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The enhancement of dropwise and spray evaporative cooling by using extract of Reetha added water as a coolant
Thermochimica Acta 680 (2019) 178379
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The enhancement of dropwise and spray evaporative cooling by using extract of Reetha added water as a coolant Anita Pandaa, A. Kumarb, Basudeb Munshia, S.S. Mohapatraa, a b
T
⁎
Department of Chemical Engineering, National Institute of Technology Rourkela, 769008, India Department of Chemical Engineering, Indian Institute of Technology Dhanbad, 826004, India
ARTICLE INFO
ABSTRACT
Keywords: Sapindus mukorossi Surfactant Recoiling Bouncing Spreading
Although the use of synthetic surfactant in various quenching process significantly enhances the heat removal rate, the corrosive nature of the synthetic surfactants and their interaction with the metal surface during cooling does not allow to use. The replacement of the synthetic surfactant by another surfactant which is free from the abovementioned disadvantages can mitigate the requirement of various industries dealing with high heat flux applications. In this regard, the literature does not reveal any information. Therefore, in the current work, an attempt has been made to replace the synthetic surfactant by preparing a surfactant which does not depict the disadvantage of the synthetic surfactant. In this work, the surfactant is prepared from the extract of Reetha (Sapindus mukorossi). The measurement of various physical properties of the extract of Reetha and also different characterizations such as FT-IR and EDX analysis confirm the existence of surfactant properties in the extract of Reetha. Furthermore, the corrosion analysis clearly indicates insignificant corrosion in comparison with synthetic surfactants. The post quenching analysis of the heat treated steel plate reveals that the extract of Reetha added water does not depict the deposition characteristics. The thermal performance of the extract of Reetha is determined by conducting dropwise evaporative cooling and spray quenching experiments. The result reveals that the extract of Reetha has an excellent heat removal capacity (CHF = 1.3 MW/m2) in the critical heat flux region.
1. Introduction During evaporation, the dropwise evaporative cooling is mainly controlled by the thermo-physical properties of the coolant [1–4]. At very high initial surface temperature, many researchers have tried to achieve enhanced heat transfer rate by altering the thermo-physical properties in the favorable direction of heat transfer using various additives such as salt [5–8], surfactants [9–14], nanoparticles [15–17], polymers [18–21] and dissolved gases in the coolant [22–24]. Among all the additives reported in the literature, surfactant added water depicts minimum disadvantages and significant augmentation of heat transfer rate. Although other additives have comparatively better heat removal capacity; however, the surface interaction effect of additives during quenching alters the surface morphology and this is not the desired requirement of the various metal industries. By using various commercial surfactants such as SDS [25,26], Tween
20 [27] and CTAB [28], several investigators have reported different degree of enhancement at various conditions. In SDS, the presence of elements or compounds promoting surface corrosion or reaction is the main disadvantage. Additionally, the existence of quaternary ammonium compound in CTAB inhibits bacterial activity and promotes corrosion. Furthermore, although in case of Tween 20, corrosion is not considered as the major disadvantage, the presence of more number of hydrophobic part in the surfactant destabilizes the air which affects foaming characteristics of the surfactant. In addition to the above, during quenching process in metal industries, the use of synthetic chemicals in bulk quantity creates environmental problem after discharge from the industry without pre-treatment. Therefore, the requirement of natural surfactant for high heat flux application put thrust on the further research in the exploration of natural surfactant free from the disadvantage depicted by the synthetic surfactant. In addition to the above, the focus of an eco-friendly environment concerns the use of biodegradable surfactants and in turn
Abbreviations: AHF, Average Heat Flux, MW/m2; CHF, Critical Heat Flux, MW/m2; AISI, American Iron and Steel Institute; CMC, Critical Micelle Concentration, g/ cc; SDS, Sodium Dodecyl Sulfate; CTAB, Cetyl Trimethyl Ammonium Bromide; Tween 20, Polyoxyethylene (20) Sorbitan Monolaurate; SEM, Scanning Electron Microscope; FT-IR, Fourier Transform Infrared Spectroscopy; EDX, Energy Dispersive X- Ray spectroscopy ⁎ Corresponding author at: Spray Boiling Heat Transfer Laboratory, Department of Chemical Engineering, NIT Rourkela, 769008, India. E-mail address: [email protected] (S.S. Mohapatra). https://doi.org/10.1016/j.tca.2019.178379 Received 22 February 2019; Received in revised form 3 August 2019; Accepted 16 August 2019 Available online 17 August 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
Thermochimica Acta 680 (2019) 178379
A. Panda, et al.
Nomenclature
K T T1 T2 T3 R1 R2 R3 T
Symbols
Ρ μ θ Cp
Surface tension,n, N/ Density of liquid, kg/m3 Viscosity, mPas Contact angle, degree Specific heat of water, J/kg°C
creates interest to develop a natural surfactant against commercial nonbiodegradable surfactant [29,30]. Therefore, in the current research, an eco-friendly and thermal performance enhancing skill surfactant is tried to explore. The analysis confirms that the extract of Reetha can be used as a natural surfactant. For the quantification of enhancement in heat transfer rate and identification of heat transfer mechanism, systematic experiments were conducted by using droplet dispensing system and high speed motion analyzer. The evaporation time was calculated from the analysis of captured images. Finally, spray cooling experiments were conducted by using the extract of Reetha as additives to identify the justification of replacement of synthetic surfactant by the natural surfactant.
Thermal conductivity, W/m°C Surface temperature, °C Thermocouple location 1 Thermocouple location 2 Thermocouple location 3 Region 1 Region 2 region 3 Time, sec
2.2. Experimental set-up and procedure The schematic diagrams of the experimental set-up for dropwise and spray evaporative cooling is depicted in Fig. 1. Dropwise evaporative cooling set-up consists of droplet generation and dispensing system (VITA 19). For the dropwise evaporative cooling investigation, an AISI 304 steel plate of dimension 20 mm × 20 mm × 6 mm was used as the test specimen. The needle was placed at a height of 40 mm from the surface of the plate to avoid splashing of the droplet. Three k-type thermocouples having diameter of 2 mm were embedded in the plate for the measurement of transient time-temperature histories. Before the experimentation, the steel plate was heated up to various temperatures in the range of about 200–600 °C in an electric muffle furnace. After attainment of desired temperature, the hot plate was brought out from the furnace and put on the cooling pad. Then, the droplets from the droplet dispensing system were allowed to impinge on the hot surface of the plate at a rate of 60 drops per minute according to the information reported in the literature [31]. In the current work, for each experiment, images were captured by various imaging systems (DSLR (Nikon D5200), SLR and High speed motion analyzer (Model no. Sony RX 100v)). For the processing of the images, software such as video to jpg converter (v.5.0.92 build 608) was used. In case of spray cooling experimentation, similar type of set-up as described in the literature [28] was used. In this case, full cone nozzle (Leechler: 490.804.1Y.CE) was used to generate the spray.
2. Experimentation 2.1. Preparation of the sample The Reetha fruit pericarp samples were collected from the forest of the eastern part of India. Before experimentation, the samples were dried in an oven at 50℃ for 24 h and then to make powder form, grinding was performed. The powder samples were then added to deionized water for the preparation of the solution. For this, the mixture was continuously stirred and left for 24 h for the collection of undissolved materials contained by the powder. After 24 h, the separation was achieved by using filtration process. The final permeate was used in the experimentation as an additive. In the current work, solutions of 1%, 2%, 4%, 8%, 10%, 20%, 30% and 50% (v/v) concentrations were used.
Fig. 1. Schematic diagram of the experimental set-up. 2
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3. Results and discussion 3.1. Physical properties of the coolant The measured physical properties of the extract of Reetha solution at various concentrations are reported in Table 1. In the current work, temperature (25 ± 2 °C), pressure (1 atm) and water quality (Deionized water) were considered as the constant parameters in case of measurement of various physical properties of the surfactants. In addition to the above, the electrical conductivity and pH of the extract of Reetha solution at different concentrations were also determined by using a digital multi parameter analyzer (Agilent Technologies, 3200 M) fitted with a combined glass electrode. The decrement in surface tension with the increasing extract of Reetha concentration in water confirms the controlling properties defining a surfactant. Furthermore, the variation of contact angle with the concentration of extract of Reetha in water also corroborates the abovementioned statement. When the intensity of the decrement of surface tension in case of extract of Reetha added water is compared with the same parametric variation obtained in case of synthetic surfactant, the depression of surface tension in case of natural surfactant is found to be significant. In addition to the above, at critical micelle concentrations (CMC), the foaming characteristic of the natural surfactant (CMC = 0.017 g/cc) [32] is compared with the synthetic surfactants (Fig. 2). The analysis clearly ascertains the existence of foaming characteristic in the presence of extract of Reetha in water and the visual observation qualitatively ensures that the stability and foam sizes achieved in case of extract of Reetha added water are better than the synthetic surfactant. The discussed quality of the extract of Reetha encourages the current generation researchers to use as an additive in the attainment of high heat applications. From Table 1, it is also observed that the density of the solution rises with the increase in extract of Reetha concentration in water due to the increment of solute amount in the solvent. The insignificant variation in the viscosity favors heat transfer in the positive direction by reducing the early recoiling characteristic. Furthermore, the strong acidic nature of the solution could be due to the hydrolysis of the constituent of saponins. In addition to the above, it is also observed that the electrical conductivity of the extract of Reetha added water solution increases up to certain concentration (8%) and then decreases. This is mainly due to the ionic nature of the surfactant. The mobility and the number of ions present in the solution enhances the electrical conductivity and thereafter, formation of micelles affect the conductivity. This behavior of the solution is suitable for the achievement of higher heat transfer co-efficient. In case of conducting solution, the free electrons or ions are considered as the carrier of heat [33]. As in the current work, extract of Reetha solution is identified as anionic surfactant and therefore, the role of ions is also considered in the thermal transportation.
Fig. 2. Foaming stability of Tween 20, Extract of Reetha added water and SDS solution.
Fig. 3. FT-IR spectra showing different functional groups.
3.2. Composition analysis To identify the various elements and functional groups present in the Reetha, EDX and FT-IR analysis were performed, respectively. The FT-IR analysis clearly asserts that Reetha contains eOH, eCH2, and eCO group. The characteristic peaks obtained in the range of 4000400 cm−1 which is shown in the Fig. 3. The spectral line at a wavelength of 3398 cm−1 corresponds to presence of alcoholic (OeH) group which is confirmed by the presence of another intense peak at 1056 cm−1. The band at 2927 cm−1 is due to eCH2 stretching indicates the aliphatic nature of the Reetha. In addition to the above, methyl groups are also present (1450 and 1383 cm−1 peaks, eCHe). In EDX analysis of Reetha powder, carbon, oxygen and hydrogen are observed as shown in Fig. 4. In the absence of any other peaks in EDX spectra, it is concluded that the Reetha is free from sulphur and any
Table 1 Physical properties of extract of Reetha solution at various concentrations. Extract of Reetha concentration (%)
Density (ρ) (kg/m3)
Viscosity (μ) (mPas)
pH
Electrical conductivity (μs/cm)
Contact angle, θ (degree)
Surface tension (σ) (mN/ m)
Pure water 0.5 1 2 4 8 10 20 30 50
998 1001.4 1001.5 1004.5 1009.2 1017.7 1022.6 1027.4 1042.5 1047.7
0.788 0.807 0.811 0.819 0.825 0.854 0.867 0.89 0.907 0.92
6.7 4.197 4.016 3.842 3.809 3.825 3.879 4.006 3.887 3.987
13.60 195.9 324 641 1174 1972 2.38 2.65 3.55 4.52
74.7 63.4 57.7 49.2 46.3 45.9 31.2 27.7 29.8 33.5
71.5 65.2 56.9 51.3 50.7 48.9 37.8 31.2 32.8 35.6
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Table 2 Residence time for different extract of Reetha concentration at a surface temperature of 200 0C. Temperature = 200 0C Extract of Reetha concentration
Residence time of droplet (sec)
10% 20% 30% 50% 100% Pure water
1.56 1.94 2.2 2.54 2.80 0.34
other corrosive elements. Therefore, from the primary analysis, it can be considered as an environmental friendly and corrosion free surfactant.
Fig. 4. EDX spectrum of Reetha powder.
Fig. 5. Comparison of impact regime of different extract of Reetha concentrations at a surface temperature of 200 0C. 4
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Fig. 6. Comparison of impact regime of different extract of Reetha concentrations at a surface temperature of 400 0C.
3.3. Droplet impact mapping
residence time are observed. However, pure water evaporative cooling excludes the aforesaid phenomena.
In case of dropwise evaporation, the effect of droplet dynamics on dropwise cooling at various substrate surface temperature is investigated. The plate temperature was varied from 200 0C to 600 0C and the extract of Reetha concentration was varied within the range of 0% to 100%.
3.3.2. At plate temperature 400 and 600 0C The evaporative cooling nature in transition boiling regime is identified by conducting experiments at 400 and 600 ℃ initial surface temperatures. In transition boiling regime, the extract of Reetha added water dropwise evaporative cooling performance is much better than that of pure water case. The aforesaid behavior is confirmed from the images presented in droplet impact mapping (Figs. 6 and 7). At surface temperature 400 and 600 ℃, the residence times at different concentrations of Reetha added water solution are presented in Tables 3 and 4, respectively. Furthermore, the similar behavior is also noticed in film boiling regime temperature range recommended in the literature. In case of dropwise cooling performed at 6000C, the residence time depicts a decreasing trend with the increasing extract of Reetha concentration in water (Table 4). The investigation demonstrates that at any Reetha added water solution concentration, the droplet dynamics exhibits spreading, recoiling, bouncing, and rolling characteristics on the hot plate. For this particular case, the absence of the blasting
3.3.1. At plate temperature 200 0C To investigate the role of extract of Reetha concentration in dropwise evaporative cooling in nucleate boiling regime, experiments were conducted at various concentrations of extract of Reetha and at a surface temperature of 200 0C (Fig. 5). From the analysis of the captured images, the residence time was calculated and also presented in Table 2. It clearly asserts that the augmentation of residence time with the increasing concentration of extract of Reetha in water. For the determination of reason behind the enhancement, the droplet impact mapping is analyzed. From Fig. 5, it is seen that at higher extract of Reetha concentration in water, insignificant recoiling, bouncing and blasting characteristics which create suitable environment for higher 5
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Fig. 7. Comparison of impact regime of different extract of Reetha concentrations at a surface temperature of 600 0C. Table 3 Residence time for different extract of Reetha concentration at a surface temperature of 400 0C.
Table 4 Residence time for water at different extract of Reetha concentration at a surface temperature of 600 0C.
Temperature = 4000C
Temperature = 6000C
Extract of Reetha concentration
Residence time of droplet (sec)
Extract of Reetha concentration
Residence time of droplet (sec)
10% 20% 30% 50% 100% Pure water
0.38 0.32 0.28 0.26 0.22 0.30
10% 20% 30% 50% 100% Pure water
0.32 0.30 0.28 0.26 0.22 0.32
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by the thermocouple diameter [36]. The locations of the subsurface thermocouples used in the experimentation are provided in Fig. 10. The measured temperatures were recorded by using a Data Acquisition System (NI 9211, National Instrument, Austin, Texas, USA). For the calculation of surface heat flux and temperatures, the appropriate heat conduction model with the justified boundary conditions are initially selected. Then, the plate geometry and the thermal model equations are discretized. Finally, by performing grid independency test, optimum grid sizes are determined (3340 quadratic elements with 4 nodes per each element). The discretized form of the geometry, model equations, measured time-temperature histories, thermo-physical properties of the plate and the boundary conditions are given as the input. Based on the above mentioned input parameters, INTEMP predicts surface heat flux and temperatures by using a non-linear optimization technique. The model equation involved in the calculation (Eq. 1) and the algorithm used for the estimation of surface heat flux have been presented below.
Cp Fig. 8. Boiling diagram.
T = k t
2T
x2
+
2T
y2
(1)
Where, ρ = density of water, kg/m3 Cp = specific heat of water, J/kg°C k = thermal conductivity, W/m°C T = surface temperature, °C To solve the equation, following information are used Initial condition: At t = 0, T1 = T2 = T3 = 900 °C Boundary conditions Except the impinging surface, the remaining surfaces are assumed to be adiabatic. The algorithm for solving inverse heat conduction problems is as follows:
characteristic clearly indicates the absence of accumulation of vapor which promotes strong film or transition boiling. The lesser residence time of the droplet on the hot plate ascertains the prevention of formation of thin liquid layer created by the coalescence of the unevaporated droplet. This phenomenon is possible only when droplet resides on the hot plate for a longer period of time. 4. Extract of Reetha added spray cooling In case of pure water and extract of Reetha added water spray cooling, the boiling behavior is presented in Fig. 8. From the nature of the boiling diagram, it is concluded that for both the type of coolants, the quenching occurs initially in transition boiling regime and then shifts to nucleate boiling regime. The extract of Reetha added water spray cooling depicts enhancement near the critical heat flux region. However, in the early stage of cooling, pure water spray cooling produces higher heat flux than the extract of Reetha added water spray cooling. Due to the aforesaid characteristics of the boiling diagram, the time average values of the heat fluxes for entire boiling regimes are determined. From the obtained average values (AHFwater = 0.35, and AHFExtract of Reetha solution =0.55 MW/m2), it is said that the average heat removal rate for the entire cooling process is higher in case of extract of Reetha added water spray. For the further verification, the boiling behavior of the extract of Reetha added water is examined and the obtained results are presented in Fig. 9. It reveals early onset of nucleate boiling during a heating process and the observed phenomenon is considered as favorable for heat transfer. The surface heat fluxes reported in Fig. 9 are determined by using an inversion heat conduction algorithm (INTEMP) developed by D. Trujillo [34,35]. To measure the transient time–temperature histories, three Ktype of subsurface thermocouples were used and these thermocouples were placed parallel to the quenching surface to avoid the error induced
1) Initially, the software assumes the heat flux as known boundary condition, and predicts all the nodal temperatures by solving a two dimensional transient heat conduction equation. 2) The final prediction of the temperature distributions from the assumed surface heat fluxes is performed by a nonlinear optimization technique which minimizes the error between the measured and predicted temperatures at the measured location. 3) The technique converges to minimum, when error is less than 0.02 0 C. If the error is more than 0.02 0C, INTEMP goes for the estimation of new surface heat flux and repeats until it reaches the target value. 4) The accurate temperature at each nodal point is determined at the converged heat flux value. In case of spray evaporative cooling process, 2–6 ℃/s discrepancy in temperature drop rate is observed between region R1 and region R3 (Fig. 11). As a result, the temperature of one region is comparatively higher than the other region. This could be due to the non-uniformity in the distribution of mass flux. Since the cooling is very fast, the substrate does not find sufficient time to eliminate the abovementioned discrepancy. The discussed behavior continues until the temperature
Fig. 9. Optical images of extract of Reetha added water solution and pure water depicting boiling behavior. 7
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Fig. 10. Locations of the embedded thermocouples. Table 6 Errors associated with different variables used in the current work.
Fig. 11. Schematic diagram representing the temperature variation during cooling process.
Sl. No.
Name of the variable
Maximum error
Minimum error
1 2 3 4 5 6 7
Substrate temperature (0C) Density (kg/m3) Viscosity (mPa. S) Electrical conductivity(μS) pH Surface tension (mN/m) Contact angle (degree)
5 0.0009 0.008 0.15 0.05 0.9 2.3
1.1 0.0002 0.003 0.06 0.030 0.6 1.6
Due to this, the temperature profile in R2 again depicts increasing trend and shows the cross over. The transient time-temperature histories measured by various thermocouples are presented in Fig. 12. 5. Comparative study To identify the advantages associated with the extract of Reetha added spray cooling, various parameters such as residence time, corrosive nature, surface morphology, elemental analysis of the extract of Reetha are compared with the same parameters of the synthetic surfactant and pure water. From the comparison, it is concluded that the residence time of the extract of Reetha added water solution is higher in comparison with other two coolants. The aforesaid observation indicates the existence of minimum recoiling and thermal kick behavior in case of extract of Reetha added water spray cooling. In addition to the above, the surface morphology of the post heat treated steel sample demonstrates that on the surface, insignificant deposition of the natural surfactant is observed. However, in case of synthetic surfactant, the deposition on the surface is identified. Furthermore, the EDX analysis confirms the absence of any corrosive element which are not recommended to use by the steel industries or pollution department. For
Fig. 12. Variation of sub-surface temperature with time.
becomes < 180℃. As the cooling rate is very slow in this temperature range, the heat tries to flow from higher temperature region to lower temperature region. This type of phenomenon is expected in region R2. Table 5 Comparative study of different coolants at various substrate temperatures. Name of the variables
Substrate temperature
Pure water
Extract of Reetha added water solution
Synthetic surfactant
Residence time of the droplet (sec)
Surface morphology
200 400 600 200 400 600 600
0.34 0.30 0.32 45 43 39
1.94 0.32 0.30 95 94.2 93.6
1.83 0.2 0.17 158.4 152.2 151.6
Elemental analysis of the post treated steel plate
600
C = 2.35% O = 17.76% Fe = 24.06% Cr = 17.8% Mn = 5.90% Ni = 8.13%
C = 6.5% O = 18.48% Fe = 41.07% Cr = 18.53% Ni = 14.30% Others = 1.1%
C = 44.37% O = 47.45% Mg = 1.12% Fe = 1.16% Ni = 2.20% Mn = 3.5% S = 0.2%
Corrosion analysis (μm/year)
8
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further verification on the corrosive nature of the Reetha, the corrosion tests were performed. In the corrosion test, corrosion resistances of the sample in the electrolyte (extract of Reetha solution) were measured. The specimens were stabilized for 30 min. at the open-circuit potential (OCP). In the discussed test, the potential was changed from −0.725 mV vs. OCP to 1 V vs. Ag/AgCl at a scan rate of 5 mVs−1. The corrosion test reveals that the natural surfactant depicts significantly lower corrosive nature in comparison with synthetic surfactant. The corrosion rate of natural surfactant is also comparable with water. A comparative study of different coolants at various substrate temperatures is presented in Table 5.
[2] [3] [4]
[5] [6]
6. Measurement uncertainty
[7]
The measurable parameters used in the current work are extract of Reetha added water solution concentration, temperature of the substrate, surface tension, contact angle, viscosity, electrical conductivity, pH and density. The main sources of uncertainty in temperature measurement are noise recorded by the thermocouple and its exact location inside the steel plate. Therefore, in the current work, each experiment was conducted three times to check the reproducibility of the reported results and the average values have been reported and also used for the calculation of surface heat flux. In addition to the above, the nodal and measured temperatures are compared at the measured location. The comparison depicts a very good agreement between the calculated and measured temperatures. The maximum and the minimum error for the different measurements and predictions have been reported in Table 6.
[8] [9] [10] [11] [12] [13] [14] [15] [16]
7. Conclusions
[17]
In the current study, dropwise evaporative cooling experiment was conducted using the extract of Reetha added water as a coolant. The experiments were conducted using various concentrations of extract of Reetha added water solution and at different substrate temperatures. Based on the analysis of droplet impact mapping, spray boiling curve, thermo-physical properties of Reetha and corrosiveness of the extract of Reetha added water, the followings are the conclusions;
[18] [19] [20] [21] [22] [23]
1 The attainment of very low value of surface tension (37.8 mN/m) in case of extract of Reetha added water ensures that it is a surfactant. Furthermore, the lower value of contact angle (31.2°), foaming characteristics and FT-IR analysis support the above claim. 2 At 200℃, the droplet impact mapping result shows that the residence time rises with step up in extract of Reetha concentration. However, at higher initial surface temperature (600℃), with the increasing extract of Reetha concentration, the residence time decreases and the heat transfer rate reaches the maximum value at comparatively lower concentration of extract of Reetha in water due to the decrement of sensible heat extraction period. 3 When the achieved heat removal rate in case of pure water spray cooling is compared with spray cooling performed by extract of Reetha added water mixture, better heat removal rate is observed around the critical heat flux region in case of quenching by extract of Reetha added water spray. 4 The extract of Reetha added water spray does not alter the surface morphology and also deposition of the additives on the plate is not observed according to the information revealed by SEM analysis and composition of the washing solution, respectively. 5 The corrosion analysis indicates that the corrosive nature of the natural surfactant is much lower than the corrosiveness of the synthetic surfactants.
[24] [25] [26] [27] [28]
[29] [30] [31] [32] [33] [34] [35] [36]
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Heat Transfer 123 (2001) 719–728. S.M. You, T.W. Simon, A. Bar-Cohen, Y.S. Hong, Effects of dissolved gas content on pool boiling of a highly wetting fluid, J. Heat Transfer 17 (1995) 687–692. K. DCHNS, V. Ravi Kumar, Experimentation on pool boiling with sodium dodecyl sulfate, Int. J. Eng. Res. Dev. 12 (2016) 45–53. M.K. Kumar, P. Ghosh, Coalescence of air bubbles in aqueous solutions of ionic surfactants in presence of inorganic salt, Chem. Eng. Res. Des. 84 (2006) 703–710. G. Suryanarayana, P. Ghosh, Adsorption and coalescence in mixed-surfactant systems: airwater interface, Ind. Eng. Chem. Res. 49 (2010) 1711–1724. N.H. Bhatt, R. Raj, Lily Das, P. Varshney, A.R. Pati, D. Chouhan, A. Kumar, B. Munshi, S.S. Mohapatra, Enhancement of heat transfer rate of high mass flux spray cooling by ethanol-water and ethanol-tween20-water solution at very high initial surface temperature, Int. J. Heat Mass Transf. 110 (2017) 330–347. L. Tmakova, S. Sekretar, S. Schmidt, Plant-derived surfactants as an alternative to synthetic surfactants: surface and antioxidant activities, Chem. Pap. 70 (2016) 188–196. R. Ghagi, S.K. Satpute, B.A. Chopade, A.G. Banpurkar, Study of functional properties of Sapindus mukorossi as a potential bio-surfactant, Indian J. Sci. Technol. 4 (2011) 530–533. A. Kumar, A. Panda, S.S. Mohapatra, Polyethylene-glycol-added water-droplet evaporative cooling for fast cooling operations, J. Thermophys. Heat Transf. 33 (2019) 1–12. M.B. Sarma, S.B. Gogoi, D. Devi, B. Goswami, Degumming of muga silk fabric by biosurfactant, J. Sci. Ind. Res. 71 (2012) 270–272. C. Wei, N. Antolin, O.D. Restrepo, W. Wind, J.-C. Zhao, A general model for thermal and electrical conductivity of binary metallic systems, Acta Mater. 126 (2017) 272–279. D.M. Trujillo, H.R. Busby, INTEMP - Inverse Heat Transfer Analysis – User’s Manual, TRUCOMP CO, Fontain Valley, Canada, 2003, p. 47. D.M. Trujillo, H.R. Busby, Practical Inverse Analysis in Engineering, CRC Press LLC, Boca Raton, Florida, 1997. D.I. Li, M.A. Wells, Effect of subsurface thermocouple installation on the discrepancy of the measured thermal history and predicted surface heat flux during a quench operation, Metall. Mater. Trans. B 36 (2005) 343–354.
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The enhancement of dropwise and spray evaporative cooling by using extract of Reetha added water as a coolant Anita Pandaa, A. Kumarb, Basudeb Munshia, S.S. Mohapatraa, a b
T
⁎
Department of Chemical Engineering, National Institute of Technology Rourkela, 769008, India Department of Chemical Engineering, Indian Institute of Technology Dhanbad, 826004, India
ARTICLE INFO
ABSTRACT
Keywords: Sapindus mukorossi Surfactant Recoiling Bouncing Spreading
Although the use of synthetic surfactant in various quenching process significantly enhances the heat removal rate, the corrosive nature of the synthetic surfactants and their interaction with the metal surface during cooling does not allow to use. The replacement of the synthetic surfactant by another surfactant which is free from the abovementioned disadvantages can mitigate the requirement of various industries dealing with high heat flux applications. In this regard, the literature does not reveal any information. Therefore, in the current work, an attempt has been made to replace the synthetic surfactant by preparing a surfactant which does not depict the disadvantage of the synthetic surfactant. In this work, the surfactant is prepared from the extract of Reetha (Sapindus mukorossi). The measurement of various physical properties of the extract of Reetha and also different characterizations such as FT-IR and EDX analysis confirm the existence of surfactant properties in the extract of Reetha. Furthermore, the corrosion analysis clearly indicates insignificant corrosion in comparison with synthetic surfactants. The post quenching analysis of the heat treated steel plate reveals that the extract of Reetha added water does not depict the deposition characteristics. The thermal performance of the extract of Reetha is determined by conducting dropwise evaporative cooling and spray quenching experiments. The result reveals that the extract of Reetha has an excellent heat removal capacity (CHF = 1.3 MW/m2) in the critical heat flux region.
1. Introduction During evaporation, the dropwise evaporative cooling is mainly controlled by the thermo-physical properties of the coolant [1–4]. At very high initial surface temperature, many researchers have tried to achieve enhanced heat transfer rate by altering the thermo-physical properties in the favorable direction of heat transfer using various additives such as salt [5–8], surfactants [9–14], nanoparticles [15–17], polymers [18–21] and dissolved gases in the coolant [22–24]. Among all the additives reported in the literature, surfactant added water depicts minimum disadvantages and significant augmentation of heat transfer rate. Although other additives have comparatively better heat removal capacity; however, the surface interaction effect of additives during quenching alters the surface morphology and this is not the desired requirement of the various metal industries. By using various commercial surfactants such as SDS [25,26], Tween
20 [27] and CTAB [28], several investigators have reported different degree of enhancement at various conditions. In SDS, the presence of elements or compounds promoting surface corrosion or reaction is the main disadvantage. Additionally, the existence of quaternary ammonium compound in CTAB inhibits bacterial activity and promotes corrosion. Furthermore, although in case of Tween 20, corrosion is not considered as the major disadvantage, the presence of more number of hydrophobic part in the surfactant destabilizes the air which affects foaming characteristics of the surfactant. In addition to the above, during quenching process in metal industries, the use of synthetic chemicals in bulk quantity creates environmental problem after discharge from the industry without pre-treatment. Therefore, the requirement of natural surfactant for high heat flux application put thrust on the further research in the exploration of natural surfactant free from the disadvantage depicted by the synthetic surfactant. In addition to the above, the focus of an eco-friendly environment concerns the use of biodegradable surfactants and in turn
Abbreviations: AHF, Average Heat Flux, MW/m2; CHF, Critical Heat Flux, MW/m2; AISI, American Iron and Steel Institute; CMC, Critical Micelle Concentration, g/ cc; SDS, Sodium Dodecyl Sulfate; CTAB, Cetyl Trimethyl Ammonium Bromide; Tween 20, Polyoxyethylene (20) Sorbitan Monolaurate; SEM, Scanning Electron Microscope; FT-IR, Fourier Transform Infrared Spectroscopy; EDX, Energy Dispersive X- Ray spectroscopy ⁎ Corresponding author at: Spray Boiling Heat Transfer Laboratory, Department of Chemical Engineering, NIT Rourkela, 769008, India. E-mail address: [email protected] (S.S. Mohapatra). https://doi.org/10.1016/j.tca.2019.178379 Received 22 February 2019; Received in revised form 3 August 2019; Accepted 16 August 2019 Available online 17 August 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
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Nomenclature
K T T1 T2 T3 R1 R2 R3 T
Symbols
Ρ μ θ Cp
Surface tension,n, N/ Density of liquid, kg/m3 Viscosity, mPas Contact angle, degree Specific heat of water, J/kg°C
creates interest to develop a natural surfactant against commercial nonbiodegradable surfactant [29,30]. Therefore, in the current research, an eco-friendly and thermal performance enhancing skill surfactant is tried to explore. The analysis confirms that the extract of Reetha can be used as a natural surfactant. For the quantification of enhancement in heat transfer rate and identification of heat transfer mechanism, systematic experiments were conducted by using droplet dispensing system and high speed motion analyzer. The evaporation time was calculated from the analysis of captured images. Finally, spray cooling experiments were conducted by using the extract of Reetha as additives to identify the justification of replacement of synthetic surfactant by the natural surfactant.
Thermal conductivity, W/m°C Surface temperature, °C Thermocouple location 1 Thermocouple location 2 Thermocouple location 3 Region 1 Region 2 region 3 Time, sec
2.2. Experimental set-up and procedure The schematic diagrams of the experimental set-up for dropwise and spray evaporative cooling is depicted in Fig. 1. Dropwise evaporative cooling set-up consists of droplet generation and dispensing system (VITA 19). For the dropwise evaporative cooling investigation, an AISI 304 steel plate of dimension 20 mm × 20 mm × 6 mm was used as the test specimen. The needle was placed at a height of 40 mm from the surface of the plate to avoid splashing of the droplet. Three k-type thermocouples having diameter of 2 mm were embedded in the plate for the measurement of transient time-temperature histories. Before the experimentation, the steel plate was heated up to various temperatures in the range of about 200–600 °C in an electric muffle furnace. After attainment of desired temperature, the hot plate was brought out from the furnace and put on the cooling pad. Then, the droplets from the droplet dispensing system were allowed to impinge on the hot surface of the plate at a rate of 60 drops per minute according to the information reported in the literature [31]. In the current work, for each experiment, images were captured by various imaging systems (DSLR (Nikon D5200), SLR and High speed motion analyzer (Model no. Sony RX 100v)). For the processing of the images, software such as video to jpg converter (v.5.0.92 build 608) was used. In case of spray cooling experimentation, similar type of set-up as described in the literature [28] was used. In this case, full cone nozzle (Leechler: 490.804.1Y.CE) was used to generate the spray.
2. Experimentation 2.1. Preparation of the sample The Reetha fruit pericarp samples were collected from the forest of the eastern part of India. Before experimentation, the samples were dried in an oven at 50℃ for 24 h and then to make powder form, grinding was performed. The powder samples were then added to deionized water for the preparation of the solution. For this, the mixture was continuously stirred and left for 24 h for the collection of undissolved materials contained by the powder. After 24 h, the separation was achieved by using filtration process. The final permeate was used in the experimentation as an additive. In the current work, solutions of 1%, 2%, 4%, 8%, 10%, 20%, 30% and 50% (v/v) concentrations were used.
Fig. 1. Schematic diagram of the experimental set-up. 2
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3. Results and discussion 3.1. Physical properties of the coolant The measured physical properties of the extract of Reetha solution at various concentrations are reported in Table 1. In the current work, temperature (25 ± 2 °C), pressure (1 atm) and water quality (Deionized water) were considered as the constant parameters in case of measurement of various physical properties of the surfactants. In addition to the above, the electrical conductivity and pH of the extract of Reetha solution at different concentrations were also determined by using a digital multi parameter analyzer (Agilent Technologies, 3200 M) fitted with a combined glass electrode. The decrement in surface tension with the increasing extract of Reetha concentration in water confirms the controlling properties defining a surfactant. Furthermore, the variation of contact angle with the concentration of extract of Reetha in water also corroborates the abovementioned statement. When the intensity of the decrement of surface tension in case of extract of Reetha added water is compared with the same parametric variation obtained in case of synthetic surfactant, the depression of surface tension in case of natural surfactant is found to be significant. In addition to the above, at critical micelle concentrations (CMC), the foaming characteristic of the natural surfactant (CMC = 0.017 g/cc) [32] is compared with the synthetic surfactants (Fig. 2). The analysis clearly ascertains the existence of foaming characteristic in the presence of extract of Reetha in water and the visual observation qualitatively ensures that the stability and foam sizes achieved in case of extract of Reetha added water are better than the synthetic surfactant. The discussed quality of the extract of Reetha encourages the current generation researchers to use as an additive in the attainment of high heat applications. From Table 1, it is also observed that the density of the solution rises with the increase in extract of Reetha concentration in water due to the increment of solute amount in the solvent. The insignificant variation in the viscosity favors heat transfer in the positive direction by reducing the early recoiling characteristic. Furthermore, the strong acidic nature of the solution could be due to the hydrolysis of the constituent of saponins. In addition to the above, it is also observed that the electrical conductivity of the extract of Reetha added water solution increases up to certain concentration (8%) and then decreases. This is mainly due to the ionic nature of the surfactant. The mobility and the number of ions present in the solution enhances the electrical conductivity and thereafter, formation of micelles affect the conductivity. This behavior of the solution is suitable for the achievement of higher heat transfer co-efficient. In case of conducting solution, the free electrons or ions are considered as the carrier of heat [33]. As in the current work, extract of Reetha solution is identified as anionic surfactant and therefore, the role of ions is also considered in the thermal transportation.
Fig. 2. Foaming stability of Tween 20, Extract of Reetha added water and SDS solution.
Fig. 3. FT-IR spectra showing different functional groups.
3.2. Composition analysis To identify the various elements and functional groups present in the Reetha, EDX and FT-IR analysis were performed, respectively. The FT-IR analysis clearly asserts that Reetha contains eOH, eCH2, and eCO group. The characteristic peaks obtained in the range of 4000400 cm−1 which is shown in the Fig. 3. The spectral line at a wavelength of 3398 cm−1 corresponds to presence of alcoholic (OeH) group which is confirmed by the presence of another intense peak at 1056 cm−1. The band at 2927 cm−1 is due to eCH2 stretching indicates the aliphatic nature of the Reetha. In addition to the above, methyl groups are also present (1450 and 1383 cm−1 peaks, eCHe). In EDX analysis of Reetha powder, carbon, oxygen and hydrogen are observed as shown in Fig. 4. In the absence of any other peaks in EDX spectra, it is concluded that the Reetha is free from sulphur and any
Table 1 Physical properties of extract of Reetha solution at various concentrations. Extract of Reetha concentration (%)
Density (ρ) (kg/m3)
Viscosity (μ) (mPas)
pH
Electrical conductivity (μs/cm)
Contact angle, θ (degree)
Surface tension (σ) (mN/ m)
Pure water 0.5 1 2 4 8 10 20 30 50
998 1001.4 1001.5 1004.5 1009.2 1017.7 1022.6 1027.4 1042.5 1047.7
0.788 0.807 0.811 0.819 0.825 0.854 0.867 0.89 0.907 0.92
6.7 4.197 4.016 3.842 3.809 3.825 3.879 4.006 3.887 3.987
13.60 195.9 324 641 1174 1972 2.38 2.65 3.55 4.52
74.7 63.4 57.7 49.2 46.3 45.9 31.2 27.7 29.8 33.5
71.5 65.2 56.9 51.3 50.7 48.9 37.8 31.2 32.8 35.6
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Table 2 Residence time for different extract of Reetha concentration at a surface temperature of 200 0C. Temperature = 200 0C Extract of Reetha concentration
Residence time of droplet (sec)
10% 20% 30% 50% 100% Pure water
1.56 1.94 2.2 2.54 2.80 0.34
other corrosive elements. Therefore, from the primary analysis, it can be considered as an environmental friendly and corrosion free surfactant.
Fig. 4. EDX spectrum of Reetha powder.
Fig. 5. Comparison of impact regime of different extract of Reetha concentrations at a surface temperature of 200 0C. 4
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Fig. 6. Comparison of impact regime of different extract of Reetha concentrations at a surface temperature of 400 0C.
3.3. Droplet impact mapping
residence time are observed. However, pure water evaporative cooling excludes the aforesaid phenomena.
In case of dropwise evaporation, the effect of droplet dynamics on dropwise cooling at various substrate surface temperature is investigated. The plate temperature was varied from 200 0C to 600 0C and the extract of Reetha concentration was varied within the range of 0% to 100%.
3.3.2. At plate temperature 400 and 600 0C The evaporative cooling nature in transition boiling regime is identified by conducting experiments at 400 and 600 ℃ initial surface temperatures. In transition boiling regime, the extract of Reetha added water dropwise evaporative cooling performance is much better than that of pure water case. The aforesaid behavior is confirmed from the images presented in droplet impact mapping (Figs. 6 and 7). At surface temperature 400 and 600 ℃, the residence times at different concentrations of Reetha added water solution are presented in Tables 3 and 4, respectively. Furthermore, the similar behavior is also noticed in film boiling regime temperature range recommended in the literature. In case of dropwise cooling performed at 6000C, the residence time depicts a decreasing trend with the increasing extract of Reetha concentration in water (Table 4). The investigation demonstrates that at any Reetha added water solution concentration, the droplet dynamics exhibits spreading, recoiling, bouncing, and rolling characteristics on the hot plate. For this particular case, the absence of the blasting
3.3.1. At plate temperature 200 0C To investigate the role of extract of Reetha concentration in dropwise evaporative cooling in nucleate boiling regime, experiments were conducted at various concentrations of extract of Reetha and at a surface temperature of 200 0C (Fig. 5). From the analysis of the captured images, the residence time was calculated and also presented in Table 2. It clearly asserts that the augmentation of residence time with the increasing concentration of extract of Reetha in water. For the determination of reason behind the enhancement, the droplet impact mapping is analyzed. From Fig. 5, it is seen that at higher extract of Reetha concentration in water, insignificant recoiling, bouncing and blasting characteristics which create suitable environment for higher 5
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Fig. 7. Comparison of impact regime of different extract of Reetha concentrations at a surface temperature of 600 0C. Table 3 Residence time for different extract of Reetha concentration at a surface temperature of 400 0C.
Table 4 Residence time for water at different extract of Reetha concentration at a surface temperature of 600 0C.
Temperature = 4000C
Temperature = 6000C
Extract of Reetha concentration
Residence time of droplet (sec)
Extract of Reetha concentration
Residence time of droplet (sec)
10% 20% 30% 50% 100% Pure water
0.38 0.32 0.28 0.26 0.22 0.30
10% 20% 30% 50% 100% Pure water
0.32 0.30 0.28 0.26 0.22 0.32
6
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by the thermocouple diameter [36]. The locations of the subsurface thermocouples used in the experimentation are provided in Fig. 10. The measured temperatures were recorded by using a Data Acquisition System (NI 9211, National Instrument, Austin, Texas, USA). For the calculation of surface heat flux and temperatures, the appropriate heat conduction model with the justified boundary conditions are initially selected. Then, the plate geometry and the thermal model equations are discretized. Finally, by performing grid independency test, optimum grid sizes are determined (3340 quadratic elements with 4 nodes per each element). The discretized form of the geometry, model equations, measured time-temperature histories, thermo-physical properties of the plate and the boundary conditions are given as the input. Based on the above mentioned input parameters, INTEMP predicts surface heat flux and temperatures by using a non-linear optimization technique. The model equation involved in the calculation (Eq. 1) and the algorithm used for the estimation of surface heat flux have been presented below.
Cp Fig. 8. Boiling diagram.
T = k t
2T
x2
+
2T
y2
(1)
Where, ρ = density of water, kg/m3 Cp = specific heat of water, J/kg°C k = thermal conductivity, W/m°C T = surface temperature, °C To solve the equation, following information are used Initial condition: At t = 0, T1 = T2 = T3 = 900 °C Boundary conditions Except the impinging surface, the remaining surfaces are assumed to be adiabatic. The algorithm for solving inverse heat conduction problems is as follows:
characteristic clearly indicates the absence of accumulation of vapor which promotes strong film or transition boiling. The lesser residence time of the droplet on the hot plate ascertains the prevention of formation of thin liquid layer created by the coalescence of the unevaporated droplet. This phenomenon is possible only when droplet resides on the hot plate for a longer period of time. 4. Extract of Reetha added spray cooling In case of pure water and extract of Reetha added water spray cooling, the boiling behavior is presented in Fig. 8. From the nature of the boiling diagram, it is concluded that for both the type of coolants, the quenching occurs initially in transition boiling regime and then shifts to nucleate boiling regime. The extract of Reetha added water spray cooling depicts enhancement near the critical heat flux region. However, in the early stage of cooling, pure water spray cooling produces higher heat flux than the extract of Reetha added water spray cooling. Due to the aforesaid characteristics of the boiling diagram, the time average values of the heat fluxes for entire boiling regimes are determined. From the obtained average values (AHFwater = 0.35, and AHFExtract of Reetha solution =0.55 MW/m2), it is said that the average heat removal rate for the entire cooling process is higher in case of extract of Reetha added water spray. For the further verification, the boiling behavior of the extract of Reetha added water is examined and the obtained results are presented in Fig. 9. It reveals early onset of nucleate boiling during a heating process and the observed phenomenon is considered as favorable for heat transfer. The surface heat fluxes reported in Fig. 9 are determined by using an inversion heat conduction algorithm (INTEMP) developed by D. Trujillo [34,35]. To measure the transient time–temperature histories, three Ktype of subsurface thermocouples were used and these thermocouples were placed parallel to the quenching surface to avoid the error induced
1) Initially, the software assumes the heat flux as known boundary condition, and predicts all the nodal temperatures by solving a two dimensional transient heat conduction equation. 2) The final prediction of the temperature distributions from the assumed surface heat fluxes is performed by a nonlinear optimization technique which minimizes the error between the measured and predicted temperatures at the measured location. 3) The technique converges to minimum, when error is less than 0.02 0 C. If the error is more than 0.02 0C, INTEMP goes for the estimation of new surface heat flux and repeats until it reaches the target value. 4) The accurate temperature at each nodal point is determined at the converged heat flux value. In case of spray evaporative cooling process, 2–6 ℃/s discrepancy in temperature drop rate is observed between region R1 and region R3 (Fig. 11). As a result, the temperature of one region is comparatively higher than the other region. This could be due to the non-uniformity in the distribution of mass flux. Since the cooling is very fast, the substrate does not find sufficient time to eliminate the abovementioned discrepancy. The discussed behavior continues until the temperature
Fig. 9. Optical images of extract of Reetha added water solution and pure water depicting boiling behavior. 7
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Fig. 10. Locations of the embedded thermocouples. Table 6 Errors associated with different variables used in the current work.
Fig. 11. Schematic diagram representing the temperature variation during cooling process.
Sl. No.
Name of the variable
Maximum error
Minimum error
1 2 3 4 5 6 7
Substrate temperature (0C) Density (kg/m3) Viscosity (mPa. S) Electrical conductivity(μS) pH Surface tension (mN/m) Contact angle (degree)
5 0.0009 0.008 0.15 0.05 0.9 2.3
1.1 0.0002 0.003 0.06 0.030 0.6 1.6
Due to this, the temperature profile in R2 again depicts increasing trend and shows the cross over. The transient time-temperature histories measured by various thermocouples are presented in Fig. 12. 5. Comparative study To identify the advantages associated with the extract of Reetha added spray cooling, various parameters such as residence time, corrosive nature, surface morphology, elemental analysis of the extract of Reetha are compared with the same parameters of the synthetic surfactant and pure water. From the comparison, it is concluded that the residence time of the extract of Reetha added water solution is higher in comparison with other two coolants. The aforesaid observation indicates the existence of minimum recoiling and thermal kick behavior in case of extract of Reetha added water spray cooling. In addition to the above, the surface morphology of the post heat treated steel sample demonstrates that on the surface, insignificant deposition of the natural surfactant is observed. However, in case of synthetic surfactant, the deposition on the surface is identified. Furthermore, the EDX analysis confirms the absence of any corrosive element which are not recommended to use by the steel industries or pollution department. For
Fig. 12. Variation of sub-surface temperature with time.
becomes < 180℃. As the cooling rate is very slow in this temperature range, the heat tries to flow from higher temperature region to lower temperature region. This type of phenomenon is expected in region R2. Table 5 Comparative study of different coolants at various substrate temperatures. Name of the variables
Substrate temperature
Pure water
Extract of Reetha added water solution
Synthetic surfactant
Residence time of the droplet (sec)
Surface morphology
200 400 600 200 400 600 600
0.34 0.30 0.32 45 43 39
1.94 0.32 0.30 95 94.2 93.6
1.83 0.2 0.17 158.4 152.2 151.6
Elemental analysis of the post treated steel plate
600
C = 2.35% O = 17.76% Fe = 24.06% Cr = 17.8% Mn = 5.90% Ni = 8.13%
C = 6.5% O = 18.48% Fe = 41.07% Cr = 18.53% Ni = 14.30% Others = 1.1%
C = 44.37% O = 47.45% Mg = 1.12% Fe = 1.16% Ni = 2.20% Mn = 3.5% S = 0.2%
Corrosion analysis (μm/year)
8
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further verification on the corrosive nature of the Reetha, the corrosion tests were performed. In the corrosion test, corrosion resistances of the sample in the electrolyte (extract of Reetha solution) were measured. The specimens were stabilized for 30 min. at the open-circuit potential (OCP). In the discussed test, the potential was changed from −0.725 mV vs. OCP to 1 V vs. Ag/AgCl at a scan rate of 5 mVs−1. The corrosion test reveals that the natural surfactant depicts significantly lower corrosive nature in comparison with synthetic surfactant. The corrosion rate of natural surfactant is also comparable with water. A comparative study of different coolants at various substrate temperatures is presented in Table 5.
[2] [3] [4]
[5] [6]
6. Measurement uncertainty
[7]
The measurable parameters used in the current work are extract of Reetha added water solution concentration, temperature of the substrate, surface tension, contact angle, viscosity, electrical conductivity, pH and density. The main sources of uncertainty in temperature measurement are noise recorded by the thermocouple and its exact location inside the steel plate. Therefore, in the current work, each experiment was conducted three times to check the reproducibility of the reported results and the average values have been reported and also used for the calculation of surface heat flux. In addition to the above, the nodal and measured temperatures are compared at the measured location. The comparison depicts a very good agreement between the calculated and measured temperatures. The maximum and the minimum error for the different measurements and predictions have been reported in Table 6.
[8] [9] [10] [11] [12] [13] [14] [15] [16]
7. Conclusions
[17]
In the current study, dropwise evaporative cooling experiment was conducted using the extract of Reetha added water as a coolant. The experiments were conducted using various concentrations of extract of Reetha added water solution and at different substrate temperatures. Based on the analysis of droplet impact mapping, spray boiling curve, thermo-physical properties of Reetha and corrosiveness of the extract of Reetha added water, the followings are the conclusions;
[18] [19] [20] [21] [22] [23]
1 The attainment of very low value of surface tension (37.8 mN/m) in case of extract of Reetha added water ensures that it is a surfactant. Furthermore, the lower value of contact angle (31.2°), foaming characteristics and FT-IR analysis support the above claim. 2 At 200℃, the droplet impact mapping result shows that the residence time rises with step up in extract of Reetha concentration. However, at higher initial surface temperature (600℃), with the increasing extract of Reetha concentration, the residence time decreases and the heat transfer rate reaches the maximum value at comparatively lower concentration of extract of Reetha in water due to the decrement of sensible heat extraction period. 3 When the achieved heat removal rate in case of pure water spray cooling is compared with spray cooling performed by extract of Reetha added water mixture, better heat removal rate is observed around the critical heat flux region in case of quenching by extract of Reetha added water spray. 4 The extract of Reetha added water spray does not alter the surface morphology and also deposition of the additives on the plate is not observed according to the information revealed by SEM analysis and composition of the washing solution, respectively. 5 The corrosion analysis indicates that the corrosive nature of the natural surfactant is much lower than the corrosiveness of the synthetic surfactants.
[24] [25] [26] [27] [28]
[29] [30] [31] [32] [33] [34] [35] [36]
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