John C. Chai

Finite volume method for radiation heat transfer

A finite volume method (FVM) is presented in this article. This procedure can be used to model transparent, absorbing, emitting, and anisotropically scattering media. A procedure to capture collimated beam is also presented. The FVM is applied to six test problems, and the results compared favorably against other published results. The test problems include twoand three-dimensional enclosures with participating media, collimated incidence, and heat generation. The efficiency of the FVM procedure is also investigated using a three-dimensional test problem.

Ray effect and false scattering in the discrete ordinates method

A discussion on the ray effect and false scattering occurring in discrete ordinates solution of the radiative transfer equation is presented in this article. Ray effect arises from the approximation of a continuously varying angular nature of radiation by a specified set of discrete angular directions. It is independent of the spatial discretization practice. False scattering, on the other hand, is a consequence of the spatial discretization practice and is independent of the angular discretization practice. In multidimensional computations, when a beam is not aligned with the grid line, false scattering smears the radiative intensity field. It reduces the appearance of unwanted bumps, but does not eliminate ray effect. An inappropriate view of false scattering is also presented. Four sample problems are used to explain these two effects.

Numerical computation of fluid flow and heat transfer in microchannels

Three-dimensional fluid flow and heat transfer phenomena inside heated microchannels is investigated. The steady, laminar flow and heat transfer equations are solved using a finite-volume method. The numerical procedure is validated by comparing the predicted local thermal resistances with available experimental data. The friction factor is also predicted in this study. It was found that the heat input lowers the frictional losses, particularly at lower Reynolds numbers. At lower Reynolds numbers the temperature of the water increases, leading to a decrease in the viscosity and hence smaller frictional losses.

Treatment of irregular geometries using a Cartesian coordinates finite-volume radiation heat transfer procedure

This article presents a blocked-off-region procedure to model radiative transfer in irregular geometries using a Cartesian coordinates finite-volume method (FVM). Straight-edged, inclined and curved boundaries can be treated. It is capable of handling participating or transparent media enclosed by black or reflecting walls. With this procedure, irregular geometries can be specified through the problem specification portion of the program. Four test problems are used to show that the procedure is capable of reproducing available results for inclined and curved walls, transparent, nonscattering, and anisotropically scattering media.

Evaluation of Spatial Differencing Practices for the Discrete-Ordinates Method

Three popular spatial differencing practices for the discrete ordinates method are examined in detail for a basic two-dimensional Cartesian coordinates problem. These differencing schemes are 1) positive, 2) step, and 3) diamond schemes. The diamond scheme is shown to produce negative intensities under certain conditions irrespective of the number of control volumes employed, requiring some form of negative intensity fix-up. In absorbing-emitting or absorbing-emitting-scattering media, grid refinement can result in negative intensities when the diamond scheme is used. The diamond scheme and a positive scheme, which sets the negative intensities encountered in the diamond scheme to zero or very small number for purely absorbing media, can also produce physically unrealistic overshoots. The step scheme, although not considered as accurate as the diamond scheme, gives physically realistic results for the basic problem considered. Further evaluation of Fivelands positive conditions, and variable weight and exponential-type schemes indicate a need for alternate spatial differencing schemes that describe the physics of radiative heat transfer more accurately.

ONE-DIMENSIONAL TRANSIENT RADIATION HEAT TRANSFER MODELING USING A FINITE-VOLUME METHOD

This article presents a finite-volume method to calculate transient radiative transfer in a one-dimensional slab. The fully implicit scheme is used to discretize the transient term. The step and CLAM spatial differencing schemes are used in this study. The procedure is validated with available published results. The capabilities of the procedure are then examined using three additional test problems. In these test problems, one of the walls is subjected to a single-pulse collimated beam and a repeated-pulse collimated beam. The effects of the two spatial differencing schemes are discussed.

Improved Treatment of Scattering Using the Discrete Ordinates Method

A finite volume method (FVM) is presented in this article. This procedure can be used to model transparent, absorbing, emitting, and anisotropically scattering media. A procedure to capture collimated beam is also presented. The FVM is applied to six test problems, and the results compared favorably against other published results. The test problems include two- and three-dimensional enclosures with participating media, collimated incidence, and heat generation. The efficiency of the FVM procedure is also investigated using a three-dimensional test problem.

Thermally mediated droplet formation in microchannels

Precise dispensing of microdroplets is an important process for droplet-based microfluidics. The dropletformation by shear force between two immiscible fluids depends on their flow rates, the viscosities, and the interfacial tension. In this letter, the authors report the use of integrated microheater and temperature sensor for controlling the dropletformation process. The technique exploits the dependency on temperature of viscosities and interfacial tension. Using a relatively low heating temperature ranging from 25 to 70 ° C , the droplet diameter can be adjusted to over two times of its original value. The relatively low temperature range makes sure that this concept is applicable for droplets containing biological samples.

Thermally mediated breakup of drops in microchannels

The authors used thermally induced surface tension gradients to manipulate aqueous droplets in microchannels. Control of the droplet breakup process was demonstrated. Droplet sorting can be achieved with temperatures above a critical value. Numerical simulation using a two-dimensional model agrees qualitatively well with the experimental results. The used control temperature of less than 55°C shows that this active control concept is suitable for biochemical applications. Thermal control promises to be a simple and effective manipulation method for droplet-based lab on a chip.

Monte Carlo Solutions for Radiative Heat Transfer in Irregular Two-Dimensional Geometries

Several new methods of computing radiation heat transfer have emerged in recent Years, and thorough testing is often required to validate these. Monte Carlo methods can provide exact solutions within statistical limits, hence are attractive for validation purposes. Unfortunately, many Monte Carlo solutions currently available in the literature are not useful for testing new solution techniques for complex geometries, because they either treat simple cases or application-specific irregular geometries. The intent of this paper is to provide solutions of general interest for two-dimensional irregular geometries using the Monte Carlo method. The paper presents radiative heat flux solutions for three enclosures with absorbing, emitting, and anisotropically scattering medium. 8 refs., 6 figs.