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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]

• A magnetic field changes the ferrofluid droplet-surface interaction and cooling performance. • Ferrofluid cooling with a magnet is from 28% to 263% higher than that without it. • A silicon substrate under a magnetic field shows the best heat transfer performance. • Superhydrophobic surfaces do not foul after ferrofluid droplet impact and evaporation. [Display omitted] This paper documents the spreading characteristics and heat transfer of a single ferrofluid droplet impinging onto dry solid surfaces with and without a magnetic field. Three surfaces were investigated: a pure silicon substrate, a silicon substrate with a thin titanium film, and a superhydrophobic coating. Fluid dynamics and cooling results of a ferrofluid droplet were compared to that of pure water and water-surfactant droplets for the same Weber number (We = 61) and initial surface temperature (T 0 = 95 ∘ C). Our results show that the magnetic field (magnetic Bond number Bo m = 2186) increased the droplet spreading diameter up to 12% and suppressed droplet rebound, thus improving the cooling performance of a ferrofluid droplet. The highest heat transfer rates were measured for the pure silicon substrate under a magnetic field with an average value of 14 W. The lowest cooling performance was observed for the superhydrophobic surface without a magnetic field with a peak value of 0.82 W. However, using the magnetic field, the superhydrophobic surface achieved 263% higher heat transfer rate than the non-magnetic case because of the suppression of droplet rebound. Moreover, the surface did not foul from ferrofluid deposition, an advantage for spray cooling applications when using any nanofluid. [ABSTRACT FROM AUTHOR]

• Macro-structure and nanoporous surfaces are tested in close-loop R410A spray cooling. • Both macro-structure and nanoporous surfaces greatly enhances cooling capability. • Pyramid surface with 3 μm roughness achieves 330 W/cm2 CHF at low wall temperature. • Fin structure plays a decisive role rather than area increase in cooling performance. • CHF first decreases and then increases with pore diameter for nanoporous surface. Flash spray cooling has been considered as one of the most promising technologies of heat dissipation for high power electronic devices because of its high cooling capacity at low surface temperature. In this study, experiments were conducted to study the heat transfer enhancement due to surface modification in the close-loop R410A flash spray cooling system. The test surfaces including the macro-structured surfaces with pyramid and square fins with two orders of roughness, and the nano-porous surfaces with different pore diameters were examined. The experimental results indicated that the surface with macro fins could tremendously enhance the heat transfer due to the increase of wetted area. However the pyramid fins with less increase in wetted area showed a better heat transfer performance than square fins, indicating fin structure played a more important role than the increase in wetted area. Higher roughness could further improve the cooling performance of macro-structured surface. The maximum CHF of 330 W/cm2 and heat transfer coefficient of 300 kW/(m2 K) were achieved by the surface with rough pyramid fins, corresponding to 60% enhancement and 5 times respectively over smooth flat surface, while the wall temperature was maintained below 10 °C. The nano-porous surface could also lead to better heat transfer performance by increasing the number of nucleation sites and improving the wettability to working fluid. The result of pore size effect showed that the CHF value at first declined and then increased with the increasing pore size. It was postulated that this phenomenon was related to the transition of dominating heat transfer mechanism from "evaporation-limited region" to "viscosity-limited regime". These findings helped to guide further investigations of enhanced surface aiming at enhancing heat transfer performance in flash evaporation spray cooling. [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]

• Spray cooling tests are performed at various system pressures ranging from 0.1 MPa to 0.5 MPa in a practically oxygen-free environment. • Quenching occurs at higher temperature as the system pressure increases, mainly due to the increase in the saturation temperature. • The rate of heat transfer in the film boiling regimes increases with the system pressure due to the reduction of vapour film thickness. • Partial liquid-solid contact may take place even at sufficiently high surface temperatures. • Predictive models for the onset of quenching and heat transfer rate in the film boiling regime were assessed and found to agree with the experimental observations reasonably well. The present contribution addresses an open question about spray cooling: when does quenching occur? We performed water spray cooling tests with heated iron disks at system pressures ranging from 0.1 MPa to 0.5 MPa. Sample heating was carried out under high vacuum and then spray cooling in an argon environment, minimising the effect of surface oxidisation. The temperature for the onset of quenching was found to increase with the system pressure. We demonstrated that the quenching point could be predicted reasonably well by a mechanistic model that considers the force balance at the interface between the droplet and the vapour film underneath it. The influence of droplet impact velocity was also discussed. From the temperature histories obtained by the cooling tests, the rate of heat transfer in the film boiling regime was evaluated and found to increase with the system pressure. This was due to the increase in the saturation temperature which then led to the reduction of vapour film thickness, as corroborated by a Leidenfrost droplet model. Using the spray parameters, we quantified the amount of heat removed by a single droplet and compared it with theoretical models. Our analysis suggested a potential existence of partial droplet–surface contact even in the film boiling regime. [ABSTRACT FROM AUTHOR]

In consideration of droplet flash evaporation and film flash evaporation, a comprehensive mathematical model of vacuum flash evaporation cooling (VFEC) was presented in this paper. Pure water was considered to be the working fluid. The droplet flash evaporation was modeled based on the diffusion-controlled evaporation model, while the film flash evaporation was modeled based on the film penetration theory. The droplet flash model coincided well with the experimental results reported in the literature and the comprehensive mathematical model was validated by experiment results in this paper. The droplet temperature and diameter after droplet flash evaporation were obtained numerically, and then analyzed. The effect of droplet flash evaporation on the surface temperature distribution, heat flux density, and heat transfer ratio of different mechanisms was studied based on the model. It can be concluded that the droplet flash evaporation, which can be expressed by the ratio of droplet-wall impaction heat transfer, had a great effect on vacuum flash evaporation cooling heat transfer. The droplet-wall impaction and film flash heat transfer were dominant in the vacuum flash evaporation cooling. The spray characteristics such as droplet diameter and temperature decreased a lot after droplet flash evaporation, compared to the initial parameters (measured by the PDA). After taking droplet flash into account, the surface temperature decreased and the heat flux increased. [ABSTRACT FROM AUTHOR]

In order to enhance the heat transfer performance of spray cooling system, an integrated system is developed. The system is based on the refrigeration cycle, and uses isobutane (R600a) as the coolant. The heat transfer performance, such as the heat surface temperature, the heat transfer coefficient and the surface temperature distribution, are experimentally investigated in this paper. When the coolant mass flow rate is nearly 6.9 kg/h, the surface temperature can be kept at 57.3 °C with the heat flux of 145 W/cm 2 . The heat transfer coefficient can be achieved up to 35,000 W/(m 2 °C) when the nozzle inlet pressure and the chamber pressure are 4.9 bar and 2.3 bar, respectively. The surface temperature uniformity is mainly influenced by the mass flow rate, the heat flux and the nozzle inlet pressure together, and the standard deviation of the surface temperature is less than 4.0 °C in this study. All the results indicate that this system is promising in the application for high heat flux removal. [ABSTRACT FROM AUTHOR]

• For Weber < 6000 and T ≤ T s a t , droplet dynamics is only affected by the impact velocity. • Droplet spreading diameter was found to be independent of surface temperature. • Low impact velocities are better at heat removal than high velocities. • A physics-based model is developed to predict the transient surface temperature. Single droplet impingement on a heated surface is a fundamental problem of basic interest in understanding complex phenomena in spray cooling and other applications involving heat transfer to impinging droplets. The surface temperature experiences rapid temporal gradients upon droplet impact. These temperature transients are strongly dependent on the hydrodynamic and heat transfer regimes. Experimental results are presented for the transient surface temperature in the film evaporation heat transfer regime for water drop diameters of 2.1 and 3.5 mm at drop Reynolds numbers up to 34,000 and Weber number up to 5540 are reported for surface temperatures of 60 to 100 ∘ C. High-speed video was used to capture the dynamics of the impact. Intrinsic thermocouples were manufactured to measure surface temperature with sufficient temporal resolution. It was found that the hydrodynamics of the impact were not significantly affected by the surface temperature. Heat removal was found to be more effective at lower impact velocities. The maximum temperature drop occurred at the maximum spreading diameter. An analytical model was developed to successfully predict the transient Nusslet number of the droplet motion on the surface for both free-falling and air-propelled droplets. [ABSTRACT FROM AUTHOR]

Abstract: The effect of surface wettability on the collision dynamics and heat transfer phenomena of a single water droplet impacting upon a heated solid surface has been studied experimentally. To modify the surface wettability, two modules of stainless steel coated by TiO2 were employed. The first module was induced by ultraviolet irradiation to produce the hydrophilic surface, while the second one was not. The diameter and the depth of coating surface were 30mm and 200nm, respectively. The droplet size was varied from 1.90 to 2.90mm and substrate temperature raised up to 340°C. The interaction of an impact water droplet with a heated solid surface was investigated using a high-speed video camera. As a result, it was found that; (1) in the lower surface temperature region the evaporation time decreases as the static contact angle decreases, (2) the wetting limit temperature decreases with the increase of static contact angle, (3) the ultraviolet irradiation on the TiO2 surface does not change the qualitative behavior of the evolution of wetting diameters, and (4) the maximum wetting diameter increases with the decrease of static contact angle below the wetting limit temperatures. [Copyright &y& Elsevier]

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]

• The optimum operating condition of pulsed spray cooling is determined. • The effects of mixed surfactants of AOS-FS-31, CTAB-FS-31 and CTAB-AOS are studied. • The best heat transfer performance is achieved with mixed surfactants of 25% AOS and 75% FS-31. Pulsed spray cooling is a new type of cooling technique which can make full use of working medium and achieve better heat transfer effect. In this study, the effects of surfactants on pulsed spray cooling were investigated experimentally in a closed pulsed spray cooling system. The optimal pulsed spray parameters were determined to be 450 ms spray cycle and 60% duty cycle. Under this condition, the heat transfer performance of surfactant FS-31was studied. The results show that the mean surface temperature and the heat transfer coefficient of FS-31 are 50.01 °C and 1.55 W/cm2·K at the concentration of 100 ppm. Furthermore, the effects of mixed surfactants of AOS-FS-31, CTAB-FS-31 and CTAB-AOS on the heat transfer performance of pulsed spray cooling were studied. The results show that the mixed surfactants of AOS-FS-31 has a significant enhancement on the heat transfer performance of pulsed spray cooling due to the synergistic effect between anionic and nonionic. The mean surface temperature and the heat transfer coefficient can reach 48.03 °C and 1.75 W/cm2·K. [ABSTRACT FROM AUTHOR]