As shown in Fig. 24, frictional losses are negligible
in the case of L/D = 49 but friction significantly reduces the
performance of the DT of L/D = 103 and 151. From this result, L/
D 50 would seem to be the best aspect ratio for a PDE. Moreover,
the addition of a diverging nozzle would be effective for further
increasing the performance of this engine, because it is known
from Ref. [30] that the extension of the engine with the diverging
nozzle has the effect of increasing the net Isp in spite of wall losses
from heat transfer and friction, which is in contrast to the lengthening
of the straight tube. In the present study, we consider only
the case of the single-pulse detonation tube. For multi-cycle operation,
as would be encountered in PDE applications, we should
consider the influence of the wall temperatures, which will be
much higher than Tw = 298 K due to the long duration of the hot
detonation products. According to Ref. [27], since the hot wall suppresses
water condensation, the pressure-based Isp increases as the
wall temperature approaches the saturation temperature of water
vapor. However, we did not determine how frictional losses change
as the effect of water condensation is reduced, and therefore, further
investigation is needed to determine the optimal wall temperatures
to exploit the performance of the PDE.