The optical constants of a liquid hydrocarbon such as liquid n-octane are basic material properties that may be used to evaluate their thermal radiation transfer capabilities. In this study, the ellipsometry method was used to measure the optical constants of liquid n-octane in the midinfrared wavelength range of 2.0–16.0
The thermal radiation transfer of liquid fuels is key to energy utilization, which plays an important role in the diagnosis and regulation of liquid-fuel ignition, liquid-droplet combustion, and fuel composition measurement and analysis in thermal devices such as automobile internal combustion engines and aerospace engines [
With the development of experimental conditions, numerous experimental approaches have been proposed to investigate the optical properties of liquids, such as the attenuated total reflection (ATR) method [
The importance of the absorption and scattering properties of fuel droplets in the radiative heat transfer process has been widely discussed. Absorption and scattering of thermal radiation can be applied to various problems, including heating evaporation of droplets, autoignition of fuel vapor, attenuation of radiation by droplets and sprays, and optical diagnostics of sprays [
Liquid n-octane, a component of industrial gasoline, is a colorless and transparent combustible organic compound fuel. At present, this liquid fuel is widely used in theoretical and experimental spray combustion research on the combustion chambers of internal combustion engines and aerospace engines [
In this study, the ellipsometry method is used to measure the temperature-dependent optical constants of liquid n-octane in the spectral wavelength range 2.0–16.0
In this study, the ellipsometry method was used to measure the optical constants of liquid n-octane using the IR-VASE ellipsometer (J.A. Woollam, Inc.). The light source of the IR-VASE ellipsometer is based on the Michelson Fourier transform infrared interferometer and the wavelength range spans from 1.5 to 30
Schematic diagrams of ellipsometry measurements based on the “prism-liquid” system.
The sample cell and optical windows were used for containing the liquid to be measured. Before performing the experiment, a protective shell made of PVC was placed on the test bench and filled with inert gas. In the measurements, the polarized light was vertically incident on one side of the ATR prism. The light, reflected by the ATR prism-liquid interface, was then emitted vertically from the other side of the ATR prism. The polarized light entered the detector via the rotary compensator and analyzer to measure changes in intensity and phase. The light from the infrared light source changed its polarization state, which was expressed by the amplitude and phase, when passing through the polarizer and being received by the detector. The ellipsometric parameters, which include the amplitude difference and phase difference are denoted as
The measurement uncertainties are evaluated using the error propagation method provided in Ref. [
The ceramic heater was pasted onto the back side of the liquid cell using high-temperature resistant inorganic adhesive. The liquid was connected to a real-time pressure monitor, and the pressure was maintained at 1 atm by adjusting the manual feed on the liquid injector. The spectral resolution was set to 23.14 cm−1, and the wavenumber range for the experimental measurements was 625 − 5000 cm−1, providing a measured wavelength range of 2.0 – 16.0
In the radiative heat transfer modeling of combustion engines and diagnosis during fuel combustion, liquid fuels are presented in the form of droplets. Considering the complexity of nonspherical particle calculations and similarity of fuel droplets to pure spheres, Mie theory is generally used to calculate the radiative properties of a single spherical droplet, including the absorption, scattering, and extinction efficiency factor [
The radiative properties of a single droplet depend on two independent parameters: the size parameter
Based on Mie theory, the extinction efficiency factor
In the case of a monodisperse spray, considering that fuel droplets in combustion chambers such as diesel engines are usually present with droplet radii of 10 − 100
When a liquid fuel forms a spray using a nozzle, a dispersed droplet system is produced, which is composed of many droplets. In engineering applications, calculations involving polydisperse sprays are usually simplified using a monodisperse approximation, with droplet radii equal to an average radius of droplets in a polydisperse spray [
The IR-VASE ellipsometer was used to measure the ellipsometric parameters
Refractive index
We assume that droplets of liquid n-octane that are formed during spraying are spherical. Absorption and scattering of thermal radiation by droplets can be calculated using Mie theory [
Figure
Absorption and scattering efficiency factors of a single spherical n-octane droplet with
Figure
Absorption and scattering efficiency factors of a single spherical n-octane droplet at 20°C.
The effects of temperature and size parameter on the absorption and scattering efficiency factors of an individual spherical droplet have been evaluated. The thermal radiation transfer properties of the droplet system in an air medium formed by a nozzle, approximated as a monodisperse system, were then, calculated using Mie theory. Figure
Absorption and scattering coefficients of the n-octane droplets-air system at 20°C.
In this study, the optical constants of liquid n-octane, the absorption and scattering efficiency factors of a single spherical droplet, and the absorption and scattering coefficients of droplets-air systems formed by sprays with various droplet radii and droplet volume fractions were investigated at 20, 50, and 80°C in the midinfrared wavelength range of 2.0–16.0
The refractive index of liquid n-octane presents an approximately linear relationship with temperature, while temperature has a much smaller effect on the absorption index. With increasing temperature, a reduction in density leads to a smaller refractive index and weaker absorption intensities of the n-octane, which leads to the decrease of the absorption efficiency factor of a single spherical n-octane droplet. According to the Mie theory, the scattering efficiency factor is closely related to the complex refractive index and size parameter of the droplets. For different size droplets, due to the change of the complex refractive index and size parameter, the scattering efficiency factors show different tendency with the increase of incident wavelength. With increasing droplet radius, the absorption efficiency factor increases within the studied wavelength range except the absorption peaks around 3.5
All data used to support the findings of this study are included within the article.
The authors have no conflicts of interest relevant to this manuscript.
This work was supported by the National Natural Science Foundation of China (NSFC) (51576052).