Me and my wife are planning to arrive at jiuhuang airport mid morning and go straight to Huanglong.

Outside the terminal building of the airport, there are many taxis waiting for hire. Just find one and negotiate the price with the driver. Usually for your trip, it costs CNY700-800 and just pay for it when you finish the whole trip. During your visiting at HL, the driver will wait for you, and it is generally ok to leave your luggage in the car. If you are still worried about your luggage, just store it at the Customer Service Center of the scenic park, and don't forget to retrieve it.