Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations

Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations

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Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations Cheng Ziming a, Wang Fuqiang a,*, Xie Yinmo a, Ma Lanxin a, Xu Huijin b, Tan Jianyu a, Bai Fengwu c a

School of Automobile Engineering, Harbin Institute of Technology at Weihai, 2, West Wenhua Road, Weihai, 264209, PR China b College of Pipeline and Civil Engineering, China University of Petroleum (Huadong), 66, West Changjiang Street, Qingdao, 266580, PR China c Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineerin(CAS), 6, Beiertiao Road, Beijing, 100190, PR China

article info

abstract

Article history:

Photocatalytic decomposition of sea water is an effective way to solve the future energy

Received 6 July 2017

crisis. However, there is a great deal of controversy about seawater's effect on photo-

Received in revised form

catalytic hydrogen production efficiency. In this paper, the catalyst particles are divided

31 August 2017

into large particles (c > 1) and small particles (c < 1). The Mie theory combined with Monte

Accepted 4 September 2017

Carlo method are used to analyze the influence of sea water on radiative transfer in the sea

Available online xxx

water-based nanofluids. The effects of the optical constants of the base fluid (fresh water and sea water) on the spectral extinction and spectral transmittance of the TiO2 nanofluids

Keywords:

are calculated. Results indicate that, for sea water-based nanofluids with small particles

Sea water

(c < 1), the spectral extinction coefficient increases and spectral transmittance decreases

Complex refractive index

with salt concentration increasing. For sea water-based nanofluids with large particles

TiO2 nanofluids

(c > 1), the spectral extinction coefficient and transmittance oscillate because of diffraction

Extinction coefficient

peaks.

Transmittance

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction In the past centuries, fossil fuels have sustained the energy demands of human beings [1,2]. However, the world has been confronting an energy crisis due to depletion of nonrenewable resources and increased environmental problems [3e6]. On account of this, it is necessary and important to develop renewable and clean energy sources [7,8]. Many

regions and counties have been focused on vigorously searching for renewable and clean energy sources such as wind, solar, hydrogen, and tidal energy [9e16]. Hydrogen is likely to become an increasingly used energy source because it has many advantages [17e20]; for example, it has a high heating value and low emissions, and is environment-friendly, cleaner, and more sustainable [3]. In addition, the need for sustainable energy draws attention to hydrogen fuel [3], which is produced almost exclusively from

* Corresponding author. E-mail address: [email protected] (W. Fuqiang). https://doi.org/10.1016/j.ijhydene.2017.09.044 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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Nomenclature D m n k Q Qext Qabs Qsca fv N I g p P an bn jn ; cn pn tn S1 ; S2 Pn

diameter of the particle, nm complex index of refraction refractive index absorptive index efficiency factor extinction factor absorption factor scattering factor particle volume fraction number density of particles, m3 radiative density asymmetry factor probability density function cumulative distribution function Mie scattering coefficient Mie scattering coefficient RiccatieBessel functions directional dependent function directional dependent function amplitude function Legendre polynomial

Greek symbols c size parameter l Wavelength, nm F scattering phase function Q scattering angle s scattering coefficient, m-1 k absorption coefficient, m-1 b extinction coefficient, m-1 x random number u scattering albedo solid angle, sr U0 f azimuth angle Subscripts ext extinction sca scattering abs absorption f fluid p particle rel relative T monodispersed particles

fossil fuels through the steam reformation of methane [21]. Since the report by Fujishima and Honda on water splitting [22], hydrogen generation from water via solar energy using photocatalysts has been extensively studied [23e26]. Among all materials developed for photocatalytic applications, TiO2 remains the benchmark photocatalyst for hydrogen generation [27,28] because it is environmentally friendly, corrosion resistant, cost-effective, and a suitable band gap semiconductor (3.2 eV) [29]. Now, more and more research [30,31] turns to photocatalytic hydrogen production from seawater because of the lack of fresh water resources and the fact that seawater accounts for 96.5% of all water.

However, the effect of seawater on photocatalysis is controversial, and a consensus on whether it has a positive or negative effect on the hydrogen production rate has not yet been reached. Several studies have been conducted to investigate the effect of seawater on photocatalysis. Peng et al. [32] studied the water-splitting property of the composite photocatalyst CdSePt/TiO2 in visible light. The experimental results showed that the catalytic activity of the photocatalytic hydrolysis of seawater was 33% higher than that of photolysis with pure water under optimal conditions. Sang et al. [33] studied the effects of two kinds of catalysts, La2Ti2O7 and CdS/TiO2, on natural seawater and fresh water. The results showed that the hydrogen generation rate from seawater was about half as much as that from pure water. The main research objective of Simamora et al. [34] was to prepare effective photocatalysts for the splitting of seawater for hydrogen. They used CuO/nano TiO2 to perform photocatalytic hydrogen production experiments on sea water and fresh water. The results were that the 2.5% CuO/nano TiO2 has 9.9 and 7.8 times more activity than nano TiO2 in the photocatalytic splitting of water and seawater, respectively, so the efficiency of seawater-based hydrogen production is still lower than that of fresh water. In addition, nanofluids are the absorbing and scattering medium due to the presence of catalyst particles in the fluid phase. And just because of this, it is valuable for researchers to study the radiation field of the nanofluids [35,36]. The problem of liquid-solid two-phase radiation transmission in photocatalytic reaction vessels was studied by Pasquali et al. [37] using the Monte Carlo method. They analyzed the catalyst particles present in the liquid reaction solution according to the absorption and scattering of the participant. The numerical results indicated that the optical thickness of the reaction solution was thin, and catalyst particles with low absorption albedo could improve the efficiency of the photocatalytic process. They mentioned the effect of optical properties of the base fluids, but they did not study it further. With the same purpose, Said et al. [38] had studied the effects of the size and concentration of the TiO2 nanoparticles on the extinction coefficient using the Rayleigh approach. Their results showed that a smaller particle size (<20 nm) has a nominal effect on the optical properties of nanofluids and the volume fraction is linearly proportional to the extinction coefficient. However, Said et al. [38] ignored the fact that the small particles would coalesce, and they did not research the influence of the largescale particles on the optical properties of nanofluids. Additionally, Rayleigh scattering theory is not suitable for large particles. In order to improve the utilization of light in the biofuel photo bioreactor, Li et al. [39] used a combination of theory and experiment to measure the spectral transmittance of microalgae nanofluids with different species and concentrations. Their experimental results showed that there were significant differences in the UVevisible band, and the differences at long wavelengths were very small. However, they had not taken into account the effects of different media on the light utilization of microalgae nanofluids. The radiation characteristics of six different nanoTiO2 particles were measured by Cabrera et al. [40]. Their results showed that the scattering extinction of nanoparticles plays a major role in radiative transfer, and the effects of particle scattering on

Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3

photocatalysis cannot be neglected. It is worth mentioning that Tan et al. [41] studied the light transmission process with scattering effect of different sizes of particles on the spectral characteristics of TiO2 nanofluids. And that study concluded that the scattering effects need to be considered during investigation of optical properties and radiative transfer of nanofluids. The literature survey indicated that there was the controversy regarding photocatalytic hydrogen production from seawater: some researches showed that seawater benefits photocatalytic hydrogen production [32], meanwhile some researches showed that seawater is negative effect on photocatalytic hydrogen production [33,34]. Many researchers tried to study it from the aspects of the optical properties of the photocatalyst particles [42], the reaction mechanism [32], presence of other components [43] and photocatalyst particle dynamics [44]. But few researches have studied the effects of optical constant variations of base fluid on the optical characteristics of reaction solution and radiative transfer in process of photocatalysis. And the particle size parameters of the catalyst were also not studied separately. In addition, the optical properties of nanofluids were mainly investigated using Rayleigh scattering theory, which is an approximation of Mie scattering theory and unsuitable for large particles. Therefore, this study investigated the optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations. The catalyst particles were divided into large particles (c > 1) and small particles (c < 1) by the size parameters as the standard. Combined with Mie theory, Monte Carlo method was developed to calculate the optical properties and radiative transfer of the nanofluids with TiO2. The effects of different refractive indexes n and different absorption coefficients k of the base solution on the optical properties of the reaction solution were investigated. The optical parameters of the catalyst particles TiO2, base solution, and cuvette were taken into account in the calculation. The effects of the optical constants of the base fluid (fresh water and sea water) on the spectral extinction and spectral transmittance of the TiO2 nanofluids are calculated.

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absorptive indexes, respectively [46,47]. The spectral distribution of the complex refractive index of the anatase TiO2 photocatalyst particle for wavelengths between 300 and 1000 nm is presented logarithmically in Fig. 1 [48]. When a single spherical, isotropic, homogeneous particle with a constant diameter D interacts with an electromagnetic wave with wavelength l, the extinction efficiency factor Qext , scattering efficiency factor Qsca , and the scattering phase function FðQÞ are governed by Maxwell's equations and calculated using Mie theory [46,47]. Qext ðm; cÞ ¼

∞ 2 X ð2n þ 1ÞRefan þ bn g 2 c n¼1

(1)

Qsca ðm; cÞ ¼

∞   2 X 2 ð2n þ 1Þ jan j2 þ jbn j c2 n¼1

(2)

Qabs ðm; cÞ ¼ Qext ðm; cÞ  Qsca ðm; cÞ FðQÞ ¼ 2

i1 þ i2 c2 Qsca

(3)

(4)

where the symbol Re represents the real components, an and bn are the Mie scattering coefficients, and both of them can be calculated by the following formulas an ¼

j0n ðmcÞjn ðcÞ  mjn ðmcÞj0n ðcÞ j0n ðmcÞxn ðcÞ  mjn ðmcÞx0n ðcÞ

(5)

bn ¼

mj0n ðmcÞjn ðcÞ  jn ðmcÞj0n ðcÞ mj0n ðmcÞxn ðcÞ  jn ðmcÞx0n ðcÞ

(6)

where xn ¼ jn  icn , jn and cn are RiccatieBessel functions, and both of them conform to the following recursive relationship: jnþ1 ðcÞ ¼

2n þ 1 jn ðcÞ  jn1 ðcÞ c

(7)

cnþ1 ðcÞ ¼

2n þ 1 cn ðcÞ  cn1 ðcÞ c

(8)

where

Methodology

j1 ðcÞ ¼ cosc j0 ¼ sinc

First, the optical properties of a single nanoparticle, including the absorption efficiency factor, scattering efficiency factor, and extinction efficiency factor, are calculated using Mie theory. When these parameters of a single nanoparticle are obtained, the independent regime is used to calculate the optical properties of the uniform photocatalyst particles in the reaction solution. Finally, the radiative transfer equation (RTE) is numerically solved by using the Monte Carlo method to calculate the spectral transmissivity of the photocatalytic reaction solution [45,46].

c1 ðcÞ ¼ sinc

The non-dimensional polarized intensities i1 and i2 in Eq. (3) are calculated from i1 ðc; m; QÞ ¼ jS1 j2 ; i2 ðc; m; QÞ ¼ jS2 j2 S1 ðQÞ ¼

The size parameter c ¼ pD l and complex index of refraction m ¼ n  ik are the most important parameters for the nanoparticles, which have a constant diameter D, where l represents the wavelength, n and k represent the refractive and

∞ X ð2n þ 1Þ n¼1

S2 ðQÞ ¼

Optical properties of a single nanoparticle

c0 ðcÞ ¼ cosc

½an pn ðcos QÞ þ bn tn ðcos QÞ nðn þ 1Þ

∞ X ð2n þ 1Þ n¼1

½bn pn ðcos QÞ þ an tn ðcos QÞ nðn þ 1Þ

(9)

(10)

(11)

In the above the equations, S1 ðQÞ and S2 ðQÞ represent complex amplitude functions and the direction-dependent function.pn and tn are related to the Legendre polynomials Pn , expressed as

Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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2.0

4.0

1.8

n k

3.6

1.6

Refractive index (n)

1.4 3.4

1.2 1.0

3.2

0.8

3.0

0.6

Absorptive index (k)

3.8

2.8 0.4 2.6

0.2 0.0

2.4 300

400

500

600

700

800

900

1000

Wavelength, nm Fig. 1 e Spectral distributions of the complex refractive index of TiO2 in the wavelength range of 300e1000 nm [48].

pn ðcos QÞ ¼

d ½Pn ðcos QÞ d cos Q

(12)

2

tn ðcos QÞ ¼ cos Qpn ðcos QÞ  sin Q

d ½pn ðcos QÞ d cos Q

(13)

When the nanoparticles are dispersed in the base fluid, the relative size parameter and the normalized refractive index of the particles in the fluid are calculated with [48,49]: pDnf ¼ l

crel

mrel

When the optical properties of a single particle are obtained, the scattering coefficient sp;l , absorption coefficient kp;l , and scattering phase function FT;l ðQÞ for a group of monodispersed particles with independent scattering regimes can be computed with [45]:

kp;l ¼ NT

(14)

(18)

pD2 Qabs;l ; 4

(19)

 fv ¼ NT

np  ikp ¼ nf

pD2 Qsca;l ; 4

sp;l ¼ NT

pD3 6

 (20)

(15)

In Eqs. (14) and (15), the symbol nf represents the refractive index of base fluid, and the symbols np and kp represent the refractive and absorptive indexes of the particle, respectively. g ¼ cos Q ¼

1 4p

Z Fp ðQÞcos QdU

(16)

4p

In Eq. (16), the symbol g represents the asymmetry factor. It is used to describe the directional scattering behavior, and related to the phase function.

Spectral transmissivity of the photocatalytic reaction solution The photocatalytic reaction solution consists of nanoparticles and the base fluid. For the pure base fluid, the spectral absorption coefficient of the base fluid can be calculated with Eq. (17): kf;l ¼

4pkf l

(17)

The symbol kf is the absorptive index of the base fluid.

Fig. 2 e Schematic of radiative transfer for nanofluids in a cuvette.

Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3

bl ¼ sl þ kl

(24)

When the optical properties of the nanofluids are obtained, the calculation model of the radiative transfer of nanofluids in a cuvette can be used as depicted in Fig. 2. Essentially, sea water-based nanofluids is a kind of semitransparent medium, the radiative transfer in sea water-based nanofluids follows the classic radiative transfer equation. The RTE of nanofluids can be written as [46,51]. dIl ðsÞ sl ¼ ðsl þ kl ÞIl ðsÞ þ kl Ibl ðsÞ þ ds 4p

Z

 !  ! ! Il s; U 0 Fl U 0; U dU0

4p

(25)

Fig. 3 e Micrograph of polystyrene standard particles with a radius of 200 nm obtained by SEM.

FT;l ðQÞ ¼

1 pD2 NT Qsca;l ; Fl ðQÞ sl 4

(21)

The symbol Il represents the spectral radiative intensity in ! the direction of U along path s; Ibl represents the spectral blackbody intensity; and U0 is the solid angle. To solve the RTE of nanofluids, the nanofluids are considered to be homogeneous media with specified scattering coefficient sl , absorption coefficient kl , and scattering phase function FTl ðQÞ, and the base fluid is considered to be non-scattering material.

D is the nanoparticle diameter, fv is the particle volume fraction, and NT is the number density of the particles; Qsca;l , Qabs;l and Fl ðQÞ can be obtained by using Eqs. (1e4). The total scattering coefficient sl , absorption coefficientkl , and extinction coefficient bl of the nanofluids are given by Ref. [50]:

Monte Carlo method

sl ¼ sp;l

(22)

1 Ds ¼  lnx1 b

kl ¼ kf ;l þ kp;l

(23)

The symbol x1 represents a uniformly distributed random number in the range of 0 and 1. As long as the

The problem of radiative transfer for nanofluids a cuvette is solved by the Monto Carlo method. When a photon bundle is transported in the reaction solution, it will be absorbed and scattered, and the free path length is calculated with [52,53]. (26)

1.0 0.9

Tested Calculated

0.8

Transmittance

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 300

350

400

450

500

550

600

650

700

750

800

Wavelength, nm Fig. 4 e Comparison of spectral transmittance of the nanofluids between results calculated by the Monte Carlo method and test results from the UVeVis spectrophotometer. Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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Fig. 5 e Nanoparticle cluster distribution in the nanofluids under different operating temperatures after the 30-min photocatalytic water splitting test conducted by the authors [56]. photon has moved, we determine whether the photon is to be scattered or absorbed by generating a random number x2 and compare this number with the single scattering albedo u, which is defined as u ¼ s=b. If x2  u, the photon will be scattered, otherwise the photon will be absorbed. x2

is a uniformly distributed random number in the range of 0 and 1. The scattering phase function FTl ðQÞ of the nanofluids can be obtained from Eq. (21). The scattering phase function satisfies the normalization condition [53]. 4000 3500

n=1.33 n=1.43 n=1.53 n=1.63

1500

Extinction coefficient (m-1)

Extinction coefficient (m-1)

2000

1000

500

n=1.33 n=1.43 n=1.53 n=1.63

3000 2500 2000 1500 1000 500 0

0 300

400

500

600

700

(a)D=20nm( χ

< 1)

300

800

400

Wavelength (nm)

(b)D=50nm( χ

n=1.33 n=1.43 n=1.53 n=1.63

700 600 500 400 300

600

700

800

< 1)

n=1.33 n=1.43 n=1.53 n=1.63

350

Extinction coefficient (m-1)

Extinction coefficient (m -1)

800

500

Wavelength (nm)

300

250

200

150 300

200 300

400

500

600

700

800

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

(c)D=500nm ( χ

>1)

(d)D=800nm ( χ

>1)

Fig. 6 e Spectral extinction of TiO2 nanofluids with different base fluid refractive indexes.

Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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Zp

f ¼ 2px4

1 FTl ðQÞsin QdQ ¼ 1 2

(27)

0

then probability density function pl ðQÞfor scattering angle Q can be obtained (28)

To determine the value of scattering angle Q for a particular Monte Carlo realization, we set a random number x3 equal to the cumulative distribution function of nanofluids Pl ðQÞ[53]. ZQ

pl ðQ0 ÞdQ0 ¼ x3 ;

where, x3 and x4 are uniformly distributed random numbers between 0 and 1.

Model validations

1 pl ðQÞ ¼ FTl ðQÞsin Q 0  Q  p 2

Pl ðQÞ ¼

(30)

0  Pl ðQÞ  1

(29)

0

and solve for scattering angle Q. The azimuth angle f is sampled from a uniform distribution between 0 and 2p:

In this study, an experiment was conducted to validate the Monte Carlo method dealing with the radiative transfer problem of nanofluids in a cuvette. Polystyrene standard particles with a radius of 200 nm were mono-dispersed in water to form the nanofluids. The micrograph of polystyrene standard particles with a radius of 200 nm obtained by a scanning electron microscope (SEM, TESCAN-VEGA-II) is shown in Fig. 3. The spectral complex refractive indexes of polystyrene were the same as those obtained by Ma et al. [54]. The volume fraction (fv) of polystyrene standard particles in the nanofluids was 0.0538%. A cuvette was used to keep

2000 3500

1800

k=0.1 k=0.01 k=0.001 k=0.0001

1400 1200 1000 800 600 400

2500 2000 1500 1000

200 0 300

400

500

600

k=0.1 k=0.01 k=0.001 k=0.0001

3000

Extinction coefficient (m-1)

Extinction coefficient (m-1)

1600

700

500 0

800

300

Wavelength (nm)

(b)D=50nm ( χ

700

800

< 1) k=0.1 k=0.01 k=0.001 k=0.0001

400

Extinction coefficient (m-1)

Extinction coefficient (m-1)

600

600

450

k=0.1 k=0.01 k=0.001 k=0.0001

700

500

Wavelength (nm)

(a)D=20nm (χ<1 ) 800

400

500 400 300 200

350 300 250 200 150 100

100 300

400

500

600

700

Wavelength (nm)

(c)D=500nm( χ

800

50 300

400

500

600

700

800

Wavelength (nm)

>1)

(d)D=800nm ( χ

>1)

Fig. 7 e Spectral extinction of TiO2 nanofluids with different base fluid absorption indexes.

Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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nanofluids, and the optical path length of the cuvette was 1.0 mm. A double-beam UVeVis spectrophotometer (TUe1901, Persee) was used to measure the spectral transmittance of the nanofluids within the cuvette. The spectral transmittance of the nanofluids calculated by the Monte Carlo method is validated with the experimental values and is shown in Fig. 4. As seen from this figure, the solid lines representing the numerical calculated results agree well with the scatter points representing experimental values at all wavelengths.

Results and discussion During the photocatalytic water splitting test, nano-sized photocatalyst particles readily aggregate into large

secondary agglomerates even in the presence of the antiagglomeration agent, especially at high temperatures [55e58]. Fig. 5 presents the nanoparticle cluster distribution and radius variations of the nanoparticles in the nanofluids under different operating temperatures after the 30-min photocatalytic water splitting test conducted by the authors [56]. The size parameter of the photocatalyst nanoparticles in the reaction solution has a large variation. However, the pre-study indicated that the optical properties of nanofluids show a smooth variation with wavelength for small particles (c < 1), while they present oscillating variations with wavelength for large particles (c > 1). Therefore, the optical property analyses of TiO2nanofluids were conducted in two categories: nanofluids with small particles (c < 1) and nanofluids with large particles (c > 1).

1.0

1.0

0.8

n=1.33 n=1.43 n=1.63

0.6

Transmittance

Transmittance

0.8

0.4

n=1.33 n=1.43 n=1.63

0.6

0.4

0.2

0.2

300

400

500

600

700

0.0

800

300

400

Wavelength (nm)

500

600

700

800

Wavelength (nm)

(a)D=20nm ( χ

(b)D=50nm ( χ

< 1)

< 1)

0.88 0.84

n=1.33 n=1.43 n=1.63

0.82

0.86

Transmittance

0.80

Transmittance

n=1.33 n=1.43 n=1.63

0.78 0.76

0.84

0.82

0.74

0.80 0.72

0.78

0.70 300

400

500

600

700

300

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

(c)D=500nm( χ

800

> 1)

(d)D=800nm ( χ

>1)

Fig. 8 e Spectral transmittance of TiO2 nanofluids with different base fluid refractive index.

Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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Effects of refractive index of base fluid on spectral extinction Fig. 6 presents the spectral extinction of TiO2 nanoscale and submicron photocatalyst particle reaction solution with change in the base fluid's refractive index and wavelength. As described in Fig. 6(a) and (b), the extinction coefficient of the nanoscale photocatalyst particles (c < 1) increases with the increase in the refractive index of the base fluid, especially in the range of 300 nme350 nm, and decreases smoothly with the increase in wavelength. In addition, comparing Fig. 6(a) and (b), it is apparent that the extinction coefficient of the nanoscale photocatalyst particles (c < 1) increases with the increase in the diameter of the particles. Fig. 6(c) and (d) show that the extinction coefficient of the submicron-scale photocatalyst particles (c > 1) changes with the increase in the wavelength and the different base fluid refractive indexes; it basically stays the same when the wavelength varies in the

range of 300e340 nm and refraction and long-term volatility appear after a brief calm. In addition, in this wavelength area several curves are almost coincident, so the effect of the base fluid refractive index on the spectral extinction of the reaction solution is weak. The base fluid refractive index affects the spectral extinction coefficient because the wavelength shortens when light transfers from the optical thinner medium to the optical denser medium and is easily attenuated. For the large particles, the diffraction effect need to be considered.

Effects of absorption index of base fluid on spectral extinction When the refractive index of base fluid is constant, four different absorption indexes, k ¼ 0.1, k ¼ 0.01, k ¼ 0.001, and k ¼ 0.0001, are investigated. Fig. 7 shows the absorption

1.0

1.0

0.8

Transmittance

Transmittance

0.8

k=0.1 k=0.01 k=0.001 k=0.0001

0.6

0.4

k=0.1 k=0.01 k=0.001 k=0.0001

0.6

0.4

0.2

0.2

0.0 300

300

400

500

600

700

400

500

600

700

800

Wavelength (nm)

800

Wavelength (nm)

(a)D=20nm ( χ

(b)D=50nm ( χ

< 1)

< 1)

0.90

0.86

k=0.01 k=0.001 k=0.0001

0.84

0.88

0.82

0.86

Transmittance

Transmittance

k=0.01 k=0.001 k=0.0001

0.80 0.78 0.76

0.84

0.82

0.74 0.80

0.72 0.70

0.78

300

400

500

600

700

300

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

(c)D=500nm( χ

800

>1)

(d)D=800nm ( χ

>1)

Fig. 9 e Spectral transmittance of TiO2 nanofluids with different base fluid absorption indexes.

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Effects of refractive index of base fluid on spectral transmittance When the absorption index of the base fluid is maintained atk ¼ 0, base fluids with different refractive indexes,n ¼ 1.33, n ¼ 1.43, andn ¼ 1.63, are investigated (when n ¼ 1.53, the calculation encounters convergence difficulties because of the subtle difference between the refractive indexes of the base fluid and cuvette). Fig. 8 illustrates the spectral transmittance with different refractive indexes of the base fluid and different size parameters of the photocatalyst particles. Fig. 8(a) and (b) show that when the size parameter is less than 1, the spectral transmittance increases with the increase in wavelength; it has a rapid rise, especially in the range of 300e400 nm, but in contrast, it increases with the decrease in the refractive index of the base fluid. As seen in Fig. 8(c) and (d), the spectral transmittance for different refractive indexes of the base fluid does not increase monotonically with the increase in the wavelength when the size parameter is larger than 1. Unlike for a small size parameter, the spectral transmittance of large

Effects of absorption index of base fluid on spectral transmittance With the refractive index of the base fluid is maintained at n ¼ 1.33, base fluids with different absorption indexes are investigated. Fig. 9 illustrates the change in spectral transmittance of nanofluids containing different sizes ofTiO2 particles along with different absorption indexes of the base fluid. Fig. 9(a) and (b)show that when the size parameter is less than 1.0, the spectral transmittance increases with increasing wavelength and also increases with the decrease in the absorption index of the base of the base fluid. As seen in Fig. 9(c) and (d), the spectral transmittance with different absorption indexes of the base fluid does not increase monotonically with the increase in the wavelength when the size parameter is larger than 1. Unlike the nanofluids with small particles, the transmittance of the nanofluids with large particles does not increase with the increase in the absorption of the base fluid.

Effects of complex refractive index of base fluid on spectral extinction According to the above parts of the paper, a conclusion can be drawn that different n or different kaffect the spectral extinction coefficient and spectral transmittance. Thus, pure water and two different salt concentration solutions were studied. The spectral complex refractive index of concentration of 0 g/L, 60 g/L and 360 g/L salt solutions, complex refractive index of the cuvette [59], and photocatalyst particles were taken into consideration, and the complex refraction index of them is shown in Fig. 10.

n-360g/L k-360g/L n-60g/L k-60g/L n-0g/L k-0g/L

1.44 1.42

Refraction index (n)

size parameter photocatalyst particles increases slightly as the refractive index of the base fluid decreases.

1.40

1E-7

1.38 1E-8

1.36 1.34

Absorption index (k)

index's effect on the spectral extinction coefficient of the reaction solution with different particle diameters. As described in Fig. 7(a) and (b), the extinction coefficient decreases sharply to zero at a wavelength of 320 nm when k ¼ 0.01, k ¼ 0.001, andk ¼ 0.0001, and the size parameter of the photocatalyst particle is small (c < 1). When k ¼ 0.1, the extinction coefficient decreases smoothly with the increase in wavelength. Fig. 7(c) and (d) show that when the size parameter is larger than 1 (c > 1), the extinction coefficient remains approximately constant in the range of 300e330 nm and fluctuates when l>330 nm. Overall, the absorption action is enhanced with the increase in the absorption index of the base fluid, so the spectral transmittance is impacted by it.

1.32 1.30 300

400

500

600

700

1E-9 800

Wavelength(nm) Fig. 10 e Spectral distributions of the complex refractive index of different NaCl concentration solutions in the wavelength range of 300e800 nm [59]. Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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1400

800

Extinction coefficient (m-1)

1200 1000

Extinction coefficient (m-1)

c=360g/L c=60g/L c=0g/L

800 600 400 200 0

360g/L 60g/L 0g/L

700 600 500 400 300 200

300

400

500

600

700

800

300

400

Wavelength (nm)

500

600

(a) D=20nm( χ

(b) D=500nm( χ

< 1)

1.0

0.84

360g/L 60g/L 0g/L

0.82

0.92

Transmittance

Transmittance

0.7

Transmittance

c=360g/L c=60g/L c=0g/L

0.8

0.5 0.4

0.91

400

500

600

700

800

0.2 400

0.78 0.76

0.72

Wavelength (nm)

300

0.80

0.74

0.90

0.3

800

>1)

0.9

0.6

700

Wavelength (nm)

500

600

700

800

(c) D=20nm( χ

300

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

< 1)

(d) D=500nm ( χ

>1)

Fig. 11 e Spectral extinction coefficients and spectral transmittances of TiO2 nanofluids with different base fluids.

The effect of the different kinds of base fluid with different complex refractive indexes on the spectral extinction coefficient and spectral transmittance is presented in Fig. 11. Fig. 11(a) and (c) show that although there is a very slight difference in the complex refractive index between the two kinds of base fluid, the effect on the spectral extinction coefficient and spectral transmittance of the nanofluids with small particles is relatively obvious when the wavelength is below 330 nm. In addition, Fig. 11(b) and (d) show the results of the nanofluids with large particle: the spectral transmittance increases with the decrease in salt concentration in the range of 300e350 nm. However, there is no obvious regular pattern when l > 350 nm.

Conclusion In this study, the effects of optical constant variations of base fluid on the optical characteristics of reaction solution and

radiative transfer in process of photocatalysis were investigated. A Monte Carlo method combined with the Mie scattering phase function was developed to calculate the optical properties and radiative transfer of TiO2 nanofluids with the consideration of scattering effects. The particle size parameter was adopted as the criteria to divide the nanofluids into two categories: nanofluids with small particles (c < 1) and nanofluids with large particles (c > 1). The following conclusions have been drawn: (1) The spectral extinction coefficient of nanofluids with small particles decreases and the spectral transmittance of them increases with the increase in the refractive index of the base fluid. And the spectral extinction coefficient increases and spectral transmittance decreases with the increase in the absorption index. (2) The spectral extinction coefficient and spectral transmittance of nanofluids with large particles change not

Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044

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noticeable with the change in the refractive index and absorption index of the base fluid. Besides, due to diffraction peaks, the spectral transmittance and spectral extinction coefficient oscillate with the wavelength. (3) The spectral extinction coefficient increases and the spectral transmittance decreases with increasing salt concentration, especially for nanofluids with small particles in the short wavelength region; thus, the utilization of light increases with increasing salt concentration, and higher salt concentration is beneficial for light absorption in photocatalysis. (4) The change in the spectral extinction coefficient and spectral transmittance of the nanofluids with large particles for increasing salt concentrations is not obvious, so higher salt concentrations do not improve the light absorption in photocatalysis.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51676061, 51406239), and the China Postdoctoral Science Foundation (No. 2016T90283), and the fundamental research funds for the central universities (HIT.NSRIF.201706).

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