<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">Crispin, cc list<div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>1. Yesterday you said this:</div><div><span class="Apple-tab-span" style="white-space: pre;"> </span><i>Personally I don’t think a ratio is a good design approach. You need a certain vertical space to burn the gases before you attempt heat transfer. A total of 125-150mm is normally OK assuming you have the fuel deck covered with secondary air and it is burning hot near the fuel bed. If you bleed in secondary air through a lot of small holes, there will be no secondary air in the middle. Some try to compensate for this by putting in a vertical tube. That tube is a sign that he secondary air entrance has not been handled correctly. It is a common ‘solution’ to a problem that should not be there in the first place. Try to avoid it.</i></div><div><div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>2. Re the “vertical tube”: I have been involved in several off-list discussions where central secondary air seemed to be providing turbulence benefits (benefits that including reducing the height and therefore lowering costs. A central pipe can also prevent gases from rising in the center, which can be a benefit if one has swirl. Can you give a cite on where a central secondary pipe was shown to not be of value?</div><div><br></div><div>Ron</div></div></div></body></html>