Additives to Increase Fuel Heat Sink Capacity

Abstract

Recently, NASA has increased its emphasis on reducing the cost of reaching low Earth orbit . Studies indicate that using a rocket based combined cycle (RBCC) engine could help achieve significant cost reductions. In the third stage of a RBCC launch, the air-breathing vehicle will travel at speeds from Mach 5 to Mach 10 prior to exiting the atmosphere. At these speeds, the heat loads in the combustor are so high that sensible heating of the fuel alone will not be sufficient to meet the cooling demand. However, up to 50% additional heat sink capacity can be extracted from the fuel if it undergoes endothermic, thermal cracking reactions prior to combustion. Unfortunately, the very high temperatures required to achieve necessary cracking rates reduce the allowable stress in the heat exchanger, increasing its weight and reducing its efficiency. Thus, the overall objective of this work is to maximize the heat sink available from hydrocarbon fuels such as JP-7 by adding a chemical initiator, which significantly increases the rates of the thermal cracking reactions. In previous work, TDA developed additives that produced significant increases in the rate of thermal cracking of normal paraffins. However, no studies had been done to measure the effectiveness of these additives with real aviation fuels, such as JP -7, which are complex mixtures of bra nched and cyclo paraffins in addition to normal paraffins. Therefore the goal of this project was to measure the effect of our chemical initiator on the heat sink capacity of JP-7. To accomplish this, we first constructed a test section that could be use d to measure fuel heat sink directly. We then measured the heat sink of JP -7 with and without initiator and, for comparison, conducted similar measurements with a normal paraffin, n-heptane. The results of this project clearly demonstrate that the initiator is very effective at increasing the fuel heat sink capacity of JP-7. In the temperature range between 400 and 550oC, we obtained a heat sink value of 3 03 Btu/lb without initiator. When we added initiator, we obtained a value of 373 Btu/lb over the same temperature range, an improvement of 23%. We also found that the effects of the initiator were greater with a normal paraffin fuel. With nheptane, we obtained a heat sink value of 282 Btu/lb as the fuel is heated from 400 and 575oC without initiator. With initiator, we obtained a value of 420 Btu/lb, an increase of 49%. Finally, a kinetic model suggests that the temperatures required to obtain 50 and 75% n-heptane cracking levels with initiator are close to 100oC lower than the temperatures required without initiator.