Zhixiong Guo

Discrete-ordinates solution of short-pulsed laser transport in two-dimensional turbid media

The discrete-ordinates method is formulated to solve transient radiative transfer with the incorporation of a transient term in the transfer equation in two-dimensional rectangular enclosures containing absorbing, emitting, and anisotropically scattering media subject to diffuse and/or collimated laser irradiation. The governing equations resulting from the discrete-ordinates discretization of the angular directions are further discretized in the spatial and the temporal domains by the finite-volume approach. The current formulation is suitable for solving transient laser transport in turbid media as well as for steady-state radiative transfer in many engineering problems. The method is applied to several example problems and compared with existing steady-state solutions and Monte Carlo transient solutions. Good agreement is found in all cases. Short-pulsed laser interaction and propagation in a turbid medium with high scattering albedo are studied. The imaging of an inhomogeneous zone inside a turbid medium is demonstrated.

Three-Dimensional Discrete Ordinates Method in Transient Radiative Transfer

A complete transient three-dimensional discrete ordinates method is formulated for the e rst time to solve transient radiative transfer in a rectangular enclosure containing nonhomogeneous media that absorb, emit, and scatter. Twofold validation of the transient method is obtained: First, there is an excellent agreement between its results at long time stage with several steady-state solution methods. Second, the transient predictions of transmittance and ree ectance compare very well with Monte Carlo simulations. The sensitivity and accuracy of the transient method against the sizes of time increment and grid cell and angular discrete order are examined. The false radiation propagation and numerical diffusion associated with the differencing schemes are discussed. Calculations show the behavior of the wave nature of propagation of transient radiation. The transient behavior of radiation is found to be ine uenced by many parameters, such as the boundary conditions, the optical thickness of the medium, the scattering albedo, and the incident radiation pulse width. Duhamel’ s superposition theorem is also applied to obtain the transient response to different temporal input pulses.

Monte Carlo simulation and experiments of pulsed radiative transfer

A three-dimensional Monte Carlo simulation of transient radiative transfer is performed for short pulse laser transport in scattering and absorbing media. Experimental results of a 60ps pulse laser transmission in scattering media are presented and compared with simulation. Good agreement between the Monte Carlo simulation and experimental measurement is found. The refractive index of the scattering particles is found to influence strongly the prediction of transmitted pulse shape. The temporal shape of the transmittance is very weakly influenced by the output detector angle for a diffuse medium. Scaled isotropic scattering modeling is shown to be insufficient in transient three-dimensional radiative transfer for early times.

ULTRAFAST RADIATION HEAT TRANSFER IN LASER TISSUE WELDING AND SOLDERING

Laser tissue welding and soldering with use of short laser pulses are proposed. The transient radiation heat transfer in the picosecond time scale is numerically investigated for the first time using the discrete ordinate method for cylindrical geometries. The numerical method developed incorporates the propagation of radiation with the speed of light. The temporal radiation fields of tissue cylinders under the irradiation of short laser pulses are obtained. The use of short laser pulses for tissue welding and soldering is found to have reduced thermal damage to the healthy tissue and improves the uniformity of heating in the tissue closure region in both the depth and radial directions. The addition of absorbing solders in tissue soldering results in a well-confined radiation energy deposition field in the proximity of the solder-stained region and lessens the outgoing radiative heat flux at the laser incident surface. Comparisons of radiation heat transfer are made between the spatially square-variance and Gaussian-variance laser inputs and between the temporally Gaussian-profile and skewed-profile pulses, respectively.

Imaging analysis of digital holography

In this study we focus on understanding the system imaging mechanisms given rise to the unique characteristic of discretization in digital holography. Imaging analysis with respect to the system geometries is investigated and the corresponding requirements for reliable holographic imaging are specified. In addition, the imaging capacity of a digital holographic system is analyzed in terms of space-bandwidth product. The impacts due to the discrete features of the CCD sensor that are characterized by the amount of sensitive pixels and the pixel dimension are quantified. The analysis demonstrates the favorable properties of an in-line system arrangement in both the effective field of view and imaging resolution.

Multidimensional Monte Carlo Simulation of Short-Pulse Laser Transport in Scattering Media

The Monte Carlo technique is used to simulate the two-dimensional transient radiative heat transfer in scattering and absorbing media. The transient behavior of transmissivity and reflectivity, subject to short-pulse laser radiation incident on highly scattering media, is investigated. The influences of medium dimensions, anisotropic scattering characteristics, incident pulse width and spatial and temporal Gaussian distributions, and the effect of Fresnel reflection resulting from refractive index changes at the boundaries are discussed. It is found that the temporal distribution shape and spread of the predicted transmissivity and reflectivity are significantly influenced by the incident pulse width and the dimensions of the media. Forward scattering increases the magnitude of maximum transmissivity and reduces the transmitted pulse width. Neglecting the boundary reflection results in overestimated transmissivity and reflectivity and shortens the transmitted pulse width.

RADIATION ELEMENT METHOD FOR TRANSIENT HYPERBOLIC RADIATIVE TRANSFER IN PLANE-PARALLEL INHOMOGENEOUS MEDIA

In this study the radiation element method is formulated to solve transient radiative transfer with light radiation propagation effect in scattering, absorbing, and emitting media with inhomogeneous property. The accuracy of the method is verified by good agreement between the present calculations and Monte Carlo simulations. The sensitivity of the method against element size, ray emission number, and time increment size is examined. The transient effect of radiation propagation is essential in short-pulse laser radiation transport when the input pulse width is not considerably larger than the system radiation propagation time. The transient characteristics of radiative transfer are investigated in the media subject to collimated laser irradiation and/or diffuse irradiation withtemporal Gaussian and/or square profiles. The inhomogeneous profile of extinction coefficient of the medium affects strongly the transient radiative flux divergence inside the medium.

Multi-time-scale heat transfer modeling of turbid tissues exposed to short-pulsed irradiations

A combined hyperbolic radiation and conduction heat transfer model is developed to simulate multi-time-scale heat transfer in turbid tissues exposed to short-pulsed irradiations. An initial temperature response of a tissue to an ultrashort pulse irradiation is analyzed by the volume-average method in combination with the transient discrete ordinates method for modeling the ultrafast radiation heat transfer. This response is found to reach pseudo steady state within 1 ns for the considered tissues. The single pulse result is then utilized to obtain the temperature response to pulse train irradiation at the microsecond/millisecond time scales. After that, the temperature field is predicted by the hyperbolic heat conduction model which is solved by the MacCormacks scheme with error terms correction. Finally, the hyperbolic conduction is compared with the traditional parabolic heat diffusion model. It is found that the maximum local temperatures are larger in the hyperbolic prediction than the parabolic prediction. In the modeled dermis tissue, a 7% non-dimensional temperature increase is found. After about 10 thermal relaxation times, thermal waves fade away and the predictions between the hyperbolic and parabolic models are consistent.

Analysis of the Nusselt number in pulsating pipe flow

Much attention has been given to the possibility of enhancing the heat transfer rate by superimposing pulsation on a mean flow inside a confined passageway. Industrial applications can be found in Stifling engines and reciprocating cycles, to name a few. A perusal of the relevant literature reveals that pulsating flows in a pipe and the attendant heat transfer have been the subject of several analytical [1,2] and experimental investigations [3, 4]. However, the available published data have been inconclusive and they often show conflicting results. Some investigators reported heat transfer enhancements [4, 5], whereas heat transfer reductions were also noted by some workers [6]. In some instances, both heat transfer augmentation and reduction were detected in a single experiment [3]. Some of these discrepancies can be traced to differences in the parameter spaces that were examined, as well as the nonuniformities in experimental methodologies utilized in various research efforts. However, an analysis of these discrepancies is of vital importance for a more complete application to pulsating heat transfer problems, The present Technical Note addresses this technical issue. One promising avenue to cope with these discrepancies is to reevaluate the Nusselt numbers used in prior published works. It is found that many versions of the Nusselt number were devised to account for the results of the analyses. For a small-amplitude pulsating flow, the Nusselt numbers in various forms are shown to lead to inconsistent results. In addition, if the pulsation amplitude is appreciable, a reverse flow is produced. In this case, difficulties arise in defining the bulk temperature or the time-dependent Nusselt number over a cycle, The present Technical Note aims to examine the consequences of using various forms of the Nusselt number. Special attention is paid to the case of a large-amplitude pulsation flowrate (Af/> 1), which causes reverse flows at the cross section in a pipe. A new definition of the Nusselt number is proposed in the present study. By adopting this definition, the influence of the pulsation amplitude and frequency on heat transfer is scrutinized.

Whispering-gallery mode silica microsensors for cryogenic to room temperature measurement

Optical resonance shifts are measured against a wide range of temperatures from cryogenic to room temperature for silica microspheres operating at whispering-gallery modes. The sensor head microsphere is coupled to a fiber taper and placed in an insulated cell where the air temperature first cools down to below 110 K and then rises steadily and slowly. The transmission resonance spectrum of a distributed feedback laser at 1531 nm exciting the microsphere–taper system is monitored and recorded for every 1 K temperature increment. The resonance wavelength shifts against the temperature changes are analyzed. Several microspheres with size from 85 to 435 µm are tested. No significant dependence of the sensor sensitivity is seen with the sphere size. A cubic dependence of the wavelength shift versus the temperature is least-squares fitted. The measured sensitivity increases from 4.5 pm K−1 to 11 pm K−1 with increasing temperature in the test temperature range, and this behavior is consistent with the temperature dependence of the sum of thermal expansion and thermo-optic coefficients of silica material. The resolution of the sensors with the current instrument could reach 3 mK.