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Heat and Mass Transfer

, Volume 53, Issue 9, pp 2745–2768 | Cite as

Convective heat transfer on a flat surface induced by a vertically-oriented piezoelectric fan in the presence of cross flow

  • Xin-Jun Li
  • Jing-zhou ZhangEmail author
  • Xiao-ming Tan
Original

Abstract

Experimental tests are carried out to investigate the convective heat transfer performances on a flat surface around the vibration envelope of a vertically-oriented piezoelectric fan in the presence of cross flow. Distinct behaviors of convective heat transfer are illustrated under the present conditions of piezoelectric-fan excitation voltage (U = 50, 150, 250 V) or characteristic velocity (u PF = 0.83, 1.67, 2.34 m/s) fan tip-to-heated surface gap (G = 3, 5, 7 mm) and cross flow velocity (u CH = 0.94, 1.56 m/s). In addition, three-dimensional flow field simulations are conducted to illustrate the instantaneous flow fields around the vibrating fan. By comparing with the pure piezoelectric fan, the vortex induced by the vibrating fan is pushed downward by the cross flow and a series of vortices are displayed down the vibrating fan. It is confirmed that the presence of cross flow is contributive to the improvement of convective heat transfer in the rear zone downstream fan vibration envelope. The impingement role of streaming flow induced by piezoelectric fan is reduced by the presence of cross flow in the fan vibration envelope. On the other hand, the oscillating movement of the piezoelectric fan promotes the disturbance intensity of cross flow passing through the fan vibration envelope. These two aspects make the conjugated convective heat transfer in the vicinity of fan vibration envelope complicated. In general, the convective heat transfer in the vicinity of fan vibration envelope is mostly improved by the combined action of fan-excited steaming flow and cross flow in the situation where the piezoelectric fan is placed very close to the heated surface.

Keywords

Convective Heat Transfer Heat Transfer Enhancement Convective Heat Transfer Coefficient Cross Flow Excitation Voltage 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

List of symbols

Ap

Vibration amplitude of fan-tip (m)

App

Peak to peak amplitude of fan-tip (m)

D

Hydraulic diameter (m)

f

Vibrating frequency (Hz)

G

Fan tip-to-surface distance (m)

h

Convective heat transfer coefficient [W/(m2 K)]

H

Inner height of channel (m)

k

Thermal conductivity [W/(m K)]

Lb

Exposed length of fan (m)

Lp

PZT length (m)

Lx

Streamwise distance for average

Ly

Lateral distance for average

Nu

Nusselt number

q

Heat flux (W/m2)

Re

Reynolds number

t

Time (s)

T

Temperature (K)

U

Excitation voltage (V)

UR

Velocity ratio of u CF/u PF

u

Velocity (m/s)

W

Width of fan (m)

x, y, z

x-, y-, z-Directions

Greek letters

ε

Surface emissivity

λ2

Criterion for vortex structure identification

ν

Kinematic viscosity (m2/s)

σ

Stefan–Boltzmann constant

Subscripts

a

Relative to ambient

av

Area-averaged

avx

Laterally-averaged along x direction

c

Relative to working fluid

b

Relative to back surface of wall

CF

Channel flow

PF

Piezoelectric fan

w

Relative to wall

Notes

Acknowledgements

The authors gratefully acknowledge the financial supports for this project from the National Natural Science Foundation of China (Grant No.: 51106073) and the Fundamental Research Funds for the Central Universities (NS201408).

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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  1. 1.College of Energy and Power Engineering, Jiangsu Province Key Laboratory of Aerospace Power SystemNanjing University of Aeronautics and AstronauticsNanjingChina

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