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To investigate the spray cooling characteristics and the impact of spray parameters such as chamber pressure, spray height, and spray tilt angle on heat transfer efficiency, a mathematical model based on the Eulerian–Lagrangian frame was established for an R410A closed-loop spray cooling system. The results revealed that the spray pattern is conical, with the center velocity significantly higher than the edge velocity. The temperature distribution of the cooling surface and liquid film height both exhibit a "W" shape, and the surface temperature is lower where the liquid film is thin. There is an optimal liquid film height of approximately 5 μm, at which the cooling surface temperature is the lowest. The surface temperature increases with an increase in the spray chamber pressure. Considering average cooling surface temperature, the optimal tilt angle is 40° with an average surface temperature of 330.1 K. When considering wall temperature and wall heat transfer coefficient uniformity, however, the optimal tilt angle is 10°, leading to the average surface temperature of 332.6 K. When increasing the optimal spray height to 70 mm, the average surface temperature is 313.4 K. [ABSTRACT FROM AUTHOR]

In this study, a closed-loop spray cooling system using R134a as the working fluid was established. The heat transfer characteristics and influencing mechanism of transient spray cooling were studied. The transient spray cooling curve under quenching was built accurately. The results show that the vapor film suppressed time, tsup, is the main period that the spray cooling must pass through. The flow rate and the sub-cooling of R134a have little effect on the cooling rate but the critical heat flux, which are mainly affected by chamber pressure. The transient Jacob number decreases with the increases of chamber pressure. As Jacob number decreases, the growth of vapor film is inhibited, then the tsup reduces in consequence. The surface temperature drop point and critical heat flux increases with the rise of chamber pressure. The maximum critical heat flux is 70.08 W/cm² in this experiment. [ABSTRACT FROM AUTHOR]

Abstract Spray cooling technology as a high heat flux cooling method is widely used to meet the increasingly cooling requirements, and the enhancement of spray cooling has gradually become a hot spot. In this study, three surfactants of AOS, CTAB and Tween20 with different concentrations were added into pure water to investigate their effects on spray cooling at low temperature range (≤100 °C) in a closed system. The three kinds of surfactants obviously improved the heat transfer performance, which had different optimum concentrations. The lowest mean surface temperature was obtained by CTAB with concentration of 200 ppm. The surface temperature non-uniformity of different surfactants spray was not improved, while the heat transfer deteriorated at high concentrations. In further study, the inclined mode method was used to alleviate the influence of bubble accumulation at high concentrations. The results showed that the inclined spray could significantly enhance the heat transfer capacity of the spray with surfactants and produce the optimum concentration point backward shift phenomenon. Moreover, the inclined spray mode showed a better promotion effect on spray cooling of surfactants with large surface tension. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The transient heat transfer performance of spray cooling with R21 is studied experimentally. Three variation patterns of surface temperature depending on the initial surface temperature are observed. There is only one rapid temperature drop at lower initial surface temperature and two drops with a shoulder transition at higher initial surface temperature. Further increasing the initial surface temperature will deteriorate heat transfer. Based on the pool boiling heat transfer curve and one-dimensional transient heat conduction model, the temperature variation patterns and processes are discussed which relate to nucleate boiling, transition boiling and film boiling, respectively. The experimental results reveal that the deterioration temperature decreases with increase of the heating flux and nozzle-to-heater distance, while increases with the spray flow rate. Based on the experimental results a region map of temperature variation patterns in transient spray cooling is given. [ABSTRACT FROM AUTHOR]

The aim of this paper is to study the spray cooling heat transfer characteristics using CFD method. The two phase mathematical model is established based on Euler-Lagrange approach. In this model, unsteady-state flow conditions are simulated. Heat and mass transfer between the vapor and the water droplet are computed. The averaged error of numerical results is 10% compared to experimental results in our previous papers. It is concluded that the heat flux and its distribution on heated surface are influenced by the heated surface temperature, mass flux, nozzle to surface distance and the number of nozzles. Furthermore, the heat flux distribution do not change obviously with the increasing of heated surface temperature, and the heat flux increases with the increasing of mass flux. Moreover, there is an optimal nozzle to surface distance while the spray impingement zone circumscribes in heated surface, and more nozzles lead to higher heat flux and better heat flux distribution on heated surface. Considering the disadvantage of manufacture difficulty and cost increase for more nozzles, there is an optimal nozzle number in certain space and heat dissipation requirement. The optimal number of nozzles is 8 in this article. [ABSTRACT FROM AUTHOR]

A commonly-used method to cool rotating cylinders involves spraying water on the surface to cool its internal particles. This paper conducted a numerical investigation of heat transfer from a rotary cylinder cooled with sprayed water. A 3-D numerical model was generated, and the distributions of the wall temperatures and liquid films were analyzed. Then, the heat transfer characteristics for different parameters were obtained and compared. The results showed that the best location for a single spray nozzle was directly above the cylinder, where it had the most uniform liquid film and highest heat exchange efficiency. With increased rotation speeds, the cylinder wall temperature increased and the distribution of wall temperature and liquid film became more uniform, which improved heat transfer. Because of the influence of gravity, the slope of the cylinder had a negative influence on the distribution of the liquid film. When the cylinder material had lower thermal conductivity, the system needed a much longer period of time before it was stable and had lower heat flux. Furthermore, with the same heat source, the system heat flux could not be determined with only the outer surface temperature, but with the temperature difference between inner surface and outer surface. [ABSTRACT FROM AUTHOR]

• Spray cooling heat transfer of various surface structures are investigated numerically. • 3D Euler-Lagrangian simulation is performed and wall-film model is used. • Liquid film thickness and streamlines are computed in the vicinity of different fin geometries. • Local film thickness increases in grooves. • The film velocity magnitude depends on the fin blockage. Liquid impingement cooling in the form of jet or spray is an appropriate method to remove heat from small surfaces progressively utilized in industry. In this study, numerical simulations are conducted to investigate the effect of surface structure on heat transfer characteristics. Five different three-dimensional structured surfaces are investigated using Euler-Lagrangian approach. The effect of surface structures is discussed on the liquid film thickness, the liquid film velocity, and the heat flux with its uniformity for a given spray and temperature characteristics. The results show that the convective heat transfer strongly depends on both liquid film thickness and velocity. While the liquid film thickness always grows in the grooved surfaces, its velocity depends on the form and direction of the fins. The averaged values of the film thickness and velocity magnitude are almost independent of the surface temperature. Among the investigated surfaces, the parallel and circular patterns showed the best and worst performance on heat transfer characteristics, respectively. [ABSTRACT FROM AUTHOR]

• Heat transfer for non-simultaneous droplets impinging onto a heated dry solid is examined. • Effects of time separation between adjacent droplets and surface temperature are studied. • Peak heat transfer rates for two droplets can be 68–100% higher than that of a single droplet. • Droplet-droplet interactions led to a decrease in the heat transfer rate and cooling efficiency. High generation of heat in the electronics industry presents a roadblock for further innovations in higher processing power due to complications associated with overheating. Commonly used air-based systems cannot cool high-performance electronics, mainly due to the relatively poor thermo-physical properties of air. Spray cooling is a technique capable of cooling or quenching objects at very high temperatures and heat fluxes. While there has been considerable research on single droplet impact to study the fundamentals of spray cooling, multiple droplet impingement offers more realistic realisations of the dynamics of spray phenomena. In this paper, we examined the fluid dynamics and heat transfer of multiple droplet impingement using high-speed imaging and fast heat flux measurements to capture droplet-droplet and droplet-surface interactions at constant droplet Weber number. We investigated an individual droplet and two non-simultaneous droplets of water at a constant horizontal droplet spacing and different time separations to analyse the heat transfer at different initial surface temperatures. Although not affected significantly by the surface temperature, the maximum diameter of the combined droplets decreased for increasing time separation due to the kinematic discontinuity from the interaction between droplets moving in opposite direction. For small time separation, the spreading droplet contact line of the first droplet collided with the moving contact line of the second droplet. The receding and the quasi-static contact lines of the first droplet was strongly and mildly impacted by the second spreading droplet diameter, respectively. However, all quasi-static droplet diameters increased for non-simultaneous droplet scenario, following by a small reduction in the droplet diameter when the surface temperature increased due to evaporation. Depending on the surface temperature and time separation between droplets, it was found that the peak heat transfer rates for non-simultaneous droplet impingement scenarios can be increased by 68% to 100% when compared to a single droplet case due to higher combined droplet diameter and fluid volume in contact with the heated solid surface. However, the overall droplet cooling efficiency was higher for the single droplet case, showing that the droplet-droplet interaction can decrease the potential cooling performance of droplets. The present study provides knowledge combining droplet-droplet interaction and heat transfer performance for future spray cooling system design and optimisation. [ABSTRACT FROM AUTHOR]

This experimental work aims at investigating the cooling of hot surfaces by using full cone sprays; comparison with the use of a liquid jet is also considered. The wall is a 175 mm diameter nickel disk and 5 mm thickness heated by electromagnetic induction up to about 800 °C. In the case of the sprays, the goal is to link the spray properties with the heat flux removed from the heated surface. For the spray, the influence of the mass flux distribution as well as the droplets properties on the cooling are studied by using three different spray nozzles. The mass flux distribution for each of the sprays is measured with the help of a patternator. The heat removed during the cooling phase is investigated with the use of infrared thermography while the droplet properties are characterized simultaneously by a Phase Doppler system. The Phase Doppler technique is mainly applied to study the statistical properties of the outcoming droplets in the vicinity of the heated surface. A decrease of the outcoming droplets size compared to the incoming one is well observed. Moreover, Phase Doppler device has also highlighted that the presence of the surface may have a significant influence on the upstream spray flow. Compared to the liquid jet, heat flux measurements have clearly demonstrated that the sprays lead a more spatial uniformity of the extracted heat flux and a better cooling efficiency. When spray cooling is considered, the influence of the mass flux on the heat flux and the cooling efficiency is in qualitative agreement with previous studies performed in similar conditions. In addition, a comparative study of performances, in term of liquid consumption and cooling duration, is performed by considering a similar surface temperature decrease for the three sprays and the liquid jet. Among the four experiments, the liquid jet has the longest cooling time as well as the highest liquid consumption. Furthermore, the best performances are reached for the spray having the highest mass flux value. [ABSTRACT FROM AUTHOR]

Heat dissipation and uniform temperature distribution are crucial to the operational reliability of electronic devices. In this study, the heat transfer enhancement and surface temperature non-uniformity (STNU) improvement of spray cooling are numerically investigated by CFD. The simulated results match well with experimental data under verification conditions with the maximum relative deviation below 7%, which demonstrates the numerical reliability of the two-phase flow model based on Euler-Lagrangian approach. The results of adjusting spray height reveal that there are different optimal spray heights for the heat transfer (4 mm) and the STNU (12.81 mm), indicating that the heat transfer enhancement and STNU improvement cannot be realized simultaneously. Based on this, two methods to enhance heat transfer and reduce STNU are developed. One method is to reduce the tangent angle θ and increase the tangent height H accordingly. It shows that the θ = 30°, H = 27.59 mm case has a maximum enhancement of 24.03% in heat flux relative to the θ = 100°, H = 6.21 mm case. Moreover, for all nozzles (θ = 30°∼100°), the lower STNU is attained at respective tangent heights. The other method is spray-array jets cooling, in which 132 micro-hole jets are added around the spray. It is found that the minimum STNU is obtained by spray-array jets cooling compared with all cases of spray cooling. In addition, it weakens the evaporation, but promotes the forced convection dramatically. Based on the analysis of several average liquid film microscopic characteristics and the droplet distribution, it is concluded that the mean film velocity, thickness and droplet coverage uniformity are the keys to determine heat transfer and STNU. • Heat transfer and surface temperature non-uniformity STNU are numerically studied. • Two methods are developed to enhance heat transfer and reduce STNU simultaneously. • The minimum STNU is attained by spray-array jets cooling relative to spray cooling. • The mean film velocity and thickness are the key factors affecting heat transfer. • The more uniform the coverage of the impacting droplets, the smaller the STNU. [ABSTRACT FROM AUTHOR]

• Liquid film morphologies are defined as 'continuous, interacted, and isolated' types. • The interval of wetted areas for the isolated films under P = 0.2 MPa is [0.11, 1.54] mm2. • The distributed and surface-averaged velocity of isolated films are predicted. Although the understanding of spray cooling has significantly improved, knowledge of the heat transfer mechanisms is still limited, and the related models based on experiments are separated from physical phenomena. As an example, while liquid film morphology have been investigated in detail, comparatively little is known about how the liquid films evolve on the surface, owing to the strong interaction and disturbance between atomised droplets and thin films with high turbulence. In this study, the liquid film dynamics of HFE-7000 spray in a two-phase cooling regime on a smooth and unpolished silicon surface were quantitatively captured, including the film morphology, film velocity, wetted area, and contact line length, under different inlet pressures and heat fluxes. Owing to the thermally induced agglomeration with increasing heat flux, the continuous liquid film finally dispersed into an isolated state, which is quantitatively defined as the 'isolated film regime' by the critical interval of the wetted area. The velocity of the isolated film under various conditions is a consequence of the radial velocity of the atomised droplets, surface conditions, and surface temperature gradient. Based on the correlation of the radial velocity of atomised droplets, prediction methods for the distribution and surface-averaged velocity of isolated films are proposed. [ABSTRACT FROM AUTHOR]

• Visualized microbubble dynamics in boiling water droplets on textured substrates. • Number and size distributions of the nucleated bubbles within a single droplet. • Analysis of bubble growth & number change with time within a drop as it boils. • Estimated latent heat-flux removal during growth of single and multiple bubbles. Dissipating large heat fluxes from a surface is critically important in numerous industrial and natural applications. Boiling based spray cooling and surface texturing are two of the most promising methods being investigated to address this problem. Although our understanding on these topics has significantly improved over past decades, critical gaps remain in the knowledgebase stymieing the realization of their full potential. As an example, while bubble growth in pool boiling have been investigated in detail, comparatively little is known about how the bubbles evolve inside boiling drops. In the present work, we have investigated for the first time, the microbubble dynamics inside water droplets boiling on superhydrophilic textured substrates using high-speed X-ray phase contrast imaging (XRPCI). Our observations show that the transient bubble density variation follows similar characteristics irrespective of the texture spacing at a given surface temperature. For an example microstructure, we found that the number of discrete bubbles on the surface decreases as temperature is increased although their growth rate increases. We observe that bubble growth is highly non-uniform during the lifetime of a drop on the surface. Initially, bubbles grow under diffusion-limited regime, but at later times they grow as ~ t 1.45 due to combined effects of coalescence and evaporation. In some conditions, we found that bubbles shrink dramatically after the initial growth spurt presumably due to severe quenching of the surface, and migration of bubbles on the surface. Using the bubble sizes, for the first time we analyzed the heat flux removed by a single bubble and also by all the bubbles at a given time. We find that the highest dissipation through latent-heat component (~600 W/cm2) occurs just in the beginning and thereafter it decreases. We expect that our findings and the analysis would guide further work on the topic and will aid in the overarching goal of engineering surfaces that are more efficient in boiling heat transfer. [Display omitted] [ABSTRACT FROM AUTHOR]

• The distributions of spray characteristics under different conditions were measured. • Relationship between spray characteristics and surface temperature was established. • Surface temperature non-uniformity was predicted by the model in the present study. The surface temperature non-uniformity is crucial in understanding the heat transfer mechanism of spray cooling. It is also a sensitive factor in the thermal management of electronic devices or in the application of transient sprays where the surface heat flux is obtained from the internal temperature measurement. In this study, the effects of heat flux, subcooling, nozzle-to-surface height, and injection pressures on the surface temperature uniformity in a spray cooling application were studied. The distribution of droplet diameter and the spray volumetric flux was found to significantly vary under the above experimental conditions. A higher spray volumetric flux and smaller droplet diameter resulted in a lower surface temperature. In general, the spray volumetric flux is the dominant parameter. The effect of droplet diameter distribution always became obvious with low spray volumetric flux. Regarding the high spray volumetric flux, there was an offset between these two parameters. The results also indicated that increasing the heat flux, nozzle-to-surface height, and subcooling, as well as decreasing the inlet pressure, all resulted in higher surface temperature uniformity owing to the variation in the droplet diameter and spray volumetric flux distributions. Finally, prediction methods for temperature non-uniformity were proposed for single-phase and two-phase regimes. [ABSTRACT FROM AUTHOR]