Okay, here's the thing. When the air blast enters the fire, it provides oxygen to make it burn BUT it doesn't all get burned up at once. This means that you've got three layers to the fire: oxidizing, neutral, and reducing. In the oxidizing layer at the bottom of the fire, you've got a lot of hot unburned oxygen just waiting to grab onto a piece of hot steel and turn it into scale. In the neutral layer in the center of the fire, all the excess oxygen is burned up, so it's just hot. In the reducing layer at the top of the fire, there's not quite as much oxygen as you need to sustain combustion, so it's not quite as hot as you want. TL:DR = The top and bottom layers of the fire won't heat your workpiece efficiently, and the bottom layer will increase scaling. Stick your workpiece straight in from the side, into the center of the fireball, thus:
(Note that this shows a bituminous fire with its layers of coke, but the same principle applies.)