Measurement of Time-Averaged Turbulent Free Convection in a Tall Enclosure Using Interferometry

Abstract

Laser interferometry is combined with high-speed digital cinematography to measure time-averaged transient and turbulent convective heat transfer rates. The method is applied to study free convection in a tall vertical air-filled enclosure. Measurements are made at three wall spacings in the turbulent flow regime (5.2×104≤RaW≤2.8×105). An automated image processing algorithm is used to calculate the instantaneous local heat flux from a sequence of interferograms that is captured by a high-speed camera. The local Nusselt number distributions on the hot and cold walls are obtained by time-averaging the fluctuations in local heat flux. The effects of key experimental parameters, such as the camera frame rate and the total image capture time, are investigated. For the current problem, it is shown that a total capture interval of about 10 s is required to accurately measure the time-average local Nusselt number. Within the measurement uncertainty, the average Nusselt number results are in agreement with a widely used empirical correlation from the literature.

Free Convection in Asymmetrically Heated Vertical Channels With Opposing Buoyancy Forces

“…The numerical model was used to conduct a parametric study for the following range of parameters: A = 50; Rx= -0.75, -0.5; 0.1 < Ra(b/L) < lO" (16) The overall channel average Nusselt numbers were determined using the numerical solutions. Both laminar and turbulent CFD on the inability to obtain convergence of a steady laminar model, it seems that instability inside the channel occurs at a much lower modified Rayleigh number than when studying unidirectional buoyancy forces.…”

“…A sequence of interferograms was recorded at 30 fps for 20 s using a high-speed digital movie camera. The timeaveraged heat flux became nearly stationary after 20 s. Each interferogram was analyzed using the same custom MATLAB image processing code developed by Poulad et al [16] in order to obtain the instantaneous fringe gradient. The instantaneous heat fluxes of both the cold and hot walls were determined and then integrated in order to obtain the time-averaged heat fluxes.…”

“…The local Nusselt numbers on each channel wall were determined using the gradient measurement technique as described by Poulad et al [16]. The temperature gradient at the cold surface was calculated using…”

“…(It should be noted that R = 287 J/kg K for air, = 632.8 nm for a helium-neon laser, and G = 0.226 x 10^^ m^/ kg for a helium-neon laser in air.) The fringe gradient normal to the measurement surface, de/dxl^^^, was extracted from the finite fringe interferograms using a custom MATLAB [17] image processing code developed by Poulad et al [16]. A scan of pixel intensity was taken along a horizontal line at the y-location of interest.…”

Passively Enhanced Natural Convection Heat Transfer via Swirl Effect

“…The average of these ten scans was used in (5) to calculate the beam-averaged heat flux. Further details on the image analysis procedure can be found in [11].…”