Rosebud grate hole size

hi My question is about using the Venturi exit hole size to control the local temperature of the fire across the whole grate.

At the moment, the hole area is about 15% using 4.2 mm holes arranged in concentric rings, at 8 mm spacing. it is a vertical fire tube boiler with a 5" diameter grate. It is obvious that although much better than a bar grate the heat is concentrated towards the centre, as would be expected as it's got fire all round it. the Venturi has a slow taper formed on a reground bs4 centrerill ( cutter grinder) the grate performs much better than the bar grate, although I have not seen a floating fire as some writers describe, although this could be due to using a combination of nut size coal for a slower burn due to its larger mass to surface area; and bean for easier firing on the move and filling holes in the fire

So.....

1 To promote generating a hotter fire in the cool areas of the grate do I increase the hole size ( by only a small amount) to get more air in so increasing combustion

2 OR do I increase the hole size in the hot areas so that increased air flow will cool the fire at that point

Is this possible?

I'm guessing that the area of holes in the grate is quite a lot greater than the area of the venturi, so the holes aren't controlling the distribution of the air that well. air is entering at the centre of the grate and more is going towards the centre holes than the outer ones. I'd be tempted to make a new grate with smaller holes or to modify the existing ones with larger holes towards the outside.

Disclaimer:- I worked in an aerodynamics lab, but I'm an electrical engineer!

I have just embarked on a Rosebud for my Polly V loco and have done a spreadsheet to calculate the hole matrix for a given size of grate and % area occupancy of the holes etc. Others might find this helpful, see the link below : -

**LINK**