Performance of heat exchangers for application on thermal energy recovery systems from internal combustion engines of light-duty vehicle

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1 Performance of heat exchangers for application on thermal energy recovery systems from internal combustion engines of light-duty vehicle Ricardo Manuel Andrade Mateus Abstract The present work focuses on the recovery of thermal energy that is lost to the environment during the normal operation of internal combustion engines on light-duty vehicles. The main contribution for this loss is the discharge of hot exhaust gases to the atmosphere. To address this issue a Rankine cycle implementation is proposed, which converts the thermal energy of the exhaust gases in mechanical work and, finally, in electricity. This is used to charge the vehicle s battery or to fulfill the needs of the vehicle s auxiliary electrical systems. The efficiency of implementing this system depends on all its components, but it is particularly important to maximize the heat transfer between the exhaust gases and the Rankine cycle s working fluid. Therefore, a careful analysis to the cycle s evaporator (heat exchanger) is important, whose design has restrictions both regarding working limits and the physical installation of the evaporator in the vehicle. The main objective of this work is to compare different heat exchangers, with geometry and/or currents arrangement variation, recurring to computational simulations based in updated heat transfer calculation methods. This calculations are applied to an ensemble of experimental data taken from different working conditions of a light-duty vehicle. This work allows to select the heat exchanger with the best performance parameters while respecting simultaneously the working restrictions. Key-words: Thermal energy recovery, Rankine cycle, heat exchanger, thermodynamic efficiency Nomenclature area [ ] global heat transfer coefficient [ ] Brake Mean Effective Pressure [ ] width [ ] HEX tube diameter [ ] work [ ] HEX Shell diameter [ ] work transfer rate [ ] tube-bank diameter [ ] Variation [-] thickness [ ] efficiency [-] Force [ ] Subscript height [ ] RC working points heat exchanger external Internal combustion engine electrical length [ ] evaporation mass [kg] working fluid mass flow rate [ ] exhaust gases revolutions per minute [rpm] internal number of transfer units [-] inlet pressure [ ] mechanical heat [ ] outlet heat transfer rate [ ] pump fouling resistance [ ] tube Rankine cycle turbine temperature [ ] thermal Introduction Growing concern on the environmental impact caused by the combustion of fossil fuels leads to the development of systems that use these fuels with a higher efficiency and less emission of pollutants. Particularly in the case of internal combustion engines (ICE), which are the main propulsion system of nowadays the automobile vehicles (and it s foreseen they ll continue to be for the next decades), some 1

2 improvements have been made to enhance the performance of the engine like turbo charging or recirculation of the exhaust gases, EGR (Ferrari, 2011). Still, the efficiency of this type of engine is very low Diesel engines, with an efficiency of around 40% are distinguished from the spark-ignition engines, for which the efficiency decreases to about 30% (Domingues et al., 2013) and (Gewald et al., 2012). The low efficiency is mainly due to thermal energy losses from the engine to the environment, with the thermal energy loss due to the discharge of high temperature exhaust gases being the one with the greatest impact, according to Macián et al. (2013). In this work the thermal energy recovery from the exhaust gases is proposed to be done by implementing a Rankine cycle (RC) system, with main focus on the evaporator (heat exchanger HEX) design and performance. The main objectives of this work are: i) To design the evaporator heat exchanger (HEX) for the vehicle data under study. Initially the design considers various geometries and flow arrangements, choosing then the HEX with the best performance and suitable vehicle installation characteristics. Following this, the HEX is tested for varying geometries and different vehicle operating conditions. Theoretical Background The present work involves the modellation of a RC and design and simulation of a HEX, which require a theoretical background both in Thermodynamics and in Heat Transfer. Therefore a brief approach to both these areas will be made. A Rankine cycle (RC) is composed of a group of processes in which the working fluid goes through changes in its properties in such a way that it returns to the departure state, producing mechanical power from thermal power. Figure 1 shows a simplified schematics of the RC components for heat recovery from exhaust gases emmited by an ICE. As can be seen, the working fluid is compressed by using a pump (, process 1-2), receives heat from a hot source (, process 2-3), produces work in an expander by reducing its pressure (, process 3-4), then transfers heat to the cold source through the condenser (, process 4-1) and is compressed again, thus closing the cycle. The net work output and thermal efficiency of the cycle are calculated using equations (1) and (2) respectively, taken from Moran et al. (2006). (1) (2) In the present work the turbine is coupled to an electrical generator that converts mechanical work into electricity with an associated conversion efficiency ( ). The relevance of the evaporator in the RC makes it evident that a correct HEX design is indispensable to obtain the maximum net work output of the cycle. This means the HEX needs to have a good energy efficiency, in order to recover the thermal energy from the exhaust gases. However, as the RC s components are to be installed in a vehicle, restrictions to the installation dimensions make it necessary for the HEX to have a good exergetic efficiency so as to minimise irreversibilities. A first approach to increase the energy efficiency of a HEX would be to increase the heat transfer area. However this could lead to greater pressure loses for the exhaust gases downstream of the ICE a term usually called back-pressure. Chammas et al. (2005) refer to a value of 5,0 kpa for the exhaust gas back-pressure in the HEX as reasonable. In the present work a limit value of 7,0 kpa has been defined. 2

3 Figure 1 Simple RC system for heat recovery from exhaust gases emitted by an ICE Literature Review The literature review presented herein, taken from several scientific sources, is fundamental for the development of this work. And its primary objective is to provide the state of the art on thermal energy recovery contained in exhaust gases by using a RC. An example of a study done by automobile companies is the development by BMW of the Turbosteamer system (Obieglo et al., 2009); a system which consists of a RC with 2 paralell systems (a high temperature and a low temperature one) using water as working fluid. It s relevant to notice that in this system the mechanical power produced at the expander/s is directly transmitted to the engine s crankshaft. The BMW RC system called Turbosteamer ensures fuel savings from 5 to 10% according to Obieglo et al. (2009). Ford also studied a RC sytem with an organic working fluid (R245fa) to convert the thermal energy from exhaust gases into electricity. The system was simulated with the experimental data obtained for a Ford Escape 2009 jeep and the respective hybrid version for 2 different drive cycles. A shell & tubes HEX is used as an evaporator (Hussain et al., 2011), working at an evaporation pressure of 25 bar. Hussain et al. (2011) concluded that the RC system can generate enough electricity to supply the accessory electrical system of the vehicle for most drive cycles. Besides company studies there are several scientific articles on the subject but few of them center their attention on the HEX design and optimisation, but rather on the thermodynamic analyses of the RC regarding the working fluid. An example of these few is the article from Mavridou et al. (2010), in which several HEX geometries are tested shell & tubes, finned tubes, plates, etc for a specified vehicle operating condition in order to determine the HEX with the highest performance. Thermodynamic Analysis of the Rankine Cycle The application of the mathematic models for thermodynamic analysis and heat transfer calculation was done with MATLAB as a calculus software. Fluid properties were obtained using the database software REFPROP (Reference Fluid Thermodynamic and Transport Properties), version 9.0, developed by NIST (National Institute of Standards and Technology). Exhaust gases data were taken from experimental results obtained by Domingues et al. (2013) for a spark ignition 2.8L VR6 engine, by varying the engine speed and torque and can be seen in Table 1. 3

4 Table 1 ICE experimental results obtained by Domingues et al. (2013) Vehicle operating condition ,8 730, ,91 31,7 4,26 20,4 17,0 790, ,75 30,1 8,18 39,1 21,0 829, ,35 26,6 10,96 52,3 23,9 850, ,78 23,5 12,96 61,9 25,9 868, ,3 807, ,98 50,2 6,88 21,9 25,8 897, ,95 49,7 13,67 43,5 31,5 939, ,85 48,2 19,97 63,6 37,9 968, ,77 47,2 26,39 84,0 43,0 989, ,4 869, ,98 67,0 18,45 44,0 43,0 1001, ,98 67,0 37,17 88,7 59,7 1052,3 In the following analysis the Pinch-Point method was used to limit the minimum temperature difference between the exhaust gases and the working fluid to =30ºC, as per Domingues et al. (2013). Simulation with following working fluids water, 90% water-10% ethanol, 80% water-20% ethanol, 70% water-30% ethanol and ethanol were performed regarding the RC conditions presented in Table 2 and with the following additional considerations: Isentropic efficiencies values of the pump ( ) and turbine ( ) are respectively 0,8 and 0,7; Exhaust gases inlet temperature at the evaporator is taken from the ICE experimental values from Table 1; Exhaust gases outlet temperature at the evaporator ( ) is initially considered to be 200ºC; The working fluid is at the saturated vapour state at the turbine outlet and at the saturated liquid state at the pump inlet. Table 2 Turbine maximum inlet temperature ( ) and RC condensation pressure ( for the working fluids under study 100% Water 90% Water 80% Water 70% Water 100% Ethanol 10% Ethanol 20% Ethanol 30% Ethanol 405,0 388,5 372,0 355,5 240,0 101,4 138,7 161,9 177,1 225,3 Figure 2 shows the thermodynamic characteristics of the RC (thermal efficency, volume ratio, working fluid mass flux, net work output rate, exergy destruction rate and exergetic efficiency) as a function of the evaporation pressure for the working fluids tested. Figure 2a shows that the RC working with water demonstrates always a higher thermal efficiency than that for ethanol or water-ethanol mixtures. The same tendency is shown for the volume ratio ( ) in Figure 2b and for the net work output rate in Figure 2d, although for high values of the evaporation pressures ( ) ethanol allows for higher values of than the water-ethanol mixtures. Figure 2c reveals that water is the working fluid which requires a lower mass flux. And in Figure 2e it can be seen that the exergy destruction rate in the evaporator is the highest for ethanol and the lowest for water, which is supported by the high values of the exergetic efficiency for water shown in Figure 2f. It can be concluded that water is the working fluid which presents the best performance, and so it s the chosen fluid for the evaporator HEX design. 4

5 Figure 2 Thermodynamic characteristics of the RC as a function of the evaporation pressure for the working fluids tested. a) Thermal efficiency, b) volume ratio, c) working fluid mass flux, d) net work output rate, e) exergy destruction rate, f) exergetic efficiency Heat Exchanger Design Analysis After the literature review that was conducted, four differente heat exchangers are considered for the present study. Table 3 shows all four different HEX geometries used for simulation with ICE data from Table 1 and their respective designations. 5

6 Table 3 HEX geometries used for simulation with ICE data from Table 1 and respective designations HEX geometry Designation Shell & tubes with 114 tubes (Santos et al., 2013) HEX1 Shell & concentric tubes with 100 tubes HEX2 Tubes with longitudinal plates (Horst et al., 2013) configuration 8.0-3/8T (Kays et al., 1984) HEX3 Flat, concentric and finned tubes (Ambros et al., 2011) HEX4 Figure 3 shows illustrations of HEX1 (a), HEX2 (b), HEX3 (c) and HEX4 (d). Figure 3 Illustrations of the HEXs under study. a) HEX1, b) HEX2, c) HEX3, d) HEX4 In HEX1 (Figure 3a) the exhaust gases flow inside the tubes while water flows in the shell. In HEX2 (Figure 3b shell & concentric tubes heat exchanger), the exhaust gases flow both inside the inner tubes as well as between the outside of the external tubes and the shell walls. Water flows in the annulus formed between tubes in counterflow relative to the inner tubes gas flow. In HEX2 (Figure 3c) the exhaust gases flow in the shell while the water flows in the multi-pass tubes. HEX4 (Figure 3d) is a typical plate heat exchanger with non-mixing counter-flow arrangement. Horst et al. (2013) defined as installation limitations for the heat exchanger the HEX cross section dimensions: = 0,146 m and = 0,132 m. In the present work, vehicle installation limits the maximum HEX cross section to 0,146 m ( ) x 0,154 m ( ) and the maximum HEX length ( ) of 0,3 m. Table 4 shows the HEX characteristics for the maximum = 0,3 m. Table 4 HEX characteristics for = 0,3 m (, and ) HEX1 HEX2 HEX3 HEX4 0,104 0,145 0,146 0,146 0,154 0,123 0,132 0,138 8,46 14,54 7,27 10,35 From Table 4, HEX2 is the heaviest heat exchanger with a mass ( ) of kg. The ε-ntu method was used as was unknown a priori and the HEX was divided in three parts according to the water state: pre-evaporator (sub-cooled state), evaporator (2 phases co-existing state) and superheater (superheated state). 6

7 Heat transfer rate calculation is presented here for HEX2 as it is the most complex HEX under study. The total heat exchange rate for HEX2 ( ) is given by the sum of the contribution of the inner tubes flowing gases ( ) and the gases flowing in the shell ( ), as can be noted in expression (3). (3) Calculation of and by the ε-ntu method is given by expression (4). (4) is the HEX effectivity, that accounts for the flow arrangement, the temperature distribution and physical phenomena like evaporation and condensation in the HEX (Incropera et al., 2008) and is the minimum fluid heat capacity, calculated as,. For HEX2, it s considered that = =. Correlations for heat transfer and pressure loss calculation for the exhaust gases and water were taken from Bell (1981), Bougriou et al. (2010), Incropera et al. (2008), Kays et al. (1984), Kuppan (2000), Saunders (1988), Shah et al. (2003), Thome (2010) and VDI Heat Atlas (2010). Table 5 presents the global heat transfer coefficient ( ) calculation for each of the four HEX geometries in Table 3. The expressions in the table are based on the calculation of the heat transfer for the exhaust gases side. Table 5 Global Heat Transfer Coefficient Calculation for each of the four HEX geometries HEX geometry Global Heat Transfer Coefficient calculation expression HEX1 (5) (6) HEX2 (7) HEX3 (8) HEX4 (9) Table 6 shows HEX s performance parameters (,,, and ) for the vehicle operating condition 13 (see Table 1), considering. Table 6 HEX performance parameters (,, and ) of the HEXs under study for the vehicle operating condition 13 (see Table 1), considering HEX1 HEX2 HEX3 HEX4 353,5 0,2 0,2 0,5 7326,1 0,0 523,7 384,2 561,5 634,2 19,17 17,52 10,98 From Table 6, HEX2 is the one for which the exhaust gases pressure drop is the highest ( =885,6 Pa). HEX3 presents the higher water pressure drop ( =7326,1 Pa). The highest heat transfer rate is 7

8 obtained for HEX2 ( ( = 384,2 ). = 28,97 kw) with the correspondent lowest exhaust gas outlet temperature By demonstrating the highest heat transfer rate while non violating the exhaust gases pressure drop limit of =7,0 kpa, HEX 2 is considered the best HEX to be used for the ICE data in Table 1. After the selection of HEX2, a geometry change was identified which increases the heat exchanger performance. This geometry was obtained by inserting rectangular longitudinal fins in the exterior of the external tubes and in the interior of the internal tubes. This new geometry is named HEX2_1 and the dimensions of both HEX2 and HEX2_1 can be seen in Table 7. Table 7 HEX2 and HEX2_1 dimensions HEX2 HEX2_1 [º] [-] [mm] - [mm] - [-] - [-] - For this new geometry, the global heat transfer coefficient calculation for the shell side is calculated through expression (10). (10) Table 8 shows the results for HEX2 and HEX2_1 simulation with vehicle operating condition 13 of Table 1. Results for other vehicle operating conditions are not shown here for simplicity sake. From Table 8, it can be seen that the heat transfer area for HEX2_1 is nearly twice that for HEX2, and the exhaust gases pressure drop both inside the inner tubes as in the shell increases consequently, altough it doesn t achieve the =7,0 kpa limit. Exergetic efficiency is the same for both heat exchangers ( =0,417) as the temperature distribution difference for both currents is not enough to significantly change this value. The values for HEX2 and HEX2_1 are respectively and. Global effectivity ( ) for HEX2_1 (0,668) is 59% higher than for HEX2. Heat transfer rate ( ) for HEX2_1 is 40,73 kw, which is 40% higher than for HEX2. As a result of this, the water mass flow is higher for HEX2_1 than for HEX2 with a higher working fluid pressure drop. A further study of HEX2 and HEX2_1 is performed for 2 other representative vehicle operating conditions: conditions 3 and 9 of Table 1. Results are presented only for the net mechanical power ( ) and net electrical power ( ) output produced for each vehicle operating condition. 8

9 Table 8 Results for HEX2 and HEX2_1 simulation with vehicle operating condition 13 (see Table 1) Section Preevaporator Evaporator Superheater Total Global Parameters HEX -- HEX2 [-] [-] HEX2_1 [-] [-] -- Note: is calculated through the Bell-Delaware Method for all and so, is not discretized for the 3 HEX sections The ratio of to ICE developed power ( ) and are calculated acording to expressions (11) and (12) respectively. (11) (12) is the mechanical efficiency of the turbine which is considered to be equal to the one for the pump with a value of 0,85, given by Seher et al. (2012). is the electrical efficiency of the turbine coupled generator, considered to be equal to the one for the pump actuator with a value of 0,95. Figure 4a shows the percentage of ICE power recovered by the RC for vehicle operating conditions 3, 9 and 13 (see Table 1) while Figure 4b shows the electrical power generated by the RC for the same vehicle operating conditions. Figure 4 reveals that HEX2 allows for a ratio of net mechanical power to ICE developed power of 12~13% and HEX2_1 allows for 17%-18% value of the same ratio. For vehicle operating condition 13, a RC using HEX2_1 produces about 6 kw net electrical power, while using HEX2 a 4,3 kw net electrical power is produced. Figure 4 Percentage of ICE power recovered by the RC (a) and electrical power generated by the RC (b) for 3 different vehicle operating conditions 3, 9 and 13 (see Table 1) 9

10 Conclusions From the present work it is concluded that for the particular ICE data in study, water is the working fluid that allows higher values of thermal efficiency and turbine power output, as well as lower exergy destruction rates in the evaporator. The shell & concentric tubes heat exchanger (HEX2) is the heat exchanger with highest performance parameters from the initial ensemble of 4 heat exchangers studied for the vehicle s highest exahust gases inlet temperature operating condition. All heat exchangers allow for an exhaust gas pressure drop lower than the 7,0 kpa limit established in this work. A geometric change to HEX2, introducing longitudinal fins on the exhaust gases side (HEX2_1 geometry), allows for a higher conversion of the thermal energy contained in the exhaust gases in electricity to charge the vehicle battery or secondary electrical system. For the highest inlet gas temperature working regime, HEX2_1 allows for a net electrical output of 6,0 kw while HEX2 allows for a 4,3 kw. HEX2_1 is therefore the most promising candidate to be used as the evaporator for the RC to be installed in the vehicle. References Bell, K.J. (1981). Delaware Method for Shellside Design, in Heat Exchangers - Thermal Hydraulic Fundamentals and Design. s.l. : Hemisphere/McGraw-Hill. Bougriou, C. and Baadache, K. (2010). Shell-and-double concentric-tube heat exchangers. Heat Mass Transfer, Vol. 46, pp Chammas, R. and Clodic, D. (2005). Combined Cycle for Hybrid Vehicles. SAE International, Vol Domingues, A., Santos, H. and Costa, M. (2013). Analysis of vehicle exhaust waste heat recovery potential using a Rankine cycle. Energy, Vol. 49, pp Ferrari, G. (2011). Internal Combustion Engines. 1ª. s.l. : Esculapio - Bologna. Gewald, D., Karellas, S., Schuster, A. and Spliethoff, H. (2012). Integrated system approach for increase of engine combined cycle efficiency. Energy Conversion and Management, Vol. 60, pp Hussain, Q. and Brigham, D. (2011). Organic Rankine Cycle for Light Duty Passenger Vehicles. Directions in Engine-Efficiency and Emissions Research (DEER) 2011 Conference. Incropera, F., DeWitt, D., Bergman, T. and Lavine, A. (2008). Fundamentos de Transferência de Calor e de Massa. 6ª. s.l. : LTC - Livros Técnicos e Científicos Editora S.A. Kays, W. and London, A. (1984). Compact Heat Exchangers. 3ª. s.l. : McGraw-Hill Book Company. Kuppan, T. (2000). Heat Exchanger Design Handbook. 10ª. s.l. : Marcel Dekker Inc. Moran, M. and Shapiro, H. (2006). Fundamentals of Engineering Thermodynamics. 5ª. s.l. : John Wiley & Sons, Inc. Obieglo, A.; Ringler, J.; Seifert, M.; Hall and W. (2009). Future EfficientDynamics with Heat Recovery. DEER High Efficiency Engine Technologies. Saunders, E. (1988). Heat exchangers: selection, design & construction. 2ª. s.l. : Longman Scientific & Technical. Seher, D., Lengenfelder, T., Gerhardt, J., Eisenmenger, N., Hackner, M. e Krinn, M. (2012). Waste Heat Recovery for Commercial Vehicles with a Rankine Process, Robert Bosch GmbH, Stuttgart, Germany. 21st Aachen Colloquium Automobile and Engine Technology Shah, R. and Sekulic, D. (2003). Fundamentals of Heat Exchanger Design. 10ª. s.l. : John Wiley & Sons, Inc. Thome, J. R. (2010). Engineering Data Book III. s.l. : Wolverine Tube, Inc., Engineering Thermal Innovation. VDI-GVC. (2010). VDI Heat Atlas. 2ª. s.l. : Springer. 10