An experimental investigation of natural convection between horizontal, heated, parallel plates in air was carried out by visualizing the flow and measuring the air temperature. Grashof numbers, based on the plate spacing, varied in the $1.22×105-1.06×106$ range. Flow patterns and probable onset of secondary motions were observed for three heating modes: (1) both plates heated, (2) upper plate heated and lower one unheated, and (3) upper plate unheated and lower one heated. The main flow pattern resembled a C shape (C loop) for all modes. In fact, the flow penetrated inside the cavity close to the leading edge of the lower plate and exited from the upper part, by reversing its motion between the plates. When the lower plate was heated, flow visualization showed that secondary flows were added to the C loop main flow. Such secondary structures arose as thermals, then changed into longitudinal vortices and, in the upper region of the open-ended cavity, a chaotic motion was detected. The existence of these structures was confirmed by measurements of instantaneous temperature values. They showed that the greater the Grashof number the more chaotic the flow in the outflow branch of the C loop when the lower plate was heated and the upper one was unheated. [S0022-1481(00)01801-6]

1.
Goldstein
,
R. J.
, and
Volino
,
R. J.
,
1995
, “
Onset and Development of Natural Convection Above a Suddenly Heated Horizontal Surface
,”
ASME J. Heat Transf.
,
117
, pp.
808
821
.
2.
Mahajan
,
R. L.
,
1996
, “
Transport Phenomena in Chemical Vapor Deposition Systems
,”
,
28
, pp.
339
425
.
3.
Sparrow
,
E. M.
, and
Carlson
,
C. K.
,
1986
, “
Local and Average Natural Convection Nusselt Numbers for a Uniformily Heated, Shrouded or Unshrouded Horizontal Plate
,”
Int. J. Heat Mass Transf.
,
29
, pp.
369
379
.
4.
Fukui
,
K.
,
Nakajima
,
M.
, and
Ueda
,
H.
,
1983
, “
The Longitudinal Vortex Effects on the Transport Process in Combined Free and Forced Laminar Convection Between Horizontal and Inclined Parallel Plates
,”
Int. J. Heat Mass Transf.
,
26
, pp.
109
119
.
5.
Maughan
,
J. R.
, and
Incropera
,
F. P.
,
1987
, “
Experiments on Mixed Convection Heat Transfer for Airflow in a Horizontal and Inclined Channel
,”
Int. J. Heat Mass Transf.
,
30
, pp.
1307
1318
.
6.
Maughan
,
J. R.
, and
Incropera
,
F. P.
,
1990
, “
Regions of Heat Transfer Enhancement for Laminar Mixed Convection in a Parallel Plate Channel
,”
Int. J. Heat Mass Transf.
,
33
, pp.
555
570
.
7.
Evans
,
G.
, and
Greif
,
R.
,
1989
, “
A Study of Traveling Wave Instabilities in a Horizontal Channel Flow with Applications to Chemical Vapor Deposition
,”
Int. J. Heat Mass Transf.
,
32
, pp.
895
911
.
8.
Evans
,
G.
, and
Greif
,
R.
,
1993
, “
Thermally Unstable Convection With Applications to Chemical Vapor Deposition Channel Reactors
,”
Int. J. Heat Mass Transf.
,
36
, pp.
2769
2781
.
9.
Cheng
,
K. C.
, and
Shi
,
L.
,
1994
, “
Visualization of Convective Instability Phenomena in the Entrance Region of a Horizontal Rectangular Channel Heated From Below and/or Cooled From Above
,”
Exp. Heat Transfer
,
7
, pp.
235
248
.
10.
Lin
,
W. L.
, and
Lin
,
T. F.
,
1996
, “
Experimental Study of Unstable Mixed Convection of Air in a Bottom Heated Horizontal Rectangular Duct
,”
Int. J. Heat Mass Transf.
,
39
, pp.
1649
1663
.
11.
Lin
,
W. L.
, and
Lin
,
T. F.
,
1996
, “
Unstable Aiding and Opposing Mixed Convection of Air in a Bottom-Heated Rectangular Duct Slightly Inclined from the Channel
,”
ASME J. Heat Transf.
,
118
, pp.
47
55
.
12.
Chang
,
M. Y.
,
Yu
,
C. H.
, and
Lin
,
T. F.
,
1997
, “
Changes of Longitudinal Vortex Roll Structure in a Mixed Convective Air Flow Through a Horizontal Plane Channel: An Experimental Study
,”
Int. J. Heat Mass Transf.
,
40
, pp.
347
363
.
13.
Vafai
,
K.
, and
Ettefagh
,
J.
,
1990
, “
Thermal and Fluid Flow Instabilities in Buoyancy-Driven Flows in Open-Ended Cavities
,”
Int. J. Heat Mass Transf.
,
33
, pp.
2329
2344
.
14.
Vafai
,
K.
, and
Ettefagh
,
J.
,
1990
, “
The Effects of Sharp Corners on Buoyancy-Driven Flows With Particular Emphasis on Outer Boundaries
,”
Int. J. Heat Mass Transf.
,
33
, pp.
2311
2328
.
15.
Vafai
,
K.
,
Desai
,
C. P.
,
Iyer
,
S. V.
, and
Dyko
,
M. P.
,
1997
, “
Buoyancy Induced Convection in a Narrow Open-Ended Annulus
,”
ASME J. Heat Transf.
,
119
, pp.
483
494
.
16.
Hart
,
J. E.
,
1971
, “
Transition of a Wavy Vortex Regime in Convective Flow Between Inclined Plates
,”
J. Fluid Mech.
,
48
, Part 2, pp.
265
271
.
17.
Manca, O., Morrone, B., and Nardini, S., 1997 “Visualization of Natural Convection in Inclined Parallel Plates,” Thermal Managment of Electronic System, Vol. II, Kluwer Academic, Dordrecht, pp. 283–292.
18.
Manca, O., Morrone, B., and Nardini, S., 1997, “Flow Visualization of Natural Convection between Horizontal Heated Parallel Plates,” Proceedings of the 4th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Brussels, 2–6 June, Vol. 4, pp. 2251–2258.
19.
Chyu
,
M. C.
,
1987
, “
On the Boundary Condition and Data Reduction of Heat Transfer Experiment
,”
Int. Commun. Heat Mass Transfer
,
14
, pp.
543
550
.
20.
Labview Reference Manual, 1993, National Instruments, Austin.
21.
DANTEC, 1996, “Probes for Hot-Wire Anemometry,” Dantec Meas. Tech. Publ. No. 196-105-01.
22.
Bruun, H. H., 1995, “Hot-Wire Anemometry: Principles and Signal Analysis,” Oxford University Press, New York.
23.
Kline
,
S. J.
, and
McClintock
,
F. A.
,
1953
, “
Describing Uncertainty in Single Sample Experiments
,”
Mech. Eng.
,
75
, pp.
3
12
.
24.
Moffat
,
R. J.
,
1988
, “
Describing the Uncertainties in Experimental Results
,”
Exp. Therm. Fluid Sci.
,
1
, pp.
3
17
.
25.
Lavine
,
A.
,
1993
, “
On the Linear Stability of Mixed and Free Convection Between Inclined Parallel Plates With Fixed Heat Flux Boundary Conditions
,”
Int. J. Heat Mass Transf.
,
36
, pp.
1373
1387
.