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Flat panels heat exchangers to improve the thermal management
Flat panels heat exchangers to improve the thermal management C MALVICINO, F MATTIELLO, R SECCARDINI and M ROSTAGNO Centro Ricerche Fiat S.c.p.A., Italy
ABSTRACT The introduction of the new engine generations and new powertrains will require to improve and extend the temperature control of subsystems and this implies the increase of the heat rejection demand and, as a consequence, of the heat exchange surfaces and of the constraints for the aerodynamics. To face with this issue the Centro Ricerche Fiat, evaluated the use of part of the vehicle body panels in heat exchangers. The paper present the first experimental results achieved on a A-segment car prototype giving a glance on the fallout of the concept. 1
INTRODUCTION
At least one third of the combustion energy should be dissipated as heat through the cooling systems so the heat rejection is a very important vehicle function that also has a relevant impact on the vehicle architecture and aerodynamic. The basic idea is to enhance the heat rejection capacity using the vehicle body panel as heat exchanger so to limit as much as possible the need of finned heat exchangers and forced ventilation allowing the redesign of the vehicle front to enable the aerodynamic improvement and reducing the overall vehicle energy demand. In figure 1 is schematised the concept the Centro Ricerche Fiat analysed: the engine hood and the aerodynamic under-body panel have become also heat exchangers and integrated in the a dual level vehicle cooling system [1]. The upper panel (engine hood
Standard Production Vehicle
Cooling Drag
Engine Hood Exchanger
Aerodynamic Front Under Body Exchanger
Figure 1: the use of heat exchangers integrated in the body panels allows to improve the vehicle aerodynamics and reduce the need of forced ventilation
______________________________________________ Copyright of the author(s) and/or their employer(s), 2011
91
is devoted to the low temperature heat rejection and the lower panel to the high temperature heat rejection to avoid any risk of burning. The dual level cooling system is constituted by two cooling lops: one devoted to the high temperature heat rejection (e.g. thermal engine) and the other to the liquid cooling of all the subsystems (e.g. charge air cooler, air conditioning condenser, …). To validate the concept a Fiat Panda with a 1.2 l gasoline engine has been selected as vehicle demonstrators and prototype heat exchanger have been design and realised. 2
PROTOTYPE HEAT EXCHANGERS
The so-called roll bond heat exchangers, widely used in refrigeration domain, has been selected to realise the preliminary prototypes of the heat exchangers. The roll bond technology is similar to lamination and allows to realise low cost channelled panels. Two aluminium sheet are laminated together and the channels shape and size is obtained by printing one of the sheets the selected pattern. The two inner surfaces are bonded thanks to an autogenous welding that combines temperature (preheating) and pressure (hot lamination) effects that is prevented in the printed areas (a special ink is used). The final panel is then inflated at high pressure creating the channels.
Figure 2: Roll Bond heat exchangers
At first, a small rectangular heat exchanger - 450 x 170 mm2, 1.8 mm thick - has been characterised to evaluate its heat transfer characteristic so to estimate the performance when used in the automotive application. The exchanger has been placed in a small wind tunnel and flown with coolant (water glycol mixture) at controlled temperature measuring the heat transfer characteristic for different value of air flow and coolant flow keeping a temperature differential of 70 °C. The results of the tests are summarized in figures 4a and 4b and compared with the performance of a conventional radiator for two values of coolant.
Figure 3 The tested roll bond exchanger
92
Figure 4a – Heat Transfer characteristic of the roll bond heat exchanger (Flat heat Exchanger) compared with a standard production finned radiator
Figure 4b – The roll bond heat transfer is higher at low air speed and much lower at high air speed The graphs highlight that the roll bond heat exchanger has quite good heat transfer properties at low air speed but has lower performance at high speed when compare with a conventional last generation radiator. In addition, the test puts in evidence the high pressure drop on coolant side of the tested sample (20 kPa @ 200 kg/h). It has be then decided to proceed with the design of a new patter to reduce the pressure drop and possibly enhancing the heat transfer properties. After a study carried out by means of virtual CFD (Computational Fluid Dynamics) different patterns have been considered and evaluated and at the end the most promising one selected (Figure 5).
93
Figure 5 – Designed pattern for the roll bond heat exchanger A series of samples with the new pattern and having the same dimensions of the ones previously evaluated has been realized cooperating with the Indian company Annapurna Earcanal Ltd. specialized in the roll bond technology. The new panels have been tested showing a lower pressure drop (6 kPa @ 200 kg/h) and equivalent heat transfer properties. VEHICLE DEMONSTRATOR
A electronic valve allows to control the coolant flow through the radiator so to, at first use only the flat panel to reject the engine heat and then to direct part of the coolant flow on the radiator. A control unit acquires the sensor signal and controls the valves keeping the coolant temperature under control.
94
P2 T2
FHE SUP
T3 HEATER
Using the above defined geometry a vehicle demonstrator has been realized integrating two large roll bond panels (Flat Heat Exchangers – FHE) , 0.7 x 0.7 m ca., as showed in Figure 6: one as under-engine shield and one the second as engine hood.
RADIATOR
3
ENGINE
P1 T1
THERMOSTAT
T4
P3 VALVE 2
FHE INF VALVE 1
Figure 6 – Scheme of the cooling loop with the roll bond heat exchangers
Figure 7 – Vehicle demonstrator: the lower heat exchanger is not visible The panels pressure drop has been measured generating a coolant flow at controlled temperature. The measured values highlight that (Figure 8) that, as expected, that the two panel has an higher pressure drop than the standard production radiator.
Figure 8 – Pressure drop of the roll bond heat exchangers and of the standard production radiator The vehicle demonstrator has been submitted to a series of test in climatic wind tunnel before the installation of roll bond heat exchanger (baseline) and after. The tests are performed simulating severe thermal load conditions both at low speed and high speed. The results of the test are summarized in the Figure 9 and in the following Table 1.
95
Figure 9 – The curves tests performed in the climatic wind tunnel show that the flat heat exchangers (FHE) allow to reject more than the 60% in all the conditions and all the heat at idle. The table below reports the numeric values showed in the graph Table 1 –Heat rejected comparison
The results show that: 1. the two roll bond heat exchangers (Flat Heat Exchangers –FHE) are able to transfer more than the 60% of the whole heat in all the test conditions 2. the radiator is needed but could be reduced 3. at idle, the radiator and the fan can be replaced by the FHE
96
4
CONCLUSIONS AND NEXT STEPS
The preliminary validation of the benefit related to the use of the vehicle body panels as heat exchanger to enhance the vehicle heat rejection capacity are very promising: •
the two panels installed on the engine hood and below the engine are able to exchange more than the 60% of the heat to be dissipated in case of severe thermal load;
•
at idle the Flat Heat Exchangers can replace totally the radiator not requiring the use of the fan.
Engine Hood Flat Heat Exchnager
Water Cooled Charge Air Cooler
High Temperature Radiator
Low Temperature Radiator
Water Cooled Condenser
Flat Heat Exchanger in the aerodynamic underbody
Figure 10 – The integration of the Flat Heat Exchangers in a system having a dual loop cooling system enables to maximize the benefits of the concept. These results confirms that is possible to enhance the vehicle heat transfer capacity enabling the redesign of the front thermal module and of the front of the vehicle but further steps are required to fully validate the concept and to demonstrate its technical feasibility: • the identification of a suitable technology to allow the realization of an engine hood with acceptable aesthetical features • the integration of the lower FHE in an aerodynamic underbody panel (Figure 10) • the integration of the Flat Heat Exchangers concept on a vehicle with dual level cooling system [1] These steps will be pursued within the EU project TIFFE and focused to the development of an advanced thermal system for light commercial vehicles.
97
5
REFERENCES
[1]
Dual Levels Vehicle Heat Rejection System, C. Malvicino, F. Di Sciullo, M. Cuniberti, Centro Ricerche Fiat S.C.p.A.F. Vestrelli, F. Beltramelli, Fiat Group Automobiles, VTMS 10 Ashrae Handbook, Fundamentals, American Society of Heating Refrigerating and Air Conditioning Engineers, ASHRAE, New York, 2001, ISBN 1-883413-88-5.
[2]
98
ABSTRACT The introduction of the new engine generations and new powertrains will require to improve and extend the temperature control of subsystems and this implies the increase of the heat rejection demand and, as a consequence, of the heat exchange surfaces and of the constraints for the aerodynamics. To face with this issue the Centro Ricerche Fiat, evaluated the use of part of the vehicle body panels in heat exchangers. The paper present the first experimental results achieved on a A-segment car prototype giving a glance on the fallout of the concept. 1
INTRODUCTION
At least one third of the combustion energy should be dissipated as heat through the cooling systems so the heat rejection is a very important vehicle function that also has a relevant impact on the vehicle architecture and aerodynamic. The basic idea is to enhance the heat rejection capacity using the vehicle body panel as heat exchanger so to limit as much as possible the need of finned heat exchangers and forced ventilation allowing the redesign of the vehicle front to enable the aerodynamic improvement and reducing the overall vehicle energy demand. In figure 1 is schematised the concept the Centro Ricerche Fiat analysed: the engine hood and the aerodynamic under-body panel have become also heat exchangers and integrated in the a dual level vehicle cooling system [1]. The upper panel (engine hood
Standard Production Vehicle
Cooling Drag
Engine Hood Exchanger
Aerodynamic Front Under Body Exchanger
Figure 1: the use of heat exchangers integrated in the body panels allows to improve the vehicle aerodynamics and reduce the need of forced ventilation
______________________________________________ Copyright of the author(s) and/or their employer(s), 2011
91
is devoted to the low temperature heat rejection and the lower panel to the high temperature heat rejection to avoid any risk of burning. The dual level cooling system is constituted by two cooling lops: one devoted to the high temperature heat rejection (e.g. thermal engine) and the other to the liquid cooling of all the subsystems (e.g. charge air cooler, air conditioning condenser, …). To validate the concept a Fiat Panda with a 1.2 l gasoline engine has been selected as vehicle demonstrators and prototype heat exchanger have been design and realised. 2
PROTOTYPE HEAT EXCHANGERS
The so-called roll bond heat exchangers, widely used in refrigeration domain, has been selected to realise the preliminary prototypes of the heat exchangers. The roll bond technology is similar to lamination and allows to realise low cost channelled panels. Two aluminium sheet are laminated together and the channels shape and size is obtained by printing one of the sheets the selected pattern. The two inner surfaces are bonded thanks to an autogenous welding that combines temperature (preheating) and pressure (hot lamination) effects that is prevented in the printed areas (a special ink is used). The final panel is then inflated at high pressure creating the channels.
Figure 2: Roll Bond heat exchangers
At first, a small rectangular heat exchanger - 450 x 170 mm2, 1.8 mm thick - has been characterised to evaluate its heat transfer characteristic so to estimate the performance when used in the automotive application. The exchanger has been placed in a small wind tunnel and flown with coolant (water glycol mixture) at controlled temperature measuring the heat transfer characteristic for different value of air flow and coolant flow keeping a temperature differential of 70 °C. The results of the tests are summarized in figures 4a and 4b and compared with the performance of a conventional radiator for two values of coolant.
Figure 3 The tested roll bond exchanger
92
Figure 4a – Heat Transfer characteristic of the roll bond heat exchanger (Flat heat Exchanger) compared with a standard production finned radiator
Figure 4b – The roll bond heat transfer is higher at low air speed and much lower at high air speed The graphs highlight that the roll bond heat exchanger has quite good heat transfer properties at low air speed but has lower performance at high speed when compare with a conventional last generation radiator. In addition, the test puts in evidence the high pressure drop on coolant side of the tested sample (20 kPa @ 200 kg/h). It has be then decided to proceed with the design of a new patter to reduce the pressure drop and possibly enhancing the heat transfer properties. After a study carried out by means of virtual CFD (Computational Fluid Dynamics) different patterns have been considered and evaluated and at the end the most promising one selected (Figure 5).
93
Figure 5 – Designed pattern for the roll bond heat exchanger A series of samples with the new pattern and having the same dimensions of the ones previously evaluated has been realized cooperating with the Indian company Annapurna Earcanal Ltd. specialized in the roll bond technology. The new panels have been tested showing a lower pressure drop (6 kPa @ 200 kg/h) and equivalent heat transfer properties. VEHICLE DEMONSTRATOR
A electronic valve allows to control the coolant flow through the radiator so to, at first use only the flat panel to reject the engine heat and then to direct part of the coolant flow on the radiator. A control unit acquires the sensor signal and controls the valves keeping the coolant temperature under control.
94
P2 T2
FHE SUP
T3 HEATER
Using the above defined geometry a vehicle demonstrator has been realized integrating two large roll bond panels (Flat Heat Exchangers – FHE) , 0.7 x 0.7 m ca., as showed in Figure 6: one as under-engine shield and one the second as engine hood.
RADIATOR
3
ENGINE
P1 T1
THERMOSTAT
T4
P3 VALVE 2
FHE INF VALVE 1
Figure 6 – Scheme of the cooling loop with the roll bond heat exchangers
Figure 7 – Vehicle demonstrator: the lower heat exchanger is not visible The panels pressure drop has been measured generating a coolant flow at controlled temperature. The measured values highlight that (Figure 8) that, as expected, that the two panel has an higher pressure drop than the standard production radiator.
Figure 8 – Pressure drop of the roll bond heat exchangers and of the standard production radiator The vehicle demonstrator has been submitted to a series of test in climatic wind tunnel before the installation of roll bond heat exchanger (baseline) and after. The tests are performed simulating severe thermal load conditions both at low speed and high speed. The results of the test are summarized in the Figure 9 and in the following Table 1.
95
Figure 9 – The curves tests performed in the climatic wind tunnel show that the flat heat exchangers (FHE) allow to reject more than the 60% in all the conditions and all the heat at idle. The table below reports the numeric values showed in the graph Table 1 –Heat rejected comparison
The results show that: 1. the two roll bond heat exchangers (Flat Heat Exchangers –FHE) are able to transfer more than the 60% of the whole heat in all the test conditions 2. the radiator is needed but could be reduced 3. at idle, the radiator and the fan can be replaced by the FHE
96
4
CONCLUSIONS AND NEXT STEPS
The preliminary validation of the benefit related to the use of the vehicle body panels as heat exchanger to enhance the vehicle heat rejection capacity are very promising: •
the two panels installed on the engine hood and below the engine are able to exchange more than the 60% of the heat to be dissipated in case of severe thermal load;
•
at idle the Flat Heat Exchangers can replace totally the radiator not requiring the use of the fan.
Engine Hood Flat Heat Exchnager
Water Cooled Charge Air Cooler
High Temperature Radiator
Low Temperature Radiator
Water Cooled Condenser
Flat Heat Exchanger in the aerodynamic underbody
Figure 10 – The integration of the Flat Heat Exchangers in a system having a dual loop cooling system enables to maximize the benefits of the concept. These results confirms that is possible to enhance the vehicle heat transfer capacity enabling the redesign of the front thermal module and of the front of the vehicle but further steps are required to fully validate the concept and to demonstrate its technical feasibility: • the identification of a suitable technology to allow the realization of an engine hood with acceptable aesthetical features • the integration of the lower FHE in an aerodynamic underbody panel (Figure 10) • the integration of the Flat Heat Exchangers concept on a vehicle with dual level cooling system [1] These steps will be pursued within the EU project TIFFE and focused to the development of an advanced thermal system for light commercial vehicles.
97
5
REFERENCES
[1]
Dual Levels Vehicle Heat Rejection System, C. Malvicino, F. Di Sciullo, M. Cuniberti, Centro Ricerche Fiat S.C.p.A.F. Vestrelli, F. Beltramelli, Fiat Group Automobiles, VTMS 10 Ashrae Handbook, Fundamentals, American Society of Heating Refrigerating and Air Conditioning Engineers, ASHRAE, New York, 2001, ISBN 1-883413-88-5.
[2]
98