Effect of spray modes on electrospray cooling heat transfer of ethanol

doi:10.1016/j.applthermaleng.2021.116757

Applied Thermal Engineering

Volume 189, 5 May 2021, 116757

https://doi.org/10.1016/j.applthermaleng.2021.116757 Get rights and content

Highlights

•
The electrospray characteristics of four typical modes of ethanol are analyzed.
•
The cooling performance of electrospray varies with spray modes and flow rates.
•
Electrospray makes enhancement by thinning liquid film or avoiding droplets rebound.
•
The nucleate boiling region of jet modes electrospray can be efficiently extended.
•
Cooling capacity can be promoted up to 2.8 times of the neutral dropwise spray.

Abstract

Spray cooling is a promising approach to efficient removal of heat and has attracted increasing interest with its excellent characteristics, such as strong heat exchange capacity, low flow rate, and uniform temperature distribution. Recently, high voltages have been successfully employed in electrospray (ES) cooling, which has improved cooling performance but has poor heat transfer control. We experimentally investigate heat transfer performance of ES cooling with different spray modes, and we study spray characteristics and droplet impinging behavior of the different modes. The results show that droplet size decreases and spray velocity increases with increasing applied voltage, and droplet formation and subsequent impinging behavior are gradually dominated by electric forces. In addition, the nucleate boiling region is significantly extended, and the critical heat flux (CHF) increases to a higher surface temperature when the spray mode is transformed to cone-jet/multi-jet modes. Under certain conditions, ES cooling achieves a 2.8 times enhancement rate over neutral (no voltage) dropwise cooling. Moreover, the visualization results of the droplet impinging behavior on the hot surface for different spray modes are consistent with their cooling performance. Overall, ES cooling is found to greatly improve the cooling capacity, and the specific mechanisms for enhancement vary with spray mode. Our results contribute to precise control of ES cooling and give a better understanding of the effect of electric fields on spray cooling.

Introduction

Spray cooling is a technique in which a liquid is fully pressurized before being atomized into droplets through a nozzle, then rapidly ejected to a hot surface, which removes heat with droplet impact, film motion, bubble formation, and environmental convection [1]. Because of its efficient heat removal, this technique has been widely utilized in various fields, such as metallurgy heat treatment, space thermal control, integrated circuit heat management, and biomedicine laser operation [2]. Over the past few years, increasing research has been conducted on the heat transfer mechanism, influence factors, and enhancement of spray cooling [3]. Previous studies have shown an extremely high heat flux of 1200 W/cm² with water spray cooling, which is expected to be a key in solving thermal management problems of high-power electronic components [4]. High pressure is required to atomize coolant liquid, and a typical spray cooling system usually consists of a liquid tank, force pump, spray chamber, and condenser. Thus, traditional sprays suffer from a bulky system and high energy consumption, which hampers commercial applications. With the rapid development of 5th generation wireless systems (5G) [5], high-performance electronic equipment has put forward growing requirements for cooling systems. Specifically, Shedd [6] points out that fluid management, system scalability, and compact size should be taken into consideration for next-generation spray cooling technology.

Compared with traditional sprays, such as pressure and centrifugal atomization sprays, ES is characterized by fine droplets, narrow diameter distribution, flexible setup, and low power requirements, and has been broadly utilized in ink-jet printing, energy nanomaterials fabrication [7], biodiesel preparation [8], industrial dust removal [9], and agricultural plant protection. The ES process was first theoretically studied by Rayleigh [10] in 1882. The formation of charged droplets is shown in Fig. 1(a). High voltage is applied to the capillary to charge the working liquid with like charges, which is mostly distributed on the surface of the liquid. The charged liquid drops from the meniscus tip (also called Taylor cone [11]) once the electric force and gravity overcome surface tension. Once the charge density is extremely high, approaching the Rayleigh limit (the theoretical maximum charge that a droplet of a certain size can carry [10]), the droplet becomes unstable and breaks up into mountains of droplets. The Rayleigh limit is defined as q_R = (8π²εσD³)^1/2, where ε is the dielectric constant of the ambient medium, σ is the liquid surface tension, and D is the droplet diameter. In addition, the Coulomb repulsion force weakens the liquid surface tension, resulting in fine droplets, and avoids the coalescence between charged droplets, improving the dispersion [12]. Various breakup modes of liquid flow have been defined, such as dripping mode, spindle mode, oscillating mode, cone-jet mode, multi-jet mode, and ramified mode. Four stable mode morphologies are shown in Fig. 1(b) and will be further discussed in the subsequent sections. The evolution of these modes is accompanied by a change in spray characteristics, including droplet diameter, droplet velocity, spray angle, and penetration distance, which we hypothesize to have a crucial effect on the heat transfer of spray cooling. Hence, we investigate the spray characteristics of different ES modes to increase our understanding of the mechanisms of ES cooling.

The multiphase flow and heat transfer mechanisms involved in ES cooling are very complicated due to the coupling effects of the electric field, temperature field, and flow field. Until now, little research has been conducted on this subject, and the ES heat transfer mechanism is still unclear. The electric field was first applied to spray impingement heat transfer by Feng [13], who studied the effects of applied voltage, flow rate, spray spacing, and heated surface geometry and found that the enhancement rate was 1.7 times at a low heat flux under certain conditions. Deng [14] obtained a high heat flux of 96 W/cm² with ethanol as the coolant. He reported that the electric image force could efficiently inhibit spray droplet rebound from the hot surface at temperatures above the Leidenfrost point, resulting in high cooling efficiency. To achieve a larger cooling area, Wang [15], [16] designed a multi-nozzle ES cooling chamber and optimized its methodology by increasing nozzle quantity and spacing. Heat transfer correlation models were eventually discovered, covering most experimental data. Among the various spray modes, the cone-jet mode is preferred by many researchers due to its fine and stable atomization. Gibbons [17] defined two different cooling regimes (evaporative and pool) of cone-jet ES and discovered the optimal flow rate (4 μl/min) and spray height (2.5 mm) for heat transfer enhancement. In addition, new hemispherical nozzle was introduced to generate a stable cone-jet at a high flow rate of 65 ml/h [18] and has successfully been applied to ES cooling, obtaining a CHF of 15.13 W/cm² [19].

In the previous literature, ES has shown promise in spray cooling, and some fundamental research has been conducted. It has been shown that applying an electric field can break up the liquid into many fine droplets with increased specific surface area, enhancing heat transfer on a heated surface. Additionally, the liquid in ES may disintegrate in various modes due to different electric fields and liquid properties, which determine the cooling performance. However, correlations between the spray modes and heat transfer performance for ES cooling have not been adequately studied, and the ES cooling technique is not well understood, especially for precise cooling. The major contribution of our work is to characterize the effects of spray mode evolution on heat transfer performance, which previous research does not consider. First, the spray characteristics of four different spray modes are studied. We consider droplet size, spray velocity, repetition frequency, and spray swath, which are used to analyze the difference in cooling performance as spray mode switches. Then, the cooling capacities of different ES modes are quantitatively investigated, and the enhancement ratio of ES to neutral spray cooling is specified. In addition, we visualize the impact behavior of spray droplets on the heated surface to explain the heat transfer performance of ES with different modes. Our study reveals the enhancement mechanism of ES cooling for more precise control.

Section snippets

Experimental setup

As shown in Fig. 2(a), the experimental setup consists of three primary components, including an electrospray section, a heat transfer section, and a visualization section.

Ethanol is employed as the coolant due to its excellent charge and heat transfer characteristics. Its relevant physical properties are listed in Table 1. The fluid is supplied to a stainless steel capillary with an inner diameter of 0.2 mm and an outer diameter of 0.4 mm via a syringe pump (KDS-100). A high negative voltage

ES modes evolution

Electrohydrodynamics (EHD), a relatively new branch, is the combination of fluid mechanics and electrodynamics [21]. Multi-component forces act on liquids, resulting in many interesting phenomena, such as electrowetting, electrospray, and corona winds. Typically, the ES process is accompanied by complex deformation, breakup, coalescence, and separation of charged droplets. Spray modes will vary with the applied EHD force, and their classification has been well investigated [22], [23], [24].

Conclusion

In summary, the present work experimentally investigates the heat transfer performance of electrospray (ES) cooling with different spray modes. We explain differences in cooling characteristics by taking the spray characteristics and spray droplet impact behavior into consideration. The mechanisms of ES are discussed, and four typical ES modes of ethanol, namely dropwise, micro-dripping, cone-jet, and multi-jet, are studied in this work. We find that as the spray mode shifts from

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank the financial supports from the National Natural Science Foundation of China (No. 51976084, No. 52036007).

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Effect of spray modes on electrospray cooling heat transfer of ethanol

Highlights

Abstract

Introduction

Section snippets

Experimental setup

ES modes evolution

Conclusion

Declaration of Competing Interest

Acknowledgements

Int. J. Heat Fluid Flow

Renew. Sustain. Energy Rev.

Int. J. Multiph. Flow

Particuology

Int. J. Heat Mass Transf.

Appl. Therm. Eng.

J. Electrostat.

Int. J. Heat Mass Transf.

Appl. Therm. Eng.

J. Electrostat.

J. Aerosol Sci.

Appl. Therm. Eng.

Int. J. Heat Mass Transf.

Int. J. Multiphase Flow

J. Aerosol Sci.

Exp. Therm. Fluid Sci.

Int. J. Heat Mass Transf.