A REVIEW ON THERMO-FLUIDIC STUDY OF DROPLET IMPACT IN SPRAY COOLING

Wu Y., Wang Q., Liu Q., Guo K., Hao Z.

This study investigates the thermal management performance of phase change material (PCM) coupled with a microgroove flat plate heat pipe under air cooling and spray cooling conditions. Different ratios of paraffin-lauric acid hybrid PCMs are prepared, and the thermophysical properties of the materials are optimal when the ratio of paraffin to lauric acid is 4:6. Composite PCMs are prepared by adding alumina nanoparticles, and the thermal conductivity is increased by 3.66 times when alumina particles with a mass fraction of 0.6% are added. Compared to air cooling, the spray cooling system demonstrates up to a 5.7% reduction in peak chip temperature. In the experimental range the spray cooling system dissipates heat better for the 60 W heating power chip, while the air cooling system is more suitable for the 30 W heating power chip. In the air cooling system, the heat stored in the PCM accounts for up to 39.8% of the heating power. The maximum amount of heat stored in the PCM in the spray cooling system is 37.8% of the heating power. The heat storage capacity of the PCM in the spray cooling system is slightly lower than that of the air cooling system, but it still has a considerable heat storage capacity. This research can provide ideas for solving the heat dissipation problem of highdensity server chips in data centers.

Gao X., Li Y., Xia Y., Gao X., Li R., Li H.

The unpredictable spray characterization caused by different manufacturing standards still limits the implementation of spray cooling. To obtain a predictable spray, a droplet generator was in-house designed and fabricated, which can generate predictable monodisperse droplets or multiple droplet trains at a certain droplet generation frequency (fG). Based on the measurement of the droplet flux (N), the controlled droplet trains are classified into the controlled sparse spray and dense spray. In the experiments, deionized water was used as the liquid coolant, and heat transfer characteristics were analyzed in the controlled sparse spray and dense spray. The experimental results show at the same volumetric flow rate the heat transfer curve of dense spray is always above the heat transfer curve of sparse spray. In the cooling area of controlled dense spray, the average Nusselt number Nu‾L was correlated as logNu‾L∝−(fG/N)1/2. This study would benefit the application of controlled spray cooling technology in high-heat-flux thermal management.

Karabey A., Yakut K.

Rapid advancements in technology constantly keep the need for thermal systems, which have high performance, on the agenda and direct the attention of researcher-engineers to the studies on improving the heat transfer. Spray cooling process depends on many parameters including nozzle diameter, surface area, surface geometry, critical heat flux, mass flow, gravity, spraying angle, and surface slope. One would need results from many experiments to better analyze the spray structure. In the present study, by using the rectangular-finned heat sinks optimized for spray cooling and those called "general," the heat and flow characteristics in spray cooling were analyzed. Water was used as the cooling fluid and the cooling fluid was atomized by using an air-supported atomized. The experiments were conducted with six air-to-liquid ratio (ALR) values, three different jet heights, three different spraying times, three different fin heights, and three different fin widths. The results are presented in the Nusselt number-air-to-liquid ratio (Nu-ALR) and jet thickness-jet velocity (<i>t_jet -U_jet</i>) diagrams. It was determined that the ALR value tended to decrease with increasing Nusselt numbers. For the determined ALR values, Nusselt numbers decreased as the fin height increased. It was concluded that Nusselt numbers tended to decrease at all fin widths as the ALR value increased. In addition, considering the parameters examined for the rectangular-finned heat sink, separate correlations were developed for the Nusselt number, spray angle, and jet thickness.

Banerjee N., Tropea C., Seshadri S.

This study examines spray cooling of horizontal, circular tubes when the surface temperature exceeds the Leidenfrost point, and the individual drop impacts are in the film boiling regime. Although film boiling is not an optimal condition for heat transfer, it is encountered in transient cooling processes or when high temperatures persist along the tube. The goal of the study is to apply existing models for drop impact and heat transfer to surfaces more complex than flat plates and to determine the overall steady state heat removal rate as an essential parameter for heat exchanger design. The spray characteristics are prescribed by a local number flux and drop size, assuming a random distribution in space and a uniform velocity of all drops. The angle of individual drop impingement on the cylindrical tube is accounted for through the maximum spreading diameter of the drop on the surface and the normal component of impact velocity. Furthermore, drop-drop interaction on the heated surface is accounted for at high local number flux values through an effective coverage area coefficient. The results are expressed in the form of a local heat transfer coefficient and an integral Nusselt number based on the tube diameter. These results confirm that the most influential parameters regarding heat transfer are the liquid mass flow rate in the nozzle and the drop diameter; however, they also indicate that the non-normal impact of drops over portions of the cylindrical tube leads to a highly reduced number flux; hence heat transfer. This oblique impact angle arises for one from the local spray angle, but more drastically from the curved, cylindrical surface; the latter leading to a dramatic sharp decrease in heat transfer. This is important information when considering the use of multiple spray nozzles, either circumferentially or longitudinally along the tube, since it dictates the optimal spacing between neighbouring nozzles.

Aksoy Y.T., Castanet G., Eneren P., García-Wong A.C., Czerwiec T., Caballina O., Vetrano M.R.

With the increasing interest in improving high-performance cooling systems, research on droplet cooling requires more complex methods for enhancing heat transfer. Hence, we investigate the effect of nanoparticle presence and concentration on droplet spreading dynamics and heat transfer during the early stages of spreading on a heated surface. For this purpose, water and three water-based TiO2 nanofluid droplets (0.2, 0.5, and 1 wt%) are tested at four release heights impinging on a sapphire substrate below boiling point (80 °C). With an emissive TiAlN coating in the infrared (IR) domain, the surface temperature of the substrate is measured via a high-speed IR thermal camera. Droplet spreading is simultaneously monitored with two high-speed black/white cameras. The thermal and rheological properties are experimentally characterized for accurate results and to investigate only the nanoparticle presence in the fluid. The captured temperature fields are analyzed by solving the inverse heat conduction problem. We observe that all nanofluid droplets spread on a heated surface marginally less than water droplets. To the best of the authors’ knowledge, this study is the first to reveal that during the early stages of droplet spreading, the presence of nanoparticles and the resulting change in viscosity primarily influence heat transfer through the modification of the droplet spreading diameter, rather than changes in thermal properties.

Ravikumar K.P., Sahoo A., Mohapatra S.S.

Attaining high heat flux around 900&#176;C temperature has been challenging for current generation researchers. Although the literature has identified several quenching methods, including upward-facing spray, downward-facing spray, and both-sides spray, upward-facing spray cooling is the most efficient. The coolant's thermophysical properties may improve upward-facing spray cooling. Thus, upward-facing spray cooling was used in this study to augment the heat transfer with better fluid properties. This study uses ethanol-added water as a coolant and heat transfer analysis to boost heat removal. The statistical analysis software (Designexpert@7.0) models an upward-facing spray and finds that for maximum heat removal the Weber and Reynolds numbers must be 700 and 2220, respectively. Fluid properties are viscosity 8660 &#215; 10^-7 mPa s, density 997.7 kg/m³, and surface tension 54 mN/m. Theoretical studies and dropwise experiments were used to determine the upward-facing spray augmentation mechanism. The comparative analysis shows that ethanol is less corrosive than the additives reported in the literature. After experimentation, the total dissolved solid concentration in used water exceeds the permissible limit.

Sontheimer H., Gholijani A., Stephan P., Gambaryan-Roisman T.

In this work, hydrodynamics and heat transport during the vertical coalescence of multiple drops impacting successively onto a hot wall are studied numerically and experimentally. The numerical model uses the volume of fluid method within the OpenFOAM library and takes evaporation into account. The significant heat transfer at the three-phase contact line is considered in a subgrid model. FC-72 is used as working fluid in a pure, saturated vapor atmosphere at ambient pressure. For the case of a drop chain of low impact frequency, numerical results are compared with experimental data. In particular, temperature and heat flux fields at the drop footprint as well as drop shape are compared between experiments and simulations. During the impingement of the second drop, several rings of high heat flux are observed in experiment and predicted by simulations, and the physical reasons of their appearance are elucidated by the analysis of simulated velocity and temperature fields. Furthermore, the impact of a drop chain consisting of five drops impacting at a high frequency is studied numerically. The drop chain configuration enhances both spreading and heat transport compared to the case of a single drop impact with a five-fold volume.

Sanjay V., Chantelot P., Lohse D.

Non-wetting substrates allow impacting liquid drops to spread, recoil and take-off, provided they are not too heavy (Biance et al., J. Fluid Mech., vol. 554, 2006, pp. 47–66) or too viscous (Jha et al., Soft Matt., vol. 16, no. 31, 2020, pp. 7270–7273). In this article, using direct numerical simulations with the volume of fluid method, we investigate how viscous stresses and gravity oppose capillarity to inhibit drop rebound. Close to the bouncing to non-bouncing transition, we evidence that the initial spreading stage can be decoupled from the later retraction and take-off, allowing us to understand the rebound as a process converting the surface energy of the spread liquid into kinetic energy. Drawing an analogy with coalescence-induced jumping, we propose a criterion for the transition from the bouncing to the non-bouncing regime, namely by the condition ${Oh}_{c} + {Bo}_{c} \sim 1$ , where ${Oh}_{c}$ and ${Bo}_{c}$ are the Ohnesorge number and Bond number at the transition, respectively. This criterion is in excellent agreement with the numerical results. We also elucidate the mechanisms of bouncing inhibition in the heavy and viscous drop limiting regimes by calculating the energy budgets and relating them to the drop's shape and internal flow.

Gajevic Joksimovic M., Schmidt J.B., Roisman I., Tropea C., Hussong J.

In the present study, the effect of graphite lubricant additives on the dynamics of a single drop impact onto a heated surface has been investigated in the nucleate boiling and thermal atomization regimes.

KUMAR B., KUMAR R., GUPTA A.

The purpose of this study is to investigate the cooling and rewetting of a heated horizontal tube surface with an air-atomized water spray impingement. Rewetting and transient heat transfer are crucial in nuclear reactor safety during a postulated accident, such as cooling of hot calandria tubes (CT) during the large-break loss of coolant accident (LOCA). The rewetting velocity in the circumferential direction and the rate of cooling of the heated tube surface were determined as a function of nozzle operating parameters. To estimate the local spray impingement density on the tube surface, an in-house mechanical patternator was designed and developed. To record the flow state during cooling, a high-speed video camera was used. The rewetting velocity on the tube surface was determined using the outcome of thermocouples mounted on the heated tube wall and an imaging system used to record the video picture during the runs. The two techniques of calculating rewetting velocity are compared. The highest heat flux removed from the tube surface was estimated as 3.7 MW/m², and the maximum rewetting velocity was found to be 5.58 mm/s. An excellent agreement regarding rewetting velocity has been reported utilizing thermocouples and a high-speed camera.

Stumpf B., Hussong J., Roisman I.V.

The impact of a drop onto a liquid film is relevant for many natural phenomena and industrial applications such as spray painting, inkjet printing, agricultural sprays, or spray cooling. In particular, the height of liquid remaining on the substrate after impact is of special interest for painting and coating but also for applications involving heat transfer from the wall. While much progress has been made in explaining the hydrodynamics of drop impact onto a liquid film of the same liquid, the physics of drop impact onto a wall film with different material properties is still not well understood. In this study, drop impact onto a very thin liquid film of another liquid is investigated. The thickness of the film remaining on a substrate after drop impact is measured using a chromatic-confocal line sensor. It is interesting that the residual film thickness does not depend on the initial thickness of the wall film, but strongly depends on its viscosity. A theoretical model for the flow in the drop and wall film is developed which accounts for the development of viscous boundary layers in both liquids. The theoretical predictions agree well with the experimental data.

Kumar B., Kumar R., Gupta A.

Gholijani A., Gambaryan-Roisman T., Stephan P.

This paper presents an experimental study on hydrodynamics and heat transport during the horizontal coalescence of two drops impinging a hot wall. The study addresses the influence of distance between impact locations, the time interval between drop impact, and wall superheat on the transport processes. The experiments were conducted under a pure vapor atmosphere with the refrigerant FC-72 at a saturation temperature of 54 ° C, corresponding to a system pressure of 0.94 bar. The drops were generated with a constant diameter and a constant impact velocity. The temperature field at the surface of the heater was measured by an infrared camera with a high spatial and temporal resolution. The local heat flux distribution was derived from the temperature field by solving the transient three-dimensional heat conduction equation within the substrate. The total heat flow was evaluated by integrating the local heat fluxes over the footprint of the drop. The impact parameters (drop size and impact velocity), the time interval between drop impact, and the distance between impact locations were evaluated through post-processing of the black-and-white images captured employing a high-speed camera. The results for simultaneous drop impingement reveal that the heat transported from the wall to the fluid is not affected by the presence of neighboring drop as long as the drops do not contact each other. In the case of horizontal coalescence of drops, the heat flow is reduced both during the spreading and receding phases and during the sessile drop evaporation phase. This can be explained by the reduction of solid–liquid interface and of three-phase contact line length after the coalescence. An increase in wall superheat leads to reducing the footprint of the drop and increasing of heat flow. Asymmetric behavior of the drop hydrodynamics and heat flux is observed during the non-simultaneous drop impingement. • Horizontal coalescence of two drops impinging a hot wall is studied. • Influence of distance between drops, time interval between impact, and wall superheat is studied. • Horizontal coalescence is accompanied by emergence of an uprising sheet between the drops. • Maximum heat flow strongly decreases with decreasing distance between impact locations. • The time interval between impacts leads to inclination of the bump towards the earlier drop.

Miao Y., Dou R., Wen Z., Liu X., Zhu C.

Luo J., Wu S., Xiao L., Chen Z.

• Hydrodynamics and heat transfer of multi-droplet impact on cylindrical surface. • Multi-droplet interaction and anisotropy induced by cylindrical surface. • Liquid-vapor interface temperature caused by anisotropy and internal flow field. • The effect of vertical space between droplets on heat transfer performance. • The effect of cylinder diameter on heat transfer performance. Based on the sparseness of research on the multi-droplet impact coupled with the anisotropy induced by cylindrical surface, in this paper, the hydrodynamics and heat transfer of multiple droplets successively impacting on cylindrical surface are deeply studied with three dimensional numerical simulation method. The comparison of single, two and even three droplets impacting on the heated cylindrical surface is carried out, and the effect of vertical space between successive droplets is discussed. Moreover, the influence of cylinder diameter on the hydrodynamics and heat transfer of multi-droplet impact is analyzed in detail. The results show that, the combination of multi-droplet interaction and anisotropy induced by cylindrical surface greatly affects the global heat flow, heat transfer coefficient, temperature distribution of liquid-vapor interface. By decreasing the vertical space between successive droplets impacting on the heated cylindrical surface, the heat transfer performance is obviously improved. Increasing the cylinder diameter boosts the heat transfer performance, but once the cylinder diameter augments to a certain value, the change of cylinder diameter no longer has an effect on the heat transfer performance, which is very close to the flat surface situation.