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A simulation approach of under-hood thermal management
Advances in Engineering Software 100 (2016) 43–52
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Advances in Engineering Software journal homepage: www.elsevier.com/locate/advengsoft
A simulation approach of under-hood thermal management Guohua Wang a,b, Qing Gao a,b,∗, Tianshi Zhang a,b, Yan Wang a,b a b
State Key Laboratory of Automotive Simulation and Control, Jilin University, 130025 Changchun, China College of Automotive Engineering, Jilin University, 130025 Changchun, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 18 December 2015 Revised 16 June 2016 Accepted 26 June 2016
Among the integrated design and analysis of the vehicle, the integrated simulation of vehicle thermal management system (VTMS) is an important approach. By the approach, the simulation of the multiple thermodynamic systems such as engine cooling system can be realized. In VTMS, the numerical simulation is an important means which is used to discuss the relevance of the multiple thermodynamic systems and the structure change. So the simulation approach coupling 1D simulation and 3D simulation is proposed to realize the comprehensive analysis. The 1D simulation is mainly about the relevance of systems and thermodynamic system cycles. The 3D simulation is mainly about the structure change. The MATLAB routine is used to compile the 1D model program and the combination module. The CFD software is used to compute the 3D simulation model in the multiple thermodynamic systems models. The combination module has the functions that include the 3D model combination and 1D relevance. Because the engine cooling, air conditioning, turbo charged cooling, oil cooling and others are set in the underhood. The under-hood thermal management is the core of the integrated vehicle thermal management. So in this paper, the integrated simulation approach is mainly based on the under-hood 3D model. Through the integrated approach, the heat interaction and the heat flow are simulated and analysis together. The examples are supplied to illustrate the comprehensive and systematic analysis ability by the heat flow characteristics and the thermal interactions between the different systems and the components. © 2016 Elsevier Ltd. All rights reserved.
Keyword: VTMS Integrated simulation 1D/3D Under-hood Multiple thermodynamic systems
1. Introduction In the automotive industry, thermal management is a very important way to solve the problems of energy saving and emission. In the thermal management analysis approaches, the numerical simulation is widely adopted as an important approach. The numerical simulation includes one-dimensional (1D) simulation approach and three-dimensional (3D) simulation approach. Both of these approaches have its advantages and disadvantages [1,2]. The 3D simulation approach can be used to show the details and characteristics of heat flow, but it cannot simulate and discuss the system relevance of the multiple thermodynamic systems [3,4]. The 1D simulation approach can simulate the transient thermodynamic changes, but it cannot show the heat flow characteristics caused by structures and positions. Therefore, the co-simulation methodology of 1D and 3D simulation is achieved to deal these problems [5–7]. Through this approach, the simulation of the multiple thermodynamic systems can be simulated with the heat flow characteristics.
∗
Corresponding author. E-mail addresses: [email protected] [email protected] (Q. Gao).
(G.
http://dx.doi.org/10.1016/j.advengsoft.2016.06.010 0965-9978/© 2016 Elsevier Ltd. All rights reserved.
Wang),
[email protected],
And it also can be established in the whole vehicle, the under-hood and the passenger cabin. Usually, the 1D simulation and the 3D simulation are mainly realized by the independent packages. The independent packages include the 3D CFD software (i.e. Fluent, StarCD, Star-ccm+ and AVL) and 1D thermo-fluid system software (i.e. KULI, Flowmaster) [1]. And the combination approach was most consisted as a kind of the ‘weak combination’ approach. Such as the 1D software (KULI) and various 3D approaches were used to analyze the thermal efficiency of the cooling package [8]. The KULI was used to simulate the heat rejection based on experimental data. The experimental data was transformed into non-dimensional values and scaled the real world components to simulate predictions. The CFD model was only calculated for one typical operating point and supplied the experimental data by simulation. For example, the KULI model was calibrated for cold conditions by the CFD experimental data; afterwards the hot conditions were simulated based on the typical flow. Other approaches included that the principle of flow network modeling (FNM) and Fluent was realized to make the vehicle under-hood air flow and thermal analysis [9]. In this approach, the entire domain in under-hood was broken into many air flow domain, which were modeled in a FNM model by nodes and links. The CFD analysis was used to obtain the pressure drop
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and flow, leading to a characteristic curve for each domain. The distribution of flow and pressure was determined by FNM through solving 1D over the entire flow network. Subsequently, the air temperature distribution in the under-hood domain was predicted by energy analysis. In the discussion of the engine cooling performance, the 1D (KULI) and 3D (Fluent, RADTHERM) simulation were used to evaluate radiator heat dissipation and Top Hose Temperatures (THT) [10]. RADTHERM and FLUENT in a co-simulation approach were used to simulate the temperature distribution in real life test conditions. The convective heat transfer coefficients and air temperatures were calculated by Fluent. RADTHERM was used to calculated the radiation and conductive. The results of 3D simulation were supplied to KULI. By the results, the engine cooling system was simulated by KULI. At the thermal and mass flow distribution of a complete vehicle, 3D CFD tool Star-ccm+ was used with GT-SUITE to simulate the under-hood [11]. Through this means, 1D model of cooling package got the non-uniform boundary conditions for transient simulations from the model. And the CFD simulation supplied the results for flows and averaged temperature levels. In a similar study, an integrated approach of AVL BOOST to KULI software and 3D software (AVL) was established to cover the entire vehicle thermal management in a scalable manner [12]. Under-hood flow was simulated by 3D software (AVL), and engine simulation and cooling system were simulated by coupling of AVL BOOST to KULI. Others like that the STAR–OLGA coupling was applied to the wavy-pipe systems model experiencing hydrodynamic slug flows [13]. The flow characteristics of slug flow in the wavy-pipe systems were investigated. The approach of coupling 1D-3D models was makes to couple between models of different dimensionality [14]. It was realized in a simple heat transfer problem and then was used in the context of modeling blood flow in large vessels. In the process of combination independent packages, 1D software usually gets the data from 3D simulation and transforms into non-dimensional values, so it is possible to scale the real components and predictions. Such as the results of air flow predicted by 3D CFD model were fed into 1D hydraulic model [15]. The methodology allowed large number 1D simulation to be performed based on relatively few 3D simulations. And the data exchange between 1D and 3D was usually done by automatic or human manual operations in combination procedure. In a similar approach, the velocity, pressure and heat transfer coefficients in the coolant jacket were computed by 3D approach using AVL/FIRE software. And software FLOWMASTER2 and EXCEL were utilized to simulate the cooling system under different environment temperatures and speeds of water pump [16]. Other approaches such as that the approach coupling of the Flowmaster and Star-CD was used to analysis the under-hood flow with heat exchanger [17]. In this approach, the Component Object Model (COM) was exploited to link the Flowmaster and Star-CD. So the CFD boundaries were represented in a 1D network using components. The pressures and flow rates were modified at each time step. And the fluid temperature was modified and entered the network to model the heat transfer. It was further reported in that a co-simulation methodology was used to achieve the combination of 1D and 3D fluid flow models [18]. In this methodology, the ‘CFDLink’ was used to data exchange in each numerical iteration step of 1D and 3D computation cycle. Because the data exchange was performed in iteration step automatically, the convergence of 3D computation was not important. Recently, the dynamic coupling algorithm of thermal management simulation was established [19]. Through this algorithm, the heat transfer between the cabin air and the Fiala Physiological Comfort passenger model was coupled in a MATLAB routine to solve a problem of conjugate heat transfer. And physical parameters such as time stepping, heat fluxes, air inlet temperatures were
communicated between the routine and the FLUENT models. The 1D structure model was implemented in MATLAB and exchanged data with the coupling routine. In order to assess the heat flow interaction of cooling package and thermal damage during the real operating conditions of under-hood, this paper presents an integrated simulation approach that is capable of establishing the dynamic interaction between all the individual components. It can realize the real time coupling of multiple thermodynamic systems with computer technology. By this approach, the heat flow characteristics and the system transient correlation characteristics can be simulated and discussed together. The 3D simulation software such as FLUENT is used to model the 3D models. The 3D models include radiator, condenser, intercooler and fan. The MATLAB routine is used to compile the 1D program, which are established the engine cooling system model, turbo charged cooling model and etc. The 1D program also has the functions, includes the controlling of 3D simulation model, the data exchange with external program and automatic running control. The 3D simulation is involved in the whole cycle simulation as a core model. After the integrated simulation, the transient characteristics of the heat flow and the transient characteristics of the system cycle will be obtained together. 2. Integrated vehicle thermal management The vehicle thermal management system (VTMS) mainly includes the under-hood thermal management system and the passenger cabin thermal management system. The analysis of underhood thermal management is mainly about the dynamic interaction and the heat flow in the under-hood. The under-hood thermal management usually includes the engine cooling system, the air conditioning system, the charged air cooling system, and etc. The analysis of passenger cabin thermal management is mainly about the thermal comfort of different environment conditions and the passenger cabin structure. The cabin heat load is ensured by the thermal comfort. So the thermal comfort has a direct influence on the air conditioning system. In the air conditioning system, the condenser is as a heat sink component, which is located in the front of the radiator. The evaporator is as a refrigeration component to ensuring the thermal comfort of the passenger cabin. And the compressor, its power is from the engine. Therefore, through the air conditioning system, the under-hood thermal management and the passenger cabin thermal management are combined to form the thermal management system based on the vehicle. As shown in Fig. 1. In the 3D simulation, 3D simulation software such as FLUENT is used to model the 3D models. The characteristics of heat flow can be simulated and showed by the 3D simulation model, such as the simulation of the radiator and fan in the under-hood. In the 3D model, the flow influence on the heat dissipation capacity of the radiator will be simulated. And the flow influence is caused by the interior structure of under-hood. The radiator is a component of the engine cooling system, so the heat rejection that need to heat transfer is from the engine cooling cycle system which is simulated by 1D model. Therefore, 1D /3D models are needed to be coupled for the influence on the heat dissipation capacity and the heat rejection at same time. The integrated simulation approach is needed to deal the problem. Due to the under-hood has most of the thermal components, including radiator, condenser, intercooler, fan, and etc. And the engine cooling, air conditioning, turbo charged cooling, oil cooling and others are also set in the under-hood. Through combining the under-hood 1D/3D model with these thermal components, the heat interaction characteristics of the multiple thermodynamic systems can be discussed. In this paper, the integrated simulation approach is based on the under-hood 3D model and the 1D
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Fig. 1. Schematic diagram of integrated vehicle thermal management.
Fig. 2. Schematic diagram of 1D/3D combination.
cooling package model. Such as the engine cooling model, the 1D program can be compiled to establish the engine cooling system model by the MATLAB routine. Through the combination module which complied by the MATLAB routine, the engine cooling model couples the 3D model as the special program module in the simulation. And the 1D model and 3D simulation model are controlled and exchanged the data automatically. Therefore, this approach includes three parts: 1D program, 3D CFD model, and the combination module. As shown in Fig. 2. 3. 1D/3D simulation model The integrated simulation approach involves 1D program models and 3D simulation models. The 1D program models includes engine cooling cycle model, turbo charged air cycle model, oil cooling model, air conditioning model. But those models do not model their whole cycle models with 1D simulation approach. Such as in the engine cooling model, the radiator model is modeled as the 3D
simulation model. Other thermodynamic systems have the similar components which modeled in 3D model. 3.1. 1D simulation model In the 1D simulation model, there are three sub-models. They are the engine cooling cycle model, the air conditioning system model, the turbo charged air cycle model. The engine cooling cycle is made up 1D program model and 3D simulation model of the radiator. The 1D program model includes engine cooling model, thermostat model and pump model. Because the oil cooler is used for heat transfer between coolant and oil, oil cooling cycle model is modeled by 1D program. That is shown in Fig. 3. Engine cooling model as main heat source of the whole cycle, its function is providing heat rejection that need to heat transfer by coolant and oil. Through the engine cooling model, the inlet temperatures can be calculated for the radiator model and the oil cooling model. When the driving conditions are given, the speed
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Fig. 4. Schematic diagram of air conditioning model. Fig. 3. Schematics diagram of engine cooling cycle and oil cooling cycle model.
and the load will be calculated. Through the universal characteristic curve and the heat balance characteristic curve, the heat rejection that is needed to heat transfer by coolant and oil will be calculated also. And some parameters of the engine should be known for obtaining the heat rejection first, which include stroke, displacement and numbers of cylinder. Through setting the initial valve of engine inlet temperature, the engine outlet temperature will be calculated by the heat rejection. And the engine outlet temperature will be given to the thermostat model. For the thermostat, the engine outlet temperature is the control variable of the valve opening. The flow to radiator or the flow back to engine is determined by the valve opening. So the thermostat model is needed to realize the flow distribution through the engine outlet temperature. In the thermostat model, the valve opening curve is used to associate the valve opening and the flow temperature. Once getting the engine outlet temperature, the flow distributions will be calculated. For the total flow, it is from the pump model. And the flow valve is obtained by the flow performance curve of the pump. The horizontal coordinates of the curve are the pump speed, and the longitudinal coordinates are the pump flow. The pump speed is proportional to the engine speed. After given the engine speed, the pump flow will be obtained from the curve. The oil cooling model is modeled by 1D program. So it can be simulated through two different ways. One way is fixed power. It adopts the base heat transfer equation to get the outlet temperature with given the heat dissipation of oil cooler. The other way is experimental representation. The table of heat rejection with flow change and the reference inlet temperatures are needed to the characterization of oil cooler. The effectiveness-NTU and number of transfer units are adopted to calculate the outlet temperature. Based on the above models, the inlet parameters of radiator 3D model can be calculated with the inlet temperatures and the flow of the cycle. The radiator outlet temperature of 3D model is got by the integrated control model, and gives it to the 1D program as the inlet parameter of pump model. In the model of the air conditioning system, the model of the condenser is also realized by using 3D model. As shown in Fig. 4. The heat transfer process of the condenser is a phase change heat transfer process. When using the heat-exchanger model to simulate the condenser, its working condition will be defined by the given heat rejection. So in the 1D simulation, the air conditioning system is required to complete the cycle simulation according to the conditions of the passenger cabin, and then obtain the heat rejection of the condenser. The 1D simulation is used to simulate the compressor in the turbocharger. Its function is giving the inlet parameters to the 3D intercooler model. The inlet parameters include the charge air temperature and the mass flow. Because the process of the compressor model is approximate adiabatic compression process, the charge
air temperature can be computed by the formula 1. The formula is shown below:
TK = T0 1 +
1 ηK × τ
πK
κ −1 κ
−1
(1)
Where κ is the adiabatic exponent; π K is the pressure ratio; TK is the outlet temperature of compressor; τ is the cooling coefficient; ηK is the equal isentropic exponent; T0 is the ambient air temperature. Taking into account the air temperature of the outlet compressor is generally below 200◦ C, adiabatic exponent is 1.4. The cooling coefficient of external cooling is general at 1.04 ∼ 1.1. The pressure ratio is the ratio of the compressor outlet pressure and the compressor inlet pressure. The equal isentropic exponent and the pressure ratio are from the characteristic curves of compressor. The inner air flow of intercooler is the air flow of compressor, and it is equal to the engine mass air flow. So the inner air flow of intercooler is calculated according to engine conditions. But the various parameters of the engine is need, such as effective power, fuel consumption, engine total excess air coefficient, air - fuel ratio, and etc. 3.2. 3D simulation model Because the 3D simulation model is based on the under-hood structure, it includes the radiator 3D model, the intercooler 3D model, the condenser 3D model, and the fan 3D model. They are all coupled in the under-hood model and modeled by the heatexchanger model. The effectiveness-NTU and number of transfer units are used in the heat-exchanger model of 3D simulation model. Through these, the heat transfer between the ambient air and the coolant or the inner hot air can be simulated. In the algorithm of the effectiveness-NTU, the effectiveness is defined as the ratio of the actual heat transfer rate. So it can be calculated as shown below.
Q = (qm c p )min ε |(T 1 − T 2 )|
A
(2)
B
Where Q is the heat rejection; qm is the mass flow; cp is the specific heat; ɛ is the effectiveness of the heat exchanger capacity; T1 is the inlet temperature of air; T2 is the inlet temperature of coolant or hot air. The part A in the Eq. (2) is the minimum heat capacity valve between the two side heat transfer medium. The part B is the maximum temperature difference between two side fluids. It is usually the inlet temperature difference. The effectiveness of the heat exchanger capacity is calculated by the heat rejection data table and the reference fluid temperatures. The heat rejection data table is the heat rejection data with the flow of two side heat transfer medium, which can be from experiment or simulation. The reference fluid temperatures are the inlet temperature of ambient air and the inlet temperature of inner coolant. They are
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And the combination module includes the 1D relevance and the 3D combination. 4.1. 1D relevance
Fig. 5. Typical structure of heat exchanger model.
also the temperatures in the experiment or simulation. When the flows and the inlet temperatures are given, the heat rejection will be calculated by the effectiveness of the heat exchanger capacity. Through the heat rejection, the outlet temperatures will be also calculated. The final result will be passed to the 1D simulation model by the integrated control module. In those 3D heat exchanger models, the inlet temperature or the heat rejection and the flow may be given according the parameters of running conditions. These parameters are set by the integrated control model, and its valves are from the 1D simulation model. In the 3D simulation model, the mesh of heat exchanger cores is modeled as rectangular domains. And the heat-exchanger model is adopted in the simulation of heat exchange. By the definition of heat transfer characteristics to the rectangular domain, the heat exchanger process description can be realized in the form of body. Furthermore the rectangular domain is made of small blocks arrangement [20]. As shown in Fig. 5, the rectangular fluid domains are replaced with multiple cells. The number of Passes (nl ), the number of rows/pass (na ) and the number of columns /pass (nb /nl ) are used to define the numbers of heat exchanger fluid cells. So an overall heat exchanger core is described as a certain number of heat transfer units. And the heat transfer characteristics of each heat transfer unit are consistent with the whole heat exchange core body. In this way, the difference of heat transfer caused by the outside air is shown. And this can lead to the change of heat rejection. For the simulating of the airside pressure drop, the porous zone is defined to the rectangular heat exchangers domains. The resistance coefficients are calculated from the curve of pressure drop with velocity, which is provided by experiment or simulation. The Moving Reference Frame (MRF) model is used in the 3D simulation of the fan rotation. The MRF model is a method which the fluid zone of the fan is modeled in the rotating frame of reference, and the surrounding domains are modeled in a stationary frame [21]. The fan blades are included in the fan model, and in stationary. Because the fluid domain surrounding the fan blades is in a rotating frame, the pressure jump and swirl characteristics are impacted by the fan blades as well. The MRF allows individual cell domains to rotate with different speeds, and the parameter of fan speed is only need to set. So the 1D program can control the fan sub-model through setting the fan speed. The calculation method of the 3D model is used in the Kepsilon model and the Realizable algorithm in the K-epsilon model. For the inlet boundary, the ‘Inlet Velocity’ boundary is used to set the speed. The four parameters are needed, which include inlet velocity (m/s), inlet temperature (K), inlet turbulence intensity and hydraulic diameter. The inlet velocity and temperature will be given through the integrated control module. 4. 1D/3D combination The purpose of 1D/3D combination is realizing the cosimulation of the 1D model program and the 3D simulation model. So the combination module is compiled to realize the combining.
The 1D relevance has the function of reading condition parameters, the parameters setting of 1D simulation model, and the control of whole integrated model. The function of reading condition parameters is reading the condition parameters to the 1D/3D models. The condition parameters include the ambient conditions and the power system conditions. The setting parameters model is used to set the parameters of various components in the 1D simulation model, which includes the cooling system parameters, air conditioning system parameters and the charge air system parameters. Through the 1D correlation, the parameters are input to the corresponding model. The operation function is to solve the problem of the operation control of the various sub-models and the convergence of the corresponding integration operations. Take the engine cooling cycle as an example. The engine inlet water temperature as the initial state is given. When the engine driving conditions are given, the engine initial outlet temperature and flow will be obtained from the engine cooling model and the pump model. The flow distribution can be achieved using the thermostat model and above data. For example, one of the flows will go to the radiator inlet. The left will flow back to pump directly. When the inlet temperature and flow of radiator are obtained, the 3D combination will output the data to the batch file. The function of the batch file is to control the running and data output of the 3D radiator simulation model. The simulation model is initiated by the 3D model start module by reading the batch file. After running of the simulation model, the output data is generated which are including the outlet temperature, the outlet flow and the heat rejection. The data of heat rejection obtained will be used to compare with the data of engine heat production. When the integration simulation is needed to continue, the data is given to the mixed flow model which including the outlet temperature and the radiator outlet flow. In the mixed flow model, the data from the 3D radiator model and the thermostat model are mixed and computed. And the new data to the engine cooling are obtained. At this time, one the engine cooling cycle is completed by the integration simulation. Meanwhile combining with the pump flow, the time variable can be added in the integrated simulation. As shown in Fig. 6. 4.2. 3D combination In the integrated approach, the 3D simulation model is used as a component of the whole cycles. And there has many sub-models in the 3D simulation model, such as the radiator model, condenser model and intercooler model. And how to control the 3D simulation model and its internal sub-models is important in the approach. So the 3D combination is used to control the 3D simulation model. The 3D combination includes the 3D start, 3D model parameters setting and the result read. These are realized through the batch files. The 3D combination uses the batch files as the core means of controlling 3D simulation model. The batch files are usually as ∗ .jou file. And the most 3D simulation software itself has ‘Command Lines’ for its command operation. In the ‘Command Lines’, the command codes are used in the interface operation. The functions of the command codes include the boundary conditions setting, the model initialization, the iterative control and the output of result. Those command codes can be edited into the batch files. So through the batch files, a series continuous control to the 3D software is done.
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Fig. 6. Schematic diagram of the engine cooling 1D relevance.
By the 3D combination, the 1D program will modify the contents of the batch files about the boundary conditions which are needed to be modified. The 3D simulation model will be started and read the modified batch files. And then the link will be established between the 3D simulation model and the 1D program. In the integrated simulation process, the results and inlet conditions of 3D model will be exchanged with the 1D model automatically by the 3D combination. For batch files, the combination of multi batch files is used to deal the different control of the 3D model. According to the required function, there have different batch files. They are the batch files about fan model, iterative computation, result report, and cooling package model. In these batch files, the batch file about fan model is used to set the speed of fan. The batch file about iterative computation is used to set the iteration numbers, the initialization and computation on the 3D model. The batch file about result report is used to output the specified calculation results to a text file in the specified format, which is easy to read and counted in the cycle. And the batch files about cooling package model also include the batch files about the radiator model, the condenser model, and intercooler model. Thus the batch files about cooling package model can modify and set the parameters on the heat exchangers, such as the inlet temperature, the flow, the heat transfer data table, the reference fluid temperatures, and etc. And the dif-
ferent batch files are assembled into a specific batch file by the 3D combination. In each cycle, the corresponding batch files will be constantly modified and assembled to the specific batch file. The 3D model will be started and read the specific batch file to complete the operation by the command. And then the result data will be outputted to the 1D simulation model. As shown in Fig. 7. 5. Examples Based on the above approach, the integrated calculation of 1D/3D can be carried out to simulate the characteristics of specific multi thermal systems. Through the integrated calculation, these multi thermal systems under the different driving conditions can be specific are discussed. In this case, the 3D model is based on the under-hood 3D simulation model. The radiator, condenser, intercooler and fan model are in the under-hood 3D simulation model. And their position is fixed, as shown in Fig. 8. In the process of discussion, there are three modes to discuss the thermal characteristics including the engine cooling system mode, the combination mode of engine cooling system and air conditioning, the combination mode of engine cooling system and charge air cooling system. Considering the driving characteristics of the vehicle, the velocity conditions are
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Fig. 7. Schematic diagram of the 3D combination.
Fig. 8. Geometry of under-hood model.
idling, 30 km/h, 60 km/h, 90 km/h, 120 km/h. And the ambient temperature is 23 ◦ C. 5.1. Engine cooling In the engine cooling system model, the velocity condition is 90 km/h. The temperature coolant in the engine cooling system is started to heat up from the initial temperature. In this model, the engine load and the speed are constant. So the flow distribution is controlled by the thermostat. The thermostat distributes the flow according to the outlet temperature of engine. In the first stage, the time is about from 0 to 40 s. The radiator outlet temperature is not changed. The radiator flow is zero. In this stage, the 3D simulation model is not taken part in the integrated calculation. The thermodynamic cycle of this stage is from engine to engine. In the middle stage, the time is about from 40 s to 80 s. The radiator outlet temperature is changed, and the radiator flow is also changed continuously. The 3D simulation model is taken part in the integrated calculation. But the thermodynamic cycle of this stage includes two cycle processes. One is from engine to engine. The other is from engine to radiator, and back to engine. In the stable stage, the time is about from 80 s to end. The radiator outlet temperature and the engine outlet temperature are
not changed, and the radiator flow is also not changed. The temperature difference between the radiator outlet temperature and the engine outlet temperature is stable. So in this stage, the engine cooling system is at the heat balance. That is shown in Fig. 9. By this model, the continuous driving condition can also be discussed. The continuous driving conditions are set from idling to 120 km/h. With the driving conditions change, the heat quantities are increased. In the low velocity, the temperatures of radiator and engine are relatively stable. In the high velocity, the inlet temperature and the radiator flow are raised. And the radiator flow is raised significantly with the continuous driving conditions. The reason is that the flow distribution of the thermostat makes the radiator flow raised. So in the engine cooling system, the flow distribution of the thermostat has a greater impact on the temperature control. That is shown in Fig. 10. 5.2. Engine cooling and air conditioning In the model of engine cooling system and air conditioning, the influence of the air conditioning load changes on the engine cooling cycle is mainly discussed. In this discussion, the driving condition is constant. The velocity is 60 km/h, and engine speed is 20 0 0 rpm, engine load is 70 N.m. The air conditioning load uses
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Fig. 9. The curves of the specific condition. Fig. 11. The curves of the radiator.
Fig. 10. The curves of radiator under the different conditions. Table 1 Air conditioning load table.
Fig. 12. The curves of the condenser. 60 km/h
Condenser heat rejection, W Compressor power load, W
30 0 0 1200
3500 1300
40 0 0 1400
4500 1500
50 0 0 1600
the different condenser heat rejections to discuss, and the compressor power load of is also in different conditions. Its conditions are shown in Table 1. With the air conditioning load changing, the temperature of the radiator coolant is changed. But compared to the engine cooling cycle mode, the temperature of the radiator coolant is raised a little. That is shown in Fig. 11. There are two main reasons to explain the raising. The one reason is that the power source of the air conditioning is from the engine. So the running of the air conditioning lead to the raising of the engine load, thus the heat rejection that the engine needs to be heat transfer lead to the raising of the radiator heat rejection. The other reason is the space position of condenser and radiator. The condenser is set in front of the radiator, which directly affects the heat rejection of the radiator. And the radiator is also affecting the cooling of the condenser. This point can be seen from the air average temperature curves of the condenser, the front air average temperature of the condenser is slightly higher than the ambient temperature in Fig. 12.
Fig. 13. The velocity contour of under-hood.
It shows that there is air reflux into the condenser. As shown in Fig. 13, there is a mutual influence between the radiator and the condenser. The Air through the inlet grille into the under-hood makes the condenser cooling firstly. And then the air from the condenser makes the radiator cooling. Due to the influence of the structure, the air at the bottom part of under-hood flows is returned to the front of the condenser. So it leads to that the air
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Fig. 15. The curves of the intercooler. Fig. 14. The curves of the radiator.
temperature into the radiator is raised. And the heat rejection of radiator is declined. In the high air conditioning system, the radiator outlet temperature does not change with the flow. The main reason is that the total flow is limited by the pump speed, and the pump speed is limited by the engine speed. When the engine speed is fixed, the pump speed and the total flow rate are also fixed. In the high temperature, the valve opening is in maximum value. The flow is not changed with the temperature. The result is that the temperature in the cooling system is further raised, as shown in Fig. 10. This may show that the components of the cooling system do not match each other. 5.3. Engine cooling and charge air cooling In the mode of engine cooling system with charge air cooling system, the driving conditions are 30 km/h, 60 km/h, 90 km/h, 120 km/h, and 150 km/h. The turbocharger system is started when the engine speed is above 1500 rpm. With the turbocharger system, the engine power output is raised. The heat rejection which is needed to heat transfer is raised correspondingly. These lead to the result that the engine outlet temperature and the radiator inlet temperature are correspondingly raised, and the flow of the radiator is also raised. As shown in Fig. 14. In the high velocity, the radiator outlet temperature is closed to 120◦ C. The radiator flow of coolant is still increasing. But the flow increasing is not still meeting the heat transfer. From the heat rejection and the ambient air average temperature of the intercooler, the heat interaction is also occurring as shown in Fig. 15. So this engine cooling system cannot meet the need of the engine cooling. The component of this engine cooling system should be changed, such as radiator, pump and thermostat. From the above discussions, how to realize the reasonable matching of cooling components in the under-hood is the key to the thermal management. 6. Conclusions The integrated simulation approach is used in vehicle thermal management. The integrated control module is compiled by the MATLAB routine in the integrated simulation of the 1D and 3D. This can couple the 1D model and 3D model to a whole heat interaction model. By the approach, the characteristics of heat flow and the heat interaction of multiple thermodynamic systems can
be simulated and discussed together, and the transient characteristics of the system can also be simulated and discussed. Through the case of the under-hood thermal management analysis, the integrated simulation approach is used to simulate the characteristics of heat interaction and the heat flow at same time. With the integrated simulation, the reasonable matching of cooling components can be analyzed and considered. And many controllable adjustment factors can be included with this approach, such as the flow distribution of the thermostat, the position relation of the cooling package, and etc. The more systems can be integrated in thermal management, such as fuel cooling system, thermal comfort, and driving conditions, and etc. Even the electric vehicle battery and electric motor thermal management, all these may be integrated on this approach, and form a wider vehicle thermal management analysis system. Acknowledgements The authors gratefully acknowledge the financial support from the NSFC (National Natural Science Foundation of China) under the grant No. 51376079, Department of Science & Technology of Jilin Province development plan item (No. 20130204018GX), and science & technology development plan item (No. 14KG096) of Changchun, Jilin province. References [1] Pang SC, Kalam MA, Masjuki HH, Hazrat MA. A review on air flow and coolant flow circuit in vehicles’ cooling system. International Journal of Heat and Mass Transfer 2012;55:6295–306. [2] Banjac T, Wurzenberger JC, Katrašnik T. Assessment of engine thermal management through advanced system engineering modeling. Adv Eng Softw 2014;71:19–33. [3] Mao S, Feng Z, Michaelides EE. Off-high way heavy-duty truck under-hood thermal analysis. Appl Thermal Eng 2010;30:1726–33. [4] Bäder D. Interference effects of cooling air flows on a generic car body. J Wind Eng Ind Aerodyn 2013;119:146–57. [5] Bayraktar I. Computational simulation methods for vehicle thermal management. Appl Thermal Eng 2012;36:325–9. [6] Khaled M, Ramadan M, El-Hage H, Elmarakbi A, Harambat F, Peerhossaini H. Review of underhood aerothermal management: Towards vehicle simplified models. Appl Thermal Eng 2014;73:842–58. [7] Wang H, Wang S, Wang X, Li E. Numerical modeling of heat transfer through casting–mould with 3D/1D patched transient heat transfer model. Int J Heat Mass Transf 2015;81:81–9. [8] Stroh C, Reitbauer R and Hanner J, Increasing the reliability of designing a cooling package by applying joint 1D/3D simulation, SAE Paper 2006-01-1571. [9] Kumar V, Kapoor S, Arora G, Dutta P, A combined CFD and flow network modeling approach for vehicle underhood air flow and thermal analysis, SAE Paper 2009–01-1150.
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[10] Kumar V, Shendge SA, Underhood thermal simulation of a small passenger vehicle with rear engine compartment to evaluate and enhance radiator performance, SAE Paper 2010–01-0801. [11] Minovski BB, Lofdahl L, A 1D method for transient simulations of cooling systems with non-uniform temperature and flow boundaries, SAE Paper 2015–010337. [12] Puntigam W, Hörman T et al., Thermal management simulations by coupling of different software packages to a comprehensive system, SAE Paper 2005-012061. [13] Xing L, Yeung H, Geng Y, Cao Y, Shen J. Study on hydrodynamic slug flow mitigation with wavy pipe using a 3D–1D coupling approach. Comput Fluids 2014;99:104–15. [14] Blanco PJ, Feijo´o RA, Urquiza SA. A unified variational approach for coupling 3D–1D models and its blood flow applications. Comput Methods Appl Mech Eng 2007;196:4391–410.
[15] Moffat J, Coupling of 1D and 3D CFD models to predict transient hydraulics in an engine cooling circuit, SAE Paper 2002-01-1285. [16] You-Chang L, Xiao-Hong G, Dan C, Research on cooling system for 4-cylinder diesel engine, SAE Paper 2007-01-2064. [17] Grose DJ and Austin K, Coupling of one dimensional and three dimensional simulation mode, SAE Paper 2001-01-1770. [18] Watanabe N, An 1D-3D integrated numerical simulation for engine cooling problem, SAE Paper 2006-01-1603. [19] Lorenz M, A coupled numerical model to predict heat transfer and passenger thermal comfort in vehicle cabins, SAE Paper 2014–01-0664. [20] Kim HJ, Kim C-J. A numerical analysis for the cooling module related to automobile air-conditioning system. Appl Thermal Eng 2008;28:1896–905. [21] Kim J-H, Seo J-M, Jung JH. Investigation into the development of a bus electrical cooling fan system. Int J Autom Technol 2011;12(3):423 417W22.
Contents lists available at ScienceDirect
Advances in Engineering Software journal homepage: www.elsevier.com/locate/advengsoft
A simulation approach of under-hood thermal management Guohua Wang a,b, Qing Gao a,b,∗, Tianshi Zhang a,b, Yan Wang a,b a b
State Key Laboratory of Automotive Simulation and Control, Jilin University, 130025 Changchun, China College of Automotive Engineering, Jilin University, 130025 Changchun, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 18 December 2015 Revised 16 June 2016 Accepted 26 June 2016
Among the integrated design and analysis of the vehicle, the integrated simulation of vehicle thermal management system (VTMS) is an important approach. By the approach, the simulation of the multiple thermodynamic systems such as engine cooling system can be realized. In VTMS, the numerical simulation is an important means which is used to discuss the relevance of the multiple thermodynamic systems and the structure change. So the simulation approach coupling 1D simulation and 3D simulation is proposed to realize the comprehensive analysis. The 1D simulation is mainly about the relevance of systems and thermodynamic system cycles. The 3D simulation is mainly about the structure change. The MATLAB routine is used to compile the 1D model program and the combination module. The CFD software is used to compute the 3D simulation model in the multiple thermodynamic systems models. The combination module has the functions that include the 3D model combination and 1D relevance. Because the engine cooling, air conditioning, turbo charged cooling, oil cooling and others are set in the underhood. The under-hood thermal management is the core of the integrated vehicle thermal management. So in this paper, the integrated simulation approach is mainly based on the under-hood 3D model. Through the integrated approach, the heat interaction and the heat flow are simulated and analysis together. The examples are supplied to illustrate the comprehensive and systematic analysis ability by the heat flow characteristics and the thermal interactions between the different systems and the components. © 2016 Elsevier Ltd. All rights reserved.
Keyword: VTMS Integrated simulation 1D/3D Under-hood Multiple thermodynamic systems
1. Introduction In the automotive industry, thermal management is a very important way to solve the problems of energy saving and emission. In the thermal management analysis approaches, the numerical simulation is widely adopted as an important approach. The numerical simulation includes one-dimensional (1D) simulation approach and three-dimensional (3D) simulation approach. Both of these approaches have its advantages and disadvantages [1,2]. The 3D simulation approach can be used to show the details and characteristics of heat flow, but it cannot simulate and discuss the system relevance of the multiple thermodynamic systems [3,4]. The 1D simulation approach can simulate the transient thermodynamic changes, but it cannot show the heat flow characteristics caused by structures and positions. Therefore, the co-simulation methodology of 1D and 3D simulation is achieved to deal these problems [5–7]. Through this approach, the simulation of the multiple thermodynamic systems can be simulated with the heat flow characteristics.
∗
Corresponding author. E-mail addresses: [email protected] [email protected] (Q. Gao).
(G.
http://dx.doi.org/10.1016/j.advengsoft.2016.06.010 0965-9978/© 2016 Elsevier Ltd. All rights reserved.
Wang),
[email protected],
And it also can be established in the whole vehicle, the under-hood and the passenger cabin. Usually, the 1D simulation and the 3D simulation are mainly realized by the independent packages. The independent packages include the 3D CFD software (i.e. Fluent, StarCD, Star-ccm+ and AVL) and 1D thermo-fluid system software (i.e. KULI, Flowmaster) [1]. And the combination approach was most consisted as a kind of the ‘weak combination’ approach. Such as the 1D software (KULI) and various 3D approaches were used to analyze the thermal efficiency of the cooling package [8]. The KULI was used to simulate the heat rejection based on experimental data. The experimental data was transformed into non-dimensional values and scaled the real world components to simulate predictions. The CFD model was only calculated for one typical operating point and supplied the experimental data by simulation. For example, the KULI model was calibrated for cold conditions by the CFD experimental data; afterwards the hot conditions were simulated based on the typical flow. Other approaches included that the principle of flow network modeling (FNM) and Fluent was realized to make the vehicle under-hood air flow and thermal analysis [9]. In this approach, the entire domain in under-hood was broken into many air flow domain, which were modeled in a FNM model by nodes and links. The CFD analysis was used to obtain the pressure drop
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and flow, leading to a characteristic curve for each domain. The distribution of flow and pressure was determined by FNM through solving 1D over the entire flow network. Subsequently, the air temperature distribution in the under-hood domain was predicted by energy analysis. In the discussion of the engine cooling performance, the 1D (KULI) and 3D (Fluent, RADTHERM) simulation were used to evaluate radiator heat dissipation and Top Hose Temperatures (THT) [10]. RADTHERM and FLUENT in a co-simulation approach were used to simulate the temperature distribution in real life test conditions. The convective heat transfer coefficients and air temperatures were calculated by Fluent. RADTHERM was used to calculated the radiation and conductive. The results of 3D simulation were supplied to KULI. By the results, the engine cooling system was simulated by KULI. At the thermal and mass flow distribution of a complete vehicle, 3D CFD tool Star-ccm+ was used with GT-SUITE to simulate the under-hood [11]. Through this means, 1D model of cooling package got the non-uniform boundary conditions for transient simulations from the model. And the CFD simulation supplied the results for flows and averaged temperature levels. In a similar study, an integrated approach of AVL BOOST to KULI software and 3D software (AVL) was established to cover the entire vehicle thermal management in a scalable manner [12]. Under-hood flow was simulated by 3D software (AVL), and engine simulation and cooling system were simulated by coupling of AVL BOOST to KULI. Others like that the STAR–OLGA coupling was applied to the wavy-pipe systems model experiencing hydrodynamic slug flows [13]. The flow characteristics of slug flow in the wavy-pipe systems were investigated. The approach of coupling 1D-3D models was makes to couple between models of different dimensionality [14]. It was realized in a simple heat transfer problem and then was used in the context of modeling blood flow in large vessels. In the process of combination independent packages, 1D software usually gets the data from 3D simulation and transforms into non-dimensional values, so it is possible to scale the real components and predictions. Such as the results of air flow predicted by 3D CFD model were fed into 1D hydraulic model [15]. The methodology allowed large number 1D simulation to be performed based on relatively few 3D simulations. And the data exchange between 1D and 3D was usually done by automatic or human manual operations in combination procedure. In a similar approach, the velocity, pressure and heat transfer coefficients in the coolant jacket were computed by 3D approach using AVL/FIRE software. And software FLOWMASTER2 and EXCEL were utilized to simulate the cooling system under different environment temperatures and speeds of water pump [16]. Other approaches such as that the approach coupling of the Flowmaster and Star-CD was used to analysis the under-hood flow with heat exchanger [17]. In this approach, the Component Object Model (COM) was exploited to link the Flowmaster and Star-CD. So the CFD boundaries were represented in a 1D network using components. The pressures and flow rates were modified at each time step. And the fluid temperature was modified and entered the network to model the heat transfer. It was further reported in that a co-simulation methodology was used to achieve the combination of 1D and 3D fluid flow models [18]. In this methodology, the ‘CFDLink’ was used to data exchange in each numerical iteration step of 1D and 3D computation cycle. Because the data exchange was performed in iteration step automatically, the convergence of 3D computation was not important. Recently, the dynamic coupling algorithm of thermal management simulation was established [19]. Through this algorithm, the heat transfer between the cabin air and the Fiala Physiological Comfort passenger model was coupled in a MATLAB routine to solve a problem of conjugate heat transfer. And physical parameters such as time stepping, heat fluxes, air inlet temperatures were
communicated between the routine and the FLUENT models. The 1D structure model was implemented in MATLAB and exchanged data with the coupling routine. In order to assess the heat flow interaction of cooling package and thermal damage during the real operating conditions of under-hood, this paper presents an integrated simulation approach that is capable of establishing the dynamic interaction between all the individual components. It can realize the real time coupling of multiple thermodynamic systems with computer technology. By this approach, the heat flow characteristics and the system transient correlation characteristics can be simulated and discussed together. The 3D simulation software such as FLUENT is used to model the 3D models. The 3D models include radiator, condenser, intercooler and fan. The MATLAB routine is used to compile the 1D program, which are established the engine cooling system model, turbo charged cooling model and etc. The 1D program also has the functions, includes the controlling of 3D simulation model, the data exchange with external program and automatic running control. The 3D simulation is involved in the whole cycle simulation as a core model. After the integrated simulation, the transient characteristics of the heat flow and the transient characteristics of the system cycle will be obtained together. 2. Integrated vehicle thermal management The vehicle thermal management system (VTMS) mainly includes the under-hood thermal management system and the passenger cabin thermal management system. The analysis of underhood thermal management is mainly about the dynamic interaction and the heat flow in the under-hood. The under-hood thermal management usually includes the engine cooling system, the air conditioning system, the charged air cooling system, and etc. The analysis of passenger cabin thermal management is mainly about the thermal comfort of different environment conditions and the passenger cabin structure. The cabin heat load is ensured by the thermal comfort. So the thermal comfort has a direct influence on the air conditioning system. In the air conditioning system, the condenser is as a heat sink component, which is located in the front of the radiator. The evaporator is as a refrigeration component to ensuring the thermal comfort of the passenger cabin. And the compressor, its power is from the engine. Therefore, through the air conditioning system, the under-hood thermal management and the passenger cabin thermal management are combined to form the thermal management system based on the vehicle. As shown in Fig. 1. In the 3D simulation, 3D simulation software such as FLUENT is used to model the 3D models. The characteristics of heat flow can be simulated and showed by the 3D simulation model, such as the simulation of the radiator and fan in the under-hood. In the 3D model, the flow influence on the heat dissipation capacity of the radiator will be simulated. And the flow influence is caused by the interior structure of under-hood. The radiator is a component of the engine cooling system, so the heat rejection that need to heat transfer is from the engine cooling cycle system which is simulated by 1D model. Therefore, 1D /3D models are needed to be coupled for the influence on the heat dissipation capacity and the heat rejection at same time. The integrated simulation approach is needed to deal the problem. Due to the under-hood has most of the thermal components, including radiator, condenser, intercooler, fan, and etc. And the engine cooling, air conditioning, turbo charged cooling, oil cooling and others are also set in the under-hood. Through combining the under-hood 1D/3D model with these thermal components, the heat interaction characteristics of the multiple thermodynamic systems can be discussed. In this paper, the integrated simulation approach is based on the under-hood 3D model and the 1D
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Fig. 1. Schematic diagram of integrated vehicle thermal management.
Fig. 2. Schematic diagram of 1D/3D combination.
cooling package model. Such as the engine cooling model, the 1D program can be compiled to establish the engine cooling system model by the MATLAB routine. Through the combination module which complied by the MATLAB routine, the engine cooling model couples the 3D model as the special program module in the simulation. And the 1D model and 3D simulation model are controlled and exchanged the data automatically. Therefore, this approach includes three parts: 1D program, 3D CFD model, and the combination module. As shown in Fig. 2. 3. 1D/3D simulation model The integrated simulation approach involves 1D program models and 3D simulation models. The 1D program models includes engine cooling cycle model, turbo charged air cycle model, oil cooling model, air conditioning model. But those models do not model their whole cycle models with 1D simulation approach. Such as in the engine cooling model, the radiator model is modeled as the 3D
simulation model. Other thermodynamic systems have the similar components which modeled in 3D model. 3.1. 1D simulation model In the 1D simulation model, there are three sub-models. They are the engine cooling cycle model, the air conditioning system model, the turbo charged air cycle model. The engine cooling cycle is made up 1D program model and 3D simulation model of the radiator. The 1D program model includes engine cooling model, thermostat model and pump model. Because the oil cooler is used for heat transfer between coolant and oil, oil cooling cycle model is modeled by 1D program. That is shown in Fig. 3. Engine cooling model as main heat source of the whole cycle, its function is providing heat rejection that need to heat transfer by coolant and oil. Through the engine cooling model, the inlet temperatures can be calculated for the radiator model and the oil cooling model. When the driving conditions are given, the speed
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Fig. 4. Schematic diagram of air conditioning model. Fig. 3. Schematics diagram of engine cooling cycle and oil cooling cycle model.
and the load will be calculated. Through the universal characteristic curve and the heat balance characteristic curve, the heat rejection that is needed to heat transfer by coolant and oil will be calculated also. And some parameters of the engine should be known for obtaining the heat rejection first, which include stroke, displacement and numbers of cylinder. Through setting the initial valve of engine inlet temperature, the engine outlet temperature will be calculated by the heat rejection. And the engine outlet temperature will be given to the thermostat model. For the thermostat, the engine outlet temperature is the control variable of the valve opening. The flow to radiator or the flow back to engine is determined by the valve opening. So the thermostat model is needed to realize the flow distribution through the engine outlet temperature. In the thermostat model, the valve opening curve is used to associate the valve opening and the flow temperature. Once getting the engine outlet temperature, the flow distributions will be calculated. For the total flow, it is from the pump model. And the flow valve is obtained by the flow performance curve of the pump. The horizontal coordinates of the curve are the pump speed, and the longitudinal coordinates are the pump flow. The pump speed is proportional to the engine speed. After given the engine speed, the pump flow will be obtained from the curve. The oil cooling model is modeled by 1D program. So it can be simulated through two different ways. One way is fixed power. It adopts the base heat transfer equation to get the outlet temperature with given the heat dissipation of oil cooler. The other way is experimental representation. The table of heat rejection with flow change and the reference inlet temperatures are needed to the characterization of oil cooler. The effectiveness-NTU and number of transfer units are adopted to calculate the outlet temperature. Based on the above models, the inlet parameters of radiator 3D model can be calculated with the inlet temperatures and the flow of the cycle. The radiator outlet temperature of 3D model is got by the integrated control model, and gives it to the 1D program as the inlet parameter of pump model. In the model of the air conditioning system, the model of the condenser is also realized by using 3D model. As shown in Fig. 4. The heat transfer process of the condenser is a phase change heat transfer process. When using the heat-exchanger model to simulate the condenser, its working condition will be defined by the given heat rejection. So in the 1D simulation, the air conditioning system is required to complete the cycle simulation according to the conditions of the passenger cabin, and then obtain the heat rejection of the condenser. The 1D simulation is used to simulate the compressor in the turbocharger. Its function is giving the inlet parameters to the 3D intercooler model. The inlet parameters include the charge air temperature and the mass flow. Because the process of the compressor model is approximate adiabatic compression process, the charge
air temperature can be computed by the formula 1. The formula is shown below:
TK = T0 1 +
1 ηK × τ
πK
κ −1 κ
−1
(1)
Where κ is the adiabatic exponent; π K is the pressure ratio; TK is the outlet temperature of compressor; τ is the cooling coefficient; ηK is the equal isentropic exponent; T0 is the ambient air temperature. Taking into account the air temperature of the outlet compressor is generally below 200◦ C, adiabatic exponent is 1.4. The cooling coefficient of external cooling is general at 1.04 ∼ 1.1. The pressure ratio is the ratio of the compressor outlet pressure and the compressor inlet pressure. The equal isentropic exponent and the pressure ratio are from the characteristic curves of compressor. The inner air flow of intercooler is the air flow of compressor, and it is equal to the engine mass air flow. So the inner air flow of intercooler is calculated according to engine conditions. But the various parameters of the engine is need, such as effective power, fuel consumption, engine total excess air coefficient, air - fuel ratio, and etc. 3.2. 3D simulation model Because the 3D simulation model is based on the under-hood structure, it includes the radiator 3D model, the intercooler 3D model, the condenser 3D model, and the fan 3D model. They are all coupled in the under-hood model and modeled by the heatexchanger model. The effectiveness-NTU and number of transfer units are used in the heat-exchanger model of 3D simulation model. Through these, the heat transfer between the ambient air and the coolant or the inner hot air can be simulated. In the algorithm of the effectiveness-NTU, the effectiveness is defined as the ratio of the actual heat transfer rate. So it can be calculated as shown below.
Q = (qm c p )min ε |(T 1 − T 2 )|
A
(2)
B
Where Q is the heat rejection; qm is the mass flow; cp is the specific heat; ɛ is the effectiveness of the heat exchanger capacity; T1 is the inlet temperature of air; T2 is the inlet temperature of coolant or hot air. The part A in the Eq. (2) is the minimum heat capacity valve between the two side heat transfer medium. The part B is the maximum temperature difference between two side fluids. It is usually the inlet temperature difference. The effectiveness of the heat exchanger capacity is calculated by the heat rejection data table and the reference fluid temperatures. The heat rejection data table is the heat rejection data with the flow of two side heat transfer medium, which can be from experiment or simulation. The reference fluid temperatures are the inlet temperature of ambient air and the inlet temperature of inner coolant. They are
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And the combination module includes the 1D relevance and the 3D combination. 4.1. 1D relevance
Fig. 5. Typical structure of heat exchanger model.
also the temperatures in the experiment or simulation. When the flows and the inlet temperatures are given, the heat rejection will be calculated by the effectiveness of the heat exchanger capacity. Through the heat rejection, the outlet temperatures will be also calculated. The final result will be passed to the 1D simulation model by the integrated control module. In those 3D heat exchanger models, the inlet temperature or the heat rejection and the flow may be given according the parameters of running conditions. These parameters are set by the integrated control model, and its valves are from the 1D simulation model. In the 3D simulation model, the mesh of heat exchanger cores is modeled as rectangular domains. And the heat-exchanger model is adopted in the simulation of heat exchange. By the definition of heat transfer characteristics to the rectangular domain, the heat exchanger process description can be realized in the form of body. Furthermore the rectangular domain is made of small blocks arrangement [20]. As shown in Fig. 5, the rectangular fluid domains are replaced with multiple cells. The number of Passes (nl ), the number of rows/pass (na ) and the number of columns /pass (nb /nl ) are used to define the numbers of heat exchanger fluid cells. So an overall heat exchanger core is described as a certain number of heat transfer units. And the heat transfer characteristics of each heat transfer unit are consistent with the whole heat exchange core body. In this way, the difference of heat transfer caused by the outside air is shown. And this can lead to the change of heat rejection. For the simulating of the airside pressure drop, the porous zone is defined to the rectangular heat exchangers domains. The resistance coefficients are calculated from the curve of pressure drop with velocity, which is provided by experiment or simulation. The Moving Reference Frame (MRF) model is used in the 3D simulation of the fan rotation. The MRF model is a method which the fluid zone of the fan is modeled in the rotating frame of reference, and the surrounding domains are modeled in a stationary frame [21]. The fan blades are included in the fan model, and in stationary. Because the fluid domain surrounding the fan blades is in a rotating frame, the pressure jump and swirl characteristics are impacted by the fan blades as well. The MRF allows individual cell domains to rotate with different speeds, and the parameter of fan speed is only need to set. So the 1D program can control the fan sub-model through setting the fan speed. The calculation method of the 3D model is used in the Kepsilon model and the Realizable algorithm in the K-epsilon model. For the inlet boundary, the ‘Inlet Velocity’ boundary is used to set the speed. The four parameters are needed, which include inlet velocity (m/s), inlet temperature (K), inlet turbulence intensity and hydraulic diameter. The inlet velocity and temperature will be given through the integrated control module. 4. 1D/3D combination The purpose of 1D/3D combination is realizing the cosimulation of the 1D model program and the 3D simulation model. So the combination module is compiled to realize the combining.
The 1D relevance has the function of reading condition parameters, the parameters setting of 1D simulation model, and the control of whole integrated model. The function of reading condition parameters is reading the condition parameters to the 1D/3D models. The condition parameters include the ambient conditions and the power system conditions. The setting parameters model is used to set the parameters of various components in the 1D simulation model, which includes the cooling system parameters, air conditioning system parameters and the charge air system parameters. Through the 1D correlation, the parameters are input to the corresponding model. The operation function is to solve the problem of the operation control of the various sub-models and the convergence of the corresponding integration operations. Take the engine cooling cycle as an example. The engine inlet water temperature as the initial state is given. When the engine driving conditions are given, the engine initial outlet temperature and flow will be obtained from the engine cooling model and the pump model. The flow distribution can be achieved using the thermostat model and above data. For example, one of the flows will go to the radiator inlet. The left will flow back to pump directly. When the inlet temperature and flow of radiator are obtained, the 3D combination will output the data to the batch file. The function of the batch file is to control the running and data output of the 3D radiator simulation model. The simulation model is initiated by the 3D model start module by reading the batch file. After running of the simulation model, the output data is generated which are including the outlet temperature, the outlet flow and the heat rejection. The data of heat rejection obtained will be used to compare with the data of engine heat production. When the integration simulation is needed to continue, the data is given to the mixed flow model which including the outlet temperature and the radiator outlet flow. In the mixed flow model, the data from the 3D radiator model and the thermostat model are mixed and computed. And the new data to the engine cooling are obtained. At this time, one the engine cooling cycle is completed by the integration simulation. Meanwhile combining with the pump flow, the time variable can be added in the integrated simulation. As shown in Fig. 6. 4.2. 3D combination In the integrated approach, the 3D simulation model is used as a component of the whole cycles. And there has many sub-models in the 3D simulation model, such as the radiator model, condenser model and intercooler model. And how to control the 3D simulation model and its internal sub-models is important in the approach. So the 3D combination is used to control the 3D simulation model. The 3D combination includes the 3D start, 3D model parameters setting and the result read. These are realized through the batch files. The 3D combination uses the batch files as the core means of controlling 3D simulation model. The batch files are usually as ∗ .jou file. And the most 3D simulation software itself has ‘Command Lines’ for its command operation. In the ‘Command Lines’, the command codes are used in the interface operation. The functions of the command codes include the boundary conditions setting, the model initialization, the iterative control and the output of result. Those command codes can be edited into the batch files. So through the batch files, a series continuous control to the 3D software is done.
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Fig. 6. Schematic diagram of the engine cooling 1D relevance.
By the 3D combination, the 1D program will modify the contents of the batch files about the boundary conditions which are needed to be modified. The 3D simulation model will be started and read the modified batch files. And then the link will be established between the 3D simulation model and the 1D program. In the integrated simulation process, the results and inlet conditions of 3D model will be exchanged with the 1D model automatically by the 3D combination. For batch files, the combination of multi batch files is used to deal the different control of the 3D model. According to the required function, there have different batch files. They are the batch files about fan model, iterative computation, result report, and cooling package model. In these batch files, the batch file about fan model is used to set the speed of fan. The batch file about iterative computation is used to set the iteration numbers, the initialization and computation on the 3D model. The batch file about result report is used to output the specified calculation results to a text file in the specified format, which is easy to read and counted in the cycle. And the batch files about cooling package model also include the batch files about the radiator model, the condenser model, and intercooler model. Thus the batch files about cooling package model can modify and set the parameters on the heat exchangers, such as the inlet temperature, the flow, the heat transfer data table, the reference fluid temperatures, and etc. And the dif-
ferent batch files are assembled into a specific batch file by the 3D combination. In each cycle, the corresponding batch files will be constantly modified and assembled to the specific batch file. The 3D model will be started and read the specific batch file to complete the operation by the command. And then the result data will be outputted to the 1D simulation model. As shown in Fig. 7. 5. Examples Based on the above approach, the integrated calculation of 1D/3D can be carried out to simulate the characteristics of specific multi thermal systems. Through the integrated calculation, these multi thermal systems under the different driving conditions can be specific are discussed. In this case, the 3D model is based on the under-hood 3D simulation model. The radiator, condenser, intercooler and fan model are in the under-hood 3D simulation model. And their position is fixed, as shown in Fig. 8. In the process of discussion, there are three modes to discuss the thermal characteristics including the engine cooling system mode, the combination mode of engine cooling system and air conditioning, the combination mode of engine cooling system and charge air cooling system. Considering the driving characteristics of the vehicle, the velocity conditions are
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Fig. 7. Schematic diagram of the 3D combination.
Fig. 8. Geometry of under-hood model.
idling, 30 km/h, 60 km/h, 90 km/h, 120 km/h. And the ambient temperature is 23 ◦ C. 5.1. Engine cooling In the engine cooling system model, the velocity condition is 90 km/h. The temperature coolant in the engine cooling system is started to heat up from the initial temperature. In this model, the engine load and the speed are constant. So the flow distribution is controlled by the thermostat. The thermostat distributes the flow according to the outlet temperature of engine. In the first stage, the time is about from 0 to 40 s. The radiator outlet temperature is not changed. The radiator flow is zero. In this stage, the 3D simulation model is not taken part in the integrated calculation. The thermodynamic cycle of this stage is from engine to engine. In the middle stage, the time is about from 40 s to 80 s. The radiator outlet temperature is changed, and the radiator flow is also changed continuously. The 3D simulation model is taken part in the integrated calculation. But the thermodynamic cycle of this stage includes two cycle processes. One is from engine to engine. The other is from engine to radiator, and back to engine. In the stable stage, the time is about from 80 s to end. The radiator outlet temperature and the engine outlet temperature are
not changed, and the radiator flow is also not changed. The temperature difference between the radiator outlet temperature and the engine outlet temperature is stable. So in this stage, the engine cooling system is at the heat balance. That is shown in Fig. 9. By this model, the continuous driving condition can also be discussed. The continuous driving conditions are set from idling to 120 km/h. With the driving conditions change, the heat quantities are increased. In the low velocity, the temperatures of radiator and engine are relatively stable. In the high velocity, the inlet temperature and the radiator flow are raised. And the radiator flow is raised significantly with the continuous driving conditions. The reason is that the flow distribution of the thermostat makes the radiator flow raised. So in the engine cooling system, the flow distribution of the thermostat has a greater impact on the temperature control. That is shown in Fig. 10. 5.2. Engine cooling and air conditioning In the model of engine cooling system and air conditioning, the influence of the air conditioning load changes on the engine cooling cycle is mainly discussed. In this discussion, the driving condition is constant. The velocity is 60 km/h, and engine speed is 20 0 0 rpm, engine load is 70 N.m. The air conditioning load uses
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Fig. 9. The curves of the specific condition. Fig. 11. The curves of the radiator.
Fig. 10. The curves of radiator under the different conditions. Table 1 Air conditioning load table.
Fig. 12. The curves of the condenser. 60 km/h
Condenser heat rejection, W Compressor power load, W
30 0 0 1200
3500 1300
40 0 0 1400
4500 1500
50 0 0 1600
the different condenser heat rejections to discuss, and the compressor power load of is also in different conditions. Its conditions are shown in Table 1. With the air conditioning load changing, the temperature of the radiator coolant is changed. But compared to the engine cooling cycle mode, the temperature of the radiator coolant is raised a little. That is shown in Fig. 11. There are two main reasons to explain the raising. The one reason is that the power source of the air conditioning is from the engine. So the running of the air conditioning lead to the raising of the engine load, thus the heat rejection that the engine needs to be heat transfer lead to the raising of the radiator heat rejection. The other reason is the space position of condenser and radiator. The condenser is set in front of the radiator, which directly affects the heat rejection of the radiator. And the radiator is also affecting the cooling of the condenser. This point can be seen from the air average temperature curves of the condenser, the front air average temperature of the condenser is slightly higher than the ambient temperature in Fig. 12.
Fig. 13. The velocity contour of under-hood.
It shows that there is air reflux into the condenser. As shown in Fig. 13, there is a mutual influence between the radiator and the condenser. The Air through the inlet grille into the under-hood makes the condenser cooling firstly. And then the air from the condenser makes the radiator cooling. Due to the influence of the structure, the air at the bottom part of under-hood flows is returned to the front of the condenser. So it leads to that the air
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Fig. 15. The curves of the intercooler. Fig. 14. The curves of the radiator.
temperature into the radiator is raised. And the heat rejection of radiator is declined. In the high air conditioning system, the radiator outlet temperature does not change with the flow. The main reason is that the total flow is limited by the pump speed, and the pump speed is limited by the engine speed. When the engine speed is fixed, the pump speed and the total flow rate are also fixed. In the high temperature, the valve opening is in maximum value. The flow is not changed with the temperature. The result is that the temperature in the cooling system is further raised, as shown in Fig. 10. This may show that the components of the cooling system do not match each other. 5.3. Engine cooling and charge air cooling In the mode of engine cooling system with charge air cooling system, the driving conditions are 30 km/h, 60 km/h, 90 km/h, 120 km/h, and 150 km/h. The turbocharger system is started when the engine speed is above 1500 rpm. With the turbocharger system, the engine power output is raised. The heat rejection which is needed to heat transfer is raised correspondingly. These lead to the result that the engine outlet temperature and the radiator inlet temperature are correspondingly raised, and the flow of the radiator is also raised. As shown in Fig. 14. In the high velocity, the radiator outlet temperature is closed to 120◦ C. The radiator flow of coolant is still increasing. But the flow increasing is not still meeting the heat transfer. From the heat rejection and the ambient air average temperature of the intercooler, the heat interaction is also occurring as shown in Fig. 15. So this engine cooling system cannot meet the need of the engine cooling. The component of this engine cooling system should be changed, such as radiator, pump and thermostat. From the above discussions, how to realize the reasonable matching of cooling components in the under-hood is the key to the thermal management. 6. Conclusions The integrated simulation approach is used in vehicle thermal management. The integrated control module is compiled by the MATLAB routine in the integrated simulation of the 1D and 3D. This can couple the 1D model and 3D model to a whole heat interaction model. By the approach, the characteristics of heat flow and the heat interaction of multiple thermodynamic systems can
be simulated and discussed together, and the transient characteristics of the system can also be simulated and discussed. Through the case of the under-hood thermal management analysis, the integrated simulation approach is used to simulate the characteristics of heat interaction and the heat flow at same time. With the integrated simulation, the reasonable matching of cooling components can be analyzed and considered. And many controllable adjustment factors can be included with this approach, such as the flow distribution of the thermostat, the position relation of the cooling package, and etc. The more systems can be integrated in thermal management, such as fuel cooling system, thermal comfort, and driving conditions, and etc. Even the electric vehicle battery and electric motor thermal management, all these may be integrated on this approach, and form a wider vehicle thermal management analysis system. Acknowledgements The authors gratefully acknowledge the financial support from the NSFC (National Natural Science Foundation of China) under the grant No. 51376079, Department of Science & Technology of Jilin Province development plan item (No. 20130204018GX), and science & technology development plan item (No. 14KG096) of Changchun, Jilin province. References [1] Pang SC, Kalam MA, Masjuki HH, Hazrat MA. A review on air flow and coolant flow circuit in vehicles’ cooling system. International Journal of Heat and Mass Transfer 2012;55:6295–306. [2] Banjac T, Wurzenberger JC, Katrašnik T. Assessment of engine thermal management through advanced system engineering modeling. Adv Eng Softw 2014;71:19–33. [3] Mao S, Feng Z, Michaelides EE. Off-high way heavy-duty truck under-hood thermal analysis. Appl Thermal Eng 2010;30:1726–33. [4] Bäder D. Interference effects of cooling air flows on a generic car body. J Wind Eng Ind Aerodyn 2013;119:146–57. [5] Bayraktar I. Computational simulation methods for vehicle thermal management. Appl Thermal Eng 2012;36:325–9. [6] Khaled M, Ramadan M, El-Hage H, Elmarakbi A, Harambat F, Peerhossaini H. Review of underhood aerothermal management: Towards vehicle simplified models. Appl Thermal Eng 2014;73:842–58. [7] Wang H, Wang S, Wang X, Li E. Numerical modeling of heat transfer through casting–mould with 3D/1D patched transient heat transfer model. Int J Heat Mass Transf 2015;81:81–9. [8] Stroh C, Reitbauer R and Hanner J, Increasing the reliability of designing a cooling package by applying joint 1D/3D simulation, SAE Paper 2006-01-1571. [9] Kumar V, Kapoor S, Arora G, Dutta P, A combined CFD and flow network modeling approach for vehicle underhood air flow and thermal analysis, SAE Paper 2009–01-1150.
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