Gas-Si Wall Temperatures in Rib-Backed Liquid-Cooled Combustion Chambers

I IS of considerable value to obtain estimates of the temperatures of "hot spots" that may occur on the wall surfaces of rocket combustion chambers. One form of hot spot can occur when a thin wall is reinforced by a thick rib or spacer to satisfy structural requirements. Adding a rib or "fin" is normally desirable from a heat transfer viewpoint since the effective coolant-side surface area is generally increased. However, conditions do exist where a liquid coolant film resistance is very small in comparison with the resistance of the wall material. The rib, if wide enough, can therefore effectively impede the local heat flux to the liquid and raise the gas-side wall temperature several hundred degrees above the temperature of the wall surfaces remote from the rib. The exact dividing line between a "cooling" fin and an "insulating" fin is rather difficult to establish analytically. If we accept conventional fin efficiency equations as valid, the heat flow from a rectangular plane fin is given by