CHEMICAL ENGINEERING TRANSACTIONS VOL. 81, 2020 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Petar S. Varbanov, Qiuwang Wang, Min Zeng, Panos Seferlis, Ting Ma, Jiří J. Klemeš Copyright © 2020, AIDIC Servizi S.r.l. ISBN 978-88-95608-79-2; ISSN 2283-9216 The Optimization Design of a Compact Integrated Precooler for Advanced Space Launcher Engines Hongliang Chang, Haoning Shi, Ting Ma, Long Li, Qiuwang Wang* Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, P.R.China wangqw@mail.xjtu.edu.cn In recent years, aeroplane engines have evolved towards the direction of hypersonic flight, re-use and combined cycle propulsion. As one of the core components, air-hydrogen precooler plays a vital role in the development of liquefied air cycle engines. In order to solve the problem of rapid cooling of high-temperature high-speed air to cryogenic temperature in a very short period of time, the present study proposes a novel compact integrated precooler, which is made up of thin-walled fin and printed circuit plates. To examine the thermal-hydraulic performance of the novel compact integrated precooler design, two different types of conventional precoolers, i.e., shell-and-tube heat exchanger and printed circuit heat exchanger, are used for comparison in terms of volumetric power, power per mass unit and compactness. The results indicate that the comprehensive performance of the novel precooler is significantly improved. 1. Introduction Recently, hypersonic transportation and routine access to space urgently demand the development of hypersonic airbreathing propulsion systems. The optimisation of the thermodynamic cycles of aero engines has always been in the main targets of engineering efforts for environmental and economic reasons (Missirlis et al., 2019). The safe, reliable, and sustained operation aircraft could extend and diversify space activities. As a representative, existing combined cycle engines mainly include Turbine Based Combined Cycle (TBCC), Rocket Based Combined Cycle (RBCC), Air Turbine Ramjet (ATR), three-combination engine and pre-cooled engine, etc (Tang et al., 2019). Figure 1 shows the types of combined cycle engine. The mentioned above engines combine different types of airbreathing or rocket engines, which give full play to their respective advantages. Combined cycle engines air-breathing mode pre-cooled inlet air can effectively improving performance and specific impulse. Due to the presence of the precooler, the cycle core engine can be isolated from the real flight conditions. Consequently, it can operate at even higher Mach numbers (Yu et al., 2019). Rocket Turbojet Ramjet/ Scramjet Precooler TBCC ATRRBCC Three-combination engine Figure 1: The types of combined cycle engine DOI: 10.3303/CET2081065 Paper Received: 20/05/2020; Revised: 31/05/2020; Accepted: 04/06/2020 Please cite this article as: Chang H., Shi H., Ma T., Li L., Wang Q., 2020, The Optimization Design of a Compact Integrated Precooler for Advanced Space Launcher Engines, Chemical Engineering Transactions, 81, 385-390 DOI:10.3303/CET2081065 385 The most representative pre-cooled engine is Liquid Air Cycle Engine (LACE), as shown in Figure 2, which was proposed by Marquardt in the late 1950s (Mamoru et al., 1999), the high heat capacity liquid hydrogen is used to liquefy intake air in the precooler. The final liquefied air is utilized as the oxidiser and hydrogen to combust in the thrust chamber. The main advantages of LACE system include: First, the LACE take off without carrying oxidants can theoretically reduce the gross takeoff weight and dry mass. Second, using the same nozzle for air- breathing and rocket modes. Nevertheless, wang et al. (2014) also pointed out that LACE has icing and precooler design problem. As indicated, the successful implementation of these cycles relies critically on the development of high-performance precoolers (Murray et al., 1997). P LH2 Pump Turbine LH2 Tank P Air P r e c o o le r Liquefied Air Pump Thrust Chamber Figure 2: A basic model of LACE system (Mamoru et al., 1999) Hendrick et al. (2009) selected and designed the shell-and-tube heat exchanger (STHE) as the LACE precooler. The STHE have flexible configuration but fundamentally cannot be used to achieve a compact design where space, volume, shape and weight are at a premium. So STHE are not an appropriate type of construction for being considered. A printed circuit heat exchanger (PCHE) is chosen as a precooler due to high compactness, small volume and low leakage. The main constraint is the high overall quality, which seriously affects its application in the aerospace field. For these reasons, combining the characteristics of LACE precooler design parameters, the present study proposes a novel compact integrated air-hydrogen precooler (IAHP), which is made up of thin-walled fin and printed circuit plates. This structure combines the advantages of conventional plate-fin heat exchangers (PFHE) and PCHE. In order to prove the feasibility and thermal-hydraulic performance of the IAHP design, two different types of conventional precoolers, i.e., STHE, and PCHE are used to compare with the IAHP in terms of volumetric power, power per mass unit and compactness. 2. Plant layout description The designed novel IAHP is composed of two types of fluid channels, i.e., etched channels and plate-fin channels, as shown in Figure 3. The two types of fluid channels are alternately stacked to achieve heat exchange of cold and hot fluid. The high-pressure small-flow hydrogen flows in the etched channels, which are formed by two identical rectangular channel printed circuit plate through diffusion welding. Etched channels have the features of high heat transfer and strong pressure capacity, especially suitable for high-pressure and small-flow working flow. The channels usually have an equivalent diameter with 0.5 ~ 2 mm. The low-pressure large-flow air flows in the plate-fin channels, which are formed by ultra-thin fins. The channels usually have an equivalent diameter with 0.5 ~ 1.5 mm. This kind of flow path can greatly reduce the pressure loss and weight, increase system compactness and heat transfer capability. The 6061 aluminium alloy is chosen as the precooler processing material (Hendrick et al., 2009), which has comprehensive advantages in terms of processability, weldability, plating and corrosion resistance. 386 Hydrogen channels Air channels Figure 3: Heat transfer channels of the novel IAHP 3. Thermal design and optimisation algorithm Heat exchanger thermal design is the main factor affecting heat exchanger performance, the selection of the thermal design method will directly determine the accuracy of the heat exchanger design results. It is notable that a large number of highly interdependent geometric and operational variables, which often show trade-offs. The Genetic Algorithm (GA) can omit the tedious procedures of manual optimisation design and achieve efficient calculation and optimisation. 3.1 Segmented LMTD method and GA optimisation The widely used conventional thermal design methods are the Log-Mean Temperature Difference (LMTD) and ε-NTU methods (ε-NTU Method). These methods often assume the working fluid physical properties as constants. In fact, during the pre-cooling process, the physical properties of the working fluid fluctuate greatly in a very short time. Zhang et al., (2018) proposed the traditional LMTD method could bring huge calculation error in high-temperature design. In order to accurately calculate the heat transfer rate and pressure drop of the precooler, a segmented LMTD method is proposed. The heat exchanger is divided into finite segments based on the temperature variation range. Every segment is considered to be a small heat exchanger. Initial conditions and connection conditions are used to obtain design results. In order to study the actual thermal-hydraulic performance of the novel IAHP, the present study selected detailed operating parameters applied in the LACE for thermal design (Hendrick et al., 2009). The design conditions of the precooler are shown in Table 1. Table 1: Design parameters of IAHP (Hendrick et al., 2009) Parameter Unit Value Hydrogen side Flow rate kg/s 3.0 Inlet temperature °C -64 Outlet temperature °C 171 Inlet pressure kPa 1,600 Pressure drop kPa 400 Air side Flow rate kg/s 52.8 Inlet temperature °C 206 Outlet temperature °C 90 Inlet pressure kPa 148 Pressure drop kPa 15 The detailed design results of IAHP are shown in Table 2. Since the air flow is much larger than the hydrogen flow, the plate-fin structure is chosen as the air channel. The fin height is much larger than the fin pitch to increase the flow cross-sectional area and reduce the air flow rate. The design can reduce the pressure loss. For PCHE, the detailed design results of PCHE are shown in Table 3. In the structure layout, two hydrogen channels are corresponding to one air channel, so the cross-sectional area of the air channel is about ten times than that of the hydrogen channel. These structural optimization designs help the both side fluids to be in an 387 appropriate pressure range. The detailed design results of the STHE are shown in the work of Hendrick et al. (2018). Table 2: Design results of IAHP Parameter Unit Value Hydrogen side channel diameter mm 0.16 channel pitch mm 0.41 plate thickness mm 1.16 Air side Fin pitch mm 0.36 Fin height mm 2.30 Fin thickness mm 0.10 Volume m3 0.24 mass kg 345.98 Table 3: Design results of PCHE Parameter Unit Value Hydrogen side channel diameter mm 0.10 channel pitch mm 0.35 plate thickness mm 0.35 Air side channel diameter mm 0.45 channel pitch mm 0.7 plate thickness mm 0.7 Volume m3 0.35 mass kg 674.27 The GA is a search heuristic algorithm based on genetics, mutation, selection and crossover, which has the characteristics of highly parallel, random, global search and adaptive. The present study combines the thermal design process with the GA and develops an optimisation algorithm based on MATLAB and NIST database. The thermal design and GA optimisation process is illustrated in Figure 5. Start • Design parameters • Parameter ranges • Objective function • Constraints Input • The evaluation of initial population Evaluation Genetic algorithm • Selection • Crossover • Mutation Termination condition satisfied Optimal results Yes No Update values of design parameters Stop Thermal design Start Input given parameters: qm,air, qm,hydrogen, Tair,in, Thydrogen,in, Thydrogen,out Select segment numbers Select geometrial parameters Calculation of each segment LMTD, Ki, Ai, ΔPi (i=1, N) Obtain the overall characteristics ΔP<ΔPallow Stop Design results No Yes Calculate Tair,out Assumate Thydrogen,out,j (j=1, N) Calculate Thydrogen,out,i (i=1, N) Thydrogen,out,i =Thydrogen,out,j No Yes 1 2 3 n1-1 n1 Air outlet 1 2 Total length, L 3 n1n1-1 1 2 3 n1-1 n1 1 2 3 n1n1-1 Hydrogen inlet ΔL segmented LMTD method Figure 5: The thermal design and GA optimisation process 388 3.2 Basic equations Equation of heat balance is as: 2 2 2 2air p,air air,out air,in H p,H H ,out H ,in ( ) ( )Q q c T T q c T T=   − =   − (1) where air q and 2H q are the mass flow rate of air and hydrogen, p,airc and 2p ,Hc are the specific heat at constant pressure of air and hydrogen, air,inT and air,outT are the inlet and outlet temperature of the air, and 2H ,inT and 2H ,out T are the inlet and outlet temperature of hydrogen (Qian, 2002). The overall heat transfer rate Q is given by: m ΔQ UA T= (2) where m T is a log-mean temperature difference, and U is the overall heat transfer coefficient based on heat transfer area A, which is defined as: 2air H 1 1 1 U h h   = + + (3) max min m max min ln T T T T T  −   =   (4) where  is the hydraulic diameter between hot and cold channels,  is a thermal conductivity of aluminium alloy 6061, air h is convective heat transfer coefficient on the air side, 2H h is convective heat transfer coefficient on hydrogen side, and max T is the maximum temperature difference on one side, min T is minimum temperature difference on the other side. Convective heat transfer coefficients on both sides, air h and 2H h , are calculated as: h Nu h D  = (5) where  is the thermal conductivity, and hD is the hydraulic diameter. For the hydrogen side, the global Nusselt number Nu is given by (Maylavarapu, 2011): 2 300Re  ， 4.089Nu = (6) 2300 5 000Re  ， ( ) G Re 5 000 4.089 4.089 2 300 5 000 2 300 Nu Nu Re − − = + − − ， ， ， ， (7) 5 000Re  ， ( ) ( )2 3 1 000 2 1 12.7 1 2 f Re Pr Nu f Pr − = + − ， (8) where Pr is the Prandtl number, and NuG|Re-5000 is the Nusselt number from the Gnielinski correlation evaluated at Reynolds number of 5,000. For the air side, the heat transfer factor j and Fanning friction factor f are determined by (Qian, 2002). ( ) ( ) 2 ln 0.103109 ln 1.91091 ln 3.211j Re Re= − + (9) ( ) ( ) 2 f ln 0.106566 ln 2.12158 ln 5.82505f Re Re= − + (10) The global Reynolds number Re is calculated: 389 h VD Re   = (11) where  , V ,  are density, area-averaged velocity in the inlet section, dynamic viscosity. 4. Performance comparison and discussion In order to compare the three types of precoolers, the parameters in terms of compactness, volumetric power and power per mass unit of these precoolers are listed in Table 4. The designed results indicate that the IAHP shows excellent comprehensive performance. Under the condition of the same heat exchange and pressure drop, the compactness of IAHP is 1.61 times than that of PCHE, and 2.57 times than that of STHE. The volumetric power of IAHP is 1.46 times than that of PCHE, and 4.85 times than that of STHE. The power per mass unit of IAHP is 1.96 times than that of PCHE, and 1.70 times than that of STHE. Table 4: Comparison of designed results for three kinds of structure precooler Parameter Unit PCHE STHE IAHP Volume m3 0.35 1.16 0.24 Weight kg 674.27 579.15 345.98 Power MW 10.5 10.5 10.5 Compactness m2/m3 2449 1530 3940 Volumetric power MW/m3 30.11 9.06 44.00 Power per mass unit kW/kg 15.70 18.13 30.77 5. Conclusions The present study proposes a novel type of IAHP. Two different types of conventional precoolers, i.e., STHE, and PFHE are selected for comparison. Under the same working condition, the compactness of IAHP is 1.61 times than that of PCHE, and 2.57 times than that of STHE. The volumetric power of IAHP is 1.46 times than that of PCHE, and 4.85 times than that of STHE. The power per mass unit of IAHP is 1.96 times than that of PCHE, and 1.70 times than that of STHE. In brief, the IAHP shows excellent comprehensive performance. Acknowledgements This work is supported by Key Project of the National Natural Science Foundation of China (Grant No. 51536007) and the Fundamental Research Funds for the Central Universities of China. References Hendrick P., Heintz N., Bizzarri, D., 2009, Air-Hydrogen Heat Exchangers for Advanced Space Launchers, Journal of Propulsion and Power, 25(6), 1211-1219. 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