Effect of binary mixed-surfactants and hybrid nanofluid on spray cooling heat transfer

Abstract

The present study emphasizes the spray-cooling enhancement of a copper heater block of 10 mm² cross-sectional area with binary mixed surfactant based fluid and hybrid nanofluid using a pressure swirl nozzle with an orifice size of 0.51 mm. The experiments have been carried out at two distinct fluid input temperatures of 27 °C and 42 °C, with a volumetric flux of 3.50 cm³/cm²s. Polyvinyl pyrrolidone (polymer surfactant), Sodium dodecyl sulfate (an anionic surfactant), Tween-20 (a nonionic surfactant) have been used as binary mixed surfactant systems in distilled water. The best performance has been obtained with SDS:Tween-20, which resulted in a maximum heat flux of 264 W/cm² and a heat transfer coefficient of 3.59 W/cm²K. When compared to distilled water, the heat transfer coefficient increased by 32.2%, but only by 25.2% and 18.1% with PVP:Tween-20 and SDS:PVP surfactant solutions. Further, the effect of a hybrid nanofluid, which is a colloidal mixture of Titanium dioxide (TiO₂) and MWCNTs (multi-walled carbon nanotubes) in distilled water, has been studied to better understand spray cooling heat transfer enhancement. Heat transfer was first enhanced by the hybrid nanofluid and then decreased as the volume concentration was increased from 0.1% to 0.3%. A maximum heat flux of 234 W/cm² and a heat transfer coefficient of 3.15 W/cm²K has been achieved, with a heat transfer coefficient improvement of nearly 27.3% compared to distilled water. However, the mixed surfactant system outperformed, giving nearly 12.8% improvement in heat flux values. It has been observed that low surface tension resulted in high heat transfer rates. Furthermore, the effect of 15 °C subcooling on heat transfer performance has been investigated. A new correlation to find Nusselt number is proposed in terms of Reynolds number, Weber number and Prandtl number with a mean absolute error of 7.16%.

1. Introduction

Recently, with the rise in demand for high performance electronic devices, the need for efficient cooling has become an obvious concern. Conventional methods, such as natural convection, forced convection, and heat sinks, do not meet the high demand of cooling. Therefore, aggressive cooling techniques such as, jet impingement, microchannel, and spray cooling has drawn a lot of interest in the domain of thermal management. Amongst all, spray cooling has gained widespread attention as it is an inexpensive and reliable method to extract heat from high heat flux devices, and finds many applications in datacenters, ¹ nuclear fuel rods, ² metallurgical processes, ³ and combustion technology. ⁴ Compared to other cooling methods, it has additional benefits, such as high cooling power, low surface superheat, no contact thermal resistance, and low working fluid inventory. ⁵

Over the last several decades, the benefits of spray cooling have caught the interest of researchers, who have sought to increase the rate of heat transfer by improving spray properties. To increase heat transfer, many experiments and theoretical studies have been done to improve spray parameters such as flow rate, droplet diameter, droplet velocity, and so on. Studies have also been carried out on other influencing factors such as enhanced surfaces or by modifying the thermo-physical properties of the base fluid by incorporating additives. The primary objective of introducing additives such as surfactants, soluble salts, soluble gases, and nanoparticles is to improve the evaporation rate and the spray droplets’ dissemination over the heated surface. ⁶ The addition of surfactant to the base fluid lowers surface tension, resulting in a reduced contact angle between the droplets and the surface. ⁷ This result in reduction of liquid film thickness and easier penetration by spray droplets into the liquid film. Moreover, low film thickness improves the rate of film evaporation and hence facilitates the spray cooling heat transfer. Furthermore, lower surface tension strengthens the vapor nucleation process by producing small droplets, which increases droplet contact area and, as a result, improves heat transfer. ⁸ Jia et al. ⁹ explored into the advantages of employing surfactants in spray cooling and discovered that in addition to reduced super heat temperatures, a stable critical heat flux may be achieved.

Recently, experiments performed by mixing of surfactants indicated enhanced heat transfer to individual surfactant due to improved thermo-physical properties by means of synergism.^10,11 Synergism can be described as a phenomenon that enhances the properties of mixed surfactants compared to individual surfactants. Stocco et al. ¹² reported increase in foam stability using a binary mixture of myristic acid (anionic surfactant) and cetyltrimethylammonium bromide CTABr (cationic surfactant) against CTABr solutions due to low surface tension and high viscoelastic compression moduli. Suryanarayana et al. ¹¹ showed that bubble coalescence time can be reduced by adding salt to nonionic surfactant (Tween-20) and ionic surfactants (SDS and CTAB). Jadidi et al. ¹³ reported significant reduction of surface tension values for any binary surfactant system due to synergism that improved foaming besides spreadability of the coolant. In the same way, many researchers have concluded, after experimental investigation of various mixed surfactants, that better wettability of the surface could be attained by improved surface spread of the coolant due to synergism effect.^14–16

Another way to improve heat transfer is by using nanofluids that provide high thermal conductivity, internal micro-convection, large specific area, and less corrosion which makes research on high heat flux devices worthwhile. ¹⁷ The word nanofluid is a new class of engineered fluids which have suspended nanoparticles in the base fluid that was first used by American scientist Choi. ¹⁸ Base fluid may be: water, acetone, refrigerants, oil or mixtures whereas metals, metal-oxides, carbon, and other materials can be used as nanoparticles. Bansal et al. ¹⁹ prepared alumina-water nanofluid by a two-step method using ultrasonic bath to investigate the effect of various concentration and flow rates on spray cooling heat transfer. The results revealed that heat flux values for nanofluid obtained were lower in the range between 45 °C to 140 °C. However, unlike for water, in that range, there was no indicated CHF for nanofluid. It was concluded that the lower values were due to nanoparticle deposition over the surface, which affected heat transfer. Hsieh et al. ²⁰ performed experimental analysis with seven distinct types of nanofluids: Ag, Al, Al₂O₃, Fe₃O₄, SiO₂, TiO₂ and MWCNTs which were dispersed in deionized water at three different concentrations, namely 0.04%, 0.07%, and 0.1% by volume. As the volume percentage increased from 0.04% to 0.1%, the heat transfer coefficient and critical heat flux improved by 1.7 and 1.84 times, respectively, against deionized water.

Researchers recently attempted to investigate nanofluid capabilities by dispersing more than one nanoparticle into the base liquid and referred to it as a hybrid nanofluid with improved characteristics than a single nanofluid due to synergistic effects. Siddiqui et al. ²¹ investigated the effect of Silver (Ag)-Graphene (GNP) hybrid nanofluid droplet and its residue size on evaporation for various mixing ratios. The findings revealed that evaporation of the droplet increased with increasing residue size and decreasing mixing ratio. At a mixing ratio of (0.1:0.9) Ag:GNP, the highest evaporation rate of 370% was obtained over the residual surface. However, the evaporation rate diminishes as the mixing ratio for Silver was increased. The study of hybrid nanofluids is complex, but presents multiple opportunities for flow-boiling heat transmission. The area of nanofluids in spray cooling needs to be further explored.

Dissolving small amounts of salts prevents bubble coalescence and improve evaporation rate. ²²

Horacek et al. ²³ utilized spray cooling to study the influence of dissolved gas on heat transfer, indicating that CHF increased in the presence of gas. The contact line length was shown to be highly linked with heat flux, and it was inferred that heat flux might be enhanced by creating surfaces that could control contact line lengths. The problem with the soluble gas in the working fluid makes it unstable and difficult to control whereas soluble salts may choke the nozzle. Therefore, experimental system with surfactants offer benefits over soluble gases and salts in terms of stability, high efficiency and a system free of corrosion.

According to the aforementioned literature, additives have enhanced the efficiency of heat transfer in the base fluid as well as declined. It may be due to the varying concentrations of the additives. Moreover, experiments has mainly studied the effects, in addition to nanoparticles, of conventional additives such as alcohol and soluble salts. Very few research studies related to use of hybrid nanofluid containing MWCNT and TiO₂ nanoparticles in spray cooling, have been reported in literature. Due to MWCNTs’ distinctive trait of very high thermal conductivity, researchers have changed their emphasis to investigate their benefits in diverse heat transfer industrial applications. TiO₂ based nanofluids have a high heat transfer capability, moderate viscosity & frictional pressure drop. Further, these nanofluids possess excellent dispersion stability and are cheap and non-toxic. Thus, the novelty of the present work has been to find out the optimal concentration of hybrid nanofluid (MWCNT and TiO₂) along with three distinct binary surfactant systems namely: Sodium dodecyl sulfate (SDS), Polyvinyl pyrrolidone (PVP), and Tween-20. The effect of subcooling of 15 °C on heat transfer performance has also been investigated. Moreover, a correlation to estimate Nusselt number in terms of Reynolds number, Weber number and Prandtl number has been proposed with a mean absolute error of 7.16%. Experiments conducted in this study will not only aid in the understanding of spray cooling enhancement, but will also aid in the development of models for industrial applications.

2. Experimental setup

Figures 1(a) and (b) displays the schematic diagram and pictorial view of the spray cooling facility built to conduct the experiments. The details of the experimental setup and procedure is provided in author's earlier work. ²⁴

Fig. 1.

(a) and (b) Schematic and pictorial representation of the spray cooling test facility.

2.1 Test fluid

Three distinct binary mixed surfactant systems and one nanofluid were prepared for experimental investigation, and results are compared with distilled water (DW) for reference. Table 1 shows the experimental conditions for pressure nozzle having orifice diameter 0.5 mm. Three distinct binary surfactant systems and one hybrid nanofluid were sprayed over the heated surface at a volumetric flux of 3.5 cm³/cm²s. The average droplet size and Weber number are measured in terms of (Sauter mean diameter) d₃₂ = 183 µm and 890, respectively. The optimum concentration for different surfactants i.e. PVP = 75 ppm, SDS = 600 ppm, and Tween-20 = 45 ppm, are obtained from the author's previous work. ²⁴ For each of the binary surfactants, three distinct mixtures were prepared in proportion of 25%, 50% and 75% as shown in Table 2. Surfactants may be toxic to humans and accumulate in the human body. Human skin and eye discomfort, as well as increased skin hypersensitivity, can be caused by SDS intoxication. It is considered as non-carcinogenic. Tween-20 can also cause skin, eye, and mucosal irritation, which can lead to vomiting and diarrhoea. Aspirating children may experience airway inflammation and respiratory pain. The United States Food and Drug Administration deemed PVP as safe. Certain people, however, may develop skin allergies because of it.

Table 1.

Experimental conditions.

Nozzle	Spray height (H) mm	Heating power (W)	Fluid inlet temperature (°C)	Spray pressure (P)	Chamber Pressure (P)	Volumetric flux (Q”)
N1	10.48	350	27 and 42	0.35 MPa	101 kPa	3.50 cm3/cm2s

Table 2.

Details of concentration of surfactants and nanoparticles employed.

Sr. No.	Binary mixture	Volume percentage
1	Anionic (SDS) + Polymer surfactant (PVP)	0.25; 0.50; 0.75
2	Anionic (SDS) + Nonionic (Tween-20)	0.25; 0.50; 0.75
3	Polymer surfactant (PVP) + Nonionic (Tween-20)	0.25; 0.50; 0.75
4	MWCNT + Titanium dioxide (TiO2)	0.25; 0.50; 0.75

The hybrid nanofluid is assumed to give high heat transfer performance in contrast to nanofluid consisting of single nanoparticle. ²⁵ In the present study, MWCNT (purity of 95%) and nanopowder of TiO₂ (purity of 99.9%) are procured from Sisco Research Laboratory Pvt. Ltd The average length and outer diameter of MWCNT are 10–30 µm and 15–30 nm, respectively. The average size of nanoparticles of TiO₂ is 80 nm. The volume proportion for MWCNT:TiO₂ is set at 25:75 as the surface tension values are lowest for spray cooling experiments. Also, the heat transfer enhancement is shown to be more prominent near to this range in the literature. ²⁶ The hybrid nanofluid was prepared by adding commercial grade of nanoparticles of MWCNT and TiO₂ in three different volume percentage i.e. 0.1%, 0.2%, and 0.3% in DW. The aqueous solution is then subjected to high shear stirring to produce a stable nanofluid. The nanofluid is then ultrasonicated (Model: LMUC12, Labman Scientific Instruments Pvt. Ltd) for nearly 1 h at a frequency of 40 Hz. After the sonication process is completed, the solution is used for spray cooling within 1 h.

Each coolant has been characterized by measuring surface tension, contact angle, viscosity and thermal conductivity before the experiments. To achieve accuracy in the results, all measurements for thermo-physical properties have been performed four times and then averaged. The resultant thermo-physical properties of the binary mixed surfactant systems and hybrid nanofluid have been listed in Table 3 and supplementary Table 4, respectively. The surface tension was measured using surface tensiometer. The surface tension of a liquid determines its capacity to wet a solid surface. Apex contact angle measuring equipment have been used to measure contact angle of the test samples. Before measuring the contact angle, the sample surface has been washed with acetone and dried for a few minutes to decrease the likelihood of human error. All of the working fluids’ viscosities have been measured using a viscometer. The thermal conductivity of the hybrid nanofluid was measured with thermal property analyzer KD2 pro.

Table 3.

Thermo-physical properties of surfactants at 27 °C.

Name	Type	Chemical Formula	Molecular Weight (g/mol)	Concentration taken (ppm)	Surface Tension (mN/m)	Viscosity (mN-s/m2)
PVP	Polymer	(C6H9NO)n	40,000	75	41.2	1.31
SDS	Anionic	CH3(CH2)11OSO3NA	288.37	600	42.6	1.04
Tween-20	Nonionic	C18H34O6(C2H4O)20	1227.54	45	38.7	0.82

2.2 Heater assembly

Figures 2(a) and (b) depict the cross-sectional view of test heater assembly and include descriptions of the thermocouple dimensions and locations within the heater block. A copper heater block consisting of 4 cartridge heaters as the heat source is fixed at the heater side of the pressure chamber. Each cartridge heater can have a maximum power of q_in = 125 W, and input power can be controlled precisely by adjusting the input AC voltage of the variac.

Figure 2.

(a) Cross-sectional view of the test heater assembly) (b) positioning of thermocouples inside the copeer specimen (all dimensions are in mm).

 Figure 3(a) shows the copper heater block with a 10 mm² impact surface protruded over the heater block. Four holes are drilled inside the heater block from the bottom surface to embed cartridge heaters. The heat flux is assumed to distribute uniformly on the test surface. An enclosure made up of stainless-steel is used to house the heater block and the gap between these two parts is filled with ceramic wool to minimize heat loss to the ambience. Teflon insulation is provided at the top of the test specimen, and sealants to check leakage into the gap between the heater and Teflon. Figures 3(b) and (c) show the pictorial view of the cartridge heaters and test heater assembly, respectively.

Figure 3.

Pictorial view of the (a) copper specimen, (b) cartridge heaters, (c) test heater assembly.

2.3 Data acquisition system

In this study, the parameters measured are the liquid volumetric flux (Q”), the inlet pressure of the working fluid (ΔP), inlet and outlet temperatures of the working fluid (T_l), and the target surface temperature (T_w). To avoid interference with the formation of a thin fluid layer on the heated surface, the desired surface temperature is obtained indirectly using Fourier's 1D heat transfer equation, which assures continuous impact of spray droplets without any blockage. Four thermocouples (type-K) with a wire diameter of 2 × 10⁻⁴ m are inserted into the holes drilled in the protruded cube over the copper heater block, as shown in Figure 3(a). The distance between the two-thermocouple planes and between the heater surface and top thermocouple plane is shown in Figure 2(a). As described earlier, this type of indirect measurement prevents disruption of the spray and maintains perfect convection boundary conditions near the heater surface. Fourier's equation of heat conduction can be used to determine the heat flow on the heater surface, which is as follows:

$${\mathrm{q}}^{\mathrm{″}}=\mathrm{k}{\displaystyle \frac{({\mathrm{T}}_{2,\mathrm{a}}-{\mathrm{T}}_{1,\mathrm{a}})}{{\mathrm{x}}_2}}$$

(1)

where q″ is the heat flux, k is the thermal conductivity, T_2,a and T_1,a are the thermocouple locations in the middle plane and x₂ is the distance between the two thermocouple planes. The average temperature of the test surface may be evaluated from the equation given below ²⁴ :

$${\mathrm{T}}_{\mathrm{w}}={\mathrm{T}}_{1,\mathrm{a}}-{\displaystyle \frac{{\mathrm{q}}^{\mathrm{″}}{\mathrm{x}}_1}{\mathrm{k}}}$$

(2)

where T_w is the wall temperature (target surface) and x₁ is the distance between the wall surface and the upper thermocouple plane. Heat transfer coefficient (h) can be estimated using the equation given below:

$$\mathrm{h}={\displaystyle \frac{{\mathrm{q}}^{\mathrm{″}}}{({\mathrm{T}}_{\mathrm{w}}-{\mathrm{T}}_{\mathrm{sat}})}}$$

(3)

The thermocouple signals are sent to the data acquisition system for showing and recording the temperature information. All the thermocouples are pre-calibrated.

3. Uncertainty analysis

All measurements taken during the experiments have error tolerance. The data acquisition system (make: Agilent) with multiplexer have been used to measure uncertainties in volumetric flux (Q”), liquid inlet temperature (T_in), and temperature of the thermocouples. Precision mercury thermometer with ± 0.05 °C rated accuracy is used to calibrate DAQ and all the thermocouples before using at the range 0–300 °C. The accuracy of this arrangement of the system is found to be within 0.25 °C throughout experiments. Therefore, when the temperature fluctuation comes down to within 0.25 °C, it is considered that a steady state is reached. The maximum uncertainty of fixing and measuring type-K (Chromel-Alumel) sheathed thermocouple is about ± 0.01 mm and ± 0.05 °C. The mass flow rate and the fluid inlet temperature of the DW are obtained by coriolis mass flow meter. Uncertainties associated to direct measure quantities are presented in Table 4.

Table 4.

Uncertainties of the direct measured quantities.

Measured parameter	Uncertainty
Inlet pressure	±1.2%
Volumetric flux	±1.1%
Temperature by the thermocouples	±0.05 °C
Distance between the test surface and the top plane of thermocouple location	±0.6%
Distance between the two thermocouple locations	±0.2%

Accounting for error propagation, uncertainty can be evaluated as follows:

$${\sigma }_{\mathrm{y}}=\sqrt{{{\sum }_{\mathrm{i}=1}^{\mathrm{n}}\left({\displaystyle \frac{\mathrm{\partial }\mathrm{f}}{\mathrm{\partial }{\mathrm{x}}_{\mathrm{i}}}}\right)}^2{\sigma }_{{\mathrm{x}}_{\mathrm{i}}}^2}$$

(4)

Therefore, the measurements of uncertainties of the surface temperature, heat flux, HTC are 1.64%, 5.35%, 6.29%, respectively.

Measurements have been taken four times to ensure the reproducibility of results before experimentations. It is estimated that the deviation in the heat flux curve is due to the fluctuation of spray characteristics, such as d₃₂ (Sauter mean diameter) and volumetric flux, and not by the input power.

4. Results and discussion

4.1 Physical properties measurement

Figure 4 depicts the change in surface tension that causes the contact angle and viscosity of binary surfactant solutions to vary. Tween-20Tween-20Concentration of primary surfactant is shown on primary x-axis, and concentration of secondary surfactant is shown on secondary x-axis.

Figure 4.

Variation of surface tension, contact angle, viscosity of binary mixed surfactant systems at various volume concentrations.

Figure 4(a) shows the plot for surface tension and contact angle for the binary mixed surfactant solutions of the anionic and polymer surfactant (SDS:PVP). Higher percentage of SDS surfactant reduces the surface tension and contact angle for binary surfactant monotonically when compared to individual polymer surfactant. At 75% SDS and 25% PVP, the drop in surface tension and contact angle is more appreciable. The reason for this change can be attributed to the synergism, phenomena that results in properties of binary surfactant, which is better than that of individual surfactant.

Figure 4(c) shows the plot of surface tension and contact angle for the binary composition of the anionic and nonionic surfactant (SDS:Tween-20). At all mixture proportions, the surface tension and contact angle are low when compared to individual surfactant. Moreover, at 25% SDS and 75% Tween-20, the decrease is more appreciable. The reason for this decrease can be attributed to the phenomena of synergism that gives properties of the binary surfactant system better that the individual surfactant.

Figure 4(e) shows the plot for surface tension and contact angle for the binary surfactant system of the polymer and nonionic surfactant (PVP:Tween-20). Higher Tween-20 percentage in the binary surfactant system reduces the surface tension and contact angle. Moreover, at 25% PVP and 75% Tween-20 binary mixture, the decrease in surface tension and contact angle is more appreciable compared to the individual surfactants.

The effect of binary mixed surfactant solutions on viscosity are shown in Figure 4(b) SDS:PVP, (d) PVP:Tween-20, and (f) SDS:Tween-20. It is clearly seen that SDS:PVP and PVP:Tween-20 solution shows greater increases in viscosity as shown in Figure 4(b) and (f). The lowest values of viscosity is obtained for Tween-20 and SDS binary solution as shown in Figure 4(d). The reduction in viscosity improved the spreadability of the liquid film on the heater surface, which is crucial for achieving high extraction of heat. Furthermore, the improved droplet spreadability increased surface wetting, resulting in improved liquid film boiling over the heater surface. As a result, high heat transfer rates can be achieved.

4.2 Spray cooling

4.2.1 Binary mixture of anionic SDS and polymer surfactant PVP

A blend of 600 ppm aqueous solution of SDS and 110 ppm aqueous solution of polyvinyl-pyrrolidone (PVP) was employed as the first binary surfactant system in this investigation. The binary mixtures taken are 25:75 vol% SDS:PVP, 50:50 vol% SDS:PVP, 75:25 vol% SDS:PVP. Figure 5 illustrates the influence of different concentration of SDS:PVP binary surfactant system on the boiling curve at a volumetric flux of 3.50 cm³/cm²s. The addition of varying amounts of surfactant mixtures in the base fluid causes all of the curves to change quantitatively. The maximum heat flux achieved using N1 pressure nozzle is 232.76 W/cm² and the value of HTC obtained is 3.20 W/cm²K at T_in = 27 °C. At T_in = 42 °C, the maximum value of heat flux obtained is 145.25 W/cm² and HTC achieved is 2.49 W/cm²K. The surface heat flux achieved with this binary mixture is higher at low fluid inlet temperature. As the subcooling is lowered by 15 °C, value of HTC is reduced by nearly 22%. For comparison, the results obtained with the binary solution is directly compared with DW. The maximum values obtained for HTC and heat flux for DW are 2.709 W/cm²K and 199.77 W/cm², respectively, at T_in = 27 °C. Therefore, an increase of about 18.1% and 16.5% in HTC and heat flux, respectively, has been achieved for binary solution SDS:PVP in contrast to DW. At T_in = 42 °C, the maximum values obtained for HTC and heat flux are 2.19 W/cm²K and 128.36 W/cm², respectively for DW. Hence, an increase of 13.7% in HTC and 13.2% in heat flux has been achieved for SDS:PVP when compared to DW.

Figure 5.

Variation of heat flux and HTC for pressure nozzle N1 at T_in = 27 °C and 42 °C.

4.2.2 Binary mixture of anionic SDS and nonionic surfactant tween-20

The anionic SDS and nonionic Tween-20 surfactants form the second binary system. The binary mixtures taken are 25:75 vol% SDS:Tween-20, 50:50 vol% SDS:Tween-20, 75:25 vol% SDS:Tween-20. The addition of different amounts of surfactant mixtures in the base fluid leads all of the curves to alter quantitatively. The surface heat flux achieved with this binary mixture is higher at T_in = 27 °C in contrast to when the fluid was supplied at T_in = 42 °C. The maximum value of heat flux achieved is 264.21 W/cm² and HTC is 3.59 W/cm²K at T_in = 27 °C. At T_in = 42 °C, the maximum value of heat flux obtained is 164.14 W/cm² and HTC is 2.81 W/cm²K. There is a reduction of nearly 21.7% in HTC when the subcooling is lowered by 15 °C. Therefore, an increase of about 32.2% in HTC and 32.5% in heat flux is achieved for binary solution SDS:Tween-20 when compared to DW at T_in = 27 °C. At T_in = 42 °C, an increase of 28.3% and 27.9% is achieved in HTC and heat flux, respectively for SDS:Tween-20 in contrast to DW.

4.2.3 Binary mixture of polymer surfactant PVP and nonionic surfactant tween-20

The last binary mixture solution is between the PVP and Tween-20 solution. The binary mixtures taken are 25:75 vol% SDS:Tween-20, 50:50 vol% SDS:Tween-20, 75:25 vol% SDS:Tween-20. It can be seen that all the curves vary quantitatively due to use of different proportions of surfactant mixtures in the base fluid. The surface heat flux achieved is higher at low fluid inlet temperature i.e. T_in = 27 °C and heat flux diminished as the fluid inlet temperature is increased to 42 °C. The maximum heat flux achieved is 248.42 W/cm² and HTC is 3.38 W/cm²K at T_in = 27°C. At T_in = 42 °C, the maximum value of heat flux obtained is 150.35 W/cm² and HTC is 2.59 W/cm²K. As the subcooling is reduced to 15 °C, the value of HTC is reduced by nearly 23.4%. The maximum increase in HTC and heat flux obtained for Tween-20:PVP is about 25.2% and 24.4%, respectively when compared to DW at T_in = 27 °C. And the maximum increase in HTC and heat flux obtained for Tween-20:PVP is about 18.2% and 17.13%, respectively at T_in = 42 °C in reference to DW. The maximum values of heat flux and HTC obtained for different mixed surfactant systems at T_in = 27 °C are shown in supplementary Table 6. For T_in = 42 °C, the values for heat flux and HTC for different mixed surfactant systems are shown in supplementary Table 7.

In comparison to the results of single surfactant study conducted earlier by the author, ²⁴ binary mixed surfactant systems showed considerable improvement in heat transfer performance. For instance, SDS:Tween-20 binary mixed surfactant solution provided nearly 12.2% and 15% enhancement in HTC over single surfactant Tween-20 and SDS, respectively. The reason for this superior performance can be attributed to the synergistic effect that occurs between SDS (anionic) and Tween-20 (non-ionic) surfactant systems, which reduced droplet size while ejecting from the pressure nozzle. Besides that, low surface tension produced a small contact angle, resulting in a large droplet contact area with the heated surface. Droplets with a small contact angle and a large contact area can evaporate quickly from the surface. Because the thickness of the liquid film is proportional to the diameter of the droplet, a smaller diameter resulted in the formation of a thin liquid film that is easily pierced by surfactant molecules. As a result, spray droplets make intermittent contact with the heated surface. Furthermore, bubbles near the surface are more likely to nucleate and accumulate, which can eventually increase the rate of evaporation as well as the performance of heat transfer.

4.2.4 Effect of hybrid nanofluid on heat transfer

The last coolant system used in this study is the hybrid nanofluid of multi-walled carbon nano tubes (MWCNT) and Titanium dioxide (TiO₂). Figure 6(a) shows the variation of surface tension, contact angle for hybrid nanofluid of MWCNT and Titanium dioxide at three different mixed concentration namely 25:75 vol%, 50:50 vol%, and 75:25 vol%. The values of surface tension and contact angle obtained at 25:75 of MWCNT:TiO₂ hybrid nanofluid are minimum. Because of the low surface tension, the droplet size becomes smaller which provides a large surface area for heat transfer between the droplets and heated surface. The low contact angle helps in spreading of the droplet resulting in thin film layer over the heated surface. The rate of evaporation increases with thin film and eventually improves heat transfer performance. Three different volume fractions of hybrid nanofluid prepared were, namely; 0.1%, 0.2%, and 0.3% by volume. Before commencing experiments, carbon filter was removed from the test rig to avoid filtering of nanoparticles. Results obtained in terms of maximum heat flux and HTC at two fluid inlet temperatures i.e. T_in = 27 °C and 42 °C are given in supplementary Table 8.

Figure 6.

(a) shows the variation of surface tension, contact angle for hybrid nanofluid of MWCNT and TiO₂ at various mixed concentrations, (b) experimental Nu v/s predicted Nu.

The highest heat flux achieved is 233.9 W/cm² and the value of HTC obtained is 3.15 W/cm²K at fluid inlet temperature T_in = 27 °C, and the maximum heat flux achieved at a fluid inlet temperature T_in = 42 °C is 138.42 W/cm² and the value of HTC obtained is 2.42 W/cm²K within the non-boiling region. It is observed that as the volume fraction is increased from 0.1% to 0.2%, rate of heat transfer improved. It diminished with further increase in volume fraction from 0.2% to 0.3%. As the subcooling is reduced to 15 °C, the value of HTC is reduced by 23.17%. When compared to DW, an increase of 27.3% in HTC and 27.95% in heat flux is achieved at T_in = 27 °C. For T_in = 42 °C, an increase of 23.3% and 20.36% in HTC and heat flux is achieved, respectively in contrast to DW.

In a non-boiling regime, fluid properties are the main parameters that affect spray cooling heat transfer are the thermo-physical properties of the fluid and spray characteristics.

Therefore, it is assumed that:

$$f(\mathrm{h},\mathrm{L},\mathrm{d},{\mathrm{u}}_{\mathrm{o}},\rho ,{\mathrm{C}}_{\mathrm{p},\mathrm{l}},\sigma )=0$$

(4)

Using Eq. (4) various thermo-physical and transport properties of the fluid have been grouped into the following four dimensionless groups such as Nusselt number $(\mathrm{Nu}=\mathrm{hL}/\mathrm{k})$ , Reynolds number $(\mathrm{Re}=\rho {\mathrm{u}}_{\mathrm{o}}\mathrm{d}/\mu )$ , Weber number $(\mathrm{We}={\rho \mathrm{u}}_{\mathrm{o}}^2\mathrm{d}/\sigma )$ , and Prandtl number $(\mathrm{Pr}={\mathrm{C}}_{\mathrm{p},\mathrm{l}}\mu /\mathrm{k})$ , The groups formed after the dimensional analysis are:

$$f\left({\displaystyle \frac{\mathrm{hL}}{\mathrm{k}},{\displaystyle \frac{\rho {\mathrm{u}}_{\mathrm{o}}\mathrm{d}}{\mu },{\displaystyle \frac{{\rho \mathrm{u}}_{\mathrm{o}}^2\mathrm{d}}{\sigma }}}}\right)=0$$

(5)

The new proposed correlation can be given as:

$$\mathrm{Nu}=f(\mathrm{Re},\mathrm{We},\mathrm{Pr})$$

(6)

Figure 6(b) shows the comparison for the experimental Nusselt number with the predicted Nusselt number with the spray cooling data using Eq. 7. The equation was given for Reynold number, Weber number, and Prandtl number along with exponential coefficients and exponents. The correlation given in Eq. 7 works well when the experimental data of different fluid is compared with the prediction, with a mean absolute error of 7.16%.

$$\mathrm{Nu}=3.58\mathrm{R}{\mathrm{e}}^{0.5}\mathrm{W}{\mathrm{e}}^{0.2}\mathrm{P}{\mathrm{r}}^{0.55}$$

(7)

4.3 Social significance of the present work

Thermal management of engineering devices with large heat flux has become more challenging in recent years. As semiconductor device sizes have shrunk, the amount of heat generated per unit area in the electronics industry has grown considerably. As a consequence, a cutting-edge thermal management system, such as spray cooling, can provide excellent system cooling performance and reliability from heat generated by electronic devices, especially when the electronic components are subjected to heat fluxes more than 500 W/cm². Because of its various effective heat transfer mechanisms, spray cooling is a high heat flux, consistent, and efficient cooling strategy that has proved useful in a range of applications such as industrial operations, data centers, and combustion technologies. To meet the demands of next-generation ultrahigh-power applications, spray cooling capacity and efficiency must be enhanced even more. Surfactants and nanofluids can have a significant influence on liquid–wall interactions in spray cooling, making them the most promising alternative for improving spray cooling efficiency. The findings of the present study will help the designers of the applications/devices with large heat flux in the more efficient manner which will subsequently results in reduced rate of failure, better reliability and efficient performance and less material cost

4.4 Barriers and opportunities of using hybrid nanofluid in spray cooling

The high cost of producing nanofluids, their low specific heat compared to base fluids, agglomeration, and sedimentation are among the barriers that inhibit their usage in industrial cooling applications. The usage of nanofluid for an extended period of time may cause erosion and corrosion of thermal system components. Because employing nanofluid as a coolant increases pressure drop, a higher pumping power is required as compared to conventional fluid.

However there are numerous opportunities in using hybrid nanofluids. By altering the nanoparticle concentration, the nanofluids’ characteristics such as thermal conductivity and surface wettability may be adjusted to cater to the different magnitudes of heat transfer rate required for various cooling applications. These have greater stability and the capacity to improve heat conduction as compared to microfluidics. The most notable attribute of the nanofluid is thermal conductivity, which increases the rate of heat transfer. As a result, the use of nanofluids increases the rate of heat transfer in thermal cooling systems, increasing the efficiency of these systems.

5. Conclusions

In this study, the heat transfer characteristics of three distinct binary mixed-surfactant systems along with one hybrid nanofluid have been analyzed using spray cooling method for enhanced heat transfer performance for the applications in the area of electronic cooling. The working fluid was sprayed over the heated surface with a volumetric flux of 3.50 cm³/cm²s. Each binary mixed-surfactant system (SDS:PVP, PVP:Tween-20 and SDS:Tween-20) was prepared in various proportions as described previously. In addition to surfactants, one hybrid nanofluid was also prepared at fix mixed ratio of 25:75 (MWCNT:TiO₂) and studied under three different volume concentrations i.e. 0.1%, 0.2%, and 0.3%. The results of all the above-mentioned coolants were directly compared with DW to know the heat transfer enhancement. Results showed that SDS:Tween-20 performed better among all, due to low surface tension of the coolant, which leads to increased surface activity than that of SDS:PVP and Tween-20:PVP surfactant systems. Higher surface activity because of low surface tension aids in generation of finer droplets during atomization. Low surface tension also reduced the contact angle that improved wettability of the heated surface by the spray droplets. Fine droplets improved evaporation over the heated surface because of large contact area and leads to thin film resulting in improving overall boiling heat transfer. The optimum concentration of 75:25 vol% gives higher cooling rates for SDS:Tween-20 surfactant system. It was noted that the value of surface tension reduced as the concentration of PVP increased. The following conclusions may be drawn from spray cooling experimental results for binary mixed-surfactant systems and hybrid nanofluid:

The surface tension of DW increased by 20% when nanoparticles (MWCNT and TiO₂) were added at optimum concentration it 0.2% by volume. However, addition of surfactants reduced the value of surface tension of the base fluid significantly which assists in the easy wettability by the droplets. In addition, the contact angles reduced considerably for the binary mixed surfactant systems, which is crucial for the easy spreading of droplets over the heated surface. The surface tension values reduced in the range 120% to 130% for the binary mixed surfactant systems.

In case of hybrid nanofluid, the maximum heat transfer performance is obtained at 0.2% volume concentration. The maximum value achieved for HTC is 3.45 W/cm²K at T_in = 27 °C and 2.70 W/cm²K at T_in = 42 °C. Hence an increase of nearly 27.3% and 23.3% in HTC at two fluid inlet temperatures when compared to DW.

In case of binary mixed surfactant systems, the maximum value achieved for HTC is 3.59 W/cm²K at T_in = 27 °C and 2.81 W/cm²K at T_in = 42 °C for SDS-Tween-20 solution at a volume proportion of 75:25. Therefore, an increase of about 32.2% and 28.3% in the values of HTC is obtained in comparison with DW at two fluid inlet temperatures, respectively. For Tween-20:PVP and SDS:PVP binary surfactant systems, the enhancement in HTC of nearly 28.3% and 18.2% is achieved at T_in = 27 °C. At T_in = 42 °C, an improvement of nearly 27.9% and 17.13% is observed, respectively for the two surfactant systems.

SDS:Tween-20 binary surfactant system provided the best overall performance and showed an enhancement of 4.1% in HTC in contrast to hybrid nanofluid. At T_in = 27 °C, HTC improved by roughly 6.2% and 12.2% when compared to other binary mixed surfactant systems, namely Tween-20:PVP and SDS:PVP.

Subcooling has a notable influence on heat transfer performance, although the increase in HTC is in the same range for both fluid inlet temperatures. The increase in HTC is nearly 4% when compared to hybrid nanofluid at T_in = 42 °C for SDS:Tween-20. When compared to other binary surfactant systems, SDS:Tween-20 outperformed Tween-20:PVP by nearly 8.5% and SDS:PVP by about 13% in HTC.

Supplemental Material

sj-docx-1-pie-10.1177_09544089221097691 - Supplemental material for Effect of binary mixed-surfactants and hybrid nanofluid on spray cooling heat transfer

Supplemental material, sj-docx-1-pie-10.1177_09544089221097691 for Effect of binary mixed-surfactants and hybrid nanofluid on spray cooling heat transfer by Sukhdeep Singh and Rajeev Kukreja in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering