Fan structure having a first impeller and a second impeller

A fan structure comprising:

• a first impeller having a first annular substrate, a plurality of first ribs, and a plurality of first blades,
• wherein the first ribs and the first blades are formed on the first annular substrate, each of the first ribs comprises an engaging hole, and a first accommodating space is formed between each of the adjacent first blades,
• wherein the first annular substrate comprises a first outer lateral surface, a first inner lateral surface corresponded to the first outer lateral surface, a first bottom surface and a first top surface corresponded to the first bottom surface,
• wherein the first bottom surface is in communication with the first outer lateral surface and the first inner lateral surface, the first ribs are formed on the first inner lateral surface, and the first blades are formed on the first bottom surface;
• and
• a second impeller having a hub, a second annular substrate, a plurality of second ribs, a plurality of second blades and at least one engaging member formed on the second ribs,
• wherein the second annular substrate comprises a second outer lateral surface, a second inner lateral surface corresponded to the second outer lateral surface, a second top surface and a second bottom surface corresponded to the second top surface,
• wherein the second top surface is in communication with the second outer lateral surface and the second inner lateral surface, the second ribs are formed on the second inner lateral surface, the second blades are formed on the second top surface, and the first bottom surface of the first annular substrate is faced toward the second top surface of the second annular substrate,
• wherein the second ribs are located between the second annular substrate and the hub, the second blades are formed on the second annular substrate, a second accommodating space is formed between each of the adjacent second blades, each of the first blades is located in the second accommodating space separately, each of the second blades is located in the first accommodating space separately, and the engaging member is inserted into the engaging hole.