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Flow and heat transfer in the wake of a surface-mounted rib with a slit

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Abstract

An experimental study of flow and heat transfer downstream of a surface-mounted rib with a slit is reported. The open area ratios of the slit rib considered are 10, 20, 30, 40 and 50% with respect to the total projected rib area. Experiments were conducted in a wind tunnel, mostly at a hydraulic diameter based Reynolds number of 32,100. The surface Nusselt number distribution was determined by liquid crystal thermography. Results show that the slit inside the rib enhances heat transfer and reduces pressure penalty, with an optimum performance seen at an open area ratio of 20%. To explain this result, a qualitative picture of the flow field behind the rib was obtained by smoke visualization. Time averages and turbulent statistics of the velocity and temperature fluctuations were measured in detail, using hotwire anemometry and cold wire anemometry respectively. For open area ratios less than 30%, measurements show that the flow through the slit modifies the reattaching shear layer from the top of the rib. The resulting reattachment length is smaller, the peak in Nusselt number is higher, and the average heat transfer from the heated surface is enhanced. For the rib with an open area ratio greater than 40%, the lower portion behaves as an independent small rib with its own reattachment region. Simultaneously, the flow downstream of the upper rectangular part shows characteristics of vortex shedding. Thus, the size of the slit is seen to be an additional parameter that can be used to control heat transfer from the solid surface, in comparison to the solid rib.

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Abbreviations

e :

Rib height, m

f :

Friction factor based on the pressure drop in the duct over the length of the test section

L r :

Reattachment length

Nu :

Span-wise averaged Nusselt number as a function of x

\(\overline{{Nu}} \) :

Plate-averaged Nusselt number

Re :

Reynolds number based on the hydraulic diameter of the duct and average velocity

Re e :

Reynolds number based on the rib height and average velocity

T(x, y):

Fluid temperature distribution, dimensionless

u, v :

Local velocity components in Cartesian coordinates scaled by the duct-averaged velocity

u′′, v’′:

Local velocity fluctuations in Cartesian coordinates scaled by the duct-averaged velocity

\( - \overline{{{u}\ifmmode{'}\else$'$\fi{v}\ifmmode{'}\else$'$\fi}} \) :

Turbulent shear stress non-dimensionalized by the square of the average velocity

U avg :

Duct-averaged velocity, m/s

x, y:

Streamwise and wall normal coordinates respectively (Fig. 1)

Fig. 1
figure 1

Schematic diagram of flow system, coordinate system and instrumentation

Full size image
β :

Open area ratio, %

δ :

Velocity boundary layer thickness, m

δ T :

Thermal boundary layer thickness, m

avg:

Average

o:

Smooth passage

rms:

Root mean square

w:

Wall

∞:

Free stream

+:

Wall coordinate

′:

Fluctuating quantity

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Authors and Affiliations

  1. Department of Mechanical Engineering, Indian Institute of Technology Kanpur, 208016, Kanpur, India

    Andallib Tariq, P. K. Panigrahi & K. Muralidhar

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  1. Andallib Tariq
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  2. P. K. Panigrahi
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  3. K. Muralidhar
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Correspondence to P. K. Panigrahi.

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Tariq, A., Panigrahi, P.K. & Muralidhar, K. Flow and heat transfer in the wake of a surface-mounted rib with a slit. Exp Fluids 37, 701–719 (2004). https://doi.org/10.1007/s00348-004-0861-8

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  • DOI: https://doi.org/10.1007/s00348-004-0861-8

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