Advanced cooling system for heavy vehicles heat exchangers/coolers Wamei Lin
Outline Introduction of my project Energy distribution in vehicle Methods for saving energy New material for heat exchangers Conclusion of graphite foam heat exchanger Future work
Introduction of my project The aim of my PhD project is to develop a new cooling system for heavy vehicles, so that the fuel consumption and CO 2 emission would be reduced. This project includes two parts: Flow field: Chalmers University of Technology (Lennart Löfdahl, Lisa Larsson) Heat transfer: Lund university (Bengt sundén, Wamei Lin)
Energy distribution in vehicle In order to save fuel consumption in vehicles, what can be done? (1) Can we reuse some energy from the engine coolant? (2) Can we recover some energy from the exhaust gas? (3) Can we recover some mechanical energy?
Engine cooling Radiator: to make sure the engine works at its optimal temperature (80-90 C). Intercooler: to cool down the fresh air, whose temperature is increased after through a turbocharger.
Methods for saving energy Engine cooling: thermal management 14.5 kw power was saved in a standard diesel vehicle (Cho 2007)
Methods for saving energy Exhaust gas (vary between 250 and 680 ) (1) Heating compartment (2) Absorption cooling: 7.1 billion gallons of gasoline was saved in U.S. vehicles (2002 Johnson) (3) Thermoelectric device: Peltier-Seebeck effect. 3-8% of fuel consumption can be saved (2007 Smith)
Methods for saving energy The mechanical work During the braking process, we can store the mechanical energy and use it to drive vehicle later. (1)Transfer the mechanical energy into electricity (2)During the braking process, the kinetic energy is used to generate a high pressure gas, which is used to drive vehicles later. 80% of kinetic energy lost in the braking process can be recovered
Methods for saving energy Thermal management Heating compartment Absorption cooling Thermoelectric device (3-10% fuel reduced) The regenerative braking system (10-25% fuel reduced) With the increasing power of vehicles and the increase of electric or hybrid electric vehicles, less heat will be dissipated in the exhaust gas, more heat has to be brought away by the cooling system.
New material for heat exchangers The thermal management development Cooling power increasing Aluminum and copper heat exchanger (180 W/(m.K) for aluminum 6061 and 400 W/(m.K) for copper) The utilization of microcellular foam materials such as metal or graphite foams (the enhancement of heat transfer by huge fluid-solid contact surface area and the fluid mixing)
New material for heat exchangers Graphite foam High thermal conductivity: (K solid =1700 W/(m.K). K eff =150W/(m.K) > K eff.al =2-26W/(m.K)) Low density: 0.2-0.6g/cm 3, 20% of that of Aluminum An appropriate material for the thermal management Large specific surface area: 5000-50000m 2 /m 3
New material for heat exchangers Problems The high pressure drop The effective area of heat transfer is reduced; A large input of pumping power, a low coefficiency of performance. Weak mechanical properties The tensile strength of graphite foam with porosity of 75 % is only 0.69 MPa. However, the tensile strength of nickel foam with the same porosity is 18.44 Mpa. The dust blocking
New material for heat exchangers In order to reduce the pressure drop of graphite foams, four different configurations of foams (pin-finned, blind-holes, corrugated and baffle) are analyzed. Graphite foam Porosity (ε) Pore diameter (D p )(um) Specific surface area (β)(m 2 /m 3 ) Effective thermal conductivity (k eff )(W/m.K) Permeability (α)(m 2 ) Forchheimer coefficient (C F ) POCO 0.82 500 5240 120 6.13x10-10 0.4457
Verification of the simulation model 15 160 140 Pressure drop (kpa) 10 5 0 Experiment [6] Simulation 0 0.02 0.04 0.06 0.08 0.1 Frontal velocity (m/s) Nusselt number Nu 120 100 80 60 40 20 0 Experiment [6] Simulation 0 0.02 0.04 0.06 0.08 0.1 Frontal velocity (m/s) The pressure drop values of the present simulation model are justified to be comparable to experiment. The Nusselt numbers of the present simulation model are slightly higher than the experimental results.
Pressure drop of graphite foam The pressure loss through the graphite foam is based on the Forchheimer extended Darcy equation dp dx u C u u f f F i i i Pressure drop (Pa) 400 350 300 250 200 150 100 50 corrugated blind-holes baffle pin-finned 15 times The corrugated and pinfinned foams have lower pressure drop, due to the short flow length (corrugated) and smooth flow path (pin-finned). The configuration has important effect on the pressure drop of foams. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frontal air velocity (m/s)
Thermal performance of graphite foam Due to the low flow resistance through the corrugated and the pin-finned foams, more cold air can reach the surface inside the foam and bring away the heat from the foam. Thus, the effective heat transfer surface is larger in the corrugated and the pin-finned foams Nusselt number Nu 160 140 120 100 80 60 40 corrugated blind-holes baffle pin-finned Nusselt number (Nu) is calculated by Nu Q ha.. T removed eff hd D Q k k A T p h removed f f b base inlet 20 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frontal air velocity (m/s) The corrugated and pin-finned foams have higher Nu
Comparison between graphite foam and aluminium louver fin Aim of comparison (1) coefficient of performance (COP, how much heat can be removed by a certain input pump power) Qremoved Qremoved COP P u A P pum in in (2) power density (PD, how much heat can be removed by a certain mass of fins) Qremoved PD 1000 m HEX (3) compactness factor (CF, how much heat can be removed in a certain volume) Qremoved CF 1000 V HEX L p (mm) Θ (degree) F p (mm) T p (mm) L w (mm) F L (mm) 1 29 2.5 14 12 50
Comparison of COP (coefficient of performance) The louver fin heat exchanger has a larger COP value than the corrugated and pin-finned foam heat exchangers at low velocity. COP 510 460 410 360 310 260 210 160 110 60 pin-finned louver fin corrugated At high velocity, the COP values are similar 10 5 6 7 8 9 10 11 12 13 14 15 Frontal velocity (m/s) Thus, by applying an appropriate configuration for graphite foam, it is possible to reduce the input pumping power, and have similar COP value as aluminium louver fin.
Comparison of PD (power density) PD (kw/kg) 100 90 80 70 60 50 40 30 20 pin-finned louver fin corrugated The corrugated and pin-finned foams have higher PD values than the louver fin 5 6 7 8 9 10 11 12 13 14 15 Frontal velocity (m/s) This means that the corrugated or pin-finned graphite foam heat exchanger is lighter than the louver fin heat exchanger, when the removed heat is the same. (Because of the low density)
Comparison of CF (compactness factor ) CF (kw/m3) 18000 16000 14000 12000 10000 8000 pin-finned louver fin corrugated The CF value of the corrugated or pin-finned foam is higher than that of the louver fin. 6000 4000 2000 5 6 7 8 9 10 11 12 13 14 15 Frontal velocity (m/s) High compactnes s in graphite foam When the removed heat is the same, the volume of the corrugated or pin-finned foam is much smaller than that of the louver fin. Because of the open cells in the foam, the heat transfer surface is larger in the corrugated or pin-finned foam than in the louver fin.
Conclusion of graphite foam heat exchanger Low pressure drop and high thermal performance have been provided by the corrugated and pin-finned graphite foams. The corrugated and pin-finned graphite foams have higher PD and CF values than the aluminium louver fin. This implies a light or compact cooling system in vehicles. By using an appropriate configuration of the graphite foam, it is possible to have the similar COP value as the aluminum louver fin.
Future work New structure for heat exchangers Due to space limitation, it might be good to change the position of heat exchanger. But when the position of the heat exchanger is changed, maybe a new structure of the heat exchanger is appropriate for the new position. 1) Maybe a countercurrent exchanger is good for the radiator which is on the top of driver compartment. 2) Designing a new configuration of heat exchanger, first the aluminum heat exchanger will be considered and analyzed. Later the graphite foam heat exchanger will be considered. Radiator 1: countercurrent exchanger compartment radiator 2 radiator 3
Ideas about countercurrent HEX a. Louver fin (cross flow) b. Louver fin (countercurrent flow) c. Pin fin (countercurrent flow) d. Wave fin (countercurrent flow)
Thank you Discussion and Questions