Search

A new experimental study (aka SPAYCOR-S2) on multi-nozzle spray cooling of a downward-facing heater surface is conducted on the SPAYCOR facility at KTH, to provide data assessing the feasibility of spray cooling for in- vessel melt retention (IVR) in light water reactors. It is intended to investigate the potential for reducing the number of nozzles for spray cooling of an 80 mm x 120 mm surface, from the 2 x 3 nozzle array in the previous study (Bandaru, 2021) to a 2 x 2 nozzle array in the present study. Given the same heater surface, the pitch-to- diameter ratio is enlarged in a 2 x 2 nozzle array, resulting a partial coverage of the spray cones of four nozzles, in contrast with the 2 x 3 nozzle array where the heater surface was fully covered by six-nozzle spray. The tests focus on the cooling performance of such partial coverage of multi-nozzle spray and the effects of heater surface's inclination angle. The experimental results reveal that the inclination angle of the heat surface has a negligible impact on cooling capacity, although it is slightly higher on the surface inclined at 90 degrees degrees than on the surfaces inclined at 45 degrees degrees or 60 degrees. degrees . The maximum steady heat flux of the 2 x 2 nozzle array at its minimum spray flowrate is determined as 1.96 MW/m2. 2 . A numerical study on the spray cooling of the 2 x 2 nozzle array is also performed with models in OpenFOAM, which are extended from our previous development (Fang, 2023) by adding models for thin-film boiling and conjugate heat transfer in solid. For validation of the numerical study, the spray cooling of the heater surface inclined at various degrees is simulated, and the results are compared with those of experiment. The simulation generally shows good correspondence with experiments., QC 20240910

An experimental study on multi-nozzle spray cooling of a downward-facing heater surface has been carried out in the SPAYCOR facility at KTH, to provide data assessing the feasibility of spray cooling for in-vessel melt retention (IVR) in light water reactors. To help understand the characteristics and influential factors of the liquid film formed on the heater surface in spray, a numerical study on the dynamics of an isothermal liquid film on the heater surface has also been performed by adopting the OpenFOAM platform, and Eulerian and Lagrangian methods for liquid film and droplets, respectively. The present study is an extension of the previous modeling from hydrodynamics to thermal-hydraulics of the spray cooling problem, via adding heat flux of the heater and two convective heat transfer models between the heater wall and the liquid film. Moreover, droplets-film interaction model is modified. The SPAYCOR experiment is simulated by the numerical models, and the simulation results show a good agreement between the numerical and experimental data, in particular when the modified droplets-film interaction model is applied. After the validation of the numerical models against the SPAYCOR experiment, the numerical models are employed to investigate influential factors on heat transfer, such as mass flux, nozzle-to-surface distance, and nozzle matrix layout. The results indicate that heat transfer is enhanced by increasing mass flux and decreasing nozzle-to-surface distance, and the change of nozzle matrix from inline to staggered layout has little impact on heat removal capacity or temperature distribution of the multi-nozzle spray cooling., QC 20240618

In a consideration of spray cooling as the potential cooling mechanism for the in-vessel melt retention (IVR) strategy of nuclear reactors because of its superior heat removal efficiency, the SPAYCOR experiment has been conducted at KTH to investigate the spray cooling capacity of multiple nozzles applied to a downward-facing heated surface. In the present study, the dynamics of liquid film on the downward-facing surface resulting from the multi-nozzle spray are numerically simulated by using a coupled Eulerian-Lagrangian method implemented in the OpenFOAM platform. Prior to simulation of the SPAYCOR experiment, the numerical approach is used to calculate two theoretical setups which have known analytical solutions, with the objective to validate the models in predicting liquid film dynamics either in spray or on an inclined surface. In the simulation of the SPAYCOR experiment, the predicted film morphology shows a good agreement with the experimental observation. What's more, the influential factors, including the inclination of the downward-facing heater surface, the nozzle-to-surface distance as well as the nozzle-array layout, are also investigated numerically in the present study. The simulation results show that a decreasing nozzle-to-surface distance does not only lead to a thicker liquid film and a lower velocity in the vicinity of each spray coverage, but also increases non-uniformity of the liquid film. The nozzles-array layout has little influence on the average liquid film thickness and velocity, but significantly affects the film morphology., QC 20230228

The demand for larger power density and torque for the power traction motors used in electrified transportation puts forward a requirement for better thermal management methods. Spray cooling is a promising direct liquid cooling technique that has been proved to possess high heat removal capability in previous research. This paper investigates the heat transfer characteristics of spray cooling on endwindings of electric machines via numerical simulation through an Eulerian–Lagrangian approach. The utilized numerical models and calculated results are validated with experimentally measured data. The influence of different parameters and options involved in the simulation settings on the final results, like the stream numbers for the spray injector, the constant heat flux versus constant temperature thermal boundary condition, the influence of splashing, the effect of heat conduction in the endwindings and the Saffman lift force, only solving the energy equation for the air after its flow field reaches a steady-state, are evaluated. Parameter analyses are also conducted for operation conditions, configuration of the spray nozzles, and material properties of the coolant liquid. It is found that larger flow rate, smaller droplet size, lower spray height, more nozzle numbers, larger thermal conductivity and smaller viscosity of the coolant liquid tend to increase the overall heat transfer performance., QC 20230830

Spray cooling on moving hot solids is widely used in metal heat treatment processes. Understanding coolant droplet collision behavior with moving hot solids is of great importance toward improving heat treatment temperature control technology. Via flash photography, we experimentally investigated the hydrodynamics of droplet train obliquely impinging on a hot moving solid. The test piece was a rectangular steel piece (SUS303) heated to 500°C, 550°C, or 600°C with a moving velocity of 0.5 m/s, 1.0 m/s, or 1.5 m/s. The test liquid was water at approximately 20°C. The pre-impact diameter of droplets, droplet impact velocity, and inter-spacing between every successive two droplets were 0.64 mm, 2.2 m/s, and 1.91 mm, respectively. The tilt angle of the droplet train to the vertical was 50°. No coalescence of droplets was seen—the droplets deformed independently on the moving solid. The measured results of the maximum diameter and the residence time of the droplets agreed well with the empirical formulas that can be used for droplet impact on a stationary solid. It was found that the dynamics of a droplet train impinging on a hot moving solid are the same as the dynamics of a droplet train impinging on a hot stationary solid when the droplets deform independently on a moving solid. Taking advantage of said property such that it is equivalent to the dynamics of a droplet train impinging on a hot stationary solid, we proposed a critical condition for droplet coalescence and experimentally confirmed the validity of the critical condition.

The solution of the inverse heat conduction problem (IHCP) is commonly found with thesequential algorithm known as the function specification method with explicit updating formulas andsensitivity coefficients of heat flux. This paper presents a different approach namely a direct mathemat-ical optimization of minimizing the least squares norm between experimental data and simulation. ACFD open-source code OpenFOAM is used together with NLOPT and DLIB optimization libraries. Toguarantee credibility of the simulation tool developed herein, real experimental data is used from spraycooling of a fast-moving hot steel plate. As the IHCP is inherently an ill-posed problem, the proposedsequential algorithm is stabilized using future time stepping and thereof the optimal number is explained.An assumption about the profile of thermal boundary condition during future steps must be made. Itis shown that assuming a linear change of the heat transfer coefficient during each sequence of futuretime steps yields more accurate results than setting a constant value. For the problem size consideredwith less than 10k cells, the preconditioned conjugate gradient (FDIC) linear solver converges fasterthan the multigrid solver (GAMG). However, the latter performs better as the accuracy is concerned.Concerning the best choice of minimizer, the BOBYQA algorithm (quadratic approximation) is foundsuperior to other methods. The proposed IHCP solver is compared with the well-established one.

The solution of the inverse heat conduction problem (IHCP) is commonly found with thesequential algorithm known as the function specification method with explicit updating formulas andsensitivity coefficients of heat flux. This paper presents a different approach namely a direct mathemat-ical optimization of minimizing the least squares norm between experimental data and simulation. ACFD open-source code OpenFOAM is used together with NLOPT and DLIB optimization libraries. Toguarantee credibility of the simulation tool developed herein, real experimental data is used from spraycooling of a fast-moving hot steel plate. As the IHCP is inherently an ill-posed problem, the proposedsequential algorithm is stabilized using future time stepping and thereof the optimal number is explained.An assumption about the profile of thermal boundary condition during future steps must be made. Itis shown that assuming a linear change of the heat transfer coefficient during each sequence of futuretime steps yields more accurate results than setting a constant value. For the problem size consideredwith less than 10k cells, the preconditioned conjugate gradient (FDIC) linear solver converges fasterthan the multigrid solver (GAMG). However, the latter performs better as the accuracy is concerned.Concerning the best choice of minimizer, the BOBYQA algorithm (quadratic approximation) is foundsuperior to other methods. The proposed IHCP solver is compared with the well-established one.

The solution of the inverse heat conduction problem (IHCP) is commonly found with thesequential algorithm known as the function specification method with explicit updating formulas andsensitivity coefficients of heat flux. This paper presents a different approach namely a direct mathemat-ical optimization of minimizing the least squares norm between experimental data and simulation. ACFD open-source code OpenFOAM is used together with NLOPT and DLIB optimization libraries. Toguarantee credibility of the simulation tool developed herein, real experimental data is used from spraycooling of a fast-moving hot steel plate. As the IHCP is inherently an ill-posed problem, the proposedsequential algorithm is stabilized using future time stepping and thereof the optimal number is explained.An assumption about the profile of thermal boundary condition during future steps must be made. Itis shown that assuming a linear change of the heat transfer coefficient during each sequence of futuretime steps yields more accurate results than setting a constant value. For the problem size consideredwith less than 10k cells, the preconditioned conjugate gradient (FDIC) linear solver converges fasterthan the multigrid solver (GAMG). However, the latter performs better as the accuracy is concerned.Concerning the best choice of minimizer, the BOBYQA algorithm (quadratic approximation) is foundsuperior to other methods. The proposed IHCP solver is compared with the well-established one.

A novel method of cooling rolls in hot rolling is proposed. This method uses a combination of solid jet nozzles and specially shaped deflecting vanes. The vanes transform the incoming cylindrical water jet into a flat fan. Thereby, the coolant can be directed into hardly accessible locations whilst maintaining the optimal angle of impingement. The vanes allow for effectively cooling the roll surface near the rolling gap, which is otherwise not possible with classical flat fan nozzles. The jet impact is tangential due to the limited room for nozzle mounts or a coolant supply. The developed method was laboratory tested. A similar cooling efficiency was found between the vane and the flat fan nozzle. The latter was however mounted in a position impossible in the plant. The potential of the proposed cooling is, therefore, eminent. Apart from hot rolling, it could be exploited in other technological processes such as high pressure die casting, machining, turning, hot stamping, etc.

A novel method of cooling rolls in hot rolling is proposed. This method uses a combination of solid jet nozzles and specially shaped deflecting vanes. The vanes transform the incoming cylindrical water jet into a flat fan. Thereby, the coolant can be directed into hardly accessible locations whilst maintaining the optimal angle of impingement. The vanes allow for effectively cooling the roll surface near the rolling gap, which is otherwise not possible with classical flat fan nozzles. The jet impact is tangential due to the limited room for nozzle mounts or a coolant supply. The developed method was laboratory tested. A similar cooling efficiency was found between the vane and the flat fan nozzle. The latter was however mounted in a position impossible in the plant. The potential of the proposed cooling is, therefore, eminent. Apart from hot rolling, it could be exploited in other technological processes such as high pressure die casting, machining, turning, hot stamping, etc.

A novel method of cooling rolls in hot rolling is proposed. This method uses a combination of solid jet nozzles and specially shaped deflecting vanes. The vanes transform the incoming cylindrical water jet into a flat fan. Thereby, the coolant can be directed into hardly accessible locations whilst maintaining the optimal angle of impingement. The vanes allow for effectively cooling the roll surface near the rolling gap, which is otherwise not possible with classical flat fan nozzles. The jet impact is tangential due to the limited room for nozzle mounts or a coolant supply. The developed method was laboratory tested. A similar cooling efficiency was found between the vane and the flat fan nozzle. The latter was however mounted in a position impossible in the plant. The potential of the proposed cooling is, therefore, eminent. Apart from hot rolling, it could be exploited in other technological processes such as high pressure die casting, machining, turning, hot stamping, etc.

Spray cooling of hot steel surfaces is an inherent part of continuous casting and heat treatment. When we consider the temperature interval between room temperature and for instance 1000 degrees C, different boiling regimes can be observed. Spray cooling intensity rapidly changes with the surface temperature. Secondary cooling in continuous casting starts when the surface temperature is well above a thousand degrees Celsius and a film boiling regime can be observed. The cooled surface is protected from the direct impact of droplets by the vapour layer. As the surface temperature decreases, the vapour layer is less stable and for certain temperatures the vapour layer collapses, droplets reach the hot surface and heat flux suddenly jumps enormously. It is obvious that the described effect has a great effect on control of cooling. The surface temperature which indicates the sudden change in the cooling intensity is the Leidenfrost temperature. The Leidenfrost temperature in spray cooling can occur anywhere between 150 degrees C and over 1000 degrees C and depends on the character of the spray. This paper presents an experimental study and shows function for prediction of the Leidenfrost temperature based on spray parameters. Water impingement density was found to be the most important parameter. This parameter must be combined with information about droplet size and velocity to produce a good prediction of the Leidenfrost temperature.

Spray cooling of hot steel surfaces is an inherent part of continuous casting and heat treatment. When we consider the temperature interval between room temperature and for instance 1000 degrees C, different boiling regimes can be observed. Spray cooling intensity rapidly changes with the surface temperature. Secondary cooling in continuous casting starts when the surface temperature is well above a thousand degrees Celsius and a film boiling regime can be observed. The cooled surface is protected from the direct impact of droplets by the vapour layer. As the surface temperature decreases, the vapour layer is less stable and for certain temperatures the vapour layer collapses, droplets reach the hot surface and heat flux suddenly jumps enormously. It is obvious that the described effect has a great effect on control of cooling. The surface temperature which indicates the sudden change in the cooling intensity is the Leidenfrost temperature. The Leidenfrost temperature in spray cooling can occur anywhere between 150 degrees C and over 1000 degrees C and depends on the character of the spray. This paper presents an experimental study and shows function for prediction of the Leidenfrost temperature based on spray parameters. Water impingement density was found to be the most important parameter. This parameter must be combined with information about droplet size and velocity to produce a good prediction of the Leidenfrost temperature.

This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction of spray cooling intensity. The research is based on experiments with water and air-mist nozzles. The following spray parameters were measured together with a heat transfer coefficient: water flowrate, water impingement density, impact pressure, droplet size and velocity. Derived parameters as droplet kinetic energy, droplet momentum and droplet Reynolds number are used in the tested correlations as well. Ten combinations of spray parameters used for correlation functions for the heat transfer coefficient (HTC) are studied and discussed. Correlation functions for prediction of HTC are presented and it is shown which spray parameters are necessary for reliable computation of HTC. The best results were obtained when the parameters impact pressure and water impingement density were used together. It was proven that the correlations based only on water impingement density, which are the most frequent in literature, can not provide reliable results.

This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction of spray cooling intensity. The research is based on experiments with water and air-mist nozzles. The following spray parameters were measured together with a heat transfer coefficient: water flowrate, water impingement density, impact pressure, droplet size and velocity. Derived parameters as droplet kinetic energy, droplet momentum and droplet Reynolds number are used in the tested correlations as well. Ten combinations of spray parameters used for correlation functions for the heat transfer coefficient (HTC) are studied and discussed. Correlation functions for prediction of HTC are presented and it is shown which spray parameters are necessary for reliable computation of HTC. The best results were obtained when the parameters impact pressure and water impingement density were used together. It was proven that the correlations based only on water impingement density, which are the most frequent in literature, can not provide reliable results.

Spray cooling of hot steel surfaces is an inherent part of continuous casting and heat treatment. When we consider the temperature interval between room temperature and for instance 1000 degrees C, different boiling regimes can be observed. Spray cooling intensity rapidly changes with the surface temperature. Secondary cooling in continuous casting starts when the surface temperature is well above a thousand degrees Celsius and a film boiling regime can be observed. The cooled surface is protected from the direct impact of droplets by the vapour layer. As the surface temperature decreases, the vapour layer is less stable and for certain temperatures the vapour layer collapses, droplets reach the hot surface and heat flux suddenly jumps enormously. It is obvious that the described effect has a great effect on control of cooling. The surface temperature which indicates the sudden change in the cooling intensity is the Leidenfrost temperature. The Leidenfrost temperature in spray cooling can occur anywhere between 150 degrees C and over 1000 degrees C and depends on the character of the spray. This paper presents an experimental study and shows function for prediction of the Leidenfrost temperature based on spray parameters. Water impingement density was found to be the most important parameter. This parameter must be combined with information about droplet size and velocity to produce a good prediction of the Leidenfrost temperature.

Accurate control of the temperature of the workpiece is crucial when carrying out thermal processing of steel. Effective heat transfer coefficient for water cooling of hot rolled steel was calibrated using spray nozzles with different amounts of water flux. Austenitic stainless steel was chosen for the studies in order to avoid the release of latent heat, which would affect the result in the case of carbon steels. Three different measurement sites acquired with thermocouples were used, namely in the middle and at the quarter distance from both top and bottom surfaces. The heat transfer coefficient was fitted to the experimental data using a computational model, which calculated the time-dependent temperature distribution within the steel slab. The model was validated by altering the experimental exposure time to the water spray and compared the model prediction to the observed result. The calibrated model provides the capability to design and carry out cooling routes in thermomechanical processing of steel in practice.

This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction of spray cooling intensity. The research is based on experiments with water and air-mist nozzles. The following spray parameters were measured together with a heat transfer coefficient: water flowrate, water impingement density, impact pressure, droplet size and velocity. Derived parameters as droplet kinetic energy, droplet momentum and droplet Reynolds number are used in the tested correlations as well. Ten combinations of spray parameters used for correlation functions for the heat transfer coefficient (HTC) are studied and discussed. Correlation functions for prediction of HTC are presented and it is shown which spray parameters are necessary for reliable computation of HTC. The best results were obtained when the parameters impact pressure and water impingement density were used together. It was proven that the correlations based only on water impingement density, which are the most frequent in literature, can not provide reliable results.

This paper deals with the procedure for measuring the relevant parameters of plate-finned tube heat exchangers operating as an air heaters. Besides the description of the experimental installation, the most important geometric characteristics of these appliances are given. In order to determine the relevant parameters for dry and wet regimes (for cases with and without spray cooling), experimental measurements of the heat and hydraulic performance of the heat exchangers were made. Water flowed through the pipes as a warmer fluid. During the experiments, the flow rates and temperatures of the working fluids were varied within the limits defined by the operating parameters of the installation itself, sufficient to allow the obtained results to be generalized. The experimental data were processed by appropriate statistical methods; the aim was to determine equations suitable for engineering practice. The coefficients of the air-side heat and mass transfer and friction factor are defined on the complex geometry of this type of heat exchangers under realistic operating conditions, as a function of the relevant similarity criteria.

On the hitherto installed chillers/rooftop units/dry coolers and air coolers, a significant improvement in cooling was noticed in the summer mode when the pipe bundle is sprayed with water. Tests have been performed on systems that produce water mist (micron-sized water droplets), which are formed by spraying water under high pressure through the nozzles/nozzle openings. When a drop of micron-sized water comes in contact with warm air, it evaporates very quickly, absorbing a certain amount of heat, which results in cooling the air. This improves the operation of the cooler in critical periods when the ambient air temperature is very high. The aim of the test is to determine the improvement of the cooler operation under operating conditions with the application of the described system in relation to the operating mode without water spraying (dry operating mode).

This study represents an experimental investigation of spray dehumidification process of hot and moist air in turbulent flow. An experimental setup is constructed in order to produce hot and humid air flow in a circular tube. Sub-cooled water at a temperature below the dew point is injected into air stream. The effects of spray water flow rate, water temperature, spray injection direction (parallel/counter flow), nozzle type (hydraulic/air atomization) and air inlet conditions on dehumidification performance are investigated. The results of the present study show that improved dehumidification performance is obtained for highly humid air., The authors gratefully acknowledge the financial support for this work by Ministry of Science, Industry and Technology of Turkey under Grant# 01488.STZ.2012-2 ., Ministry of Science, Technology and Space [01488.STZ.2012-2]

The objective of this study is to analyze the effect of spray cooling in a liquid piston compressor during the compression of air in terms of temperature abatement and compression efficiency. The liquid piston can effectively cover any irregular containers making its surface area to volume ratio better than conventional reciprocating pistons. This plays an important role in increased heat transfer to the outer surface resulting in improved efficiency. As the compression process has the minimum work input during an isothermal compression, the process path needs to shift from the polytropic curve towards the isothermal curve for higher efficiency. Recently, various methodologies have been tested in the liquid piston for increasing heat transfer from the chamber such as optimal compression trajectories, placing porous media inserts into the chamber, and introducing hollow spheres in the chamber. Also, several studies have presented spray cooling as a viable option for heat transfer enhancement in compressors due to its high specific heat and fine atomization. In this study, to shift compression trajectory towards an isothermal curve, spray cooling is investigated for a compression ratio of 2 in a liquid piston compressor. Experiments are performed in the liquid piston setup with water as the medium and a polypropylene chamber. The experimental setup for the spray system consists of a pump, pressure regulator, flowmeter, data acquisition system, and nozzles. For the spray; full cone and hollow cone nozzles were selected as they provide an even distribution of droplets with moderate to fine diameters. Spray angles from 60° to 120° for full cone nozzles, and injection line pressure from 275 kPa to 550 kPa are tested for various stroke times of compression. A temperature drop from 59°C to 35°C was observed when spray cooling was introduced with a full cone nozzle with 60° spray angle and 275 kPa injection pressure for a compression ratio of 2. Similarly, a significant drop

[EN] An experimental study was carried out with a hexagonal finned heat sink to obtain valuable information about the thermal and flow characteristics on spray cooling. Water was used as the refrigerant fluid and was atomized by an air-assisted atomizer and some of the variables that affect spray cooling process, such as breakup length and Sauter Mean Diameter (SMD) were obtained using appropriate correlations. The parameters most influential on the Nusselt number were determined and analyzed. Experiments were conducted for optimized conditions. The jet diameter and spray angle were determined via image processing using macroscopic aspects. Nusselt were demonstrated of hexagonal finned heat sink which optimized according to the Taguchi optimization method. The results of the spray analysis showed that SMD decreases with an increase in either air-liquid mass ratio (ALR) or operating pressure resulting in a more uniform spray. As the liquid flow rate increases in all ALR values, the heat transfer rate also increases markedly. As a results of the experiments, Nusselt number, jet width and spray angle correlations were developed. The relationship of ALR-Nu was demonstrated according to fin height and spraying time.

[EN] Spray cooling process has many parameters such as extended surface, angle of inclination, effect of gravity, diameter of nozzle, angle of spray, mass flux, geometry of cooled surface, thermal performance and critical heat flux of spray etc. Many effective parameters to carry out the experiments with conventional test methods are both expensive and time consuming. As a solution in these circumstances, Taguchi method, which is one of the modern experimental design and optimization methods and very effective in solving such problems, was used in this study. Taguchi method, as well as being in effective to improve the quality of products, also gives the opportunity to achieve better results with much less experiment. Using Taguchi method, as well as to reach the target value exactly, the sensitivity of the design against uncontrollable factors is reduced to a minimum. Thus, the optimum tolerance range in cost and quality factors is determined. When compared to conventional experimental design methods, Taguchi method has many advantages. In these experiments with using rectangular pin fin heat sinks, the effects of the longitudinal and lateral distances of the consecutively arranged nozzle or diffuser-like fin pairs, widths of the fins, angle of fins, heights of fins, spraying time, air flow rate, liquid flow rate (ALR, the ratio of air-liquid flow rate) and the ratio of the nozzle-heat sink distance to the nozzle diameter (h/d) on heat and flow characteristics have been investigated by using Taguchi experimental design method. For this reason, characteristics of flow and heat transfer are considered separately. Nusselt number considered as performance statistic, L27(311) orthogonal array has been selected as an experimental design plan for the eleven parameters mentioned above. The Nusselt number was calculated by taking into account the characteristic length of heat sink and the optimized results were found to be fin width of 45 mm, fin angle of 45o , fin height o

[EN] Spray cooling systems are able to remove large amounts of heat due to phase-change. Although vaporization is the most common phase-change process used in applications requiring thermal management, the use of liquids often implies the presence of a liquid film which is known to mitigate cooling performance. Thus, it is worth exploring other approaches for spray cooling avoiding liquid films. The work presented here explores sublimation using CO2 particles (dry-ice) formed through the Joule-Thomson effect. The application of interest is the molding industry, where reducing the cycle time taking advantage of the time-frame available between the mold opening and closing during the part’s extraction, allows a production increase and, consequently, a higher competitive advantage in the market. The purpose of the experiments performed in dry-ice particle spray cooling is to investigate the effect of the impingement distance (350-450mm), and injection duration, on the total energy flux removed from the surface, and cooling efficiency, in order to assess the performance of sublimation spray cooling. The results show an evolution of temperature distribution from a more homogeneous pattern with shorter pulses to a heterogeneous one for pulse durations longer than 1 s. This is particularly useful in hotspot cooling. In terms of changing the spray impact distance, the higher particle dispersion achieved with a larger distance led to a decrease in thermal performance, probably due to the saturation of CO2 close to the impact surface. However, the pattern observed for the evolution of the total energy flux removed, with a maximum around an injection duration of 0.5 s, remains unaltered. The maximum cooling efficiency, obtained for the shortest distance, is up to 30%, which is comparable to spray cooling systems based on vaporization.

We present a new Nusselt number correlation for spray cooling at large Reynolds numbers and high surface temperatures for water sprays impinging perpendicularly onto a flat plate. A large set of experimental data on spray cooling of hot surfaces with water has been analyzed, including the water temperature effects. For large-scale cooling, such as in industrial processes, large number of injection parameters such as number, type, pressure, and angle of the spray injection has led to a multitude of correlations that are difficult for general and practical applications. However, by synthesizing a set of experimental data where all of the above parameters have been varied, we find that the Nusselt number and therefore the heat transfer coefficient can be cast accurately as a function of the Reynolds number. Water is widely used as the coolant during spray cooling, and has a specific phase change characteristic. At large Reynolds number (Re > 100,000) and surface temperature (Ts > 600°C) ranges, which are of interest in large-scale spray cooling, the effect of water temperature is quite significant as it affects the film boiling close to the surface. This effect also has been parameterized using experimental data.

We present a new Nusselt number correlation for spray cooling at large Reynolds numbers and high surface temperatures for water sprays impinging perpendicularly onto a flat plate. A large set of experimental data on spray cooling of hot surfaces with water has been analyzed, including the water temperature effects. For large-scale cooling, such as in industrial processes, large number of injection parameters such as number, type, pressure, and angle of the spray injection has led to a multitude of correlations that are difficult for general and practical applications. However, by synthesizing a set of experimental data where all of the above parameters have been varied, we find that the Nusselt number and therefore the heat transfer coefficient can be cast accurately as a function of the Reynolds number. Water is widely used as the coolant during spray cooling, and has a specific phase change characteristic. At large Reynolds number (Re > 100,000) and surface temperature (Ts > 600°C) ranges, which are of interest in large-scale spray cooling, the effect of water temperature is quite significant as it affects the film boiling close to the surface. This effect also has been parameterized using experimental data.

We present a new Nusselt number correlation for spray cooling at large Reynolds numbers and high surface temperatures for water sprays impinging perpendicularly onto a flat plate. A large set of experimental data on spray cooling of hot surfaces with water has been analyzed, including the water temperature effects. For large-scale cooling, such as in industrial processes, large number of injection parameters such as number, type, pressure, and angle of the spray injection has led to a multitude of correlations that are difficult for general and practical applications. However, by synthesizing a set of experimental data where all of the above parameters have been varied, we find that the Nusselt number and therefore the heat transfer coefficient can be cast accurately as a function of the Reynolds number. Water is widely used as the coolant during spray cooling, and has a specific phase change characteristic. At large Reynolds number (Re > 100,000) and surface temperature (Ts > 600°C) ranges, which are of interest in large-scale spray cooling, the effect of water temperature is quite significant as it affects the film boiling close to the surface. This effect also has been parameterized using experimental data.

Air or water cooled heat exchangers (HX) are typically utilized as condensers or coolers for air-conditioning, refrigeration or process cooling applications in both commercial and industrial sector. However, air cooled heat exchanger performance degrades considerably with rise in ambient air temperature and water cooled coolers require considerable pumping power, a cooling tower and may consume a significant amount of water which may come from fresh water sources. Evaporative cooling offers a unique solution to this problem, where a small amount of wetting water evaporates on HX surface to boost performance in high ambient air temperature conditions. In this study, several evaporative cooling technologies were applied to three wavy-fin HXs to quantify capacity enhancement ratio (CER) and air-side pressure drop penalty ratio (PRΔPa) compared to respective dry case baseline values. Effect of varying wetting water flow rate, air velocity, fin spacing, hydrophilic coatings, spray orientation and inlet air temperature and relative humidity was investigated on hybrid heat exchanger performance. Several new performance comparison parameters were defined to compare different evaporative cooling approaches. Deluge cooling achieved overall highest CER but at a PRΔPa that was similar in magnitude to the CER. This limitation was found to be inherent to the nature of wetting water distribution method itself. Although front spray cooling tests indicated PRΔPa~1, front spray evaporative cooling technology was found to have up to 23-75 % lower CER at 60-100% lower PRΔPa compared to deluge cooling. In order to understand the wetting behavior a novel visualization method was proposed and implemented, which consisted of borescope assisted flow mapping of water distribution within the HX core as a function of air velocities and wetting water flow rates. It was found that up to 85% of HX volume remained dry during front spray cooling which accounted for lower capacit

本实用新型提供一种蒸发器及应用其的吸附式制冷系统，包括壳体，该壳体围绕形成密封空间，且在该密封空间内盛有作为吸附质的纳米液体；在密封空间内设置有热交换器和喷淋装置，其中，热交换器用于蒸发纳米液体；喷淋装置包括水泵和喷头，水泵设置在壳体之外，用于抽取密封空间内的纳米液体，并将其传输至喷头；喷头设置在密封空间内，且位于纳米液体上方，用于喷出液滴状的纳米液体。本实用新型提供的蒸发器，其可以提高吸附质的蒸发速度，从而可以提高吸附式制冷系统的制冷量和其性能系数。The utility model provides an evaporator and an adsorption type refrigerating system applying the same. The evaporator comprises a case. A sealing space is formed in the case in a surrounding mode, and nanometer liquid serving as adsorption media is contained inside the sealing space. A heat exchanger and a spraying device are arranged inside the sealing space. The heat exchanger is used for evaporating the nanometer liquid. The spraying device comprises a water pump and a spraying head, and the water pump is arranged outside the case and used for extracting the nanometer liquid in the sealing space and conveying the nanometer liquid to the spraying head. The spraying head is arranged in the sealing space and above the nanometer liquid and used for spraying nanometer liquid drops. According to the evaporator, the evaporation speed of the adsorption media can be increased so that the refrigerating capacity and the performance coefficient of the adsorption type refrigerating system can be increased.

The importance of high-strength steel concepts for car bodies has increased in recent years due to the necessity of weight reduction and improved crash safety. By using hot stamping or also known as press hardening of boron alloyed heat-treatable steels, it is possible to produce parts with a much higher strength than by cold forming processes. Depending on the stress profile of a structural part, it might be desirable to have different material rigidity in the part, with some high strength and other more ductile areas, so called "Tailored properties". There are a variety of methods to produce such parts, but all of these methods have currently still major challenges. Two methods to manufacture parts with tailored properties, these challenges and corresponding approaches are presented in this paper. This is on the one hand subsequently cooling in a spray field, which currently is still in the development phase because of challenges related to distortion. And on the other hand, the masked austenitization, which is used by only a few manufacturers due to the lack of experience, the impairments of coating condition and the great demands on the process management concerning re-cooling of the mask. Both variants are forward-looking and deliver high potential for further research.

Experimental work was undertaken to investigate the critical heat flux (CHF) mechanism in pool boiling. Local temperature and heat transfer coefficient on the heated surface were measured via IR camera. Based on the experimental results, the wetted area fraction and the contact line length density with wall superheat were observed. Also, an analytical work into pool boiling heat transfer was presented to determine the contribution of each region to the overall heat transfer in pool boiling. The heat transfer through the liquid was found to be the dominant heat transfer mechanism due to significantly larger area. While the heat transfer through the liquid determines the overall heat transfer, contact line heat transfer wall is critically important to trigger the onset of CHF. When the process of dewetting occurs at contact line and micro region, the temperature of dry spots increases, hence dryout areas increase and the CHF occurs. Finally, we proposed the CHF mechanism based on the surface dewetting and experimental data.

Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the heat transfer measurements are presented. The analysis of the results explains the influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the spray cooling boiling curve, on the Nusselt number and on the efficiency. It has been found that the effect of a smooth roughness is to delay the onset of the nucleate boiling, but once this regime has started, boiling is so fast that the CHF (Critical Heat Flux) is reached at lower heater temperatures and lower heat fluxes. In a companion paper the sprayed refrigerant film thickness measurements and its relation with the heat transfer measurements are presented., QC 20130408

Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the sprayed refrigerant film thickness measurements are presented. The influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the total average film thickness is analyzed and discussed. In a companion paper, results of the heat transfer measurements are presented. It has been found that there is a relation between the variations of the average Nusselt number and of the film thickness along the spray cooling boiling curve. The heat transfer regimes along that curve are related not only to a variation in the average Nusselt number but also to changes in the film thickness. The qualitative analysis of those variations served to better understand the heat transfer mechanisms occurring during the spray cooling., QC 20130802

Spray cooling experiments were performed to study the effect of the spray cone angle on heat transfer and film thickness. The experiments were carried out for three different types of nozzles. Results for heat transfer and film thickness for different flow rates are presented working with the dielectric refrigerant R134a and on a heater with low surface roughness. The behavior of the heat transfer with respect to the spray cone angle shows that as the spray cone angle decreases, there is a delay in the onset of the nucleated boiling regime. As a result, thermal performance worsens as the spray cone angle decreases. On the other hand, in the nucleate boiling regime film thickness increases as the spray cone angle decreases. The qualitative analysis of these variations serves to better understand the heat transfer mechanisms that occur during the spray cooling technique., QC 20130819

A closed two-phase loop system was developed that combined with a multi-nozzle spray cooling unit for the cooling of high heat flux power sources. The fluid circulation was sustained by a magnetic gear pump operating with an ejector pump unit. The motive flow of the ejector shared the pumping liquid flow with the multi-nozzle spray. The use of the ejector stabilized the circulation of the two-phase flow. A multi-nozzle plate with 48 miniature nozzles was designed to generate an array of 4x12 sprays. A closed loop spray cooling experimental setup with a cooling area of 19.3 sq cm was built. The spray nozzle to target distance was 10 mm. Water and FC-72 were used as the working fluids. Spray cooling experiments were performed in three orientations of the spray target surface, namely (a) horizontal facing upward, (b) vertical, and (c) horizontal facing downward. The thermal performance of the horizontal facing downward surface was the best. A comparison with the thermal performance data for a smaller cooling surface area of 2.0 sq cm was made., Presented at the ASME/JSME Thermal Engineering Joint Conference (8th) held in Honolulu, HI on 13-17 March 2011.

A two-nozzle spray cooling system (R-134a) was installed on an 8-foot-diameter centrifuge table to test the effects of elevated acceleration on steady state cooling performance. Acceleration effects were evaluated in four orientations relative to the acceleration vector, which ranged from 1-10 g's. The 2-cm2 heated surface removed heat fluxes up to 250 W/cm2 in all test cases, and small performance improvements were seen at high accelerations in certain orientations. Additionally, the vertical spraying upward and horizontal spraying orientations demonstrated improved cooling over the nominal downward spraying orientation, with a decreasing effect as radial acceleration increased.

Spray atomization and droplet dynamics are research topics that have existed for many decades. Their prevalence in manufacturing, energy generation and other practical applications is undeniable, though researchers have often overlooked the importance of understanding the physics of atomization or droplet impact characteristics in the ongoing effort to improve efficiency. In this talk, I will address the atomization of thermodynamically unstable "flashing" sprays and the splashing mechanisms of single droplets impinging on flat, smooth surfaces. The related heat transfer phenomena for cooling applications are also addressed. These topics are motivated by efforts to improve the thermal protection provided by cryogenic spray cooling in laser dermatological procedures, increasing the throughput of the spray production of nano and micro-scale particulates used as dyes and catalysts, and in modeling of the release and dispersion of flammable or hazardous chemicals through large-scale collisions with storage containers. Through the use of high-speed video imaging, phase Doppler interferometric measurements and numerical modeling of the two-phase flow taking place within spray nozzles, a detailed picture of the processes involved in flash atomization are attained. Results reveal that flashing fluid jets under low superheats undergo many dynamic processes leading to eventual droplet formation, including the nucleation of vapor bubbles within the nozzle interior and their subsequent expansion and explosion. At high superheats, a stable "flare flashing" regime is attained resulting in very fine atomization. These insights may lead to improved nozzle designs to better control the atomization process.High-speed imaging of droplet impacts also reveals new insights into the mechanisms of splashing. The surrounding ambient air pressure, fluid viscosity, and fluid-surface affinity are found to profoundly influence the initiation and characteristics of splashing. A new analytical mo

One of the main challenges of spray cooling technology is the prediction of local and average heat flux on the heater surface. It has been suggested that spray cooling heat transfer depends on the local spray mass flux. However, in this work it is hypothesized and demonstrated that local single-phase and boiling heat transfer can be predicted within ±25% of the measured values from the local normal pressure produced by the spray. In the single-phase study, hollow cone, full cone, and flat fan sprays operated at three standoff distances, five spray pressures, and two nozzle orientations were used to identify the relation between impingement pressure and heat transfer coefficient. PF-5060, PAO-2, and PSF-3 were used as test fluids, resulting in Prandtl number variation between 12-76. A 7×7 mm2 micro-heater array consisting of 96 platinum resistance heaters operated at constant temperature was used to measure the local heat flux. A separate test rig was used to make impingement pressure measurements for the same geometry and spray pressure. The heat flux data were then compared with the corresponding impingement pressure data to develop a pressure-based correlation for single-phase spray cooling heat transfer. Hollow cone and full cone PF-5060 sprays at three subcooling levels were used for the two-phase heat transfer study. The conventional wisdom is that the temperature at which critical heat flux (CHF) is observed changes with the droplet impact velocity, droplet number density, and droplet size. However, the present measurements indicate that although the magnitude of CHF is strongly dependent on the spray characteristics, the temperature at which CHF occurs lies within a very narrow band (about ±5°C) for smooth flat surfaces. This was also observed from local measurements at various radial distances using hollow cone and full cone spray nozzles where the local mass flux varies dramatically. This observation along with liquid properties and subcooling were used to

This thesis discusses the effects of a variable-gravity environment on the performance of a subcooled partially-confined spray. An experiment, consisting of a test chamber, the associated flow loops, and instrumentation, was fabricated and flown on the NASA Reduced-Gravity Testing Platform. This modified KC-135 aircraft followed a parabolic flight path to provide various acceleration levels. The spray chamber contained two opposing nozzles spraying onto Thick Film Resistor (TFR) heaters, which were mounted on insulating glass pedestals. Only the upward facing heater was used during this testing. Thermocouples under the heater in the glass pedestal were used to determine the heater surface temperature. The glass pedestals were surrounded by an annular sump system, which was used to collect and remove the cooling fluid from the test chamber. The fluid used for this testing was FC-72, which is a non-toxic, non-flammable, and non-reactive refrigerant. Due to its dielectric nature, FC-72 was sprayed directly onto the electric thick film heaters. The parametric ranges of this experiment were as follows: Heat flux to the spray, 21.1 less than or equal to q"sp less than or equal to 69.0 Watts/(square centimeter); acceleration field, 0.15 less than or equal to "a" less than or equal to 1.80 grams; coolant volumetric flow rate, 6.18 less than or equal to V less than or equal to 8.94 milliliters/second; and coolant subcooling, 23.1 less than or equal to delta-T(sub sc) less than or equal to 31.7 degrees Kelvin. The heat fluxes tested were below the critical heat flux q"CHF. The wall superheat delta-T(sub sat) = T(sub s) - T(sub sat) was found to increase with heat input and acceleration, and decrease with subcooling and volumetric flow rate., Performed in collaboration with Wright State University, Dayton, OH and NASA Glenn Research Center, Cleveland, OH. In-house contract.

The objective of this thesis is to investigate the heat transfer performance of a partially confined FC-72 spray with varying dissolved air concentrations. An experimental test rig consisting of a spray chamber coupled to a fluid delivery loop system was used to obtain temperature, pressure, and critical heat flux (CHF) data. A downward facing nozzle within the spray chamber allowed the FC-72 fluid to be sprayed onto an upward facing, thick-film, resistor heater. The heater was mounted onto a glass post, with a sump system to allow removal of excess fluid. Type-E thermocouples were imbedded in the post to obtain temperature data. The parametric ranges for experimental testing were as follows: volume-percent concentration of dissolved air, 5

The objective of this thesis was to investigate the cooling performance of a 16-nozzle spray array, using FC-72 as the working fluid, in variable gravity conditions with additional emphasis on fluid management and flow stability. A flight test experiment was modified to accommodate a 16-nozzle spray array, which was then tested in the parabolic flight trajectory environment of NASA's C-9 reduced gravity aircraft. The 16-nozzle array was designed to cool a 25.4 by 25.4 [mm] area on a thick film resistive heater used to simulate electronic components. Data was taken and reduced as a result of flight tests conducted over the course of two flight weeks (each week consisting of four flights, each flight consisting of 40 to 60 parabolas). The flight tests were conducted in order to examine gravity effects on spray cooling performance and to evaluate a novel liquid-vapor separator design. The mass flow rate through the 16-nozzle spray array ranged from 13.1< m< 21.3 [g/s] for the spray cooling analysis and 14

Main objective was developing the foundation for designing sprayed cooled power packages for semiconductors like silicon-carbide devices capable of high temperature and voltage operation. Research addressed: (1) Understanding of the heat transfer phenomenon during spray cooling under phase change; (2) Packaging materials for high temperature and spray cooling or jet impingement; (3) A scaled down spray-cooled power package of a power conversion module; and (4) New electric topologies of power conversion modules for integration with spray-cooling solutions. Main conclusions were: (1) Quick replacement of hot liquid by cold liquid (i.e., transient conduction) is main mechanism enabling high heat fluxes; (2) Jet impingement without nozzles provides cost effective solutions; (3) Several high temperature active metal brazing materials suitable for high temperature die attach but requiring more research to ensure proper chemical behavior; (4) A double-sided power package providing minimized volumetric footprint for the spray box and integrates the power electronic systems; and (5) Several new electric topologies of power conversion modules., Project No. 07PRO5674-00.

The primary goal of this project was to discover how to most effectively use electromagnetic forces to enhance spray impingement boiling in microgravity (%g). This project was closely coordinated with the non-electromagnetic spray impingement boiling heat transfer experiments conducted by Dr. Kirk L. Yerkes of the Air Force Research Laboratory (AFRL). The West Virginia University (WVU) investigation of electrical body forces to enhance and control spray impingement boiling extended the AFRL research. The computational phase of the WVU project was based on the commercial multiphysics code CFD-ACE+, which was successfully modified to incorporate the Coulomb and electric Kelvin forces. WVU's ground-based experiments demonstrated for the first time that modest increases in heat transfer coefficient and Nusselt number can be achieved in spray impingement boiling by using electrical forces, thus confirming the fundamental hypothesis of this research. Further increases in heat transfer performance are likely to result from a better understanding of the relevant microphysics and better electrode designs. Experimental results have been used to estimate the time scales for various phenomena that occur in spray impingement boiling, which could lead to improved electrode designs., The original document contains color images.

This paper discusses the effects of a variable-gravity environment on the performance of a subcooled partially-confined spray. An experiment was flown on the NASA KC-135, providing various gravity levels (- 0.16, 0.35, 1.0, and 1.8 g). The experiment consists of a nozzle spraying on a Thick Film Resistor (TFR) heater which is mounted on an insulating glass pedestal. The working fluid is FC-n. Non-dimensional heat input varied over the range 35 < (1-f) GDelta < 190, with a heat flux of 20 < (l-f)q" < 70 W/cm2. The non-dimensional grouping (Frl/2Ga)l/2, a ratio of inertia and acceleration forces to viscous forces, varied over the range 35 < (Fr1/2Ga)l/2 < 70. Subcooling temperature ranged from 24 deg C < Tsc < 30 deg C. For this data, from one flight week of three flights, DeltaT = Ts-Tsat is found to decrease with decreasing values of (Fr1/2Ga)l/2. It appears that higher values for subcooling may enhance this acceleration effect. Changing the flow rate may affect how much the acceleration level affects the 2-phase heat transfer coefficient (h2-p), with the higher flow rate showing less of a change in h2-p as the acceleration level changes., Prepared in collaboration with Wright State University, Dayton, OH and NASA Glenn Research Center, Cleveland, OH. Presented at the AIAA Aerospace Sciences Meeting and Exhibit (45th) held in Reno, NV on 8-11 Jan 2007 and published in the proceedings as AIAA 2007-198. See also related paper, ADA468418, AFRL-PR-WP-TP-2007-223, Cooling Performance of a Partially-Confined FC-72 Spray: The Effect of Dissolved Air, AIAA 2007-199. The orginal document contains color images.

The present paper deals with a theoretical analysis of the spray cooling of a Reactor Pressure Vessel (RPV) head in BWR. To this end a detailed computational model has been developed. The model predicts the trajectories, diameters and temperatures of subcooled droplets moving in saturated vapor. The model has been validated through comparison with experimental data, in which droplet temperatures were measured as functions of the distance that they cover in saturated vapor from the moment they leave the sprinkler outlet to the moment they impact on the RPV head inner wall. The calculations are in very good agreement with measurements, confirming the model adequacy for the present study. The model has been used for a parametric study to investigate the influence of several parameters on the cooling efficiency of the spray system. Based on the study it has been shown that one of the main parameters that govern the temperature increase in a subcooled droplet is its initial diameter. Comparisons are also made between conclusions from the theoretical model and observations made through flow and temperature measurements in the plant (Forsmark 1 and 2). One of these observations is that the rate at which the RPV head temperature decreases on the way down from hot to cold standby is constant and independent of the sprinkling flow rate as long as the flow rate is above a certain minimum value. Accordingly, the theoretical model shows that if one assumes that the cooling of the RPV head is through a water film built on the inner wall due to sprinkling, the heat removal rate is only very weakly dependent on the sprinkling flow rate., QC 20141106

The effects of elevated acceleration fields on spray cooling heat transfer are discussed in this paper. Spray cooling has proven to be one of the most efficient methods of heat removal. This technology is being transitioned into more advanced applications, such as fighter aircraft that must withstand a wide range of variable acceleration-induced body forces. Heat transfer associated with closed-loop spray cooling will be affected by acceleration body forces, the extent of which is not yet known. To test these various effects, an eight-foot-diameter centrifuge table will be outfitted with a spray cooling system to test for the effects associated with elevated gravity., Published in the Proceedings of the SAE Power Systems Conference, paper no. 06PSC-90, 2006. Prepared in collaboration with Wright State University, Dayton, OH.

In our recent works Theofanous et al., 2002a-b, we have demonstrated that burnout in pool boiling is not hydrodynamically limited, at least not in the sense that has been perceived in the past. In this paper, we discuss the opportunities created by the new understanding of mechanisms that govern the boiling crisis. This understanding is built upon a scales separation phenomenon, referring to a vapor blanket separating the liquid film on the heater surface from the chaotic, churning flow in the two-phase pool. In essence, the scales separation suggests that mechanisms of boiling crisis should be sought within the micro-hydrodynamics of the evaporating liquid microlayer rather than in the pool thermal-hydraulics. Detail analysis of surface temperature patterns obtained by infrared imaging at high heat fluxes points to a nearly static picture of boiling heat transfer, with intense cooling at locations which were nucleation sites activated at lower heat fluxes. Furthermore, we show that control of the surface and coolant chemistry offers the potential to enhance resistance to burnout and achieve critical heat fluxes (CHF) exceeding those defined by the so-called hydrodynamic limit. Our more recent experiments show an improved resilience of the heater to burnout when a high-solubility salt or nanoparticles are added to the coolant (water). We explain the observed phenomenon through the increase in disjoining pressure at the meniscus contact line that promotes liquid spreading towards the dry area. It is noteworthy that the scales separation phenomenon provides a basis to suggest that mechanisms of enhancement to burnout in pool boiling are also active, even to a larger extent, in spray cooling and flow boiling., QC 20141125