**1. Introduction**

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Cross-Flow Air to Air Heat Exchanger With Plate-Fin Cores. *International Journal of Heat and Mass Transfer*, Vol. 52, No. 19-20, (September 2009), pp. 4500-4509, ISSN Rocket propellant has various requirements, such as higher specific impulse (Isp), large density, non-toxic, storablity, operational reliability and easy handling. In addition, large heat capacity of the fuel is also desirable. A regenerative cooling system is often adopted to cool a combustion chamber because of high combustion temperature (about 3000K), and high heat transfer rates from combustion gas. In the regenerative cooling system, the combustion chamber is a kind of a heat exchanger and fuel of a rocket engine is used as a coolant. The fuel heated in regenerative cooling passage is injected into the combustion chamber and burned. Heat loss from combustion gas to chamber wall is absorbed into the heat capacity of fuel and utilize them to propulsive work again. ( regeneration ).

Unfortunately, no propellant can satisfy all of those requirements. The requirements of higher Isp and larger thrust trade off each other. For example, Liquid hydrogen (LH2) has high Isp and "clean" propellant. In addition, hydrogen has large heat capacity, and thus, is favorable coolant for regenerative cooling. However, the molecular weight of hydrogen is the lowest among the all of chemical compounds, thus, it is difficult to obtain larger thrust by using LH2. For those reasons, LH2 is suitable for an upper staged rocket engine, rather than an booster staged one. Another disadvantage of LH2 is diffculity to handle because it is cryogenic fluid (20K at tank) and easy to leak from the tank.

The hydrocarbon fuels have been also widely used for rocket propulsion because they have advantages of non-toxic, lower cost, higher density and easier handling. RP-1 is the typical kerosene fuel for rocket propulsion, which was used for F-1 engien of Saturn 5. Kerosine can produce larger thrust than LH2, although it has a drawback of lower Isp. Therefore, kerosene fuel is suitable for a booster staged rocket engine. If kerosene fuel would be employed to the regenerative cooling system, the combustion chamber has fuel flow passage within its wall. The regenerative cooling combustion chamber plays role of heat exchanger. The temperature of fuel increases to begin the thermal decomposition and cause soot formation in the regenerative cooling fuel passage. The soot in the fuel passage can clog the fuel flow and deteriorate the heat transfer from a chamber wall to fuel. Thus, the rocket engine designers must pay attention to coking of hydrocarbon fuel.

Liqufied Natural Gas (LNG) is one of the hydrocarbon fuel and its main component is CH4. The volumetric fraction of CH4 typically ranges from 85% to 95%. The rest of them are etane

Control of LNG Pyrolysis and Application to Regenerative Cooling Rocket Engine 145

40 min from 500 to 850 oC

From 500 to 850 for Ascendant Heating

Inconel 718 Ni: 53.5%, Cr : 19%, Fe: 18% Inconel 600 Ni: 78%, Cr : 15%, Fe: 7% A286 Ni: 25%, Cr : 15%, Fe: 54.9%

Temperature oC 500, 700, 800 for Constant Temperature Heating

**2. Fundamental study on coking characteristics of LNG rocket engine** 

**2.1 Experimental study of CH4 pyrolysis and catalytic effects of chamber material** 

To make clear the fundamental characteristics of methane pyrolysis, the authors conducted the experimental investigation. In the present section, we introduced the experimental results by Higashino, K. et. al. ( Higashino, K. et. al. 2009A ) The test apparatus shown in the Figure 1 is employed. The CH4 gas (99.99% of purity) is supplied from the test gas bottle to silica glass tube, where is heated by the electronic furnace. The nitrogen gas can be also fed to gas flow passage to purge the CH4 gas. The flowrates of those gases are controlled by using flowrate control valves and pressure control valves. To investigate its catalytic effects on pyrolysis reaction, 10 pieces of combustion chamber materials (10mm length x 5mm width x 0.5mm thickness) are placed into the alumina board, which are located in this tube. The materials of those test pieces are Inconel 718, Inconel 600 and A286. The formers of two are Nickel based alloy and have mechanical strength under high temperature condition. They were utilized for combustion chamber material, however, Nickel has the catalytic effects to accelerate the thermal decomposition of CH4. On the other hand, A286 is Iron based alloy, but contains Ni of 25%. The details of test conditions and properties of those materials are summarized in Table.1. Prior to the test, the oxide film on the surface of the test pieces was hydrogenated at 500 0C because it can hamper accurate evaluation of the catalytic effects. The gas sampling point is located in the downstream of the heating section, where the test gas is sampled to investigate its chemical composition. The test gas was

Two types of heating methods are employed, constant temperature heating and ascendant heating. In ascendant heating, temperature of the electronic furnace increases with time linearly. The existence of pyrolysis can be found by analyzing the sampled gas by using gas chromatograph with thermal conductivity detector. During the heating tests, gas sampling is conducted in every 5 minutes. The volume of the sampled gas is about 0.5 ml per one time. The progression of CH4 pyrolysis is evaluated by the CH4 conversion rate (%) as

Test Duration 60 min for Constant temperature heating

Test piece materials Inconel 718, Inconel 600, A286

Pressure (MPa A) 0.20 Flowrate (ml/min ) 20.0

Table 1. Test Conditions of CH4 Heating Test.

ventilated after diluting with the air at draft.

expressed in equation (1)

Constituents of test

piece materials

**chamber** 

and propane. The average molecular weight of LNG is about 16 g/mol, which is larger than hydrogen but much less than kerosene. Thus, the Isp of LNG propellant is greater than Kerosene. LNG can be stored in a tank at 110K and easier to handle than LH2, though LNG is cryogenic fluid. However, the application of LNG propellant has not been realized so far. For the booster stage rocket, kerosene can produce greater thrust than LNG. For the upper stage, the Isp of LH2 is superior to LNG.

Recently, LNG is reconsidered as the propellant for an interplanetary transfer vehicle or an booster rocket engine due to its low cost and easy handling ( Brown, C.D. 2004 and Crocker, A. M. 1998). In such engine, the regenerative cooling system and a turbopump are necessary to improve its propulsive performance. As well as kerosene fuel, LNG pyrolysys can be also occurred in high temperature condition, which means that coking problem arise in the regenerative cooling with LNG. Few databases are available about LNG pyrolysis, because LNG propellant has not been utilized in the rocket engine until now. Therefore, such databases must be prepared for a successful development of this engine. For the turbopump, it feeds the propellant to combustion chamber with high pressure and is operated by various cycles, for example, gas generator cycle, staged combustion cycle and expander cycle. In the expander cycled rocket engine, turbopump is driven by the high temperature propellant gas. The propellant cooled the combustion chamber and receive the heat. The higher temperature and the lower molecular weight propellant is, the larger turbine power can be obtained. Therefore, the expander cycle have usually employ the LH2, however, LNG also has the feasibility for the expander cycled turbopump( Brown, C.D. 2004 and Crocker, A. M. 1998). the efficient heat exchange in cooling passage is one of the most important factors to establish the expander cycled rocket engines. It is necessary to control the LNG pyrolysis in the high temperature environment.

For successful development of LNG rocket engine with regenerative cooling, the fundamental characteristics of LNG pyrolysis must be cleared. The present study focused on 1) the temeperature to begin CH4 pyrolysis, 2) the catalytic effect of combustion chamber materials, 3) the effects of addition of propane to CH4. At same time, numerical analyses are conducted to simulate CH4 and CH4-propane mixture. Secondary, the authors proposed coking inhibition methods and experimentally evaluate it.

Fig. 1. Schematic of Test Apparatus.


Table 1. Test Conditions of CH4 Heating Test.

144 Heat Exchangers – Basics Design Applications

and propane. The average molecular weight of LNG is about 16 g/mol, which is larger than hydrogen but much less than kerosene. Thus, the Isp of LNG propellant is greater than Kerosene. LNG can be stored in a tank at 110K and easier to handle than LH2, though LNG is cryogenic fluid. However, the application of LNG propellant has not been realized so far. For the booster stage rocket, kerosene can produce greater thrust than LNG. For the upper

Recently, LNG is reconsidered as the propellant for an interplanetary transfer vehicle or an booster rocket engine due to its low cost and easy handling ( Brown, C.D. 2004 and Crocker, A. M. 1998). In such engine, the regenerative cooling system and a turbopump are necessary to improve its propulsive performance. As well as kerosene fuel, LNG pyrolysys can be also occurred in high temperature condition, which means that coking problem arise in the regenerative cooling with LNG. Few databases are available about LNG pyrolysis, because LNG propellant has not been utilized in the rocket engine until now. Therefore, such databases must be prepared for a successful development of this engine. For the turbopump, it feeds the propellant to combustion chamber with high pressure and is operated by various cycles, for example, gas generator cycle, staged combustion cycle and expander cycle. In the expander cycled rocket engine, turbopump is driven by the high temperature propellant gas. The propellant cooled the combustion chamber and receive the heat. The higher temperature and the lower molecular weight propellant is, the larger turbine power can be obtained. Therefore, the expander cycle have usually employ the LH2, however, LNG also has the feasibility for the expander cycled turbopump( Brown, C.D. 2004 and Crocker, A. M. 1998). the efficient heat exchange in cooling passage is one of the most important factors to establish the expander cycled rocket engines. It is necessary to control the LNG

For successful development of LNG rocket engine with regenerative cooling, the fundamental characteristics of LNG pyrolysis must be cleared. The present study focused on 1) the temeperature to begin CH4 pyrolysis, 2) the catalytic effect of combustion chamber materials, 3) the effects of addition of propane to CH4. At same time, numerical analyses are conducted to simulate CH4 and CH4-propane mixture. Secondary, the authors proposed

stage, the Isp of LH2 is superior to LNG.

pyrolysis in the high temperature environment.

Fig. 1. Schematic of Test Apparatus.

coking inhibition methods and experimentally evaluate it.
