1. Introduction
The addition of an exit diffuser to a horizontal-axis wind
turbine is one of the few ways in which power output may be
increased in cost-effective manner, as recently noted by
Al-Sulaiman and Yilbas [1]. There is an extensive literature on
diffuser-augmented wind turbine (DAWT) performance. Rio Vaz
et al. [2] developed an innovative approach for the performance
analysis of DAWTs based on blade element theory (BET), in which
a more general semi-empirical one-dimensional analysis was carried
out. Glauert’s correction for the induction at high thrust was
also employed. Their results yielded good agreement with experimental
data. Most recently, Kosasih and Hudin [3] investigated the
impact of turbulence intensity on micro wind turbine efficiency in
converting the wind energy into power. The performance of bare
micro wind turbine (MWT) and diffuser-augmented micro wind
turbine (DAMWT) models subject to different levels of turbulence
was reported. It was shown that shrouding the turbine with diffuser
increases the power coefficient CP by a factor of almost
two. Beyond a certain tip-speed ratio, the performance of both
MWT and DAMWT was shown to decrease with turbulence
intensity, however the CP of the DAMWT was still greater than that
of the bare MWT wind indicating the diffuser augmentation was
still achievable even at high freestream turbulence.
Jafari and Kosasih [4] performed a Computational Fluid
Dynamics (CFD) study, where the augmentation is strongly
dependent on the geometry of the diffuser, such as length and
expansion angle. Also, they reported that a higher area ratio creates
greater pressure reduction at the diffuser exit, which increases the
mass flow rate, agreeing with Hansen et al. [5]. They employed a
one-dimensional CFD analysis to evaluate a DAWT composed of a
NACA 0015 airfoil on an ideal turbine, concluding that the power
coefficient for a shrouded turbine is proportional to the mass flow,
and the increasing flow through the rotor induced by the diffuser
increases the extracted power for the same thrust coefficient
compared to a bare wind turbine.
Wang et al. [6] measured the influence of a flanged diffuser on a
shrouded wind turbine. Their experimental results revealed that
the rotational speed and the dynamic strain of the blade are much
higher than those without a flanged diffuser. Abe and Ohya [7]
combined a numerical and experimental investigation of flanged
DAWTs. They suggested that the loading coefficient for the best
performance of a flanged diffuser is considerably smaller than for
a bare wind turbine. In addition, it was necessary to avoid
boundary-layer separation and maintain a high pressure-recovery