Investigation on thermal radiation property of near- and super-critical water in the pressure range of 20–25 MPa based on line-by-line method
Graphical Abstract
Introduction
The critical point of a fluid is defined as the terminus of vapor-liquid coexistence curve in the phase diagram [1]. Water at state near or above critical point (T = 647 K, P = 22.1 MPa) holds many advantaged physical properties, such as liquid-like density, gas-like viscosity and low dielectric constant, making it an attractive reaction media in organic chemistry [2], [3]. At present, near- and super-critical water (NSCW) has been widely used in hydrothermal synthesis and organic matter treatment and transformation [4], [5]. One of the successful applications is supercritical water gasification (SCWG) technology. As an innovative clean and efficient energy conversion and utilization technology, SCWG is characterized by converting glycerol [6], sludge [7], coal [8], and other organic matters [9] into hydrogen-rich products in supercritical water environment through a series of thermo-chemical reactions. In this process, thermal radiation as an important heat transfer mechanism, significantly affects the interphase heat transfer behaviors and gasification efficiency [10], [11]. Therefore, it is of great significance to investigate thermal radiation property of high-temperature and high-pressure water up to supercritical state.
Probably the most well-known study to date on thermal radiation property of water is Hottel’s emissivity charts, in which total emissivity measured at atmosphere pressure was plotted as a function of temperature T and pressure length PL [12]. However, owing to its own limitations [13], it is unrealistic to evaluate radiation characteristics of supercritical water by Hottel’s charts. The first exploration of thermal (infrared) radiation property of water under supercritical state, up to the author’s knowledge, can be traced back to Gorbaty’s work in 1999 [14]. In which, mid-infrared radiation absorption coefficients of water (T = 773 K, P = (15−100) MPa) were measured by a special high-pressure cell equipped with a pair of sapphire windows. Unfortunately, due to some design flaws in sealing and regulating, the experimental apparatus did not perform well at high temperatures. Later, Tassaing et al. [15] and Jin et al. [16] developed more advanced infrared spectroscopy instruments, and obtained radiative absorption coefficients (in spectral range of 1200 cm−1 to 7800 cm−1) of water at pressure of (2.5–24.5) MPa. However, faced with similar problems, the operating temperature in experimental study is generally limited to less than 673 K, because the mechanical strength of infrared windows decreases rapidly with the increase of temperature [16], [17]. That is why more and more researchers begin to resort to molecular dynamics (MD) simulations [18] or approximate radiative property models [19], [20] (such as the line-by-line (LBL) calculations) to study radiation property of NSCW at high temperatures.
MD simulation is a powerful tool for exploring numerous kinds of physical and chemical properties in molecular system, and can well predict radiation spectrum of various species [21], [22]. In 2005, Yoshida et al. [22] conducted a spectroscopic study based on classical MD simulation, and compared the spectral differences of supercritical water at temperatures of 673 K and 1473 K. Whereas, the main deficiency in MD simulation lies in that the spectrum is yield through Fourier-Laplace transform of dipole autocorrelation function of molecules, so that the resultant absorption coefficient is in arbitrary unit and cannot be applied into industrial calculation directly.
Different from MD simulation, approximate radiative property models establish the relationship between transition line parameters, absorption coefficient, temperature, and pressure based on the principles and formulas of quantum mechanics [23]. According to the accuracy and complexity of computation algorithm, approximate radiative property models can be loosely put into four categories: (1) LBL calculations (the most accurate one), (2) narrow band models, (3) wide band models, and (4) global models [24]. So far, these models are primarily intended for solving radiative transfer in participating medium (e.g. water vapor and carbon dioxide) in moderate-pressure applications, such as boiler and combustion chamber [25], [26]. The particularity of NSCW comes from the appearing of water clusters that is caused by the hydrogen-bonding interaction in high-density water atmosphere [27], [28]. This not only significantly influences the spectral features, but also sophisticates calculation procedure for the reason that radiation spectra of both water monomer and water clusters should be reasonably evaluated [20]. Several years ago, Vigasin reproduced the radiation spectra (in spectral range of 3200 cm−1 to 7600 cm−1) of water at pressure up to 15 MPa through comprehensively considering the contributions of water monomer and dimer in LBL calculation procedure, and the results show good agreement with experimental data [19], [29]. Recently, the authors studied thermal radiation property of water under pressure of 23 MPa based on LBL method, and established a gray model for NSCW for the first time [10]. The gray model enables researchers to analyze the radiation-convection coupled heat transfer in SCWG reactor, but it is only applicable to operating pressure of 23 MPa.
In SCWG engineering, working conditions usually vary between pressure from 20 M to 25 MPa and temperature from 773 K to 1073 K (or higher). While, radiation characteristics of water in above thermodynamic state range have not been fully studied. In order to deeply understand the radiative heat transfer process in SCWG engineering, it is urgent to investigate radiation properties of NSCW covering a wide range of thermodynamic states, and to develop more precise radiation models [25], [30] (such as gray-band model or weighted-sum-of gray-gases model) for this purpose.
In view of above research deficiency, this paper explores thermal radiation property of water in the pressure of 20–25 MPa and temperature of 773–1273 K based on LBL method. The calculation procedure is described in detail in the following Section 2, and the calculation results are validated against experimental data and MD simulation values in Section 4. In Section 4, thermal radiation characteristics of NSCW and the variation of spectral intensity with pressure and temperature are comparatively analyzed. To facilitate the solution of radiative heat transfer in practical engineering, a gray-band radiation model for NSCW is proposed based on the LBL calculation results.
Section snippets
Fundamentals of thermal radiation of water molecules
The emission or absorption of radiant energy is essentially the result of transition of molecular energy levels. In other words, when a molecule transitions from a lower energy state to a higher energy state, it absorbs a photon, conversely, emits a photon, resulting in an individual transition line [31]. At temperature below 3000 K, bound-bound transitions (i.e. vibration transitions and rotational transitions) dominate, and the resultant spectra are found in infrared region (mainly in 1.5
Validation of calculation method
In this section, four groups of spectral absorption coefficients of water at different thermodynamic states are presented. The results obtained from the LBL calculations, experimental data [15], [16] and MD simulations are compared. As depicted in Fig. 5, no matter in the low-pressure conditions or near-critical states, spectral data predicted by LBL calculations show good agreement with the measured values in experimental study, and LBL method provides much more accurate and precise prediction
Results and discussions
Generally speaking, change of temperature or pressure affects thermal radiation spectrum by adjusting molecular number density, integrated intensity and half-width of the transition line, showing as the variation of absorption coefficient magnitude and spectral shape. Fig. 6 presents an intuitionistic contrast between absorption coefficient spectra of water under pressures of 23 MPa and 0.1 MPa. Clearly, supercritical water possesses much higher absorption coefficient than atmospheric water
Conclusions
This paper studied thermal radiation characteristics of near- and super-critical water (NSCW) based on line-by-line calculations, and proposed a gray-band radiation model for water in pressure of (20−25) MPa and temperature of (773–1273) K. Main conclusions reached in this study are summarized as follows:
- (1)
NSCW has five major radiation bands in spectral range of (200– 10,000) cm−1. The order of spectrum intensity of each band from the highest to the lowest is: band-Ⅴ (200 cm−1 to 900 cm−1)
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Key R&D Program of China (2020YFA0714400) and the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No. 51888103).
References (41)
- et al.
A zero-dimensional model of porous char gasification in supercritical water: experiments and mathematical modeling
Chem. Eng. J.
(2022) - et al.
A mathematical model and numerical investigation for glycerol gasification in supercritical water with a tubular reactor
J. Supercrit. Fluids
(2016) - et al.
Supercritical water gasification of glycerol: intermediates and kinetics
J. Supercrit. Fluids
(2013) - et al.
Hydrogen production by sewage sludge gasification in supercritical water with a fluidized bed reactor
Int. J. Hydrog. Energy
(2013) - et al.
Re-creating Hottel’s emissivity charts for water vapor and extending them to 40 bar pressure using HITEMP-2010 data base
Combust. Flame
(2016) - et al.
Infrared spectroscopic study of hydrogen-bonding in water at high temperature and pressure
J. Mol. Liq.
(2002) - et al.
Density evolution of absorption bandshapes in the water vapor OH-stretching fundamental and overtone: evidence for molecular aggregation
J. Mol. Struct.
(2005) Evidence for the contribution of water dimers to the near-IR water vapour self-continuum
J. Quant. Spectrosc. Radiat. Transf.
(2008)- et al.
Calculations of gas radiation heat transfer in a two-dimensional rectangular enclosure using the line-by-line approach and the statistical narrow-band correlated-k model
Int. J. Therm. Sci.
(2012) - et al.
Effects of total pressure on non-grey gas radiation transfer in oxy-fuel combustion using the LBL, SNB, SNBCK, WSGG, and FSCK methods
J. Quant. Spectrosc. Radiat. Transf.
(2016)