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RESEARCH PAPERS: Experimental Techniques

Optically Based Rapid Heat Transfer Measurements in Complex Internal Flows

[+] Author and Article Information
Charles W. Booten1

Mechanical Engineering Department, Stanford University, Stanford, CA 94305booten@alum.mit.edu

John K. Eaton

Mechanical Engineering Department, Stanford University, Stanford, CA 94305eatonj@standard.edu

1

Presently at Protonex Technology Corporation, Bloomfield, CO 80020.

J. Heat Transfer 129(12), 1655-1665 (Apr 09, 2007) (11 pages) doi:10.1115/1.2767751 History: Received December 21, 2006; Revised April 09, 2007

An optically based technique was developed that involves fabrication of a thin-walled plastic model with laser heating applied to a small section of the outer surface. The heat flux distribution applied to the model by the laser was measured first using a short-duration, transient experiment. The external temperature distribution was then recorded using infrared thermography with steady laser heating. The measured heat flux and temperature distributions were used as thermal boundary conditions in a finite-element code to solve an inverse heat conduction problem for the heat transfer coefficient on the internal passage wall. Hydrodynamically fully developed turbulent flow in a round tube was used as a test case for the development of the new optical method. The Reynolds numbers used were 30,000 and 60,000. This flow was chosen because accurate computational tools were available to calculate the internal heat transfer coefficient for a variety of thermal boundary conditions. In addition, this geometry simplified both the model fabrication and the implementation of a finite-element model for the inverse heat conduction problem. Heat transfer coefficient measurements agreed with numerical simulations and semi-analytical solutions within 1.5% and 8.5% for the low and high Reynolds numbers, respectively. Additional simulations suggest that the method can be accurate with thermal boundary conditions more complex than in these experiments.

Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Infrared thermography experimental apparatus

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Figure 2

Copper calibrator

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Figure 3

Spline interpolation at the center pixel location in the copper calibrator

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Figure 4

Comparison of the interpolated temperature at each pixel to the interpolated temperature that would be calculated if the spline at that location were the same as the spline at the pipe centerline (i.e., normal to the CCD array). The pixels are in the middle of the image horizontally and form a vertical line from the top of the calibrator (pixel 0) to the centerline (pixel 90). The copper calibrator temperature was 44°C.

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Figure 5

Top view of optical path

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Figure 6

Beam power taken at vertical centerline of profile. Each profile was measured at a different laser power.

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Figure 7

Single image of beam profile intensity at 0.12W power applied. Randomly distributed, single, white pixels are dead pixels and do not represent the laser beam. The dark background is the unheated area of the pipe, and the gray, background is the wall behind the pipe. Flow in the pipe is left to right.

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Figure 8

Heat flux loss, through natural convection and radiation (q̇nc″+q̇rad″ in Eq. 4). Scale is W∕m2.

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Figure 9

Net heat flux into the test section (q̇expt″, in Eq. 4) at laser power 0.12W (used for Re=30,000) with 3% threshold applied. The zero heat flux contour is labeled in the plot. Scale is W∕m2.

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Figure 10

Beam power taken at vertical centerline of profile on multiple days

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Figure 11

Measured, filtered temperature distribution on the external surface of the test section for Re=30,000. Flow is from left to right, and scale is K.

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Figure 12

Measured heat transfer coefficient distribution for Re=30,000 on the inside of the pipe. Scale is W∕m2K.

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Figure 13

Temperature distribution calculated on the inside wall of the pipe using COMSOL . Scale is K.

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Figure 14

Temperature contours with heat fluxes shown as arrows as determined in COMSOL . The top is the outside of the pipe, and the flow on the inside is from left to right. Scale is K.

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Figure 15

The high intensity area of the beam where h was averaged

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Figure 16

Comparison of heat transfer coefficients calculated (using COMSOL ) from experimental data and from FLUENT simulations

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Figure 17

Difference in measured h and that calculated in FLUENT as a function of the cutoff beam intensity level used to determine the area over which h was calculated.

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Figure 18

External temperature distribution (left) in °C and internal h distribution (right) in W∕m2K specified to generate experimental data for the test case. The h distribution was created to resemble that inside a turbulated passage.

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