Porous Graphite Foam: Towards A Better Thermal Management in Vehicle

Transcription

1 Porous Graphite Foam: Towards A Better Thermal Management in Vehicle Sidharth Borkataky Master Thesis Intern Altair Engineering, Bangalore sidharthborkataky@rediffmail.com Abbreviations: ORNL- Oak Ridge National Laboratory CFD- Computational fluid dynamics COP Coefficient of performance PD Power density CF-Compactness factor Keywords: CFD, POCO graphite foam Abstract Thermal management has become an integral part in automotive industry as it focuses on increase of power production and reduction of under-bonnet space in vehicle. The present scenario of automotive industry is looking for developing heat exchangers that occupies very little space in the underhood thus helping the overall thermal management of the vehicle. Cast or foam materials are a new search area for an alternative heat exchanger. Porous carbon foam is one such material. Study done by Oak Ridge National Laboratory on graphite foam showed a lot of difference in comparison to metal foam. Designing a heat exchanger with excellent thermal properties and low pressure drop can lead to proper thermal management in vehicle. To achieve this different configuration of fin patterns has to be analyzed. However choosing the best fin is a challenging task. The fins with lowest pressure drop capacity where flow can pass easily without any hindrance is another prime aim of this project. After choosing a better fin from various literature work we come across of applying the principle of CFD towards the thermal management of the vehicle using the graphite porous corrugate fin. A suitable numerical modeling using advance meshing techniques in Acuconsole and fast and robust solver Acusolve gave results which were validated experimentally. The performance of the fins are compared with aluminum fin and suitable conclusions along with assumptions made are discussed. INTRODUCTION Engine cooling system Cooling is the most important aspect of engines. It is needed to maintain the overall working temperature of the engine. Vehicle cooling system is primarily divided into two types. 1. Air cooling system, 2. liquid cooling systems. Air cooling systems can be seen in scooter and motorcycles while in modern vehicle liquid cooling system is used. Three dimensional model of an engine cooling system is mentioned below in the figure. 1

2 Fig1 :Engine cooling system 1. Air cooling systems: The air cooling systems works on engines with a capacity of around kw. Heat generation in the engine is carried away by the fins, which are situated on various parts of the cylinder like walls, head. 2. Water cooling systems: In this system use of water jackets in the cylinder, valve followed by cooling the water using radiator fan and water which again is recirculated. Thermal management of vehicle cooling system Thermal management is a prerequisite for any modern automotive research and development organization. A good thermal management improves many features of the vehicle. To discuss some of the important benefits are good fuel economy, exhaust emission reduction, cooling system improvement, lifetime increase of engine etc. In brief thermal management has paved a path for new and improved vehicle. There are many ways of performing a thermal management when coming to automotive. The main areas of thermal management in automotive industry are cooling system consisting of radiator, fan, condenser and engine block. Some of the ways of improving thermal management are discussed in the subsequent section. New materials: Carbon Foam Carbon foam material developed by ORNL has high conductivity, low density and high porosity. These are some of the key features that made a suitable material. Moreover a large surface area with suitable properties makes it light and compact thus a useful product for thermal management. 2

3 Fig 2 : Carbon foam material heat exchanger Heat Exchangers Heat exchanger plays the most significant role in thermal management of vehicle. Ever since its development it has been classified according to various ways based on its working process. Mainly flow arrangement,construction, mechanism of heat transfer etc. are some of the ways to describe its features. In short heat exchanger is used for thermal energy transport and the medium can be solid to liquid, liquid to liquid( two or more) provided they are in thermal contact and at different temperatures. In the classifications of heat exchanger, the mode of construction of heat exchanger classification plays a very important role specially for automotive industries. In other words extended surface plays a key role in vehicle industries. The key feature is compactness that plays a significant role in the process of thermal management. A detailed diagrammatical various structure of fins are shown in figure 3.Each and every structure of the fins has a certain uniqueness that plays a key role as a fin. 3

4 Fig 3.Fin configurations of heat exchanger Corrugated Fins In the experimental study conducted by Gallego and Klett it was proved that corrugated fins have a good pressure drop reduction technique. The processing of corrugated fins is achieved with proper machining in an alternative manner thus giving a corrugated shape. As shown in figure 4 a corrugated foam geometry working procedure is analyzed when the fluid enters the foam and the forced convection takes place inside together with a constant heat flux which is exposed on the geometry. 4

5 Fig 4 : Corrugated fin exposed to heat flux The theory of porous media flow explains that the pressure drop is proportional to the length of the path through which the fluid flows. This can be a good explanation for corrugated fins to be more superior when compared to non corrugated fins in terms of pressure drop. As the flow length has been reduced in the corrugated fins and flow is uniform distributed this helps in pressure drop reduction. Modeling and Simulation The primary objective of this work is numerical modeling of the corrugated fins taken from suitable literature. Corrugated fins have been preferred over several other non corrugated fins and details regarding the physical model have been discussed in this section. The complete modeling of the heat exchanger is a challenging task as per computation cost and time is concerned, so certain assumption has been made. Grid test has been a prior important work and along with this meshing method, element size etc are discussed. The boundary conditions and some important parameters of the heat exchanger fins are discussed. Methodology The fins configuration has been taken after a detailed literature survey. The corrugated fins is preferred over other fins due to study from literature and so the corrugated geometry of the fin is chosen for modeling.. The coolant is air (incompressible) with constant properties. The other portion of the heat exchanger consists of water tubes which is kept constant which is for two important reasons (i)large heat transfer coefficient,(ii) high thermal conductivity. The fin material considered here is graphite foam. The properties of the graphite foam are mentioned in table 1. 5

6 Graphite foam type Porosity(ϕ) of the foam Pore diameter(d p)(µm) Area to volume ratio(m 2 /m 3 ) Effective thermal conductivity(λ eff) (W/m.K) Permeability (α) (m 2 ) POCO Table 1 :Graphite foam parameters Modeling The most important step in cfd before any numerical computations a proper model which is computationally correct in terms of flow has to be chosen. The flow conditions are mentioned clearly in references. The inlet flow of the air ranges from 12m/s to 20 m/s as per vehicle speed[11]. The presence of turbulence in the flow(air side) is predicted based on Reynolds number. The generation of turbulent eddies is not possible in the open pores.. The turbulence model is only applied on the upstream domain. The computational domain is modeled in hypermesh 13.0 and it is mentioned in figure 5. Downstream Outlet Heat Exchanger Region Upstream Inlet Fig.5 Computational domain Meshing Surface Meshing The hypermesh 13.0 has various meshing feature. Some of the important meshing feature which has been used in this work for generating surface mesh are discussed below. 6

7 1.Automatic 2 D meshing: This provides a unique feature of creating a surface mesh. The ability to choose variety of mesh types and size are some of the unique feature together with various other features like mapping, reviewing the mesh. 2.Topology refinement: In this feature the hypermesh provides a range of options that can alter such geometry features that create poor quality of mesh. 3.Interactive 2D meshing: This feature allows the user to have a full control over the mesh created so that at each step user can choose properly which meshing parameters he/she can choose to have a good quality mesh. Periodic Mesh The hypermesh provides a unique feature for creating periodic boundary conditions. The corrugated heat exchanger fins has periodic boundary conditions. To assign equal number of mesh on the periodic sides hyper mesh provides a unique feature to create it. The periodic boundary condition generated in this work using hypermesh feature is mentioned in the figure 6. Periodic 1 Periodic 2 Fig 6 :Periodic Boundary Condition in hypermesh Volume Meshing The volume mesh is generated in AcuConsole. It has the following advantages apart from its unique meshing feature. Some of its features applied during volume mesh of the project are discussed Global mesh attributes: This allow a range of mesh size selection. It offers options of absolute and relative mesh size selection. The absolute mesh size comes with unit size length and user can specify this based 7

8 on his/her assumptions or choice of mesh.in the relative mesh size options the value size must be higher than 0 and lower than or equal to 1. Volume mesh attributes: This attribute has absolute and relative expression similar to global mesh which has to be defined after properly defining them earlier.in a similar fashion the value has to be provide based on our global mesh attributes. Relative expression: This is used when we set up our mesh size to relative expression. The mesh is calculated based on syntax provided by the user for the relative expression. This feature multiplies the result based on maximum edge length. Curvature refinement: The curvature angle and mesh size factors can be properly set in the volume with the curvature refinement technique. Fig 7.Volume mesh in AcuConsole Grid Independence Study The table 2 shows a grid independence study based on pressure drop at the velocity of 12m/s. The study conducted could not find any difference in any pressure drop after increasing the number of elements. So in order to save the computational time we have considered the case 1 for other cases as well. Total Nodes Total Elements Table 2 : Grid Independence Study 8

9 Boundary conditions To show the entire operating conditions of the domain we have considered the periodic boundary condition. Figure 8 shows the computational domain with boundary conditions. Inlet Downstream (wall) Periodic 1(wall) Heat flux (wall) Upstream (Wall) Downstream (wall) Heat flux (Wall) Periodic 2 (wall) Downstream (wall) Pressure outlet Fig 8 : Boundary conditions of the domain 9

10 Results and Discussions Experimental Validation The experiment aims at measuring the heat transfer and pressure drop across three sample of porous carbon foam (PCF). Out of three sample one is the POCO TM foam. The coolant used was water instead of air and the graphite foam was a block size. The dimension of the geometry (block) was 50mm (wide) * 6mm (high). Figure 9: Experimental setup The experiment tested for water flow rates and power density. The variation was observed with the flow condition and power to measure the pressure drop observed together with the heat transfer on the foam. The result obtained for the three different PCF are shown below in figure

11 Fig 10 : Pressure drop vs velocity for the three PCF Validation using AcuSolve Pressure Drop(kPa) Acusolve result(kpa) Experimental(kPa)[6] Velocity(m/sec) Fig 11. Pressure drop result validation Frontal Velocity(m/s) Pressure Drop(kPa) Pressure Drop Acusolve(kPa) Table 3: Comparison between experimental and simulation 11

12 Post processing result: Pressure Drop Fig 12 Pressure distribution for velocity v=0.00m/sec9, v=0.03m/sec The pressure drop found from the calculation of the solver are compared with experimental work. The highest deviation was observed in the case for the velocity The validation result suggest clearly that it can be applied for the case of corrugated graphite foam. Simulation of the corrugated graphite foam The graphite porous foam can be a very good alternative material in comparison to aluminum fins in terms of its excellent thermal conductivity and low density. The major challenge in the graphite fin is the huge pressure drop. The various literatures were suggested and based on the literature it was suitable to consider to study the simulation on this pattern of design. To make the similar condition as like the one in the heat exchanger we have run cases for four different cases of velocity(12m/sec,15 m/sec,18m/sec,20 m/sec) and the fluid is air. The flux is provided at the fins bottom and also the periodic boundary condition is considered here for simulation. The pressure drop generated in the fins and COP, Power Density, Compactness Factor are found out for the corrugated fins and compared with experimental data with aluminum. The simulations are run for all the velocities and convergence set limit are set up as per standard requirement. The convergence plot is shown below. 12

13 Pressure Loss The Forchheimer extend Darcy equation has been followed here to properly find out the pressure loss. The results obtained for the porous graphite foam for the pressure drop are found out and it clearly matches as the Darcy law. Fig 13 Pressure in the inlet and outlet of porous fin Fin Type Pressure drop range(kpa) Frontal speed(m/sec) range Corrugated Non-corrugated(Baffle fin) Table 4 : Pressure drop comparison to non-corrugated fin[4] Thermal performance and velocity distribution Fig. 14 Temperature contour, pressure contour, velocity contour (u=15m/sec) 13

14 The thermal performance from the figure 14 suggest that the preheating effect is dominant but in compared to a baffle fin where the air mixing is weak and higher flow resitance results the overall effective surface area to reduce, hence a lower thermal performance. Performance Comparison with Graphite Fin The most important feature to test whether the fin considered in our study will lead a better performance when compared to other material is the prime aim for the study. In order to find out comparison is made with aluminum fin(louver) in terms of COP (Coefficient of Performance), PD (Pressure Density) and CF (Compactness factor).the experimental work on the aluminum fin was referred from and based on the evaluation made on this paper we laid down the correlation with graphite fin in a similar fashion comparing the simulation result with the experimental work from this paper. The pressure drop evaluation was made as the following section will show the difference between both the fins. Coefficient of performance The coefficient of performance is given by the relation : COP = Q AΔp Here, Q = Amount of heat dissipated by heat exchanger(w), u= Velocity at the inlet(m/sec), A=Area at the inlet(m 2 ), Δp= Pressure drop(pa) Fig 15.COP Comparison: Aluminum vs Graphite Power Density The power density is given as: PD = Q 1000.m Here, m=mass of the heat exchanger (kg),q = Amount of heat dissipated by heat exchanger(w) 14

15 Fig 16 :Power density comparison : Graphite fin vs Aluminum fin Compactness factor The compactness factor is given by the relation Here, CF = Q 1000.V V= Volume of the heat exchanger (m 3 ), Q = Amount of heat dissipated by heat exchanger(w) Figure 17: Compactness factor : Graphite fin vs Aluminum fin 15

16 COP result from figure 15 in suggests higher value for an aluminum fin compared to graphite due to high pressure drop. This suggest that the amount of pumping power requirement will be more for a graphite fin as compared to aluminum fin. But on a closer look at the graph suggest that the COP value is gradually decreasing for the aluminum fin with change in velocity. The result for the PD in figure 16 is higher in graphite foam as the graphite has excellent properties including small density, higher heat transfer coefficient, larger specific area. All these factors makes it a light and compact heat exchanger. Compactness factor defines how much of heat it can dissipate. The results in figure 17 suggest a very high compactness factor for a graphite fin. This reason is due to graphite foam provides a large surface area for heat transfer due to its structure for the same volume. Thus in other words a graphite foam fin is more compact. Conclusion and future work This work is a computational fluid dynamics approach towards a better thermal management in vehicle considering the graphite porous material as an alternative approach in comparison to aluminum. The reason due its excellent properties like higher thermal conductivity, low density. These properties are prime requisite before designing any heat exchanger fins. However the major hindrance towards designing graphite porous fins is the high pressure drop. This challenge can by deal by various methods. One of the method is discussed in this work. This is the design of the fin. A fin design change can create a large difference in the performance specially pressure drop. A study from various literature found that corrugated fin to be promising design for pressure reduction technique due to is flow length and less obstacle to air flow thus providing a very good option for reducing pressure drop in the fins. The CFD simulation has been performed on the corrugated fins using the same condition for a vehicle during its nominal speed and using air as coolant. Moreover the CFD result is validated with suitable literatures. The result for the corrugated fins were found out and based on the result the performance characteristics of the fins was compared to the aluminum fin. The final conclusion is that graphite foam is an excellent material for thermal management but reducing the pressure drop is the major challenge. There are lot of future work remaining in this area. Optimization of the fins design and studying the flow pattern using CFD can bring better result as compared to this fin. The research of the graphite foam machining process can bring more changes in properties of the material. Thus manufacturing aspect has got many more future work in this area. REFERENCES 1. Klett J. W., Burchell T.D., Pitched Based Carbon Foam Heat Sink With Phase Change Material, US Patent No. 6,780,505, Webb, R.L. Taylor Francis: New York, NY, USA, 1994,1995, Principles of enhanced heat transfer, John Wiley & Sons,Inc 3. Wamei Lin, Modeling and performance analysis of alternative heat exchangers for heavy vehicles, Thesis, Lund University,Award of degree: Timothy Henry Norton Jr., Modeling of corrugated graphite foam heat exchangers MS Thesis, The University of Tennessee, Knoxville, Award of degree: Gallego,N. and Klett, J. Carbon foams for thermal management, Carbon, 01/2003; 41(7): Nidia C. Gallego, James W. Klett, Carbon Materials Technology Group, Metals and Ceramic Division Oak Ridge National Laboratory 7. Y.Chang, K Hsu., Y.Lin, C Wang, A generalized heat transfer correlation for louver fin geometry,international Journal Of Heat And Mass Transfer,40 (3), Straatman A.G., Gallego N.C., Yu Q, and Thompson B.E.,2007, Characterization of porous carbon foam as a material for compact recuperators, ASME Journal Of Engineering for Gas Turbines and Power, 129(2),