I do not think an air-propulsion system of the type you hypothesize is actually possible.
There are many insects which have evolved not to use their wings but still have wings - however, these always turn out to be vestigial, much smaller than wings on related species that still use them. It seems unlikely that, given this other type of propulsion, the giant dragonflies would retain large wings similar in proportion and shape to those of dragonflies that use their wings. It also seems highly unlikely that an insect species would evolve an independent method of flight when it already started out with perfectly functional wings that get the job done.
The notion of the dragonflies storing food-blood in its wing-veins presents a problem I've already explained - no other insect, including those with vestigial wings, stores food outside of their digestive system - nor do blood-feeding insects use the eaten blood for oxygen delivery. Insects, including those that feed on animal blood, have their own special blood-like substance called hemolymph which performs these functions. Hemolymph fills the insect's exoskeleton (and those wing-veins), and is kept circulating slowly through the body using a series of tiny pumps.