Small airways have laminar air flow because

The probability of turbulence is determined by Reynold's number. When it is more than 3000, the flow is almost always turbulent. When it is less than 2000, the flow is usually non-turbulent (laminar). Reynold's number is directly propoional to Density of fluid Diameter of vessel Velocity of flow Ref: guyton and hall textbook of medical physiology 12 edition page number:321,321,322