Python for 循环不迭代

Question

迭代只运行一次，下面是我试图运行的代码，

def heat_coeff(gas,Nu,d):

    """Calculate heat transfer (at interior tube wall)"""

    mass = gas *100

    diff = mass * 2

    for i in d:

        U = 4.*diff*Nu / (i*i)

        return U

heat_coeff(10,4, [1,2,3])