Default value of a 'managed file' field
To render the existing image, please use the below code

$form['project']['image'] = array(
  '#type' =&gt; 'managed_file',
  '#title' =&gt; t('Image'),
  '#default_value' =&gt; array(10),
);


You need to give the fid in array format. Please try it and let me know whether you got the result