Vertically Center Text Bootstrap

Define a custom class and use as follows:

.vertical-align {
    display: flex;
    align-items: center;
}
<div class="container">
    <div class="row vertical-align">
        <div class="col-md-6"><img src=" http://placehold.it/150x50" /></div>
        <div class="col-md-6">Dummy text</div>
    </div>
</div>

this is bootstrap 4:

<html><head>    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
  </head>
  <body>
<div class="row">
<div class="col-6">
    <img src="https://media.haircutinspiration.com/photos/20181204011855/boy-hairstyle-e1536810585307.jpg" class="img-fluid">
</div>
    <div class="col-6 align-self-center">
        hello hello
    </div>

</div>
  </body>

Check my snippet it will work as you want ...

.content_center {
  display: flex;
  justify-content: center;
  align-items: center; 
  width: 250px;
  height: 150px;
  border : 1px solid black;
}
<div class="content_center col-md-6">
What I want
</div>