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A comparative study of liquid jet and spray is explored over plain and pillared copper surfaces with coolant flow (water) ranging over 3000 < Re < 10500, and heat fluxes up to the critical heat flux limit. Liquid jets and sprays are hydrodynamically characterized using high-speed videography and the phase-Doppler particle analyzer system. The interaction of liquid jets and sprays with the respective metallic substrates is experimentally investigated and strong liquid splattering is observed over the pillared surfaces at higher coolant flow rates. The resulting splattering mass fraction is measured over a range of coolant flow rates and radial positions of the nozzle. Subsequently, the effect of splattering on heat transfer performance is compared for various heat flux conditions by measuring the steady-state surface temperature. Heat transfer coefficient is found to be a strong function of coolant flow rate, operating heat flux conditions, and splattered mass fraction. Significant differences are not seen in these quantities between the liquid jet and spray for a plain surface. However, spray cooling clearly shows superior heat transfer enhancement over a liquid jet for a pillared surface where the critical heat flux limit is significantly delayed. The apparent advantages of heat transfer performance enhancement with patterning of surfaces are not always guaranteed, and the design consideration must address the coolant interaction with it to ensure a net advantage, which may get diminished due to liquid splattering induced by the interaction of the coolant with the patterned surface. [ABSTRACT FROM AUTHOR]

• The effect of varying surface wettability was probed on spray cooling heat transfer. • Surfaces with high wettability had low wall temperature in the nonboiling regime. • Early incipience of boiling on hydrophobic surfaces led to greater heat transfer. • Evaporation fraction increased as the contact angle decreased. • Modified heat transfer correlations were derived for both single- and two-phase regimes. This study conducted a spray cooling experiment to investigate the effects of varying surface wettability on the heat transfer performance of modified copper surfaces in both single- and two-phase regimes. The contact angles of the test surfaces ranged from 0° to 150°. The spray cooling experiment was performed using deionized water. The results indicated that surface wettability influenced heat transfer performance during spray cooling. In the nonboiling regime, the surface with higher hydrophilicity exhibited higher heat transfer performance owing to the larger wetting area available for heat transfer. The surface temperature of the superhydrophobic surface reached saturation at a relatively low heat flux; therefore, nucleation occurred rapidly, and the increase in the heat transfer coefficient intensified as the heat flux increased. Consequently, the heat transfer performance of the hydrophobic surface exceeded that of the plain surface in the two-phase regime. Evaporation fraction values were derived for the various test surfaces. Finally, a dimensionless surface wettability number, defined as the ratio of the droplet contact diameter to the droplet height, was used to establish heat transfer prediction models considering surface wettability in the single-phase and two-phase regimes. The heat transfer performance in the single-phase and two-phase regimes could be predicted within error bands of 18 % and 33.3 %, respectively. [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 is widely used in industries. Understanding of the complex surface heat transfer characteristics is essential to enhance cooling efficiency. In the present study, surface heat transfer of transient spray cooling with different cryogens, nozzles, and substrates were investigated. Minimum surface temperature reached −46.1 °C, −55.9 °C, and −57.9 °C and maximum surface heat flux values were 294.9, 364.1, and 377.4 kW/m 2 for R134a, R407C, and R404A, respectively, on an epoxy resin block. Results proved that R404A has the best cooling capacity. Compared with that of the straight nozzle, q max of the expansion-chamber nozzle increased from 266.9, 364.1, and 403.9 kW/m 2 to 339.3, 442.8, and 445.1 kW/m 2 or 16.6%, 18.6%, and 16.7% for R134a, R407C, and R404A, respectively. Transient cooling could be divided into two stages, namely, fast boiling cooling and film evaporation cooling. The similarity of dynamic heat flux with different cryogens, nozzles, and substrates was observed, and the dimensionless correlation was proposed by the spray Biot number, which is defined as the ratio between the internal thermal resistance of the substrate and the surface convective heat transfer resistance. The dimensionless Reynolds number Re l and Fourier number Fo l were proposed to represent the maximum heat flux ( q max ) and the corresponding time ( t max ), respectively. By coupling the Jakob number with Re l and the droplet Weber number ( We ), dimensionless correlations of maximum heat flux and the corresponding time were proposed, through which it is indicated that We number was the key factor in spray cooling. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the performance characteristics of spray cooling with ammonium hydroxide (NH 4 OH) binary mixture for high heat flux removal. Two mixtures, having ammonia mass fractions of 0.3 and 0.5, were selected to represent practical operation conditions near atmospheric pressure and room temperature. Experimental setup involved a closed loop system with a vapor atomized spray nozzle and a 1-cm 2 heater sample that simulated a high heat flux source. Tests were performed with gradually increasing heat fluxes of up to 800 W/cm 2 and maintaining surface temperatures below ∼75 °C at varying liquid and vapor flow rates. Results indicated that the heat transfer coefficient (HTC) values from NH 4 OH mixtures can be lower than those from pure water and pure ammonia. The data suggested that boiling depression, due to mass diffusion resistance at liquid vapor interface, could greatly affect the overall spray cooling performance, especially when the binary mixtures comprise components with widely different boiling points. The study therefore provides performance characteristics, as well as some fundamental insights, for a potential spray cooling scheme suitable for low temperature, low pressure operations in various applications including thermal management of aerospace electronics and electro-optics. [ABSTRACT FROM AUTHOR]

Cryogen spray cooling with R134a has been widely used in laser dermatology such as port wine stain to prevent unspecific thermal injury due to the absorption of laser energy by melanin in the epidermis. However, R134a spray cooling has shown poor efficacy for darkly-pigmented human skin. This paper presents an experimental study on the spray characteristics and heat transfer dynamics of pulsed spray cooling with three kinds of volatile cryogens (R134a, R407C and R404A), the latter two of which have potential to improve the cooling effect due to their lower saturation temperature. The spray patterns reveal that R404A generates the narrowest spray width and the smallest cooling spot on the cooling surface, which facilitates the accurate control of the cooling area. The R404A spray produces the smallest droplet diameter and the largest velocity, whereas the opposite applies to the R134a spray. The droplet temperature measurements show that R404A spray has the lowest temperature of spray, while the R134a spray has the highest. However, the non-dimensional temperature in the decrease period shows a similar pattern, independent of the cryogens. For the common spray distance in laser surgery (30 mm), R404A spray leads to the lowest surface temperature and highest heat flux, and also removes the most heat from the cooling surface, indicating that R404A has the strongest cooling capacity. Furthermore, R404A spray has the least liquid film resistance time on the cooling surface, implying that it provides the best selectively spatial cooling effect. The results of spray and heat transfer all reveal that R404A possesses significant superiority with regard to pulsed spray cooling for epidermis protection, with the potential of substituting the current R134a in the application of laser dermatology, especially in the case of darkly-pigmented human skin. [ABSTRACT FROM AUTHOR]

• A novel closed loop spray cooling system is built to investigate enhanced surfaces using Novec HFE-649 fluid. • Micro-finned and micro-coated surfaces (100 µm and 200 µm coating thickness) are investigated. • CHF and HTC are improved by 97% and 453% respectively with enhanced surfaces. • Maximum spray cooling efficiency improved by 177% with enhanced surfaces. The present study utilizes commercial HFE-649 fluid to analyze the effect of enhanced surfaces on spray cooling. Two pressure swirl nozzles, N1 and N2 with an orifice diameter of 0.51 mm and 1.19 mm respectively, have been used to generate micro-sized droplets and to diffuse these droplets on copper specimen of 10-mm2 cross-section. The volumetric flux range of the fluid varies from 1.00 to 21.67 cm3/cm2s. Three enhanced surfaces, namely, two micro-coated surfaces having coating thickness of 100 µm and 200 µm, and a micro-finned (1 mm fin height) surface, have been used in the present study. Heat transfer enhancement of these enhanced surfaces has been compared with a reference plain surface. With micro-coated surface having thickness 100 µm, the maximum heat flux and heat transfer coefficient of 115 W/cm2 and 9.87 W/cm2K with N1, and 143.48 W/cm2 and 10.31 W/cm2K with N2 has been achieved, respectively. When compared to plain surface, coating thickness of 100 µm resulted in a maximum enhancement in heat flux of 61.54% with N1 and 52.91% with N2, and enhancement in heat transfer coefficient of 352% with N1 and 326% with N2. Effect of subcooling at two fluid inlet temperatures, 25℃ and 49℃, on heat transfer performance of enhanced surfaces has also been investigated. The critical heat flux also increased by up 97.1% for the micro-coated surface having 100 µm thickness at near saturated condition when compared to the plain surface, depending upon the volumetric flux. A new correlation for calculating HTC and CHF has been proposed which is based on the earlier correlation proposed by Estes and Mudawar for plain surface. In addition, a correlation to estimate CHF for enhanced surfaces has also been proposed in terms of density ratio, Weber number and Jakob number. [ABSTRACT FROM AUTHOR]

Spray cooling shows significant advantages in thermal control of high heat flux components. However, rapid drainage problem with the spray chamber can affect the reliability and stability of the cooling system. It can be solved by means of immersion. The focus of this paper is to conduct a comparative study of the heat transfer performance between spray cooling (SC) and immersed spray cooling (ISC). It is proved that with reasonable control of immersion state and spray pressure, the immersion does not worsen the heat transfer, but even helps to enhance it with a maximum enhancement ratio of 12.2%. The immersion water temperature and spray pressure play a vital role in the ISC. Wide-angle nozzles are not suitable for ISC applications. The immersion deteriorates the spray cooling by up to 50.1% when the spray angle is 83°. This paper can be a guide to the structural design of the spray chamber. [ABSTRACT FROM AUTHOR]

The effects of variation of spray characteristics (mass flux, inlet pressure, flow rate, nozzle tip to target distance and plate thickness) on heat flux were systematically investigated. The round spray released from a full cone internally mix atomizing nozzle was impinged with a wide range of air and water pressures on stationary hot steel surface of definite dimension. The effect of each parameter was examined while keeping others nearly fixed. Four different plate thicknesses i.e., 4 mm, 6 mm, 8 mm and 10 mm were tested and effect of plate thickness on heat transfer was determined. Surface heat flux at each experimental condition was computed from the transient temperature history measured by K-type thermocouples embedded at bottom surface of the plate. The mass impingement density was measured by the help of a simple mechanical patternator. The maximum surface heat flux of 4895.525 kW/m 2 was achieved at an inlet water pressure of 4 bar, air pressure of 3 bar and nozzle height of 120 mm for an initial temperature of 850 °C of the 4 mm steel plate. [ABSTRACT FROM AUTHOR]

A closed loop R134-a spray cooling system was designed using a commercial full-cone nozzle to generate the spray for the cooling of an upward plain copper surface. Given the flow rate, the spray cooling characteristics such as heat flux, critical heat flux (CHF) and spray efficiency were obtained under different spray chamber pressures. The results show the CHF and spray efficiency were enhanced with the increase of the chamber pressure. Due to the dependence of the saturation temperature on the vapor pressure, the cooling temperature of the target surface was directly proportional to the chamber pressure. Moreover, an improved empirical correlation was proposed for the non-dimensional heat flux as a function of the Jacob number, the Weber number, the dimensionless mass flow rate, the dimensionless pressure and the dimensionless bubble departure radius. [ABSTRACT FROM AUTHOR]

A parametric experimental study on the development of a round jet spray impacting a smooth, heated, flat plate has been accomplished. The main objective of this effort was to provide information characterizing the flow structure of a developing mist jet, issuing vertically towards an upward facing, horizontal heated plate, by means of simultaneous droplet size and velocity measurements. Phase Doppler anemometry was used, providing also information on liquid volume flux. The fine spray of small atomized droplets (0.5-5.0 μm), was generated using a medical nebulizer. Two low Reynolds number jets (Re =2952 and 3773) issuing from a cylindrical pipe have been tested. The distance between the jets exit and the plate was 50 cm. A stainless steel non-magnetic flat plate of dimensions 1000 mm × 500 mm × 12 mm was used as target wall. Constant heat flux boundary conditions were established during measurements. Results indicate that the heat flux from the plate is influencing the evolution of the spray jet, diminishing its velocity and turbulence. Average droplet sizes are affected little by the heat flux, although for the non-heated sprays, droplet sizes increase at locations very close to the plate. A significant effect on droplet volume flow rate is also reported. [ABSTRACT FROM AUTHOR]

This paper proposes a solution method for the inverse spray cooling problem with relatively long cooling time. The entire time domain is divided into several sub-time intervals. By minimizing the mean square error between the experimental data obtained from inside the body and the estimated data from the derived analytical solution of a spray cooling problem with time-dependent boundary conditions, the temperature function at the spray cooling surface in each sub-time interval can be predicted. Consequently, the temperature distribution and the heat flux over the entire time and space domains can also be obtained. In addition, the integral transform and tedious numerical operations are not required in the proposed solution method. Mathematical and experimental examples are given to illustrate the simplicity, efficiency, and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

• Transient phase-change heat transfer analysis was performed in cryogenic regimes. • A heat flux estimation method that considers an imposed heat flux was proposed. • The moderate spurt duration (Δ t = 30 s) produced preferable cooling performance. • Small-orifice-diameter nozzle has low cooling rate but high thermal efficiency. Focusing on the cryogenic cooling for the high-power semiconductor lasers to improve their output performance, this work conducted an experimental investigation on analyzing the transient cooling performance using a single-pulsed flash-evaporation spray with the environmental cryogen R32. A heat flux calculation methodology that considers an imposed heat flux as a boundary condition, was first developed to assist the transient heat transfer analysis. The results provided further insight into phase-change heat transfer on the hot surface from high-temperature to cryogenic regimes through coupling the heat transfer measurements with observations of hydrodynamic liquid film formation and development. The moderate spurt duration (Δ t = 30 s) that produced a large temperature drop (109.5 °C) and high time-averaged cooling rate (24.1 W/cm2) was preferable. The significant losses in droplets' momentum and mass contributed to the fast decrease in cooling effect as nozzle-to-surface distance (Z) increased. Specifically, shortening Z from 50 to 10 mm not only increased the maximum surface heat flux from 19.6 to 73.6 W/cm2, but also broadened the temperature region with high heat flux. The nozzle with a smaller orifice diameter (0.5 mm) produced a lower cooling rate. Whereas surprisingly, its lower flow rate resulted in the lesser coolant accumulation and splashing on the hot surface, which elevated its thermal efficiency. The low spray thermal efficiencies (≤16.2%) in the present work attribute to the low liquid utilization and coverage area across the hot spot surface under the spray impact. [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]

The study deals with the air-atomized spray cooling using nanofluid as the cooling media for high heat flux applications. The nanofluid has been prepared by commercial Al 2 O 3 particles of diameter less than 13 nm and water. Heat transfer study has been carried out on a pre-heated steel specimen of dimensions 100 mm × 100 mm × 6 mm. The initial temperature of the plate which was subjected to air-atomized spray cooling was over 900 °C. Various coolants consisting of 0.1% volumetric concentration of water–Al 2 O 3 mixture, with or without a dispersing agent (surfactant) were used for the study. The dispersing agents used are sodium dodecyl sulphate (SDS) and polyoxyethylene (20) sorbitan monolaurate (Tween 20). Inverse heat conduction software INTEMP has been used for estimating the surface heat flux and temperatures taking into account the measured internal temperature histories by the thermocouples during the cooling process. The results obtained using nanofluid coolants are compared with that of the results where pure water (filtered potable water) is used as a coolant. The analyses reveal that the cooling rate, critical heat flux and heat transfer coefficients are significantly enhanced when nanofluids are used as coolants in air-atomized spray process. Also, the nanofluid coolants with dispersing agent shows a better enhancement of heat transfer over that of the nanofluid without the dispersing media. The nanofluid with dispersing agent Tween 20 is found more effective than that of its counterpart. Overall, the percentage enhancement in cooling rate of all these nanofluids compared with pure water (filtered potable water) is 10.2% for water–Al 2 O 3 , 18.6% for water–Al 2 O 3 –SDS, and upto 32.3% for water–Al 2 O 3 –Tween 20. [ABSTRACT FROM AUTHOR]

Spray cooling is a common cooling method used in many high-temperature metal processes. Using a combined numerical and experimental approach, the influence of the oxide layer on the Leidenfrost temperature during spray cooling of surfaces at high temperatures was investigated. The heat transfer from a metal surface covered by an oxide layer is described using the concept of the effective heat transfer coefficient and this concept is extended to the Leidenfrost temperature. The effective Leidenfrost temperature is introduced. The prediction of the effective Leidenfrost temperature is compared with the numerical simulation and with the experiment, which was conducted on an austenitic stainless steel plate with varied oxide layer thicknesses. The test plate with the oxide layers was heated to 1000 °C and then cooled using flat jet nozzles. The present study confirms that the use of water in the spray cooling of hot surfaces can create a situation where the oxide layer not only serves as insulation but can also increase the cooling intensity for short time period, mainly by a shift of the Leidenfrost temperature. [ABSTRACT FROM AUTHOR]

Spray cooling is one of the promising technologies in heat removing for high power density equipment, especially for the equipment with high heat flux surface. R134-a is one of the most prospective alternative refrigerants of R22 due to the zero ODP, low GWP, non-toxicity and non-inflammability, it is necessary to do comparative study on the heat transfer characteristics of R22 and R134-a in spray cooling. In this article, two closed loop spray cooling test rigs with the same spray nozzle are established to investigate the thermal performance of R22 spray cooling and R134-a spray cooling. The main parameters such as critical heat flux (CHF), heat transfer coefficient and target surface temperature are compared in the same thermal condition. The CHF of R134-a spray is lower due to the lower latent heat. The heat transfer coefficient of R22 spray is higher than that of R134-a spray with the same spray chamber pressure. However, R134-a spray cooling could replace R22 spray cooling in the phase change heat transfer region when the heat flux is less than 80 W cm −2 . [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]

Experimental investigation on the R134-a spray cooling characteristics is presented in a closed loop system. A full-cone nozzle was applied to spray onto a heated circular copper plate. The characteristics of the R134-a spray cooling under a series of different volumetric flow rate were obtained. The results shown that volumetric flow rate improves the critical heat flux (CHF). And larger volumetric flow rate prevent the dry-out phenomenon under high heat flux. However, volumetric flow rate bring the adverse condition that reduces the system efficiency. The maximum CHF of 117.2 W cm −2 was achieved with 319 K target surface temperature at 0.356 L min −1 . The results indicate that R134-a owns a great cooling characteristics in spray cooling system. [ABSTRACT FROM AUTHOR]

Due to its high local heat transfer coefficients, spray cooling is a vital technique for thermal management of high heat flux electronic devices. In this study, an experimental system using R134a as the working fluid was designed and built to study the heat transfer characteristic of spray cooling. The influences of the liquid flow rate, the vapor phase mass fraction and the liquid subcooling of the two phase R134a spray cooling on the heat transfer performances were discussed. The experimental results show that the heat transfer coefficient increased with the increasing liquid flow rate when the liquid fluid rate was less than 1.6 L/min. However, the enhancement on heat transfer coefficient owing to the increase of liquid flow rate was insignificant while the flow rate was higher than 1.6 L/min. A maximum surface heat flux of 325.99 W/cm 2 was achieved using R134a spray cooling while the temperature of the test surface was about 42.4 °C at the flow rate of 2.15 L/min. [ABSTRACT FROM AUTHOR]

An experimental platform was set up to investigate the factors that affect liquid nitrogen spray cooling. Sixteen groups of experiments were conducted according to the mass flux, spray height, and the surface size of the heat sink. The superheat at the critical heat flux was found to be between 11 K and 22 K. As the heated surface size increased, the heat flux and heat transfer coefficient decreased. In our study, the maximum heat flux was established to be 191.31 × 104 W m−2 at a superheat of 11.5 K and the maximum heat transfer coefficient was established to be 18.39 × 104 W m−2 K−1 at a superheat of 9.8 K. The experimental correlation of the Nusselt number for incomplete spray coverage, full spray coverage, overspray coverage, and all the data had a mean absolute deviation of 8.30%, 14.25%, 20.39%, and 19.51%, respectively. • The initial heat transfer is approximately regarded as pool boiling as superheat below 8 K. • The superheat at the critical heat flux is in the range of 11 K to 22 K. • The heat transfer coefficient increases to its maximum value and then decreases as the superheat increases. • The heat flux decreases as the heat transfer coefficient decreases then increases slightly as the spray height increases. • The heat flux and heat transfer coefficient decreases as the heated surface size increases. [ABSTRACT FROM AUTHOR]

A study of heat transfer and film dynamics caused by single and multiple streams of impinging droplets using HFE-7100 as the cooling liquid under constant heat flux conditions is presented. Single and multiple streams of mono-dispersed droplets were produced using a piezoelectric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter, droplet velocity, and spacing between adjacent impinging droplet streams. In this study, a heater consisting of a thin layer of Indium Tin Oxide (ITO) as heating element, combined with a Zinc Selenide (ZnSe) substrate was used for characterizing the heat transfer behavior and hydrodynamic phenomena of impinged liquid films near or at the onset of critical heat flux (CHF). Film thickness inside the impact crater was measured using the Total Internal Reflection (TIR) technique. Hydrodynamic phenomena of the droplet impact craters were analyzed using a high speed imaging technique. Impact regimes of the impinging droplets were identified and classified, and their effects on heat transfer performance are discussed. The study supports the notion that forced convection is the main heat transfer mechanism inside the impact crater due mainly to the high frequency and periodic nature of droplet impingement. Furthermore, droplet impingement regimes such as spreading and splashing have been observed to play an important role in the overall heat transfer behavior. Spacing between adjacent impinging droplet streams is also an important factor in surface cooling when multiple droplet streams are used. [ABSTRACT FROM AUTHOR]

Abstract: An experimental spray cooling study was carried out to investigate the effect of enhanced surfaces on heat transfer performance. Test surfaces involved; (a) micro scale indentations and protrusions, (b) macro (mm) scale pyramidal, triangular, rectangular, and square pin fins, and (c) multi-scale structures that combine macro and micro scale structures, along with a smooth surface that served as reference. Tests were conducted in a closed loop system using vapor atomized spray nozzles with ammonia as the working fluid. Cooling performance data for each enhanced surface were obtained applying heat fluxes of up to 500W/cm2, and using flow rates of 1.6ml/cm2-s of liquid and 13.8ml/cm2-s of vapor. Typical temperature readings with embedded thermocouples were verified using infrared thermography method. Data indicated that the multi-scale structured surface achieved the highest heat transfer coefficient (HTC) of 772,000W/m2 °C, corresponding to 161% enhancement over the reference surface. The results suggest that the multi-scale structured surface can combine the unique benefits of the micro and macro scale structures, and provide some insights to the understanding of the spray cooling heat transfer mechanisms by emphasizing the importance of boiling through surface nucleation. Therefore ammonia spray cooling, with the utilization of enhanced surfaces, offers significant cooling performance for high heat flux thermal management applications that target to maintain low device temperatures with a compact and efficient cooling system. [Copyright &y& Elsevier]

The experimental investigation of using a water based polymer additive to enhance the spray cooling performance of hot steel plate, has been carried out in the current research. This is essentially important to produce high-strength steel on the run-out table (ROT) of a Hot Strip Mill. The ultra-high heat flux cooling system has been developed using an air-atomized spray, containing dissolved polyvinylpyrrolidone (PVP) in water at different concentration ranges between 10 and 150ppm and compared with the cooling performance of pure water. To understand the heat transfer mechanism of aqueous polymer solutions, the physical properties such as surface tension, viscosity and thermal conductivity were measured. The cooling experiments were conducted using an AISI 304 stainless steel plate of 6mm thickness initially kept at a temperature above 900°C, where the Leidenfrost effect is predominant. In these experiments, the transient temperature data during cooling has been measured with three subsurface thermocouples and this time-temperature history has been used to estimate the surface temperature and the surface heat flux histories using a commercial inverse heat conduction software namely, INTEMP. The results explain that the polymer solution has a significant effect on the enhancement of surface heat flux, critical heat flux, as well as the cooling rate of the test plate. It was observed that an increase in the polymer concentration increases the heat transfer rate up to an optimal concentration; after which it results in a reduction in the rate. A maximum cooling rate of 253°C/s was obtained with a critical heat flux of 4.212MW/m2, which can be termed as the higher range of an ‘Ultrafast cooling’ process. Overall, the aqueous polymer solution can serve as a better heat transfer fluid for high heat flux applications. [ABSTRACT FROM AUTHOR]

• Thermocouple response time affects the heat flux estimates with inverse methods. • Proposal of a new correction method for applications with fast cooling or heating. • Use of a simplified model and a calibration test to estimate the response time. • Effect of the thermocouple noise, data filtering and heat loss were evaluated. • A MatLab code of the present model with data examples are available. [Display omitted] Many heat transfer processes do not allow temperature measurements with advanced instruments like IR cameras, and thus require the use of thermocouples – often inserted sheathed thermocouples – in order to measure the temperature in some key-regions. Nevertheless, thermocouples have an intrinsic response time that can dampen substantially temperature measurements and, when applicable, affect heat flux estimations by inverse methods. In this study, a thermocouple measurement correction method is proposed, especially for fast thermal transients like quenching, to avoid underestimation of the boundary heat fluxes due to delayed temperature responses. A simplified energy balance at the hot junction allowed modeling the temperature measurement by a thermocouple, in which heat losses and the response time were taken into account so they can be estimated using the least squares method on experimental data. A series of tests was performed to validate the present method with a heated jet impinging on a copper body instrumented with two thermocouples: one ungrounded and sheathed, then with high response time ("slow"); and another with exposed welded wires, hence with low response time ("fast"). After having estimated the slow thermocouple response time and heat loss parameters, the model allowed a good reconstruction of the fast thermocouple signal using the slow thermocouple measurements. Also, the heat flux estimation by the inverse method using the reconstructed signal resulted in practically the same results obtained using the fast thermocouple data. A parametric sensitivity analysis showed the heat loss parameters can be neglected for heat flux estimations, while simulations showed that temperature noises degrade the response time estimation but data filtering can mitigate this noise effect. Finally, the present method was applied to a large scale jet-cooling of a hot plate – near industrial conditions – and showed a substantially higher dissipated heat flux than the initially estimated. In fact, the results using reconstructed signals demonstrated that the heat flux dissipated by the jet has a higher dependence on the jet Reynolds number than the observed with the original thermocouple measurements, showing how important the temperature measurement correction is to evaluate fast thermal processes. [ABSTRACT FROM AUTHOR]

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

• Successive droplet impingement cooling on hydrophilic (uncoated) and superhydrophobic (coated) surfaces is investigated experimentally. • A new hybrid heat transfer solution using the integral transform technique is explained and used. • Single droplet impingement onto the uncoated surface showed higher heat transfer rates than for the coated surface. • Successive droplet impingement onto the coated heated surface allows surface refreshing with potential benefits for spray cooling. New high-performance electronic components generate a massive amount of heat per unit of area, and spray cooling is a field of interest for this application. Spray cooling achieves higher heat transfer coefficients than commonly used forced air convection cooling systems. In this study, we performed multiple droplet impingement experiments over heated surfaces in the non-boiling regime, a scenario that is closer to the phenomena of spray cooling than individual droplet impact. This paper focuses on the effects of droplet interactions with surfaces exhibiting distinct wettabilities. A surface thermocouple was used concomitantly as the impact surface and the temperature sensor. The surface thermocouple was originally hydrophilic (uncoated), which later we treated with a low cost commercial superhydrophobic coating (coated). High-speed imaging showed the droplet motion during impact. We proposed a one-dimensional inverse heat conduction semi-analytical solution, adopting a time-dependent droplet heat flux as one of the boundary conditions to represent the droplet cooling effect. We compared the heat transfer performances of both uncoated and coated surfaces for different droplet velocities but same initial temperature. The results showed higher heat transfer rates on uncoated surface relative to the coated one. Also, higher droplet velocities showed higher heat transfer mainly due to the increase in the droplet spreading diameter in contact with the solid. Successive droplets showed lower heat flow for the uncoated surface because of the low thermal effusivity and thickness of the heated solid, where most of the heat is removed by the first droplet. The first droplet thermal resistance and damping effects for the subsequent droplets, reduced the overall cooling performance. On the other hand, for the coated surface we found that the consecutive droplets continued to transfer heat with a similar rate as the previous one, which is desirable for spray cooling. However, the reduced overall heat transfer performance for the coated surface can be explained by the air pockets within the coating structure. [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]

High heat flux removal are important issue in many perspective applications such as computer chips, laser diode arrays, or boilers working on supercritical parameters. Electronic microchips constructed nowadays are model example of high heat flux removal, where the cooling system have to maintain the temperature below 358 K and take heat flux up to 300 W/cm2. One of the most efficient methods of microchips cooling turns out to be the spray cooling method. Review of installations has been accomplished for removal at high heat flux with liquid sprays. In the article are shown high flux removal characteristic and dependences, boiling critical parameters, as also the numerical method of spray cooling analysis. [ABSTRACT FROM AUTHOR]

Abstract: Phase change spray cooling is known as an effective method to remove heat from high-power electronic devices due to its high heat transfer efficiency and high critical heat flux (CHF). In this study, a closed-cycle spray cooling system with R22 as the coolant was designed and built, and the system cooling performance including the heat transfer coefficient, cooling surface temperature and CHF was experimentally investigated with nozzle inlet pressure in the range of 0.6–1.0MPa. The system shows a great capability in removing high heat flux from the cooling surface at a relatively low temperature. As the nozzle inlet pressure increases, the value of CHF first increases and then decreases slightly. The maximum CHF of 276.1Wcm−2 is achieved at the nozzle inlet pressure of 0.8MPa and the corresponding cooling surface temperature is 26.8°C. [Copyright &y& Elsevier]

Abstract: This paper presents a methodology combining experimental measurements with computational modeling to find the heat flux extracted during spray cooling of a metal surface. Controlled experiments are performed to impinge air-mist spray onto a metal probe surface while applying induction heating to follow a desired temperature history. A transient axisymmetric computational model of induction heating which couples electromagnetics and heat conduction has been developed and validated with a test problem. The model is calibrated to match transient dry measurements and then used to simulate a steady-state air-mist spray cooling experiment in order to quantify the heat extracted from the probe surface by the boiling water droplets. A detailed example is presented to illustrate this approach. [Copyright &y& Elsevier]

Abstract: The boiling convection heat flux, −q, taking place during the impingement of a water air-mist upon the surface of a Pt-disk, held at steady-state surface temperatures Tw ranging between 550 and 1200°C, has been measured under different conditions of water impact density, w, droplet velocity, u, and droplet size, dd . The new steady-state measurement method controls induction heating to balance the heat extracted from the sample, as described in detail in Part I. Local mist characteristics were determined at room temperature in free non-impinging mists using a patternator for w and a particle/droplet image analyzer (PDIA) for dd and u at positions equivalent to those of the Pt-disk. Three different air-mist nozzles of fan discharge type are characterized over their full range of water flow rates and air inlet pressures and using different positions of the hot surface with respect to the nozzle, to cover the following ranges of local spray characteristics: w from 2 to 106L/m2 s; normal volume weighted mean velocities, uz , v , from 9.3 to 45.8m/s and volume mean diameters, d 30, from 19 to 119μm. Increasing the air nozzle pressure at constant water flow rate generates mists with finer and faster drops that lead to a higher frequency of drops with large impinging Weber numbers, suggesting a higher probability of wet contact with the surface and an enhanced heat extraction. Heat fluxes as large as ∼12 and ∼10MW/m2 were found in the transition and stable film boiling regimes, respectively. The boiling convection heat flux in the range of 750–1200°C, which corresponded to stable film boiling, was found to correlate very well with the mist characteristics and temperature. The order of importance of the four parameters influencing −q was: d 30 ≪ Tw < w ≈ uz , v . For given local water flux and surface temperature, the correlation indicates that spray cooling becomes more intense as the velocity of the drops increases; the droplet size plays a very minor role. Compared with previous results using a transient method, the steady-state heat transfer coefficients increase faster with w in the range of 5–20L/m2 s, reaching much higher values. This suggests that within this range, the steady-state heat flux is controlled by the local water flux, while the transient heat flux must be controlled by the supply of heat conducted to the surface. [Copyright &y& Elsevier]

Abstract: This work presents a technique for characterizing water air-mist cooling of surfaces at temperatures from 200–1200°C while subjected to intense spraying conditions, so that the heat flux could reach values in excess of 10MW/m2. In this new steady-state method, a local region of an air-mist/spray impinges upon the exposed surface of a hot Pt-disk (8mm ∅ by 2.5mm thickness) which is surrounded by an induction coil while both are embedded in a cast ceramic monolith. A digital controller adjusts the output power of a high-frequency generator to balance the induction heating of the Pt-specimen with the heat removed by the boiling of the spray droplets impinging on its active surface, in order to maintain the control temperature of the sample at the predetermined set-point. Measurement of the RMS current flowing through the coil to maintain this temperature, together with the solution of a two-dimensional axi-symmetric computational model of the electromagnetic field and heat conduction equations, enabled estimation of the heat extracted locally by the mist. A rigorous experimental procedure was performed to determine the boiling curves from 200°C to 1200°C and then back to 200°C. The curves for both trajectories revealed that strong boiling hysteresis occurred in the nucleate and transition boiling regimes and that this was almost absent in the stable film boiling regime. On the other hand, hysteresis was not present in thermal loops involving surface temperatures only between 600 and 1200°C. The existence of hysteresis, in a certain temperature interval, points out the importance of considering the thermal history of actual cooling processes when simulating them in the laboratory. The ability of the new technique to maintain the sample temperature for prolonged periods, e.g., hours, enables future studies to improve the understanding of boiling phenomena during intense heat-removal processes. In Part II of this article, the effects that the mist droplet size, droplet velocity and water impact density have on the heat extracted under steady-state conditions from a surface with large superheat are investigated for a wide variety of those characteristics. [Copyright &y& Elsevier]

Abstract: Spray cooling technology relying on phase-change mechanism has a notable ability to remove high heat fluxes. Spray cooling can be used to transfer large amounts of energy through the latent heat of evaporation. Heat transfer rates much higher than can be attained in pool boiling are possible with spray cooling since the vapor can be removed from the surface more easily. Therefore, spray cooling has a number of applications in the various fields such as continuous casting process, electronics thermal control and metal quenching process which require a high flux cooling technique [1–3]. One important area which would benefit from spray cooling to improve heat removal is press quenching of extruded aluminum alloy plate. [Copyright &y& Elsevier]

Abstract: A spray cooling study was conducted to investigate the effect of enhanced surfaces on Critical Heat Flux (CHF). Test surfaces involved micro-scale indentations and protrusions, macro (mm) scale pyramidal pin fins, and multi-scale structured surfaces, combining macro and micro-scale structures, along with a smooth surface that served as reference. Tests were conducted in a closed loop system using a vapor atomized spray nozzle with ammonia as the working fluid. Nominal flow rates were 1.6ml/cm2 s of liquid and 13.8ml/cm2 s of vapor, resulting in a pressure drop of 48kPa. Results indicated that the multi-scale structured surface helped increase maximum heat flux limit by 18% over the reference smooth surface, to 910W/cm2 at nominal flow rate. During the additional CHF testing at higher flow rates, most heaters experienced failures before reaching CHF at heat fluxes above 950W/cm2. However, some enhanced surfaces can achieve CHF values of up to ≈1100W/cm2 with ≈67% spray cooling efficiency based on liquid usage. The results also shed some light on the current understanding of the spray cooling heat transfer mechanisms. Enhanced surfaces are found to be capable of retaining more liquid compared to a smooth surface, and efficiently spread the liquid film via capillary force within the structures. This important advantage delays the occurrence of dry patches at high heat fluxes, and leads to higher CHF. The present work demonstrated ammonia spray cooling as a unique alternative for challenging thermal management tasks that call for high heat flux removal while maintaining a low device temperature with a compact and efficient cooling scheme. [Copyright &y& Elsevier]

The paper aims to study Spray Cooling to achieve the high strip cooling rate of Ultra Fast Cooling. Spray evaporative cooling can be used as an alternative to conventional laminar impingement cooling for a Runout Table. A study has been carried out to explain Spray Cooling using an Inverse Algorithm. Spray cooling of hot steel strip has been done by means of an air-water spray, water spray and a water jet .Air- water spray is found to give a higher cooling rate as reported in results. The volumetric flow rate also is found to increase with increase in the cooling rate as regards to Water Jet Cooling. The Surface heat flux and surface temperature is calculated for each case by extrapolating the inner temperature readings. The determination of Heat flux and Surface Temperature, for each of the conditions involves a technique called Inverse Heat Transfer, in which boundary conditions are estimated from the temperature data in the strip from two thermocouples fitted inside. The algorithm is based on the Control Volume approach and the conjugate gradient method and the slab is split into control volumes to estimate the surface heat flux as a function of time. A new algorithm is devised to directly find the Heat flux and the Surface Temperature given the inner Temperature Readings. [ABSTRACT FROM AUTHOR]

Abstract: The challenge of effectively removing high heat flux from microelectronic chips may hinder future advancements in the semiconductor industry. Spray cooling is a promising solution to dissipate high heat flux, but traditional sprays suffer from low cooling efficiency partly because of droplet rebound. Here we show that electrosprays provide highly efficient cooling by completely avoiding the droplet rebound, when the electrically charged droplets are pinned on the heated conducting surface by the electric image force. We demonstrate a cooling system consisting of microfabricated multiplexed electrosprays in the cone-jet mode generating electrically charged microdroplets that remove a heat flux of 96W/cm2 with a cooling efficiency reaching 97%. Scale-up considerations suggest that the electrospray approach is well suited for practical applications by increasing the level of multiplexing and by preserving the system compactness using microfabrication. [Copyright &y& Elsevier]

Abstract: Heat-treating of solid alloy cylinders is an important practical problem for which no optimal production methods have been developed, especially in terms of the most crucial quenching stage. This study explores the use of spray quenching as an alternative to the commonly used bath quenching, which is known to yield relatively slow quench rates and provide few options for spatial optimization of cooling rate. A carefully configured spray cooling system is examined, which provides maximum coverage of the surface of a solid alloy cylinder with full-cone pressure sprays. A new analytical model is derived to determine the shape and size of the spray impact zone, as well as the distribution of volumetric flux across the curved surface of the cylinder. This distribution is combined with heat transfer correlations for all spray boiling regimes to generate a local boiling curve for every location across the impact surface. Using these boiling curves as boundary conditions, a transient analysis is conducted for aluminum alloy and steel cylinders. Increasing the nozzle pressure drop or decreasing the orifice-to-surface distance are shown to hasten the exit from the poor film boiling regime to the more efficient transition boiling regime, resulting in a quicker quench. Relatively high thermal diffusivity causes faster transmission of the spray cooling effect through the cylinder and milder temperature gradients in aluminum compared to steel. This also causes the outer surface to cool earlier but deeper points much slower for steel. Large temperature gradients are encountered on the surface during the quench because of different boiling regimes occurring at different locations exposed to the spray. This study highlights several practical advantages of spray quenching compared with bath quenching, including the ability to achieve a wide range of fast quench rates, uniformity and predictability of quench rate, and the ability to predict and guard against imperfections caused by thermal stresses. [Copyright &y& Elsevier]

Abstract: Experiments were performed to study the effects of spray inclination angle (the angle between the normal of the square test surface and the axis of symmetry of the spray), the mass flux as well as the surface temperature on the heat transfer performance in non-boiling regime by using water sprays. A swirl semi-solid nozzle was used with mass flux of 15.7kg/m2 s, 18.1kg/m2 s and 24.9kg/m2 s, respectively. The spray parameters were measured by PDPA. It was found that there was a recirculating zone due to a strong rotational flow at the center of the spray. By varying the orifice-to-surface distance for the same inclination angle, the optimum distance was found, corresponding to the best heat flux, when the major axis of the elliptical spray impact area intersects the square test surface. With the optimum distance for each different inclination angle, the heat flux, heat transfer coefficient and cooling efficiencies were obtained and the results showed that as the inclination angle increased, the heat transfer performance and utilization efficiency of liquid were both enhanced gradually, with an increased rate as the surface temperature approaches the saturation temperature of the liquid spray. Generalized correlations were developed for local Nusselt number as a function of the spray Reynolds number and the non-dimensional temperature in non-boiling regime of vertical and inclined sprays. [Copyright &y& Elsevier]

Abstract: The work presented here investigates a new approach in the development of heat transfer empirical correlations for intermittent spray impingement, based on simultaneous measurements of the spray droplets characteristics and the surface thermal behavior. Conventionally, heat transfer correlations for spray impingement do not consider the temporal variations of droplets characteristics. However, in applications using intermittent sprays (internal combustion engines, cryogen spray cooling or microprocessor thermal management), the spray transient behavior suggests that heat transfer predictions may be improved using a dynamic approach. Additionally, the impact of multiple consecutive injections on a heated surface implies a certain degree of interaction, depending on the frequency of their intermittency. If the time between consecutive injections is shorten, the result is the formation of a liquid film which mitigates phase-change and privileges a single-phase heat transfer over a two-phase. This suggests that heat transfer correlations for spray impingement should take the spray unsteadiness and the multiple injections interaction degree into account. The dynamic approach here suggested presupposes the identification of systematic periods characterizing the spray dynamic behavior and, once identified, the development of a heat transfer correlation for each period. The analysis ends with a comparison between the dynamic heat transfer correlation with a correlation obtained using the conventional approach and a significant improvement in heat transfer predictions is achieved if the spray dynamic nature is considered. [Copyright &y& Elsevier]

• A thermal management strategy for electronic chips was proposed. • Experiments were performed to evaluate the overall heat transfer performance. • The start-up characteristics of the FPHP was investigated. • The overall heat transfer performance was optimised in a certain heat-flux range. • The effects on the heat transfer performance were investigated. With the rapid development of electronic technology, the required heat dissipation capacities of electronic chips are increasing. In this study, a thermal management strategy for electronic chips based on a combination of a flat-plate heat pipe (FPHP) and spray cooling was designed to improve the heat dissipation performance of the condensation section of the heat pipes. Experiments were conducted to investigate the start-up characteristics of the FPHP, as well as the effects of the inlet temperature and the spray flow rate on the overall heat transfer performance. The best comprehensive performance of the FPHP and spray cooling was obtained when the heat flux was 20–40 W/cm2. In this heat-flux range, the heat pipe had the lowest thermal resistance and the highest thermal conductivity, and the corresponding spray heat transfer coefficient was 168.4 kW/(m2•K), which was significantly higher than those for traditional air-cooled (0.02∼0.1 kW/(m2•K)) and water-cooled radiators (1∼10 kW/(m2•K)). Within a certain range, a higher inlet temperature corresponded to a shorter start-up time of the heat pipe, and the effect of inlet temperature on the heat transfer uniformity of the heat pipe was negligible. Moreover, when the heat flux was less than 30 W/cm2, increasing the inlet temperature would help increase the temperature difference between the evaporation surface and the condensation surface of the heat pipe, and improved the heat transfer performance of FPHP. But when the heat flux was greater than 30 W/cm2, the higher the inlet temperature, the higher the amount of steam in the vapor chamber, the greater the thermal resistance, and the worse the heat transfer performance of FPHP. Increasing the inlet temperature of the cooling medium caused the droplets to vaporise at the entrance of the nozzle in advance, weakening the dissipation effect of the spray cooling. The combination of the heat pipe and spray cooling provides a novel idea for electronic thermal management technology, that is, using two or more cooling technologies to adapt to diverse applications. [ABSTRACT FROM AUTHOR]

• The MR-2 SGHF exhibits the highest synergistic thermal conductivity. • The low mixing ratio droplets give highest evaporation rates for 25 °C≤ T s ≤ 100 °C. • The high mixing ratio droplets give highest evaporation rates for 103 °C≤ T s ≤ 125 °C. • The SGHF droplet evaporation rate on residue is up to 10 times that on copper. • The latent heat flux removal up to 890 W/cm2 is obtained using SGHF droplets. Thermal management of many high heat flux devices depends on droplet based cooling, such as the spray cooling or electro-wetting for hotspot cooling. Recently, heat dissipation in these devices increased to unprecedented levels, pressing a need for advanced thermal fluids in droplet based cooling systems. In this paper, we address this challenge by investigating the evaporation and boiling performance of the silver-graphene hybrid nanofluid (SGHF) droplet for its various mixing ratios and droplet sizes on a heated copper and a residue surface, obtained from the evaporation of the first SGHF droplet. The results show that low mixing ratio (MR ≤ 0.1) SGHF droplets exhibit highest evaporation rates for substrate temperature (T s) in a range of 25 °C ≤ T s ≤ 100 °C. However, this trend is reversed in the nucleate boiling regime, where high mixing ratio (MR ≥ 0.9) droplets give highest evaporation rates. Moreover, all SGHF droplets, irrespective of their mixing ratio, exhibit similar evaporation rates in the film-boiling regime. Furthermore, the SGHF droplet evaporation rate on its porous residue surface increases up to 173% for 25 °C ≤ T s ≤ 100 °C and by an order of magnitude in the nucleate boiling regime as compared to a plain copper surface. We also show that besides the synergistic thermal effect, the thermal Marangoni convection also affects the SGHF droplet evaporation rate. Moreover, we develop a diffusion-convection evaporation model that can predict the evaporation rate for different mixing ratios of the SGHF droplet on heated copper and residue surfaces. Moreover, we demonstrate that the latent heat flux up to 890 W/cm2 and 850 W/cm2 can be achieved using a SGHF droplet on heated copper and residue surfaces, respectively, suggesting its potential application in high heat flux device cooling. Finally, we discuss the effects of spray hydrodynamic parameters on critical heat flux of the SGHF spray cooling. [ABSTRACT FROM AUTHOR]

Prediction models have been constructed to investigate the effect of vibrating surfaces on the critical heat flux (CHF) and its associated temperature in spray evaporative cooling. Dimensional analysis has been used to construct the models to account for the influence of key dynamic parameters. Experimental measurements have been obtained from a flat, electrically-heated, copper test-piece, located inside a spray-chamber mounted on top of a shaker. A wide range of large-amplitude and high-frequency measurements have been obtained which correspond to test conditions for a piece of hardware mounted on board a light-duty automotive vehicle with vibration amplitudes ranging from 0 to 8 mm and frequencies from 0 to 200 Hz. Three nozzle types have been fed with distilled water at flow rates ranging from 55 to 100 ml/min being used to cool with subcooling degrees Δ T s u b ranging from 10°C to 45°C. Measured data for both static and dynamic cases have been used to explore the influence on the CHF and the surface-to-fluid saturation temperature at which this occurs, of subcooling degrees, surface vibration amplitude and frequency, vibrational Reynolds Number and vibrational Acceleration Number. The measured data has also subsequently been used to calibrate the predictive models for use in thermal management systems. Static measurements (without vibration) show that the influence of flow rate, volumetric flux, and subcooling are largely in agreement with published literature. For dynamic cases, the influence of vibration is best explained in terms of the nondimensional parameters: Vibration Reynolds Number and Acceleration Number. The effect of vibration on CHF and associated temperature is assessed in detail for the three nozzle types at different flow rates and degrees of subcooling. Predictions of CHF and associated excess temperature, using the calibrated correlation models for the dynamic conditions, are very reasonable, and suitable for the intended purpose of ensuring safe operation of thermal management systems using spray evaporative cooling. [ABSTRACT FROM AUTHOR]