Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
0.5
0.4
δNu =0.18(Re p /Re) 0.7
(R 2 = 0.696)
0.5
δNu =0.18(Re p /Re) 0.7
(R 2 = 0.696)
0.2
δNu
0.3
0.2
δNu
0.1
0.02
0.1
0
0
0.05
0 0.5 1 1.5 2 2.5 3
Re p /Re
Fig. 7. Dimensionless heat transfer enhancement for pulsating flow through
the heat sink as a function of the ratio of pulsating to steady flow
component Re p /Re. Markers indicate the entire set of measurements for 35 <
Re p < 225. Solid line indicates correlation in Eq. (4).
The enhancement factors collapse satisfactorily when
plotted versus Re p /Re. Besides the ratio Re p /Re, other
dimensionless quantities and combinations have been
explored to better collapse the results, e.g. dimensionless
stroke length or Womersley number Wo. However none
seemed to outperform the ratio Re p /Re.
Figure 7 shows the following power law correlation fitted
to the entire set of experimental results for pulsating flow:
0.7
⎛Re p ⎞
δ Nu = 0.18⎜ ⎟ (R 2 = 0.696) (4)
⎝ Re ⎠
The above expression provides an adequate fit which at least
shows the existence of a trend, however does not seem
accurate enough to use as a prediction.
Moreover, when plotting the same results on a nonlinear
scale as in Fig. 8, a significantly different behaviour
becomes apparent for a low and high ratio of pulsating to
steady flow. Within the investigated range (0.09 < Re p /Re <
4.4) and for increasing pulsating flow magnitude, the
enhancement is first slightly negative albeit only a few
percent. In this regime, pulsating flow seems to decrease
cooling performance in this heat sink. Beyond Re p /Re > 0.2,
the enhancement is positive and increases with Re p /Re. Only
in this region does the correlation in Eq. (4) predict the
experimental results with an acceptable degree of accuracy.
The negligible enhancement or even slight deterioration of
heat transfer for small pulsation amplitudes (Re p /Re < 0.2)
reminds of findings by other authors [8,9], although the
present case with an actual heat sink geometry (including
conjugate heat transfer, hydraulically and thermally
developing laminar flow) is difficult to compare to the cases
presented in the literature.
0.02
0.05
0 0.05 0.2 0.5 1 2 3
Re p /Re
Fig. 8. Identical to Fig. 7, yet with nonlinear plot scaling to reveal different
behaviour at low and high pulsation magnitude Re p /Re.
IV. CONCLUSIONS
This paper has shown the potential for overall heat transfer
rate enhancement using pulsating flow in single-phase liquid
flow heat sink, for use in microelectronics cooling.
This initial study uses a single rectangular channel serving
as a reference case for subsequent studies of pulsating flow
in parallel microchannels.
Further improvements to the pulsating flow generator
should significantly narrow the uncertainty margins on the
parameters e.g. stroke length and pulsating Reynolds number
Re p . Nevertheless by limiting the actuation frequency in this
study, the results are deemed sufficiently reliable.
For the investigated range (50 < Re < 400, 35 < Re p < 225,
2 < Wo < 17), an enhancement factor of up to 40% is
observed with respect to steady flow heat transfer at the
same steady flow component.
The data show a consistent trend for heat transfer
enhancement as a function of the ratio of the pulsating to
steady flow component Re p /Re. For small pulsation
amplitude (Re p /Re < 0.2) a negligible yet slightly negative
enhancement is observed. For larger pulsation amplitudes
(Re p /Re > 0.2), the heat transfer enhancement increases with
Re p /Re and can be predicted (yet with limited accuracy) by
the power law expression in Eq. (4).
ACKNOWLEDGMENTS
This work is sponsored by the Institute for the promotion
of Innovation by Science and Technology in Flanders (IWT),
project SBO 60830 “HyperCool-IT”, as well as travel
funding by Research Foundation Flanders (FWO). The
authors explicitly thank Dr. Suresh V. Garimella, Benjamin
J. Jones and Tannaz Harirchian from Purdue University,
West Lafayette, In. for helpful discussions on this topic.
©EDA Publishing/THERMINIC 2009 166
ISBN: 978-2-35500-010-2