Power generation and heat sink improvement characteristics of recooling cycle for thermal cracked hydrocarbon fueled scramjet

SCIENCE CHINA Technological Sciences

© Science China Press and Springer-Verlag Berlin Heidelberg 2011 tech.scichina.com www.springerlink.com

*Corresponding author (email: [email protected])

• RESEARCH PAPER • April 2011 Vol.54 No.4: 955–963

doi: 10.1007/s11431-011-4292-5

Power generation and heat sink improvement characteristics of recooling cycle for thermal cracked hydrocarbon fueled scramjet

BAO Wen, QIN Jiang*, ZHOU WeiXing, ZHANG Duo & YU DaRen

Harbin Institute of Technology, Harbin 150001, China

Received August 6, 2010; accepted December 31, 2010; published online January 25, 2011

In order to further investigate how much fuel heat sink could be increased and how much power generation could be obtained by using recooling cycle for a regeneratively cooled scramjet, the energy conversion from heat to electricity and the fuel heat sink increase in recooling cycle are experimentally investigated for fuel conversion rate and components of gas cracked fuel products at different fuel temperatures. The results indicate that the total fuel heat sink (i.e., physical+chemical+recooling) of a recooling cycle is obviously higher than the heat sink of fuel itself, and the maximum heat sink increment is as high as 0.4 MJ/kg throughout the recooling cycle. Furthermore, the cracked fuel mixture has a significant capacity of doing work. The thermodynamic power generation scheme, which adopts the cracked fuel gas mixture as the working fluid, is a potential power generation cycle, and the maximum specific power generation is about 500 kW/kg. Turbine-pump scheme using cracked fuel gas mixture is also a potential fuel feeding cycle.

recooling cycle, scramjet, heat sink, power generation, thermal cracking

Citation: Bao W, Qin J, Zhou W X, et al. Power generation and heat sink improvement characteristics of recooling cycle for thermal cracked hydrocarbon fu-eled scramjet. Sci China Tech Sci, 2011, 54: 955963, doi: 10.1007/s11431-011-4292-5

1 Introduction

Hypersonic airbreathing vehicles, including the single-stage- to-orbit (SSTO) vehicles or two-stages-to-orbit (TSTO) aerospace planes, fully reusable space transport vehicles and hypersonic cruise missiles powered by scramjets, have be-come one of the popular subjects in recent years [1]. Cool-ing is a major design consideration for the development of a scramjet engine because of high combustion temperatures and high heat transfer rates from the hot gases to the walls of a combustor chamber. It is generally accepted that regen-erative cooling with fuel used as the coolant is the only fea-sible solution. Fuel flows through the cooling passage to cool the engine walls before it is used for combustion [2]. Hydrocarbon fuel has been considered as the primary cool-

ant in a regenerative cooling system for hypersonic applica-tions [3]. However, the following key thermal management issues must be considered for the development of a hydrocarbon fueled scramjet.

Firstly, the limited fuel onboard and correspondingly the limited fuel heat sink can barely meet the cooling require-ments for the whole vehicle, i.e., the fuel heat sink is insuf-ficient, and so, more fuel than required must be carried for the mission and the excess fuel has to be abandoned [4, 5]. In addition, the lack of necessary heat sink confines a hy-personic vehicle to a relatively lower flight speed. Therefore, it is very important to increase the fuel heat sink for a high speed scramjet. Though it is a very effective method to in-crease the fuel chemical heat sink to develop an endother-mic fuel, the speed of a hydrocarbon fueled scramjet engine is limited to approximately Mach 8 due to coking within cooling channels [6].

956 Bao W, et al. Sci China Tech Sci April (2011) Vol.54 No.4

Secondly, it is not appropriate for a generator to be driven directly of the main propulsion engine for hyper-sonic propulsion, since none of these engines has rotating machinery unless they are part of a combined cycle pro-pulsion system. And the growing aircraft electric power requirements for directed energy weapons pose many spe-cial problems in such a high Mach number speed regime, and maybe at least one megawatts of electricity must be available to the weapon system. Much attention has been paid to MHD power generation [7] and a Rankine cycle using the onboard liquid fuel as working fluid [8], however, extra components which have nothing to do with thrust will be introduced and corresponding weight penalty and complexity.

Thirdly, the sustained flight of a scramjet largely depends on an efficient fuel supply system. As the most popular liq-uid fuel feeding system for aerospace propulsion, a tur-bopump system is thus mainly considered as the fuel feed-ing system [9]. However, the temperature of the ram air taken on board the hypersonic vehicle is too high to drive the turbopump system. Therefore, a ram air powered tur-bopump fuel feeding system is not applicable [10].

A recooling cycle (RCC) is therefore proposed to in-crease fuel heat sink by repeatedly utilizing fuel heat sink. The recooling cycle has both power generation and fuel supply functions in addition to increasing fuel heat sink [11]. In order to further investigate how much fuel heat sink in-creasement and how much power generation can be achieved using recooling cycle in a hydrocarbon-fueled scramjet, the energy conversion from heat to electricity and the fuel heat sink increasement in the recooling cycle are experimentally investigated for fuel conversion rate and components of gas cracked fuel products at different fuel temperatures.

Unlike previous studies on recooling cycle [12], the fuel conversion and components of the gaseous products were studied experimentally. Because there is a wide range of fuel state changes and fuel composition changes in the re-cooling cycle, it is imperative to understand how fuel composition and fuel properties change the work output and the subsequent recooling processes in the secondary cooling. A heated tube experimental system was used to achieve a wide range of test section temperatures, pressures, and conversion rates for a systematic investigation. Such an experimental and analytical study aims at a further insight into the characteristics of power generation and fuel heat sink increasing characteristics in the recooling cycle.

2 Recooling cycle and integrated scramjet

A conceptual, two-dimensional actively cooled scramjet engine cross section is shown at the bottom of Figure 1. The major components of the engine are the inlet, combustor, and nozzle.

Figure 1 Schematic configuration of a scramjet engine with recooling cycle.

The heat sink increase of hydrocarbon fuel is constrained by both the heat-resistance of material and the fuel coking temperature. The upper limit of fuel temperature in the heat transfer process is thus fixed. If the fuel temperature is be-low the upper limit temperature after partial fuel heat is converted into other form of energy, the fuel can be used for secondary or multiple cooling. The fuel heat sink can thus be repeatedly utilized to absorb additional heat, which could be considered as the heat sink indirect increase of per unit fuel. And the wasted heat will be converted into available energy for power generation. The energy obtained through heat conversion is also the product of energy regeneration and utilization of waste heat.

As shown in Figure 1, as a new cooling method, recool-ing cycle (RCC) consists of first and second cooling pas-sages, a turbine, a generator, and a pump. Dashed lines in Figure 1 represent the fuel flow path. First of all, the fuel from the fuel tank is pumped to the supercritical pressure, and then enters into the first cooling passage using its own heat sink to cool the heated surfaces. The total heat sink of hydrocarbon fuel comes from the physical capacity of the fuel to raise its temperature and thereby its sensible en-thalpy and the heat-absorbing endothermic chemical reac-tion. As the fuel outlet temperature is beyond its initial cracking temperature, the fuel at the inlet of a turbine is the mixture of cracked products and uncracked fuel. Secondly, the high temperature and high pressure cracked fuel mixture expands while doing work to the turbine, and its tempera-ture decreases, and the generator is driven by the fuel tur-bine to generate electricity. Thirdly, the cracked fuel mix-ture enters into the second cooling passage, and the chemi-cal heat absorption capacity of incompletely cracked fuel can still be made use of once the new heat transfer tem-perature difference between the fuel and the heated wall is re-formed by the introduction of RCC, before they enter into the combustion chamber.

Compared to the traditional regenerative cooling, RCC can absorb additional heat per unit fuel through secondary cooling, which will effectively reduce the fuel flow rate used for cooling in terms of the overall cooling requirement

Bao W, et al. Sci China Tech Sci April (2011) Vol.54 No.4 957

for a vehicle. In addition, the work output of the turbine can drive an electric generator and the fuel pump to provide the power for vehicle subsystems, such as radar communication system, flight control system, electronic equipment, and environmental control system, and so forth.

RCC begins to work once fuel begins to crack. In the lit-erature, endothermic reaction is needed during Ma6 to Ma8 flight for hydrocarbon fueled scramjet [13]. Therefore, RCC is applicable to the Mach number range from 6 to 8 in prin-ciple. Actually, RCC is workable once fuel temperature is heated higher than its boiling point, because fuel is vapor-ized and it has the ability to do work. The lowest Mach number for RCC will be lower than Ma6. If partial flow rate of RCC were adopted by adding a bypass valve in the fuel line before the fuel entering the cooling passage, the fuel flow for cooling would be lower than that for combustion, and the fuel would vaporize or crack in much lower Mach number. Maybe the whole Mach number range for RCC with partial flow rate can cover the flight range from Ma4 to Ma8.

3 Experimental specifications

3.1 Experimental apparatus

A flow schematic routing of the experimental facility built to investigate the issue for flow characteristic, heat transfer and cracking characteristics of hydrocarbon fuels at super-critical conditions is shown in Figure 2.

The system can be used for the study of pyrolytic reac-tion regimes over a long period. The facility consists pri-marily of a fuel reservoir, fuel pump, a fuel reactor (tube), an electric heating power and a fuel cooler. The tube section can easily heat fuel to 1000 K. The test section is a tube using T304 stainless steel.

Fuel is nitrogen-purged to remove dissolved oxygen in

order to improve the fuel thermal stability. The entire sys-tem is purged with nitrogen prior to the introduction of fuel. Downstream the reactor, the products are quenched in a waterfed heat exchanger to room temperature and filtered before passing through a needle valve (back pressure valve) to regulate the system pressure.

The tube wall temperature distribution is measured by K-type thermocouples spot-welded to the outside of the tubes. The fuel temperatures at the inlet and outlet of tube are measured by a thermocouple inserted into the flow of fuel. Fuel pressure is measured with pressure gauges in-stalled at the outlet of the pump and upstream back pres-sure valve. The uncertainty associated with the measure-ment of wall temperature and the temperature of fuel is es-timated to be less than ±3 K, whereas that of pressure measurement is less than 1%. All the experimental data are recorded via a data acquisition system for analysis.

The liquid and gaseous components of fuel are separately metered and analyzed. The gas products are analyzed using a gas chromatograph/mass spectrometry (GC7900/MS), which consists of flame ionization and thermal conductivity detectors, and the liquid portion of the stressed fuel is ana-lyzed by liquid chromatography (LC).

3.2 Experimental procedure

With a test section installed and tested for leak, tests were conducted by first purging the reactor tube with nitrogen to remove any oxygen, and then the pressure was achieved with cold fuel flow by partially closing the back pressure valve. The fuel pressure was regulated with the back pres-sure valve to achieve the desired reactor pressures.

With fuel flow established, data logging was initiated and electrical power was supplied to the test section, and voltage was adjusted to obtain the desired exit fuel temperature. Once the fuel outlet temperature reached its target value, the

Figure 2 Single tube test apparatus schematic.


test was ran for a desired period of time or until the tube was plugged with coke. Minor adjustments in the back pressure valve were made during the run to maintain a rela-tively constant pressure throughout the test.

4 Analytical specifications

4.1 Conversion rate

As conversion (cracking) increases, the liquid products be-come more degraded, with olefins initially appearing. The extent of conversion from fuel to gaseous products is de-termined by collecting and measuring the liquid product for a specified period of time and comparing this with the known flow rate of the unreacted fuel.

The liquid products are collected after test starts for GC/MS analysis, and for calculation of the percent conver-sion to gas, which is defined as [14]

ci co

ci

100,V V

ZV

(1)

where Vci is the volumetric flow rate of liquid fuel fed to reactor, and Vco is the volumetric flow rate of unreacted liquid fuel at the reactor exit.

4.2 Surrogate composition of endothermic hydrocar-bon fuel

China no. 3 aviation kerosene was employed for this study. One kind of additive was added to the original kerosene to improve its chemical heat sink. It was approximately com-posed of 92.5% saturated hydrocarbons, 0.5% unsaturated hydrocarbons, and 7% aromatic hydrocarbons on the vol-ume basis. Its mass composition distribution by carbon number from chromatograph is shown in Figure 3. The overall chemical formula of this kerosene is approximately C11H22 [15].

Figure 3 Composition distribution of China no. 3 kerosene.

Accordingly, a ten-component surrogate was proposed, as listed in Table 1 [16]. The modeling of China no. 3 kero-sene and other kerosene is usually achieved using a repre-sentative blend of paraffin. Since the endothermic hydro-carbon fuels are mixtures and it is hard to find a single sub-stance to represent it. The current surrogate gives better predictions for fuel properties such as specific heat. The predicted critical temperature and critical pressure of this surrogate are 660 K and 2.4 MPa, respectively, whereas the measured critical temperature and critical pressure for China no. 3 kerosene are 640 K and 2.4 MPa, respectively.

4.3 Composition of cracked products

When China no. 3 kerosene is heated to a sufficient tem-perature (>725 K), endothermic reactions known as thermal cracking occur. During this process, thermal decomposition of high molecular weight hydrocarbons results in lower molecular weight aromatics, alkenes and alkanes. The de-tails of the product distribution are provided in Table 2.

The fuel at the outlet of the cooling passage is the mix-ture of cracked products and uncracked fuel once endother-mic reaction undergoes. The composition of mixed products differs with the reaction depth, which is mainly decided by the fuel temperature and velocity.

Properties for surrogate fuel and cracked products of China no. 3 kerosene are obtained from the National Insti-tute of Standards and Technology (NIST) Thermodynamic and Transport Properties of Pure Fluids database and the NIST Chemistry Web Book [17].

Table 1 Ten component surrogate of China no. 3 kerosene

Composition Molar percentage

Alkanes

n-Octane n-Decane

n-Dodecane n-Tridecane

n-Tetradecane n-Hexadecane

6 10 20 8

10 10

Cycloalkanes Methylcyclohexane

trans-1,3-Dimethylcyclopentane 20 8

Benzenes Propylbenzene 5

Naphthalenes 1-Methylnaphthalene 3

Table 2 Comparison of China no. 3 kerosene products of pyrolysis

Formula Species

CH4 methane

C2H6 ethane

C2H4 ethylene

C3H8 propane

C3H6 propylene

C4H10 n-butane

C4H8 butene


4.4 Heat absorption analysis

The cooling passage in an actively cooled scramjet is a sur-face heat exchanger arranged in series, in which the coolant flows forward through. And the cooling process can be re-garded as the heating process for fuel, therefore, the first and second cooling passages can be respectively seen as the primary heater and reheater in RCC. The endothermic hy-drocarbon fuel completes the cooling relying on both physical heat absorption and chemical heat absorption. The uniform relation of heat absorption in such two cooling passages can be interpreted as

total phy chem.Q Q Q (2)

Taking the heat transfer in the first cooling passage as in-stance, the physical heat absorption Qphy of the first cooling passage can be given by

phy co ci1

( ).n

f i i iP

i

Q m C T T

(3)

The whole calculation of fuel temperature range within the heat transfer process can be divided into many segments, and the specific heat capacity in the calculation of each segment is obtained by its arithmetic average value at fuel inlet and outlet temperature of the segment. The whole physical heat absorption of the cooling passage is the sum of the product of average specific heat capacity and tem-perature difference over each segment. The average specific heat capacity at each temperature is obtained by calculating the weighted average of every species of the mixture of cracked and uncracked fuel.

Chemical heat absorption Q fchem in the first cooling pas-

sage is given by

chem ch ,f fQ mZ Q (4)

where the cracking rate (extent of fuel conversion) Z is also defined as the mass ratio of gaseous products to fuel feed.

From eqs. (3) and (4), as the sum of Q fphy and Q f

chem, the total heat absorption Q f

total in the first cooling passage can be given by

total co ci ch1

( ) .n

f i i i fP

i

Q m C T T mZ Q

(5)

Due to the temperature decrease in the fuel by doing work to the turbine, there is a new temperature difference between the fuel and the heating wall. Such a new tempera-ture difference enables the fuel to be used for secondary cooling.

If the cracking reaction of endothermic hydrocarbon fuel is incomplete in the first cooling passage, the remaining unreacted fuel will keep on cracking in the secondary cool-ing passage as long as the temperature of fuel is higher than its cracking temperature. Therefore, the total heat absorp-

tion in the secondary cooling passage can be given by

total phy chem

co ci ch1

( ) (0.8 ) ,

s s s

Nj j j f

Pj

Q Q Q

m C T T m Z Q

(6)

where the maximum value of cracking rate Z is set to 0.8, because 0.8 is generally the maximum upper limit of fuel for cooling. Once the cracking rate of fuel is over 0.8, the coke produced by thermal cracking of hydrocarbon fuel will dramatically increase, which may lead to the cooling chan-nel plug or local wall overheating, even the failure of the whole cooling system.

We denote the average specific heat in each calculation cell by pC and use subscripts i and j to identify the calcu-

lation cells of the first cooling passage and the secondary cooling passage, respectively.

5 Turbo-machinery analysis

For a pump, the flow rate in a turbine is the same as that of the pump, so we are only concerned with the specific power of it, which can be given by

po pi

pi

,pp

P PW

(7)

where Ppi and Ppo denote the fuel inlet and outlet pressures of the pump, pi is the pump efficiency, and pi is the liquid fuel desity.

If there is no loss of power due to the friction of a me-chanical system, the turbine outlet temperature can be given by [18]

(1 )to ti 1 ,1 K K

tT T (8)

The specific power rate of the turbine can be given by [19]

(1 )ti [1 ].K K

t t PW C T (9)

6 Performance parameters

The prime function of RCC is to increase the heat sink of fuel. And the multiplication ratio of fuel heat sink capacity used to evaluate the performance of RCC has been defined in ref. [12] to demonstrate the extent of increasing fuel heat sink capacity. The total heat sink capacity of fuel (i.e., physical+chemical+recooling) is chosen to present the ca-pacity of RCC in increasing fuel heat. Power

fc total total( ) / ,f sh Q Q m (10)

includes the heat absorption capacities obtained using both


direct methods (physical+chemical) and indirect method (recooling) to increase fuel heat sink.

Another performance gain of RCC is the specific electric power We, which is the difference between specific power produced by turbine Wt and power required by pump Wp

.e t PW W W (11)

7 Results and discussion

7.1 Test conditions

Table 3 lists the experimental conditions for test cases. The fuel was heated to supercritical pressures and the outlet temperatures of fuel at test section are all higher than its cracking temperature with an inner diameter of 1 mm and a wall thickness of 1 mm. And the pressure is kept constant at a value of approximately 4.3 MPa.

7.2 Test results

Table 4 lists the measured compositions of gaseous hydro-carbons and the corresponding crack rates at seven different fuel temperatures. The cracking progresses as the tempera-ture goes up. As fuel conversion (cracking rate) Z increases, liquid products become more degradable.

Figure 4 compares the composition variations of the gaseous products at seven different exit fuel temperatures, ranging from 773 to 953 K. The major gaseous products obtained through cracking are methane, ethane, ethylene, propane, propylene and butene.

It can also be seen from Table 4 and Figure 4 that in-creasing fuel temperature generally leads to larger conver-sion to methane and ethane, but smaller conversion to pro-pane, butane and butene. Although it is possible to have a further increase in fuel temperature, it is not attempted here because a significant thermal coke occurs at a fuel tempera-ture higher than 953 K. These experimental results are found to be consistent with the published results.

7.3 Thermal properties of cracked fuel mixture

It is necessary to evaluate the variation in thermal properties of cracked fuel with the fuel temperature and cracking rate, because there is a wide range of fuel states and composition

Figure 4 Comparison of gaseous product compositions at varying fuel temperatures.

Table 3 Summary of test conditions

Test no. Pressure (MPa) Temperature of fuel (K) Volume flow rate (mL/min) Maximum wall heat flux (W/cm2)

1 4.3 773 40 7.3

2 4.3 813 40 8.39

3 4.3 843 40 9.2

4 4.3 863 40 9.68

5 4.3 883 40 10.2

6 4.3 913 40 12.69

7 4.3 953 40 16

Table 4 Molar percentage of gaseous products and cracking rates resulted from thermal cracking of China no. 3 kerosene

Tf (K) 773 813 843 863 883 913 953

Methane 7.66 8.95 10.14 10.56 12.14 14.65 17.61

Ethylene 25.81 26.11 26.42 27.67 25.49 23.44 20.58

Ethane 18.76 18.95 19.03 19.43 19.95 21.83 23.54

Propylene 21.8 22.4 22.99 23.08 23.33 23.31 23.3

Propane 12.93 11.56 10.37 9.96 9.19 8.46 7.55

Butane 9.11 8.54 8.13 6.88 7.37 6.58 5.74

Butane 3.93 3.49 2.92 2.42 2.53 1.73 1.68

Z (%) 14.03 16.28 18.32 22.49 36.32 48.57 63.1


variations in the cracking region, and the properties of cracked fuel mixture change dramatically. The average spe-cific heat pC and average specific heat ratio at each

temperature are obtained through calculation of weighted average of each species of the cracked fuel mixture.

As shown in Figure 5, the average specific heat pC

firstly decreases and then increases with the temperature. The non-monotonic variation of average specific heat pC with

temperature is caused by the variation of the composition of cracked fuel mixture with temperature. The specific heat in-creases as the number in carbon atoms of cracking product increases. Therefore, pC firstly decreases as fuel

temperature increases once fuel cracks into gas products with small carbon atoms. Simultaneously, the specific heat of each single species of the mixed products increases as the increase of temperature, and saturated alkanes have greater specific heat than the olefins with the same carbon atoms. Therefore,

pC then begins to increase with fuel temperature.

As shown in Figure 6, the specific heat ratio of cracked

Figure 5 Variation of average specific heat of cracked fuel mixture with temperature.

Figure 6 Variation of average specific heat ratio of cracked fuel mixture with temperature.

fuel mixture decreases as fuel temperature Tf increases,

because the specific heat ratio of each species of the mixed products decreases with the increase of temperature. How-ever, the decrease rate of the specific ratio of cracked fuel mixture with the increase of temperature is higher than that of any single gas product with the increase of temperature. As is far lower than 1.4, the cracked gas products can

not be seen as an ideal gas any more.

7.4 Thermodynamic power generation evaluation

As shown in Figure 7, the temperature drop of cracked fuel mixture through turbine Tt decreases as Tf increases at fixed expansion ratio , i.e. decreases as cracking rate Z increases. Such variation of Tt with Tf is very different from that for single gas species, even opposite to that for single gas species. For single gas species, the temperature drop through the turbine increases with the increase of its inlet temperature of the turbine. From eq. (8), Tt is mainly decided by fuel inlet temperature Tf, expansion ratio , and specific heat ratio . Although the increase of Tf will lead

to the increase of Tt , Tt is mainly decided by at fixed

, because Tt varies exponentially with . The tempera-

ture drop of cracked fuel mixture through turbine Tt in-creases as expansion ratio increases at each temperature.

As shown in Figure 8, the specific electric power We basically decreases with the increase of cracked fuel mixture temperature Tf. With respect to a constant ex-pansion ratio, the power required by a pump does not vary with turbine inlet temperature Tf, the net work output is thus exclusively decided by the work output of the turbine. Such variation of Wt with Tf is also very different from that for single gas species, even it can be concluded that it is just opposite to that for single gas species.

It is still noticed that Wt begins to increase once Tf is lar-ger than 913 K, and Wt will further increase if bigger Z is

Figure 7 Temperature drops of cracked fuel mixture through the turbine with fuel temperature at different expansion ratios.


Figure 8 Specific electric powers with fuel temperature at different ex-pansion ratios.

obtained at a higher temperature. From eq. (9), Wt is still decided by average specific heat pC besides fuel inlet

temperature Tf and specific heat ratio at fixed expansion

ratio . And we noticed that pC begins to increase once Tf

is larger than 883 K. The composition of gaseous products and their thermal properties have strong influence on spe-cific power rate of turbine Wt, and bigger pC and are

favorable to obtain a higher Wt. As the specific heat ratio of olefins is higher than that of saturated alkanes with the same number of carbon atoms, it would be better to have more olefins in pyrolysis products in order to obtain higher Wt. Furthermore, specific electric power We of RCC increases as the expansion ratio increases.

7.5 Fuel heat sink enhancement evaluation

As shown in Figure 9, the total heat sink of fuel (i.e., physi-cal+chemical+recooling) in RCC decreases with the in-crease of fuel temperature Tf when Tf is lower than 913 K, i.e., it decreases as cracking rate Z increases. Here the chemical heat sink of fuel Qch is assumed to be 2 MJ/kg with complete cracking, and the exit temperature and cor-responding crack rate used for the calculation are set to 1000 K and 0.8, respectively under different conditions. The contribution to the total heat sink depends on the increase of physical heat absorption, because the total chemical heat absorption remains constant while final cracking rate Z is equal to 0.8 under each condition.

The gain of physical heat sink owes to the secondary cooling in the second cooling passage. And the possibility of secondary cooling is due to the energy conversion from thermal energy to mechanical energy through the turbine. The gain of physical heat sink will be much higher if bigger Wt is obtained. Moreover, as the RCC has been introduced, the maximum total heat sink is close to 4.4 MJ/kg, which is definitely greater than the maximum heat sink 3.9 MJ/kg of

Figure 9 Variations of total heat absorption of the two cooling passages with cracking rate.

general endothermic fuel when Z =0.8. It is also concluded from Figure 9 that the total fuel heat sink hfc increases with the increase of expansion ratio .

Figure10 shows some confirmed enhanced total heat sink capacities of fuel. The physical heat sink, heat sinks (i.e., physical+chemical) and total heat sink (i.e., physical+ chemical+recooling) are depicted in Figure 10. The physical heat sink is just the sensible heat absorption of fuel, and it increases monotonically with fuel temperature. The chemi-cal heat sink of a hydrocarbon fuel comes from a heat-absorbing endothermic chemical reaction, which will add a large amount of heat absorption capacity to the fuel. It should be noticed that the total heat sink (i.e., physical+ chemical+recooling) plays the roles of direct method (physical+chemical) and indirect method (recooling) in in-creasing fuel heat sink. The maximum heat sink increment is as high as 0.4 MJ/kg through recooling on the basis of the physical heat sink and chemical heat sink. The combination of direct method and indirect method enables fuel heat sink to achieve its maximum.

Figure 10 Experimentally evaluated enhanced heat sink capacities (chemical+recooling) and statistically estimated physical heat sink capacity of China no.3 kerosene.


It can be seen from Figures 9 and10 that the physical heat absorption capacity is repeatedly used in the secondary cooling and the chemical heat absorption capacity of in-completely cracked fuel could still be made use of once the new heat transfer temperature difference between fuel and heated wall is re-formed by the introduction of RCC.

8 Conclusions

The thermal cracking of China no. 3 aviation kerosene was experimentally investigated while it flows through a heated tube. And the composition of seven different sets of cracked fuel products were obtained at different temperatures. From the composition of the measured cracked fuel mixture, fuel temperature and the calculated fuel properties using a 10-component kerosene surrogate, the power generation and fuel heat sink increasing characteristics in recooling cycle were evaluated. The following specific conclusions can be drawn from the results of this study:

1) Recooling cycle provides the probability that the physical heat absorption capacity can be repeatedly used and the chemical heat absorption capacity of incompletely cracked fuel could still be made use of in the secondary cooling.

2) The total fuel heat sink (i.e., physical，chemical and recooling) in the recooling cycle is obviously greater than conventional fuel heat sink (i.e., physical+chemical). The maximum heat sink increment is as high as 0.4MJ/kg through recooling. And the total fuel heat sink decreases as endothermic reaction continually undergoes.

3) The cracked fuel mixture has a significant capacity of doing work, and the work output can be used to provide electric power generation and drive the fuel pump. The composition of cracked fuel mixture and their thermal properties have significant effect on their capacity of doing work.

4) The cracked fuel gas mixture turbine-pump scheme is one potential fuel feeding cycle, which makes the most effi-cient use of fuel’s capacity of doing work, and it may be-come the top cycle among various fuel feeding cycles.

5) The turbine-generator thermodynamic power genera-tion scheme adopting cracked fuel gas mixture as the work-ing fluid is also one potential power generation cycle. The maximum specific power generation is about 500kW/kg.

6) Catalytic cracking can be carried out with good prod-uct selectivity for the purpose of increasing fuel heat sink or

electric power generation, providing a fuel that should have excellent combustion characteristics.

This word was supported by the Key Program of the National Natural Science Foundation of China (Grant No. 51076035).

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