher net heat flux reduction than the case without the vortex generators.

Refrigerant film flow and heat transfer characteristics on the elliptical tube under constant wall temperature

• CFD models of falling film on horizontal tubes are verified by experimental data. • Effects of wall heating conditions on heat transfer of falling film are explored. • 11 refrigerants film flow and heat transfer under different temperature is studied. • Correlations of Nusselt number and film thickness for elliptical tubes are fitted. In this paper, the 2D model of the elliptical tube is established to simulate the film hydrodynamics and heat transfer performance. The volume of fluid (VOF) is used and the model is verified by the experimental data in the previous literature. The effects of the heating condition, fluid medium, inlet temperature ( T in ) on the film thickness and heat transfer coefficients are explored. The studied 11 refrigerants include ethane, propane, R123, R1234yf, R1234ze(E), R125, R134a, R143a, R152a, R227ea, R245fa at T in = –30, –10 and 10 °C. The results show at Re = 2000, the local heat transfer coefficients of the propane at T in = 10 °C under fixed heat flux are 19% greater than those under fixed wall temperature. Both the local film thickness and local heat transfer coefficients increase with Reynolds number ( Re ). The overall external heat transfer coefficients increase with Re , but the growth rate slows down at Re ≥ 1500. Fluids have different results of film thickness and heat transfer coefficients even they have a similar Kapitza number ( Ka ). Different from the water and seawater, the heat transfer coefficients reduce as the inlet temperature increases for the refrigerants. A correlation is proposed to predict local film thickness on both circular and elliptical tubes. A correlation is acquired by the fitting method to predict the overall external heat transfer coefficients under the heating condition of fixed wall temperature, which can capture 89% of 132 data within the deviations of ±10%.

Effect of length-to-diameter ratio on film cooling and heat transfer performances of simple and compound cylindrical-holes in transverse trenches with various depths

Advances in trenched-hole film cooling result in a fact that short-holes exposing in a transverse trench have a significantly large potential for application in designs of future double-wall cooled blade. In present work, combined influences of hole-length, trench-depth, compound angle and cooling air flowrate on the film effectiveness and heat transfer features of trenched-holes were discussed deeply. Film effectiveness and heat transfer coefficient were measured by an infrared thermal imaging system. Numerical simulation as an auxiliary tool was conducted to provide necessary flow details. Two typical length-to-diameter ratios of L/D = 2 and 5, three trench-depths of H = 0.5D, 0.75D and 1.0D and two compound-angles of film-hole of 0° and 45° were designed. Cooling air flowrate was controlled in a relatively large range from 0.5 to 4.0 in averaged blowing ratio (BR). The results revealed the various jet-mechanisms caused by L/D while relatively regular variations of turbulent intensity of jets. Thus, the complicated trends in film effectiveness while regular trends in heat transfer coefficient can be clearly captured. The combined influences of trends above can bring to the more undesirable thermal protections for the short-hole jet. The L/D-induced decrement in area-averaged net heat flux reduction (NHFR) can reach 60% at most. To effectively improve film coverage and NHFR, the short-holes were properly embedded in the deeper trench. And the sensitivity analysis of trench-depth to discharge coefficient showed the negligible variation below 5%. The NHFR of the shallowest trench can be improved through application of compound angle, inducing an enlarged aerodynamic loss below 10%.

Assessment of thermal fatigue induced by dryout front oscillation in printed circuit steam generator

A printed circuit steam generator (PCSG) is being considered as the component for pressurized water reactor (PWR) type small modular reactor (SMR) that can further reduce the physical size of the system. Since a steam generator in many PWR-type SMR generates superheated steam, it is expected that dryout front oscillation can potentially cause thermal fatigue failure due to cyclic thermal stresses induced by the transition in boiling regimes between convective evaporation and film boiling. To investigate the fatigue issue of a PCSG, a reference PCSG is designed in this study first using an in-house PCSG design tool. For the stress analysis, a finite element method analysis model is developed to obtain the temperature and stress fields of the designed PCSG. Fatigue estimation is performed based on ASME Boiler and pressure vessel code to identify the major parameters influencing the fatigue life time originating from the dryout front oscillation. As a result of this study, the limit on the temperature difference between the hot side and cold side fluids is obtained. Moreover, it is found that the heat transfer coefficient of convective evaporation and film boiling regimes play an essential role in the fatigue life cycle as well as the temperature difference.