what is the default kernel_initializer in keras

Usually, it's glorot_uniform by default. Different layer types might have different default kernel_initializer. When in doubt, just look in the source code. For example, for Dense layer:

class Dense(Layer):
...
    def __init__(self, units,
                 activation=None,
                 use_bias=True,
                 kernel_initializer='glorot_uniform',
                 bias_initializer='zeros',
                 kernel_regularizer=None,
                 bias_regularizer=None,
                 activity_regularizer=None,
                 kernel_constraint=None,
                 bias_constraint=None,
                 **kwargs):

GlorotUniform, keras uses Glorot initialization with a uniform distribution.r = √(3/fan_avg)

fan_avg = (fan_in + fan_out) /2

number of inputs = fan_in

number of nurons in a layer = fan_out