James R. Welty

Thermal Measurements in Rectangular Microchannels

This paper reports on an experimental study of heat transfer in high aspect ratio (width/depth), rectangular micro-channels. A single channel with width of 10 μm was cut into polycarbonate spacers of various thicknesses, resulting in channel depths of 128 μm, 263 μm and 521 μm. Heat transfer experiments were performed with a constant heat flux boundary condition applied at the surface of the channel. Experiments conducted for refrigerant R-124 working fluid in the range Re = 300 − 900 and Pr = 3.6 − 3.8 showed small or no departure from macro-scale predictions for channels with hydraulic diameters larger than 500 μm. Results for the 80:1 aspect ratio channel showed a significant departure from theoretical predictions. Experimental values of local Nusselt numbers were approximately 25 percent lower than predicted using macro-scale theory.Copyright

Measurement and use of bi-directional reflectance

Bi-directional reflectance is a fundamental surface property that has received a great deal of attention, particularly during the early years of the space program. Although the value of using accurate surface property data has always been recognized, bi-directional reflectance information is rarely used because of the sheer volume of data involved. Now, with powerful workstations readily available, large Monte Carlo codes can be run very quickly and full advantage can be taken of the detailed surface property description provided by bi-directional reflectance data. This paper provides a detailed description of a simple, compact, flexible apparatus for measuring bi-directional reflectance and a review of measurement considerations.

Monte Carlo simulation of radiation heat transfer in a three-dimensional enclosure containing a circular cylinder

This paper presents the distribution of radiation heat flux within an enclosure containing a horizontal circular cylinder. Cases were examined both with and without an absorbing, emitting, and scattering medium present. The bottom surface of the square enclosure was considered a uniform heat flux surface, and the other surfaces were considered perfectly absorbing. A circular cylinder was located at various positions along the vertical centerline of the enclosure; its length was that of the enclosure depth. In the first part of this study (cases 1 and 2), different configurations were used with an optically thin ( tau 1) medium in the enclosure, and in the second, different optical thicknesses were used with a fixed enclosure size. The amount of radiant energy transferred to the cylinder depends on its location, the optical thickness of the participating medium, and the enclosure depth. For the optically thin cases ( tau 1) the Monte Carlo solution calculates geometric view factors.

Comparison of experiment with Monte Carlo simulations on a reflective gap using a detailed surface properties model

The spatial distribution of light through a rectangular gap bounded by highly reflective, diffuse surfaces was measured and compared with the results of Monte Carlo simulations. Incorporating radiant properties for real surfaces into a Monte Carlo code was seen to be a significant problem; a number of techniques for accomplishing this are discussed. Independent results are reported for measured values of the bidirectional reflectance distribution function over incident polar angles from 0 to 90 deg for a semidiffuse surface treatment (Krylon flat white spray paint). The inclusion of this information into a Monte Carlo simulation yielded various levels of agreement with experimental results. The poorest agreement occurred when the incident radiation was at a grazing angle with respect to the surface and the reflectance was nearly specular. 10 refs., 7 figs.

Monte Carlo simulation of radiative heat transfer in arrays of fixed discrete surfaces using cell-to-cell photon transport

An accurate simulation of radiative heat transfer in arrays of fixed discrete surfaces is challenging because of the complicated geometries that can shade and block many surfaces. This paper presents an innovative Monte Carlo scheme using cell-to-cell photon transport developed to simulate monochromatic radiation impinging on an array of fixed discrete elements. Results of the study show that cell-to-cell photon transport is an efficient method of simulating radiation heat transfer in complicated geometries. Sample calculations demonstrate the dependence of radiation heat transfer in the array on the geometry of the array elements.

INCORPORATION OF POLARIZATION EFFECTS IN MONTE CARLO SIMULATIONS OF RADIATIVE HEAT TRANSFER

The electric field vector of individual photons has been incorporated into Monte Carlo simulations of radiative heat transfer to examine the effects of polarisation on the optical properties of arrays of fixed discrete surfaces. Simulations are performed on arrays that have specular surfaces with high and low reflectivity. Two different arrays are illuminated by polarized and unpolarized light and compared with conventional Monte Carlo simulations. The results show that if the initial illumination is either partially or fully polarized, polarization effects are substantial, especially for low-reflectivity surfaces and for arrays that favor a large number of grazing-angte reflections.

Analysis of heat transfer, including axial fluid conduction, for laminar tube flow with arbitrary circumferential wall heat flux variation

Abstract An analysis is presented for the exact solution to the problem of thermal-entry-region heat transfer in a circular tube with an arbitrary circumferential wall heat flux, including the effect of axial fluid conduction. The solution is expressed in a power series form; the first 12 eigenvalues and eigenfunctions were obtained numerically. The least squares method was employed to determine the series expansion coefficients for the non-orthogonal system. A simple result is given for heat flux variation in the form q = qav(1 + cosφ) which illustrates the simultaneous influence of circumferential wall heat flux variation and axial fluid conduction on wall temperature and Nusselt number.

Monte Carlo simulation of radiation heat transfer from an infinite plane to parallel rows of infinitely long tubes : Hottel extended

Abstract A two-dimensional Monte Carlo method has been applied to a classic radiant energy exchange problem that models the interior of an industrial furnace. The configuration involves a source as an infinite radiating plane and the heat sink as parallel rows of infinitely long tubes. Hottel used a graphical technique to solve this furnace model for the two-tube-row configuration. This work extends Hotels results by increasing the number of rows in the original equilateral triangular array and then generalizing the results to isosceles triangular arrangements.

Heat transfer characteristics for tubular arrays in a high-temperature fluidized bed: An experimental study of bed temperature effects

Abstract Experimental results are reported for the heat transfer between a high-temperature fluidized bed and an immersed horizontal tube array. Of specific interest in this study was the effect of bed temperature. Three bed operating temperatures were used: 705, 761, 812. At each temperature, measurements were taken to evaluate both local and average values of the total heat transfer coefficient and the radiation contribution. Mean bed particle sizes examined were 0.97, 1.53, and 2.37 mm. Superficial velocities were varied such that the heat transfer results apply over the range from packed bed conditions through minimum fluidization to well over twice the minimum fluidization velocity. A comparison is made between the heat transfer behavior of a tube with near neighbors, that is, in a bundle, and that for a single tube at the same operating conditions. This comparison illustrates the competing nature of both gas convection and radiation with neighboring tubes present.

Heat transfer to a horizontal tube in the splash zone of a bubbling fluidized bed, an experimental study of particle size effects

Abstract The experimental results of an investigation involving particle size effects on the heat transfer for a horizontal tube located in the splash zone of a high-temperature bubbling fluidized bed are reported. This article is the second of a series [1] that investigates specific operating parameters of bubbling fluidized beds. The array of experimental conditions for this work involved three particle sizes, of nominal 1.1, 2, and 2.9 mm in diameter; four bed temperatures, 700, 810, 908, and 1003 K; and three tube locations, −127, 64 and 406 mm relative to the tube centerline to nonfluidized bed surface. The tube locations are representative of a tube totally immersed in the bed, located in the splash zone, and located in the freeboard, respectively. Convective and blackbody radiative heat transfer coefficient variations are presented as functions of the nondimensionalized velocity ratio and of the particle size for the 1003-K case. Maximum convective and blackbody radiative heat transfer coefficients are tabulated for the other temperatures and particle sizes. The tube outside diameter was 51 mm, and the superficial velocity was varied from near-minimum fluidization conditions ( U mf ) to over 2 U mf .